Petrologic Constraints on the Thermal Structure of the Cascades Arc

Journal of Volcanology and Geothermal Research 140 (2005) 67–105 www.elsevier.com/locate/jvolgeores

Petrologic constraints on the thermal structure of the Cascades arc

William P. Leemana,*, Jared F. Lewisa, Russell C. Evartsb, Richard M. Conreyc, Martin J. Streckd

aDepartment of Earth Science, Rice University, Houston, TX 77005, United States bU.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, United States cDepartment of Geology, Washington State University, Pullman, WA 99164, United States dDepartment of Geology, Portland State University, Portland, OR 97207-0751, United States Received 2 December 2003; received in revised form 15 April 2004; accepted 15 July 2004

Abstract

Primitive late Cenozoic basaltic lavas from the Cascades volcanic arc near latitude 468N comprise two distinct compositional groups. Group I includes samples with low Ba/Nb (b20) and other compositional similarities to oceanic island and MORB lavas from within-plate settings. In contrast, Group II exhibits enrichment of Ba and large-ion lithophile elements (LILE) and depletion of Nb and high-field strength elements (HFSE) as seen commonly in calcalkalic lavas from other volcanic arcs. Lavas of both groups are widely distributed across the transect, and Group I lavas are found as much as 30–40 km trenchward of stratovolcanoes that define the High Cascades dvolcanic front (VF)T. The most primitive lavas are sparsely porphyritic, have elevated Ni, Cr, and Mg#, high calculated magmatic temperatures (1200–1300 8C), and lack evidence of shallow (crustal level) storage and crystallization. Compositions of parental liquids were calculated for each primitive sample on the premise of Fe–Mg equilibrium with mantle peridotite. Assuming that such magmas ascended rapidly from accumulation zones in the mantle, we estimate P and T of segregation. We infer that (a) Group I magmas ascended from systematically greater depths (~50–70 km) than Group II (~30–50 km), implying the possible existence of compositional stratification in the mantle wedge; (b) Group I basalts show the least evidence for slab-derived contributions in their sources despite their apparently greater segregation depths (approaching the locus of the Cascadia slab beneath the frontal arc region); (c) Group II lavas with the strongest slab compositional signature have temperatures far exceeding the wet peridotite solidus at high pressure; and (d) the inferred thermal structure of the mantle wedge is very warm, implying a significant component of mantle upwelling and convection. Group I lavas are interpreted as decompression melts from this mantle, and their compositions suggest that their source was little modified by slab-derived contributions. We speculate that melting to produce Group II magmas occurs in the shallow mantle, possibly in response to heating by hot ascending Group I magmas. If true, it seems unlikely that the slab-like signal in Group II lavas can be attributed to modern slab inputs; rather, we postulate that this signature may reflect melting of lithospheric mantle domains containing a dstoredT slab-derived component inherited from earlier stages of Cascadia subduction. This scenario differs from the standard paradigm for subduction zones (SZs), and stresses the importance of convecting

* Corresponding author. Tel.: +1 713 348 4892; fax: +1 713 348 5214. E-mail addresses: [email protected] (W.P. Leeman)8 [email protected] (R.C. Evarts)8 [email protected] (R.M. Conrey)8 [email protected] (M.J. Streck).

0377-0273/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2004.07.016 68 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 asthenospheric mantle in driving arc magmatism, particularly in warm subduction zones where slab fluid contributions likely are minimal. In contrast, because tectonic conditions in more typical volcanic arcs favor subduction of cooler, less dehydrated oceanic lithosphere, slab-derived fluids may promote extensive flux-melting in the wedge. Such melts may dominate the magmatic output and mask wedge contributions. The Cascade arc thus provides rarely afforded insights into arc magma genesis. D 2004 Elsevier B.V. All rights reserved.

Keywords: Cascades arc; subduction zone; mantle thermal structure; primitive arc magma

1. Introduction arc near latitude 468N. Our findings bear on the origin of these magmas and on the general thermal structure The Cascades convergent margin represents an of the underlying mantle wedge. Also, our results raise unusual end member variant with regard to both its important questions concerning the relative contribu- subduction characteristics and the compositional tions to arc magma production of subducted materials diversity of associated magmatism. First, slow sub- as opposed to components intrinsic to the mantle duction of the relatively young Juan de Fuca and wedge. We suggest (1) that direct subduction inputs Gorda plates beneath North America results in (fluids and/or melts) are relatively minor in the modern unusually warm conditions within the subduction Cascades owing to the warm thermal structure of the zone (SZ) (Hyndman and Wang, 1993; Harry and SZ, and (2) that decompression melting in the Green, 1999). Accordingly, Benioff–Wadati zone convecting mantle wedge contributes significantly to seismicity is limited to relatively shallow depths magma production in this setting. The relative (b100 km; Crosson and Owens, 1987; Weaver and importance of these contributions may vary along- Baker, 1988), below which the slab descends aseismi- strike, in response to variations in the age of the slab at cally to at least ~300 km depth (Michaelson and depths below the active arc (Green and Harry, 1999), Weaver, 1986; Rasmussen and Humphreys, 1988; or differences in the nature of the subducting slab Harris et al., 1991; Benz et al., 1992). The region is (Grove et al., 2002, 2003). also complicated by upper plate deformation that, in the forearc region, is characterized by northward compression (Mazzotti et al., 2002; Wang et al., 2. Geologic setting and background 2003). South of the Columbia River, there has been significant late Cenozoic intra-arc E–W extension The Quaternary Cascades volcanic arc extends (Wells, 1990; Conrey et al., 1997; Wells et al., 1998). from northern California (Lassen volcanic center) to Second, although the stratovolcano centers produce southern British Columbia (Fig. 1a). Relations a spectrum of variably evolved (mainly andesitic to between magmatism and plate tectonic setting are dacitic) magmas, late Cenozoic magmatism in the summarized by Green and Harry (1999),who Cascades volcanic arc is fundamentally basaltic subdivided the arc into the northern Garibaldi belt (McBirney, 1978; Green and Harry, 1999). Basaltic and the southern High Cascades belt. The volcanic lavas are produced mainly from diffusely distributed front (VF) commonly is defined by locations of monogenetic and shield vents; they are most extensive modern stratovolcanoes (most b1 Ma; Sherrod and in the region between central Washington and north- Smith, 1990). Above ~478N latitude, late Cenozoic ern California, but are present along the entire arc. As magmatism is less voluminous and focused near the summarized below, detailed study of these lavas by main stratovolcanoes, which are separated by wide many workers has established that they are composi- non-volcanic gaps. To the south, vents are more tionally diverse and generally distinct from basaltic densely and, in places, more broadly distributed. The suites found at most other convergent margins. VF displays along-strike offsets that define distinct This paper reviews previous models and presents sectors (Guffanti and Weaver, 1988); some of these new inferences concerning the conditions of formation offsets are associated with broader cross-arc vent of Cascadia basalts based on a regional sampling of the distributions and may coincide with structural and W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 69

Fig. 1. (a) General tectonic setting of the Cascade volcanic arc, with principal stratovolcanoes named and areal distribution of Neogene volcanic rocks shown in shaded field (after Luedke and Smith, 1981). (b) Detailed map view of the inset box shows locations of primitive basaltic lavas included in Table 1. Reference points are the city of Portland and the stratovolcanoes of Mts. St. Helens, Hood, and Adams. Other localities denoted are the Indian Heaven (IH), Simcoe (Sim), and Tygh Valley (TV) volcanic fields. Symbols denote the magma subtypes defined in the text. Group I lavas include low-K tholeiite (LKT), hy-normative ocean island type (OIB), and ne-normative ocean island type (Alk) basalts. Group II lavas include calcalkalic (CAB) and high-K calcalkalic (HKCA) basalts, magnesian basaltic andesite (BA), and shoshonite (SHO). Note that both CAB and HKCA are denoted as dCABT in the legend, and both are shown using the same symbol (open triangles) because their distributions and compositions overlap extensively. The same symbols are used in all subsequent data plots. 70 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105

Table 1 Primitive Cascades Columbia Transect (CCT) lavas

Sample N W Unit/vent Reference SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2OK2OP2O5 Latitude Longitude

Group I. Intraplate type lavas—low Ba/Nb, Ba/TiO2 Low-K tholeiite (LKT) RC02-4 45.47 122.50 FA, l. Horse 6 48.98 1.38 17.07 11.18 0.19 8.02 9.94 2.96 0.16 0.13 Barn lava RC02-152 45.43 122.49 FA, Rock Ck; 6 49.10 1.39 16.94 10.94 0.19 9.24 9.24 2.63 0.20 0.13 Sunnyside RMC92-8 45.60 122.00 FA 1 48.25 1.16 16.65 11.63 0.18 9.70 9.49 2.75 0.09 0.10 MA-696 46.42 121.76 IH 7 47.71 1.40 16.97 11.12 0.18 8.89 10.55 2.86 0.16 0.16 DS-7A-80 45.90 121.71 IH, Qbl 3 48.87 1.11 17.66 10.27 0.17 8.53 9.89 3.01 0.31 0.18 DS-9-80 45.87 121.72 IH, Qbl 3 48.79 1.24 17.66 10.11 0.17 8.07 10.46 3.00 0.31 0.19 DS-14-80 46.06 121.90 IH, Qbp 3 49.61 1.01 18.11 9.62 0.17 7.93 10.17 2.93 0.29 0.17 DS-17B-80 46.08 121.86 IH, Qbp 3 49.27 1.10 17.88 10.16 0.17 7.94 10.00 3.07 0.23 0.17 DS-19C-80 46.05 121.86 IH, Qbp 3 49.02 1.12 18.06 10.24 0.18 7.60 10.34 3.07 0.21 0.16 DS-36A-80 45.97 121.66 IH, Qic 3 48.91 1.09 18.37 9.99 0.16 7.73 10.24 3.12 0.23 0.15 DS-80A-80 45.88 121.52 IH, Qic 3 48.30 1.16 17.51 10.98 0.18 8.37 9.89 3.23 0.22 0.16 DS-61-80 46.03 121.68 IH, Qic 3 49.69 1.22 18.00 9.74 0.16 7.39 9.97 3.33 0.30 0.19 DS-52B-80 45.96 121.49 IH, Qic 3 48.89 1.02 17.61 11.13 0.17 8.16 9.61 3.05 0.19 0.16 DS-24A-81 45.97 121.62 IH, Qic 3 49.10 1.18 17.43 10.58 0.17 7.69 10.19 3.20 0.29 0.17 DS-34-80 45.99 121.57 IH, Qic 3 48.80 1.09 17.56 11.27 0.18 8.01 9.61 3.13 0.19 0.15 MA-767 46.06 121.25 MA 7 48.32 1.23 16.38 10.94 0.17 9.66 10.15 2.86 0.17 0.11 MA-120 46.32 121.50 MA, Qbsc 7 48.22 1.43 16.81 10.69 0.18 8.84 10.32 3.04 0.31 0.16 L83-49 46.34 121.62 MA, Qbsc 3 48.95 1.52 16.82 10.69 0.17 7.89 10.40 3.04 0.33 0.17 xL83-57 45.98 121.13 MA, Qob 3 50.21 1.30 17.09 10.33 0.16 7.50 9.84 3.09 0.35 0.13 xRC02-172 45.45 122.21 FA, Boring, 6 49.19 1.31 17.54 11.38 0.18 7.16 10.12 2.91 0.11 0.10 Aims village xRC02-170 45.48 122.28 FA, Boring, 6 49.77 1.36 17.36 11.24 0.18 7.24 9.58 2.93 0.22 0.13 Gordon Ck xRC92-TB6 45.46 122.25 FA, Boring, 6 49.08 1.29 18.43 11.38 0.20 7.06 9.61 2.73 0.10 0.13 Palmer Mill Rd xRCDA- 45.47 122.50 FA, u. Horse 6 49.26 1.40 17.92 10.80 0.19 7.25 9.80 3.06 0.20 0.14 9102B Barn lava xDS-9A-81 45.98 121.76 IH, Qlc 3 49.66 1.10 17.37 9.82 0.17 8.59 9.51 3.10 0.48 0.19 xDS-41-81 46.02 121.78 IH, Qlc 3 49.56 1.07 17.32 10.20 0.17 8.40 9.57 3.08 0.43 0.20 xDS-62-80 45.95 121.72 IH, Qoc 3 49.92 1.17 16.88 9.97 0.17 8.51 9.60 3.08 0.48 0.22 L83-20 45.14 121.25 Tygh V. 6 49.39 1.37 17.08 10.96 0.17 7.83 9.51 3.09 0.43 0.16 L83-25 45.09 121.22 Tygh V. 6 50.04 1.19 17.48 10.60 0.17 7.33 9.83 3.04 0.23 0.09 L83-22 45.13 121.23 Tygh V. 6 49.28 1.29 16.78 11.64 0.18 8.02 9.26 3.13 0.30 0.12 L83-23 45.10 121.24 Tygh V. 6 49.62 1.33 17.25 10.83 0.17 7.42 9.70 3.05 0.46 0.18 L83-24 45.11 121.26 Tygh V. 6 49.83 1.39 17.13 10.87 0.17 7.25 9.65 3.10 0.41 0.19 xL82-39 45.76 121.83 Wind R. 6 50.48 1.21 17.56 10.11 0.17 7.58 9.28 3.03 0.42 0.15 Hy-normative OIB-like basalts (OIB) QV01-34 45.56 122.83 FA, Kaiser 6 50.14 2.08 16.08 9.85 0.16 7.64 9.05 3.32 1.28 0.40 Rd/Bethany V185A 46.02 122.34 FA, Speelyai Ck 6 52.15 1.51 16.68 8.04 0.13 8.28 8.13 3.57 1.11 0.41 87CG-V185B 46.02 122.34 FA, Speelyai Ck 6 52.39 1.51 16.66 7.51 0.12 8.03 8.61 3.66 1.10 0.39 DS-26A-80 45.94 121.85 IH, Qbc 3 49.34 1.40 17.62 9.31 0.16 8.32 9.64 3.23 0.70 0.27 DS-1-80 45.90 121.83 IH, Qbc 3 52.21 1.46 17.53 8.88 0.15 6.63 8.18 3.68 0.89 0.39 DS-2A-80 45.94 121.82 IH, Qbc 3 52.34 1.56 17.31 8.60 0.15 6.63 8.29 3.85 0.92 0.35 DS-20-80 46.05 121.84 IH, Qbp 3 52.24 1.44 16.77 8.21 0.14 7.71 8.57 3.55 1.07 0.30 DS-58B-80 46.05 121.75 IH, Qhl 3 50.93 1.11 16.65 9.18 0.16 8.74 9.22 3.06 0.71 0.24 DS-60-80 46.04 121.72 IH, Qhl 3 50.60 1.15 16.98 9.55 0.16 8.20 9.29 3.16 0.67 0.24 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 71

Mg# Ni Cr V Ba Rb Sr Zr Y Nb (Sr/P)n Ba/ Ba/ Nb/ Ba/ Ba/Y Sr/Y K/Ti Ca/ Fe/ Equilibrium T-erupt TiO2 Nb Zr Zr Al Mg olivine (Fo)

60.1 151 230 194 72 – 288 89 27 3.5 2.3 52 20.6 0.039 0.81 2.7 10.7 0.12 0.58 1.39 82.9 1237

63.9 148 242 208 82 – 261 90 28 4.9 2.0 59 16.7 0.054 0.91 2.9 9.3 0.14 0.55 1.18 85.1 1265

63.6 208 260 178 71 1 238 73 23 3.9 2.5 61 18.2 0.053 0.97 3.1 10.3 0.08 0.57 1.20 85.0 1283 62.6 178 275 217 58 3.3 247 98 26 6.0 1.6 41 9.7 0.061 0.59 2.2 9.5 0.11 0.62 1.25 84.4 1268 63.5 126 291 184 98 – 327 80 20 – 1.9 88 – – 1.23 5.0 16.7 0.28 0.56 1.20 84.9 1249 62.6 118 263 208 99 – 343 90 21 – 1.9 80 – – 1.10 4.6 16.1 0.25 0.59 1.25 84.4 1239 63.3 127 232 170 70 – 257 75 21 – 1.6 69 – – 0.93 3.3 12.1 0.29 0.56 1.21 84.8 1229 62.1 116 241 175 67 – 242 83 21 – 1.5 61 – – 0.81 3.2 11.4 0.21 0.56 1.28 84.1 1232 60.9 95 208 173 58 – 233 81 21 – 1.5 52 – – 0.72 2.8 11.3 0.19 0.57 1.35 83.4 1226 61.9 103 199 154 63 – 286 71 15 – 2.0 58 – – 0.89 4.2 18.9 0.21 0.56 1.29 84.0 1229 61.5 110 208 187 63 2.5 259 75 19 3.8 1.7 54 16.6 0.051 0.84 3.4 13.8 0.19 0.56 1.31 83.8 1250 61.4 78 – 176 69 4.3 295 88 17 5.2 1.6 56 13.3 0.060 0.78 4.1 17.5 0.25 0.55 1.32 83.7 1216 60.6 121 243 174 56 1.9 246 63 20 3.8 1.6 55 14.7 0.060 0.89 2.8 12.2 0.19 0.55 1.36 83.2 1240 60.4 97 201 172 69 – 291 80 18 – 1.8 58 – – 0.86 3.8 16.2 0.25 0.58 1.38 83.1 1227 59.8 114 237 182 55 – 237 65 19 – 1.6 50 – – 0.85 2.9 12.7 0.17 0.55 1.41 82.8 1237 64.9 143 345 189 68 3 243 83 23 4.9 2.3 55 13.9 0.059 0.82 3.0 10.6 0.14 0.62 1.13 85.7 1281 63.4 134 300 212 81 6.9 289 112 24 7.7 1.9 57 10.5 0.069 0.72 3.4 12.0 0.22 0.61 1.21 84.8 1262 60.7 103 252 178 76 5.6 282 111 25 8.8 1.7 50 8.6 0.079 0.68 3.1 11.4 0.22 0.62 1.35 83.3 1234 60.3 43 – 183 102 5.3 295 107 22 8.2 2.3 78 12.4 0.076 0.95 4.7 13.5 0.27 0.58 1.38 83.1 1216 56.9 122 256 191 41 – 236 80 29 4.9 2.5 31 8.4 0.061 0.51 1.4 8.1 0.09 0.58 1.59 81.0 1215

57.5 125 249 202 43 2 249 89 25 5.3 2.0 32 8.1 0.060 0.48 1.7 10.0 0.16 0.55 1.55 81.3 1213

56.6 118 235 191 58 1 265 84 25 5.5 2.1 45 10.6 0.065 0.69 2.3 10.6 0.08 0.52 1.61 80.8 1213

58.5 130 221 204 65 1 306 94 28 4.7 2.4 47 13.8 0.050 0.69 2.3 10.9 0.14 0.55 1.49 82.0 1216

64.7 133 254 187 130 6.2 382 107 22 6.5 2.1 118 20.0 0.061 1.21 5.9 17.4 0.44 0.55 1.14 85.5 1244 63.3 129 252 189 124 – 371 90 23 – 1.9 116 – – 1.38 5.4 16.1 0.40 0.55 1.21 84.8 1241 64.2 146 292 195 147 – 371 102 22 – 1.8 126 – – 1.44 6.6 16.6 0.41 0.57 1.17 85.2 1241 60.0 100 – – – 5.4 439 115 25 7.2 2.9 – – 0.063 – – 17.6 0.31 0.56 1.40 82.9 1229 59.2 105 – – – 4.7 294 78 22 4.4 3.4 – – 0.056 – – 13.4 0.19 0.56 1.45 82.4 1213 59.1 112 – – – 4.3 259 86 21 6.1 2.2 – – 0.071 – – 12.3 0.23 0.55 1.45 82.3 1234 59.0 97 – – – 5.4 429 108 25 6.6 2.5 – – 0.061 – – 17.2 0.35 0.56 1.46 82.3 1218 58.3 91 – – – 6.1 426 112 25 7.3 2.3 – – 0.065 – – 17.0 0.29 0.56 1.50 81.9 1213 61.1 55 – – – 11 270 105 25 8.1 1.9 – – 0.077 – – 10.8 0.35 0.53 1.33 83.5 1216

61.9 154 295 212 389 18 695 169 23 30.1 1.8 187 12.9 0.178 2.30 16.9 30.2 0.62 0.56 1.29 84.0 1220

68.4 174 230 160 370 18 725 160 22 21.0 1.8 246 17.6 0.131 2.31 16.8 33.0 0.74 0.49 0.97 87.5 1222 69.2 189 276 – 324 14 739 152 17 20.3 2.0 214 16.0 0.134 2.13 19.1 43.5 0.73 0.52 0.94 87.9 1215 65.2 158 356 223 219 – 483 138 22 – 1.9 156 – – 1.59 9.9 21.8 0.50 0.55 1.12 85.8 1240 61.0 111 184 166 266 – 554 160 21 – 1.5 182 – – 1.66 12.9 26.9 0.61 0.47 1.34 83.5 1185 61.8 90 184 160 268 12 554 174 22 21.0 1.7 172 12.8 0.121 1.54 12.2 25.3 0.59 0.48 1.30 83.9 1184 66.3 146 264 179 240 19 562 151 16 1.9 167 – – 1.59 14.6 34.3 0.74 0.51 1.06 86.4 1209 66.6 158 336 193 189 – 449 124 23 – 1.9 170 – – 1.52 8.1 19.4 0.64 0.55 1.05 86.6 1239 64.3 108 243 186 162 11 447 126 22 – 1.9 141 – – 1.29 7.5 20.6 0.58 0.55 1.16 85.3 1229 (continued on next page) 72 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105

Table 1 (continued)

Sample N W Unit/vent Reference SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2OK2OP2O5 Latitude Longitude

Group I. Intraplate type lavas—low Ba/Nb, Ba/TiO2 Hy-normative OIB-like basalts (OIB) DS-67-80 46.07 121.59 IH, Qldp 3 51.24 1.59 16.89 8.19 0.15 8.10 8.72 3.70 1.03 0.37 DS-69A-80 46.06 121.57 IH, Qldp 3 52.37 1.40 16.78 8.08 0.14 7.92 8.38 3.62 0.94 0.37 DS-21B-80 46.03 121.83 IH, Qoc 3 50.91 1.29 16.73 9.71 0.16 8.36 8.58 3.33 0.66 0.27 L01-24 45.97 121.39 MA, Quigley 6 51.55 1.42 16.76 8.95 0.15 7.54 8.99 3.49 0.84 0.31 Butte L01-25 46.06 121.33 MA, King Mtn 6 51.85 1.34 16.99 9.20 0.16 7.50 8.71 3.33 0.64 0.28 MA394 46.30 121.50 MA 2 51.21 1.56 16.47 9.45 0.14 7.64 8.45 3.72 0.97 0.39 MA88 46.04 121.49 MA 2 50.50 1.42 16.80 10.19 0.14 7.56 8.92 3.40 0.76 0.30 MA7 46.03 121.47 MA 2 51.46 1.38 16.49 10.04 0.14 7.25 8.55 3.47 0.95 0.28 MA381 46.07 121.54 MA 2 52.27 1.51 16.19 9.94 0.14 6.59 8.13 3.70 1.19 0.34 L82-50 46.35 121.96 Marble Mtn 6 52.09 1.41 16.01 9.18 0.14 8.03 8.23 3.54 1.11 0.26 00MSH-V117 46.09 122.11 Marble Mtn 6 52.22 1.42 16.36 9.04 0.15 7.70 8.28 3.49 1.01 0.33 L82-51 46.27 121.92 Marble Mtn 6 51.61 1.45 16.28 9.30 0.15 7.81 8.39 3.60 1.12 0.28 L82-49 46.34 121.92 Marble Mtn 6 52.18 1.39 16.19 9.26 0.15 7.57 8.40 3.59 0.99 0.27 L82-63 46.14 122.24 MSH, Castle Ck 6 51.43 1.83 17.19 9.01 0.14 6.78 8.38 3.81 1.09 0.33 L01-17 46.21 122.25 MSH, Castle Ck 6 51.82 1.76 16.93 9.15 0.14 6.57 8.39 3.87 1.00 0.36 D151C 45.90 120.82 Simcoe 4 47.26 2.73 15.17 11.51 0.18 8.86 9.45 2.75 1.31 0.78 L83-96 45.85 120.79 Simcoe 3 49.42 2.20 16.18 11.00 0.16 7.16 9.70 3.14 0.70 0.35 L83-93 45.90 120.71 Simcoe 3 49.44 2.22 16.42 10.85 0.16 7.01 9.60 3.21 0.75 0.34 L83-95 45.85 120.79 Simcoe 3 49.50 2.19 16.35 11.06 0.15 7.10 9.47 3.12 0.71 0.34 L83-92 45.90 120.71 Simcoe 3 49.33 2.24 16.76 11.06 0.15 6.56 9.75 3.14 0.66 0.35 L84-66 46.02 120.63 Simcoe 3 49.19 2.11 16.82 10.48 0.15 6.19 10.54 3.41 0.72 0.39 L83-94 45.92 120.71 Simcoe 3 49.14 2.57 15.50 11.99 0.16 6.88 9.39 3.22 0.76 0.39 Ne-normative OIB-like basalts (Alk) *QV97-02 45.83 122.51 FA, 6 49.38 2.00 16.55 9.26 0.15 8.01 9.37 3.32 1.41 0.54 Battleground *L00-8 45.83 122.50 FA, 6 49.62 2.00 16.58 9.06 0.15 7.90 9.35 3.40 1.40 0.55 Battleground *WDS87-82 44.85 121.83 FA 1 48.80 1.71 15.89 10.05 0.16 9.94 8.90 3.35 0.82 0.38 *DS49A-80 46.02 121.60 IH, Qfm 3 49.53 1.95 16.40 9.39 0.16 8.46 8.98 3.47 1.24 0.43 *DS48A-80 46.02 121.61 IH, Qfm 3 49.66 2.08 16.12 9.55 0.16 8.18 9.20 3.48 1.18 0.40 *MA421 46.03 121.48 MA 2 49.34 1.93 15.52 10.45 0.15 8.41 9.10 3.53 1.13 0.45 *MA-21 46.27 121.43 MA, bsb 2 48.71 1.90 16.55 9.51 0.15 8.15 9.43 3.59 1.50 0.51 *MA-303 46.05 121.51 MA, bsb 2 48.52 2.10 16.22 9.52 0.15 8.14 9.36 3.84 1.58 0.57 *MA20 46.04 121.52 MA, bsb 2 48.46 1.89 16.46 10.23 0.15 7.94 9.16 3.73 1.44 0.55 *MA312 46.05 121.52 MA, bsb 2 48.75 1.93 16.32 10.25 0.15 7.82 9.26 3.56 1.42 0.55 *MA-91 46.27 121.43 MA, btc 2 49.82 1.78 16.38 9.31 0.15 7.90 9.72 3.36 1.20 0.37 *H-SH17 46.22 122.17 MSH, Castle Ck 5 49.96 1.95 16.98 9.37 0.15 7.00 9.01 3.89 1.28 0.40 *DS-5 46.22 122.17 MSH, Castle Ck 3 49.34 2.00 16.90 10.13 0.14 7.17 8.71 3.90 1.26 0.44 *DS-6 46.22 122.17 MSH, Castle Ck 3 49.96 2.04 16.85 10.14 0.15 6.62 8.63 3.89 1.31 0.41 *DS-7 46.22 122.17 MSH, Castle Ck 3 50.02 1.98 16.28 10.55 0.15 6.84 8.55 3.91 1.34 0.38 *DS-9 46.22 122.17 MSH, Castle Ck 3 48.99 2.10 16.70 11.14 0.15 6.68 8.72 3.79 1.36 0.37 *D151A 45.90 120.90 Simcoe 4 46.74 2.92 14.43 12.31 0.19 9.15 9.51 2.82 1.33 0.60 *SM-27 45.98 120.96 Simcoe 7 49.45 2.30 16.45 10.54 0.15 6.34 7.40 4.21 2.27 0.91 *L83-67 45.68 120.89 Simcoe 3 50.48 2.25 16.43 10.55 0.16 6.06 7.20 4.13 2.00 0.74

Group II. Calcalkalic group—high Ba/Nb, Ba/TiO2 Calcalkalic basalt (CA) L01-7 45.65 122.19 Bobs Mtn 6 50.02 1.25 17.11 8.23 0.13 8.77 9.67 3.30 1.05 0.46 QV98-16 45.65 122.18 Bobs Mtn S 6 50.11 1.29 17.51 8.51 0.13 8.52 9.13 3.22 1.12 0.46 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 73

Mg# Ni Cr V Ba Rb Sr Zr Y Nb (Sr/P)n Ba/ Ba/ Nb/ Ba/ Ba/Y Sr/Y K/Ti Ca/ Fe/ Equilibrium T-erupt TiO2 Nb Zr Zr Al Mg olivine (Fo)

67.5 155 335 181 282 12 589 182 20 20.6 1.7 177 13.7 0.113 1.55 14.4 30.1 0.65 0.52 1.01 87.0 1222 67.3 186 373 184 268 13 569 157 19 18.1 1.6 191 14.8 0.115 1.71 14.0 29.8 0.67 0.50 1.02 86.9 1213 64.4 189 312 175 199 – 458 133 20 – 1.8 154 – – 1.50 10.1 23.2 0.51 0.51 1.16 85.4 1231 63.8 123 227 183 238 13 582 151 23 13.6 2.0 167 17.5 0.090 1.58 10.3 25.3 0.59 0.54 1.19 85.1 1209

63.1 124 214 191 235 11 450 139 23 12.8 1.7 175 18.4 0.092 1.69 10.2 19.6 0.48 0.51 1.23 84.7 1207 62.9 158 276 161 278 16.5 581 173 23 23.0 1.6 178 12.1 0.133 1.61 12.4 25.8 0.62 0.51 1.24 84.5 1213 60.9 129 264 168 253 13 605 163 25 14.0 2.1 178 18.1 0.086 1.55 10.1 24.2 0.54 0.53 1.35 83.4 1215 60.2 158 296 162 254 23 463 193 27 17.0 1.7 184 14.9 0.088 1.32 9.4 17.1 0.69 0.52 1.38 83.0 1203 58.2 109 270 177 289 25 513 206 28 18.0 1.6 191 16.1 0.087 1.40 10.3 18.3 0.79 0.50 1.51 81.8 1185 64.7 159 – – – 19 607 170 20 17.0 2.4 – – 0.100 – – 30.4 0.79 0.51 1.14 85.6 1217 64.1 150 244 – 251 14 538 151 20 16.0 1.7 177 15.7 0.106 1.66 12.6 26.9 0.72 0.51 1.17 85.2 1209 63.8 145 – – – 18 615 164 20 16.0 2.3 – – 0.098 – – 30.8 0.77 0.52 1.19 85.0 1214 63.2 136 – – – 15 552 153 19 15.5 2.1 – – 0.101 – – 29.1 0.71 0.52 1.22 84.7 1206 61.2 103 – – – 20 574 176 21 24.0 1.8 – – 0.136 – – 27.3 0.60 0.49 1.33 83.6 1192 60.1 95 180 190 259 17 513 157 24 21.2 1.5 147 12.2 0.135 1.65 10.8 21.4 0.57 0.50 1.39 82.9 1186 61.7 –––– – – –––– – – – – – – 0.48 0.62 1.30 83.9 1273 57.7 91 187 185 171 12 552 158 24 20.0 1.6 78 8.6 0.127 1.08 7.1 23.0 0.32 0.60 1.54 81.5 1214 57.5 76 158 193 178 13 548 155 23 20.0 1.7 80 8.9 0.129 1.15 7.7 23.8 0.34 0.58 1.55 81.4 1210 57.4 90 212 169 189 12 544 150 22 18.0 1.7 86 10.5 0.120 1.26 8.6 24.7 0.32 0.58 1.56 81.3 1212 55.4 80 160 173 181 8.7 554 156 23 21.0 1.6 81 8.6 0.135 1.16 7.9 24.1 0.29 0.58 1.69 80.1 1200 55.3 73 122 198 201 9 548 155 23 20.0 1.5 95 10.1 0.129 1.30 8.7 23.8 0.34 0.63 1.69 80.0 1192 54.6 87 159 186 182 11.6 501 164 27 25.6 1.3 71 7.1 0.156 1.11 6.8 18.8 0.30 0.61 1.74 79.5 1209

64.5 130 227 198 372 16 914 196 21 31.5 1.8 186 11.8 0.161 1.90 17.7 43.5 0.71 0.57 1.16 85.4 1233

64.6 123 220 206 355 18 841 193 21 32.5 1.6 178 10.9 0.168 1.84 16.9 40.0 0.70 0.56 1.15 85.5 1228

67.5 165 354 200 282 8 619 152 22 24.2 1.7 165 11.7 0.159 1.86 12.8 28.1 0.48 0.56 1.01 87.0 1283 65.4 170 313 213 327 – 688 183 21 – 1.7 168 – – 1.79 15.6 32.9 0.64 0.55 1.11 85.9 1242 64.2 147 269 210 298 15.3 661 191 20 31.9 1.7 143 9.3 0.167 1.56 14.8 32.7 0.57 0.57 1.17 85.3 1235 62.8 166 278 166 311 19 680 171 22 26.0 1.6 161 12.0 0.152 1.82 14.5 31.6 0.59 0.59 1.24 84.5 1243 64.2 150 – – 348 26 998 207 23 24.0 2.0 183 14.5 0.116 1.68 15.1 43.4 0.79 0.57 1.17 85.3 1240 64.2 149 – – 424 30 918 213 26 33.0 1.7 202 12.9 0.155 1.99 16.3 35.3 0.75 0.58 1.17 85.3 1241 61.9 134 278 178 371 24 862 205 27 33.0 1.6 196 11.2 0.161 1.81 13.7 31.9 0.76 0.56 1.29 84.0 1237 61.5 127 276 177 365 23 824 199 25 29.5 1.6 189 12.4 0.148 1.83 14.6 33.0 0.74 0.57 1.31 83.8 1233 64.0 119 306 195 256 21 485 184 23 26.0 1.4 144 9.9 0.141 1.39 11.1 21.1 0.67 0.59 1.18 85.2 1227 61.0 82 159 230 309 20 577 174 24 22.0 1.5 158 14.1 0.126 1.78 12.9 24.0 0.66 0.53 1.34 83.5 1205 59.7 86 196 219 272 27 589 207 – – 1.4 136 – – 1.31 – – 0.63 0.52 1.41 82.7 1213 57.8 97 196 212 316 25 620 203 25 32.7 1.6 155 9.7 0.161 1.56 12.7 24.9 0.64 0.51 1.53 81.5 1196 57.6 81 157 211 319 27 577 203 – – 1.6 161 – – 1.57 – – 0.68 0.53 1.54 81.4 1201 55.7 84 155 214 312 24 590 200 26 28.5 1.7 148 11.0 0.143 1.56 11.9 22.5 0.65 0.52 1.67 80.2 1204 60.9 –––– – – –––– – – – – – – 0.46 0.66 1.35 83.4 1286 55.8 103 129 147 672 37.6 995 275 31 57.0 1.1 292 11.8 0.207 2.44 21.7 32.1 0.99 0.45 1.66 80.3 1193 54.6 137 144 129 554 34 861 257 27 50.0 1.2 246 11.1 0.195 2.16 20.5 31.9 0.89 0.44 1.74 79.5 1181

69.1 214 317 154 788 3 1558 151 18 8.3 3.5 631 94.9 0.055 5.22 43.8 86.6 0.84 0.57 0.94 87.8 1245 67.7 202 298 170 740 4 1382 157 19 8.4 3.2 574 88.1 0.054 4.71 38.9 72.7 0.87 0.52 1.00 87.1 1239 (continued on next page) 74 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105

Table 1 (continued)

Sample N W Unit/vent Reference SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2OK2OP2O5 Latitude Longitude

Group II. Calcalkalic group—high Ba/Nb, Ba/TiO2 Calcalkalic basalt (CA) QV97-15 45.64 122.17 Bobs Mtn S 6 51.39 1.12 16.80 7.65 0.13 8.32 9.61 3.69 1.02 0.27 92TB16 45.51 122.74 FA, Boring Lava 1 52.07 1.32 16.49 8.15 0.13 7.89 8.76 3.63 1.20 0.36 *RC02-8 45.40 122.49 FA, Boring, 6 51.50 1.20 16.79 7.16 0.13 8.49 9.38 3.81 1.16 0.38 Carver vent RC02-8B 45.40 122.49 FA, Boring, 6 51.82 1.19 17.02 7.51 0.13 8.17 9.07 3.58 1.16 0.34 Carver vent 813+73 45.51 122.74 FA, Boring, 6 51.07 1.33 15.26 9.24 0.15 9.61 8.47 3.43 1.16 0.28 Sylvan/tunnel L01-6 46.03 122.42 FA, Rock Ck 6 49.64 1.14 17.04 8.74 0.15 9.43 9.87 2.95 0.82 0.22 RCW-85 44.44 121.84 Hood 1 52.30 1.16 16.48 7.63 0.14 8.84 8.80 3.52 0.86 0.26 RCDS-197 44.39 121.54 Hood 1 51.38 1.42 16.79 7.45 0.13 8.19 9.51 3.57 1.13 0.44 xDS-66 46.07 121.62 IH, Qldp 3 51.84 1.14 16.70 9.05 0.15 7.85 8.74 3.41 0.87 0.26 DS-57-80 46.09 121.76 IH, Qml 3 50.58 1.23 16.40 9.18 0.15 8.17 9.55 3.37 0.98 0.38 xDS-56-80 46.10 121.78 IH, Qml 3 50.37 1.26 16.88 9.19 0.15 7.73 9.53 3.48 1.01 0.38 xDS-55-80 46.09 121.81 IH, Qml 3 50.62 1.30 17.10 9.28 0.16 7.69 9.22 3.42 0.91 0.30 MA-352 46.04 121.50 MA 2 49.77 1.52 16.15 8.82 0.14 9.01 9.55 3.55 1.09 0.40 *MA-305 46.04 121.50 MA, bsb 2 50.00 1.38 16.36 8.63 0.14 8.90 9.35 3.78 1.10 0.37 L01-23 46.03 121.50 MA, bsb 6 50.25 1.48 16.19 8.68 0.15 8.79 9.43 3.54 1.11 0.37 *MA-322 46.04 121.50 MA, bsb 7 49.59 1.52 16.09 9.06 0.14 8.98 9.51 3.61 1.11 0.39 *MA-321 46.04 121.50 MA, bsb 2 49.42 1.56 16.01 9.21 0.15 8.93 9.74 3.42 1.16 0.41 *MA-27 46.04 121.50 MA, bsb 2 48.48 1.79 16.18 9.63 0.15 8.71 10.38 3.03 1.17 0.48 DS91-65 46.40 121.72 MA, Spud Hill 1 50.40 1.30 16.49 8.40 0.14 8.52 9.74 3.31 1.26 0.42 High-K calcalkalic basalt (HKCA) RCDA-9043 45.40 122.49 FA, Boring, 6 50.83 1.29 16.11 8.32 0.14 8.75 8.89 3.53 1.47 0.66 Carver vent RCDA-9044 45.40 122.49 FA, Boring, 6 51.07 1.31 16.17 8.26 0.14 8.46 8.79 3.59 1.63 0.58 Carver vent *RC02-3 45.49 122.52 FA, Boring, 6 51.78 1.33 16.29 7.44 0.12 8.05 8.95 3.92 1.53 0.58 Powell Butte V123A 46.08 122.35 FA, Merrill L 6 51.13 1.16 15.33 8.14 0.14 9.86 9.09 3.12 1.68 0.34 *V458 46.08 122.37 FA, Merrill L 6 50.51 1.10 16.03 7.95 0.13 8.78 10.18 3.35 1.59 0.38 *L01-3B 46.08 122.37 FA, Merrill L 6 50.66 1.24 15.82 7.90 0.13 8.46 9.84 3.30 2.11 0.54 *V457 46.10 122.34 FA, Merrill L 6 50.72 1.28 16.03 7.87 0.13 8.04 9.74 3.44 2.19 0.55 *V551C 46.01 122.45 FA, Rock Ck 6 49.80 1.07 15.63 8.05 0.14 9.75 10.48 3.23 1.49 0.36 *V312 46.02 122.44 FA, Rock Ck 6 49.61 1.15 15.56 8.27 0.14 9.70 10.91 2.76 1.54 0.37 *V253A 46.03 122.41 FA, Rock Ck 6 49.78 1.15 15.75 8.28 0.14 9.51 10.70 2.82 1.49 0.37 *RC-A LDCK 45.32 122.06 FA 1 50.49 1.46 15.21 9.13 0.15 8.68 9.63 3.21 1.60 0.44 x83-12 45.50 121.50 Hood 6 52.47 1.30 17.58 7.51 0.13 6.76 8.28 3.92 1.53 0.52 *MA-953 46.36 121.71 MA 7 49.61 1.36 15.64 8.59 0.14 9.64 9.83 3.18 1.48 0.52 Basaltic andesite (BA) QV99-21 45.52 122.60 FA, Boring, 6 53.09 1.21 16.78 7.42 0.13 7.33 8.38 3.73 1.49 0.44 Mt. Tabor RC-M TBRP 45.52 122.60 FA, Boring, 6 53.44 1.20 16.59 7.32 0.12 7.25 8.34 3.86 1.44 0.44 Mt. Tabor q99CM-T03 45.59 122.48 FA, Boring, 6 54.29 1.25 17.28 7.84 0.13 6.08 7.84 3.87 1.07 0.34 Prune Hill QV99-18 45.54 122.56 FA, Rocky 6 54.52 1.23 17.22 7.16 0.12 6.32 7.84 4.07 1.17 0.35 Butte, u. flow L00-10 45.54 122.56 FA, Rocky 6 54.39 1.23 17.41 7.18 0.12 6.24 7.82 4.10 1.17 0.35 Butte, u. flow W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 75

Mg# Ni Cr V Ba Rb Sr Zr Y Nb (Sr/P)n Ba/ Ba/ Nb/ Ba/ Ba/Y Sr/Y K/Ti Ca/ Fe/ Equilibrium T-erupt TiO2 Nb Zr Zr Al Mg olivine (Fo)

69.5 146 287 180 320 8 1131 122 16 6.7 4.4 285 47.8 0.055 2.62 20.0 70.7 0.90 0.57 0.92 88.0 1226 67.0 200 327 196 349 12.6 996 150 17 13.0 2.9 264 26.9 0.087 2.33 20.5 58.6 0.91 0.53 1.03 86.8 1213 71.3 187 314 185 453 4 1135 124 16 9.9 3.1 377 45.8 0.080 3.65 28.3 70.9 0.97 0.56 0.84 88.9 1229

69.5 184 305 204 448 6 1157 119 15 9.6 3.5 376 46.7 0.081 3.76 29.9 77.1 0.98 0.53 0.92 88.0 1220

68.6 235 336 187 340 15 929 133 15 11.2 3.4 255 30.4 0.084 2.56 22.7 61.9 0.87 0.56 0.96 87.6 1258

69.3 131 364 231 359 7 849 130 20 6.1 3.9 315 58.9 0.047 2.76 18.0 42.5 0.72 0.58 0.93 88.0 1263 70.8 180 373 196 256 10 627 129 19 10.6 2.5 221 24.2 0.082 1.98 13.5 33.0 0.74 0.53 0.86 88.7 1233 69.7 156 256 204 552 21 924 209 21 10.3 2.2 389 53.6 0.049 2.64 26.3 44.0 0.80 0.57 0.91 88.2 1223 64.5 160 245 174 264 17 636 133 22 – 2.6 231 – – 1.98 12.0 28.9 0.76 0.52 1.15 85.4 1214 65.1 144 287 184 329 9.8 765 141 20 8.5 2.1 267 38.7 0.060 2.33 16.2 37.7 0.79 0.58 1.12 85.8 1228 63.8 161 262 187 394 10 943 148 21 8.3 2.6 312 47.5 0.056 2.66 19.0 45.6 0.80 0.57 1.19 85.1 1219 63.5 143 260 189 306 9 733 141 20 - 2.5 236 - - 2.17 15.3 36.7 0.70 0.54 1.21 84.9 1217 68.2 175 326 208 347 7.4 972 165 25 14.0 2.6 228 24.8 0.085 2.10 13.9 38.9 0.72 0.59 0.98 87.4 1252 68.4 189 - - 303 14 979 162 20 10.0 2.7 220 30.3 0.062 1.87 15.2 49.0 0.80 0.57 0.97 87.5 1248 68.0 169 294 198 326 6 936 142 20 14.4 2.6 221 22.6 0.101 2.30 16.3 46.8 0.75 0.58 0.99 87.3 1244 67.5 165 326 - 345 15 948 168 22 11.0 2.5 228 31.4 0.065 2.05 15.7 43.1 0.74 0.59 1.01 87.0 1253 67.0 157 - - 332 19 981 171 21 15.0 2.5 213 22.1 0.088 1.94 15.8 46.7 0.74 0.61 1.03 86.8 1253 65.5 162 - - 326 20 1026 192 23 16.0 2.2 182 20.4 0.083 1.70 14.2 44.6 0.66 0.64 1.10 86.0 1256 68.0 156 355 205 384 22 723 190 23 14.0 1.8 295 27.4 0.074 2.02 16.7 31.4 0.97 0.59 0.99 87.3 1237

68.8 223 341 178 755 13 1287 156 20 13.3 2.0 583 56.8 0.085 4.84 37.8 64.4 1.13 0.55 0.95 87.7 1239

68.3 199 312 190 924 13 1418 167 21 13.0 2.5 704 71.1 0.078 5.53 44.0 67.5 1.24 0.54 0.98 87.4 1231

69.4 190 311 136 866 12 1666 157 18 8.8 3.0 651 98.4 0.056 5.52 48.1 92.6 1.15 0.55 0.92 88.0 1218

71.8 151 290 200 500 32 1139 182 22 8.9 3.5 431 56.2 0.049 2.75 22.7 51.8 1.45 0.59 0.83 89.1 1263 69.9 126 260 224 530 26 1400 150 16 9.0 3.8 482 58.9 0.060 3.53 33.1 87.5 1.45 0.64 0.91 88.2 1242 69.2 130 259 213 737 19 1789 180 19 7.4 3.5 597 99.6 0.041 4.09 38.8 94.2 1.71 0.62 0.93 87.9 1234 68.2 112 258 - 552 17 1302 149 19 7.2 2.4 431 76.7 0.048 3.70 29.1 68.5 1.71 0.61 0.98 87.4 1224 71.7 184 390 205 515 24 1200 150 18 - 3.4 482 - - 3.43 28.6 66.7 1.40 0.67 0.83 89.1 1269 71.1 155 296 232 604 37 1350 178 25 - 3.8 524 - - 3.39 24.2 54.0 1.33 0.70 0.85 88.8 1270 70.7 137 307 250 559 33 1328 179 24 8.3 3.7 486 67.4 0.046 3.12 23.3 55.3 1.30 0.68 0.87 88.6 1264 66.6 164 268 204 594 20 1232 189 22 7.7 2.9 407 77.1 0.041 3.14 27.0 56.0 1.10 0.63 1.05 86.5 1241 65.4 124 - - - 14 1331 182 18 20.0 2.7 - - 0.110 - - 73.9 1.18 0.47 1.11 85.9 1186 70.2 211 347 180 547 23.5 967 204 24 15.0 1.9 402 36.5 0.074 2.68 22.8 40.3 1.09 0.63 0.89 88.4 1268

67.4 165 228 152 666 11 1255 185 20 11.5 3.0 552 57.9 0.062 3.60 33.3 62.8 1.23 0.50 1.01 87.0 1196

67.5 158 224 161 671 11 1253 198 20 11.4 3.0 559 58.9 0.058 3.39 33.6 62.7 1.20 0.50 1.01 87.0 1193

61.9 118 185 - 415 10 739 146 21 11.0 2.3 332 37.7 0.075 2.84 19.8 35.2 0.86 0.45 1.29 84.0 1165

64.9 129 178 161 341 18 828 167 18 8.7 2.5 276 39.2 0.052 2.04 18.9 46.0 0.95 0.46 1.13 85.7 1169

64.6 130 179 149 357 15 837 169 18 8.7 2.5 291 41.0 0.052 2.12 19.8 46.5 0.96 0.45 1.15 85.5 1168

(continued on next page) 76 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105

Table 1 (continued)

Sample N W Unit/vent Reference SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2OK2OP2O5 Latitude Longitude

Group II. Calcalkalic group—high Ba/Nb, Ba/TiO2 Basaltic andesite (BA) V456 46.04 122.32 FA, Speelyai Ck 6 53.31 1.37 16.55 7.33 0.11 7.94 8.26 3.85 0.95 0.34 V348 46.06 122.43 FA, Wolf Ck 6 52.97 0.92 16.21 7.74 0.13 9.18 8.83 3.21 0.62 0.19 A114 46.08 122.41 FA, Wolf Ck 6 53.64 1.00 16.60 7.49 0.12 8.52 8.23 3.44 0.74 0.21 L01-5 46.09 122.42 FA, Wolf Ck 6 53.69 0.97 16.51 7.30 0.12 8.30 8.37 3.79 0.75 0.20 q83-11 45.50 121.55 Hood 6 54.89 1.15 17.28 7.40 0.13 6.00 7.86 3.98 1.03 0.28 DS30-80 45.94 121.91 IH, Qfc 3 53.60 1.09 16.99 7.38 0.12 6.49 8.57 3.81 1.59 0.36 L01-11 45.63 122.14 McCloskey Ck. 6 54.13 1.13 17.40 7.37 0.13 6.66 8.11 3.91 0.88 0.29 Shoshonite/absarokite (SHO) *DS90-47 46.23 121.83 FA, SWC 1 50.28 1.29 15.27 7.22 0.11 7.84 10.44 4.36 2.28 0.89 *RMC92-1 45.52 122.18 Hood 1 49.79 1.37 15.32 8.33 0.13 8.47 10.21 3.90 1.76 0.73 DS42-81 46.04 121.77 IH, Qdl 3 52.66 1.13 16.04 7.36 0.12 8.41 8.49 3.62 1.74 0.43 DS39-81 46.03 121.77 IH, Qdl 3 52.65 1.16 15.67 7.42 0.12 8.30 8.48 3.66 2.01 0.53 *DS-23B-80 45.97 121.80 IH, Qdl 3 52.21 1.32 16.88 7.65 0.13 6.02 8.85 4.18 2.19 0.57 References: (1) Conrey et al. (1997); (2) Hildreth and Fierstein (1997); (3) Leeman et al. (1990) and Leeman and Smith (unpublished); (4) Ort et al. (1983); (5) Halliday et al. (1983); (6) Evarts, Leeman, and Conrey (this paper); (7) Bacon et al. (1997). dUnpublished dataT include XRF major and trace elements and, for some samples, ICP-MS trace elements analyzed at Washington State F F University (precision and accuracy are estimated as 2% relative for major elements and 5–10% for trace elementsP depending on content). All analyses are recalculated to 100% totals, anhydrous, with all iron reported as FeO*. Mg# computed with Fe3/ Fe=0.15, as indicated from compositions of spinel inclusions in olivine phenocrysts (Smith and Leeman, 2005). d–T indicates no data. K/Ti, Ca/Al, and Fe/Mg are oxide ratios. dEquilibrium olivineT composition (mol% Fo) is computed assuming rock Mg# as magmatic value. dT-eruptT is estimated liquidus T (8C) based on Sugawara (2000) olivine–melt thermometer. Unit/vent key: FA=frontal arc, MSH=Mt. St. Helens, MA=Mt. Adams, IH=Indian Heaven, Hood=Mt. Hood area, others=local vents. Sample numbers preceded by asterisk or dqT are nepheline- or quartz-normative, respectively; sample numbers preceded by dxT were not used for segregation P and T calculation. compositional variations in the subducting (cf. Cascades magmatism initiated around 40 Ma Michaelson and Weaver, 1986) or upper plates (Wells (Duncan and Kulm, 1989), following accretion of et al., 1998). the Coast Range volcanic terranes (Duncan, 1982), In southern Washington and northern Oregon, and continued in an episodic fashion since that time Neogene basaltic magmatism occurred over an excep- with varying subduction conditions. The presence of tionally wide (N160 km E–W) cross-arc swath. an accreted block of oceanic lithosphere beneath Included are numerous monogenetic vents in the portions of the arc implies that the present mantle dforearcT region (as far west as the Portland area, 30– wedge must initially have had compositional similar- 40 km west of the VF as defined above) and in the ities to suboceanic mantle. Convergence rate back-arc region (Simcoe volcanic field). A transect decreased progressively from ~16 cm/year (at 40 across this sector is the primary focus of this paper. Ma) to ~4 cm/year at present (Riddihough, 1984; Neogene volcanism also extends across areas of Verplanck and Duncan, 1987). Also, with increasingly nearly comparable breadth in central Oregon and oblique convergence since 35 Ma, the present-day northern California (cf. Guffanti and Weaver, 1988). orthogonal convergence rate (~2 cm/year; Rogers, The common occurrence of basaltic magmas also 1985; Verplanck and Duncan, 1987) is one of the reflects significant input of heat (or mantle upwelling) lowest known. The active arc migrated eastward with beneath the region during late Cenozoic time (b5 Ma). time to its present location, although the apparent High heat flow (Blackwell et al., 1990) and low Pn extent of migration may be exaggerated by clockwise velocities (cf. Leaver et al., 1984; Mooney and tectonic rotations of the older strata in Oregon (cf. Weaver, 1989; Trehu et al., 1994; Parsons et al., Magill and Cox, 1980; Beck et al., 1986; Wells, 1998) are consistent with this view. 1990). Tertiary volcanic rocks are best exposed in the W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 77

Mg# Ni Cr V Ba Rb Sr Zr Y Nb (Sr/P)n Ba/ Ba/ Nb/ Ba/ Ba/Y Sr/Y K/Ti Ca/ Fe/ Equilibrium T-erupt TiO2 Nb Zr Zr Al Mg olivine (Fo)

69.4 178 265 150 295 14 720 152 14 14.0 2.2 215 21.1 0.092 1.94 21.1 51.4 0.70 0.50 0.92 88.0 1209 71.3 178 393 188 198 11 593 115 17 5.4 3.2 215 36.7 0.047 1.72 11.6 34.9 0.67 0.55 0.84 88.9 1238 70.5 172 293 183 205 14 625 129 19 3.1 205 1.59 10.8 32.9 0.74 0.50 0.88 88.5 1220 70.4 172 350 163 198 8 638 111 18 5.8 3.4 205 34.1 0.052 1.78 11.0 35.4 0.78 0.51 0.88 88.5 1215 63.0 134 - - - 11 1165 149 18 9.6 4.3 - - 0.064 - - 64.7 0.89 0.46 1.23 84.6 1162 64.8 116 200 176 595 21.4 894 178 21 9.1 2.6 544 65.4 0.051 3.34 28.2 42.4 1.45 0.50 1.14 85.6 1176 64.2 126 199 148 228 10 669 131 16 10.7 2.4 202 21.3 0.082 1.74 14.3 41.8 0.78 0.47 1.11 86.0 1178

69.5 194 222 203 1873 11.5 3655 194 19 10.0 4.3 1452 187.3 0.052 9.65 98.6 192.4 1.77 0.68 0.92 88.0 1220 68.1 159 251 205 1343 9 3096 179 18 9.5 4.4 980 141.4 0.053 7.50 74.6 172.0 1.28 0.67 0.98 87.3 1239 70.6 183 334 169 563 21.5 1058 184 15 7.9 2.6 498 71.3 0.043 3.06 38.3 72.0 1.54 0.53 0.88 88.6 1222 70.1 197 317 181 693 29 1341 204 17 7.8 2.6 597 88.9 0.038 3.40 40.8 78.9 1.73 0.54 0.89 88.3 1219 62.2 64 134 181 896 24.2 1545 218 22 13.8 2.8 678 64.9 0.063 4.11 40.2 69.3 1.65 0.52 1.27 84.2 1171

Western Cascades but have not been studied in 2001). Although it has been suggested that temper- comparable detail to those of the modern High atures could be sufficiently high to induce hydrous Cascade arc. Thus, temporal development and petro- partial melting of parts of the subducted slab (Defant genesis of the magmatic arc have not been fully and Drummond, 1993; Grove et al., 2002), the general characterized (cf. McBirney, 1978; McBirney and importance of this process is uncertain if the slab is White, 1982; Smith, 1993; Sherrod and Smith, 2000). extensively dehydrated as predicted. In any case, the Nevertheless, compared to the Neogene lavas flux of subduction components (fluids and/or melts) to described in this paper, pre-middle Miocene basaltic arc magma sources is expected to be smaller beneath magmatism was relatively less voluminous and was the Cascades than at more typical, cooler SZs (Kerrick compositionally more representative of arcs world- and Connolly, 2001; Forneris and Holloway, 2003; wide (cf. Barker and Leeman, 1997). Hacker et al., 2003). The latter authors estimate Slow subduction of young (b10 Ma at trench) greater than 98% H2O depletion from initially oceanic lithosphere in the last few million years hydrated oceanic crust by the time the Cascadia slab results in relatively warm slab surface temperatures reaches subarc depths. Even more extreme dehydra- and stronger dehydration at shallow slab depths tion is predicted according to warmer subduction zone compared to most SZs (Peacock and Wang, 1999; models (e.g., Van Keken et al., 2002), in which case Huang et al., 2001; Van Keken et al., 2002; Gutscher silicate melts may be a viable transport medium where and Peacock, 2003; Hacker et al., 2003). It is temperatures are sufficiently high. While this does not predicted that only a small fraction of the initial necessarily imply that the slab is devoid of water, volatile inventory of the slab is retained to depths strong devolatilization of the slab is likely to have (~100 km) beneath the Cascadia arc (Leeman, 1996, scavenged fluid-mobile elements leaving it highly 78 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 depleted in these constituents. These factors could confirming accuracy of the latter, and their use contribute to the distinctive compositional character- introduces no significant bias with respect to the istics of many Cascades basaltic lavas (Leeman et al., parameters discussed in this paper. The majority of 1990). Despite these arguments, high water contents samples considered erupted within the last 1 Myr, reported for basaltic andesites from Mt. Shasta although some lavas (Simcoe, Tygh Valley, and suggest that there is a significant slab fluid flux Portland Basin areas) range up to ~4 Ma (cf. beneath that region (Sisson and Layne, 1993; Grove et Hammond and Korosec, 1983; Korosec, 1989; Sher- al., 2003). In contrast, data presented below for mafic rod and Smith, 1990; Uto et al., 1991; Hildreth and lavas from a transect near 468N suggest that slab Lanphere, 1994; Conrey et al., 1996a,b; Fleck et al., contributions are small in that part of the arc. As a 2002). These samples provide a cross-arc view of framework for evaluating existing petrologic models, magma production over this time period. we use the compositions and distribution of mafic As in other sectors of the High Cascades, CCT lavas from both regions to develop a depth perspec- basaltic lavas exhibit significant diversity and can be tive on the mantle thermal structure. subdivided on the basis of major and trace element chemistry into distinctive end member sub-types. Adapting the terminology of Bacon et al. (1997), 3. The Cascades Columbia Transect (CCT) Clynne and Borg (1997), and Conrey et al. (1997), the following compositional groups are defined (acro- In this study, we consider basaltic lavas from nyms used herein are given in parentheses): calcal- southern Washington and nearby northern Oregon, kalic (CAB), high-K calcalkalic (HKCA), low-K representing the full breadth of the magmatic arc tholeiite (LKT), and oceanic island-like (OIB) and where it is crossed by the Columbia River near 468N alkalic (Alk=nepheline-normative OIB type) basalts; latitude (Fig. 1b). Within this transect are located three we also tabulate data for magnesian calcalkalic stratovolcanoes: Mts. St. Helens (MSH), Adams basaltic andesites (BA) and rare shoshonitic (SHO) (MA), and Hood (MH). In addition, numerous basaltic lavas within the study area (Leeman et al., 1990; lavas erupted from: (a) dforearcT monogenetic vents in Conrey et al., 1997); the latter rocks appear to have the Portland area (Boring lavas of Trimble, 1963) and complex (in some cases hybrid) origins and are not also west of MSH; (b) flank vents associated with considered in detail in this paper. Criteria used to MSH and MA; (c) intra-arc vents in the Indian define each variant are summarized below. The CCT Heaven (IH) region (between MSH and MA); and transect is unique in that (1) all of these variants are (d) dbackarcT vents associated with the Simcoe (SIM) represented, whereas only partial subsets of these lava field. MH has produced no true basalts, but such magma types have been recognized to date in other lavas erupted to the southeast (Tygh Valley area) and sectors of the Cascade arc, and notably (2) all are west (Portland Basin). Published analytical data for widely distributed across the transect (Fig. 1)— these lavas (cf. Leeman et al., 1990; Bacon et al., implying that the corresponding mantle source rocks 1997; Conrey et al., 1997; Hildreth and Fierstein, also are present at depth. 1997) were augmented by some 300 new XRF To better understand conditions of magma genesis analyses at Washington State University (WSU; for in the mantle and to assess relationships among analytical details, see Conrey et al., 1997). Here, we magma types, we first filtered the overall database discuss compositional variations among representative to include only relatively primitive lavas while primitive basalts (as defined below) culled from this attempting to retain a comprehensive geographic data base (Table 1). As complete trace element and coverage. The selected rocks are weakly to slightly isotopic data are not available for a majority of these porphyritic (b5% phenocrysts in most; dominantly samples, the present paper is based largely on the olivine except as noted below). Other selection criteria b N N XRF database and emphasizes elements determined include: SiOP2 55 wt.%, MgO 6% and Mg# 55 (with with high precision. Where available, ICP-MS trace molar Fe3+/ Fe=0.15). Classification is based on element data are tabulated for Ba, Sr, Y, and Nb. chemical characteristics with major element analyses These data are usually similar to the XRF results, normalized to total 100% on an anhydrous basis with W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 79 all iron calculated as FeO*. Table 1 presents elsewhere in the High Cascades—notably, in northern representative analyses of 133 samples that pass this California (Clynne and Borg, 1997; Grove et al., filter. Fig. 2 emphasizes the high content of ferro- 2002) and parts of the Oregon (Conrey et al., 1997; magnesian components in most of these samples. All Bacon et al., 1997; Green and Harry, 1999) and have FeOt/MgOb1.5, and most have NiN100 ppm and Canadian (Green and Sinha, 2005) Cascades. CrN200 ppm; all except the BA group have SiO2b53 Although few samples meet the more stringent criteria wt.% (Fig. 3) and MgON7%. Rocks having similar of Albare`de (1992) as candidates for dprimary meltsT compositional and petrographic characteristics occur of mantle peridotite, the samples studied here undoubtedly represent the most primitive magmas erupted in this part of the Cascades, and many appear to be only slightly evolved from primary liquids due to fractionation of magnesian olivine (Fspinel). We further emphasize that we have excluded from consideration a much larger population of basaltic samples that are variably evolved due to more extensive crystallization, magma mixing (hybridization), or interaction with older crustal rocks. Compositions of these lavas (Table 1) define two fundamentally distinct groups: Group I has element ratios and contents similar to those in within-plate basalts (Pearce, 1982), whereas Group II includes basalts of calcalkalic arc affinity with variable enrichments of Sr, K, and Ba. The former essentially lack a slab signature whereas the latter do carry such a signature. The first group includes distinct variants closely resembling oceanic island basalt (OIB) and mid-ocean ridge basalt (MORB). The latter are referred to herein as LKTs, and elsewhere have been called high-alumina olivine tholeiites (e.g., Bacon et al., 1997). They are similar to E-MORB, but differ primarily in exhibiting small enrichment of Sr and Ba relative to other incompatible trace elements. The OIB-like basalts include hy- as well as ne-normative variants. For convenience, the former are referred to simply as OIB and the latter are referred to as alkalic basalts (Alk); the latter tend to have elevated incompatible element contents, but similar element ratios compared to our OIBs. LKT, OIB, and Alk basalts all have low Ba/Nb ratios (7–20) and overlapping Mg# (55–69). LKT basalts are further b Fig. 2. Variation of Mg# and Ni and Cr contents versus cross-arc distinguished by low K2O( 0.5%) and other incom- b longitudinal position and K2O/TiO2 ratio, which effectively patible element contents, low K2O/TiO2 ( 0.45), and distinguishes the LKTs from allP Group II compositional variants. nearly flat chondrite-normalized rare earth element Mg# is computed using Fe3+/ Fe=0.15 as discussed in the text. profiles (e.g., Leeman et al., 1990; Bacon et al., 1997; Nearly all samples have high transition metal contents (Group II Conrey et al., 1997); they also have systematically lavas being more enriched on average) consistent with them being close in composition to primitive mantle melts. Separate fields are lower Sr/Y (8–19) than OIB and Alk sub-types (17– defined for the distinctive LKT and OIB/Alk lavas of Group I, and 44), and exhibit the strongest Fe enrichment (i.e., for Group II (all variants); sub-types are indicated in symbol key. lower Mg#). All Group I variants plausibly are 80 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105

Fig. 3. Diagram illustrating compositional distinctions among the principal basaltic subtypes. LKTs have systematically lower K2O and incompatible element contents. Although OIB/Alk and CAB lavas overlap in K2O–SiO2 space, all primitive Group I lavas are distinguished by systematically lower Ba/Zr (and Ba/Nb). OIB/Alk lavas are uniquely enriched in Nb and Ta, and have elevated Nb/Zr and similar ratios. Fields and symbols are same as in previous plots. Group I lavas are indistinguishable from the MORB-OIB oceanic array, shown as shaded field (cf. Leeman et al., 1990). derived from mantle sources generally similar to those are used in our figures for both CAB and HKCA for oceanic basalts. types because of their compositional overlap. SHO The second group also comprises several sub- lavas show the greatest Ba and Sr enrichments.P High types including CAB and HKCA basalts as well as Mg# (63–75, calculated with Fe3+/ Fe=0.15; cf. BAs and SHO lavas. Compared to the lavas of Smith and Leeman, 2005) in these lavas indicate that Group I, CAB, HKCA, and BA are distinguished by many are not far from Fe–Mg equilibrium with elevated K2O (0.6–2.2%), Ba/Nb (21–100), Ba/TiO2 mantle olivine (~Fo90). These characteristics are (202–651), and Sr/Y (29–94) values; some of these consistent with derivation of Group II lavas from parameters overlap or exhibit gradational variations mantle sources that have been modified by sub- between the Group II sub-types. The same symbols duction contributions. W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 81

Ranges are given in Table 2 for selected compositional characteristics of the respective magma sub- -erupt types. Significant overlaps exist for MgO, Mg#, and T estimated magmatic temperature (dT-eruptT; Smith and Leeman, 2005) in most variants. Incompatible element ratios are particularly useful in discerning distinctions in magma sources because the respective element pairs are not strongly fractionated during partial melting, and thus are more representative of source compositions than elemental concentrations (cf. Pearce, 1982; Sims and DePaolo, 1997). Fig. 3 (and subsequent diagrams) highlights several critical distinctions between the MORB- and OIB-like lavas Sr/Y Nb/Zr Equilibrium Fo of Group I and Group II calcalkalic lavas. Group I n lavas define a coherent Nb/Zr–Ba/Zr trend consistent with variable source fertility (or inversely related to (Sr/P)

degree of melt extraction; Leeman et al., 1990). 2 Compared to OIB, and especially LKT, ne-normative Alk basalts have systematically higher Nb/Zr (and Nb contents) consistent with them being products of lower degree melting. This is also reflected in Nb contents that increase in the order LKT (3–8 ppm), Ba/Nb Ba/TiO OIB (13–30 ppm), Alk (24–57 ppm). Also, order of 2

magnitude differences in K2O enrichment at compa- O/TiO 2 rable ranges in MgO (Table 1) clearly implies differences in source composition among Group I variants. Group II lavas show selective Ba enrichment and persistently low Nb/Zr, and can be explained by addition of slab-derived fluids to a relatively refractory source (Hawkesworth et al., 1994). However, in O Ni (ppm) K 2 contrast with other sectors of the Cascades (e.g., Borg K

et al., 1997), such a process is less clearly reflected by a (Sr/P)n values that overlap significantly for Group I (1.1–3.4) and Group II (1.8–4.3) basalts. Group II basalts also exhibit primitive mantle-normalized incompatible element profiles with pronounced deple- tions of high field strength elements as seen in many Fe=0.15. =31 6.2–8.9 55–69 0.64–1.31 73–189 0.29–0.79 7–18 71–246 1.3–2.4 17–44 0.09–0.16 80–88 1184–1239 n P

arc lavas (Leeman et al., 1990; Conrey et al., 1997; /

Smith and Leeman, 2005); however, their Nb contents 3+ =19 6.1–9.9 55–65 0.82–2.27 81–170 0.48–0.99 9–15 136–292 1.1–2.0 21–43 0.09–0.21 80–87 1181–1286 are relatively high (5–20 ppm)—roughly double those n =21 7.7–11.3 64–75 0.81–1.26 143–286 0.66–0.98 23–95 213–631 1.8–4.4 29–87 0.05–0.10 85–89 1213–1263 =32 7.1–9.7 57–65 0.10–0.48 55–208 0.09–0.34 8–20 31–126 1.4–3.4 8–19 0.04–0.08 81–86 1213–1283

in the LKTs. =12 6.0–9.2 62–71 0.62–1.59 116–178 0.67–1.45 21–66 202–559 2.2–4.3 33–65 0.05–0.08 84–89 1162–1238 n n Cross-arc variations in selected incompatible ele- n ment ratios are shown in Fig. 4. For simplicity, we denote fields for the distinctive OIB/Alk and LKT =13=5 6.8 65 6.9 1.53 62 124 2.19 1.18 64 – 1.65 – 65 678 2.7 74 2.8 0.11 69 86 0.063 84 1186 1171 variants of Group I and Group II lavas collectively. Of n n , , particular note is the broad cross-arc distribution of b b Slightly evolved example, included for geographic coverage. Mg# calculated assuming Fe

Group I lavas, and especially the high-Nb OIB/Alk a b Evolved Evolved Table 2 Ranges of selected parameters in CascadesMagma Columbia type Transect (CCT) magma types MgO Mg# Basaltic andesite (BA), Shoshonite/absarokite (SHO) 7.8–8.5 68–71 1.74–2.28 159–197 1.28–1.77 71–187 498–1452 2.6–4.4 72–192 0.04–0.06 87–89 1219–1239 Group I—within-plate types Low-K tholeiite (LKT), Hy-normative OIB-like basalt (OIB) Ne-normative OIB type (Alk), High-K calcalkalic basalt (HKCA) 8.0–9.9 67–72 1.47–2.19 112–223 1.09–1.71 36–100 402–651 1.9–3.8 40–94 0.04–0.09 87–89 1218–1270 lavas that have the strongest intraplate signature. Group Group II—calcalkalic types Calcalkalic basalt (CA), 82 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105

Fig. 4. Cross-arc longitudinal variations for many of the ratios used to classify Columbia Cascades Transect (CCT) basaltic lavas. This diagram emphasizes the wide spatial distribution as well as systematic compositional distinctions of Group I (LKT, OIB/Alk) and Group II lavas. Symbols and fields as in previous diagram.

II lavas are present across all but the far backarc region frontal arc region where slab-derived fluid fluxing (i.e., they have not been found east of Mt. Adams). might be expected to be greatest. Group I basalts show little cross-arc variation for Relative enrichment of B (e.g., B/Zr, B/Nb, etc.) is relatively fluid-mobile Ba, Sr, or K with respect to another useful indicator of slab-derived fluids in arc fluid-immobile Ti, Y, or Nb. As noted earlier, Alk lavas magma sources. Available data for CCT basalts (Lee- tend to have systematically higher Nb/Zr (also, where man et al., 1990; Leeman, 2001; Leeman et al., 2004) data exist, Nb/La, Nb/Th, etc.) than OIB types, indicate that relatively modest B-enrichment (up to consistent with lower degree melting in the Alk ~10 ppm in Group II lavas) is restricted to the forearc sources. In most panels (except Nb/Zr), Group II lavas region (at and west of MSH); elsewhere in the define fairly distinct fields consistent with Ba-, K-, and transect, B contents are less than 3 ppm and B/Zr Sr-enriched sources; however, they have Nb/Zr similar values are similar to those for OIB (b0.03) regardless to LKTs. The main impression is that the respective of chemical type. Lavas of both Groups I and II show magma sources appear to be distributed broadly small increases in d11B values in the frontal arc (~2x) beneath the arc with little correspondence to subduc- compared to lavas east of Mt. St. Helens which have tion geometry, and all seem to be present beneath the OIB-like d11B values (~À9x). Although this obser- W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 83 vation suggests a weak influx of slab-derived B below uncertainties associated with steps (b) and (c), this the frontal arc, decoupling of B with other aspects of approach provides a first-order constraint on the the lava chemistry implies that the latter may be relative depths and temperatures associated with inherited from source heterogeneities rather than final segregation and ascent of individual batches controlled by fluid inputs (Leeman et al., 2004). of basaltic magma, assuming they derive from It is proposed that B enrichment may be controlled peridotitic mantle. Fe–Mg systematics for such rocks by temperature gradients in the subducting slab are well characterized experimentally for upper (Bebout et al., 1999; Leeman et al., 2002; Benton et mantle conditions, allowing us to model magma al., 2001), and that east of MSH, the slab is genesis for simple scenarios. sufficiently warm that it is unusually depleted in Conceptually, we propose that erupted batches of labile B, aqueous fluids, or both. Because Ba, Sr, and the most primitive magmas experienced the least K are not as strongly fluid-mobile as B (cf. Leeman et extent of crystallization, dominated by olivine, while al., 1992), these and similar elements may be more making their way to the surface. This assumption is efficiently retained in subducted materials and ulti- supported by detailed petrologic and mineral chem- mately enriched in arc magma sources. If aqueous istry studies of representative primitive lavas (Smith fluids are strongly depleted from the slab, a melt and Leeman, 2005). More extensive crystallization phase may be implied as the transport medium for Ba, and possibly storage and mixing between magma Sr, and K. Constraints on the thermal structure of the batches commonly occur as well, but such processes SZ would help in evaluating these scenarios. In any are, we believe, largely eliminated from consideration case, the observed spatial patterns (Fig. 4) seemingly by the selection criteria we have employed. Our require the involvement of at least two broadly methodology is discussed briefly in the following distinct types of sources, one of which has been little paragraphs. modified by subduction contributions whereas the other has been modified (albeit to lesser degree than 4.1. Fractionation correction for more typical subduction systems; Ryan et al., 1995). There is little to suggest lateral differences in Following Tatsumi et al. (1983), Falloon and composition of magma sources; rather, each distinc- Green (1988), Stolper and Newman (1994), and tive source appears to extend widely beneath most of Green and Harry (1999) among others, the effects the arc. This contrasts with the picture for the northern of fractional crystallization were compensated California Cascades in which lavas from the frontal numerically by adding equilibrium olivine in small part of the arc appear to exhibit a stronger slab increments (0.1–0.2 wt.%) to primitive lavas until influence than those in the backarc region (Clynne their projected compositions reached a Mg# (Mg/ and Borg, 1997; Borg et al., 1997, 2000). [Mg+Fe2+] molar ratio) consistent with equilibration between the computed liquid and mantle olivine (typically Fo90, but varied as appropriate). This 4. Estimation of segregation depths for primitive approach makes the simplifying assumption that only magmas olivine is a fractionating phase, which is broadly consistent with observed phenocryst assemblages for To better understand the spatial distribution of many of the most primitive basalts of all composi- magma sources, we have attempted to constrain the tional variants (e.g., dominantly olivine with trace Cr- conditions of formation of primitive CCT basaltic spinel inclusions). In more fractionated LKT- and magmas. The strategy followed is to: (a) begin with OIB-type lavas, assemblages of olivine+plagioclase the most primitive lavas sampled; (b) correct their do occur, and effects of cotectic crystallization of compositions for fractional crystallization based on these phases was evaluated and is discussed below. Fe–Mg elemental partitioning (as described below); Clinopyroxene also occurs as microphenocrysts in and (c) apply empirical geothermometers and some rocks of CA-affinity in the forearc region, or as barometers to estimate temperatures and depths of sparse phenocrysts in some OIB-like basalts from segregation of the putative parental magmas. Despite regions farther east. In any case, total phenocryst 84 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 modal abundance in primitive samples typically is higher amounts of olivine addition are required, low (b5% for most), and other phenocrysts (where ranging between 7% and 25% for most samples. present) are subordinate to olivine. This observation However, the amount of olivine addition required is atypical of many arc basalts and signifies that decreases with assumed source Fo content by about 5 many CCT magmas ascended rapidly from mantle wt.% solid added for each mole percent Fo, and less depths and were relatively water-poor. If they were refractory sources for Group I lavas will bring the nearly water-saturated at depth, outgassing of H2O fractionation models into closer accord with those for during ascent would promote cooling and significant Group II. On the other hand, lower bounds on source crystallization (Sisson and Grove, 1993b). Low Fo content are constrained by compositions (max- modal phenocryst contents and similarity between imum Fo) of olivines in each sample; for most Group calculated and measured (phenocryst core) composi- I lavas, this implies source compositions of at least tions of liquidus olivine (cf. Smith and Leeman, Fo87. Also, generally lower Mg# in Group I relative 2005) also demonstrates that olivine accumulation to Group II lavas could reflect greater degrees of effects are negligible. On the basis of these observa- fractionation of Group I magmas during their ascent. tions, we conclude that bulk rock chemistry of most An independent constraint on the extent of olivine primitive CCT basalts reasonably reflects that of addition can be obtained by considering simultane- actual magmas. ously the partitioning of Ni between olivine and melt The equilibrium olivine composition at each step (Sato, 1977; Tamura et al., 2000). In particular, an was calculated using experimentally determined Fe– upper limit of ca. 0.4 wt.% NiO in the calculated liq Mg partitioning (molar KD=[FeO/MgO] /[FeO/ equilibrium olivine is dictated by NiO contents of MgO]oliv; cf. Roeder and Emslie, 1970). Melt FeO naturally occurring mantle peridotites (Sato, 1977; 3+ contentP was calculated assuming a constant Fe / Ozawa, 1994). Maximum NiO contents calculated for Fe ratio of 0.15, which is the median value Fo90 olivines in Fe–Mg equilibrium with our parental estimated from Cr-spinel inclusions in Mg-rich liquids typically are less than 0.3 wt.% using olivine phenocrysts (cf. Smith and Leeman, 2005). experimentally determined relations for Ni partition- Small variation in KD (typically between 0.30 and ing as formulated by Beattie et al. (1991) and Beattie 0.33) with pressure and melt composition were (1993). In other words, calculated Xsolid values based modeled by stepwise increase at each increment of on Fe–Mg equilibria do not overly saturate our olivine addition using a rate consistent with the parental melts with respect to Ni. dependencies documented by Ulmer (1989) and This simple approach is considered to reasonably Kushiro and Walter (1998) and with the composi- approximate compositions of the parental melts. An tional dependency of Tamura et al. (2000). Although important finding is that, independent of the amount results are presented relative to an assumed reference of olivine added, the estimated conditions of melt mantle composition (Fo90), for some samples (mostly segregation from peridotitic mantle sources differ Group II types) compositions (Mg#z70) imply that systematically for most Group I and II lavas. To their sources were probably more refractory. In such evaluate the significance of this result, it is important cases, iterations were extended until the calculated to consider the uncertainties involved in obtaining d T melt was equilibrated with more magnesian (Fo91– segregation conditions. These mainlyP concern the 3+ 92) sources to evaluate effects on calculated liquid value of KD, redox state (Fe / Fe), mantle Mg#, compositions. Compared to the analyzed lavas, and source lithology assumed. Effects of changing the calculated melt compositions are reduced in SiO2, first three variables are evaluated below for realistic Al2O3, CaO, K2O, Na2O, and other incompatible ranges and found to be on the order of calibration elements relative to the initial chemistry, whereas errors associated with the thermobarometers used (on Mg# increased. The amount of olivine addition the order of F50 8C, F0.3 GPa). Thus, unless the (Xsolid) required to achieve liquid Mg#s consistent parameters used differ significantly for the respective with an Fo90 source is shown in Fig. 6b. For Group lava groups (which is deemed unlikely), systematic II lavas, this value is typically between 2% and 10% differences found for P and T of magma segregation but ranges up to 15%. For Group I lavas, much are considered to be robust (within the limits W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 85 discussed below) despite uncertainties in the absolute 4.2. Pressure and temperature of magma segregation values calculated. The question of source lithology is more difficult to assess because presently available Magmatic temperatures were estimated based on experimental data are sparse for lithologies other than Mg partitioning between olivine and melt composi- peridotite. It is quite possible that some lavas tions. Several published olivine thermometers were originated from somewhat different sources (e.g., investigated, and of these the formulation of Sugawara BA or SHO sub-types; cf. Elkins-Tanton and Grove, (2000) was found to most consistently reproduce 2003), in which case predicted segregation conditions experimental temperatures (within F30 8C for anhy- are more uncertain. drous test data; Fig. 5a). This thermometer (herein

Fig. 5. Comparison of calculated versus measured experimental (a) temperatures and (b) pressures using the Sugawara (2000) thermometer and Albare`de (1992) barometer. Anhydrous (closed diamonds) and hydrous (open symbols) experiments are distinguished. Heavy lines show ideal 1:1 correspondence, and light lines correspond to F50 8C and F5 kbar (0.5 GPa) brackets. Light solid line in (b) shows linear regression (equation) through all data points; although the bias is significant at pressures greater than ~30 kbar (3 GPa) this is unimportant for this study because most samples correspond to lower pressures (in the range 10–25 kbar or 1.0–2.5 GPa) where deviations between the regression line and the perfect correlation are less than F3 kbar (0.3 GPa). 86 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105

referred to as TS00) has the added advantage that in Section 5 (T-erupt; Fig. 6a). This thermometer is it is insensitive to variations in magmatic Fe3+/Fe2+ also used with back-calculated dparental liquidT ratio and requires only a valid magma composition. compositions to estimate segregation temperatures Applied to uncorrected lava compositions, we esti- (T-seg; Fig. 6b). Although this thermometer includes mate deruptiveT T’s reported in Table 1 and discussed a pressure term, for our purposes uncertainties in

Fig. 6. (a) Comparative eruptive temperatures (T-eruption, 8C) estimated for samples in Table 1 using the Sugawara (2000) MgO-in-liquid thermometer and assuming that bulk rock and magmatic compositions are equivalent (see text). For most samples, these temperatures agree within F308C with available (Smith and Leeman, 2005) mineral–melt and mineral–mineral thermometry for individual samples. (b) Estimated segregation temperatures (T-seg, 8C) calculated by the same method but using compositions of back-calculated liquids assumed to be in equilibrium with Fo90 reference mantle. These values are plotted versus the weight fraction (Xsol) of equilibrium olivine that must be added to produce liquids in Fe–Mg equilibrium with the model mantle. Although Group II lavas probably derive from a more refractory source compared to Group I, the effect of using a more magnesian reference mantle is offset by the slightly higher oxidation in those lavas (see text). Magnitude of dsource effectT is an increase in T-seg of ~408C for each mole percent increase in source Fo value. Temperature differences between the majority of Group I and II lavas are robust for reasonable mantle compositions. Additionally, consideration of H2O contents in Group II lavas will reduce temperature estimates for these magmas, further distinguishing them from Group I. Symbols and fields are as in earlier diagrams. W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 87 estimated pressure (see below) equate to negligibly deruptiveT temperatures for our LKT lavas are similar small temperature shifts (~5 8C/MPa) that usually are to low pressure anhydrous liquidi T’s (1230–1280 8C within the calibration uncertainty. at pressures between 0.01 and 1 GPa) determined Greater uncertainty is associated with magmatic experimentally for LKTs from Medicine Lake (Bartels water content, which is poorly constrained for most et al., 1991). CCT samples. Based on differences between TS00 and Pressures of equilibration between melts with experimental temperatures for hydrous melts (Sisson peridotite mantle were determined using the algorithm and Grove, 1993a; Falloon and Danyushevsky, 2000; of Albare`de (1992). Because this simple empirical Gaetani and Grove, 1998), the estimated TS00 values expression is based on estimated SiO2 content in the overpredict nominal experimental T’s by ~20 8C/wt.% parental liquids, tests were carried out to evaluate the H2O (also cf. Ulmer, 2001). As all estimated T’s are effects of various factors on this parameter. Chief uncorrected for this shift, they should be considered as among these are: (a) the inherent accuracy ofP the 3+ maximal values. Considering the H2O contents original chemical analyses; (b) variation in Fe / Fe determined for Cascades and similar lavas (Sisson in the melt; (c) magnitude of KD; (d) polybaric and Grove, 1993b; Sisson and Layne, 1993; Sisson cotectic crystallization of other phases in addition to and Bronto, 1998; Grove et al., 2002, 2003, and olivine; and (e) assumed source composition. Ana- references therein), this uncertainty is likely greatest lytical uncertainty in major element components is for the CA-type lavas, for some of which water estimated to be less than ~2% relative for all major contents could exceed several percent. However, oxides determined at WSU. Replicate analyses of plagioclase phenocrysts in slightly evolved CAB individual samples and multiple analyses of selected and HKCA lavas have compositions (max Anb75) volcanic units indicate that uncertainty in SiO2 inconsistent with formation from water-saturated content is typically about 0.5% absolute. Assuming liquids (cf. Housh and Luhr, 1991; Sisson and Grove, that errors are similar for most samples, this factor 1993b). Also, preliminary analyses of melt inclusions could introduce random biases in the pressure in representative CA-type lavas are permissive of low estimate, but would have little effect on relative to moderate H2O contents (between ca. 1% and 5%) differences between units. However, inclusion of based on deficit analytical totals and limited direct analytical data from other labs in our database likely analysis via SIMS (Leeman et al., in preparation). In introduces additional bias and may increase the scatter the absence of precise H2O analyses, it appears that in estimated P and T values. Excluding units for temperatures are likely to be overestimated by no which petrologic complexities are recognized (e.g., more than ~100 8C and probably much less for most Mt. St. Helens, see below), we examined the CA-type samples. For LKT- and most OIB-type lavas, reproducibility in segregation P for 26 lava flows or H2O content is expected to be low (b1%), in which map units for which multiple analyses are available. case temperature corrections are likely to be similar to For these examples, the standard deviation for P the calibration uncertainty. varies between 0 and 0.5 GPa, and the average Although not a strict test of accuracy, we note that deviation is less than 0.2 GPa (b6 km equivalent). thermometry based on phenocrysts and bulk rock (as This assessment includes data from multiple labs, and representing magmatic liquid), and using olivine–melt possibly samples having different histories, and so (e.g., Sugawara, 2000; Langmuir et al., 1992; Albar- provides a conservative estimate of the uncertainties e`de, 1992; Gudfinnsson and Presnall, 2001), Ca/Mg associated with samplingP and analysis. in olivine (Jurewicz and Watson, 1988), or clinopyr- Variation in Fe3+/ Fe can introduce significant oxene–melt (Putirka et al., 1996; 2003) thermometers differences in estimated compositions of parental give quite similar temperatures (usually within 30 8C) liquids, notably Mg#. This parameter was estimated for most of our analyzed samples, including CAB/ from (a) chemistry of Cr-spinel inclusions in olivines, HKCA types (Smith and Leeman, 2005). This agree- and (b) assumed Fe/Mg equilibrium between olivine ment supports our assumption that bulk rock compo- cores and bulk rock analyses, as described by Smith sition adequately reflects that of melt in equilibrium and Leeman (2005). These methods give concordant with the phenocrysts in most samples. In addition, results for most primitive CCT basalts studied, with 88 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 P Fe3+/ Fe values typically in the range 0.15F0.05. low pressure crystallization to be minimal. Such In detail, Group II lavas tend to be slightly more magmas are relatively dry and would exhibit near- oxidized (N0.15) than those ofP Group I (b0.15). For liquidus olivine+plagioclase saturation if cooled at a given lava, increasing Fe3+/ Fe from 0.1 to 0.2 pressures below ~1 GPa (cf. Bartels et al., 1991). results in corresponding decreases of about 60 8C Moreover, olivine phenocryst rims and microphe- and 0.3 GPa (10 km)P for segregation T and P. nocrysts in CCT primitive LKTs display increasing Because the median Fe3+/ Fe value (0.15) was used CaO with decreasing Fo content, effectively preclud- to compute parental liquid compositions and segre- ing cotectic crystallization of any calcic phases gation conditions, we probably overestimate T and P assuming that olivine Ca content is controlled by slightly (by as much as ~308 C and ~0.15 GPa) for that in the coexisting liquid (cf. Libourel, 1999). Group II lavas. However, redox conditions are not Similar trends are observed in CA type lavas, known in every case, and inclusion of more or less including those with microphenocrysts of augite, oxidized samples could also contribute to scatter in suggesting that they too dominantly crystallized calculated T and P. olivine. Cotectic formation of augite is likely a Concerning olivine–melt KD, the choice of how to consequence of elevated magmatic water content that model variation in this parameter with pressure also promotes expansion of the clinopyroxene phase can result in systematic shifts in estimated segregation volume (cf. Sisson and Grove, 1993a; Grove et al., pressure. Our empirical approach uses KD values that 2003). This inference is supported by the presence in increase slightly with pressure (typically within the some HKCA lavas of small amounts of late-formed range 0.30–0.33); these values are well within the phlogopite (in groundmass and vesicles) and oxi- range of experimentally determined values for liquids dized pseudomorphs of amphibole in some SHO similar to our computed parental melts (Kushiro and lavas. Notably, the augites in Group II samples Walter, 1998). Almost identical results are obtained display sector-zoning that, in other arc lavas, has using the empirical correlation of KD with melt been interpreted to indicate rapid crystallization and composition (=0.253+0.0034[MgO+0.33FeO]liq; water loss during magma ascent—implying that little Tamura et al., 2000). Assuming that all liquids are crystal fractionation could have occurred (Brophy et affected similarly and modeled in a consistent way, al., 1999). Thus, we conclude that first-order differ- this factor will have negligible effect on relative ences in estimated segregation pressure for primitive pressure estimates. CCT magma types are unlikely to reflect different Of possibly greater significance is the addition of crystallization histories for these lavas. other cotectic phases in projecting to parental liquid A final caveat is that uncertainty in projected compositions. This effect was evaluated via numerical parental liquid compositions is greatest for lavas simulations in which equilibrium pyroxenes or pla- furthest removed from equilibrium with assumed gioclase were included with olivine in varied weight source lithologies. This is because errors in the proportions. Assuming plagioclase to be a significant mixing vectors are magnified as greater proportions cotectic phase with olivine, Elkins-Tanton et al. of solid are added to reach equilibrium liquid (2001) estimated the composition of a parental magma compositions. As discussed for LKT lavas, uncertain- to the Giant Crater LKT lava (Medicine Lake) by ties in P and T related to differences in phase addition of solids with an ol/pl ratio of 70:30. Using proportions (olFplFcpx) are tolerably small for our approach with the same cotectic proportions proportions of solid added not exceeding 30%. results in an effective segregation pressure only As in most investigations of mantle melting our ~0.03 GPa (1 km equivalent) lower than values study is predicated on peridotite bulk compositions. assuming olivine addition alone for equilibration with Assuming that CCT basalts represent peridotite aFo90 reference mantle, and ~0.07 GPa lower for Fo91 partial melts, the question of how source fertility mantle. Corresponding temperature differences are affects melt composition needs to be addressed. reductions of 108 and 208, respectively. Source fertility can be expressed in terms of mantle Such effects are probably unimportant for prim- Mg# (or equivalent Fo content). Lower limits for this itive plagioclase-free LKTs for which we consider parameter are constrained by the maximum Fo W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 89 content in phenocryst cores in our primitive lavas, uncertainties for the thermobarometers used. Further- but source Mg# could be higher. For example, more, the effect of assuming a slightly more refractory because Group II lavas on average have higher (e.g., 1 mol% higher Fo) source for Group II lavas is Mg# than those of Group I (Fig. 2), their source is almost exactly offset by a similar magnitude decrease likely more refractory; the same is indicated by in T and P associated with their slightly higher redox higher Cr# (molar Cr/[Cr+Al]) in spinel included in state as discussed above. olivine phenocrysts from Group II lavas (Smith and Finally, we consider possible variation in SiO2 Leeman, 2005). Clynne and Borg (1997) describe content as a function of degree of melting. similar relations in lavas from the Lassen area. Our Experimental investigations (cf. Hirschmann et al., approach in dealing with this issue is to vary the 1998; Wasylenki et al., 2003) indicate that initial assumed source Mg# to assess the effect on estimated partial melts of a range of peridotite compositions segregation conditions. Results based on a range of have elevated alkali and SiO2 contents compared to reference models can be used to quantify the outcome those at higher degrees of melting. Between melt should source compositions differ for two magma fractions of a few percent to the point (~20–25% suites. For example, for a given LKT magma, melting) where clinopyroxene is totally consumed, increasing source Fo by one mole percent increases SiO2 remains relatively constant at isobaric con- estimated segregation T and P by b50 8C and b0.3 GPa ditions. Thus, P and depth estimates based on the (~10 km), respectively (cf. Table 3, and discussion Albare`de barometer could be unrealistically low for below). These differences are similar to calibration magmas formed by very low degree melting.

Table 3 Comparative estimates of segregation pressure and temperature Example This studya Published estimates Source Mg# T P Z T P Z (8C) (GPa) (km)b (8C) (GPa) (km)b Northern California LKTsc Elkins-Tanton et al. (2001) Whaleback (WB)d 90 1255 0.97 32.3 1290 1.2 40.0 Tennant (TN) 90 1354 1.65 55.0 1525e 2.6 86.7 Giant Crater (GC) 90 1319 1.50 50.0 1370 1.6 53.3 Mammoth (MM) 90 1332 1.46 48.7 1410 1.85 61.7 Yellowjacket (YJ) 90 1293 1.10 36.7 1470e 2.2 73.3 Tionesta (TI) 90 1338 1.55 51.7 1420 1.9 63.3 Damons Butte (DB) 90 1391 1.97 65.7 1475 2.2 73.3 Whaleback (WB) 91 1287 1.03 34.3 1290 1.2 40.0 Tennant (TN) 91 1399 1.91 63.7 1525e 2.6 86.7 Giant Crater (GC) 91 1360 1.72 57.3 1370 1.6 53.3 Mammoth (MM) 91 1374 1.69 56.3 1410 1.85 61.7 Yellowjacket (YJ) 91 1330 1.28 42.7 1470e 2.2 73.3 Tionesta (TI) 91 1380 1.79 59.7 1420 1.9 63.3 Damons Butte (DB) 91 1440 2.29 76.3 1475 2.2 73.3 Big Pine lava field, California Wang et al. (2002) Interpolated at 8% MgO 89 1314 1.29 43.0 – 1.3 43.3 ditto with redox=QFM 89 1295 1.20 40.0 [46] Sierran olivine leucitite (WC-1) Elkins-Tanton and Grove (2003) Experiment with 2% H O 90 1456 2.86 95.3 1460 3.1 103.3 2 P a All values calculated assuming redox conditions near NNO (Fe3+/ Fe=0.15) unless noted otherwise; average uncertainties in T and P are estimated as F30 8C and F0.3 GPa, respectively. b All depths calculated using 1 GPa=33.33 km, except number in brackets taken from original reference. c Two sets of calculated P and T are presented, showing effects of different source composition; average shifts due to this change in source composition are: T (41 8C), P (0.22 GPa), depth (7.2 km). d Labels used in Fig. 8. e Italicized values were considered anomalous by the authors bdue to addition of slab fluidQ. 90 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105

However, conceptually, the conditions predicted by scatter for lower pressure experiments (most at our approach correspond to equilibration between atmospheric pressure) that involve a wide range of pooled magmas and ambient mantle just prior to starting materials. Success at higher pressures stems rapid melt ascent. If eruptions are fed by such from the fact that many of these derive from studies of pooled accumulations of magma at depth, composi- peridotite melting, and the liquids are related to fusion tional variances during incipient melting are likely of generally similar starting materials with regard to to be averaged out, as would many other effects of SiO2 and MgO contents that are used in the deeper processes in a melting column (Langmuir et thermobarometric formulations. al., 1992). Although this type of behavior likely Recovery of P and T for representative exper- obscures many details of magma evolution prior to imental studies is illustrated in Fig. 5.Inthis segregation, knowledge of segregation conditions comparison, data were excluded if Fe–Mg KD values can place useful constraints on mantle temperature for olivine–liquid pairs differed from the expected at specific depths. Below, we use this approach to range (~0.30F0.04), signifying possible problems develop thermal models for the mantle wedge. with regard to disequilibrium or oxidation. Also, analytical biases between different laboratories, or 4.3. Evaluation of the computational approach for specific samples, could contribute to data variance. Calculated and observed pressures and Before proceeding with applications to the CCT, temperatures are highly correlated, although a bias we carried out a series of tests of the computation is evident in pressures estimated for experiments run method to assess its validity. These involved the at greater than 3 GPa in which garnet is a common following approaches: (a) comparison of calculated residual phase. However, as we will see below, all P, T using analyses of run products from peridotite Cascades samples in this study indicate pressures melting experiments; (b) comparison of our method below ~2.6 GPa; in this range, the bias is within the with other published schemes; and (c) application of calibration uncertainty and can be ignored for our our method to seismically and petrologically con- purposes. strained segregation depths for Hawaiian magmas. Temperatures predicted by Albare`de’s (1992) Results of each test are summarized below. Essen- empirical thermometer also are in reasonable agree- tially, if our assumptions are valid, our pressure ment with reported experimental values but, as noted estimates are believed to be reproducible on average earlier, we have found significantly closer correspond- within ~0.3 GPa and accurate to within ~0.5 GPa. In ence with those predicted by Eq. (4) of Sugawara other words, relative differences in segregation (2000). The latter are used in this paper for depths on the order of F10 km appear to be consistency. resolvable although absolute depths are less certain (F15 km). 4.3.2. Comparison with experimental phase equilibria As most experiments summarized in the previous 4.3.1. Recovery of experimental P–T conditions section were anhydrous or had relatively low water As the thermobarometer of Albare`de (1992) was contents, we evaluated the effects of water on based on experimental data published up to that time, predicted P and T by applying our approach to a an obvious test is to see how well it recovers Sierran olivine leucitite lava for which hydrous (2% experimental conditions using data from more recent H2O) experiments were carried out at high pressure by high pressure melting experiments. Details of this Elkins-Tanton and Grove (2003). Using their starting comparison will be presented elsewhere (Leeman, in composition, and back-calculating to the composition preparation). In essence, published analyses of liquids of a liquid in equilibrium with Fo90 mantle, we obtain produced by peridotite melting over a range of T and P values close (within b10 8C and 0.2 GPa) to pressures were used directly with Albare`de’s (1992) those determined experimentally for hydrous melt equations to calculate P and T. On average, calculated multiply saturated with ol+cpx+sp (Table 3). Even and actual pressures agree within 0.3 GPa over a total though the experimental liquids were not multiply range of 0.5–7.0 GPa. There is considerably greater saturated with a strictly peridotitic mineral assem- W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 91 blage, this agreement implies that our approach is for two samples [Tennant and Yellowjacket] to be applicable to hydrous melts similar to some CCT spurious owing to dfluid additions from the subduct- Group II lavas despite some variance in assumed ing slabT. However, we find that T’s estimated for source mineralogy. these two samples are more or less internally consistent with those for the other samples, although 4.3.3. Comparison with previous estimates Yellowjacket yields a conspicuously low segregation The validity of pressure and temperature estimates P (anomalously high SiO2?). Results obtained by both for magma segregation depends on how well the approaches are presented in Table 3 (also cf. Fig. 8). aforementioned assumptions are met, and can be Overall, they agree reasonably well and, in contrast to assessed by how well they agree with independent the study of Bartels et al. (1991), both imply that the approaches and constraints. Here we show that the LKTs ascend from depths well below the Moho. We methods described above result in close agreement will return to this issue later in the paper. with estimates obtained by different approaches (cf. Table 3 for comparisons). 4.3.3.2. Method of Wang et al. (2002). These authors use a version of the Langmuir et al. (1992) method in 4.3.3.1. Method of Elkins-Tanton et al. (2001). These which primitive melt compositions are normalized to authors used a similar back-calculation scheme in 8% MgO to minimize effects of low pressure differ- which equilibrium olivine and plagioclase were added entiation. [FeO]8 and [Na2O]8 values derived from in experimentally determined cotectic proportions to MgO-variation diagrams for cogenetic lava suites are primitive (but olivine+plagioclase phyric) northern used in a melting column model to estimate initial and California LKTs to estimate compositions of precursor final P of melt formation as well as the degree of magmas in equilibrium with Fo89 peridotitic mantle. melting for the parental magma batch. Our method By comparing normative mineralogy of these liquids was applied to basaltic lavas of the Big Pine volcanic with phase equilibria in experimental melts, they field (California) using data from Ormerod et al. estimated segregation P–T conditions. Interestingly, (1991), and using the same mantle composition (Fo89) several of these samples have Mg# in excess of that assumed by Wang et al. (2002).Tomakethe required for equilibration with their assumed Fo89 comparison as direct as possible, we started with a olivine, and the necessity for a more refractory source liquid composition estimated at 8% MgO (as in Wang is evident. Also, the high proportion of plagioclase in et al., 2002), and performed the back-calculationP to a their mineral additives results in substantial material parental liquid using two values for Fe3+/ Fe (0.10, addition to reach parental magma compositions. We 0.15) Our pressure estimates agree with their final have modeled parental magma compositions for the pressure within 0.1 GPa (Table 3). They did not same samples, but assuming sources with Fo90 and present a temperature estimate, but we estimate values Fo91, and adding only equilibrium olivine. Although close to 1300 8C. omission of feldspar strictly is inconsistent with the presence of phenocrystic plagioclase in some of these 4.3.4. Calibration against seismicity constraints at lavas, in their approach, its contribution likely is Kilauea volcano overestimated in assuming that the liquid follows an Our method also was applied to historic basalts of ol+pl cotectic to depths approaching those of melt the 1959 Kilauea Iki eruption (e.g., Leeman and segregation. Fortunately, as discussed earlier, segre- Scheidegger, 1977). This eruption was preceded by gation pressures and temperatures are not highly seismicity at N50 km depths that progressively shoaled sensitive to the proportion of plagioclase. until the eruptive phase began at the summit caldera Although we predict slightly shallower depths than (Eaton and Murata, 1960; Koyanagi et al., 1975). those estimated by Elkins-Tanton et al. (2001), Assuming that the seismicity was related to melt agreement is close for many samples. Temperature migration, these data provide a minimum depth for estimates are also similar for most samples, partic- segregation of the magma from the upper mantle. ularly considering our models assuming a Fo91 Kilauea Iki basalts define a classic olivine-fractiona- source. They consider their anomalously high T’s tion trend in MgO-variation (Pietruszka and Garcia, 92 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105

1999). Theoretically, lavas defining olivine control mafic lavas in this suite contain less than 7.5% MgO, lines in these diagrams differ only in the amount of and are thus somewhat evolved compared to other olivine removed (or added) to the parental composi- CCT basalts that we consider to be primitive (Table tion. We found that samples containing between ~7% 1). Because formation of the more evolved lavas and 16% MgO project back to a near constant parental involved magma mixing, they have anomalously high composition assuming equilibrium with Fo89 mantle. SiO2 and low MgO contents relative to normal closed Moreover, the calculated equilibrium olivine in these system fractionation trends. The highest pressure samples corresponds closely with phenocryst analyses (~2.2 GPa) and temperature (~1460 8C) estimates (Leeman and Scheidegger, 1977). The estimated are obtained for samples with the highest MgO (see segregation P for these samples is ~2.1 GPa, equiv- arrows in Fig. 7), and more evolved samples produce alent to a depth (~68 km) only slightly greater than the lower P and T values (by as much as 0.6 GPa and 100 deepest precursor seismicity. This example also 8C, respectively). Because truly primitive magmas demonstrates the insensitivity of our method to magma were not available for sampling, actual segregation P compositional variations as long as these result solely and T values are likely to be still higher. This example from closed system fractionation of equilibrium illustrates that processes of magma mingling or crustal olivine. As an aside, picritic samples with more than interaction during ascent are likely to result in 16% MgO, produced by olivine accumulation result in underestimation of segregation P and T. increasingly spurious (higher) P estimates due to silica dilution. Our approach thus can be used to constrain the composition of actual parental liquids. 5. Results and discussion

4.3.5. Effects of open system differentiation—an Our results have implications for several aspects of example from Mt. St. Helens Cascadia magmatism. First, the most primitive erup- Our methodology has been applied to two coge- tive lavas provide consistent evidence for high netic basaltic suites (7.5–6.0% MgO) from Mt. St. magmatic temperatures that must be considered in Helens corresponding to magmas of LKT and Alk/ developing realistic models for the thermal structure OIB parentage, respectively (Leeman et al., 1990; of the underlying mantle wedge. Projected to depths Leeman et al., in preparation). Because all of these of magma segregation, even higher temperatures are lavas are part of the ~2 ka Castle Creek eruptive implied, and it is these conditions that must be episode, they provide clear evidence for the contem- satisfied in models of wedge dynamics. Second, the poraneous presence of diverse basaltic magmas as implied depths of segregation, coupled with spatial well as isolated ascent paths at deep crust and mantle distribution of the chemical sub-type lavas, provide depths (Smith and Leeman, 1993). In each case, a important constraints for petrogenetic models. Taken compositional spectrum developed as a consequence at face value, these results suggest that Cascades of shallow mingling with intermediate composition magmatism may differ in important ways from the liquids beneath the stratocone. This interpretation is standard arc paradigm. Processes manifest there also supported by observed mineral disequilibria (Smith may be important at other subduction zones, but are and Leeman, 2005) and by discrepant U-series difficult to resolve—particularly where cooler slabs chronologies (cf. Cooper and Reid, 2003). The most promote an overwhelming flux of slab-derived fluids

Fig. 7. (a) Segregation T and P estimates for CCT magmas; arrow shows magnitude of temperature shift assuming that actual magmas contain up to ~5% H2O. Shaded field shows the likely maximum and minimum temperature range within the underlying mantle considering that only Group II magmas are hydrated. Spatial variation in (b) T-seg, showing maximum (anhydrous) values (note inverse scale) and (c) segregation depth, with T contours based on estimates for all CCT primitive lavas; data points for basaltic andesites (BA) are shown in gray tones to signify that P–T conditions are most uncertain for these samples (see text). Projected locations of Mts. St. Helens (MSH), Hood (MA), and Adams (MA) are shown for reference. Arrows in (b) and (c) indicate the range in T-seg and depth for the Mt. St. Helens OIB hybrid suite, and point toward the most magnesian sample and most realistic segregation conditions (see text). Uncertainties in absolute T-seg and depth values are indicated by boxes at lower left; relative differences between samples are considered to be more precise (see text). W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 93 and relatively greater flux melting contributions. A in the frontal arc region of the CCT transect, and fundamental question that we cannot resolve at further petrologic and geologic studies are needed present is the cause for extensive basaltic magmatism before we can adequately address this problem. 94 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105

5.1. Magmatic temperatures km), and magmas last equilibrated at ~50–100 km depth. However, because most lavas are out of Fe–Mg Estimated eruptive temperatures are based on equilibrium with mantle, they must have partly mineral–melt and mineral–mineral methods (Smith crystallized (most in the range of ~5–25%), and so and Leeman, 2005), and using bulk rock analyses with they presumably followed a steeper thermal gradient the method of Sugawara (2000). For consistency, and (nonadiabatic) more or less consistent with the upper because the necessary mineral data are lacking for bound estimates of segregation temperature. Such many samples, the latter are shown in Fig. 6a for high temperatures at typical slab depths (ca. 100 km) compositions tabulated in Table 1. For the majority of beneath arcs is a surprising result. However, it must be samples where direct comparison can be made (Smith recalled that seismicity only extends to depths of and Leeman, 2005), multiple thermometric about 60 km beneath the Cascades, the slab in this approaches agree within the respective calibration region is unusually warm, and intra-arc extension uncertainties (typically within F30 8C); similar results could facilitate convective upwelling beneath at least have been reported for the Lassen region (Clynne and the nether regions of the arc. Borg, 1997). This result supports our assumption that compositions of many eruptive lavas do in fact closely 5.2. Temperature structure of the mantle wedge reflect actual magmatic compositions. But as cau- tioned earlier, because some Group II magmas likely A plot of inferred segregation T and P (Fig. 7a) contained several percent H2O, magmatic temper- defines a thermal gradient for the sampled portions of atures could be overestimated by ~20 8C for each the mantle wedge. A stippled field indicates the likely weight percent H2O. In the absence of quantitative minimal range in T for Group II lavas, allowing for up H2O concentrations, we presume that temperatures for to ca. 5% H2O in some of these magmas. If the latter some Group II magmas could be as much as 1008C derive from more refractory mantle (Smith and cooler than indicated. Corrections for LKT and OIB/ Leeman, 2005) with respect to Mg# (say Fo91), Alk lavas are expected to be negligibly small for our estimated T’s would be correspondingly warmer (by purposes based on analogy with similar lavas from up to ~50 8C), partly offsetting effects of dissolved northern California and elsewhere (Sisson and Layne, water. Other perspectives are given by cross-arc 1993; Sisson and Bronto, 1998). Thus, minimum profiles of segregation T and P (Fig. 7b,c). Again, temperatures for their magma sources are in the range there is an apparent displacement of Group II lavas to 1200–1300 8C. systematically lower P and T, with LKT and most Similarly derived T’s for back-calculated parental OIB/Alk lavas derived from deeper and warmer liquids (Fig. 6b) are significantly higher—up to a regions in the wedge mantle. There is no indication maximum of 1350 8C for some Group II lavas, and as of systematic differences in these relations across the high as 1450 8C for many LKTs. The range for OIB/ arc, and segregation conditions appear to differ little Alk lavas is wide, and, for the highest values (up to across the entire arc, particularly for the LKTs (for 1500 8C), may partly reflect relatively large amounts which T estimates are most robust). The most striking of solid that must be added to reach compositions in result is the apparent compositional stratification equilibrium with the mantle (assuming less refractory within the wedge mantle. sources will result in lower estimates for both Xsol and There is considerably greater uncertainty for the T-seg). The range is unrealistically skewed to low CA, and especially BA, lavas of Group II. Because the values (to 1270 8C) when hybrid lavas from Mt. St. Moho is near 40 km depth beneath this region Helens are included, as discussed earlier. Assuming (Mooney and Weaver, 1989; Trehu et al., 1994), that all Group I lavas derive from a similar mantle shallower segregation depth estimates for the BAs are source (~Fo90), the majority indicate segregation from unrealistic. It is likely that these magmas either hot mantle (1350–1450 8C). Though seemingly high, equilibrated with lithologies other than peridotite or we consider these values to be reasonable if the their compositions were affected by other processes eruptive temperatures (T-eruption) are accepted, such as hybridization or magma mixing. For example, magma ascent followed an adiabatic gradient (~18/ they could record storage and equilibration with W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 95 pyroxene- or amphibole-rich domains in the mantle Cascades forearc (Blakely et al., 2003). Temperatures or, possibly, with mafic lower crustal rocks (cf. in this domain therefore are likely to be below the Conrey et al., 2001; Hart et al., 2003). Until the upper stability limit for serpentine (roughly 600 8C; petrogenesis of the BAs is better understood, we Hacker et al., 2003). Combining this interpretation consider their segregation P and T conditions to be with our estimated high T’s beneath the frontal arc, a poorly defined. Other CA lavas appear to have strong lateral thermal gradient is implied for the segregated from the uppermost mantle, and they mantle wedge below the frontal arc. It remains to be either could have formed near such depths or have seen if such a feature is consistent with the steep heat collected there following ascent from greater depths. flow gradient in the frontal arc region (Blackwell et Contoured isotherms based on segregation P and T al., 1990). estimates for Group I lavas are subhorizontal and A preliminary assessment of along-strike differ- suggest nearly uniform thermal gradients beneath the ences in mantle wedge thermal structure indicates that entire transect. This result differs from thermal our results are representative of other parts of the arc. structures predicted by numerical models, and espe- Fig. 8b shows that primitive LKT lavas from northern cially those incorporating T-dependent viscosity in the California define similar segregation conditions to mantle wedge (e.g., Van Keken et al., 2002), but is those inferred for the CCT. One difference is that more consistent with distributed high heat flow values primitive lavas of both Group I and II types from the in the back arc region (Blackwell et al., 1990; Currie Mt. Shasta–Medicine Lake (Baker et al., 1994; Grove et al., 2003). If CCT magmas ascended vertically et al., 2002) and Lassen (Borg et al., 1997) areas are from their sources, then temperatures beneath the more magnesian than their CCT counterparts, and thus frontal arc are inferred to be extremely warm their mantle sources are likely more refractory. In our approaching the slab surface. A tomographic image modeling we have compensated for this fact by using across this latitude of Cascadia (Michaelson and aFo91 reference mantle for these LKTs, and more Weaver, 1986) places the subducted slab very close refractory sources are implied for many Group II to the westernmost zone of Group I melt segregation magmas (Clynne and Borg, 1997). As explained in (Fig. 8a). A similar image at 458N(Bostock et al., Section 4.2, this choice increases estimates of 2002), indicates a shallower slab dip that is consistent segregation T and P only slightly (~40 8C, ~0.3 with a more easterly limit for vent distributions at that GPa) compared to a Fo90 source (cf. Table 3 for latitude. To reconcile our inferred thermal structure comparative estimates using different sources). As with the above constraints and with numerical models seen for the CCT, lavas similar to those of our Group of wedge cornerflow, the isotherms must invert (bend II appear to have equilibrated in the uppermost over) and become subparallel to the slab surface. mantle, whereas LKTs apparently ascended from Sampling and depth resolution for our method are depths on the order of 60F10 km, except in the inadequate to resolve such fine structure in the immediate vicinity of Mt. Shasta where shallower isotherms. conditions are indicated. The latter anomaly could be A surprising result is that Group I lavas, which an artifact of magma mixing processes (cf. Newman et show the least chemical evidence for slab contribu- al., 1986) as discussed earlier for Mt. St. Helens. Our tions (cf. Fig. 4), apparently ascend from depths results are comparable to those of Elkins-Tanton et al. closest to that source. This result is consistent with the (2001), but the larger database (McKee et al., 1983; predicted warmth and extreme dehydration of the Baker et al., 1994; Borg et al., 1997; Clynne and Cascadia slab before it reaches depths below the Borg, 1997; Grove et al., 2002) considered here frontal arc of the CCT (note that Van Keken et al. indicates the presence of a broad, hot mantle with (2002) predict a slab-surface temperature near 1000 subhorizontal isotherms extending far into the backarc 8C at 100 km depth). Bostock et al. (2002) explain region. Inclusion of Pliocene LKTs from the backarc their tomographic images in terms of an extensively region suggests that the inferred thermal conditions serpentinized wedge corner beneath the forearc, and have persisted for a substantial time and are probably this view is supported by regional gravity and not dtransientT but a long-lived characteristic of the magnetic anomaly patterns along much of the underlying mantle. 96 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105

Fig. 8. Comparative views of cross-arc variations in segregation depths for two Cascades transects (note different lateral scales). (a) CCT results (omitting basaltic andesites) compared with locus of the subducting slab beneath the region near 468N (heavy solid curve, A; Michaelson and Weaver, 1986); assuming vertical ascent, segregation depths for Group I magmas project very close to the slab surface at this latitude.

Segregation depths are estimated relative to a Fo90 reference mantle. The projected slab surface near 458N (dashed curve, B; Bostock et al., 2002) is shown for comparison; dark shaded area indicates an inferred serpentinite wedge at that latitude that has been attributed to slab de- watering. (b) The northern California Cascades (showing only LKT lavas), with data from the Mt. Shasta–Medicine Lake (SML), Mt. Lassen (LVF), and Devils Garden (DG) areas; locus of subducting slab is approximate (heavy curve; Romanyuk et al., 1998). A slightly more refractory source (Fo91 reference mantle) is assumed because lavas from California have systematically higher Mg# and more magnesian equilibrium olivine (Clynne and Borg, 1997; Smith and Leeman, 2005). Depth estimates from this study and from Elkins-Tanton et al. (2001) are in close agreement (cf. labeled tie-lines) for all but two samples (symbol key and numerical values are given in Table 3). Points plotted for other LKTs are based on published analyses (McKee et al., 1983; Baker et al., 1994; Borg et al., 1997; Bacon et al., 1997); SLM=Shasta–Medicine Lake, LVF=Lassen Volcanic Field; DG=Pliocene LKTs from the backarc region in NE California. Both transects indicate temperatures near 1300 8Cin the uppermost mantle, and generally similar thermal structures at greater depths. Segregation conditions for LKTs appear to be comparable beneath the backarc regions, but unusually shallow beneath Mt. Shasta (see text). For the high temperatures predicted at near-slab depths to be consistent with numerical models of wedge cornerflow, the isotherms must bend over and become subparallel to the slab surface. If magmas rise vertically from their sources, geometric constraints imply the existence of a strong thermal gradient in the mantle wedge just above its interface with the slab. W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 97

5.3. Implications for SZ magmatism subduction-like attributes in some Cascades basalts (including samples within our transect near Mt. The principal impetus for this study was a desire to Adams) may be inherited from previously modified evaluate existing petrogenetic models for Cascades lithospheric or wedge mantle (Bacon et al., 1994, magmatism. Two opposing views have developed 1997), and Os isotopic data provide evidence for concerning the role of volatiles and slab-derived involvement of old lithospheric material in mafic lavas contributions, perhaps compounded by along-strike from Mts. Lassen and Adams (Borg et al., 2000; Hart et variations in subduction conditions and/or the struc- al., 2003). tures of the subducting and upper plates. A general Unresolved fundamental issues concern the extent characteristic of most volcanic arcs is that arc magmas to which Cascades magmas carry a slab-derived tend to be enriched in relatively fluid-mobile elements, geochemical signature, and whether this is derived and in many places, this is the dominant situation from modern SZ processes or inherited from older (Pearce and Peate, 1995). However, many mafic lavas dfossilT enrichments in the lithosphere (i.e., assuming from the Cascades and other relatively warm arcs (e.g., this to be the most likely static reservoir). Also, if there Mexico; Righter, 2000) lack this slab signature. is only a weak slab contribution in many Cascades Although data are sparse, only near Mt. Shasta is there basalts, then some other driving force besides fluid- evidence for the existence of extremely hydrous enhanced melting is required to explain the prolific primitive magmas (Sisson and Layne, 1993; Grove et basaltic magmatism in this arc. For example, decom- al., 2002, 2003). Also, lavas from the Lassen region pression-melting of convectively upwelling subarc (Borg et al., 1997), at the southern end of the arc, tend to mantle may play a dominant role, particularly in the show progressive enrichments of Sr and other large-ion southerly regions where backarc extension appears to lithophile elements (LILE), signifying increasing con- be propagating northward from the Basin and Range tributions of slab-derived fluids, toward the forearc. province. Thus, in hot arcs like Cascadia, the relative In contrast, most Cascades basalts have extremely importance of flux-melting may be diminished com- low contents of highly fluid-mobile elements like B pared to that of decompression-melting. (Leeman et al., 1990; Noll et al., 1996; Leeman et al., Petrogenetic models that have been proposed 2004; Green and Sinha, 2005), and ratios like B/Zr, B/ include the following: Nb, etc., are indistinguishable from ranges for OIB or (1) Magma diversity may simply reflect inherent MORB. Young Cascades lavas (Shasta, Mt. St. Helens) heterogeneities within the mantle wedge, with enriched also lack characteristic U-series disequilibria (sugges- domains existing in a relatively depleted matrix. tive of fluid enrichments), and in fact most have Magma diversity could be explained by differing (230Th/238U) enrichments in the opposite sense to that degrees of melting, with more depleted magmas in typical arcs (cf. Newman et al., 1986; Volpe, 1992; (LKTs) representing higher degree melts (cf. Leeman Cooper and Reid, 2003). Nor do the few analyzed et al., 1990). Cascades lavas exhibit 10Be enrichments characteristic (2) Convection and progressive decompression- of those seen in most other arcs, and taken as evidence melting of upwelling mantle could produce a gradient for incorporation of a young subducted sediment of increasingly more refractory residual mantle toward component (Morris and Tera, 1989; Morris et al., the frontal arc, where subsequent addition of slab- 1990). Finally, the presence of isotopically light B in derived fluids or melts might promote further melting melt inclusions from one high-Mg basaltic andesite and also enhance abundances of fluid-mobile elements. from Mt. Shasta was interpreted as being derived from This scenario predicts correlated enrichment of Mg#, a highly dehydrated slab source (Rose et al., 2001). In Sr/P, etc., in frontal arc relative to backarc lavas, as general, where subduction-like signatures are seen in observed in the Lassen area (Borg et al., 1997, 2000; Cascades basalts, these are relatively weak compared to Wallace and Carmichael, 1999; Grove et al., 2002). more typical arcs (Leeman et al., 1990; Bacon et al., (3) Reiners et al. (2000) explain the spectrum of 1997; Conrey et al., 1997; Borg et al., 1997, 2000; basaltic compositions in terms of variable fluxing of Green and Harry, 1999; Reiners et al., 2000; Grove et essentially uniform wedge mantle—with magma diver- al., 2002). On the other hand, there is evidence that sity being controlled by amount and composition of slab- 98 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 derived fluids and their effects on mantle melting. This contributions approaching the frontal arc. Second, if type of model might also explain along-strike variations these magmas ascend more or less vertically from in which slab derived contributions are controlled by their sources, there must exist a relatively deep mantle differences in subducted sediment or slab composition domain, extending to near-slab depths beneath the (cf. Leeman et al., 1994; Plank and Langmuir, 1998)and/ frontal arc, that has been little modified by slab- or SZ thermal structure (Leeman et al., 2002). derived fluids or melts. Third, the implied spatial Such models can be evaluated in the light of distinctions in source composition and depth require constraints provided by this study, and additional more complex models than simple variations in flux- geochemical constraints bearing on these models have melting of a homogeneous mantle. been discussed by other authors (e.g., Grove et al., These relations are consistent with the view that 2002; Green and Sinha, 2005). The CCT is unusual in the subducting slab is extensively dehydrated during that it encompasses the fullest diversity of mafic descent before it reaches subarc depths, such that magma types seen in the Cascades. Moreover, the inventories of water and especially fluid-mobile wide spatial distribution and distinct chemistry, elements are so depleted that typical arc geochemical mineralogy, and apparent segregation depths for signatures cannot be produced. Also, despite higher LKT, OIB/Alk, and CA type lavas across the transect than normal slab temperatures, subducted sediment provide unique geometric constraints on petrogenetic and oceanic crust are unlikely to melt if volatile models (Figs. 8 and 9). This perspective conflicts with component activities are low (i.e., resulting in elements of all the aforementioned models. First, elevated solidus temperatures). For example, even Group I lavas exhibit limited cross-arc variation in the exceedingly warm thermal model of Van Keken et chemistry that belies a systematic increase in slab al. (2002) predicts slab surface temperatures (~900 8C

Fig. 9. Depth–temperature diagram illustrating a petrologic framework for the CCT transect. Bracketing maximum and minimum thermal profiles derived from estimated segregation conditions for CCT lavas (cf. Fig. 7a) are compared with a representative liquid adiabat (~1 8C/km), dry (DS) and wet (WS) peridotite solidi (Hirschmann, 2000; Ulmer, 2001), and slab surface temperature (SST) and wedge geotherm (~25 8C/ km) based on a dhotT thermal model for the Cascadia SZ (Van Keken et al., 2002). Also shown are approximate boundaries for plagioclase, spinel, and garnet peridotite subsolidus transitions (Ulmer, 2001). The dTerT field shows the range of crystallization temperatures (or dT- eruptionT) inferred for CCT basalts; gray field shows projection of these values to depth along liquid adiabats (parallel to the example shown), and indicates the minimum loci of mantle temperatures at depth. Oval fields indicate inferred segregation conditions for Group I (LKT and Alk) and Group II (CAB, HKCA) basalts and dnear slab meltT field indicates the locus of possible H2O-saturated wedge melting adjacent to the subducting slab. Broad shaded curved arrow shows an ascent path for the latter hydrous melts, corresponding to the models of Grove et al. (2002, 2003); assuming they remain in thermal equilibrium with the mantle wedge geotherm, such melts would reach temperatures similar to those of Group I magmas before rising to their inferred shallow mantle segregation zone along a super-adiabatic path. An alternative scenario is that ascending hot Group I magmas provide sufficient heat to melt the shallow mantle at H2O-undersaturated conditions (i.e., slightly below the dry solidus), producing Group II magmas and a spectrum of hybrid mixtures (e.g., some OIB and possibly BA liquids). W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 99 below the volcanic front) lower than fluid-under- domains entrained in a matrix of relatively depleted saturated solidus temperatures for appropriate sedi- LKT-source mantle. However, as evidenced by their ments (cf. Peacock et al., 1994; Johnson and Plank, distinctive compositions, Mt. St. Helens LKT and Alk 1999). The high temperatures we predict for Group I type magmas must form contemporaneously (but from lavas, and LKTs in particular, seemingly require different mantle domains?) and ascend to the surface convective upwelling of asthenospheric wedge mantle from depths of at least 60 km without encountering and preclude fluid-enhanced melting because they one another and/or homogenizing. project close to the dry peridotite solidus (cf. Group II basalts must originate in a different Hirschmann, 2000). Because their MORB-like chem- manner. Our predicted eruptive temperatures, while istry and relatively flat REE patterns are inconsistent less certain, are still constrained to be fairly high with significant garnet in their source, LKTs most (N1200 8C for some) based on a variety of crystal- likely are produced by decompression melting within lization thermometers and petrographic and mineral the spinel peridotite regime (Bartels et al., 1991; chemistry relations. Estimated segregation depths Bacon et al., 1994). Our predicted segregation depths suggest that they equilibrated in the shallow mantle (50–75 km for most LKTs) are permissive of this (~35–50 km for most, excluding the BA and some interpretation (as opposed to a garnet peridotite SHO types), which is also consistent with available source), especially considering the uncertainties in experimental data (Gaetani and Grove, 2003). Most the estimated pressures. In contrast, if LKTs derive Group II lavas likely represent at least slightly from much shallower depths, the thermal gradient in hydrated magmas, and all exhibit small to moderate the upper mantle would be unreasonably steep, compositional signatures characteristic of slab con- projecting well above the anhydrous peridotite sol- tributions. They also have fractionated REE profiles idus. Similar interpretations have been put forth suggestive of a garnet signature in their source previously (e.g., Sisson and Bronto, 1998; Righter, (Leeman et al., 1990, Bacon et al., 1997; Green and 2000; Elkins-Tanton et al., 2001)andarewell Sinha, 2005) and implying a relatively deep origin. In supported by experimental studies (cf. Gaetani and one scenario, they could represent magmas produced Grove, 2003). close to the subducting slab but, prior to eruption, Some OIB and nearly all Alk lavas have segrega- underwent equilibration with shallow mantle rocks. tion conditions overlapping with the LKTs. Although Grove et al. (2002, 2003) propose that especially many OIBs appear to have segregated from shallower water-rich variants (high-Mg basaltic andesites) from mantle depths, this could also be an artifact. As the Mt. Shasta region must originate as nearly water- already noted, mixing of OIB- and LKT-like magmas saturated (N10% H2O) melts at near-slab depths, and with more evolved shallow magmas at Mt. St. Helens they discuss models whereby such liquids interact biased our pressure estimates to artificially low with and promote melting of shallower mantle during values. Alk lavas presumably are the lowest degree ascent. Although plausible, this interpretation is melts, OIB intermediate, and LKT the greatest in difficult to reconcile with contrasting evidence that terms of decreasing Nb/Zr, La/Yb, and similar ratios the Cascadia slab is exceptionally warm and dehy- (e.g., Fig. 4) and incompatible trace element abun- drated. Moreover, reactive ascent of hydrous melts dances. Most OIB/Alk lavas exhibit REE fractionation from near-slab depths would presumably lead to consistent with garnet in their source, and exhibit modification of the overlying mantle (as modeled by much higher alkali and high-field strength elements Grove et al., 2003), yet this appears to be inconsistent (HFSE) enrichments than LKTs (Leeman et al., 1990; with preservation of LKT (and OIB/Alk) sources— Conrey et al., 1997; Bacon et al., 1997), implying particularly below the CCT frontal arc region where compositionally distinct sources. A simple explana- fluid fluxes should be strongest. To reconcile these tion of these features is that Group I lavas could paradoxes, one could call on channelized ascent of represent a spectrum of melts (i.e., varying degree of low density near-slab melts (e.g., Hall and Kincaid, fusion) of upwelling wedge mantle lacking significant 2001), whereby interaction of ascending material with slab contributions. For example, the OIB/Alk melts the overlying mantle is minimized. Another possibil- could derive from significantly alkali-enriched ity is that the Shasta lavas could reflect subduction of 100 W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 unusually hydrated and fractured slab of the Gorda would classify basaltic lavas of the northern Cas- plate (Grove et al., 2003), and thus may be atypical of cades as either Group I (OIB/Alk) or Group II other parts of the Cascades. (CAB, BA) types, the former essentially lacking a An alternative scenario is that modern Group II slab signature whereas the latter do carry such a lavas may form by melting of shallow lithospheric signature. Group II variants exclusively comprise the mantle that had been extensively metasomatized by basaltic suites between Mts. Rainier and Baker and melt or fluid infiltration during the previous 40 Ma of are minor constituents in the southeastern part of the Cascadia SZ magmatism. More rapid subduction Garibaldi volcanic belt (GVB). Group I variants during the early history of the arc (Verplanck and dominate the GVB and are the only basalt type Duncan, 1987) might have resulted in a cooler slab, found in the northwesternmost part. These relations consistent with the aforementioned flux-melting mod- imply that slab fluid flux may be modulated by slab els. Freezing of earlier Group II type magmas and/or temperature and dehydration history. The absence of formation of hydrated lithospheric mantle containing true LKTs may signify that in these sectors the arc is modal amphibole or phlogopite due to melt–mantle in a compressional state, consistent with nearly reaction could impart a Group II geochemical orthogonal plate convergence, that prohibits eruption signature to parts of the shallow mantle. Subsequent of such magmas, if present. More likely, formation reheating of eutectoid-like domains (i.e., having a of such magmas may require an extensional environ- dquenchedT basalt component) in response to advec- ment (to enhance mantle wedge convection and tive heating by hot ascending Group I magmas or to decompression-melting) that simply has not devel- decompression (Harry and Leeman, 1995)could oped thus far. produce dsecond-stageT basaltic magmas having an Finally, the warm conditions inferred for the subarc inherited Group II signature. This process would mantle, with temperatures approaching 1300 8C near likely result in common mixing and differentiation the Moho, may bear on production of silicic magmas among different magma batches that could contribute within the lower crust. For example, magnesian to the gradational compositional spectrum of erupted dacites from Mt. St. Helens have mineral equilibrium magmas, as well as the small proportion of extremely temperatures as high as 900–1000 8C(Smith and primitive end members in our database. Origin of Leeman, 1993). Such magmas could be produced by Group II lavas by remelting of fossil metasomatic partial melting of amphibolitic lower crust (Smith and zones in the shallow mantle versus a response to Leeman, 1993), fractionated derivatives of under- modern slab fluxing carries testable geochemical plated basaltic magmas (Blundy et al., 2003), or implications regarding the timing of slab additions variations of these scenarios. and storage of the source signature. Clearly, additional work is needed to fully evaluate these contrasting 5.4. Overview models, including further geochemical studies as well as assessment of volumetric relations and timing of The results presented here are in qualitative agree- magmatic evolution. For example, time-sensitive ment with those for other volcanic arcs (Tatsumi et al., tracers like 10Be or U-series isotopes potentially could 1983; Sisson and Bronto, 1998; Tamura et al., 2002) be used to discriminate between old inheritance versus and suggest that arc volcanism is driven largely by recent slab source scenarios in areas where high water convection in the underlying mantle wedge. Assum- contents have been documented (e.g., Shasta). ing convective upwelling to be a continuous process, Regional variations in the modern Cascadia arc we propose that there has been a continuous dtrickle- suggest that tectonic conditions may strongly influ- feedT of hot decompression melts since the time this ence magmatic manifestations. Green and Sinha pattern developed—a sort of dmagmatic windT that (2005) document a systematically decreasing slab influences and perhaps drives shallower magmatic signature in the northernmost Cascades with decreas- processes. The surficial manifestation of this phenom- ing age (or increasing temperature) of the subducting enon may be Group I lavas, particularly where plate. However, their data also can be interpreted in convection is strong and/or upper plate deformation the context of magma types as in this paper. We allows such magmas easier access to the surface. W.P. Leeman et al. / Journal of Volcanology and Geothermal Research 140 (2005) 67–105 101

Extension within the southern Cascadia arc may References explain the widespread occurrence of primitive Group I lavas in that region (Bacon et al., 1994, 1997). In Albare`de, F., 1992. How deep do common basaltic magmas form addition, exceptionally warm conditions in the Cas- and differentiate? J. Geophys. Res. 97, 10997–11009. cadia SZ are expected to limit availability of deep Bacon, C.R., Gunn, S.H., Lanphere, M.A., Wooden, J.L., 1994. Multiple isotopic components in Quaternary volcanic rocks flux-melting products (although the extent to which of the Cascade arc near Crater Lake, Oregon. J. Petrol. 35, this is true is an unresolved question). In more typical, 1521–1556. cooler SZs, slab-derived fluids (or melts) are expected Bacon, C.R., Bruggman, P.E., Christiansen, R.I., Clynne, M.A., to transfer a subduction signature to large parts of the Donnelly-Nolan, J.M., Hildreth, W., 1997. Primitive magmas at mantle wedge (or mingle with decompression melts five Cascade volcanic fields: melts from hot, heterogeneous subarc mantle. Can. Mineral. 35, 397–423. thereof), and in one way or the other dominate arc Baker, M.B., Grove, T.L., Price, R., 1994. Primitive basalts and magma outputs. These processes, in our view, con- andesites from Mt. Shasta, N. California: products of varying stitute a feedback system wherein the relative abun- melt fraction and water content. Contrib. Mineral. Petrol. 118, dance of Group I versus II type magmas is a function 111–129. of SZ thermal state. This, in turn, is influenced by the Barker, S., Leeman, W.P., 1997. Early High Cascade magmatism related to intra-arc extension in the Cascadia subduction zone. degree of coupling between wedge convection and Abstr. Programs-Geol. Soc. Am. 29 (5), 3. plate tectonic functions that dictate devolatilization Bartels, K.S., Kinzler, R.J., Grove, T.L., 1991. High pressure phase efficiency in the subducting slab. These factors relations of primitive high-alumina basalts from Medicine Lake provide a framework that can be used to understand volcano, northern California. Contrib. Mineral. Petrol. 108, differences in magmatic output within and between 253–270. Beattie, P., 1993. Olivine–melt and orthopyroxene–melt equilibria. volcanic arcs. Contrib. Mineral. Petrol. 115, 103–111. Although we believe this interpretation is well Beattie, P., Ford, C., Russell, D., 1991. Partition coefficients for supported by the results summarized here, there olivine–melt and orthopyroxene–melt systems. Contrib. Min- remain many unresolved questions. For example, the eral. Petrol. 109, 212–224. magnitude and nature of slab-derived contributions, Bebout, G.E., Ryan, J.G., Leeman, W.P., Bebout, AE., 1999. 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