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1: University of Delaware, Newark, DE 19716 USA; 2:Univeristy of Texas Insights to mantle wedge composition and thermal structure from magmatism along the 14o -30' N cross-chain, Southern Marianas Edward J. Kohut1; Robert J. Stern2, Adam Kent3, Roger Nielsen3, Sherman Bloomer3, Yoshi Tamura4 1: University of Delaware, Newark, DE 19716 USA; 2:Univeristy of Texas-Dallas, Richardson, TX 75083, USA, 3: Oregon State University, Corvallis, OR 97331 4: Institute for Research on Earth Evolution (IFREE), Yokosuka, JPN Major and Trace Element Compositions Cross-arc Trends Thermal Structure 6 Disscussion and concluding thoughts Methods Estimate LOI Abstract FTIR Glass The 14° 30’N cross-chain is unique magmatic region compared to other cross-arc 5 4 FTIR Ol-hosted MI 0 14-30 Cross-chain BASALT BASALTIC- ANDESITE DACITE systems. Notable features include: Figure 6: (left) K O vs. SiO for 14-30 lavas and Shaw et al. 2006 ANDESITE Chaife WR 2 2 Lavas from cross-arc volcanic chains would presumably provide sample mantle from 4 Chaife MI olivine-hosted melt inclusions. Most compositions Temperature and Pressure estimates 0 ) 2 o MP WR 1) Magmatism has primarily been basaltic and water-poor. across a subduction zone and would aid calculations of the flux of components through % We estimated magmatic temperatures for the lavas using olivine-spinel geothermometers 1174 Shoshonitic MP MI o follow the Low- to Medium-K trends typical of t (Fabries, 1979; Roeder et al., 1979; Ballhaus et al., 1991), two-pyroxene thermometer of o 1200 3 LMP WR w 3 the Subduction Factory. Since 2001, several cross-chains and arc/rear-arc volcano pairs ~1215 o ( 1132 LMP MI 4 10 o Mariana arc lavas, but are overall more primitive Brey and Köhler (1990), the olivine-liquid thermometers of Ford et al. (1983), Beattie (1993) 1200 NWR -1 WR O 2) The most primitive lavas erupted at the magmatic front. in the Southern Marianas have been the subject of ongoing investigation to address these 2 NWR -1 MI and Putirka (1997), the CPX liquid thermombarometer of Putirka et al. (1996, 2003) and H Andesite Knoll WR than most magmas from this region. The Chaife 2 issues. One of these, the 14° 30’ N cross-chain, has features that distinguish it from the melt MgO thermometer of Sugawara (2000). Pressures were estimated using the 6 o High-K 20 1273 Andesite Knoll MI CPX-liquid barometer of Putirka et al (2003) and the Ol-CPX barometer of Köhler and Brey ) o Seamount melts are the most primitive sampled 3) Current activity is at NW-Rota 1, away from the arc’s magmatic front. r others in the Marianas. Notably, the cross-chain contains the actively erupting NW-Rota 1164 O 2 Mariana Arc Ion Probe 2 a (1990). Spinel compositions were corrected using the method of Barnes et al. (2004) and ) 1 b 8 K (Stern et al. 2003) to date from the Marianas (Kohut et al., 2006). 1 volcano and the Chaife seamount (located at the arc terminus of the cross-chain), which m K whole rock compositions were back-calculated to be in equilibrium with the most primitive k ( ( 30 4) Volumes of volcanoes are small and there is no typically large composite volcano examples of olivine and CPX and spinel bearing olivine using MixNFrac (Nielsen, 1990 ) h E P has erupted the most primitive lavas known from the Marianas. Furthermore, primitive t W 10 o Med-K p 0 Mid-Point Smt 1356 e at the arc end of the cross-chain. It follows then that the 14-30 volcanoes represent 45 40 35 30 25 20 15 10 5 0 lavas have erupted at locations spaced along the length of the cross-chain and these Phase Chemistry Little Mid-Point Smt D 40 1 display variations in trace element composition and volatile contents that may reflect Phase analyses for NW-Rota samples were carried out on the JAMSTEC JEOL JXA-8900 12 NW Rota 1 melts of the underlying mantle little modified by Distance from MF (km) Andesite Knoll 800 Superprobe equipped with five wavelength-dispersive spectrometers (WDS). Olivine 1374o 5) Geothermobarometry indicates that melts stagnated at the base of relatively thin Chaife Smt Low-K shallow-level processes and contamination. heterogeneity of the mantle wedge source region. analyses were made with a counting time of 100 s, using an accelerating voltage of 20 kV, 50 14 Guguan Cross-chain 700 crust away from the magmatic front. The magmas at these locations underwent a beam current of 25 nA and a probe diameter of 5 µm toensure reliable Ni values. 0 Pyroxene and plagioclase analyses were made witha counting time of 20 s, using an W E While andesitic and dacitic lavas have been sampled, most 14° 30’ cross-chain lavas and 60 45 50 55 60 65 70 little low-pressure modification prior to erupting. accelerating voltage of 15 kV and a beamcurrent of 15 nA. The compositions of mineral 16 600 45 40 35 30 25 20 15 10 5 0 5 SiO2 melt inclusions are basaltic with Mg#s > 60. Volatile contents are variable and many phases for samples from other volcanoes were determined using the Cameca SX-50 and 1100 1150 1200 1250 1300 1350 1400 Distance from MF (km) 500 6) Trace element signatures for a subduction component are subdued or lacking. melts are water-poor (H2O<1.5 wt%). Oxygen fugacity estimates from olivine-spinel SX-100 EPMA at Oregon State University. Beam conditions were 1 mm spot size, 50 nA T (C) at 15 kV for olivine, clinopyroxene and spinel and 3 mm, 30 nA and 15 kV for feldspar and ) m pairs have a median of +1.75 D log QFM, which lies at the low end of the range of glass. Pyroxene analyses used count times of 10s for Na, Mg, Al, K, Ca, Ti and Mn and p 400 p ( 7) There are only poor to modest correlations between volatile contents and HFSE oxygen fugacity for arc lavas. Estimates of P-T conditions ranged from 924º-1250º C at 20s for Si, Cr, and Fe, while olivine analyses used count times of 20s for Ni and 10s for 2 Guguan Cross-chain O Na, Mg, Al, Si, Ca, Ti, Cr, Mn and Fe. Spinel analyses had count times of 10s for all 300 10 C Shaw et al. 2006 depletion, LILE enrichment, trace element subduction signatures, differentiation 1–3 kbar behind the arc to 1435º C at 14 kbar at the magmatic front; these pressures elements. Count times for groundmass glass were 10s for Na and Ca, 15s for Cr, 20s for Figure 4: (above) P vs. T for the 14-30 and Guguan cross- 30 0 likely coincide with the base of a relatively thin crust across this part of the Marianas Mg, Al, Si, K, Ti, Mn, and Fe, 30s for P and S and 1000s for Cl. chains based on CPX-liquid thermobarometer of Putirka 200 and distance to the magmatic front. o 1252 1237o subduction zone. Trace element evidence for an added subduction component (i.e. et al. (1996, 2003). In both chains the hottest melts had the 25 Whole Rock Chemistry 10 1 seawater assimilation 100 These features indicate enhanced asthenospheric upwelling along the axis of the cross- Ba/La, Ba/Th, U/Th La/Th, and Pb/Ce elevated above mantle values) is subdued or Major elements for whole rocks were analyzed via ICP-MS by Activation Laboratories and by XRF greatest equilibration depths. (Right) Comparison of at JAMSTEC-IFREE. 0 lacking in most melts. Significantly, correlations between volatile contents and LILE 20 MORB chain. This may due to a slab discontinuity detected in the region, and bathymetry pressure of equilibration vs. distance from magmatic front 20 o 45 40 35 30 25 20 15 10 5 0 1308 0.1 enrichment, HFSE depletion, melt fraction or presumed subduction components are poor Melt Inclusion Analyses provides evidence for crustal rifting. This upwelling would produce pressure-release (MF) for Guguan and 14-30 cross-chains. Note that in ) Distance from MF (km) o m For melt inclusion analyses, we used whole olivine and plagioclase phenocrysts separated 1384 h 15 k to non-existent. In addition, there are only weak correlations between trace element K T melting that would dilute any subduction component present. ( 30 / the latter, magmas behind the arc last equilibrated at / from the bulk rock and heated in a Deltec vertical quench furnace to eliminate post- l a slab fluid h 4.0 C t ratios and volatiles, differentiation and distance from the magmatic front. entrapment recrystallization. Analyses for major elements were conducted with the L 0.01 Mid-Point Smt at shallow depths. These likely correspond with the base p addition Cameca SX-50 and SX-100 EPMA at Oregon State University using the glass analysis routine e 10 NW Rota 1 of the crust. Eruption through shallow crust (possibly D 40 3.5 These melts may then sample mantle wedge little modified by subduction processes. We suggest that the data indicate that many of the magmas from 14° 30’ N cross-chain described above. Andesite Knoll Mariana Arc Chaife Smt Observed compositional variability may reflect mantle wedge heterogeneity. Important related to rifting) would limit crustal interaction and allow a 0.001 3.0 originated in the mantle wedge via decompression melting. As a result, there was little or Volatile Analyses more uncontaminated sampling of the mantle. P-T data 50 5 Mariana Aqueous Fluid questions remain however: For samples where water content was not determined via Loss on Ignition, Trough Mariana Trough 2.5 no input from the subducting slab to these magmas and only minor stagnation and M we used the Fourier Transform Infrared spectroscopy method (FTIR) to determine water based on CPX-liquid, Ol-liquid and olivine-spinel geotherm- F 0 Q modification at the base of the crust prior to eruption.
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