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 BASALTIC- 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 -poor.

across a 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 and Putirka (1997), the CPX liquid thermombarometer of Putirka et al. (1996, 2003) and NWR -1 MI H Andesite Knoll WR than most 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 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 ∆ 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. The mechanism driving this style content in melt inclusions and glass. Analyses were performed with the Thermo Nicolet 60 0.0001 ∆ 2.0 1) Is volatile variability truly a product of inherent variations in the melt source? -barometry (Ballhaus et al., 1991; Beattie, 1983; Fabries, 670 FTIR operated by Omnic software at the University of Oregon under the direction of 45 40 35 30 25 20 15 10 5 0 5 0.0 0.5 1.0 1.5 1 10 100 1000 g

of melting is unclear, but is likely asthenospheric upwelling and rifting related to an o l Paul Wallace. Water contents were measured using the 3530 nm OH peak and 1979; Ford et al, 1983; Roeder et al., 1979). Distance from MF (km) U/Th Ba/Nb 1.5 observed slab discontinuity in the region. These findings are significant, as they glass densities of 2022-2300 cm3/mol. 2) Is the subduction signature truly minimal or absent, or has high degrees of potentially indicate that the compositional variations in many of the lavas reflect 1.0 Figure 7: (above left) La/Th vs. U/Th from olivine-hosted MI indicates that some volatile variabilty may be due melting due to upwelling obscured the subduction signature? heterogeneity within the mantle wedge source region independent of any added to addition of aquesous fluid via subduction. Cl/K vs. Ba/Nb (above right) shows the possibilty of slab fluid 0.5 subduction component or low pressure modification. Further study of the volcanoes in 3) Is the mantle wedge in this region typical, or has the unusual slab topography led this cross-chain may yield data critical to accurate mass balance estimates in the addition, but also assimilation of seawater, particularly for Little Mid-point seamount. 0.0 45 40 35 30 25 20 15 10 5 0 -5 to compositional modification? Do serpentinites play a role? Subduction Factory. 100 Distance from Magmatic Front (km) This region has the potential for providing much need information on mantle wedge 90 inputs to the Subduction Factory. The above questions need to be resolved though. 80 We suggested that the entire cross-chain, in addition to NW-Rota 1, is an rich in e ) 70 l e Water Contents t potential research projects to answer important issues related to Subduction Factory F n + Introduction a 3 3 10 g 60

M processes.

A cross-chain in the Mariana Arc near 14°30'N latitude consists of 10 small volcanoes (1.3 - 120 km ; mean ~ 27km ) that terminates at the arc front with the small Chaife Seamount (Figure 1). Chaife has erupted M / e g v

i 50 t picrite and ankaramite, which is unusual for an arc volcano. Other than Chaife there is only another small adjacent seamount dubbed Mt. Mangnaese. Data is only now be gathered for the latter, but it is cut by a M

i

3 % m

2.5 t graben that Chaife is built on and is thus older than Chaife (Figure 2). There is no large arc volcano at this location, in comparison, typical Mariana volcanoes such as Guguan and Anatahan are 1200 and 700 km i r

a 40 ( P Required will be more isotopic data (i.e. Pb, Sr-Nd, Hf). Volatile data sets (other than S and /

(Figure 3) and Guguan itself forms the arc-terminus of a cross-chain. Furthermore, the youngest vent (< 30 ka.)appears to be NW-Rota, which is ~30 km behind the axis of the Mariana arc. NW-Rota is currently # n g

o 30 i M Cl) are presently small, with the notable exception of NW-Rota 1. New whole rock active and has been since at least 2003 (Embley, et al., 2006). Mantle P-wave tomography, shallow seismicity and focal mechanisms, and GPS results suggest that the 14°30'N cross-chain may have formed in t WR a

2 r PL MI response to a tear or other discontinuity in the subducted Pacific plate (Miller et al., 2006). t Primitive Mantle 20 compositional data has just been obtained for Chaife and Mt. Manganese that show

n Ol MI

e 1 c primitive lavas are the rule and not the exception along this cross-chain. Opportunities n 10 A number of recent cruises surveyed and sampled some of the 14°30'N cross-chain volcanoes. Cook 7 in 2001 dredge-sampled 4 of these volcanoes, TT167 in 2004 spent several days studying volcanic, o 1.5 C Mid_point Island Arc Tholeiite Field for future collaborative research appear promising. hydrothermal, and biological activity on NW Rota-1, and the NT0517 cruise aboard R/V Natsushima in 2005 returned to NW-Rota 1 and Chaife Seamonuts. Despite this flurry of investigation, numerous questions 0 45 40 35 30 25 20 15 10 5 0

O Back-arc Basin Basalt remain surrounding the nature of this cross-chain. The tectonics and the geometry of the subducting slab are not yet clearly defined. The exact mechanisms for generation along the cross-chain appear 2

H Distance from MF (km) to be due to asthenospheric upwelling, but the relation between upwelling and any unusual tectonics needs to be more better constrained. 1 N-MORB 4.5 0.1 4.0 Figure 10: Simplified model of the cross-chain 0.5 Rb Ba Th U Nb Ta La Ce Pb Sr Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb (boundary of lithosphere omitted). Note significant 100 3.5 o upwelling and shallow storage depths for cross-arc 18 N 3.0 0 magmas. 55 60 65 70 75 80 2.5 1) Dehydration of subducting slab and hydration of mantle wedge Mg# 8 2) Upwelling, melting and shallow storage beneath Mid-Point Smt. NW-Rota 1 a

N 2.0 o 2.5 3) Current decompression melting below NW-Rota e

17 30' Chaife Chaife l Mid-Point NW-Rota 1 t 4) Past flux melt accumaulation, fractionation, and assimilation at n 1.5

a 10 Mt. Manganese NW-Rota 1 and Andesite Knoll Guguan M Andesite Andesite Mt. Manganese e 1.0 2 Chaife 5) Typical subduction zone magamgenesis by flux melting to produce Little Mid-Point Knoll v o i

Knoll t o 14 30' i 17 Little Mid-Point 0.5 Andesite Knoll lavas. m i

r 6) Deep, hot decompression melting and storage below Chaife Smt.

Mid-Point P / 0.0 1.5 n followed by rapid ascent and eruption w/o significant modification. o

i 45 40 35 30 25 20 15 10 5 0

Sarigan t a

r Primitive Mantle Distance from MF (km) t o W. Rota O 2 n 1

16 30' H e

1 c Anatahan n Little Mid-Point

o 1.6

Rota C

H West Rota 1.4 o G Diamate Smts. 0.5 U 16 o O 145 1.2 R T

0.1 1.0 A 3 Ma. 0 References o N 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 Rb Ba Th U Nb Ta La Ce Pb Sr Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Ballhaus, C.G., Berry, R.F., Green, D.H., (1991) High pressure calibration of the olivine-orthopyroxene-spinel oxygen barometer, implications for the oxidation sate of A 15 30' I 100 0.8 E K8 8 Eruptive Ages i the upper mantle. Contrib. Mineral. Petrol. 107, 27-40. R C 40 39 2 Ma. T A N based on Ar/ Ar dating Beattie, P. (1993) Olivine-melt and orthopyroxene-melt equilibria, Contributions to Mineralogy and Petrology, 115, 103-111. I Saipan 2.5 0.6 M V Embley, R.W., Chadwick, W.W. Baker, E.T., Butterfield, D.A. and others (2006) Long term eruptive activity ata submarine arc volcano, Nature, 44, 494-497 O o 1 Ma. Fabriés, J. (1979). Spinel-olivine geothermometry in peridotites from ultramafic complexes. Contrib. Mineral. Petrol. 69, 329-336. R 0.4 P 15 Figure 2: Higher resolution bathymetry for Ford, C.E., Russell, D.G., Craven, J.A. and Fisk-, M.R. (1983) Olivine-liquid equilibria; temperature, pressure and composition dependence of the crystal/liquid cation Tinian 0 Ma. the arc end of the cross-chain (data courtesy 2 partition coefficients for Mg, Fe2+, Ca and Mn, Journal of Petrology, 24, 256-265. 0.2 T Kohut, E.J., Stern, R.J., Kent, A.J.R, Nielsen, R.L., Bloomer, S.H., Leybourne, M. (2006) Evidence for adiabatic decompression melting in the Southern Maraina arc e

N l of Bob Embley). t from High-Mg lavas and melt inclusions, Contrib. Mineral. Petrol. 69, 329-336. U o n 10 0.0 O a Miller, M.S., Gorbatov, A., Kennett, B.L., Stern, R.J., Gvirtzman, Z. (2004) Tear in the Subducting Slab beneath the Southern Mariana Arc: Evidence from P-wave 14 30' 1.5 M M 45 40 35 30 25 20 15 10 5 0 Tomography. Eos Trans. AGU 85

A e

E v Distance from MF (km) i McDonough, W.F. and Sun, S.-S. (1995). Composition of the Earth. Chemical Geology 120: 223-253. doi: 10.1016/0009-2541(94)00140-4. S t O Rota i Figure 1: Map showing the location of the 14-30 cross chain in the Mariana 2 Nielsen, R.L. (1990), Simulation of igneous differentiation processes, Rev. Mineral, 24, 65-105 m H 3.0 i o Arc system (left) and a detailed bathymetric view (above). The seamounts 1 r Putirka, K. (1997) Magma transport at Hawaii: Inferences based on igneous thermobarometry, Geology, v. 25, p. 69-72.

N P R 14 / Putirka, K., Johnson, M., Kinzler, R., Longhi, J. and Walker, D. (1996)Thermobarometry of mafic igneous rocks based on clinopyroxene-liquid equilibria, 0-30 kb, E n NW-Rota 1 and Chaife have previously been named and described (Embley o H i Contributions to Mineralogy and Petrology, 123, 92-108. T t 2.5 a Primitive Mantle

U r Putirka, K., Ryerson, F. J., and Mikaelian, H. (2003) New igneous thermobarometers for mafic and evolved lava compositions, based on clinopyroxene + liquid equilibria, et al., 2006; Kohut et al., 2006). In this presentation, we have labeled the other t O 0.5 1 S n American Mineralogist, v. 88, p. 1542-1554. e o volcanoes "Mid-Point Seamount", Little Mid-Point", "Andesite Knoll" and "Mt c Chaife Seamount 2.0 Roeder, P. L., Campbell, J. H. and Jamieson, H. E. (1979). A re-evaluation of the olivine-spinel geothermometer. Contrib. Mineral. Petrol. 68, 325-334 n

14 30' o Manganese". Age data provided by John Huard and Bob Duncan at Oregon Shaw, A M, Hauri, E, Tamura, Y, Ishizuka, O, Stern, R, Embley, R (2006), Volatile Contents of NW Rota Melt Inclusions: Insight to Explosive Submarine Arc Volcanism, C Guam 0 Eos Trans. AGU,87(52), Fall Meet. Suppl., Abstract V52B-05 State University. Age data not yet available for Mt. Manganese. 1.5 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 8 Stern, R.J.; Fouch, M.J.; Klemperer, S.L. (2003). An overview of the Izu-Bonin-Mariana subduction factory. In: Eiler, J. (ed.) Inside the Subduction Factory. K Ti8 Geophysical Monograph 138, American Geophysical Union, doi: 10.1029/138GM10 o o o o o o Stern, RJ., Kohut, E., Bloomer, S. H., Leybourne, M., Fouch, M. Vervoort, J. (2006) Subduction factory processes beneath the Guguan cross-chain, Marian Arc:: no 143 30' E 144 144 30' 145 145 30' 146 1.0 0.1 role for sediments, are serpentinites important? Contrib. Mineral. Petrol 151, 202-221 Figure 3 (right): Comparison of volumes of various Mariana Arc volcanoes to Figure 5: Water contents in olivine-hosted melt inclusions compared to Mg#, 0 50 Rb Ba Th U Nb Ta La Ce Pb Sr Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb km the seamounts in the 14-30 cross chain. Note the small size of these seamounts K2O and TiO2 regressed to 8% MgO. Water contents were not regressed due 0.5 relativeto typical Mariana arc volcanoes or some within the Guguan chain. to small sample size. Correlations are poor, suggesting that water contents are Figure 8: Trace element compositions normalized to primitive mantle (McDonough and Sun 1995) for not strongly controlled by LILE enrichment, HFSE depletion and differentiation 0.0 These small volumes indicate that volcanic centers in the 14-30 cross-chain Mid-Point, Little Mid-Points and Chaife Seamounts. Mid-Point and Chaife have signatures typical for arc lavas, 45 40 35 30 25 20 15 10 5 0 processes along the cross-chain. Thus, some of the observed variabilty may be Acknowledgements erupt infrequently and/or they are not long-lived. but more depleted. Little Mid-Point is distinctly more back-arc like. These data may show dilution due to Distance from MF (km) inherent in the source rock. Overall, water contents are low which supports high degrees of melting, and/or of the subduction signal due to mixing with magmas derived from pressure Figure 9: Cross-arc variation along the 14-30 cross-chain. a pressure-release melting regime. Dilution due to high degrees of melt must release melting. See author for discussion. This work funded in part by NSF Margins Grant OCE-01827 and supplements and funding from the Institute for also be considered. Research on Earth Evolution (IFREE).