Gill, James

Hoernle, Kaj

Jacques, Guillaume

Wehrmann, Heidi

OceanRep

Geophysical Research AbstractsVol. 16, EGU2014-4010-1, 2014EGU General Assembly 2014© Author(s) 2014. CC Attribution 3.0 License.Isotopic compositions of volcanic arc rocks in the Southern Volcanic Zone(33&deg;-43&deg;S), Chile: along- and across-arc variationsGuillaume Jacques (1), Kaj Hoernle (1), Jim Gill (2), and Heidi Wehrmann (1)(1) GEOMAR Helmholtz Centre for Ocean Research Kiel, 24148 Kiel, Germany, (2) University of California, Santa Cruz CA95064, USAWe investigate young, olivine-bearing volcanic arc (VA) rocks from the Southern Volcanic Zone (33-43◦S; SVZ)in Chile, and from the backarc (BA) in Argentina for their major and trace element, and Sr-Nd-Hf-Pb-O isotopegeochemistry. The compositional data are processed to identify the source components contributing to the arcmagmas and to estimate their proportions, with the aim to better understand the effects of the large-scale along-arctectonic variations onto melt generation and erupted compositions. The Transitional (T) SVZ (34.5-38◦S; Jacqueset al., 2013) samples overlap the BA samples in Sr and Nd isotopes, whereas the Central (C) SVZ (38-43◦S;Jacques et al., submitted, Chemical Geology) samples are shifted to slightly higher Sr and/or Nd isotope ratios.All samples form a tight correlation on the Pb isotope diagrams. The VA samples plot at the radiogenic end of thepositive BA array and overlap trench sediment, indicating mixing between a South Atlantic MORB-type sourceand a slab component derived from subducted trench sediments and altered oceanic crust. On the Nd versus Hfisotope diagram, the VA and BA form two sub parallel linear trends, neither pointing to subducting sediment.The VA may display an asthenospheric mantle array, whereas enriched Proterozoic lithospheric mantle may beinvolved in the BA. The CSVZ samples have higher fluid-mobile to fluid-immobile element ratios and lower more-to less-incompatible fluid-immobile element ratios than the TSVZ samples, consistent with higher hydrous meltflux and higher degrees of melting resulting in higher magma production and eruption rates in the CSVZ. Lowδ18O(melt) of CSVZ lavas suggests that the source of the enhanced water flux is likely to be hydrated lower crustand serpentinized upper mantle of the incoming plate, resulting from the multiple large fracture zones in this partof the SVZ. The δ18O(melt) values of the NSVZ, TSVZ and BA, on the other hand, largely overlap the MORBmantle range and extend slightly above it. We propose a quantitative mixing model between a mixed-source (slab-derived melt and a heterogeneous mantle wedge beneath the volcanic arc) using Arc Basalts Simulator 3. Ourmodels predict partial slab melting (including sediments and oceanic crust) in both TSVZ and CSVZ. The ratioof sediment to altered oceanic crust differs significantly between the TSVZ (60:30) and the CSVZ (30:70), andthe amount of slab-derived hydrous melt added to the mantle wedge is higher in the CSVZ than in the TSVZ.Finally, the NSVZ samples are shifted to higher Sr isotope ratios and∆8/4, lower 206Pb/204Pb, Nd and Hf isotoperatios, but have similar ∆7/4 as the other VA samples. This is inconsistent with the pre-existing models of crustalassimilation or subduction erosion of the Paleozoic upper crust. Assimilation or subduction erosion of lower crustsimilar in composition to that beneath Arizona, and thought to be present its conjugate pair in the Chilenia Terranebeneath the volcanoes, could explain the NSVZ geochemical variations.

Isotopic compositions of volcanic arc rocks in the Southern Volcanic Zone (33°-43°S), Chile: along- and across-arc variations

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