T001-0004
Arc melt production from hybrid slab and mantle sources - new evidence from Cr spinel bearing high-Ni olivines in central Transmexican Volcanic Belt

Monday, 7 December 2020
Poster
Susanne M Straub, Lamont Doherty Earth Observato, Palisades, NY, United States, Valentina G. Batanova, ISTerre, Unversite J Fourier, Grenoble, France, Alexander V Sobolev, University Grenoble Alpes, ISTerre Institute of Earth Sciences, Grenoble, France, Arturo Gomez-Tuena, Universidad Nacional Autónoma de México, Centro de Geociencias, Quéretaro, Mexico, Ramón Espinasa-Pereña, Centro Nacional de Prevención de Desastres, Mexico City, Mexico, Ilya Bindeman, University of Oregon, Department of Earth Sciences, Eugene, OR, United States, Finlay M Stuart, Scottish Universities Environmental Research Center at the University of Glasgow, East Kilbride, United Kingdom and Elisabeth Widom, Miami University, Geology and Environmental Earth Science, Oxford, OH, United States
Abstract:
‘High-Ni’ olivines - those with Ni too high at a given Fo for crystallizing from peridotite-sourced melts - provide unique insights into the mixing process of crustal and mantle materials in the mantle (Sobolev et al. 2005). However, the potential of the olivine Ni tracer has been questioned as it can also be affected by mantle temperatures (Matzen et al. 2013) and recharge melt mixing in the crustal basement.

In order to test these models, we obtained additional trace element data (Al, Na, Ti, Zn, Co, Cr) in high-and low-Ni olivines and their Cr-spinel inclusions in well characterized Quaternary arc magmas in the Trans-Mexican Volcanic Belt. We analyzed fosteritic olivines with Ni up to 5300 ppm in high-Mg# andesite and dacites from the composite volcano Popocatepetl, and low- to high-Ni olivines (2300 to 5300 ppm) from coeval high-Mg# basalts and andesites from arc-front monogenetic volcanoes up to 100 km to the west. In this segment, subduction parameters (crustal thickness and composition, distance to trench, slab height, slab dip) are all constant, so do not differentially impact melt composition.

Our findings are: (1) Olivine-spinel oxygen barometry yields the arc-typical elevated oxygen fugacity with an average fO2 of +2.1±0.3 at FMQ consistent with the elevated δ18O explained by fluids. Melt Mg# and olivine Fo do not correlate with fO2, confirming that olivines crystallize from primitive melts in, or close to, equilibrium with the upper mantle; (2) Al-in-olivine thermometry provides a narrow range of crystallization temperatures (1060 to 1160°C) without correlation to olivine Fo and Ni, or Cr-spinel Cr#, which argues against a temperature control of the olivine Ni; (3) Olivine and Cr-spinel data suggest melt origin from fertile mantle (Cr#<0.3, low-Ni olivine) and depleted mantle (Cr# >0.4-0.6, high-Ni olivine) sources.

Our results are consistent with models that propose high-Ni olivines to reflect subduction-related mantle processing, which is driven by continuous infiltration of silicic slab material. Serial melting leads to local depletion of the residual mantle, while melt-rock reaction processes produce the hybrid, olivine-free secondary pyroxenitic source lithologies that give rise to a broader spectrum of basaltic to silicic primary arc magma that traverse the thick crustal basement nearly unchanged.

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