Geochemical flux in the mantle wedge: Insights from suprasubduction zone ophiolites

Abstract

Understanding geochemical flux in the mantle wedge during subduction is critical to our understanding of the subduction factory process and arc volcanism. It forms an important aspect of the global geochemical flux and is one of the first order problems identified by the Geochemical Earth Reference Model (GERM) initiative. The MARGINS program attempts to understand these processes by studying active subduction zones (crustal inputs, erruption products, seismic tomography). An alternative approach is to examine outcrops of lithospheric mantle that underlie crust formed by supra-subduction zone (SSZ) magmatism. We have analyzed major element mineral chemistry in 50 samples of mantle peridotite from six locations in the Coast Range ophiolite of California by EPMA, and over 100 grains of pyroxene for 13 REE and 14 other trace elements (Li, Be, Sc, Ti, V, Rb, Sr, Y, Zr, Nb, Ba, Hf, Pb, Th) by laser ablation ICP-MS analysis. Abyssal lherzolite is found in one block

all others are suprasubduction zone (SSZ) harzburgite and dunite, with abyssal lherzolites can be modeled by <5% fractional melt extraction from a fertile MORB mantle source. Melting models for the SSZ peridotites imply 15-30% fractional melting under hydrous conditions. All of the peridotites have fluid-mobile element concentrations that exceed model refractory compositions by 2-3 orders of magnitude, consistent with fluid-phase enrichment during partial melting. In addition, some of the more refractory samples have LREE concentrations that indicate enrichment by an enriched melt phase. Modeling of whole rock ICP-MS data suggests that these more refractory samples may have begun to melt initially in the garnet field before inverting to spinel facies melting.