IN-SITU LA-ICP-MS ANALYSIS OF PYROXENE IN THE PERIDOTITE OF THE COAST RANGE OPHIOLITE, CALIFORNIA

The geochemical flux in the mantle wedge is critical to our understanding of arc volcanism, and forms an important aspect of the subduction factory. Studies of arc volcanism are commonly used to understand this flux. This approach is limited because the composition of the mantle is unknown and the flux can only be inferred. An alternative approach is to examine outcrops of lithospheric mantle that underlie supra-subduction zone (SSZ) ophiolites, which allows us to directly examine their petrology and geochemistry. The Coast Range Ophiolite (CRO) offers the best evidence of this interaction between flux and the mantle wedge because the CRO represents, in large part, supra-subduction zone (SSZ) formation by fore-arc extension.

We employed Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) to analyze unaltered pyroxenes. We analyzed in-situ clinopyroxene and orthopyroxene from Cuesta Ridge, Chrome (Grey Eagle Mine), Del Puerto Canyon, and Stonyford. Pyroxenes fall into two categories: (1) Al-rich, spinel lherzolite-harzburgite (compositionally similar to abyssal peridotites), and (2) Cr-spinel harzburgite and dunite (SSZ peridotites). Abyssal clinopyroxenes are enriched in minor elements (TiO2 ≈0.3%, Na2O~1%) and have rare-earth element (REE) compositions that reflect equilibrium with MORB-like melts; abyssal orthopyroxenes are lower in minor elements, but still require REE in equilibrium with MORB. In contrast, SSZ pyroxenes (clino- and ortho-) have extremely low minor element contents (TiO2 ≈0.05%, Na2O<0.1%) and LREE concentrations that are below detection (<0.01x primitive mantle). MREE-HREE patterns indicate equilibration with an extremely refractory melt.

We propose a 2 stage melting model. Abyssal peridotite (Stage 1) represents ~7% – 12% partial melting of MORB-source mantle in the garnet-lherzolite field. SSZ peridotite (Stage 2) represents further partial melting, up to 30% in the spinel-lherzolite field, after Stage 1 partial melting. Elevated fluid mobile element concentrations (Li, B, Rb, Pb) indicate hydrous melting in all samples. We expect that our study will constitute a major advance in understanding the magmatic flux through the mantle wedge portion of the subduction factory, identify primary rock compositions, add to the literature concerning the CRO, and champion the use of LA-ICP-MS.