THE ORIGIN OF TRACE ELEMENT ANOMALIES IN ARC MAGMAS USING RUTILE-BEARING SIERRAN PYROXENITES AS A WINDOW INTO THE DEEP ARC CRUST

The continental crust has a number of peculiar geochemical signatures, including an enrichment of light rare earth elements, a depletion of high-field strength elements (HFSE), subchondritic Nb/Ta ratios, and high oxidation state. In particular, both the depleted mantle and continental crust have low HFSE concentrations and low Nb/Ta (relative to the primitive mantle), which suggest the existence of a reservoir of these elements that remains unaccounted for. It seems likely that these features are generated in subduction zones, but it is unclear whether they stem from the slab, the mantle wedge, or from differentiation in the upper plate. To gain insight into these anomalies, we examined the geochemistry of two types of garnet pyroxenite samples: high MgO (>15%), low-garnet basaltic arc cumulates and low-MgO (<15%), garnet-rich cumulates, both collected from the central Sierra Nevada Batholith in California. We studied these rocks because they are derived from mantle wedge magmas and crystallized in the lower crust, with the high-MgO rocks having formed at greater depths than the low-MgO ones. In addition, they contain rutile as an accessory mineral, which is known to contain a highly disproportionate majority of the HFSE (Nb, Ta, Zr, Hf) budget of such rocks. One of our goals was to determine whether these rocks could account, at least in part, for the trace element imbalances described above. We analyzed the rutiles and whole-rock samples of these pyroxenites using laser ablation ICPMS and XRF. We found that the rutiles did indeed contain high concentrations of HFSE’s as well as high Nb/Ta ratios. Both the rocks and rutiles displayed elemental differences that correlated with the rocks’ MgO (and by proxy with depth). It is possible that these pyroxenites thus account for a large portion of depleted HFSE reserves. In addition, negative Europium anomalies were observed throughout the rocks and their rutiles; the magnitude of the negative anomalies correlated inversely with MgO. The degree of the Eu anomalies may represent a relatively low oxidation state for these rocks and suggests that negative Eu anomaly increases with degree of differentiation. Altogether the trace element signatures of these rocks may suggest that the aforementioned crustal trace-element anomalies stem from processes of crustal differentiation.