ASSESSING ALTERNATIVE MECHANISMS OF MAGMA DIFFERENTIATION: MG AND SI ISOTOPES AS INDICATORS OF THERMAL MIGRATION

Within the Jurassic-aged Smartville complex, found in the western Sierra foothills of northern California, are a series of concentric, reversely-zoned plutons of ~10 km scale. These smoothly grade from diorite composition rims to olivine gabbro cores with no evidence for contacts between lithologies. Previous interpretations have attributed this gradation to reflect repeated injections of increasingly more mafic batches of magma derived from the inversion of a mature, fractionally crystallized magma chamber at depth (Beard and Day, J. Petrol., 1988). Here we focus on the Pilot Peak pluton because it contains the greatest compositional diversity (quartz diorite to olivine gabbro) and considerable sample coverage.

An alternative process for forming these zoned plutons might be thermal migration, whereby compositional changes occur by diffusion through a partially molten mush in a temperature gradient over long timescales. Recent thermal migration experiments produce similar chemical zonation (Lundstrom et al., submitted). Notably, relatively large fractionations of Mg, Fe and Si isotopes also occur, which likely reflect mass dependence of thermal diffusion. Specifically, light isotopes consistently are enriched in the hotter end of these experiments, providing a testable prediction for variations within the Pilot Peak pluton.

We have begun to analyze Mg, Si, and Fe isotopes by multi-collector ICP-MS to see if significant variations in these systems exist while also measuring ⁸⁷Sr/⁸⁶Sr to assess the homogeneity of the sources supplying the magmas. Preliminary analyses show small but significant variations in Mg, Si, and Fe isotopes, although occurring in no straightforward spatial arrangement. Some covariation of these systems may exist but further data are needed to assess this. ⁸⁷Sr/⁸⁶Sr of the Pilot Peak varies from 0.7033 (olivine gabbro) to 0.7042 (diorite), distinct from the massive diabase host rocks surrounding the pluton at 0.7050. These variations indicate heterogeneity in the magmas forming the Pilot Peak and are inconsistent with a model of magmas derived from a single, homogeneous, normally-zoned magma chamber at depth.