Osmium isotope evidence for Early to Middle Proterozoic mantle lithosphere stabilization and concomitant production of juvenile crust in Dish Hill, CA peridotite xenoliths

摘要

The ~(187)Os/~(188)Os compositions in peridotite samples from the sub-continental lithospheric mantle (SCLM) can be used to constrain the timing of melt extraction and potentially test the link between large-scale mantle melting and juvenile crust production. The SCLM has often experienced a complex history such that some lithophile elements such as REEs (rare earth elements) in these rocks typically record overprinting during metasomatism. New ~(187)Os/~(188)Os, major and trace element compositional data were obtained on sixteen Dish Hill peridotite xenoliths (California, USA) and are used to examine these issues. The samples show strong correlations between ~(187)Os/~(188)Os and indicators of melt depletion such as Lu abundance in clinopyroxene, modal abundance of clinopyroxene, bulk rock Al_2O_3 and the Cr# (Cr/(Cr + Al)) in spinel. These relationships indicate that metasomatism did not compromise the ~(187)Os/~(188)Os systematics. The data appear to form two melt depletion trends consistent with Re depletion model ages (TRD) obtained from the two Al_2O_3 versus ~(187)Os/~(188)Os trends are2.1 ± 0.5 Ga and 1.3 ± 0.3 Ga (±95% conf.). It has been suggested that the SCLM under Dish Hill may be fragments of oceanic lithosphere emplaced as the result of Farallon plate subduction during the Late Cretaceous (Luffi et al., 2009). However, the strong melt depletion trends, major element compositions and Re-depletion ages are not consistent with the interpretation of this suite of xenoliths having an oceanic lithospheric origin. Rather, the 2.1 Ga age coincides with Nd model ages of 2–2.3 Ga (Bennett and DePaolo, 1987; R?m? and Calzia, 1998) for the overlying Mojavia crustal province. The 1.3 Ga age is consistent with large-scale A-type magmatism in the nearby region at this time that is purported to be the result of mantle plume melting processes. Therefore, data from this study point to the SCLM under Dish Hill being formed by two ancient mantle-melting events, which could be the result of interleaving SCLM at depth. These interpretations indicate that the primary mechanism of SCLM formation under Dish Hill was through stabilization following partial melting in the convecting mantle that also results in contemporaneous juvenile crust production.