Osmium Isotope Constraints on Tectonic Evolution of the Mantle Lithosphere: evidence from Lunar Crater xenoliths, Nevada, USA



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The relatively rapid removal of an original Proterozoic mantle lithosphere and subsequent replacement by the asthenospheric mantle via Rayleigh-Taylor instabilities is proposed to explain the formation of highly elevated plateaus and mountain ranges in the Basin and Range Province in the western United States. The Re-Os isotope system is useful in dating lithosphere formation due to Os resistance to being overprinted by later melting events. Hence, 187Os/188Os values provide direct information on the lithosphere’s evolution. In this study, we provide new 187Os/188Os compositions of 8 granular peridotite samples with mantle lithospheric origin from Lunar Crater volcanic field in central Nevada. The average 187Os/188Os value is 0.1222 ± 0.000163, overlapping with the range of abyssal peridotites that represent present-day convecting mantle. Lack of correlation between 187Os/188Os and melt depletion indicators such as bulk rock Al2O3 can indicate an influence of a subducted oceanic lithospheric origin for this suite of xenoliths. The bulk rocks display light rare earth element (REE) enriched primordial mantle-normalized patterns resulting from metasomatism. The temperatures recorded for these xenoliths using REE-in-two-pyroxene thermometer display a range of 1,278–1,338°C (~ 300°C higher than other localities in the Basin and Range) along with mylonitic deformation characteristics of recrystallized olivines grains suggests an asthenospheric influence and strain localization which facilitate lithospheric delamination (Dygert et al., 2019). These results contrasts with peridotite xenoliths Dish Hill in southern California (another Basin and Range locale ~ 260 miles south of Lunar Crater), where the 187Os/188Os ratios correlate with bulk rock AL2O3 from melt depletion (Armytage et al., 2014). While Lunar Crater xenoliths host basalt suggests decompression melting of asthenospheric mantle resulted from lithospheric removal, the Os isotope system beneath Dish Hill points to pulses of mantle melting as a SCLM stabilization mechanism for continental growth. Comparing results from these two locations allow us to gain insights into the influence that the subduction of the Farallon plate and how this can change temporally and spatially.



Geology, Geochemistry, Radiogenic Isotopes Geochemistry, Geochronology, Lunar Crater Volcanic Field