Osmium Isotope Constrains on Tectonic Evolution of the Mantle Lithosphere Beneath North America

Date

2020-08

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Abstract

Subduction zone systems are central to critical geodynamic processes such as the evolution of the continental crust and the subcontinental lithospheric mantle (SCLM). The southwestern United States has had a highly dynamic tectonic history in the Cenozoic, including flat-slab subduction of the oceanic Farallon Plate and removal of Proterozoic mantle lithosphere with subsequent replacement by the asthenospheric mantle via Rayleigh-Taylor instabilities. However, finding direct evidence for delamination and flat-slab subduction is challenging. Compositional data on peridotite xenoliths which are brought up to the surface in alkalic lavas can provide crucial information to better evaluate the extent of subduction related SCLM removal and its impact on lithospheric strength and fluxes to and from the mantle. Here, I combine Os isotopic data with major and trace elements abundance of peridotite xenoliths from the SCLM to provide insights on subduction zone metasomatism related to Mesozoic-Cenozoic Farallon Plate subduction and evidence for lithospheric delamination underneath the Southwest USA. Two well-characterized suites of peridotite xenoliths from Lunar Crater Volcanic Field in Nevada (LCVF), and Vulcan’s Throne in Arizona (VT) are examined. Samples range from coarse-grained spinel bearing harzburgites to lherzolites that are enclosed in host alkali basalt. Bulk rock Os isotope compositions were measured using Thermal Ionization Mass Spectrometry (TIMS) at the University of Houston. Major and trace elements abundance were analyzed using X-ray Fluorescense Spectrometry (XRF), and inductively coupled plasma mass spectrometer (ICPMS), respectively. The LCVF peridotites have average 187Os/188Os values of 0.1222 ± 0.000163 (n = 8), overlapping with the range of abyssal peridotites that represent present-day convecting mantle. These samples have high equilibration temperatures ranging from ~1000 – 1200˚C, mylonitic deformation characteristics of recrystallized olivine grains, exhibit shallow depth of melt extraction and lack a correlation between 187Os/188Os and a melt depletion index (i.e., Al2O3). These relationships are consistent with recent emplacement of asthenospheric mantle after removal of older SCLM via strain localization which facilitates lithospheric delamination. In contrast, VT samples display a larger range of 187Os/188Os from 0.11528 ─ 0.12371 and correlated well with the melt depletion index Al2O3. This correlation provides a model melt extraction age of 2.31 ± 0.12 Ga, consistent with being an old and stable Paleoproterozoic mantle lithosphere and inconsistent with recent lithospheric removal underneath this area. Trace element compositions combined with reported water contents from olivine indicate an enrichment signature associated with dehydration of seawater-altered oceanic crust and silicate melts from serpentinized slab. These observations support a low-angle subduction which heavily influenced the base of the Colorado Plateau.

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Keywords

isotope geochemistry, mantle xenoliths, North America tectonics, osmium

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