RE-OS DATING OF THE COLORADO PLATEAU MANTLE LITHOSPHERE XENOLITH: CONSTRAINTS ON THE GEOCHEMICAL AND TECTONIC EVOLUTION ASSOCIATED WITH FARALLON PLATE SUBDUCTION

Subcontinental lithospheric mantle xenoliths from Vulcan’s Throne, AZ (VT) are examined to evaluate the extent of subduction related lithospheric removal and its impact on lithospheric strength and metasomatism related to Farallon Plate subduction. Here, Os isotope data and bulk rock major and trace elements abundances of a well-characterized suite of 14 spinel peridotite xenoliths from Vulcan’s Throne, AZ (VT) are presented. Samples lithologies range from coarse-grained spinel bearing harzburgites to lherzolites. Bulk rock Os isotope compositions were measured using Thermal Ionization Mass Spectrometry (TIMS). Major and trace elements abundance were analyzed using X-ray Fluorescense Spectrometry (XRF), and inductively coupled plasma mass spectrometer (ICPMS), respectively. The VT samples display a range of ¹⁸⁷Os/¹⁸⁸Os from 0.11528 to 0.12371 and correlate well with melt depletion indices such as 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 but inconsistent with recent lithospheric removal. Trace element compositions indicate an enrichment signature associated with dehydration of seawater-altered oceanic crust and silicate melts from serpentinized portions of a subudcted slab. These observations support a low-angle subduction which heavily influenced the base of the Colorado Plateau.