Ubiquitous radiogenic Os in Miocene to recent basalts from diverse mantle domains beneath the Colorado Plateau, USA

Abstract:

The source of magmatism and mechanisms responsible for the observed geochemical signatures in Miocene to Recent Colorado Plateau (CP) basalts has been a renewed focus of investigation in light of Earthscope results. We report new Os and Nd isotopic data for magnesian basalts (Mg#=62-72) and interpret them in light of previously reported Hf isotope data to help constrain contributions from olivine-poor source lithologies and subduction-derived metasomatism in the genesis of recent CP volcanism. The basalts studied span a large range in Hf isotope compositions and represent melts last equilibrated at a variety of depths beneath the Colorado Plateau and its transition zones.

We distinguished at least three mantle domains on the basis of paired Hf-Nd isotope, other isotopic, and geochemical characteristics of CP lavas. Domain 1 likely represents a depleted, variably metasomatized, lithospheric source, with relatively radiogenic ε_Hf (+5.2 to +11.8) and highly variable ε_Nd (-6.2 to +6.2). Domain 2 could represent either ancient or Farallon subduction-modified mantle and is displaced above the Hf-Nd mantle array (ε_Hf=+1.0 to +7.3; ε_Nd=-6.1 to -3.5). Domain 3 may be melts of pyroxenite/mica-rich veins or layers within lithospheric mantle and is characterized by unradiogenic Hf and Nd (ε_Hf=-12.9 to +0.6; ε_Nd=-10.0 to -2.9). The isotopic variability in CP-related lavas can largely be attributed to contributions from these mantle domains.

Preliminary Os isotope data show no correlation with proxies for differentiation or crustal contamination. Osmium and Hf isotope compositions are negatively correlated between domains 1 and 2 (¹⁸⁷Os/¹⁸⁸Os=0.31 to 0.59), whereas the Os isotope ratios in two domain 3 basalts have both lower and higher values (¹⁸⁷Os/¹⁸⁸Os=0.25 and 0.68). Significantly, Os isotope signatures are highly radiogenic (vs. values of <0.12 for SW US peridotite xenoliths [1]), overlapping and extending the range for inferred melts of pyroxene- and mica-rich veins and/or layers [2]. Thus mafic magmatism associated with the CP appears to ubiquitously tap sources at least locally modified by processes such as recycling of ancient oceanic crust, introduction of terrigenous sediments, or subduction-related metasomatism.

[1] Lee et al., Nature, 2001.

[2] Carlson and Nowell, G-Cubed, 2001.