Chemical dynamics of enriched mantle in the southwestern United States: Thorium isotope evidence

As probes of mantle below continents, geographically dependent isotope signatures in basalts suggest preservation of old and chemically enriched lithospheric domains, In this paper, we show that Th isotope compositions of basalts from the southwestern United States further delimit possible models for the origin and consequences of this enrichment. The data presented are for a compositionally diverse suite of largely mafic lavas from the central Basin and Range (CBR). The range of initial Th isotope signatures ((Th-230)/(Th-232) = 0.65-1.10) in these continental basalts overlaps and extends that of ocean island basalts. Pairing Th isotope variations with those of Nd provides a potent means of discriminating between various models for generation of enriched chemical signatures in continental basalts and precludes the possibility that the CBR basalts acquired their enriched chemical and isotopic signatures during ascent through the lithosphere, CBR basalts exhibit a negative co-variation between Nd and Th isotope ratios that is difficult to reconcile with assimilation because Nd concentrations in appropriate lower crustal lithologies are too low to decrease significantly the Nd isotope signatures from those of depleted mantle basalts, Nd/Th-Th/U systematics and a lack of Th/U fractionation similarly offer little support for contamination of asthenospheric melts by small volume melts generated in the mantle lithosphere. Most CBR lavas have (Th-230)/(U-238) < 1.1. These modest Th enrichments reflect the probable contribution of metasomatic phases to alkaline lavas and the higher melt fractions of subalkaline lavas. Enriched mantle sampled by CBR basalts has higher Pb/Nd as well as time-integrated and present-day Th/U than most of the sub-oceanic mantle, confirming the critical role that long-term isolation of continental lithosphere has on the development of chemical signatures distinct from those of convecting upper mantle. Coupled Pb-Th isotope characteristics in CBR basalts are consistent with progressive isolation of increasingly U-enriched mantle.