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 enri ched lithospheric domains, In this paper, we show that Th isotope comp ositions of basalts from the southwestern United States further delimi t 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 initi al Th isotope signatures ((Th-230)/(Th-232) = 0.65-1.10) in these cont inental basalts overlaps and extends that of ocean island basalts. Pai ring Th isotope variations with those of Nd provides a potent means of discriminating between various models for generation of enriched chem ical signatures in continental basalts and precludes the possibility t hat the CBR basalts acquired their enriched chemical and isotopic sign atures during ascent through the lithosphere, CBR basalts exhibit a ne gative co-variation between Nd and Th isotope ratios that is difficult to reconcile with assimilation because Nd concentrations in appropria te 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 mel ts generated in the mantle lithosphere. Most CBR lavas have (Th-230)/( U-238) < 1.1. These modest Th enrichments reflect the probable contrib ution of metasomatic phases to alkaline lavas and the higher melt frac tions of subalkaline lavas. Enriched mantle sampled by CBR basalts has higher Pb/Nd as well as time-integrated and present-day Th/U than mos t of the sub-oceanic mantle, confirming the critical role that long-te rm isolation of continental lithosphere has on the development of chem ical signatures distinct from those of convecting upper mantle. Couple d Pb-Th isotope characteristics in CBR basalts are consistent with pro gressive isolation of increasingly U-enriched mantle.