Frank C. Ramos

Sr isotope disequilibrium during differentiation of the Bandelier Tuff: Constraints on the crystallization of a large rhyolitic magma chamber

The 1.61 Ma Otowi Member of the Bandelier Tuff (Jemez Mountains volcanic field, New Mexico) is a caldera-forming high-silica rhyolitic ignimbrite with a precursor fallout deposit. The Otowi Member has high Rb/Sr ratios; the 87 Rb/ 86 Sr ratios from sanidines and glasses range from 14 to 570. Most sanidines from glomerocrysts in the ignimbrite have 87 Sr/ 86 Sr I = 0.7052–0.7056, whereas the ratios from glasses from glomerocrysts range from 0.7052 to 0.7079. Quartz phenocrysts containing glass inclusions from both the initial fallout and the ignimbrite are markedly more radiogenic, at 87 Sr/ 86 Sr I = 0.7105–0.7113, despite having much lower Rb/Sr ratios than glomerocryst glasses. These relations require that the inclusion glasses are more contaminated with Proterozoic country rock than are glomerocryst glasses. Textural, isotopic, and trace element data support a model in which crystals grow in a boundary layer with the most inclusion-rich quartz grains closest to the magma–country rock contact. Phenocrysts and glomerocrysts represent fragments from different zones of the chamber9s crystalline carapace, disseminated throughout the magma prior to eruption. An important implication of these results is that glass inclusions do not necessarily represent precursor magma compositions; hence extrapolation of measured volatile contents of inclusion glasses to the entire volume of an erupted magma should be approached with caution.

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

Abstract 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 ( ( 230 Th) ( 232 Th) = 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 ( 230 Th) ( 238 U) . 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.

Pb isotope variations among Bandelier Tuff feldspars: No evidence for a long-lived silicic magma chamber

We report, for the first time, high-precision Pb isotope data from a high-silica rhyolite. Prior work on Sr isotopes in the 1.6 Ma Otowi Member of the Bandelier Tuff (Valles caldera, New Mexico) established that large 8 7 Sr/ 8 6 Sr variations exist among Otowi glasses and sanidine phenocrysts. While the glasses display unequivocal evidence for wall-rock contamination of the Otowi magma following sanidine growth, a positive correlation between 8 7 Sr/ 8 6 Sr i and 8 7 Rb/ 8 6 Sr among the feldspars could be interpreted as either a mixing line or an in situ magmatic isochron dating a differentiation event ∼270 k.y. prior to eruption. The 2 0 6 Pb/ 2 0 4 Pb and 8 7 Sr/ 8 6 Sr ranges for Otowi sanidines are 17.790 ′ 0.002 to 17.831 ′ 0.002 and 0.7074-0.7052, respectively. This Pb isotope range cannot be produced by radiogenic ingrowth at the U/Pb ratios of the host magma on any geologically reasonable time scale, and hence is unequivocal evidence for open-system behavior of the Otowi magma prior to and/or concurrent with feldspar growth. Open-system behavior is predicted to control Sr isotope variations due to much higher concentrations of Sr, relative to Pb, in the country rock than in the magma. These observations therefore undermine any age significance of the Rb-Sr isotope variations. In the absence of supporting data, Rb-Sr relations alone do not impart any information about residence times of high-silica rhyolite magmas with subchondritic concentrations of Sr.