Middle to Late Proterozoic volcanic and sedimentary sequences near Van Horn in west Texas record the pre- and syn-tectonic history of deformation from shallow crustal levels. Pre-deformational sequences include the Carrizo Mountain Group, the Allamoore and Tumbledown formations. The Carrizo Mountain Group is thrust northward, along the Streeruwitz fault, over a para-autochthonous sequence of unmetamorphosed carbonate and polymodal volcanic rocks of Allamoore and Tumbledown formations. Carbonates in the Allamoore Formation were deposited in shallow water and contain diverse stromatolites, desiccation cracks, and halite pseudomorphs. The Tumbledown formation unconformably overlies the Allamoore carbonate succession and consists of volcanic sandstones, agglomerates, pillow breccias, and mafic volcanic flows. Deformation of the pre-orogenic sequences led to deposition of more than 2,500 meters of immature alluvial fan conglomerates and eolian sandstones of the Hazel Formation. Based on clast composition of boulder conglomerates in the Hazel Formation, the principal source rocks were the Allamoore and Tumbledown formations and an unknown rhyolite terrain, with Carrizo Mountain Group lithologies notably absent. Alluvial megasequences in the alluvial fan conglomerates are aggradational and indicate at least two episodes of faulting accompanied Hazel sedimentation. A mismatch between source terrain and corresponding sediment composition, in addition to aggradation of alluvial fan megasequences,more » along the southern fault-bounded basin margin supports a transpressive regime for this orogenic belt in favor of a purely convergent margin. The Hazel orogenic sequence post-dated a 1,126--1,070 Ma old regional igneous event, whereas the Grenville deformation in the Llano uplift of central Texas pre-dated the igneous event.« less
Using stable isotope dilution, we determined the bioavailability of soilborne lead (Pb) in human adult volunteers. Soil from a residential yard at a mining-impacted federal Superfund site that had negligible amounts of other priority pollutants was dried and screened through a 25-micron mesh sieve. The < 250-micron fraction, which likely represents that ingested via hand-to-mouth activity, was then sterilized by exposure to radiation. Ten replicate samples yielded a mean (SD) soil Pb concentration of 2924 +/- 36 ppm, and a mean 206Pb/207Pb ratio of 1.1083 +/- 0.0002, indicating remarkable soil homogeneity. Six adults with 206Pb/207Pb ratios of > 1.190 were admitted to the clinical research center and fasted overnight prior to dosing with 250 micrograms Pb/70 kg bw (i.e., 85.5 mg soil/70 kg) in a gelatin capsule. Blood for Pb and 206Pb/207Pb ratios was obtained at 14 time points through 30 hr. Results of the isotopic analyses from these subjects indicate that on average 26.2% +/- 8.1 of the administered dose was absorbed. Six additional subjects were subsequently studied but ingested soil immediately after a standardized breakfast. Bioavailability in this group was only 2.52% +/- 1.7. Collectively, this study provides the first experimental estimates of soil Pb absorption in humans, and should allow for more precise estimates of health risks due to Pb-contaminated soil.
Upper-mantle xenoliths in volcanic pipes cutting the axis of the Sierra Nevada batholith contain predominantly spinel-bearing peridotites (with sporadic garnet) and garnet websterites. In spite of the enormous thickness of the Sierran crust, the Sierran upper mantle has not attained the garnet peridotite stability field. The peridotites have forsteritic (Fo88–92) olivines, Cr-diopsides, Cr-spinels, and magnesian orthopyroxenes (En88–92). Their texture and compositional characteristics of the coexisting phases indicate that these are fragments of the upper mantle that had undergone various degrees of partial fusion. The P conditions of reequilibration and mineralogical characteristics suggest that the partial fusion was accompanied by diapiric uprise. The REE distribution patterns are nearly chondritic. Garnet websterite xenoliths also contain magnesian and Cr-rich phases. Their bulk chemical compositions are like pyroxenite cumulates. The garnet websterites from Big Creek differ from those occurring at Pick and Shovel in having more Fe-rich phases and occasional hydrous minerals. The Pick and Shovel garnet websterites are interpreted to be pyroxene-rich, garnet-free cumulates formed by fractional crystallization of melts generated by partial melting of subcontinental lithosphere at depth ≧60 km. The REE abundance of these xenoliths is consistent with this mode of origin. Presence of jadeitic clinopyroxenes and F-rich phlogopites, and the LREE- and 87Sr/86Sr-enriched character of the garnet websterites from Big Creek may suggest their origin as metasomatized upper-mantle garnet peridotites. The latest P-T conditions of equilibration of all garnet-bearing samples show that they lie along a nearly adiabatic gradient in the range of 900–1000 °C and 18–32 kbar. An isotopically heterogeneous, old (∼1 b.y.) subcontinental lithosphere, characterized by high 87Sr/86Sr (0.7044–0.7082), radiogenic 206Pb/204Pb (18.86–20.04), 207Pb/204Pb (15.64–15.69) and 208Pb/204Pb (38.69–39.11), and moderate 143Nd/144Nd (0.51234–0.51260; ENd−0.35 to −5.8) is considered to be the source of these rocks. There was a fluid influx from a subducted slab carrying Ba, K, Rb, U, Th, and radiogenic Pb into the overlying ancient lithosphere.
