Donald M. Burt

The petrogenesis of topaz rhyolites from the western United States

AbstractHigh-silica topaz-bearing rhyolites of Cenozoic age are widely distributed across the western USA and Mexico. They are characteristically enriched in fluorine (>0.2 wt.%) and incompatible lithophile elements (e.g. Li, Rb, Cs, U, Th, Be). In addition to topaz, the rhyolites contain garnet, bixbyite, pseudobrookite, hematite and fluorite in cavities or in their devitrified groundmasses. Magmatic phases include sanidine, quartz, oligoclase and Fe-rich biotite. Allanite, fluorite, zircon, apatite and magnetite occur in most; pyroxene, hornblende, ilmenite and titanite occur in some. The rhyolites crystallized over a wide temperature interval (850° to 600° C) at

Apatite, phosphorus and titanium in eclogitic garnet from the upper mantle

f_{0_2 }

Nanometer-scale complexity, growth, and diagenesis in desert varnish

that ranges from QFM to NNO. The REE patterns of most topaz rhyolites are almost flat (La/YbN=1 to 3) and have deep Eu anomalies (Eu/Eu*=0.01 to 0.02). Both parameters decrease with differentiation. Titanite-bearing rhyolites have prominent middle REE depletions. Topaz rhyolites appear to have evolved from partial melts of a residual granulitic source in the Precambrian lower crust. According to the proposed model, the passage of hot mafic magmas through the crust produced partial melts as a result of the decomposition of F-rich biotite or amphibole. An extensional tectonic setting allowed these small batches of magma to rise without substantial mixing with contemporaneous mafic magmas.Some of the compositional differences between topaz rhyolites and peralkaline rhyolites may be attributed to the accumulation of fluorine and fluorphile elements (Al, Be, Li, Rb, U, Th, HREE) in melts which give rise to topaz rhyolites and chlorine and chlorophile elements (Ti, Fe, Mn, Zn, Zr, Nb and LREE) in melts which yield peralkaline rhyolites. Hence the F/Cl ratio of the melt or its source may determine the alumina saturation of the magma series. Topaz rhyolites are distinguishable from calc-alkaline rhyolites by lower Sr, Ba, Eu and higher F, Rb, U and Th. The usually low La/Yb ratios of topaz rhyolites distinguish them from both peralkaline and calc-alkaline rhyolite suites.

Mars and mine dumps

Mars Exploration Rover Opportunity discovered sediments with layered structures thought to be unique to aqueous deposition and with minerals attributed to evaporation of an acidic salty sea. Remarkable iron-rich spherules were ascribed to later groundwater alteration, and the inferred abundance of water reinforced optimism that Mars was once habitable. The layered structures, however, are not unique to water deposition, and the scenario encounters difficulties in accounting for highly soluble salts admixed with less soluble salts, the lack of clay minerals from acid–rock reactions, high sphericity and near-uniform sizes of the spherules and the absence of a basin boundary. Here we present a simple alternative explanation involving deposition from a ground-hugging turbulent flow of rock fragments, salts, sulphides, brines and ice produced by meteorite impact. Subsequent weathering by intergranular water films can account for all of the features observed without invoking shallow seas, lakes or near-surface aquifers. Layered sequences observed elsewhere on heavily cratered Mars and attributed to wind, water or volcanism may well have formed similarly. If so, the search for past life on Mars should be reassessed accordingly.

Vector representation of lithium and other mica compositions

The existence of apatite exsolution lamellae in garnet supports earlier suggestions that garnet is a major repository for P in the mantle, and that P is strongly coupled to Na+Ti. The bulk P content (∼250 ppm) of mantle eclogites is unexpectedly similar to estimates for the P content (130–220 ppm) of the upper mantle; both rocks are depleted relative to chondrites and to ocean floor basalts. Eclogites have either equilibrated with the mantle at large, or have undergone secondary melting, and based on P contents are unlikely to have had subducted protoliths.

Phase equilibria of several tungsten deposits in southern China

Abstract We have investigated the petrology and geochemistry of whole rocks from two small-volume, Sn- and F-mineralized rhyolitec dome complexes of the Mexican tin rhyolite belt, Cerro el Lobo and Cerro el Pajaro, to determine volcanic degassing and mineralizing processes in felsic igneous systems. The abundance and distribution of volatiles (H 2 O, B, F, and Cl) and lithophile trace and ore elements (Li, Rb, Cs, Be, Sr, Y, Ce, Th, U, Nb, Sn, and Mo) in the parental liquids were established by analyzing melt inclusions in quartz. The melt inclusions from both rhyolites are variably enriched in Li and the volatile constituents F and Cl, and some are extremely enriched in Li, although whole rocks are not correspondingly enriched. Compositional variations in the melt inclusions from both rhyolites also constrain magmatic differentiation. Melt evolution was dominated by crystal fractionation, modified by mass transport in a Cl- and H 2 O-rich magmatic-hydrothermal fluid, and resulted in increasing abundances of U, Nb, and Cs (± Li, F, Cl, B, Y, Ce, Be, Rb, Mo, and Sn) in both liquids. The rhyolite liquids apparently were heterogeneous prior to eruption. The Cerro el Lobo liquid contained gradients in volatiles and trace elements; comparatively less Cl, Be, B, Al 2 O 3 , and CaO (± Li, F, U, and Th) were present in the early-erupted, H 2 O-rich fractions of liquid. Comparing compositions of whole rocks with the mean compositions of melt inclusions constrains relative mobilities of magmatic constituents during and after eruption. Sodium, fluorine, lithium, uranium, and yttrium (± H 2 O, Cl, Sn) were lost from both magmas and the Cerro el Pajaro magma apparently also lost Nb and Al as a result of eruptive and posteruptive degassing. These geochemical relationships and constraints on pre-eruptive abundances and distributions of volatiles in tin rhyolite magmas probably apply to other tin rhyolites and, moreover, the high levels of Cl and Li enrichment maybe representative of other highly-evolved granitic magmas genetically associated with lithophile mineralization.

Model for Formation of Uranium/Lithophile Element Deposits in Fluorine-Enriched Volcanic Rocks: ABSTRACT

Nanometer-scale element mapping and spectroscopy of desert varnish from the northern Sonoran Desert in southwestern Arizona reveal a dynamic disequilibrium system characterized by postdepositional mineralogical, chemical, and structural changes activated by liquid water. Lack of equilibrium is suggested by the large variety of coexisting Mn phases. Sparse secondary Ba and Sr sulfates also occur, as do carbonaceous particles. Individual Mn-oxide particles contain variable concentrations of Ba and Ce, reflecting their role as repositories of trace elements, presumably derived from atmospheric aerosols. Desert varnish is analogous to more familiar sediments in displaying authigenic and diagenetic structures, but with total sediment thicknesses <1 mm and structures at the nanometer scale. As such, it is neither a weathering rind nor patina, but a unique subaerial sediment that is in dynamic disequilibrium. Our results suggest continuing adjustment of varnish to changing environmental conditions.

Introduction; terminology, classification, and composition of skarn deposits

Abundant sulfates appear to exist on the surface of Mars and have commonly been attributed to a planet-wide volcanogenic ‘acid fog’—where clouds of acid mist react directly with surface rocks or acidify surface waters—from which the sulfates precipitated [e.g., Clark and Baird, 1979]. In particular, Meridiani Planum, a plain located just south of the Martian equator, hosts many sulfate minerals. Squyres et al. [2004, 2006] and Squyres and Knoll [2005] hypothesized that the sulfate minerals there, particularly acid sulfates such as jarosite, precipitated from high concentrations of sulfuric acid in flowing and standing water and groundwater. However, such explanations are problematic, because magnesia and alkali-bearing minerals in the basaltic regolith should have been able to neutralize sulfuric acid rapidly, making precipitation of acid sulfates such as jarosite impossible.