Phenocryst

About: Phenocryst is a research topic. Over the lifetime, 4132 publications have been published within this topic receiving 158441 citations.

Melt inclusions and crystal-liquid separation in rhyolitic magma of the Bishop Tuff

Abstract: Melt inclusions in quartz phenocrysts from a single clast of pumice near the base of the plinian pumice fall of the Bishop Tuff were studied to test ideas concerning separation of melt and crystals in silicic magmas. Ten analyzed inclusions from the pumice clast are of high silica rhyolite composition with very low contents of the highly compatible elements Ba, Sr, and Eu, consistent with extensive fractionation. The concentrations of U, La, Ce, Mg, and Ca of these ten melt inclusions vary considerably as determined by ion microprobe. Petrologic considerations indicate that uranium is an incompatible element with a maximum bulk partition coefficient D of about 0.2 and that the evolution of the uranium content of the melt was controlled by crystallization of the magma. A minimum of 33 wt% perfect fractional crystallization is required to explain the observed range in uranium. However, only 17 wt% crystals occurred in the pumice clast. The greater calculated fraction of crystals requires significant separation of crystals and melt before the eruption of the plinian pumice fall in spite of the fact that crystal mixing (settling, etc.) did not occur in the Bishop magma.

Mineralogy and Petrology of the Raton-Clayton Volcanic Field, Northeastern New Mexico

Abstract: The late Cenozoic lavas of the Raton-Clayton region of New Mexico can be divided into five groups: (1) the Raton-Clayton lavas, widespread alkali olivine basalts; (2) the hornblende andesites and dacites of the Red Mountain lavas; (3) a separate group of pyroxene andesite from Sierra Grande volcano; (4) a feldspathoidal group of basanites and olivine nephelinites; and (5) the Capulin-type basaltic lavas, which are characterized by abundant “cloudy” plagioclase phenocrysts. The olivines of the feldspathoidal rocks zone toward calcium enrichment rather than iron enrichment as in more silicic rocks. This trend can be related to the silica activity of the magma and to changes in pressure during crystallization. The pyroxenes in the lavas of this suite are rich in alumina, and those of the feldspathoidal rocks show strong sector zoning. Ilmenite from one basanite contains over 40 mole percent of the geikelite (MgTiO 3 ) component, and other oxides also have high contents of MgO. The distribution of Mg between oxide phases and olivine follows predictions from thermodynamic data, but does not agree with experimental results. The Fe-Ti distribution in coexisting oxide phases indicates oxygen fugacities near FMQ for the Raton-Clayton and Capulin basaltic rocks, and higher values for the feldspathoidal, andesitic, and dacitic rocks. Estimates of partial pressures of water based on the Kudo-Weill feldspar geothermometer range from 5 kb for a basanite, 2 to 3 kb for the andesites and dacites, to near 1 kb for the Capulin magmas. Fractional crystallization and crustal contamination do not appear to have been significant processes in the origin of these lavas. Partial melting in the lower crust (Red Mountain lavas) and upper mantle under different physical conditions seems to be a more likely explanation of the diversity of rock types displayed in this suite. High water pressures, in particular, may have played a significant role in the genesis of the feldspathoidal and Sierra Grande andesitic rocks.

Silicate Melt Inclusion Evidence for Extreme Pre-eruptive Enrichment and Post-eruptive Depletion of Lithium in Silicic Volcanic Rocks of the Western United States: Implications for the Origin of Lithium-Rich Brines*

Abstract: To evaluate whether anatectic and/or highly fractionated lithophile element-enriched rhyolite tuffs deposited in arid lacustrine basins lose enough lithium during eruption, lithification, and weathering to generate significant Li brine resources, pre-eruptive melt compositions, preserved in inclusions, and the magnitude of post-eruptive Li depletions, evident in host rhyolites, were documented at six sites in the western United States. Each rhyolite is a member of the bimodal basalt-rhyolite assemblage associated with extensional tectonics that produced the Basin and Range province and Rio Grande rift, an evolving pattern of closed drainage basins, and geothermal energy or mineral resources. Results from the 0.8 Ma Bishop tuff (geothermal) in California, 1.3 to 1.6 Ma Cerro Toledo and Upper Bandelier tephra (geothermal) and 27.9 Ma Taylor Creek rhyolite (Sn) in New Mexico, 21.7 Ma Spor Mountain tuff (Be, U, F) and 24.6 Ma Pine Grove tuff (Mo) in Utah, and 27.6 Ma Hideaway Park tuff (Mo) in Colorado support the following conclusions. Melt inclusions in quartz phenocrysts from rhyolite tuffs associated with hydrothermal deposits of Sn, Mo, and Be are extremely enriched in Li (1,000s of ppm); those from Spor Mountain have the highest Li abundance yet recorded (max 5,200 ppm, median 3,750 ppm). Forty-five to 98% of the Li present in pre-eruptive magma was lost to the environment from these rhyolite tuffs. The amount of Li lost from the small volumes (1–10 km3) of Li-enriched rhyolite deposited in closed basins is sufficient to produce world-class Li brine resources. After each eruption, meteoric water leaches Li from tuff, which drains into playas, where it is concentrated by evaporation. The localized occurrence of Li-enriched rhyolites may explain why brines in arid lacustrine basins seldom have economic concentrations of Li. Considering that hydrothermal deposits of Sn, Mo, Be, U, and F may indicate potential for Li brines in nearby basins, we surmise that the world’s largest Li brine resource in the Salar de Uyuni (10 Mt) received Li from nearby rhyolite tuffs in the Bolivian tin belt.