Phenocryst

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

Petrologic and sulfur isotopic significance of highly oxidized and sulfur-rich magma of Mt. Pinatubo, Philippines

Abstract: Dacitic pumices from pyroclastic-flow deposits and air-fall tephra of the June 14-15,1991, eruption of Mt. Pinatubo, Philippines, are characteristically rich in sulfur. The presence of microphenocrystic anhydrite suggests that sulfur existed mostly as oxidized species in the magma. This supposition is corroborated by unusually high sulfur contents (up to 0.78 wt% as SO3) in apatite microphenocrysts and apatite inclusions in other phenocrystic minerals and by the highly oxidized state of the magma, near or slightly below the hematite-magnetite buffer, on the basis of extrapolation from biotite compositions and the two-oxide method. This highly oxidized state possibly caused the extraordinarily high sulfur content of Mt. Pinatubo dacite through prohibition of sulfide fractionation and because of increasing solubility of sulfur as oxidized species with increasing oxygen fugacity. Hornblende geobarometry indicates a pressure of ∼2 kbar for phenocryst formation. Among the two pumice types, cummingtoniterimmed hornblende phenocrysts are typically found in type 1 (white and crystal-rich) pumice, whereas rare hypersthene has been observed as discrete microphenocrysts in type 2 (tan and crystal-poor) pumice. These observations indicate conditions at least near if not at vapor saturation in type 1 magma and undersaturation in type 2 magma prior to eruption. Vapor saturated type 1 dacitic magma probably caused the explosive June 14-15,1991, eruption. The highly oxidized condition and magmatic water saturation are similar to those of intrusions related to porphyry copper deposits.

Evolution of the Magma Chamber beneath Usu Volcano since 1663: a Natural Laboratory for Observing Changing Phenocryst Compositions and Textures

Abstract: We have investigated the evolution of an active silicic magma-feeding system beneath Usu volcano, Japan, where eight eruptions have been recorded since AD 1663. All magmatic products contain similar types of plagioclase and orthopyroxene phenocrysts that consist of homogeneous cores with uniform compositions, and a zoned mantle that increases in size with time. The compositions of plagioclase and orthopyroxene phenocrysts vary gradually and regularly with time, as do the bulk-rock compositions. The texture of these phenocrysts also changes systematically, caused by progressive crystal growth, dissolution and diffusion. On the basis of these observations, we conclude that the same magma-feeding system has persisted at Usu volcano since AD 1663. Compositional variation of magnetite phenocrysts differs from that of plagioclase and orthopyroxene, because magnetite has large diffusion coefficients and should represent magmatic conditions immediately before the eruption. Most pumices from Usu volcano contain two types of magnetite phenocryst, each with a different composition and crystallization temperature, indicating that two magmas mixed before each eruption (approximately several days before). The end-members changed with time: rhyolite þ basaltic andesite (1663); dacite rhyolite (1769, 1822, 1853); dacite dacite (1977, 2000). The temperature of the magma apparently increases with time, and the increase can be explained by sequential tapping from a magma chamber with a thermal and chemical gradient in addition to injection of high-temperature magma.

New Insights into Andesite Genesis: the Role of Mantle-derived Calc-alkalic and Crust-derived Tholeiitic Melts in Magma Differentiation beneath Zao Volcano, NE Japan

Abstract: Two distinctive differentiation trends, tholeiitic and calc-alkalic, are recognized in Zao volcano, which is located immediately behind the volcanic front of the NE Japan arc. The genetic relationship between these two magma series is critical for a better understanding of andesite genesis, because they often coexist in close spatial and temporal proximity in arc volcanoes. Petrographic features indicative of ‘disequilibrium’ , such as reversely zoned pyroxene phenocrysts, the wide and bimodal compositional distribution in Ca/(CaþNa) of plagioclase phenocrysts, honeycomb textures and dusty zones that these plagioclase phenocrysts often exhibit, and the presence of olivine^pyroxene pairs with different Mg/Fe, are observed exclusively in calc-alkalic rocks. In tholeiitic rocks the Sr isotopic ratios of plagioclase phenocrysts, determined by both micromilling combined with thermal ionization mass spectrometry, and laser-ablation inductively coupled plasma mass spectrometry techniques, are constant at 0 7042^0 7044. On the other hand, those in calc-alkalic rocks (0 7033^0 7042) show more complex characteristics, which can be best understood if at least three end-member components, a calc-alkalic basaltic melt, a tholeiitic basaltic melt and a tholeiitic felsic melt, contribute to the production of mixed calc-alkalic magmas.The Sr/Sr and trace element compositions of the leastdifferentiated basalt magmas, which are inferred from the composition of the calcic plagioclase [Ca/(CaþNa)40 9], suggest that two types of basaltic magma, calc-alkalic and tholeiitic, exist beneath the volcano. The tholeiitic basalt magma has a higher Sr/Sr than the calc-alkalic magma (0 7042 vs 0 7038) and a characteristic trace element signature consistent with the presence of plagioclase and amphibole as melting residues.This suggests that the tholeiitic magmas are produced via anatexis of amphibolitic crust caused by underplating and/or intrusion of mantle-derived calcalkalic basalt magmas into the sub-Zao crust. The mantle-derived calc-alkalic basalt magma mixes with crust-derived tholeiitic melts to form calc-alkalic andesite magmas.The hypothesis proposed here requires revision (or even abandonment) of the general consensus that calc-alkalic magmas have greater contributions of a crustal component than tholeiitic magmas.

Contrasting Tectonic Settings and Sulfur Contents of Magmas Associated with Cretaceous Porphyry Cu ± Mo ± Au and Intrusion-Related Iron Oxide Cu-Au Deposits in Northern Chile

Abstract: Porphyry Cu ± Mo ± Au and iron oxide copper-gold (IOCG) deposits share many similarities (e.g., Fe, Cu, and Au contents), but also have important differences (e.g., the predominance of sulfide minerals in porphyry deposits and iron oxides in IOCG deposits). Genetic comparisons are complicated by the broad definition of IOCG deposits; here we restrict our study to IOCG deposits that are related to igneous intrusive systems. In the Mesozoic Coastal Cordillera of northern Chile, both porphyry and IOCG deposits occur in close spatial and temporal proximity, offering the chance to examine what controls their different modes of formation. From detailed examination of the timing, geochemistry, and tectonic setting of associated igneous rocks, based on new and published data, we find that rocks associated with mid-Cretaceous IOCG deposits (~125–110 Ma) are largely indistinguishable from those associated with slightly earlier (>125 Ma) and later (<110 Ma) porphyry Cu ± Mo ± Au deposits. Magmas related to IOCG deposits were formed during a brief period of back-arc transtension in the mid-Cretaceous and are, on average, somewhat more mafic (dioritic), locally alkaline, and isotopically primitive compared to granodioritic magmas associated with porphyry deposits formed during normal contractional arc tectonics in the later Cretaceous. However, these compositional ranges overlap, and the differences are not clear enough to be diagnostic. We measured the SO3 content of igneous apatite from selected samples of these rocks to test the hypothesis that the difference in sulfur content of the ore deposits was due to differences in sulfur content of the associated magmas. Early igneous apatite crystals occurring as inclusions in silicate phenocrysts from the Carmen de Andacollo porphyry Cu-Au deposit (Re-Os molybdenite ages of 103.9 ± 0.5 Ma, 103.6 ± 0.5 Ma) are significantly richer in S (0.25 ± 0.17 wt % SO3, n = 69) than similar apatite crystals from two IOCG deposits (Candelaria, Casualidad) and a sample of regional mid-Cretaceous igneous rock from near Productora (0.04 ± 0.02 wt % SO3, n = 76). Using published partition coefficients for S between apatite and oxidized silicate melt, we semi-quantitatively estimate corresponding magmatic sulfur contents of ~0.02 wt % S in the Carmen de Andacollo magmas versus ~0.001 to 0.005 wt % S in the IOCG-associated magmas. This is an order of magnitude difference, and the opposite of what would be expected if the difference were due to bulk magma composition (sulfur solubility is generally higher in mafic magmas, whereas here the S content is higher in the more felsic porphyries). We conclude that the porphyry-forming magmas indeed had higher S contents than the IOCG-related magmas and suggest that these differences reflect different petrogenetic processes. During normal subduction, magmas derived from the metasomatized mantle wedge are hydrous, moderately oxidized, and S rich, and have the potential to generate S-rich porphyry-type deposits. In contrast, in back-arc extensional settings, upwelling asthenospheric melts carry a weaker subduction signature, including lower S contents. Interaction of these S-poor magmas with previously subduction modified upper plate lithosphere is more likely to give rise to S-poor IOCG deposits.