Phenocryst

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

Short communication Magmatic origin of low-Ca olivine in subduction-related magmas: Co-existence of contrasting magmas

Abstract: Unravelling the origin of different components contributing to subduction-related magmas is a prerequisite to understanding the sources and processes involved in their origins. Mafic, high-Ca subduction-related magmas from geographically-diverse areas (Indonesia, Solomon Islands, Kamchatka, Valu Fa Ridge) contain two populations of olivine crystals, of which only the high-Ca population (CaO=0.3–0.5 wt.%) crystallized from the melt that dominantly contributed to the whole rock composition. Forsteriterich (Fo90–94), low-Ca (CaOb0.15 wt.%), high-Ni (NiON0.3 wt.%) olivine crystals, which constitute 16–37 vol.% of total olivine population, are generally interpreted as mantle or lithospheric xenocrysts. However, in these samples, the olivine shape and chemical zoning, the composition of included minerals (orthopyroxene, clinoenstatite and Cr-spinel) and presence of melt inclusions, are indications that these crystals are phenocrysts from a mafic magma with high silica and low calcium contents. The coexistence of contrasting magmas (mafic high-Ca silica-poor versus low-Ca silica-rich) within a number of arc systems and their mixing may not be a rare event, and should be taken into account when developing models of arc petrogenesis.

Late crystallization of K-feldspar and the paradox of megacrystic granites

Abstract: K-feldspar crystals >5 cm in greatest dimension are common in calc-alkaline granites and granodiorites worldwide. Such megacrysts are generally interpreted as having grown to large sizes early in a magma’s crystallization history while they were largely molten, owing to field relations such as megacryst alignment and megacryst-rich clusters and to crystallographic features such as zonally arranged inclusions and sawtooth Ba zoning. These features are consistent with early growth but do not require it. In contrast, experimental petrology, mineral compositions, and natural examples of partial melting of granite demonstrate that K-feldspar is typically the last major phase to crystallize and that most K-feldspar growth occurs after the magma crosses the rheologic lock-up threshold of ~50 % crystals. The near-absence of K-feldspar phenocrysts in dacite lavas and tuffs, even in highly crystalline ones, demonstrates that natural magmas do not precipitate significant K-feldspar while they are mobile. The highly potassic compositions of megacrysts (and indeed, of K-feldspar in non-megacrystic granites as well) require exsolution of albite component down to temperatures of ~400 °C. The low Ca contents of megacrysts cannot result from exsolution of anorthite and must represent recrystallization of the crystals at low temperature. These mineralogical and experimental constraints require that K-feldspar megacrysts indicate widespread and thorough recrystallization of the host granites and granodiorites.

Evolution of HF and HCl activity in magmatic volatiles of the gold-mineralized Emerald Lake pluton, Yukon Territory, Canada

Abstract: We examine the halogen contents of biotite, amphibole, and apatite from four magmatic phases of the Emerald Lake pluton to document the evolution in composition of magmatic volatiles in this system, and to determine if this evolution is consistent with the nature and distribution of weak gold mineralization in the pluton. In general, the variations in halogen compositions of hydrous minerals are greater between magmatic phases than within single magma batches, and the abundance of chlorine decreases and fluorine increases in hydrous minerals during the evolution of the Emerald Lake pluton. By applying the internally consistent thermodynamic database and mineral activity–composition relationships that account for coupled site-substitution, we further employ biotite and apatite compositions as independent monitors of fluid composition. Most analyzed biotite exhibits textural and/or compositional features characteristic of late, subsolidus halogen exchange with fluid. However, we have identified biotite that appears to retain halogen contents indicative of magmatic conditions. Fluorine contents of these biotites range from around 1.5 (syenite and monzonite) to greater than 2.2 wt% (in a late-stage pegmatite dike). Chlorine, by contrast, shows a more restricted range of values, and typically falls between 1.0 (syenite and monzonite) and 0.6 wt% (pegmatite). Apatite inclusions within anhydrous phenocrysts have fluorine contents of around 3.3 (syenite) to greater than 3.7 wt% (granite), while chlorine contents fall from 0.5 to 0.1 wt% with evolution from syenite to granite phases of the pluton. The fluid aHCl/aHF conditions recorded by biotite and apatite are in close agreement. These two independent measures of HCl and HF both show a modest decrease in HCl/HF as the system evolves. The highest log aHCl/aHF values (0.6 to 0.3) are associated with magmatic volatiles of syenite, the first phase to intrude at Emerald Lake. Activity ratios recorded from the second phase (monzonite) show a slight fall to ~0.4 to 0.1. A similar drop is recorded in the transition to hydrothermal activity recorded in biotite from a late pegmatite dike (~0.1 to –0.1). A decrease in log aHCl/aHF is consistent with a loss of chlorine and residual enrichment of fluorine in the magma, due to progressive release of a chlorine-bearing (metalliferous?) fluid. The relatively small variation in inferred fluid HCl and HF activity suggests that the magmatic volatiles present during crystallization were relatively homogeneous, and argues against both the incursion of external fluids, and of substantial temporal variation in fluid composition from the magma source. Furthermore, if magmatic volatiles are the source of the gold mineralization found across all phases of the Emerald Lake pluton, as advocated by other workers, our data would suggest that the mineralizing fluids were remarkably homogeneous and distributed throughout the pluton. The disseminated and low-grade nature of mineralization may be, at least in part, a consequence.

Trace element incorporation into growing augite phenocryst

Abstract: Since the introduction1 to geochemistry of crystallization theories developed in metallurgy, it has been thought that the effects of kinetic parmeters such as the rate of crystal growth and the rate of diffusive transport of elements in the melt could significantly affect the partition of trace elements between mineral and magma. This conclusion has not, however, been demonstrated unequivocally, mainly because the extent of variations of equilibrium partition coefficients with T, P and chemistry is not well known. Henderson and Williams2 found a correlation between morphology and apparent partition coefficient of uranium between olivine and basaltic melt. Data on diffusivity of elements (see ref. 3) and on crystal growth kinetics together with the development of secondary ion mass spectrometry (SIMS) techniques, enable crystal zoning to be studied with respect to trace elements and the kinetic effects to be evaluated quantitatively. Trace element zoning (or lack of it) of minerals is important, as many kinetic-based crystallization models predict trace element zoning in crystals. This letter presents trace element data on a sector-zoned augite phenocryst based on the ion-probe spot analyses. The fact that the slower-growing prism sector [100] is enriched in both compatible and incompatible elements relative to the faster-growing basal sector [111] strongly supports Dowty's4 model for crystal growth involving preferential adsorption of elements onto growing crystal faces in proportion to the charge/size ratio of the elements.