Phenocryst

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

Chemical and mineralogic trends within the Timber Mountain‐Oasis Valley Caldera Complex, Nevada: Evidence for multiple cycles of chemical evolution in a long‐lived silicic magma system

Abstract: Rocks of the Miocene and Pliocene Timber Mountain-Oasis Valley (TM-OV) caldera complex in the southwest Nevada volcanic field are dominantly rhyolites and quartz latites (trachytes). Minor basaltic to dacitic rocks were erupted peripherally to the caldera complex and in the moat of the youngest caldera. We divide the petrologic evolution of the caldera complex into a series of petrochemical cycles based on systematic changes in rock chemistry, modal petrography, and mineral chemistry as a function of time. Each petrochemical cycle is characterized by periods of systematic differentiation toward more siliceous or rhyolitic compositions. Breaks between cycles are generally abrupt and follow either times of major ash flow tuff eruption or episodes of relatively mafic (basaltic to dacitic) volcanism. New cycles begin with magma compositions that are less silicic than rhyolites erupted at the end of the preceding cycle. The systematic chemical changes toward more silicic compositions indicate that each cycle represents periods of progressive magmatic differentiation and suggest that members of a cycle successively evolved from a common parental magma. Some of the chemical trends within cycles are consistent with crystal fractionation of the observed phenocrysts and accessory minerals. However, other mechanisms must have operated in conjunction with crystal fractionation to produce the observed chemical variations in these rocks. Progressive buildup of volatiles and the depression of liquidus temperatures is suggested in some cycles by the decline in phenocryst abundances and the resorption of quartz. The occurrence of multiple petrochemical cycles is consistent with the interpretation that a series of magma bodies were successively emplaced and differentiated in the upper crust beneath TM-OV. Members of petrochemical cycles were periodically erupted during the evolution of these magma bodies, providing a record of their differentiation. An alternative interpretation is that a large upper crustal silicic magma body was present beneath TM-OV for most of its history and that new cycles of differentiation began after major ash flow eruptions, episodes of magma replenishment, and breakdown in compositional zonation.

Molybdenite Saturation in Silicic Magmas: Occurrence and Petrological Implications

Abstract: We identified molybdenite (MoS2) as an accessory magmatic phase in 13 out of 27 felsic magma systems examined worldwide. The molybdenite occurs as small (520 m) triangular or hexagonal platelets included in quartz phenocrysts. Laser-ablation inductively coupled plasma mass spectrometry analyses of melt inclusions in molybdenite-saturated samples reveal 1^13 ppm Mo in the melt and geochemical signatures that imply a strong link to continental rift basalt^rhyolite associations. In contrast, arc-associated rhyolites are rarely molybdenite-saturated, despite similar Mo concentrations. This systematic dependence on tectonic setting seems to reflect the higher oxidation state of arc magmas compared with within-plate magmas. A thermodynamic model devised to investigate the effects of T, fO2 and f S2 on molybdenite solubility reliably predicts measured Mo concentrations in molybdenite-saturated samples if the magmas are assumed to have been saturated also in pyrrhotite. Whereas pyrrhotite microphenocrysts have been observed in some of these samples, they have not been observed from other molybdenitebearing magmas. Based on the strong influence of f S2 on molybdenite solubility we calculate that also these latter magmas must have been at (or very close to) pyrrhotite saturation. In this case the Mo concentration of molybdenite-saturated melts can be used to constrain both magmatic fO2 and f S2 if temperature is known independently (e.g. by zircon saturation thermometry). Our model thus permits evaluation of magmatic f S2, which is an important variable but is difficult to estimate otherwise, particularly in slowly cooled rocks.

Processes and timescales in the evolution of a chemically zoned trachyte: Fogo A, Sao Miguel, Azores

Abstract: U-series disequilibria analyses have been combined with chemical and petrographic analyses in order to assess both the timescales and processes involved in the formation of the chemically zoned Fogo A trachytes. Least squares major element modelling demonstrates that the mafic trachytes could have evolved from a parental alkali basalt via trachybasalt with ∼70% fractionation of augite (35–36%), plagioclase (23%), magnetite (16%), kaersutite (15%), olivine (8%) and apatite (2–3%). Derivation of the mafic trachytes from a basanite parent is inconsistent with calculated fractionation paths. Major and trace element variations in 25 pumice samples collected from throughout the stratigraphic extent of the Fogo A deposit show that the trachytes represent the inverted, extrusive equivalent of a strongly chemically zoned magma chamber. The zonation is attributed to 70–75% Rayleigh fractional crystallization of the observed phenocryst phases. Wallrock assimilation and magma mixing did not contribute significantly to the observed chemical trends. The maximum age of the Fogo A trachytic magma based on radioactive disequilibria between 230Th and 238U is 300000 years. However, a calculated model age suggests that the time of evolution of the Fogo A trachytes from a parent alkali basalt is only 90000 years. Constant element variations and Th-isotopic ratios in Fogo C, Fogo A and 1563 A.D. trachytes suggest that a single long-lived trachytic magma chamber has been the source of at least the past 15.2 Ka of trachytic volcanism from Agua de Pao. After each eruption an evolved cupola reformed and became zoned prior to the next eruption. The maximum time necessary to form the zonation is 4600 years, the time between the Fogo A and 1563 A.D. eruptions. Low (226Ra)/(230Th)i ratios in the Fogo A and 1563 A.D. trachytes suggest that alkali feldspar fractionation continued up to the time of the respective eruptions.

Oxygen isotope geochemistry of pyroclastic clinopyroxene monitors carbonate contributions to Roman-type ultrapotassic magmas

Abstract: The oxygen isotope geochemistry and chemical composition of clinopyroxene crystals from Alban Hills pyroclastic deposits constrain the petrological evolution of ultrapotassic Roman-type rocks. Volcanic eruptions in the 560–35 ka time interval produced thick pyroclastic deposits bearing clinopyroxene phenocrysts with recurrent chemical characteristics. Clinopyroxenes vary from Si–Mg-rich to Al–Fe-rich with no compositional break, indicating that they were derived from a continuous process of crystal fractionation. Based on the δ18O and trace element data no primitive samples were recovered: monomineralic clinopyroxene cumulates set the oxygen isotope composition of primary magmas in the range of uncontaminated mantle rocks (5.5‰), but their ΣREE composition resulted from extensive crystal fractionation. Departing from these mantle-like δ18OCpx values the effects of crustal contamination of clinopyroxene O-isotope composition were identified and used to monitor chemical variations in the parental magma. δ18O values in Si–Mg-rich clinopyroxene are slightly higher than typical mantle values (5.9–6.2‰), and the low ΣREE contents are representative of early stages of magmatic differentiation. δ18O values as high as 8.2‰ are associated with Al–Fe3+-rich clinopyroxene showing high ΣREE contents. These δ18O values are characteristic of crystals formed during the late magmatic stages of each main eruptive phase. Geochemical modelling of δ18O values vs. trace element contents indicates that these ultrapotassic magmas were derived from fractional crystallization plus assimilation of limited amounts of carbonate wall rocks starting from a primary melt, and from interaction with CO2 derived from country rocks during crystal fractionation.