Phenocryst

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

Evidence for dynamic withdrawal from a layered magma body: The Topopah Spring Tuff, southwestern Nevada

Abstract: The Topopah Spring Tuff is a classic example of a compositionally zoned ash flow sheet resulting from eruption of a compositionally zoned magma body. Geochemical and petrographic analyses of whole rock tuff samples indicate that the base of the ash flow sheet and the predominant volume of erupted material consist of crystal-poor high-silica rhyolite, with a gradational transition into overlying crystal-rich quartz latite. However, major and trace element analyses of glassy pumice lumps and microprobe analyses of their silicate and oxide phenocrysts provide closer approximations of the chemical and thermal gradients within the magma body. The gradients inferred from these data indicate that the transition from high-silica rhyolitic to quartz latitic magma within the chamber was abrupt, rather than gradational, with a distinct liquid-liquid interface separating the contrasting magma layers. Compositionally and texturally distinct pumice lumps are present throughout the ash flow sheet. The degree of heterogeneity within and among pumice lumps increases with stratigraphic height, becoming most pronounced in the uppermost quartz latite, where the chemical variability among pumice lumps is as great as that of the entire ash flow sheet. These observations are consistent with fluid dynamic models in which the velocity field developed near the entrance region of the vent(s) results in simultaneous withdrawal of magma from all points of a continuously expanding lateral and vertical region within the chamber. The abrupt transition to chemically bimodal pumice types near the top of the ash flow sheet, dominated by those of quartz latitic composition, implies that the interface between the magma layers remained relatively stable until draw-down breached the interface and preferentially erupted hotter, more mafic magma along with lesser amounts of the remaining high-silica rhyolitic magma.

Complex zoning and resorption of phenocrysts in mixed potassic mafic magmas of the Highwood Mountains, Montana

Abstract: Disequilibrium phenocryst assemblages and complex compositional zoning in clinopyroxene, mica, and olivine phenocrysts provide a detailed record of multiple mixing, fractional crystallization, and degassing events during the high-level evolution of potassic mafic magmas in the Eocene Highwood Mountains province. The contrasting phenocryst assemblages of minettes (diopside + phlogopite + olivine) and mafic phonolites-shonkinites (salite + leucite + olivine) permit unambiguous documentation of mixing between these two magma types and also between more primitive and more evolved members of each type. The varied behavior of crystals mixed into disequilibrium liquids, deduced from detailed textural and microprobe analyses, is consistent with the results of experimental studies of plagioclase dissolution. Phlogopite and diopside xenocrysts within more evolved mafic phonolite liquids initially underwent peripheral resorption. Further dissolution of diopside produced a network ofinterior cavities that were subsequently plated by salite crystallized from the host magma. In contrast, salite xenocrysts within more primitive mafic phonolite liquids underwent partial dissolution at their margins, yet remained euhedral. In some cases dissolution was restricted to a single, compositionally distinct sector. Halos of Fe-Al-rich salite subsequently formed around the melt-filled cavities by diffusion. Complexly zoned salite and biotite crystals containing one or more sharply bounded, epitaxial bands of diopside and phlogopite, respectively, are common in the mafic phonolites and shonkinites. On the basis of features such as the different salite compositions on either side of diopside bands and their lack of leucite inclusions and oscillatory zoning, it is argued that each band in a crystal (rarely more than six) is a record of a mixing event with minette magma. We infer that the contrasting phenocryst assemblages of the compositionally similar minettes and mafic phonolites largely reflect differences in HrO activity during their crystallization, and we propose that degassing of ascending minette magmas was an important petrogenetic process. We conclude that the high-level evolution of the Highwood magmatic system involved repeated mixing in multiple reservoirs among batches of variably fractionated and degassed mafic phonolite magma and periodic influxes of more primitive, undegassed minette magma.

Elevated Magmatic Sulfur and Chlorine Contents in Ore-Forming Magmas at the Red Chris Porphyry Cu-Au Deposit, Northern British Columbia, Canada

Abstract: The Red Chris porphyry Cu-Au deposit is located in the Stikinia island-arc terrane in northwest British Columbia. It is hosted by the Red Stock, which has five phases of porphyry intrusions: P1, P2E, P2I, P2L, and P3. New U-Pb dating of zircon shows that these intrusions were emplaced over a similar to 10 m.y. period, with P1 intruded at 211.6 +/- 1.3 Ma (MSWD = 0.85), P2I at 206.0 +/- 1.2 Ma (MSWD = 1.5), P2L at 203.6 +/- 1.8 Ma (MSWD = 1.5), and P3 at 201.7 +/- 1.2 Ma ( MSWD = 1.05). The ore-forming event at Red Chris was a short-lived event at 206.1 +/- 0.5 Ma (MSWD = 0.96; weighted average age of three Re-Os molybdenite analyses), implying a duration of Zircons from P1 to P3 porphyry rocks have consistently high Eu-N /Eu-N* ratios (mostly > 0.4), indicating that their associated magmas were moderately oxidized. The magmatic water contents estimated from plagioclase and amphibole compositions suggest H2O contents of similar to 5 wt %. Taken together, the P1 to P3 porphyries are interpreted to be moderately oxidized and hydrous. The porphyry phases are differentiated by sulfur and chlorine contents. The SO3 contents of igneous apatite microphenocrysts from the mineralization-related P2 porphyries are higher (P2E: 0.28 +/- 0.10 wt %, n = 19; P2I: 0.32 +/- 0.17, n = 15; P2L: 0.29 +/- 0.18 wt %, n = 100) than those from the premineralization P1(0.11 +/- 0.03 wt %, n = 34) and postmineralization P3 porphyries (0.03 +/- 0.01 wt %, n = 13). The chlorine contents in apatite grains from the P2E, P2I, and P2L porphyries are 1.47 +/- 0.22 (n = 19), 0.82 +/- 0.10 (n = 15), and 1.47 +/- 0.28 wt % (n = 100), also higher than those from P1 (0.51 +/- 0.3 wt % Cl, n = 34) and P3 (0.02 +/- 0.02 wt % Cl, n = 17). These results imply that the sulfur and chlorine contents of the P2 magmas were higher than in the P1 and P3 magmas, suggesting that elevated magmatic S-Cl contents in the P2 porphyries may have been important for ore formation. Although the process that caused the increase in sulfur and chlorine is not clear, reverse zoning seen in plagioclase phenocrysts from the P2 porphyry, and the occurrence of more mafic compositions in P2L, suggest that recharge of the deeper magma chamber by a relatively S-Cl-rich mafic magma may have triggered the ore-forming hydrothermal event.

The A‐type Mount Scott Granite sheet: Importance of crystal magma traps

Abstract: The presence of rapakivi feldspar and of distinctive porphyritic texture of Mount Scott Granite indicates a period of crystallization prior to final emplacement beneath an extensive penecontemporaneous rhyolite volcanic pile. Final crystallization conditions are interpreted to have been <50 MPa at depths < <1.4 km based on stratigraphic constraints. However, geobarometry based on the Al content of amphibole phenocrysts and comparison of granite compositions with phase relations in the SiO2-NaAlSi3O8-KAlSi3O8 ternary system both yield pressure estimates of ≈200 MPa. These pressure estimates are interpreted as plumbing the depth of a temporary storage chamber at ≈7–8 km. This depth coincides, in this case, both with the probable Proterozoic basement-cover contact and with the calculated brittle-ductile transition at time of ascent of Mount Scott magma. Although rising magma that fed the preceeding voluminous Carlton Rhyolite apparently rose unimpeded past these horizontal anisotropies, rising magma that formed Mount Scott Granite temporarily paused at this depth. Based on magmastatic calculations, we suggest that horizontal anisotropies (e.g., brittle-ductile transition) become crustal magma traps where the magma driving pressure exceeds the lithostatic load when the anisotropy is encountered. During rifting, initial large influxes of magma may proceed passed crustal anisotropies but have the effect of increasing the relative magma driving pressure through reducing horizontal stress. Thus, magma driving pressure may eventually exceed the lithostatic load at the depth of the brittle-ductile transition thereby activating this crustal magma trap. Ponding of magma at the brittle-ductile transition chokes the eruption. Such a pause in magma supply rate may permit a return to initial stress conditions and deactivate the crustal magma trap. Once again magma will rise to the surface initiating a new magmatic cycle.

The Origin of Basic-Ultrabasic Sills with S-, D-, and I-shaped Compositional Profiles by in Situ Crystallization of a Single Input of Phenocryst-poor Parental Magma

Abstract: An attempt is made to develop an in situ crystallization model based on the concept of Soret fractionation to explain the origin of commonly observed S-, D-, and I-shaped compositional profiles in sills formed from a single pulse of phenocryst-poor parental magma. The model envisages that the various compositional profiles observed in sills can be interpreted in terms of different combinations of four principal units—Basal Zone and Layered Series forming the floor sequence, and Top Zone and Upper Border Series constituting the roof sequence. The Basal and Top Zones represent mirror images of the Layered and Upper Border Series, respectively, and therefore are referred to as basal and top reversals. It is proposed that the formation of basal and top reversals takes place through the non-equilibrium Soret differentiation of liquid boundary layers which form as a consequence of the temperature gradient imposed by the cold country rock. In contrast, the Layered and Upper Border Series originate predominantly through the crystal‐liquid boundary layers developing in equilibrium conditions. The model permits the production of S-, D-, and I-shaped compositional profiles from the same magma composition. All that is necessary to produce a specific shape of compositional profile is an appropriate temperature gradient imposed by the cold country rock on the liquid boundary layers of a parental magma of a given composition.