1,721,034 research outputs found
Erratum to: Melting the hydrous, subarc mantle: the origin of primitive andesites
No full textWhat we had intended to appear as the last section of our paper describing the supplementary material was instead published as the first electronic supplementary material (ESM). Because this section (Supplemental Material) was inserted into the ESM, the numerical references in the text of the section do not coincide with the supplementary materials that are included with the paper. Below, the number of each supplement has been increased by one to correct the error
Controls on the stability and composition of amphibole in the Earth’s mantle
No full textPresented here is a suite of new experiments aimed at quantifying the effects of pressure, temperature, bulk composition, and H[subscript 2]O content on the stability and composition of amphibole in the Earth’s mantle. Experiments have been performed from 2 to 4 GPa and 950 to 1100 °C on fertile and depleted mantle compositions. H[subscript 2]O contents of most experiments are 0.65 wt%. In the fertile mantle composition, pargasitic amphibole is stable up to ~3.8 GPa at 1000 °C, approximately 0.5 GPa higher than any previous study. The upper stability limit of amphibole in depleted mantle is 0.7 GPa and 40 °C lower than in fertile mantle. The addition of 3 wt% H[subscript 2]Oto fertile mantle destabilizes amphibole by 0.5 GPa and 40 °C relative to the 0.65 wt% H[subscript 2]Oexperiments. Compared to existing experiments on amphibole stability, these experiments indicate that pargasitic amphibole may be stable in mantle lithosphere to almost 4 GPa (0.5 GPa higher (15 km deeper) than previously thought). The extremely strong destabilizing effect of H[subscript 2]O suggests that deeper portions of the strongly fluid-fluxed mantle wedge may be amphibole-free even at low temperatures near the slab–wedge interface. The molar alkali content of amphibole is shown to be a linear function (R[superscript 2] = 0.98) of pressure and temperature and is relatively insensitive to bulk compositional differences between fertile and depleted mantle. This relationship is used to produce an empirical thermobarometer for pargasite-bearing spinel and garnet lherzolites. Comparison to existing experimental data shows that this thermobarometer has predictive ability over the pressure range of 1–4 GPa. Comparisons with pressure–temperature estimates of garnet + amphibole peridotites further corroborate the applicability of this thermobarometer for natural samples. Pressure estimates are presented for four examples of metasomatized spinel peridotites otherwise lacking pressure information, and future avenues for refinement of the thermobarometer are discussed.National Science Foundation (U.S.) (Grants EAR-1118598 and EAR-1551321
Erratum to: Editorial
No full textIn the original publication of the Editorial article, the Editor’s name was incorrectly published as Timothy L. Gorve. It should read Timothy L. Grove
Straddling the tholeiitic/calc-alkaline transition: the effects of modest amounts of water on magmatic differentiation at Newberry Volcano, Oregon
No full textMelting experiments have been performed at 1 bar (anhydrous) and 1- and 2-kbar H2O-saturated conditions to study the effect of water on the differentiation of a basaltic andesite. The starting material was a mafic pumice from the compositionally zoned tuff deposited during the ~75 ka caldera-forming eruption of Newberry Volcano, a rear-arc volcanic center in the central Oregon Cascades. Pumices in the tuff of Newberry caldera (TNC) span a continuous silica range from 53 to 74 wt% and feature an unusually high-Na2O content of 6.5 wt% at 67 wt% SiO2. This wide range of magmatic compositions erupted in a single event makes the TNC an excellent natural laboratory in which to study the conditions of magmatic differentiation. Our experimental results and mineral–melt hygrometers/thermometers yield similar estimates of pre-eruptive H2O contents and temperatures of the TNC liquids. The most primitive (mafic) basaltic andesites record a pre-eruptive H2O content of 1.5 wt% and a liquidus temperature of 1,060–1,070 °C at upper crustal pressure. This modest H2O content produces a distinctive fractionation trend that is much more enriched in Na, Fe, and Ti than the calc-alkaline trend typical of wetter arc magmas, but slightly less enriched in Fe and Ti than the tholeiitic trend of dry magmas. Modest H2O contents might be expected at Newberry Volcano given its location in the Cascade rear arc, and the same fractionation trend is also observed in the rim andesites of the rear-arc Medicine Lake volcano in the southern Cascades. However, the Na–Fe–Ti enrichment characteristic of modest H2O (1–2 wt%) is also observed to the west of Newberry in magmas erupted from the arc axis, such as the Shevlin Park Tuff and several lava flows from the Three Sisters. This shows that modest-H2O magmas are being generated directly beneath the arc axis as well as in the rear arc. Because liquid lines of descent are particularly sensitive to water content in the range of 0–3 wt% H2O, they provide a quantitative and reliable tool for precisely determining pre-eruptive H2O content using major-element data from pumices or lava flows. Coupled enrichment in Na, Fe, and Ti relative to the calc-alkaline trend is a general feature of fractional crystallization in the presence of modest amounts of H2O, which may be used to look for “damp” fractionation sequences elsewhere.National Science Foundation (U.S.) (NSF Grant EAR-0507486)National Science Foundation (U.S.) (NSF Grant EAR-1118598
Melts of garnet lherzolite: experiments, models and comparison to melts of pyroxenite and carbonated lherzolite
No full textPhase equilibrium experiments on a compositionally modified olivine leucitite from the Tibetan plateau have been carried out from 2.2 to 2.8 GPa and 1,380–1,480 °C. The experiments-produced liquids multiply saturated with spinel and garnet lherzolite phase assemblages (olivine, orthopyroxene, clinopyroxene and spinel ± garnet) under nominally anhydrous conditions. These SiO[subscript 2]-undersaturated liquids and published experimental data are utilized to develop a predictive model for garnet lherzolite melting of compositionally variable mantle under anhydrous conditions over the pressure range of 1.9–6 GPa. The model estimates the major element compositions of garnet-saturated melts for a range of mantle lherzolite compositions and predicts the conditions of the spinel to garnet lherzolite phase transition for natural peridotite compositions at above-solidus temperatures and pressures. We compare our predicted garnet lherzolite melts to those of pyroxenite and carbonated lherzolite and develop criteria for distinguishing among melts of these different source types. We also use the model in conjunction with a published predictive model for plagioclase and spinel lherzolite to characterize the differences in major element composition for melts in the plagioclase, spinel and garnet facies and develop tests to distinguish between melts of these three lherzolite facies based on major elements. The model is applied to understand the source materials and conditions of melting for high-K lavas erupted in the Tibetan plateau, basanite–nephelinite lavas erupted early in the evolution of Kilauea volcano, Hawaii, as well as younger tholeiitic to alkali lavas from Kilauea.National Science Foundation (U.S.) (grants EAR-0507486, EAR-0538179, and EAR- 1118598
Melt generation, crystallization, and extraction beneath segmented oceanic transform faults
Full text linkAuthor Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 114 (2009): B11102, doi:10.1029/2008JB006100.We examine mantle melting, fractional crystallization, and melt extraction beneath fast slipping, segmented oceanic transform fault systems. Three-dimensional mantle flow and thermal structures are calculated using a temperature-dependent rheology that incorporates a viscoplastic approximation for brittle deformation in the lithosphere. Thermal solutions are combined with the near-fractional, polybaric melting model of Kinzler and Grove (1992a, 1992b, 1993) to determine extents of melting, the shape of the melting regime, and major element melt composition. We investigate the mantle source region of intratransform spreading centers (ITSCs) using the melt migration approach of Sparks and Parmentier (1991) for two end-member pooling models: (1) a wide pooling region that incorporates all of the melt focused to the ITSC and (2) a narrow pooling region that assumes melt will not migrate across a transform fault or fracture zone. Assuming wide melt pooling, our model predictions can explain both the systematic crustal thickness excesses observed at intermediate and fast slipping transform faults as well as the deeper and lower extents of melting observed in the vicinity of several transform systems. Applying these techniques to the Siqueiros transform on the East Pacific Rise we find that both the viscoplastic rheology and wide melt pooling are required to explain the observed variations in gravity inferred crustal thickness. Finally, we show that mantle potential temperature Tp = 1350°C and fractional crystallization at depths of 9–15.5 km fit the majority of the major element geochemical data from the Siqueiros transform fault system.This research was supported by WHOI Academic Programs
Office (PMG), NSF grants OCE-0649103 and OCE-0623188 (MDB),
and the Charles D. Hollister Endowed Fund for Support of Innovative
Research at WHOI (J.L.)
Melting systematics in mid-ocean ridge basalts: Application of a plagioclase-spinel melting model to global variations in major element chemistry and crustal thickness
Full text linkWe present a new model for anhydrous melting in the spinel and plagioclase stability fields that provides enhanced predictive capabilities for the major element compositional variability found in mid-ocean ridge basalts (MORBs). The model is built on the formulation of Kinzler and Grove (1992) and Kinzler (1997) but incorporates new experimental data collected since these calibrations. The melting model is coupled to geodynamic simulations of mantle flow and mid-ocean ridge temperature structure to investigate global variations in MORB chemistry and crustal thickness as a function of mantle potential temperature, spreading rate, mantle composition, and the pattern(s) of melt migration. While the initiation of melting is controlled by mantle temperature, the cessation of melting is primarily determined by spreading rate, which controls the thickness of the lithospheric lid, and not by the exhaustion of clinopyroxene. Spreading rate has the greatest influence on MORB compositions at slow to ultraslow spreading rates (<2 cm/yr half rate), where the thermal boundary layer becomes thicker than the oceanic crust. A key aspect of our approach is that we incorporate evidence from both MORB major element compositions and seismically determined crustal thicknesses to constrain global variations in mantle melting parameters. Specifically, we show that to explain the global data set of crustal thickness, Na[subscript 8], Fe[subscript 8], Si[subscript 8], Ca[subscript 8]/Al[subscript 8], and K[subscript 8]/Ti[subscript 8] (oxides normalized to 8 wt % MgO) require a relatively narrow zone over which melts are pooled to the ridge axis. In all cases, our preferred model involves melt transport to the ridge axis over relatively short horizontal length scales (~25 km). This implies that although melting occurs over a wide region beneath the ridge axis, up to 20–40% of the total melt volume is not extracted and will eventually refreeze and refertilize the lithosphere. We find that the temperature range required to explain the global geochemical and geophysical data sets is 1300°C to 1450°C. Finally, a small subset of the global data is best modeled as melts of a depleted mantle source composition (e.g., depleted MORB mantle—2% melt).National Science Foundation (U.S.) (grant OCE01458201)National Science Foundation (U.S.) (grant OCE01457916
Controlled-atmosphere thermal demagnetization and paleointensity analyses of extraterrestrial rocks
Full text linkWe describe a system for conducting thermal demagnetization of extraterrestrial rocks in a controlled atmosphere appropriate for a wide range of oxygen fugacities within the stability domain of iron. Thermal demagnetization and Thellier-Thellier paleointensity experiments on lunar basalt synthetic analogs show that the controlled atmosphere prevents oxidation of magnetic carriers. When combined with multidomain paleointensity techniques, this opens the possibility of highly accurate thermal demagnetization and paleointensity measurements on rocks from the Moon and asteroids.United States. National Aeronautics and Space Administration (Grant NNX12AH80G)
A melting model for variably depleted and enriched lherzolite in the plagioclase and spinel stability fields
Full text linkHere we develop a lherzolite melting model and explore the effects of variations in mantle composition, pressure, temperature, and H[subscript 2]O content on melt composition. New experiments and a compilation of experimental liquids saturated with all of the mantle minerals (olivine, orthopyroxene, clinopyroxene, plagioclase and/or spinel) are used to calibrate a model that predicts the temperature and major element composition of a broad spectrum of primary basalt types produced under anhydrous to low H[subscript 2]O-content conditions at upper mantle pressures. The model can also be used to calculate the temperature and pressure at which primary magmas were produced in the mantle, as well as to model both near-fractional adiabatic decompression and batch melting. Our experimental compilation locates the pressure interval of the plagioclase to spinel transition on the solidus and shows that it is narrow (∼0.1 GPa) for melting of natural peridotite compositions. The multiple saturation boundaries determined by our model provide a method for assessing the appropriate mineral assemblage, as well as the extent of the fractional crystallization correction required to return a relatively primitive liquid to equilibrium with the mantle source. We demonstrate that an inaccurate fractionation correction can overestimate temperature and depths of melting by hundreds of degrees and tens of kilometers, respectively. This model is particularly well suited to examining the temperature and pressure of origin for intraplate basaltic volcanism and is used to examine the petrogenesis of a suite of Holocene basaltic lavas from Diamond Crater in Oregon's High Lava Plains (HLP).National Science Foundation (U.S.) (Grant EAR-0507486)National Science Foundation (U.S.) (Grant EAR-0538179)National Science Foundation (U.S.) (Grant EAR-1118598
(Table 2) Groundmass plagioclase size in lavas from Serocki volcano sampled in ODP Hole 109-648B
No full textPlagioclase width is the average of 6-12 of the largest crystals in each thin section measured perpendicular to the (010) crystal faces
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