The iron-titanium oxides of salic volcanic rocks and their associated ferromagnesian silicates
TL;DR: In this article, the coexisting microphenocrysts of magnetite and ilmenite together with the ferromagnesian silicates in salic volcanic rocks have been analyzed with the electron microprobe.
Abstract: The co-existing microphenocrysts of magnetite and ilmenite together with the ferromagnesian silicates in salic volcanic rocks have been analysed with the electron microprobe. The temperatures and oxygen fugacities of the oxide equilibration have been estimated from the curves of Buddington and Lindsley (1965). The co-existing ferromagnesian silicate phenocrysts are either iron-rich olivine, or orthopyroxene or biotite and amphibole; for each of these groups of phenocrysts, the oxide equilibration data are specific and fall on three distinct curves, parallel to experimental oxygen buffer curves. Many of the investigated rhyolites were quenched at temperatures near 900°C, which may represent liquidus temperatures for those with sparse phenocrysts, and also the intrusion temperature of water-undersaturated granites. The composition of the biotite phenocrysts, which are Al-poor and Ti-rich, taken in conjunction with the oxide data, suggest that two Lassen dacites precipitated biotite at a water fugacity of approximately 400 bars. The composition of the later crystallizing ferromagnesian silicates, particularly the pyroxenes which show a wide range in Fe/Mg ratio, is strongly influenced by the prior crystallization of the oxide phases. If the biotite phenocrysts are typical of acid liquids, then they are incapable of generating by fractionation a peraluminous residual liquid; rather they would tend to make a liquid peralkaline.
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TL;DR: In this article, a simple general equation is presented for estimating the Fe 3 § concentrations in ferromagnesian oxide and silicate minerals from microprobe analyses, assuming that iron is the only element present with variable valency.
Abstract: A simple general equation is presented for estimating the Fe 3 § concentrations in ferromagnesian oxide and silicate minerals from microprobe analyses. The equation has been derived using stoichiometric criteria assuming that iron is the only element present with variable valency and that oxygen is the only anion. In general, the number of Fe 3 + ions per X oxygens in the mineral formula, F, is given by; F = 2X(1 - T/S) where T is the ideal number of cations per formula unit, and S is the observed cation total per X oxygens calculated assuming all iron to be Fe 2 § Minerals for which this equation is appropriate include pyralspite and ugrandite garnet, aluminate spinel, magnetite, pyroxene, sapphirine and ilmenite. The equation cannot be used for minerals with cation vacancies (e.g. micas, maghemite) unless, as in the case of amphiboles, the number of ions of a subset of elements in the formula can be fixed. Variants of the above equation are presented for some of the numerous published schemes for the recalculation of amphibole formulae. The equation is also inappropriate for minerals showing SP += 4H § substitution (e.g. staurolite, hydrogarnet), minerals containing an unknown proportion of an unanalysed element other than oxygen (e.g. boron-bearing kornerupine) and minerals containing two or more elements with variable valency.
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TL;DR: In this article, a semi-empirical equation of state for the diopside enstatite miscibility gap has been proposed for ortho-and clinopyroxene solid solutions.
Abstract: Simple mixing models have been applied to ortho- and clinopyroxene solid solutions and a semi-empirical equation of state extracted from the available experimental data for the diopside-enstatite miscibility gap. This equation successfully reproduces the miscibility gap over a temperature range of 800 °C to 1700 °C and is apparently also applicable to aluminous pyroxenes in the system CaSiO3-MgSiO3-Al2O3. The effect of iron solubility in the pyroxenes has been calibrated empirically using most of the available experimental data for multicomponent pyroxenes. This semi-empirical model reproduces most of the experimental data within 70 °C. Temperatures calculated for naturally equilibrated Mg-rich two-pyroxene assemblages deviate markedly from those estimated using the thermometer of Wood and Banno (1973). These discrepancies can be attributed to large inaccuracies in the thermometer of Wood and Banno (1973) for Mg-rich compositions.
1,467 citations
TL;DR: In this article, the authors studied the effect of pre-emptive and preemptive gradients in T and O 2 in a variety of compositionally zoned ash flow tuffs.
Abstract: Every large eruption of nonbasaltic magma taps a magma reservoir that is thermally and compositionally zoned. Most small eruptions also tap parts of heterogeneous and evolving magmatic systems. Several kinds of compositionally zoned ash flow tuffs provide examples of preemptive gradients in T and ƒO2, in chemical and isotopic composition, and in the variety, abundance, and composition of phenocrysts. Such gradients help to constrain the mechanisms of magmatic differentiation operating in each system. Roofward decreases in both T and phenocryst content suggest water concentration gradients in magma chambers. Wide compositional gaps are common features of large eruptions, proving the existence of such gaps in a variety of magmatic systems. Nearly all magmatic systems are ‘fundamentally basaltic’ in the sense that mantle-derived magmas supply heat and mass to crustal systems that evolve a variety of compositional ranges. Feedback between crustal melting and interception of basaltic intrusions focuses and amplifies magmatic anomalies, suppresses basaltic volcanism, produces and sustains crustal magma chambers, and sometimes culminates in large-scale diapirism. Degassing of basalt crystallizing in the roots of these systems provides a flux of He, CO2, S, halogens, and other components, some of which may influence chemical transport in the overlying, more silicic zones. Basaltic magmas become andesitic by concurrent fractionation and assimilation of partial melts over a large depth range during protracted upward percolation in a plexus of crustal conduits. Zonation in the andesitic-dacitic compositional range develops subsequently within magma chambers, primarily by crystal fractionation. Some dacitic and rhyolitic liquids may separate from less-silicic parents by means of ascending boundary layers along the walls of convecting magma chambers. Many rhyolites, however, are direct partial melts of crustal rocks, and still others fractionate from crystal-rich intermediate parents. The zoning of rhyolitic magma is accomplished predominantly by liquid state thermodiffusion and volatile complexing; liquid structural gradients may be important, and thermal gradients across magma chamber boundary layers are critical. Intracontinental silicic batholiths form where extensional tectonism favors coalescence of crustal partial melts instead of hybridization with the intrusive basaltic magma. Cordilleran batholiths, however, result from prolonged diffuse injection of the crust by basalt that hybridizes, fractionates, and preheats the crust with pervasive mafic to intermediate forerunners, culminating in large-scale diapiric mobilization of partially molten zones from which granodioritic magmas separate. Much of the variability among magmatic systems probably reflects the depth variation of relative rates of transport of magma, heat, and volatile components, as controlled in turn by the orientation and relative magnitudes of principal stresses in the lithosphere, the thickness and composition of the affected crust, and variations in the rate and longevity of basaltic magma supply. Extension of the lithosphere may reduce the susceptibility of basaltic magmas to hybridization in the crust, but it can also enhance the role of mantle-derived volatiles in chemical transport.
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TL;DR: In this paper, a simple mixing model of orthopyroxene and garnet solid solutions enables extrapolation of experimentally determined equilibria in the MgSiO3-Al2O3 system to uninvestigated parts of pressure-temperature-composition space.
Abstract: Use of simple mixing models of orthopyroxene and garnet solid solutions enables extrapolation of experimentally determined equilibria in the MgSiO3-Al2O3 system to uninvestigated parts of pressure-temperature-composition space. Apparent discrepancies in the experimental data for simple and multicomponent systems may be explained by considering the effect of CaO and FeO on reducing pyrope activity in the garnet solid solutions. Equilibration pressures of natural garnet-orthopyroxene assemblages may be calculated, provided temperatures are known, from a combination of the experimental data on the MgSiO3-Al2O3 system and analyses of coexisting natural phases. Despite the presence of a compositional gap in the system, the solubility of enstatite in diopside coexisting with orthopyroxene can also be approximately treated by an ideal solution model. An empirical approach has been developed to take account of Fe2+ on the orthopyroxene-clinopyroxene miscibility gap in natural systems in order to calculate equilibration temperatures of 2-pyroxene assemblages. The model presented reproduces almost all of the available experimental data for multicomponent systems to within 60° C.
1,075 citations
TL;DR: In this paper, the partitioning of 25 trace elements between high-silica rhyolitic glass and unzoned phenocrysts of potassic and sodic sanidine, biotite, augite, ferrohedenbergite, hypersthene, fayalite, titanomagnetite, ilmenite, zircon, and allanite has been determined by INAA on suites of samples from the mildly peralkaline lavas and tuff of the Sierra La Primavera, Mexico, and the metaluminous, compo
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TL;DR: In this paper, the compositions of biotites coexisting with sanidine and magnetite were determined for a variety of conditions of furo, fs, and temperature, and the compositions lie in the ternary system KFes2+AlSLOro(OH)r, annite-KMg32+AlSraOro (OII)2, phlogopite-kFe3r+Al SigOu (H-J, ''oxybiotite''.
Abstract: Biotites on the join phlogopite-annite react to form a number of assemblages and the reactions are governed by the independent intensive parameters, temperature, fugacity of HrO (fnro), and fugacity of oxygen (f6 r) . The most common of these assemblages in natural occurrences are biotite-sanidine-hematite; biotite-sanidine-magnetite; and biotite-leuciteolivine-magnetite. The compositions of biotites coexisting with sanidine and magnetite were determined for a variety of conditions of furo, fs, and temperature. The compositions lie in the ternary system KFes2+AlSLOro (OH)r, annite-KMg32+AlSraOro (OII)2, phlogopite-KFe3r+AlSigOu (H-J, \"oxybiotite.\" Application of regular solution theory to KFerAlSiaOro(OH)s in ternary solid solution yields the following relationship:
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06 Dec 1960
TL;DR: In this article, structural formulas calculated for more than 200 published analyses of phlogopites, biotites, siderophyllites, and lepidomelanes indicate that the additional positive charges carried by trivalent cations proxying for bivalent Mg in the octahedral group of trioctahedral micas are accommodated in two different ways, (1) by a positive charge on the octagonal layer, which is neutralized by an equivalent increase in the negative tetrahedral charge (increased replacement of Si by Al), and all
Abstract: Structural formulas calculated for more than 200 published analyses of phlogopites, biotites, siderophyllites, and lepidomelanes indicate that the additional positive charges carried by trivalent cations proxying for bivalent Mg in the octahedral group of trioctahedral micas are accommodated in two different ways, (1) by a positive charge on the octahedral layer, which is neutralized by an equivalent increase in the negative tetrahedral charge (increased replacement of Si by Al), and all the octahedral sites are occupied, or (2) are neutralized by negative charges associated with unoccupied octahedral sites. In almost all the structural formulas the octahedral group had a positive charge and octahedral occupancy was less than 3.00, indicating some degree of accommodation by both methods. The degree to which accommodation is made by (1) or (2) varies greatly. In general, however, there is a greater accommodation by (2) than by (1) the greater the octahedral trivalent cation content. As a result, there is also a general decrease in octahedral occupancy with increase in octahedral trivalent cation content. In most biotites, siderophyllites, and lepidomelanes octahedral occupancy is significantly less than 3.00 sites hence they are not truly trioctahedral, nor are they octaphyllites. The few formulas in which trivalent cations occupy more than one octahedral site suggests that this is the essential limit of replacement of R+2 by R+3 in these micas. It is also strong evidence against the existence of a complete series between phlogopite and muscovite. Coincident with replacement of Mg by E+3, there is also replacement of Mg by Fe+2, ion for ion. With few exceptions the calculated formulas show both types of octahedral replacement. But the two types, although coincident, are independent; there is no relation between the amount of E+3 present and the amount of Fe+2 present. Neither type of replacement forms a separate series, as a series of Fe+2 only replacing Mg, of which Winchell's annite is the theoretical end member. No representative of this end member was found among the more than 200 analyses collected. On the evidence of the analyses and their calculated formulas, an octrahedral occupancy of more than 2.20 positions by Fe+2 is not to be expected. The only trioctahedral micas in which more than 90 percent of the octahedral positions are occupied by one species of cation are some phlogopites. From pure phlogopite as the prototype, the composition of all other trioctahedral micas may be derived by replacement of Mg by, most commonly, Fe+2 and R+3 (Al and Fe+3). In phlogopites the proxying of such cations for Mg is minor, and Mg occupies more than 70 percent of the occupied positions. Progressively greater proxying of these other cations for Mg leads successively to Mg biotites, in which Mg is still the dominant octahedral cation but in which Fe+2 is present in significant amounts, Fe+2 biotites, in which Fe+2 is the dominant octahedral cation, with Mg present in subordinate but significant amounts, and siderophyllites and lepidomelanes, in which Mg is essentially absent, with Fe+2 the greatly dominant bivalent octahedral cation and with significant amounts of aluminum and (or) ferric iron. These relations are expressed in the following formulas, which show the range in composition of the different groups:
501 citations