George B. Morgan

Optimizing the electron microprobe analysis of hydrous alkali aluminosilicate glasses

Abstract The time-dependent loss of NaKα X-ray intensity during electron-beam irradiation of hydrous alkali alumino silicate glasses is apparently more significant during the initial few seconds of beam exposure than it is for anhydrous glasses, and it is pronounced for incident beam currents > 2-5 nA (using 15-20 μm beam diameters). Exponential fits of NaKα intensity vs. time show a progressive decrease in the apparent zero-time intercepts for incident beams from 2 to 20 nA, and thus methods for correcting Na concentrations solely on the basis of curve fitting and extrapolation to zero-time values may underestimate Na contents by almost 10% (relative) for higher beam currents. Similar exponential fits to the intensity-time data for AlKα and SiKα show that “grow-in” is greater for Al than for Si. For incident currents ≥ 5 nA, the magnitudes of all intensity changes also increase with total H2O content of glass. On the basis of these observations, the optimal conditions for analysis of hydrous alkali aluminosilicate glasses include a 2 nA beam with 20 μm diameter and counting times of 20-40 s for the analysis of alkali aluminosilicate components, with Na and Al analyzed first (simultaneously, if possible). These methods minimize Na loss and grow-in for Al and Si to the point that little or no correction is needed, provide good statistical accuracy, and work with a wide variety of standard materials (i.e., glass standards with compositions and H2O contents comparable to the unknowns are not needed). For complete analysis of more complex multicomponent systems, two beam conditions are recommended: an initial 2 nA, 20 μm diameter beam for analysis of alkali alumino silicate components, followed by a 20 nA, 20 μm diameter beam for analysis of all other components. With the use of these methods, the H2O contents of hydrous glasses (H2O as the only unknown) can be determined by difference with uncertainties mostly <5% (relative to FTIR values) for glasses containing up to 10 wt% H2O. At beam currents > 5 nA, corrections for Na loss ignoring Al (and Si) grow-in underestimate H2O contents by about 10-50% of concentration.

Vapor-undersaturated experiments with Macusani glass+H2O at 200 MPa, and the internal differentiation of granitic pegmatites

Vapor-undersaturated fractional crystallization experiments with Macusani glass (macusanite), a peraluminous rhyolite obsidian, at 200 MPa yield mineralogical fabrics and zonation, and melt fractionation trends that closely resemble those found in zoned granitic pegmatites and other granitoids of comparable composition (typically peraluminous, Li-Be-Ta-rich deposits). The zonation from the edge of charges inward is characterized by: (1) fine-grained sodic feldspar-quartz border zones; (2) a fringe of very coarse-grained graphic quartz-feldspar intergrowths that flair radially toward melt and terminate with nearly monophase K-feldspar; (3) cores of very coarse-grained, nearly monominerallic quartz or virgilite (LiAlSi5O12)±mica; and (4) late-stage, fine-grained albite+mica intergrowths that are deposited from alkaline, Na-rich interstitial melt at vapor saturation. Similar experimental products have been observed in compositionally simpler, less evolved systems. Liquid lines of descent from initially H2O-undersaturated runs are marked by a decrease in SiO2, and increases in Na/K, B, P, F, H2O, and a variety of trace lithophile cations. These trends are believed to be governed by three factors: (1) disequilibrium growth of feldspars (±quartz) via metastable supersaturation; (2) fractionation of melt toward SiO2-depleted, Na-rich compositions due to increases in B, P, and F; and (3) changes in nucleation and growth rates, mostly as a function of the H2O content of melt (Xwm). In contrast, experiments that are cooled below the liquidus from the field of melt+aqueous vapor (London et al. 1988) fail to replicate pegmatitic characteristics in most respects. On the basis of these and other experiments, we suggest that the formation of pegmatite fabrics stems primarily from fractional crystallization in volatile-rich melts, and that enrichments in normally trace lithophile elements result from melt differentiation trends toward increasingly alkaline, silica-depleted compositions. Although vapor saturation at near-solidus and subsolidus conditions may promote extensive recrystallization, an aqueous vapor phase does not appear to be necessary for the generation of most of the salient characteristics of pegmatites.

Effect of current density on the electron microprobe analysis of alkali aluminosilicate glasses

Abstract In a previous work (Morgan and London 1996), we proposed an optimized procedure for electron microprobe analysis (EMPA) of rhyolitic glasses using a broad (20 μm diameter), low-current (2 nA) fixed beam. Some important applications for EMPA of glass, such as vitreous inclusions in minerals and experimental run products, require smaller beam diameters that produce greater areal current densities (expressed as nA/μm2). For these situations, we have assessed the effect of areal current density on the migration of Na and its concomitant effects on other elements and their ratios during EMPA of granitic glasses. Anhydrous and hydrous glasses of a haplogranite composition (Ab38.23Or29.31Qtz33.37C0.10) were analyzed at 20 kV accelerating potential, using 2.50 nA beam currents, fixed beam diameters of 2.20 μm, and counting times scaled to yield similar analytical uncertainty at each condition (~2.6% relative for Na2O). There is almost no loss of Na (≤1.7.2.7% relative) using a current density of 0.006 nA/μm2, minor (7.9%) Na loss for current densities up to 0.1 nA/μm2, and increasing Na loss with higher current densities that becomes severe at >0.5 nA/μm2 (e.g., 48.63% relative loss from hydrous glass at 50 nA and 2 μm during 3.6 s of irradiation). Sodium migration is more pronounced in hydrous glasses than in anhydrous ones, with significant loss from hydrous glass occurring during the first second of irradiation. The migration of Na results in increased concentrations of Al and Si, but little or no change in the concentration of K; if not fully corrected for, these effects produce systematic errors in important elemental ratios. With current densities <0.01 nA/μm2, anhydrous glasses or crystalline materials are suitable standards and data correction may not be needed. Significant Na loss using current densities up to ~0.1.0.2 nA/μm2, especially in hydrous glasses, requires data correction or primary standardization utilizing a glass having composition and water content similar to the unknown. Current densities ≥0.5.1.0 nA/μm2 are not suitable for EMPA of glass because of large and uncertain corrections (~25% to >100% of the Na2O value obtained). The correlation of analytical condition (beam current and diameter) with current density and EMPA results provided here allows analysts to select beam conditions that optimize the quality of analyses. When current densities >~ 0.01 nA/μm2 must be used (e.g., with beam spot sizes <20 μm), the results can lead to improved estimates of the systematic errors due to alkali migration. Natural and some experimental glasses contain a variety of other minor components among which Ca and Fe are important, and so the discussion of analytical methods is extended to more complex compositions. For example, Na migration is accelerated as glass structures become less polymerized than those of simple tectosilicate stoichiometry (e.g., due to increasing alkalinity and/or the presence of fluxing components such as F, Cl, B). Analysis using 20 kV accelerating voltage, as opposed to 15 kV, both slightly decreases Na migration and improves limits of detection and statistical accuracies for minor components such as Fe while providing reasonable beam penetration depths.

Experimental reactions of amphibolite with boron-bearing aqueous fluids at 200 MPa: implications for tourmaline stability and partial melting in mafic rocks

Reactions between hornblende-plagioclase amphibolite and acidic and alkaline B-bearing aqueous fluids have been investigated by experiments at 475°–600° C and 200 MPa. At 600° C, hornblende+calcic plagioclase react to form tourmaline+danburite+clinopyroxene+quartz in acidic fluids containing ≥0.5–1.0 wt% B2O3.Tourmaline is precipitated directly from acidic fluids, and the reaction is driven by neutralization of fluids by Na±Ca derived from the breakdown of reactant solids. The concentration of B2O3 in fluids needed to stabilize tourmaline increases as pH increases (above approximately 6.0), and tourmaline is unstable in alkaline fluids (pH > approximately 6.5–7.0) regardless of B concentration. In addition to acid-base relations, tourmaline stability is favored by comparatively higher activity coefficients for Al species in acidic fluids. The concentrations of Al and Si in fluid increase with alkalinity, with the eventual production of felsic borosilicate melts through partial melting of the plagioclase component of the amphibolite. In seeded experiments, tourmaline also contributes components to melt. Partial melting is evident in the range 500°–525° C at 200 MPa in experiments with ≥8wt% B2O3 in fluid as Na2B4O7. The experimental results are applied primarily to metasomatic reactions between mafic rocks and borate fluids derived from granitic magmas, but tourmaline stability and partial melting in mafic regional metamorphic systems are also discussed briefly.

Experimental study of titanite-fluorite equilibria in the A-type Mount Scott Granite: Implications for assessing F contents of felsic magma

Titanite and fluorite stability in melt were experimentally evaluated at 850 °C, 200 MPa, f(O 2 ) ≈ NNO (nickel-nickel oxide oxygen buffer) as functions of total F and H 2 O content. Experiments employed the metaluminous Mount Scott Granite of the Wichita igneous province, Oklahoma. Over a large range of added H 2 O (∼1–7 wt%), melts containing 1 wt% F precipitated fluorite without titanite. In addition, at high F (≥ 1.2 wt%) plagioclase and hornblende reacted to form biotite. Thus, an increase in F during crystallization may explain the observed higher modal abundance of plagioclase and hornblende in titanite-dominant samples vs. higher modal biotite in fluorite-dominant samples within the Mount Scott Granite pluton. Coexistence of magmatic titanite and fluorite in the Mount Scott Granite pluton implies F m of ∼1 wt% at the point in its crystallization history where these minerals coprecipitated. We suggest that the presence of primary fluorite within high-temperature, shallowly emplaced, moderate f (O 2 ), subaluminous felsic rocks indicates high magmatic fluorine, whereas titanite without fluorite in such rocks indicates low initial fluorine.

Amblygonite-montebrasite solid solutions as monitors of fluorine in evolved granitic and pegmatitic melts

Abstract The distribution of F between amblygonite (Amb, LiAlPO4F)-montebrasite (Mbr, LiAlPO4OH) solid solutions and metaluminous haplogranitic melt has been calibrated at 585 °C and 200 MPa H2O. The partition coefficient for F between the crystalline phase and melt, DFMbr/melt, is linear between 0 to ~10 wt% F in amblygonite, which contains 13 wt% F at the end-member: CFMbr = 3.65CFmelt + 0.07, r2 = 0.995, n = 6. Values of DFAmb/melt decrease sharply above 10 wt% F in amblygonite as the amblygonite reaches saturation in F at 200 MPa H2O. In natural occurrences, however, the vast majority of primary amblygonite- montebrasite solid solutions contain ~4-7 wt% F, well within the linear range of the calibrated exchange reaction, and the montebrasite-bearing assemblages are among the last to crystallize. If the F contents of the montebrasite are magmatic, then these most-fractionated residual melts of the LCT (Li-Cs-Ta, and mostly peraluminous S-types) rare-element class generally contained up to ~1.0-1.8 wt% F near the end of their crystallization. The modest F contents of pegmatites are consistent with the common association of Li aluminosilicates and with the general paucity of topaz in these occurrences. In topaz-bearing granites of Western Europe, however, high-F amblygonite (~10- 11 wt% F) reflects >3 wt% F in melt during crystallization of these magmatic phases.

Synthetic Ag-rich tourmaline: Structure and chemistry

Abstract Ag-rich tourmaline crystals were synthesized at 750 °C, 200 MPa H2O, and fO₂ = log (NNO) - 0.5, starting with an oxide mix of dravite composition to which various reagents, including AgF and AgCl, were added. Tourmaline containing up to 7.65 wt% Ag2O was synthesized, and this is the first time a tourmaline is described that contains significant amounts of Ag at the ninefold-coordinated X site. Crystal structure refinement and chemical analysis (EMPA) give the optimized formula X(Na0.58Ag0.18□0.24) Y(Al1.54Mg1.46) Z(Al5.34Mg0.66) T(Si5.90Al0.10)O18 (BO3)3V(OH)3W(O0.53F0.47), with a = 15.8995(4) and c = 7.1577(4) Å, and R = 0.036 for a crystal (~20 × 100 μm) with approximately 2.2 wt% Ag2O. Refining Na ↔ Ag at the X site clearly indicates that Ag occupies this site. The X-O2 distance of ~2.52 Å is slightly longer than tourmaline with ~(Na0.6□0.4), reflecting the slightly larger ionic radius of Ag compared to Na. Releasing the occupancy at the Y site gives ~Al0.98 (~12.7 e.), which can be explained by occupation of Mg and Al. On a bond-angle distortion vs. distance diagram, the Ag-rich olenite- dravite lies approximately on the V site = 3 (OH) line in the figure, defining the relation between bond-angle distortion (σoct2) of the ZO6 octahedron and the distance. No H could be found at the O1 site by refinement, in agreement with the Mg-Al disorder between the Y site and the Z site. Synthetic tourmaline contains no Ag when only AgCl is added; the compatibility of Ag in tourmaline, therefore, is largely a function of the F/Cl ratio of the fluid medium. A positive association of Ag at the X site with Al at the Y site and with F suggests that tourmaline might be useful for exploration in Cornwall-type polymetallic ore deposits associated with F-rich peraluminous granites or at other Ag-, F-, and B-enriched deposits such as Broken Hill, Australia. Preliminary electron microprobe analyses of tourmaline from Cornwall and Broken Hill, however, failed to detect Ag at the 3σ detection level of 0.08 wt% Ag2O.

Process of granophyre crystallization in the Long Mountain Granite, southern Oklahoma

The primary objective of this study is to assess the origin of granophyre in the A-type Long Mountain Granite of southwestern Oklahoma. The magma was emplaced as a thin sheet of crystal-poor, water-undersaturated liquid at or near the base of ∼1.4-km-thick, comagmatic rhyolite cover. Phenocrysts of anorthoclase and quartz were the first products of crystallization, and skeletal to subhedral morphologies indicate their growth involved ∼50 °C of thermal and constitutional undercooling. This initial crystallization, ∼50 vol% of the magma in a wt% ratio of anorthoclase:quartz ≈3:1, resulted from decompression and concomitant migration of the alkali feldspar-quartz cotectic boundary. This residual magma represents the liquid present when crystallization culminated with granophyric intergrowth of quartz and alkali feldspar. Sluggish diffusion of silica away from the growth surfaces of anorthoclase phenocrysts resulted in boundary-layer liquids that were supersaturated in quartz, producing initial granophyric intergrowths in which the modal proportions of quartz were greater than that of the liquid composition. From initial quartz-rich compositions, intergrowths do not follow a consistent trend of fractionation. Instead, following variable or oscillating chemical pathways, they approach and hover around a composition appropriate to the pressure and low water content of magma at the emplacement level. Compositional trends of granophyre demonstrate that the intergrowth records a process in which its growth rate exceeded the rate at which aluminosilicate components could diffuse away through magma. Granophyre in both nature and experiment is formed under a variety of pressures and activities of water but in all cases involves the simultaneous crystallization of quartz and alkali feldspar from a viscous granitic liquid that is cooled well below its liquidus temperature. The estimated undercooling for granophyre growth at Long Mountain, and observed in experiments with granitic compositions, is in the range of 70–150 °C. This magnitude of undercooling is similar to that which produces graphic granite, and it is in the cooling regime between spherulitic obsidian (or felsite) and hypidiomorphic-granular granite.

A spreadsheet for calculating normative mole fractions of end-member species for Na-Ca-Li-Fe2+-Mg-Al tourmalines from electron microprobe data

Abstract This work presents a spreadsheet that calculates the mole fractions of end-member components for simple Na-Ca-Li-Mg-Fe2+-Al tourmalines from electron microprobe data. The input includes the B2O3 concentration obtained either from direct analysis or by estimation on the basis of stoichiometry. The concentration of Li2O can either be input from other analysis or estimated by the spreadsheet. The spreadsheet does not address the mole fractions of Cr, V, oxidized or deprotonated tourmaline species, nor account for species involving tetrahedral boron or aluminum. Therefore, the spreadsheet is not a comprehensive tool that includes all IMA approved tourmaline species, and so is not intended for naming tourmalines according to IMA convention. The present method includes a useful subset of end-member species that can be described simply from electron microprobe data and so, akin to a normative mineralogical analysis for rock composition, the calculations are intended to provide a normative result that serves as simple basis for comparing tourmalines that is more direct than names derived from the most abundant species present.

Experimental Crystallization of the Macusani Obsidian, with Applications to Lithium-rich Granitic Pegmatites

Experiments at 200 MPa with the peraluminous (S-type) rhyolite obsidian from Macusani, Peru, asses the dynamics of crystallization starting with non-vesicular solid glass cores as compared to earlier experiments starting with powders of the same composition. Textures, spatial zonation of feldspars and quartz, and their compositional relations are substantively different and more clearly revealed in the solid-core experiments. The new experiments with solid cores possess more sharply bounded segregation of feldspathic and quartz-rich domains of crystallization, a shift from a predominance of feldspars to increasing mica with initial H2O content >6 wt %, and the simultaneous crystallization of solvus pairs of plagioclase and alkali feldspar at opposite ends of the melt volume. As in other similar studies, the maximum in the rate of crystallization, a function principally of crystal growth rate, occurs at a liquidus undercooling of 200 6 50 C. Both experimental studies with the Macusani obsidian apply to the chemical, textural, and spatial zonation of minerals within granitic pegmatites, particularly the Li-rich peraluminous pegmatites of S-type granite affinity. The new results have now reproduced and can account for the following features of pegmatites: (1) feldspathic outer zones and quartz-rich to pure quartz cores; (2) massive fine-grained border zones, followed by coarsening wall zones with unidirectional solidification texture, culminating in central domains of more isotropic fabric and coarse grain size; (3) locally, alternating laminations of mineral assemblages as in layered pegmatites and layered aplites; (4) a steady decrease in the An content of plagioclase from margin to core via subsolidus isothermal fractional crystallization; (5) spatial segregation of plagioclase and alkali feldspar along opposite margins of the melt body via far-field chemical diffusion; (6) an inward increase in the size of individual crystals by 10; (7) albiteþ lepidolite bodies as the latest primary assemblage, and following the crystallization of pure quartz bodies. All of these experimental results followed from the appreciable undercooling of the melt prior to the onset of crystallization. All of the features cited, except for the formation of miarolitic cavities, are entirely igneous in origin, owing nothing to the simultaneous occurrence of an aqueous solution.