Beth Goldoff

Characterization of fluor-chlorapatites by electron probe microanalysis with a focus on time-dependent intensity variation of halogens

Abstract Prior research has shown that fluorine and chlorine X-ray count rates vary with exposure to the electron beam during electron probe microanalysis (EPMA) of apatite. Stormer et al. (1993) and Stormer and Pierson (1993) demonstrate that the EPMA-operating conditions affect the halogen intensities in F-rich natural Durango and Wilberforce apatites and in a Cl-rich apatite. Following these studies, we investigated the effects of operating conditions on time-dependent X-ray intensity variations of F and Cl in a broad range of anhydrous fluor-chlorapatites. We tested 7, 10, and 15 kV accelerating voltages; 4, 10, and 15 nA beam currents; 2, 5, and 10 μm diameter fixed spot sizes; and the influence of 2 distinct crystal orientations under the electron beam. We find that the halogen X-ray intensity variations fluctuate strongly with operating conditions and the bulk F and Cl contents of apatite. We determined the optimal EPMA operating conditions for these anhydrous fluor-chlorapatites to be: 10 kV accelerating voltage, 4 nA beam current (measured at the Faraday cup), 10 μm diameter fixed spot, and the apatite crystals oriented with their c-axes perpendicular to the incident electron beam. This EPMA technique was tested on a suite of 19 synthetic anhydrous apatites that covers the fluorapatite-chlorapatite solid-solution series. The results of these analyses are highly accurate; the F and Cl EPMA data agree extremely well with wet-chemical analyses and have an R2 value >0.99.

Experimental and modeled chlorine solubilities in aluminosilicate melts at 1 to 7000 bars and 700 to 1250 °C: Applications to magmas of Augustine Volcano, Alaska

Abstract Hydrothermal experiments were conducted at ca. 1 to 7000 bars and 700 to 1250 °C in 121 rhyolitic to basaltic systems to determine Cl solubility in silicate melts, i.e., the maximum Cl concentration in melts that are saturated in a hydrosaline liquid with or without an aqueous or aqueous-carbonic vapor. The Cl concentration of melts increases with the Cl contents of the fluid unless the melt coexists with vapor plus hydrosaline liquid at fixed pressure and temperature; this phase assemblage buffers the Cl content of each phase with increasing Cl in the system. The Cl content of fluid(s)-saturated melts is independent of the CO2 concentration of the saline liquid ± vapor with up to 21 wt% CO2 in the fluid(s). The experiments show that Cl dissolution in aluminosilicate melts increases with temperature and pressure. Chlorine solubility is also a function of melt composition; it increases with the molar ([Al1/2+Ca1/2+Mg1/2+Na]/Si) of the melt. These experimental data have been integrated with results involving 41 other experiments (Webster and De Vivo 2002) to develop a broadly expanded model that supports calculation of Cl solubility in 163 aluminosilicate melts. This empirical model applies to Cl dissolution in melts of most silicate magmas at depths as great as 25 km. It determines the exsolution of hydrosaline liquid, with or without a coexisting vapor, as magmas ascend from depth, cool, crystallize, and differentiate from mafic to felsic compositions. In combination with H2O solubility models, our model supports determination of H2O-Cl solubility relations for most aluminosilicate magmas and is useful for barometric estimations based on silicate melt inclusions containing low CO2 and moderate to high-Cl concentrations. The model is applied to the phase relations of fluids in volatile-enriched magmas of Augustine volcano, Alaska. The Cl and H2O concentrations of melt inclusions from 14, basaltic to dacitic eruptive units are compared with modeled solubilities of Cl and H2O in Augustine melts. The majority of these eruptions involved magmas that first exsolved aqueous to aqueous-carbonic vapors when the melts were dacitic in composition (i.e., before the residual melts in these magmas had evolved to felsic compositions) and well prior to the eruptions. Hydrosaline liquid with or without a vapor phase exsolved from other, more-felsic fractions of Augustine melts at low, near-surface pressures of several tens of bars.

Hydroxyl, Cl, and F partitioning between high-silica rhyolitic melts-apatite-fluid(s) at 50–200 MPa and 700–1000 °C

Abstract Hydrothermal experiments were conducted with fluid- and apatite-saturated, high-silica rhyolitic melts at ca. 700–1000 °C and 50–200 MPa to determine the distribution of H2O/OH, Cl, and F between melt, apatite, aqueous vapor, brine, or vapor plus brine. Seed grains of fluorapatite (1–3 µm diameter) were added to starting charges to serve as apatite nucleation sites. CaHPO4 and Ca(OH)2 were used to stimulate apatite crystallization, and temperature was cycled daily, ±10 to ±15 °C, to promote growth of relatively equant apatite crystals large enough for electron probe microanalysis (EPMA). The experiments were conducted with gold capsules and run in cold-seal pressure vessels on a hydrothermal line and an internally heated gas pressure vessel for durations of 165 to 1149 h. The run-product glasses were analyzed by EPMA and Fourier transform infrared spectroscopy, apatites by EPMA, and most fluid phases by chloridometer; Cl contents of fluids were also estimated by mass-balance calculations. The fluids contained 0.3–39 wt% Cl at run conditions. Most experiments were conducted at 50 MPa, and these glasses contain 0.02–0.42 wt% Cl, 1.8–3.1 wt% H2O, and 0.01–0.19 wt% F. The molar Al2O3/(CaO+Na2O+K2O) (=A/CNK) and molar Na2O/(Na2O+K2O)(=N/NK) ratios of the 50 MPa glasses range from 0.88 to 1.04 and 0.48 to 0.68, respectively, and straddle the A/CNK and N/NK of the starting glass (0.99 and 0.59, respectively). The measured wt% Cl and F in the 50 MPa apatites range from 0.14 to 3.8 ( XClAp

Partitioning behavior of chlorine and fluorine in felsic melt–fluid(s)–apatite systems at 50MPa and 850–950°C

X_{{\text{Cl}}}^{{\text{Ap}}}

C–O–H–S fluids and granitic magma: how S partitions and modifies CO2 concentrations of fluid-saturated felsic melt at 200 MPa

of 0.02 to 0.56) and 0.32 to 2.4 ( XFAp

Augustine Volcano-The Influence of Volatile Components in Magmas Erupted A.D. 2006 to 2,100 Years Before Present

X_{\text{F}}^{{\text{Ap}}}

Compositions of biotite, amphibole, apatite and silicate melt inclusions from the Tongchang mine, Dexing porphyry deposit, SE China: Implications for the behavior of halogens in mineralized porphyry systems

of 0.08 to 0.63), respectively. Stoichiometrically constrained XOHAp

Cl-RICH FLUORAPATITE, DEVOID OF OH, FROM THE THREE PEAKS AREA, UTAH: THE FIRST REPORTED STRUCTURE OF NATURAL Cl-RICH FLUORAPATITE

X_{{\text{OH}}}^{{\text{Ap}}}

A glimpse into Augustine Volcano's Pleistocene past: Insight from the petrology of a massive rhyolite deposit

ranges from 0.14 to 0.7. Partition and exchange coefficients were determined for OH, Cl, and F distribution between apatite and melt±fluids. The distribution of these volatile components varies with pressure and melt and apatite compositions. The exchange of F and Cl between apatite and melt, for example, fluctuates with the Si, P, Mg, Na, Ce, Fe, and S±Ca contents of the apatite and with the molar A/CNK and N/NK ratios of the melts. Water and hydroxyl exchange between experimental apatite and melt was also investigated. It is determined empirically that the: XH2Omelt/XClmelt

Augustine Volcano - The influence of volatile components in magmas erupted A.D. 2006 to 2,100 years before present: Chapter 16 in The 2006 eruption of Augustine Volcano, Alaska

X_{{{\text{H}}_{\text{2}}}{\text{O}}}^{{\text{melt}}}{\text{/}}X_{{\text{Cl}}}^{{\text{melt}}}