James D. Webster

Chloride and water solubility in basalt and andesite melts and implications for magmatic degassing

Abstract The solubilities of chloride (Cl − ) and H 2 O in aluminosilicate melts of basalt, andesite, and latite compositions saturated in aqueous vapor and/or hydrosaline liquid were determined at 2000 bars and ≈1 bar by melting mixtures of NaCl, KCl, H 2 O, and natural and synthetic rock powders and by measuring Cl − and H 2 O in the run product glasses. The abundances of Cl − in several of the aqueous run product liquids were also measured, and the partitioning of Cl − between aqueous vapor and silicate melt was determined for these experiments. Chloride is highly soluble in H 2 O-poor melts. Maximum Cl − solubilities range from 2.9 wt.% in molten basalt to 1 wt.% in molten latite at relatively high oxygen fugacities, 1040°C to 1210°C, and 2000 bars. The solubility of Cl − varies directly with pressure and the molar ((Al+Na+Ca+Mg)/Si) ratio of aluminosilicate melts. Chloride solubility in basalt melt is an order of magnitude greater than that in silicic melts, so the role of Cl − in driving the exsolution of vapor and/or liquid from magma will increase dramatically as mafic, H 2 O- and CO 2 -undersaturated magmas fractionate and evolve to more silicic compositions. The solubility of H 2 O in silicate melts saturated in aqueous vapor and/or hydrosaline liquid varies inversely with Cl − content. Chloride has little effect on H 2 O solubility with up to about 1.9 wt.% Cl − in melt because the coexisting vapor phase contains little Cl − . Hydrosaline liquid is stable with higher Cl − contents in melt, and H 2 O solubility is highly sensitive to Cl − content at these conditions. This relationship is a result of highly nonideal mixing of H 2 O and Cl − at magmatic temperatures; in several Cl − -enriched andesite experiments, immiscible vapor and hydrosaline liquid are apparently stable instead of a single Cl − -bearing volatile phase. At 2000 bars, Cl − -bearing aqueous vapor exsolves with − in the andesite melt, vapor and hydrosaline liquid exsolve with 1 to 2 wt.% Cl − and 2 O in melt, and only hydrosaline liquid exsolves if the andesite melt contains deletion ≥2 wt.% Cl − and 2 O. At 2000 bars and temperatures near 1100°C, the distribution coefficients [D Cl = (wt.% Cl − in aqueous vapor/wt.% Cl − in silicate melt)] for basalt and andesite range from 0.9 to 6 for coexisting aqueous vapors containing 1 to 11 wt.% Cl − , respectively. Silicate melt inclusions in phenocrysts from most basalts and andesites contain − implying that, at these conditions, only Cl-bearing vapor (not vapor and hydrosaline liquid) will exsolve from most basalt and andesite magmas and that the Cl − contents of the aqueous vapors will be

Determination of molar absorptivities for infrared absorption bands of H2O in andesitic glasses

Abstract We have determined infrared molar absorptivities for water absorption bands in Fe-bearing and Fe-free andesitic glasses. Water dissolves in andesitic glasses as both hydroxyl groups and molecular water as observed in other silicate glasses. Concentrations of molecular water and hydroxyl species are a strong function of total water content. IR molar absorptivities for Fe-bearing andesite are ε3570 = 62.32 ± 0.42 L/mol·cm, ε4500 = 0.79 ± 0.07 L/mol·cm, ε5200 = 1.07 ± 0.07 L/mol·cm, and ε1630 = 42.34 ± 2.77 L/mol·cm. Molar absorptivities for Fe-free andesite are 69.21 ± 0.52 L/mol·cm for e3570, 0.89 ± 0.07 L/mol·cm for e4500, 1.46 ± 0.07 L/mol·cm for e5200, and 52.05 ± 2.85 L/mol·cm for ε1630. Molar absorptivities show significant compositional dependencies that can be predicted based on tetrahedral cation (Si+4, Al+3)/total cation fraction

Experimental and modeled solubilities of chlorine in aluminosilicate melts, consequences of magma evolution, and implications for exsolution of hydrous chloride melt at Mt. Somma-Vesuvius

Abstract Solubility experiments were conducted with forty-one aluminosilicate rock compositions to determine how extensively Cl dissolves in hydrous chloride melt- ± vapor-saturated silicate melts containing low to moderate water contents at 2000 bars. Chlorine solubility in most silicate melts is dominated by the abundances of Mg ≈ Ca > Fe > Na > K > network-forming Al > Li ≈ Rb ≈ Cs, but Ti, F, and P also have strong influences. The relationship of composition to Cl solubility is more complex in peraluminous and peralkaline felsic melts, because network-modifying Al, Na, and K have a greater influence than their network-forming counterparts. Also, the effects of Ca, Mg, and Al in mafic melts characterized by high (Ca + Mg + Al)/(Na + K + Li) are much greater than their effects in silica-enriched melts. Association coefficients that express the influence of each ion on Cl solubility were determined, and the solubility data and coefficients were employed to develop a model that predicts Cl solubility at 2000 bars for water-undersaturated melts ranging from rhyolite to basalt. The coefficients were also used to investigate the predominant chloride complexes in melt, and the bulk of the solubility data are consistent with the interaction of Cl with alkaline-earth metals that provide charge balance for network-forming Al. The Cl solubility model is applied to Mt. Somma-Vesuvius magmas as they evolved from phonotephrite to phonolitic compositions, via fractional crystallization, to investigate the role of Cl in magmatic degassing. The results clearly demonstrate that Cl solubility was dramatically reduced by subtle changes in melt composition. Decreasing abundances of Ca, Mg, and Fe in the residual melt induced a dramatic reduction in Cl solubility that occurred simultaneously with gradual increases in the abundance of volatiles in melt due to crystallization of volatile-free minerals. The increasing abundance of volatiles and concurrent reduction in Cl solubility may have forced the exsolution of a hydrous chloride melt directly from the Cl-enriched mafic magmas. It is likely that the exsolution of hydrous chloride melt may occur in other Cl-enriched magmas, because Cl solubility depends so strongly on melt composition.

FLUID-MELT INTERACTIONS INVOLVING CL-RICH GRANITES : EXPERIMENTAL STUDY FROM 2 TO 8 KBAR

Experiments have been conducted in the system Na2O-K2O-Al2O3-SiO2-H2O-Cl2O−1at 2, 4 kbar (800 and 1000°C) and 6, 8 kbar (800°C) to determine thermodynamic constraints on fluid-melt interaction and to determine the effects of pressure (P) and temperature (T) on Cl partitioning between aqueous fluid and subaluminous and peralkaline haplogranite melts. The Cl concentration of the run product glasses was determined by electron microprobe, and the H2O concentration of some glasses was determined by ion microprobe. The Cl concentration of the aqueous fluid was computed by mass balance and was also analyzed by chloridometer for several runs; agreement between the two methods is within ±9 relative %. DCl (wt% Cl in aqueous fluid/wt% Cl in granite melt) increases strongly with increasing P for P < 6 kbar, increasing concentrations of Cl in the system, and decreasing T. Previous work, however, indicates the effect of T on DCl in F-bearing granites within this P–T range is opposite to that observed here. The experimental results indicate that the concentration of Cl in granitic melts may reach a maximum limiting value as Cl-rich magmas crystallize. Granitic systems exhibiting a maximum concentration of Cl in the melt must either contain crystalline alkali chlorides, molten alkali chlorides, or coexisting liquid plus vapor or the melt must contain the solubility limit for Cl. The experimental data suggest that at 2 kbar and 800°C a single, supercritical fluid is stable, if the fluid contains ≤67 wt% NaCl and KCl. Conversely, at 2 kbar and 1000°C two immiscible phases (i.e., saline liquid and relatively alkali chloride-poor vapor) coexist with haplogranite melt if the combined liquid and vapor contain from approximately 10 to 55 wt% NaCl and KCl. Computed activities of H2O in the experimental melts (Burnham, 1981; Nekvasil, 1986) constrain the activity of H2O in the associated saline fluids at magmatic P and T. At 2 kbar and 800°C, H2O behaves ideally with up to 10 mol% NaCl and KCl in the aqueous fluid. As the NaCl and KCl concentration of the aqueous fluid increases from 10 to 40 mol%, the activity of H2O exhibits small but increasingly positive deviations from ideality. Application of experimental data to Cl-rich and F-poor, mineralizing granitic systems suggests that Cl will be most strongly enriched in ore fluids at relatively low temperature (between 800 and 1000°C) and relatively high pressure (for P < 6 kbar). The data also suggest that inasmuch as granitic systems contain a single aqueous fluid at T ≤ 800°C, 2 kbar, and with ≤67 wt% NaCl and KCl in fluid, the deposition of ore minerals from such fluids cannot be a direct result of boiling.

Water solubility and chlorine partitioning in Cl-rich granitic systems: Effects of melt composition at 2 kbar and 800°C

Abstract Experiments have been conducted in the system Na2O-K2O-Al2O3-SiO2-H2O-Cl2O−1 at 2 kbar and 800°C to determine the effects of melt composition on the solubility of water in haplogranite melt and on the distribution of Cl between aqueous fluid and haplogranite melt. The melts studied include peraluminous, subaluminous and peralkaline haplogranites and quartz-albite and quartz-orthoclase compositions. The solubility of water at 2 kbar and 800°C in water-saturated haplogranite melts varies strongly as a function of melt composition. Water solubility in melt decreases in the order strongly peralkaline haplogranite &> quartz-albite composition ≈ moderately peralkaline haplogranite ≈ peraluminous haplogranite ≈ subaluminous haplogranite &> quartz-orthoclase composition. D Cl (wt% Cl in fluid/wt% Cl in melt) is a strong function of system composition. D Cl increases as the concentration of Cl in the system increases. A comparison of the distribution of Cl between haplogranite melts of subaluminous, peraluminous, and peralkaline compositions and aqueous fluids shows that with similar concentrations of Cl in the system; D Cl is largest for subaluminous melts ( A/NK ≈ 1 ) having relatively high K2O concentrations. As the A/NK of melt increases or decreases from 1 or as the N/NK of the melt increases, both D Cl and the concentration of Cl in the fluid decrease. The experimental data imply that complexes involving Cl and Na in hydrous, subaluminous granite melts are dominant. Furthermore, the data imply that in hydrous, peraluminous granite melts, Cl complexes with network modifying A13+ and in hydrous, peralkaline granite melts Cl complexes with network modifying Na + ± K + . On a molar basis, Cl apparently complexes with network modifying A13+ in peraluminous melts as strongly as it does with Na+ and K+ in peralkaline melts. Experimental studies of Cl partitioning in granitic systems suggest that the strongest enrichment of magmatic-hydrothermal fluids in Cl and ore metals complexed with Cl will most likely occur in fluids exsolved from subaluminous granite magmas that are relatively enriched in K2O, SiO2, and Cl, and relatively depleted in F, CO2, and Na2O.

MELT INCLUSIONS IN QUARTZ FROM AN EVOLVED PERALUMINOUS PEGMATITE : GEOCHEMICAL EVIDENCE FOR STRONG TIN ENRICHMENT IN FLUORINE-RICH AND PHOSPHORUS-RICH RESIDUAL LIQUIDS

Abstract We have investigated the magmatic evolution of a late-stage, F- and P-rich, pegmatite-forming aluminosilicate liquid and the geochemical controls on magmatic mineralizing processes by remelting totally-crystallized melt inclusions in quartz and analyzing the quenched glass by EPMA and SIMS. The quartz phenocrysts were sampled from a pegmatite that occurs in a Variscan granite genetically associated with cassiterite- and wolframite-mineralized greisen veins at the Ehrenfriedersdorf SnW deposit, central Erzgebirge, SE Germany. The melt inclusion compositions imply that the pegmatite-forming liquid achieved extreme levels of chemical differentiation. It contained high abundances of Sn, F, P, Li, Rb, Cs, Nb, Ta, and Be and abnormally low concentrations of Ca, Y, Sr, and REE for a granite, and it was strongly peraluminous (the molar [Al 2 O 3 /CaO + Na 2 O + K 2 O] ranged from 1.3 to 2.0). Fractions of the pegmatite-forming liquid were extremely enriched in P 2 O 5 + F + Al 2 O 3 , and the molar abundances of (F + P) in the glasses correlate strongly with moles of network-modifying Al ions implying that the bulk liquid included F-, P-, and Al-bearing complexes. Formation of these complexes reduced the activities of F, P, and Al in bulk liquid, suppressed the crystallization of magmatic topaz and P-rich minerals, and allowed the liquid to become enriched in these constituents. Some fractions of the Ehrenfriedersdorf aluminosilicate liquid contained 1000–2000 ppm Sn. These levels of Sn enrichment were up to 2 orders of magnitude greater than that ever reported for nonmineralized, metaluminous and peraluminous igneous materials and are consistent with some experimentally-derived Sn solubilities in cassiterite-saturated granitic liquids at geologically relevant pressures and temperatures. This concordance implies that cassiterite could have crystallized directly from this highly evolved, P- and F-rich peraluminous granitic liquid without the involvement of hydrothermal fluids.

Exsolution of magmatic volatile phases from Cl-enriched mineralizing granitic magmas and implications for ore metal transport

To understand Cl dissolution in aluminosilicate liquids and the exsolution of Cl-rich magmatic volatile phases, experiments were conducted to determine the solubility of NaCl, KCl, and H2O in felsic liquids at 0.5 and 2 kbar. The Cl content of H2O-poor, NaCl-saturated, and KCl-saturated silicate liquids is low (i.e., ≤1.3 wt%) to very low (i.e., ≈0.2 wt%) and varies with changes in pressure and composition; the Cl concentration increases with the F concentration and the molar (Al + Na + Ca + Mg/Si) ratio of the liquid and decreases with increasing activity of H2O in the system. Exsolution of a volatile phase depends on the partial pressures of all dissolved volatiles, and low Cl solubilities in NaCl-saturated and KCl-saturated silicate liquids imply that exsolution of a Cl-bearing volatile phase will occur “early” in Cl-bearing granitic magmas, i.e., prior to extensive melt crystallization and/or at comparatively low water fugacities. The solubility behavior of H2O and Cl is very similar to that of CO2 and H2O in felsic liquids. Small quantities of CO2 are known to facilitate volatile phase exsolution (Holloway, 1976), and in a similar manner volatile phases may exsolve “early” in Cl-enriched granite magmas. Whereas 5 to 6 wt% dissolved H2O is necessary for volatile phase exsolution from a CO2-free and Cl-free haplogranite liquid at 2 kbar and 800°C, a Cl-rich brine will exsolve if the liquid contains only 1 wt% H2O and 0.26 wt% Cl at the same conditions. These new solubility data are interpreted in light of H2O, F, and Cl concentrations in felsic liquids, determined from silicate melt inclusions, to constrain the exsolution of Cl-bearing, magmatic volatile phases from mineralizing granitic magmas. Felsic magmas genetically associated with Cu-porphyry and Mo-porphyry mineralization contain sufficient H2O and Cl to become saturated with respect to a hypersaline brine without strong pressure reduction, boiling of the volatile phase (i.e., exsolution of immiscible vapor and brine), or strong volatile enrichment resulting from extensive crystal fractionation. Experiments were also conducted to investigate the solubility of Mo in highly saline volatile phases coexisting with granitic liquids at 2 and 0.5 kbar. The apparent partition coefficient for Mo in the volatile phase (s) relative to silicate liquid, (DMo∗), is defined as [the computed concentration of Mo in a volatile phase or phases/the measured concentration of Mo in granite glass]. DMo∗ ranges from 8 to 80 as the NaCl and KCl content of the volatile phase(s) increases from 15 to 90 wt%. Because Mo does not complex with Cl in aqueous fluids, it appears that Mo solubility may be a strong function of the activity of Na and K in alkali chloride-rich volatile phases.

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.

In situ trace-element analysis of individual silicate melt inclusions by laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS)

This paper reports the successful application of laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS) to the in situ analysis of a diverse suite of twenty trace elements including Zr, Hf, Nb, Ta, Y, and REEs, in individual silicate melt inclusions in phenocrysts from Fantale volcano, Ethiopia. The UV laser, a frequency quadrupled Nd: YAG operating at 266 nm, significantly improves the ablation characteristics of minerals that do not absorb strongly at near-IR wavelengths (e.g., quartz and feldspar). Furthermore, it allows for a significant reduction in ablation pit size to ca. 10 μm, thereby permitting numerous applications that require high-resolution sampling. Multiple ablations in individual melt inclusions in the size range 10–50 μm demonstrate both the effectiveness of the technique and the generally homogeneous character of the inclusions. Comparison of the LAM-ICP-MS data for international reference material RGM-1 (a rhyolite), with recommended values, indicates an analytical precision of <10% for most of the trace elements determined in this study. The trace element abundances of the Fantale melt inclusions, determined by LAM-ICP-MS, are typical of those of pantellerites (i.e., peralkaline rhyolites), and are consistent with their origin as tiny volumes of melt trapped in quartz and alkali-feldspar phenocrysts during the final stage of fractional crystallization of the host peralkaline magma.

Volatile and lithophile trace-element geochemistry of Mexican tin rhyolite magmas deduced from melt inclusions

Abstract We have investigated the petrology and geochemistry of whole rocks from two small-volume, Sn- and F-mineralized rhyolitec dome complexes of the Mexican tin rhyolite belt, Cerro el Lobo and Cerro el Pajaro, to determine volcanic degassing and mineralizing processes in felsic igneous systems. The abundance and distribution of volatiles (H 2 O, B, F, and Cl) and lithophile trace and ore elements (Li, Rb, Cs, Be, Sr, Y, Ce, Th, U, Nb, Sn, and Mo) in the parental liquids were established by analyzing melt inclusions in quartz. The melt inclusions from both rhyolites are variably enriched in Li and the volatile constituents F and Cl, and some are extremely enriched in Li, although whole rocks are not correspondingly enriched. Compositional variations in the melt inclusions from both rhyolites also constrain magmatic differentiation. Melt evolution was dominated by crystal fractionation, modified by mass transport in a Cl- and H 2 O-rich magmatic-hydrothermal fluid, and resulted in increasing abundances of U, Nb, and Cs (± Li, F, Cl, B, Y, Ce, Be, Rb, Mo, and Sn) in both liquids. The rhyolite liquids apparently were heterogeneous prior to eruption. The Cerro el Lobo liquid contained gradients in volatiles and trace elements; comparatively less Cl, Be, B, Al 2 O 3 , and CaO (± Li, F, U, and Th) were present in the early-erupted, H 2 O-rich fractions of liquid. Comparing compositions of whole rocks with the mean compositions of melt inclusions constrains relative mobilities of magmatic constituents during and after eruption. Sodium, fluorine, lithium, uranium, and yttrium (± H 2 O, Cl, Sn) were lost from both magmas and the Cerro el Pajaro magma apparently also lost Nb and Al as a result of eruptive and posteruptive degassing. These geochemical relationships and constraints on pre-eruptive abundances and distributions of volatiles in tin rhyolite magmas probably apply to other tin rhyolites and, moreover, the high levels of Cl and Li enrichment maybe representative of other highly-evolved granitic magmas genetically associated with lithophile mineralization.