Apatite-Melt Partitioning at 1 Bar: An Assessment of Apatite-Melt Exchange Equilibria Resulting from Non-Ideal Mixing of F and Cl in Apatite

The mineral apatite [Ca5(PO4)3(F,Cl,OH)] is present in a wide range of planetary materials. Due to the presence of volatiles within its crystal structure (X-site), many recent studies have attempted to use apatite to constrain the volatile contents of planetary magmas and mantle sources. In order to use the volatile contents of apatite to precisely determine the abundances of volatiles in coexisting silicate melt or fluids, thermodynamic models for the apatite solid solution and for the apatite components in multi-component silicate melts and fluids are required. Although some thermodynamic models for apatite have been developed, they are incomplete. Furthermore, no mixing model is available for all of the apatite components in silicate melts or fluids, especially for F and Cl components. Several experimental studies have investigated the apatite-melt and apatite-fluid partitioning behavior of F, Cl, and OH in terrestrial and planetary systems, which have determined that apatite-melt partitioning of volatiles are best described as exchange equilibria similar to Fe-Mg partitioning between olivine and silicate melt. However, McCubbin et al. recently reported that the exchange coefficients may vary in portions of apatite compositional space where F, Cl, and OH do not mix ideally in apatite. In particular, solution calorimetry data of apatite compositions along the F-Cl join exhibit substantial excess enthalpies of mixing. In the present study, we conducted apatite-melt partitioning experiments in evacuated, sealed silica-glass tubes at approximately 1 bar and 950-1050 degrees Centigrade on a synthetic Martian basalt composition equivalent to the basaltic shergottite Queen Alexandria Range (QUE) 94201. These experiments were conducted dry, at low pressure, to assess the effects of temperature and apatite composition on the partitioning behavior of F and Cl between apatite and basaltic melt along the F-Cl apatite binary join, where there is non-ideal mixing of F and Cl in apatite

Trace Element Partitioning between CAI-Type Melts and Grossite, Melilite, Hibonite, and Olivine

We determined the mineral-melt partition coefficients (Di's) and the compositional and/or temperature dependency between grossite, melilite, hibonite, olivine and Ca-, Al-inclusion (CAI)-type liquids for a number of light (LE), high field strength (HFSE), large ion lithophile (LILE), and rare earth (REE) elements including Li, Be, B, Sr, Zr, Nb, Ba, La, Ce, Eu, Dy, Ho, Yb, Hf, Ta, Th. A series of isothermal crystallization experiments was conducted at 5 kbar pressure and IW+1 in graphite capsules. The starting compositions were selected based on the calculated and experimentally confirmed phase relations during condensation in CI dust-enriched systems (Ebel and Grossman, 2000; Ebel, 2006; Ustunisik et al., 2014). Partition coefficients between melt and gehlenite, hibonite, and grossite show that the trace element budget of igneous CAIs is controlled by these three major Al-bearing phases in addition to pyroxene. In general, LE, LILE, REE, and HFSE partition coefficients (by mass) decrease in the order of Di(Gehlenite-Melt) > Di(Hibonite-Melt) > Di(Grossite-Melt). Results suggest that Di(Gehlenite-Melt) vary by a factor of 2-3 in different melt compositions at the same T (~1500 C). Increased melt Al and Ca, relative to earlier work, increases the compatibility of Di(Gehlenite-Melt), and also the compatibility of Di(Hibonite-Melt), especially for La and Ce. Olivine partitioning experiments confirm that olivine contribution to the trace element budget of CAIs is small due to the low Di(Olivine-Melt) at a range of temperatures while D-Eu, Yb(Olivine-Melt) are sensitive to changes in T and oxygen fugacity. The development of a predictive model for partitioning in CAI-type systems would require more experimental data and the use of analytical instruments capable of obtaining single phase analyses for crystals < 5 micron.Comment: 23 pages, 15 figures, 5 table

Experimental Constraints on the Partitioning Behavior of F, Cl, and OH Between Apatite and Basaltic Melt

The mineral apatite is present in a wide range of planetary materials. The presence of volatiles (F, Cl, and OH) within its crystal structure (X-site) have motivated numerous studies to investigate the partitioning behavior of F, Cl, and OH between apatite and silicate melt with the end goal of using apatite to constrain the volatile contents of planetary magmas and mantle sources. A number of recent experimental studies have investigated the apatite-melt partitioning behavior of F, Cl, and OH in magmatic systems. Apatite-melt partitioning of volatiles are best described as exchange equilibria similar to Fe-Mg partitioning between olivine and silicate melt. However, the partitioning behavior is likely to change as a function of temperature, pressure, oxygen fugacity, apatite composition, and melt composition. In the present study, we have conducted experiments to assess the partitioning behavior of F, Cl, and OH between apatite and silicate melt over a pressure range of 0-6 gigapascals, a temperature range of 950-1500 degrees Centigrade, and a wide range of apatite ternary compositions. All of the experiments were conducted between iron-wustite oxidation potentials IW minus 1 and IW plus 2 in a basaltic melt composition. The experimental run products were analyzed by a combination of electron probe microanalysis and secondary ion mass spectrometry (NanoSIMS). Temperature, apatite crystal chemistry, and pressure all play important roles in the partitioning behavior of F, Cl, and OH between apatite and silicate melt. In portions of apatite ternary space that undergo ideal mixing of F, Cl, and OH, exchange coefficients remain constant at constant temperature and pressure. However, exchange coefficients vary at constant temperature (T) and pressure (P) in portions of apatite compositional space where F, Cl, and OH do not mix ideally in apatite. The variation in exchange coefficients exhibited by apatite that does not undergo ideal mixing far exceeds the variations induced by changes in temperature (T) or pressure (P) . In regions where apatite undergoes ideal mixing of F, Cl, and OH, temperature has a stronger effect than pressure on the partitioning behavior, but both are important. Furthermore, fluorine becomes less compatible in apatite with increasing pressure and temperature. We are still in the process of analyzing our experimental run products, but we plan to quantify the effects of P and T on apatite-melt partitioning of F, Cl, and OH

Multiple reservoirs of volatiles in the Moon revealed by the isotopic composition of chlorine in lunar basalts

The isotopes of chlorine (37Cl and 35Cl) are highly fractionated in lunar samples compared to most other Solar System materials. Recently, the chlorine isotope signatures of lunar rocks have been attributed to large-scale degassing processes that occurred during the existence of a magma ocean. In this study we investigated how well a suite of lunar basalts, most of which have not previously been analyzed, conform to previous models. The Cl isotope compositions (δ37Cl (‰) = [(37Cl/35Clsample/37Cl/35ClSMOC)-1]×1000, where SMOC refers to standard mean ocean chloride) recorded range from ∼+7 to +14 ‰ (Apollo 15), +10 to +19 ‰ (Apollo 12), +9 to +15 ‰ (70017), +4 to +8 ‰ (MIL 05035), and +15 to +22 ‰ (Kalahari 009). The Cl isotopic data from the present study support the mixing trends previously reported by Boyce et al., 2015, Barnes et al., 2016, as the Cl isotopic composition of apatites are positively correlated with bulk-rock incompatible trace element abundances in the low-Ti basalts, inclusive of low-Ti and KREEP basalts. This trend has been interpreted as evidence that incompatible trace elements, including Cl, were concentrated in the urKREEP residual liquid of the lunar magma ocean, rather than the mantle cumulates, and that urKREEP Cl had a highly fractionated isotopic composition. The source regions for the basalts were thus created by variable mixing between the mantle (Cl-poor and relatively unfractionated) and urKREEP. The high-Ti basalts show much more variability in measured Cl isotope ratios and scatter around the trend formed by the low-Ti basalts. Most of the data for lunar meteorites also fits the mixing of volatiles in their sources, but Kalahari 009, which is highly depleted in incompatible trace elements, contains apatites with heavily fractionated Cl isotopic compositions. Given that Kalahari 009 is one of the oldest lunar basalts and ought to have been derived from very early-formed mantle cumulates, a heavy Cl isotopic signature is likely not related to its mantle source, but more likely to magmatic or secondary alteration processes, perhaps via impact-driven vapor metasomatism of the lunar crust

Investigating magmatic processes in the early Solar System using the Cl isotopic systematics of eucrites

Generally, terrestrial rocks, martian and chondritic meteorites exhibit a relatively narrow range in bulk and apatite Cl isotope compositions, with δ37Cl (per mil deviation from standard mean ocean chloride) values between − 5.6 and + 3.8 ‰. Lunar rocks, however, have more variable bulk and apatite δ37Cl values, ranging from ∼ − 4 to + 40 ‰. As the Howardite-Eucrite-Diogenite (HED) meteorites represent the largest suite of crustal and sub-crustal rocks available from a differentiated basaltic asteroid (4 Vesta), studying them for their volatiles may provide insights into planetary differentiation processes during the earliest Solar System history. Here the abundance and isotopic composition of Cl in apatite were determined for seven eucrites representing a broad range of textural and petrological characteristics. Apatite Cl abundances range from ∼ 25 to 4900 ppm and the δ37Cl values range from − 3.98 to + 39.2 ‰. Samples with lower apatite H2O contents were typically also enriched in 37Cl but no systematic correlation between δ37Cl and δD values was observed across samples. Modelled Rayleigh fractionation and a strong positive correlation between bulk δ66Zn and apatite δ37Cl support the hypothesis that Cl degassed as metal chlorides from eucritic magmas, in a hydrogen-poor environment. In the case of lunar samples, it has been noted that δ37Cl values of apatite positively correlate with bulk La/Yb ratio. Interestingly, most eucrites show a negative correlation with bulk La/Yb ratio. Recently, isotopically light Cl values have been suggested to record the primary solar nebular signature. If this is the case then 4 Vesta, which accreted rapidly and early in Solar System history, could also record this primary nebular signature corresponding to the lightest Cl values measured here. The significant variation in Cl isotope composition observed within the eucrites are likely related to degassing of metal chlorides

Water in evolved lunar rocks: Evidence for multiple reservoirs

We have measured the abundance and isotopic composition of water in apatites from several lunar rocks representing Potassium (K), Rare Earth Elements (REE), and Phosphorus (P) − KREEP − rich lithologies, including felsites, quartz monzodiorites (QMDs), a troctolite, and an alkali anorthosite. The H-isotope data from apatite provide evidence for multiple reservoirs in the lunar interior. Apatite measurements from some KREEP-rich intrusive rocks display moderately elevated δD signatures, while other samples show δD signatures similar to the range known for the terrestrial upper mantle. Apatite grains in Apollo 15 quartz monzodiorites have the lowest δD values measured from the Moon so far (as low as −749‰), and could potentially represent a D-depleted reservoir in the lunar interior that had not been identified until now. Apatite in all of these intrusive rocks contains 6500 ppm H2O). Complexities in partitioning of volatiles into apatite make this comparison uncertain, but measurements of residual glass in KREEP basalt fragments in breccia 15358 independently show that the KREEP basaltic magmas were low in water. The source of 15358 contained ∼10 ppm H2O, about an order of magnitude lower than the source of the Apollo 17 pyroclastic glass beads, suggesting potential variations in the distribution of water in the lunar interior

Experimental Investigation of F and Cl Partitioning Between Apatite and Fe-Rich Basaltic Melt at 0 GPa and 950-1050 degC: Evidence for Steric Controls on Apatite-Melt Exchange Equilibria in OH-Poor Apatite

Apatite-melt partitioning experiments were conducted in a Deltech vertical-quench 1-bar furnace at 0 GPa and 9501050 C using an Fe-rich basaltic starting composition. Each experiment had a unique F:Cl ratio to assess the partitioning of F and Cl between apatite and melt, and the oxygen fugacity of all experiments were between IW and IW-1. Apatite-melt partitioning of F and Cl along the F-Cl binary join is investigated in particular to assess the effect of non-ideal mixing of apatite X-site components. The quenched melt and apatite from each experiment were analyzed by electron probe microanalysis. Several of our experiments exhibited evidence of silicate liquid immiscibility (SLI), so we also evaluated the effect of SLI on the partitioning of F and Cl between apatite and melt in those experiments. The F-Cl exchange equilibria between apatite and melt were variable with (sup )(sub ) values in the range of 0.080.21 across the F-Cl join. The (sup )(sub ) values decreased with decreasing F in apatite and melt. Notably, we did not observe evidence that SLI has a first-order effect on the behavior of F and Cl partitioning between apatite and melt. The observed drop in (sup )(sub ) values with decreasing F abundance in melt and apatite indicate that F becomes more compatible in apatite than Cl as there is less F in the system under nominally anhydrous conditions. This behavior can be explained by the presence of an off mirror F site in F-Cl apatite that only F can occupy, and this F site is required to stabilize the apatite structure in OH-poor systems. These results demonstrate a link between atomic arrangements of apatite X-site components, the thermodynamic mixing properties of apatite X-site components, and the values of apatite-melt exchange equilibria for apatite X-site components. These results also indicate that Cl-rich apatites (i.e., Cl > 0.1 structural formula units (sfu) of apatite X-site) with compositions near the F-Cl binary join (i.e., <0.12 sfu from the F-Cl binary join), even if formed from H2O-bearing natural systems, should not be used for apatite-based melt-hygrometry

The Missing Magmas of MOR: Insights From Phase Equilibrium Experiments on Plagioclase Ultraphyric Basalts

Abstract Plagioclase ultraphyric basalts (PUBs) are a class of mid‐ocean ridge (MOR) lavas found in a variety of ocean floor environments, are characterized by abundant (15–40 volume %) plagioclase megacrysts and a diverse trace element and isotopic signature. Paradoxically, we never see lavas erupted on the seafloor that are in equilibrium with these PUB megacrysts. Based on petrographic evidence, melt inclusion composition, and new data on depth of entrapment calculated from CO2 contents in plagioclase‐hosted inclusions, many of the megacrysts formed at upper mantle pressures (∼3–7 kbars). To constrain the composition of the parent magmas of the plagioclase megacrysts, we conducted a series of experiments at 5 and 10 kbars using mid‐ocean ridge basalts glasses as starting materials. The experimental results were consistent with the presence of a pseudoazeotrope in the anorthitic segment of the plagioclase + basalt pseudobinary. This has the effect of dropping the anorthitic end of the feldspar loop, lowering the solidus for upper mantle conditions, and driving evolving magmas toward higher Ca. As magmas rise and pressure drops, the pseudoazeotrope disappears, and the feldspar loop at the high‐An end rises, causing those magmas to undergo decompression crystallization of plagioclase and resorption of olivine. Therefore, the conditions which generated the magmas from which the megacrysts form disappear as the magmas rise and magmas evolve toward lower Ca, Mg (as we normally assume during plagioclase + olivine crystallization). In effect, the phase equilibria conditions that allow for the generation of such liquids also prevent them from being erupted as lavas

Thermal behavior of a mineral mixture of sepiolite and dolomite

An industrial raw material taken from Beypazan (Ankara, Turkey) region was heated at different temperatures in the 100-1100 degrees C interval for 2 h. The volumetric percentage of particles having diameter below 2 mu m in an aqueous suspension of the material held 24 h were determined as 85% by the particle size distribution analysis. The mineralogical composition of the material was obtained as mass% of 81% sepiolite, 15% dolomite, and 4% interparticle water by using the X-ray diffraction (XRD) and thermal analysis (TG, DTA) data. The temperature ranges were determined for the dehydrations of the interparticle water and the zeolitic water as 25-340 degrees C, for the dehydration of the bound water as 340-580 degrees C, and for the dehydroxylation of the hydroxyls as 800-833 degrees C in the sepiolite. The zig-zag changes in the specific surface area (S/m(2) g(-1)) and specific micro- and mesopore volume (V/cm(3) g) with the temperature increases were discussed according to the dehydrations and dehydroxylation of the sepiolite