ALBERT

Description: Apatite-melt partitioning experiments were conducted in a piston-cylinder press at 1.0–1.2 GPa and 950–1000 °C using an Fe-rich basaltic starting composition and an oxygen fugacity within the range of IW-1 to IW+2. Each experiment had a unique F:Cl:OH ratio to assess the partitioning as a function of the volatile content of apatite and melt. The quenched melt and apatite were analyzed by electron probe microanalysis and secondary ion mass spectrometry techniques. The mineral-melt partition coefficients ( D values) determined in this study are as follows: D F Ap-Melt = 4.4–19, D Cl Ap-Melt = 1.1–5, D OH Ap-Melt = 0.07–0.24. This large range in values indicates that a linear relationship does not exist between the concentrations of F, Cl, or OH in apatite and F, Cl, or OH in melt, respectively. This non-Nernstian behavior is a direct consequence of F, Cl, and OH being essential structural constituents in apatite and minor to trace components in the melt. Therefore mineral-melt D values for F, Cl, and OH in apatite should not be used to directly determine the volatile abundances of coexisting silicate melts. However, the apatite-melt D values for F, Cl, and OH are necessarily interdependent given that F, Cl, and OH all mix on the same crystallographic site in apatite. Consequently, we examined the ratio of D values (exchange coefficients) for each volatile pair (OH-F, Cl-F, and OH-Cl) and observed that they display much less variability: K d Cl-F Ap-Melt = 0.21 ± 0.03, K d OH-F Ap-Melt = 0.014 ± 0.002, and K d OH-Cl Ap-Melt = 0.06 ± 0.02. However, variations with apatite composition, specifically when mole fractions of F in the apatite X-site were low ( X F 〈 0.18), were observed and warrant additional study. To implement the exchange coefficient to determine the H 2 O content of a silicate melt at the time of apatite crystallization (apatite-based melt hygrometry), the H 2 O abundance of the apatite, an apatite-melt exchange K d that includes OH (either OH-F or OH-Cl), and the abundance of F or Cl in the apatite and F or Cl in the melt at the time of apatite crystallization are needed (F if using the OH-F K d and Cl if using the OH-Cl K d ). To determine the H 2 O content of the parental melt, the F or Cl abundance of the parental melt is needed in place of the F or Cl abundance of the melt at the time of apatite crystallization. Importantly, however, exchange coefficients may vary as a function of temperature, pressure, melt composition, apatite composition, and/or oxygen fugacity, so the combined effects of these parameters must be investigated further before exchange coefficients are applied broadly to determine volatile abundances of coexisting melt from apatite volatile abundances.

Description: The chlorine isotope composition of Earth’s interior can place strong constraints on deep-Earth cycling of halogens and the origin of mantle chemical heterogeneity. However, all mantle-derived volcanic samples studied for Cl isotopes thus far originate from submarine volcanic systems, where the influence of seawater-derived Cl is pervasive. Here, we present Cl isotope data from subglacial volcanic glasses from Iceland, where the mid-ocean ridge system emerges above sea level and is free of seawater influence. The Iceland data display significant variability in 37 Cl values, from –1.8 to +1.4, and are devoid of regional controls. The absence of correlations between Cl and O isotope ratios and the lack of evidence for seawater-derived enrichments in Cl indicate that the variation in 37 Cl values in Icelandic basalts can be solely attributed to mantle heterogeneity. Indeed, positive correlations are evident between 37 Cl values and incompatible trace element ratios (e.g., La/Y), and long-lived radiogenic Pb isotope ratios. The data are consistent with the incorporation of altered lithosphere, including the uppermost sedimentary package, subducted into the Iceland mantle plume source, resulting in notable halogen enrichments in Icelandic basalts relative to lavas from adjacent mid-ocean ridges.

Description: Majoritic garnet, characterized by an excess of silicon (〉3 Si per formula unit), is considered one of the major phases of the Earth’s transition zone from 410–660 km depth. Quantifying the H 2 O content of nominally anhydrous mantle minerals is necessary to evaluate their water storage capacity from experiments and modeling the Earth’s deep water cycle. We present mineral-specific infrared absorption coefficients for the purpose of quantifying the amount of water incorporated into majorite as hydroxyl point defects. A suite of majoritic garnet samples with varying proportions of Si, Fe, Al, Cr, and H 2 O was synthesized at conditions of 18–19 GPa and 1500–1800 °C. Single-crystals were characterized using X-ray diffraction, electron microprobe analysis, secondary ion mass spectrometry (SIMS), IR, Raman, and Mössbauer spectroscopy. We utilize SIMS and Raman spectroscopy in combination with IR spectroscopy to provide IR absorption coefficients for water in majoritic garnets with the general mineral formula (Mg,Fe) 3 (Si,Mg,Fe,Al,Cr) 2 [SiO 4 ] 3 . The IR absorption coefficient for majoritic garnet in the OH stretching region is frequency dependent and ranges from 10 470 ± 3100 Lmol –1 cm –2 to 23 400 ± 2300 Lmol –1 cm –2 .

Description: 〈p〉The sources and nature of organic carbon on Mars have been a subject of intense research. Steele 〈i〉et al.〈/i〉 (2012) showed that 10 martian meteorites contain macromolecular carbon phases contained within pyroxene- and olivine-hosted melt inclusions. Here, we show that martian meteorites Tissint, Nakhla, and NWA 1950 have an inventory of organic carbon species associated with fluid-mineral reactions that are remarkably consistent with those detected by the Mars Science Laboratory (MSL) mission. We advance the hypothesis that interactions among spinel-group minerals, sulfides, and a brine enable the electrochemical reduction of aqueous CO〈sub〉2〈/sub〉 to organic molecules. Although documented here in martian samples, a similar process likely occurs wherever igneous rocks containing spinel-group minerals and/or sulfides encounter brines.〈/p〉

Description: 〈p〉Neoproterozoic West African diamonds contain sulfide inclusions with mass-independently fractionated (MIF) sulfur isotopes that trace Archean surficial signatures into the mantle. Two episodes of subduction are recorded in these West African sulfide inclusions: thickening of the continental lithosphere through horizontal processes around 3 billion years ago and reworking and diamond growth around 650 million years ago. We find that the sulfur isotope record in worldwide diamond inclusions is consistent with changes in tectonic processes that formed the continental lithosphere in the Archean. Slave craton diamonds that formed 3.5 billion years ago do not contain any MIF sulfur. Younger diamonds from the Kaapvaal, Zimbabwe, and West African cratons do contain MIF sulfur, which suggests craton construction by advective thickening of mantle lithosphere through conventional subduction-style horizontal tectonics.〈/p〉

Description: Melt inclusions (MI) are considered the best tool available for determining the pre-eruptive volatile contents of magmas. H 2 O and CO 2 concentrations of the glass phase in MI are commonly used both as a barometer and to track magma degassing behavior during ascent due to the strong pressure dependence of H 2 O and CO 2 solubilities in silicate melts. The often unstated and sometimes overlooked requirement for this method to be valid is that the glass phase in the MI must represent the composition of the melt that was trapped at depth in the volcanic plumbing system. However, melt inclusions commonly contain a vapor bubble that formed after trapping owing to differential shrinkage of the melt compared to the host crystal, and/or crystallization at the inclusion-host interface. Such bubbles may contain a substantial portion of volatiles, such as CO 2 , that were originally dissolved in the melt. In this study, we determined the contribution of CO 2 in the vapor bubble to the overall CO 2 content of MI based on quantitative Raman analysis of the vapor bubbles in MI from the 1959 Kilauea Iki (Hawaii), 1960 Kapoho (Hawaii), 1974 Fuego volcano (Guatemala), and 1977 Seguam Island (Alaska) eruptions. We found that the bubbles typically contain 40 to 90% of the total CO 2 in the MI. Reconstructing the original CO 2 content by adding the CO 2 in the bubble back into the melt results in an increase in CO 2 concentration by as much as an order of magnitude (thousands of parts per million). Reconstructed CO 2 concentrations correspond to trapping pressures that are significantly greater than one would predict based on analysis of the volatiles in the glass alone. Trapping depths can be as much as 10 km deeper than estimates that ignore the CO 2 in the bubble. In addition to CO 2 in the vapor bubbles, many MI showed the presence of a carbonate mineral phase. Failure to recognize the carbonate during petrographic examination or analysis of the glass and to include its contained CO 2 when reconstructing the CO 2 content of the originally trapped melt will introduce additional errors into the calculated volatile budget. Our results emphasize that accurate determination of the pre-eruptive volatile content of melts based on analysis of melt inclusions must consider the volatiles contained in the bubble (and carbonates, if present). This can be accomplished either by analysis of the bubble and the glass followed by mass-balance reconstruction of the original volatile content of the melt, or by re-homogenization of the MI prior to conducting microanalysis of the quenched, glassy MI.

Description: Whitlockite and merrillite are two Ca-phosphate minerals found in terrestrial and planetary igneous rocks, sometimes coexisting with apatite. Whitlockite has essential structural hydrogen, and merrillite is devoid of hydrogen. Whitlockite components have yet to be discovered in samples of extraterrestrial merrillite, despite evidence for whitlockite-merrillite solid solution in terrestrial systems. The observation of merrillite in meteoritic and lunar samples has led many to conclude that the magmas from which the merrillite formed were "very dry." However, the Shergotty martian meteorite has been reported to contain both apatite and merrillite, and recently the apatite has been shown to contain substantial OH abundances, up to the equivalent of 8600 ppm H 2 O. In the present study, we determined the abundances of F, Cl, H 2 O, and S in merrillite from Shergotty using secondary ion mass spectrometry (SIMS). We determined that the merrillite in Shergotty was properly identified (i.e., no discernible whitlockite component), and it coexists with OH-rich apatite. The absence of a whitlockite component in Shergotty merrillite and other planetary merrillites may be a consequence of the limited thermal stability of H in whitlockite (stable only at T 〈1050 °C), which would prohibit merrillite-whitlockite solid-solution at high temperatures. Consequently, the presence of merrillite should not be used as evidence of dry magmatism without a corresponding estimate of the T of crystallization. In fact, if a whitlockite component in extraterrestrial merrillite is discovered, it may indicate formation by or equilibration with hydrothermal or aqueous fluids.