Kathleen E. Vander Kaaden

Hydrous melting of the martian mantle produced both depleted and enriched shergottites

The search for water in our solar system is one of the primary driving forces for planetary science and exploration because water plays an important role in many geologic processes and is required for biologic processes as we currently understand them. Excluding Earth, Mars is the most promising destination in the inner solar system to find water, as it is undoubtedly responsible for shaping many geomorphologic features observed on the present-day martian surface; however, the water content of the martian interior is currently unresolved. Much of our information about the martian interior comes from studies of the basaltic martian meteorites (shergottites). In this study we examined the water contents of magmatic apatites from a geochemically enriched shergottite (the Shergotty meteorite) and a geochemically depleted shergottite (the Queen Alexandria Range 94201 meteorite). From these data, we determined that there was little difference in water contents between the geochemically depleted and enriched shergottite magmas. The water contents of the apatite imply that shergottite parent magmas contained 730–2870 ppm H 2 O prior to degassing. Furthermore, the martian mantle contains 73–290 ppm H 2 O and underwent hydrous melting as recently as 327 Ma. In the absence of plate tectonics, the presence of water in the interior of Mars requires planetary differentiation under hydrous conditions. This is the first evidence of significant hydrogen storage in a planetary interior at the time of core formation, and this process could support elevated H abundances in the interiors of other terrestrial bodies like the Moon, Mercury, Venus, large differentiated asteroids, and Earth.

Is Mercury a volatile-rich planet?

] Data returned from the gamma-ray spectrometeronboard the Mercury Surface, Space Environment, Geo-chemistry, and Ranging (MESSENGER) spacecraft havebeen interpreted to say that Mercury is a volatile-rich planet(elevated K/Th and K/U), which is important given itsheliocentricdistance.TheMESSENGERX-rayspectrometerprovided chemical information from the surface of Mercurywhich we used to calculate an average surface compositionfortheregionsanalyzed. ThehighSabundanceandlowFeOabundance of the surface indicates that the oxygen fugacityof the Mercurian interior is very reducing ( 6.3 to 2.6 logf

Exotic crust formation on Mercury: Consequences of a shallow, FeO‐poor mantle

The range in density and compressibility of Mercurian melt compositions was determined to better understand the products of a possible Mercurian magma ocean and subsequent volcanism. Our experiments indicate that the only mineral to remain buoyant with respect to melts of the Mercurian mantle is graphite; consequently, it is the only candidate mineral to have composed a primary floatation crust during a global magma ocean. This exotic result is further supported by Mercurys volatile-rich nature and inexplicably darkened surface. Additionally, our experiments illustrate that partial melts of the Mercurian mantle that compose the secondary crust were buoyant over the entire mantle depth and could have come from as deep as the core-mantle boundary. Furthermore, Mercury could have erupted higher percentages of its partial melts compared to other terrestrial planets because magmas would not have stalled during ascent due to gravitational forces. These findings stem from the FeO-poor composition and shallow depth of Mercurys mantle, which has resulted in both low-melt density and a very limited range in melt density responsible for Mercurys primary and secondary crusts. The enigmatically darkened, yet low-FeO surface, which is observed today, can be explained by secondary volcanism and impact processes that have since mixed the primary and secondary crustal materials.

Experimental investigation of F, Cl, and OH partitioning between apatite and Fe-rich basaltic melt at 1.0-1.2 GPa and 950-1000 °c

Abstract 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 ΔIW-1 to ΔIW+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: DFAp-Melt = 4.4-19, DClAp-Melt = 1.1-5, DOHAp-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: KdCl-FAp-Melt = 0.21± 0.03, KdOH-FAp-Melt = 0.014 ± 0.002, and KdOH-ClAp-Melt = 0.06 ± 0.02 . However, variations with apatite composition, specifically when mole fractions of F in the apatite X-site were low (XF < 0.18), were observed and warrant additional study. To implement the exchange coefficient to determine the H2O content of a silicate melt at the time of apatite crystallization (apatitebased melt hygrometry), the H2O abundance of the apatite, an apatite-melt exchange Kd 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 Kd and Cl if using the OH-Cl Kd). To determine the H2O 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.

A Low O/Si Ratio on the Surface of Mercury: Evidence for Silicon Smelting?

Data from the Gamma-Ray Spectrometer (GRS) that flew on the MESSENGER spacecraft indicate that the O/Si weight ratio of Mercurys surface is 1.2 ± 0.1. This value is lower than any other celestial surface that has been measured by GRS and suggests that 12–20% of the surface materials on Mercury are composed of Si-rich, Si-Fe alloys. The origin of the metal is best explained by a combination of space weathering and graphite-induced smelting. The smelting process would have been facilitated by interaction of graphite with boninitic and komatiitic parental liquids. Graphite entrained at depth would have reacted with FeO components dissolved in silicate melt, resulting in the production of up to 0.4–0.9 wt.% CO from the reduction of FeO to Fe0—CO production that could have facilitated explosive volcanic processes on Mercury. Once the graphite-entrained magmas erupted, the tenuous atmosphere on Mercury prevented the buildup of CO over the lavas. The partial pressure of CO would have been sufficiently low to facilitate reaction between graphite and SiO2 components in silicate melts to produce CO and metallic Si. Although exotic, Si-rich metal as a primary smelting product is hypothesized on Mercury for three primary reasons: (1) low FeO abundances of parental magmas, (2) elevated abundances of graphite in the crust and regolith, and (3) the presence of only a tenuous atmosphere at the surface of the planet within the 3.5–4.1 Ga timespan over which the planet was resurfaced through volcanic processes.

Discreditation of Bobdownsite and the Establishment of Criteria for the Identification of Minerals with Essential Monofluorophosphate (PO3F2-) [STUB]

Abstract Bobdownsite, IMA number 2008-037, was approved as a new mineral by the Commission on New Minerals, Nomenclature and Classification (CNMNC) as the fluorine end-member of the mineral whitlockite. The type locality of bobdownsite is in Big Fish River, Yukon, Canada, and bobdownsite was reported to be the first mineral with essential monofluorophosphate (PO3F2–). The type specimen of bobdownsite has been reinvestigated by electron probe microanalysis (EPMA), and our data indicate that fluorine abundances are below detection in the mineral. In addition, we conducted detailed analysis of bobdownsite from the type locality by gas chromatography isotope ratio mass spectrometry, Raman spectroscopy, EPMA, and NMR spectroscopy. These data were compared with previously published data on synthetic monofluorophosphate salts. Collectively, these data indicate that bobdownsite is indistinguishable from whitlockite with a composition along the whitlockite-merrillite solid solution. Bobdownsite is therefore discredited as a valid mineral species. An additional mineral, krásnoite, has been purported to have monofluorophosphate components in its structure, but reexamination of those data indicate that F– in krásnoite forms bonds with Al, similar to OH– bonded to Al in perhamite. Consequently, krásnoite also lacks monofluorophosphate groups, and there are currently no valid mineral species with monofluorophosphate in their structure. We recommend that any future reports of new minerals that contain essential monofluorophosphate anions be vetted by abundance measurements of fluorine, vibrational spectroscopy (both Raman and FTIR), and where paramagnetic components are permissibly low, NMR spectroscopy. Furthermore, we emphasize the importance of using synthetic compounds containing monofluorophosphate anions as a point of comparison in the identification of minerals with essential monofluorophosphate. Structural data that yield satisfactory P-F bond lengths determined by X-ray crystallography, coupled with direct chemical analyses of fluorine in a material do not constitute sufficient evidence alone to identify a new mineral with essential monofluorophosphate.