Stephen M. Elardo

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.

Apatites in lunar KREEP basalts: The missing link to understanding the H isotope systematics of the Moon

Recent re-analyses of lunar samples have undoubtedly measured indigenous water, challenging the paradigm of a “dry” Moon, and arguing that some portions of the lunar interior are as wet as some regions of the Earth’s mantle and that water in both planetary bodies likely share a common origin. Mare basalts indirectly sample the lunar mantle and are affected by petrogenetic processes such as crystallization and degassing that can modify characteristics of indigenous water in primary mantle melts. Analyses of apatite in phosphorus-rich KREEP (K + REE [rare earth elements] + P) basalts may provide more reliable estimates for the water content of lunar magmas, as some apatites likely crystallized before substantial degassing occurred. In lunar KREEP basalt sample 15386, apatite H 2 O content and H isotopic composition suggest that degassing occurred during apatite crystallization, the lowest δD value of 90‰ ± 100‰ representing an upper limit for the isotopic composition of water in the parental magma. Interpretation of the data for KREEP basalt 15386 suggests that this basalt is characterized by relatively elevated H 2 O contents and CI chondrite–type δD values, similar to those proposed for other mare basalts and pyroclastic glasses. On the other hand, most of the apatites in lunar KREEP basalt 72275 and lunar meteorite NWA 773 crystallized before degassing and H isotope fractionation, and their D/H ratios thus directly refl ect those of their source regions. These apatites have an average δD value of –130‰ ± 50‰, suggesting the presence of a water reservoir in the Moon characterized by moderate H 2 O contents and H isotopic composition similar to that of Earth’s interior. These fi ndings imply that signifi cant amounts of water in the Moon were inherited from the proto-Earth, surviving the purported Moon-forming impact event.

Origin of the lunar highlands Mg-suite: An integrated petrology, geochemistry, chronology, and remote sensing perspective

Abstract The Mg-suite represents an enigmatic episode of lunar highlands magmatism that presumably represents the first stage of crustal building following primordial differentiation. This review examines the mineralogy, geochemistry, petrology, chronology, and the planetary-scale distribution of this suite of highlands plutonic rocks, presents models for their origin, examines petrogenetic relationships to other highlands rocks, and explores the link between this style of magmatism and early stages of lunar differentiation. Of the models considered for the origin of the parent magmas for the Mg-suite, the data best fit a process in which hot (solidus temperature at ≥2 GPa = 1600 to 1800 °C) and less dense (ρ ~3100 kg/m3) early lunar magma ocean cumulates rise to the base of the crust during cumulate pile overturn. Some decompressional melting would occur, but placing a hot cumulate horizon adjacent to the plagioclase-rich primordial crust and KREEP-rich lithologies (at temperatures of <1300 °C) would result in the hybridization of these divergent primordial lithologies, producing Mg-suite parent magmas. As urKREEP (primeval KREEP) is not the “petrologic driver” of this style of magmatism, outside of the Procellarum KREEP Terrane (PKT), Mg-suite magmas are not required to have a KREEP signature. Evaluation of the chronology of this episode of highlands evolution indicates that Mg-suite magmatism was initiated soon after primordial differentiation (<10 m.y.). Alternatively, the thermal event associated with the mantle overturn may have disrupted the chronometers utilized to date the primordial crust. Petrogenetic relationships between the Mg-suite and other highlands suites (e.g., alkali-suite and magnesian anorthositic granulites) are consistent with both fractional crystallization processes and melting of distinctly different hybrid sources.

Volatile abundances of coexisting merrillite and apatite in the martian meteorite Shergotty: Implications for merrillite in hydrous magmas

Abstract 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 H2O. In the present study, we determined the abundances of F, Cl, H2O, 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.

Geologic history of Martian regolith breccia Northwest Africa 7034: Evidence for hydrothermal activity and lithologic diversity in the Martian crust

The timing and mode of deposition for Martian regolith breccia Northwest Africa (NWA) 7034 were determined by combining petrography, shape analysis, and thermochronology. NWA 7034 is composed of igneous, impact, and brecciated clasts within a thermally annealed submicron matrix of pulverized crustal rocks and devitrified impact/volcanic glass. The brecciated clasts are likely lithified portions of Martian regolith with some evidence of past hydrothermal activity. Represented lithologies are primarily ancient crustal materials with crystallization ages as old as 4.4 Ga. One ancient zircon was hosted by an alkali-rich basalt clast, confirming that alkalic volcanism occurred on Mars very early. NWA 7034 is composed of fragmented particles that do not exhibit evidence of having undergone bed load transport by wind or water. The clast size distribution is similar to terrestrial pyroclastic deposits. We infer that the clasts were deposited by atmospheric rainout subsequent to a pyroclastic eruption(s) and/or impact event(s), although the ancient ages of igneous components favor mobilization by impact(s). Despite ancient components, the breccia has undergone a single pervasive thermal event at 500–800°C, evident by groundmass texture and concordance of ~1.5 Ga dates for bulk rock K-Ar, U-Pb in apatite, and U-Pb in metamict zircons. The 1.5 Ga age is likely a thermal event that coincides with rainout/breccia lithification. We infer that the episodic process of regolith lithification dominated sedimentary processes during the Amazonian Epoch. The absence of pre-Amazonian high-temperature metamorphic events recorded in ancient zircons indicates source domains of static southern highland crust punctuated by episodic impact modification.

Magma chamber dynamics recorded by oscillatory zoning in pyroxene and olivine phenocrysts in basaltic lunar meteorite Northwest Africa 032

Abstract Oscillatory zoning in silicate minerals, especially plagioclase, is a common feature found in volcanic rocks from various terrestrial tectonic settings, but is nearly absent in the lunar environment. Here we report backscattered electron images, quantitative wavelength-dispersive spectrometry (WDS) analyses, and qualitative WDS elemental X‑ray maps that reveal oscillatory zoning of Mg, Ca, Fe, Ti, Al, Cr, and Mn in euhedral pyroxene phenocrysts, and faint oscillatory zoning of P in olivine phenocrysts in basaltic lunar meteorite Northwest Africa (NWA) 032. This is only the third known occurrence of oscillatory zoning in lunar silicate minerals. Zoning bands in pyroxene range from ~3-5 μm up to ~60 μm in width, but are typically ~10-20 μm in width. Oscillatory bands are variable in width over short distances, often within a single grain. Most oscillatory bands preserve a euhedral form and have sharp edges; however some bands have jagged or uneven edges indicative of resorption surfaces. The short-scale oscillatory nature of the zoning in pyroxene is overprinted on longer-scale core to rim normal magmatic zoning from pigeonite to augite compositions. Oscillatory zoning of P in olivine is faint and only resolvable with high beam current (400 nA) mapping. Bands of higher P are typically only a few micrometers in width, and although they preserve a euhedral form, they are not traceable around the full circumference of a grain and have variable spacing. Resorption surfaces, longer-scale normal magmatic zoning, and relatively thick oscillatory bands are indicative of the formation of these chemical oscillations as a result of variable magma composition. Pyroxenes likely experienced variable liquid compositions as a result of convection in a differentially cooling, chemically stratified magma chamber. Periodic replenishments of progressively decreasing volumes of primitive parental magma are also permissible and may have enabled convection. In a convection model, Mg-rich bands reflect growth in the lower, warmer, more crystal-poor regions of the chamber, whereas Ca-Al-Ti-Cr-rich bands reflect growth in the upper, cooler, more crystal-rich regions of the chamber. The limited duration of crystallization in the magma chamber and the slow diffusion rates of multiple elements among multiple crystallographic sites in clinopyroxene, combined with fast cooling upon eruption, act to preserve the oscillatory zoning. Oscillatory zoning of P in olivine is a product of solute trapping resulting from the slow diffusion of P in silicate melts and minerals, and relatively fast magma cooling rates that may be related to magma chamber convection. Differential cooling of the chamber and the fast cooling rates within the chamber are likely a product of the thermal state of the lunar crust at 2.93 Ga when NWA 032, which is currently the youngest dated lunar igneous rock, erupted onto the surface of the Moon.

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.