Timothy J. Fagan, Daiju Kashima, Yuki Wakabayashi, Akiko Suginohara

Department of Earth Sciences, Waseda University, 1-6-1 Nishiwaseda, Shinjuku, Tokyo 169-8050

Pyroxene and feldspar compositions indicate that most clasts from the Northwest Africa 773 (NWA 773) lunar meteorite breccia crystallized from a common very low-Ti (VLT) mare basalt parental magma on the Moon. An olivine cumulate (OC), with low-Ca and high-Ca pyroxenes and plagioclase feldspar formed during early stages of crystallization, followed by pyroxene gabbro, which is characterized by zoned pyroxene (Fe# = molar Fe/(Fe+Mg) x 100 from ~35 to 90; Ti# = molar Ti/(Ti+Cr) x 100 from ~20 to 99) and feldspar (~An_90-95Ab_05-10 to An_80-85Ab_10-16). Late stage lithologies include alkali-poor symplectite consisting of fayalite, hedenbergitic pyroxene and silica, and alkaline-phase-ferroan clasts characterized by K-rich glass and/or K,Ba-feldspar with fayalite and/or pyroxene. Igneous silica only occurs with the alkaline-phase-ferroan clasts. This sequence of clasts represents stages of magmatic evolution along a ferroan-titanian trend characterized by correlated Fe# and Ti# in pyroxene, and a wide range of increase in Fe# and Ti# prior to crystallization of igneous silica.
Clasts of Apollo 15 quartz monzodiorite (QMD) also have pyroxene co-existing with silica, but the QMD pyroxene has more moderate Fe# (~70). Thus, in AFM components (A = Na₂O+K₂O, M = MgO, F = FeO), the QMD clasts are similar to the terrestrial calc-alkaline trend (silica-enrichment at moderate Fe#), whereas the ferroan-titanian trend is similar to the terrestrial tholeiitic trend (silica-enrichment only after strong increase in Fe#). However, the variations in SiO₂-contents of QMD clasts are due to variable mixing of SiO₂-rich and FeO-rich immiscible liquids (i.e., not a progressive increase in SiO₂). Immiscibility occurred after fractionation of a KREEP-rich parent liquid.
A third trend is based on zoning relations within the NWA 773 OC, where pyroxene Ti# increases at constant Fe# with proximity to intercumulus, incompatible element-rich pockets rich in K,Ba-feldspar and Ca-phosphates. This type of fractionation (increasing refractory trace elements at constant Fe#) in a cumulate parent rock may have been important for generating lunar rocks that combine low Fe# with high incompatible trace element concentrations, such as KREEP basalts and the magnesian suite.
MELTS (Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998) models of one VLT, one low-Ti and two high-Ti mare basalts and one KREEP basalt all show evolution from low to high Fe# residual liquids during fractional crystallization; however strong enrichments in FeO-concentrations are limited to the VLT and low-Ti liquids. In the high-Ti liquids, crystallization of Fe-Ti-oxides prevents enrichment in FeO, and the increases in Fe# are due to depletion of MgO. Fe-Ti-oxide fractionation results in steady silica-enrichment in the high-Ti mare compositions. Intervals of FeO-enrichment on the VLT and low-Ti mare liquid lines of descent are linked to shifts from olivine to pyroxene crystallization. The onset of plagioclase feldspar crystallization limits the depletion of FeO during crystallization of one high-Ti mare basalt and of the KREEP basalt composition modeled.

Reference
Fagan TJ, Kashima D, Wakabayashi Y and Suginohara A (in press) Case Study of Magmatic Differentiation Trends on the Moon based on Lunar Meteorite Northwest Africa 773 and Comparison with Apollo 15 Quartz Monzodiorite. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.02.025]
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Wren Montgomery, Jonathan S. Watson, and Mark A. Sephton

Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London SW7 2AZ, UK

There are a number of key structures that can be used to reveal the formation and modification history of organic matter in the cosmos. For instance, the susceptibility of organic matter to heat is well documented and the relative thermal stabilities of different isomers can be used as cosmothermometers. Yet despite being an important variable, no previously recognized organic marker of pressure exists. The absence of a pressure marker is unfortunate considering our ability to effectively recognize extraterrestrial organic structures both remotely and in the laboratory. There are a wide variety of pressures in cosmic settings that could potentially be reflected by organic structures. Therefore, to develop an organic cosmic pressure marker, we have used state-of-the-art diamond anvil cell (DAC) and synchrotron-source Fourier transform infrared (FTIR) spectroscopy to reveal the effects of pressure on the substitution patterns for representatives of the commonly encountered methyl substituted naphthalenes, specifically the dimethylnaphthalenes. Interestingly, although temperature and pressure effects are concordant for many isomers, pressure appears to have the opposite effect to heat on the final molecular architecture of the 1,5-dimethylnaphthalene isomer. Our data suggest the possibility of the first pressure parameter or “cosmo-barometer” (1,5-dimethylnaphthalene/total dimethylnaphthalenes) that can distinguish pressure from thermal effects. Information can be obtained from the new pressure marker either remotely by instrumentation on landers or rovers or directly by laboratory measurement, and its use has relevance for all cases where organic matter, temperature, and pressure interplay in the cosmos.

Reference
Montgomery W, Watson JS and Sephton MA (2014) An Organic Cosmo-barometer: Distinct Pressure and Temperature Effects for Methyl Substituted Polycyclic Aromatic Hydrocarbons. The Astrophysical Journal 784:98.
[doi:10.1088/0004-637X/784/2/98]

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Julien Monteux¹ and Jafar Arkani-Hamed²

¹Laboratoire de Planétologie et de Géodynamique, Université de Nantes, Nantes, France
²Department of Physics, University of Toronto, Toronto, Ontario, Canada

A giant impact is an increasingly popular explanation for the formation of the northern lowland on Mars. It is plausible that at the impact time both Mars and the impactor were differentiated with solid silicate mantles and liquid iron cores. Such a large impact likely resulted in merging of the cores of both bodies, a process which will have implications on the thermal state of the planet. We model the evolution of the Martian mantle following a giant impact and characterize the thermochemical consequences of the sinking of an impactor’s core as a single diapir. The impact heating and the viscous heating induced during the core merging may affect the early thermal state of Mars during several tens of million years. Our results show that large viscosity contrasts between the impactor’s core and the surrounding mantle silicates can reduce the duration of the merging down to 1 kyr but do not modify the merging temperature. When the viscosity contrast between the diapir and the surrounding silicates is larger than a factor of 1000, the descent of the diapir can lead to some entrainment of the relatively shallow silicates to deepest regions close to the core-mantle boundary. Finally, the direct impact heating of Martian core leads to thermal stratification of the core and kills the core dynamo. It takes on the order of 150–200 Myr to reinitiate a strong dynamo anew. The merging of the impactor’s core with the Martian core only delays the reinitiation of the dynamo for a very short time.

Reference
Monteux J and Arkani-Hamed J (in press) Consequences of giant impacts in early Mars: Core merging and Martian dynamo evolution. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004587]
Published by arrangement with John Wiley & Sons

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Eric Tonui¹ et al.

¹BP Upstream Research and Technology, 501 Westlake Boulevard, Houston, TX 77079, USA

The authors regret the figure captions of Figs. 1, 13, and 14 fail to provide some necessary references. The following sentences are to be added at the end of the captions, respectively.

Fig. 1. Figures b, c, and d adapted from Nakamura (2005).

Fig. 13. Replotted from Hiroi et al. (1996).

Fig. 14. Spectral data of Y-82162, Y-86720, and B-7904 are from Hiroi et al. (1996).

 The authors would like to apologise for any inconvenience caused.

Reference
Tonui E et al. (in press) Corrigendum to “Petrographic, chemical and spectroscopic evidence for thermal metamorphism in carbonaceous chondrites I: CI and CM chondrites” [Geochim. Cosmochim. Acta 126 (2014) 284–306]. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.02.022]
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