Ansgar Greshake

Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Berlin, Germany

Hydrous carbonaceous microclasts are by far the most abundant foreign fragments in stony meteorites and mostly resemble CI1-, CM2-, or CR2-like material. Their occurrence is of great importance for understanding the distribution and migration of water-bearing volatile-rich matter in the solar system. This paper reports the first finding of a strongly hydrated microclast in a Rumuruti chondrite. The R3-6 chondrite Northwest Africa 6828 contains a 420 × 325 μm sized angular foreign fragment exhibiting sharp boundaries to the surrounding R-type matrix. The clast is dominantly composed of magnetite, pyrrhotite, rare Ca-carbonate, and very rare Mg-rich olivine set in an abundant fine-grained phyllosilicate-rich matrix. Phyllosilicates are serpentine and saponite. One region of the clast is dominated by forsteritic olivine (Fa_<2) supported by a network of interstitial Ca-carbonate. The clast is crosscut by Ca-carbonate-filled veins and lacks any chondrules, calcium-aluminum-rich inclusions, or their respective pseudomorphs. The hydrous clast contains also a single grain of the very rare phosphide andreyivanovite. Comparison with CI1, CM2, and CR2 chondrites as well as with the ungrouped C2 chondrite Tagish Lake shows no positive match with any of these types of meteorites. The clast may, thus, either represent a fragment of an unsampled lithology of the hydrous carbonaceous chondrite parent asteroids or constitute a sample from an as yet unknown parent body, maybe even a comet. Rumuruti chondrites are a unique group of highly oxidized meteorites that probably accreted at a heliocentric distance >1 AU between the formation regions of ordinary and carbonaceous chondrites. The occurrence of a hydrous microclast in an R chondrite attests to the presence of such material also in this region at least at some point in time and documents the wide distribution of water-bearing (possibly zodiacal cloud) material in the solar system.

Reference
Greshake A (in press) A strongly hydrated microclast in the Rumuruti chondrite NWA 6828: Implications for the distribution of hydrous material in the solar system. Meteoritics & Planetary Science
[doi:10.1111/maps.12295]
Published by arrangement with John Wiley & Sons

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R.M. Suggs^a, D.E. Moser^b, W.J. Cooke^a and R.J. Suggs^a

^aNASA, Marshall Space Flight Center, Meteoroid Environment Office, Natural Environments Branch, EV44 Marshall Space Flight Center, Alabama 35812
^bMITS/Dynetics, Marshall Space Flight Center, Meteoroid Environment Office, Natural Environments Branch, EV44 Marshall Space Flight Center, Alabama 35812

The flashes from meteoroid impacts on the Moon are useful in determining the flux of impactors with masses as low as a few tens of grams. A routine monitoring program at NASA’s Marshall Space Flight Center has recorded over 300 impacts since 2006. A selection of 126 flashes recorded during periods of photometric skies was analyzed, creating the largest and most homogeneous dataset of lunar impact flashes to date. Standard CCD photometric techniques were applied to the video and the luminous energy, kinetic energy, and mass are estimated for each impactor. Shower associations were determined for most of the impactors and a range of luminous efficiencies was considered. The flux to a limiting energy of 2.5×10^-6 kT TNT or 1.05×10⁷ J is 1.03×10^-7 km^-2 hr^-1 and the flux to a limiting mass of 30 g is 6.14×10^-10 m^-2 yr^-1 at the Moon. Comparisons made with measurements and models of the meteoroid population indicate that the flux of objects in this size range is slightly lower (but within the error bars) than flux at this size from the power law distribution determined for the near Earth object and fireball population by Brown et al. 2002. Size estimates for the crater detected by Lunar Reconnaissance Orbiter from a large impact observed on March 17, 2013 are also briefly discussed.

Reference
Suggs RM, Moser DE, Cooke WJ and Suggs RJ (in press) The Flux of Kilogram-sized Meteoroids from Lunar Impact Monitoring. Icarus
[doi:0.1016/j.icarus.2014.04.032]
Copyright Elsevier

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Claire L. McLeod, Alan D. Brandon, Rosalind M.G. Armytage

Department of Earth and Atmospheric Sciences, Science and Research 1, University of Houston, 4800 Calhoun Road, Houston, TX, 77204-5007, USA

The Moon likely formed as a result of a giant impact between proto-Earth and another large body. The timing of this event and the subsequent lunar differentiation timescales are actively debated. New high-precision Nd isotope data of Apollo mare basalts are used to evaluate the Low-Ti, High-Ti and KREEP mantle source reservoirs within the context of lunar formation and evolution. The resulting models are assessed using both reported ¹⁴⁶Sm half-lives (68 and 103 Myr). The linear relationship defined by ¹⁴²Nd–¹⁴³Nd systematics does not represent multi-component mixing and is interpreted as an isochron recording a mantle closure age for the Sm–Nd system in the Moon. Using a chondritic source model with present day μ  ¹⁴²Nd of −7.3, the mare basalt mantle source reservoirs closed at  () or  (). In a superchondritic, 2-stage evolution model with present day  of 0, mantle source closure ages are constrained to  () or  ().

The lunar mantle source reservoir closure ages <4.5 Ga may be reconciled by 3 potential scenarios. First, the Moon formed later than currently favored models indicate, such that the lunar mantle closure age is near or at the time of lunar formation. Second, the Moon formed ca. 4.55 to 4.47 Ga and small amounts of residual melts were sustained within a crystallizing lunar magma ocean (LMO) for up to ca. 200 Myr from tidal heating or asymmetric LMO evolution. Third, the LMO crystallized rapidly after early Moon formation. Thus the Sm–Nd mantle closure age represents a later resetting of isotope systematics. This may have resulted from a global wide remelting event. While current Earth-Moon formation constraints cannot exclusively advocate or dismiss any of these models, the fact that U–Pb ages and Hf isotopes for Jack Hills zircons from Australia are best explained by an Earth that re-equilibrated at 4.4 Ga or earlier following the Moon-forming impact, does not favor a later forming Moon. If magma oceans crystallize in a few million years as currently advocated, then a global resetting, possibly by a large impact at 4.40 to 4.34 Ga, such as that which formed the South Pole Aitken Basin, best explains the late mantle closure age for the coupled Sm–Nd isotope systematics presented here.

Reference
McLeod CL, Brandon AD and Armytage RMG (2014) Constraints on the formation age and evolution of the Moon from 142Nd–143Nd systematics of Apollo 12 basalts. Earth and Planetary Science Letters 396:179.
[doi:10.1016/j.epsl.2014.04.007]
Copyright Elsevier

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