Mahesh Anand

Planetary and Space Sciences, Open University, Milton Keynes MK7 6AA, UK.

The paradigm of a “dry Moon” was recently challenged on the basis of reexamination of lunar samples collected during the Apollo missions, raising the possibility of a volatile-rich lunar interior (1–6). Several of these studies measured appreciable quantities of water (reported as equivalent H, OH, or H₂O) and other volatiles (e.g., Cl, F) in the mineral apatite (4–6), which is ubiquitous in lunar basalts (mare basalts) (see the figure). However, an accurate estimation of the water content of the magmatic liquid from which apatite formed, and ultimately of the mantle source regions of mare basalts, depends on a number of parameters. In cases where apatite in a mare basalt formed through the process of fractional crystallization (when newly formed crystals in a cooling magma are physically separated, preventing any further interaction with the remaining melt), it may not be possible to obtain any reliable estimates of the water contents of the parental magma (and its source region). On page 400 of this issue, Boyce et al. (7) present an elegant numerical model, applicable to mare basalt apatite formed through fractional crystallization, to demonstrate that some of the highest water contents reported for lunar apatite can be reconciled with an original melt containing not much water at all. These new results cast doubt on the utility of apatite volatile abundances in reliably estimating the water content of mare basalt source regions.

Reference
Anand M (2014) Analyzing Moon Rocks. Science 344:365.
[doi:10.1126/science.1253266]
Reprinted with permission from AAAS

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J. W. Boyce¹, S. M. Tomlinson¹, F. M. McCubbin², J. P. Greenwood³ and A. H. Treiman⁴

¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095, USA.
²Institute for Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA.
³Department of Earth and Environmental Sciences, Wesleyan University, 265 Church Street, Middletown, CT 06459, USA.
⁴Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058–1113, USA.

Recent discoveries of water-rich lunar apatite are more consistent with the hydrous magmas of Earth than the otherwise volatile-depleted rocks of the Moon. Paradoxically, this requires H-rich minerals to form in rocks that are otherwise nearly anhydrous. We modeled existing data from the literature, finding that nominally anhydrous minerals do not sufficiently fractionate H from F and Cl to generate H-rich apatite. Hydrous apatites are explained as the products of apatite-induced low magmatic fluorine, which increases the H/F ratio in melt and apatite. Mare basalts may contain hydrogen-rich apatite, but lunar magmas were most likely poor in hydrogen, in agreement with the volatile depletion that is both observed in lunar rocks and required for canonical giant-impact models of the formation of the Moon.

Reference
Boyce JW, Tomlinson SM, McCubbin FM, Greenwood JP and Treiman AH (2014) The Lunar Apatite Paradox. Science 344:400.
[doi:10.1126/science.1250398]
Reprinted with permission from AAAS

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Norman H. Sleep¹ and Donald R. Lowe²

¹Department of Geophysics, Stanford University, Stanford, California, USA
²Department of Geological and Environmental Sciences, Stanford University, Stanford, California, USA

Archean asteroid impacts, reflected in the presence of spherule beds in the 3.2–3.5 Ga Barberton greenstone belt (BGB), South Africa, generated extreme seismic waves. Spherule bed S2 provides a field example. It locally lies at the contact between the Onverwacht and Fig Tree Groups in the BGB, which formed as a result of the impact of asteroid (possibly 50 km diameter). Scaling calculations indicate that very strong seismic waves traveled several crater diameters from the impact site, where they widely damaged Onverwacht rocks over much of the BGB. Lithified sediments near the top of the Onverwacht Group failed with opening-mode fractures. The underlying volcanic sequence then failed with normal faults and opening-mode fractures. Surficial unlithified sediments liquefied and behaved as a fluid. These liquefied sediments and some impact-produced spherules-filled near-surface fractures, today represented by swarms of chert dikes. Strong impact-related tsunamis then swept the seafloor. P waves and Rayleigh waves from the impact greatly exceeded the amplitudes of typical earthquake waves. The duration of extreme shaking was also far longer, probably hundreds of seconds, than that from strong earthquakes. Dynamic strains of ~10⁻³ occurred from the surface and downward throughout the lithosphere. Shaking weakened the Onverwacht volcanic edifice and the surface layers locally moved downhill from gravity accommodated by faults and open-mode fractures. Coast-parallel opening-mode fractures on the fore-arc coast of Chile, formed as a result of megathrust events, are the closest modern analogs. It is even conceivable that dynamic stresses throughout the lithosphere initiated subduction beneath the Onverwacht rocks.

Reference
Sleep NH and Lowe DR (in press) Physics of crustal fracturing and chert dike formation triggered by asteroid impact, ∼3.26 Ga, Barberton greenstone belt, South Africa. Geochemistry, Geophysics, Geosystems
[doi:10.1002/2014GC005229.]
Published by arrangement with John Wiley & Sons

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Wladimir Neumann^a, Doris Breuer^a and Tilman Spohn^a,b

^aInstitute of Planetary Research, German Aerospace Center (DLR), Rutherfordstraße 2, 12489 Berlin, Germany
^bInstitute of Planetology, Westfälische Wilhelm-University Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany

The Dawn mission confirms earlier predictions that the asteroid 4 Vesta is differentiated with an iron-rich core, a silicate mantle and a basaltic crust, and supports the conjecture of Vesta being the parent body of the HED meteorites. To better understand its early evolution, we perform numerical calculations of the thermo-chemical evolution adopting new data obtained by the Dawn mission such as mass, bulk density and size of the asteroid.
We have expanded the thermo-chemical evolution model of Neumann et al. (2012) that includes accretion, compaction, melting and the associated changes of the material properties and the partitioning of incompatible elements such as the radioactive heat sources, advective heat transport, and differentiation by porous flow, to further consider convection and the associated effective cooling in a potential magma ocean. Depending on the melt fraction, the heat transport by melt segregation is modelled either by assuming melt flow in a porous medium or by simulating vigorous convection and heat flux of a magma ocean with a high effective thermal conductivity.
Our results show that partitioning of ²⁶Al and its transport with the silicate melt is crucial for the formation of a global and deep magma ocean. Due to the enrichment of ²⁶Al in the liquid phase and its accumulation in the sub-surface (for formation times t₀<1.5 Ma), a thin shallow magma ocean with a thickness of 1 to a few tens of km forms – its thickness depends on the viscosity of silicate melt. The lifetime of the shallow magma ocean is O(10⁴)–O(10⁶) years and convection in this layer is accompanied by the extrusion of ²⁶Al at the surface, resulting in the formation of a basaltic crust. The interior differentiates from the outside inwards with a mantle that is depleted in ²⁶Al and core formation is completed within ∼0.3 Ma. The lower mantle experiences a maximal melt fraction of 45% suggesting a harzburgitic to dunitic composition. Our results support the formation of non-cumulate eucrites by the extrusion of early partial melt while cumulate eucrites and diogenites may form from the crystallising shallow magma ocean. Silicate melt is present in the mantle for up to 150 Ma, and convection in a crystallising core proceeds for approximately 100 Ma, supporting the idea of an early magnetic field to explain the remnant magnetisation observed in some HED meteorites.

Reference
Neumann W, Breuer D and Spohn T (2014) Differentiation of Vesta: Implications for a shallow magma ocean. Earth and Planetary Science Letters 395:267.
[doi:10.1016/j.epsl.2014.03.033]
Copyright Elsevier

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