¹Thomas S. Kruijer,¹Lars E. Borg,¹William S. Cassata,¹Josh Wimpenny,¹Greg A. Brennecka,^2,3Charles K. Shearer,²Steven B. Simon
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.07.026]
¹Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue (L-231), Livermore, CA 94550, USA
²Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, USA
³Lunar and Planetary Institute, Houston, TX
Copyright Elsevier

Alkali-suite rocks constitute one of three major suites of lunar crustal rocks. As such, constraining their formation timescales and petrogenesis is important for understanding the earliest magmatic history of the Moon. However, the magmatic history of alkali-suite rocks is partly obscured by superimposed effects of major basin-forming impact events on the lunar nearside. Consequently, unambiguous crystallization ages of samples from this suite of rocks have not been determined. The aim of this study is to better understand the petrogenetic history of the alkali-suite and the potential superimposed effects of impact metamorphism by determining Sm-Nd, Rb-Sr, and 40Ar/39Ar ages for an alkali anorthosite clast from Apollo 14 lithic breccia 14304 termed clast “b”. The new chronologic measurements of clast “b” yield concordant Sm-Nd, Rb-Sr, and 40Ar/39Ar ages of 3947±13 Ma, 3975±34 Ma, and 3937±37 Ma respectively, resulting in a weighted mean age of 3949±11 Ma. This age is not interpreted to date an igneous event related to production of the lunar highlands crust and instead the chronology and petrography of clast “b” are most readily explained by an impact event at ∼3.95 Ga that caused near-complete isotopic re-equilibration of the Sm-Nd, Rb-Sr, and 40Ar/39Ar chronometers. The weighted mean age of 3949±11 Ma of clast “b” is several hundred Ma younger than 4.3-4.4 Ga ages typically determined for samples of other crustal rock suites but in very good agreement with independent estimates for the formation of Imbrium basin ejecta and other marginally older impact events which are thought to have been sampled at the Apollo 14 landing site. Thus, although petrologic and geochemical examination suggest that clast “b” is a pristine igneous clast, its age likely records an impact event at the Apollo 14 landing site. Whereas the various determined ages of clast “b” do not reflect the timescales of alkali-suite magmatism, the relatively low initial Sr and Nd isotopic compositions of clast “b” indicate that its protolith evolved with very low 147Sm/144Nd and 87Rb/86Sr that are distinct from estimates for urKREEP but similar to that of lunar plagioclase. This implies that the igneous protolith of clast “b” derived from a plagioclase-dominated KREEP-rich source that must have formed after the formation of the urKREEP source at ∼4.35 Ga but well before the impact event recorded by clast “b” at ∼3.95 Ga.

¹Don R. Baker,^2,3Sara Callegaro,⁴Andrea Marzoli,⁵Angelo De Min,⁶Kalotina Geraki,⁷Martin J. Whitehouse,^2,3Agata M. Krzesinska,⁸Anna Maria Fioretti
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.08.007]
¹McGill University, Department of Earth and Planetary Sciences, Montréal, Quebec, Canada
²Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Sem Sælands vei 2A N-0371 Oslo, Norway
³Centre for Planetary Habitability (PHAB), University of Oslo, Sem Sælands vei 2A, N-0371 Oslo, Norway
⁴University of Padova, Department of Land, Environment, Agriculture and Forestry, Legnaro, Italy
⁵Department of Mathematics and Geoscience, University of Trieste, via Weiss 2, 34128 Trieste, Italy
⁶Diamond Light Source, Harwell Science and Innovation Campus, Didcot, OX11 0DE, U.K
⁷Swedish Museum of Natural History, Stockholm, Sweden
⁸Instituto di Scienze Polari CNR, Padova Italy
Copyright Elsevier

The volatile concentrations of the martian mantle and martian magmas remain important questions due to their role in petrogenesis and planetary habitability. The sulfur and chlorine concentrations, and their spatial distribution, in clinopyroxenes from nakhlites MIL 03346, Nakhla, and NWA 998 were measured to provide insight into these volatiles in the parental melts and source regions of nakhlites, and to constrain the evolution of the nakhlite melts. Sulfur and chlorine in four clinopyroxene crystals from MIL 03346, four from Nakhla, and five from NWA 998 were measured in crystal cores and rims by synchrotron X-ray fluorescence using beamline I18 at the Diamond Light Source. Portions of two crystals from MIL 03346 and one from Nakhla were mapped for S and Cl; a few reconnaissance analyses of Cl and F in MIL 03346 and Nakhla were made by ion microprobe. Clinopyroxene cores in Nakhla and NWA 998 contain ∼ 10 ppm S, ∼ 10 ppm Cl and ∼ 74 ppm F (only Nakhla analyzed), whereas the cores of MIL 03346 contain ∼ 10 ppm S, ∼ 5 ppm Cl and ∼ 53 ppm F.

Using the volatile concentrations in the cores combined with previously determined partition coefficients we calculate that these clinopyroxenes crystallized from evolved basaltic melts containing ∼ 500 ppm S, ∼ 500 to 1900 ppm Cl, and 160 to 420 ppm F. These evolved melts can be used to calculate primitive melts in equilibrium with martian peridotite and the concentrations of S, Cl and F in the mantle source region of the nakhlite melts. Depending upon the extent of melting (5 to 30 %) necessary to produce the primary melts associated with nakhlites, our calculations indicate that the nakhlite source region has a S concentration between 20 (5 % melting) to 120 ppm (30 % melting), Cl between 16 to 97 ppm, and F between 14 to 48 ppm. These concentrations in the nakhlite magma source region are similar to previous estimates for the martian mantle; our calculated source region concentrations of F and Cl agree best with previous estimates if the martian mantle undergoes 10 to 20% melting to produce primary magmas that evolve to be parental to nakhlites. However, our maximum estimated sulfur concentration of the source (calculated for 30 % melting) is near previous minimum estimates for the martian mantle, suggesting the possibility that the nakhlite source region is depleted in sulfur relative to much of Mars’ mantle.

Mapping the spatial distribution of volatiles in three clinopyroxene crystals demonstrates that S and Cl concentrations of the evolving melts changed significantly from the core to the rim, particularly those in MIL 03346. Increasing S and Cl concentrations between the core and rim of MIL 03346 crystals are attributed to incorporation of additional volatiles through assimilation, but the Nakhla crystal shows no such evidence. However, concentrations of Cl and S at some outer crystal rims of one MIL 03346 crystal decrease, most probably due to volatile degassing during the final stages of clinopyroxene growth.