Romain Tartèse

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.

Experimental investigation of F, Cl, and OH partitioning between apatite and Fe-rich basaltic melt at 1.0-1.2 GPa and 950-1000 °c

Abstract Apatite-melt partitioning experiments were conducted in a piston-cylinder press at 1.0-1.2 GPa and 950-1000 °C using an Fe-rich basaltic starting composition and an oxygen fugacity within the range of ΔIW-1 to ΔIW+2. Each experiment had a unique F:Cl:OH ratio to assess the partitioning as a function of the volatile content of apatite and melt. The quenched melt and apatite were analyzed by electron probe microanalysis and secondary ion mass spectrometry techniques. The mineral-melt partition coefficients (D values) determined in this study are as follows: DFAp-Melt = 4.4-19, DClAp-Melt = 1.1-5, DOHAp-Melt = 0.07-0.24. This large range in values indicates that a linear relationship does not exist between the concentrations of F, Cl, or OH in apatite and F, Cl, or OH in melt, respectively. This non- Nernstian behavior is a direct consequence of F, Cl, and OH being essential structural constituents in apatite and minor to trace components in the melt. Therefore mineral-melt D values for F, Cl, and OH in apatite should not be used to directly determine the volatile abundances of coexisting silicate melts. However, the apatite-melt D values for F, Cl, and OH are necessarily interdependent given that F, Cl, and OH all mix on the same crystallographic site in apatite. Consequently, we examined the ratio of D values (exchange coefficients) for each volatile pair (OH-F, Cl-F, and OH-Cl) and observed that they display much less variability: KdCl-FAp-Melt = 0.21± 0.03, KdOH-FAp-Melt = 0.014 ± 0.002, and KdOH-ClAp-Melt = 0.06 ± 0.02 . However, variations with apatite composition, specifically when mole fractions of F in the apatite X-site were low (XF < 0.18), were observed and warrant additional study. To implement the exchange coefficient to determine the H2O content of a silicate melt at the time of apatite crystallization (apatitebased melt hygrometry), the H2O abundance of the apatite, an apatite-melt exchange Kd that includes OH (either OH-F or OH-Cl), and the abundance of F or Cl in the apatite and F or Cl in the melt at the time of apatite crystallization are needed (F if using the OH-F Kd and Cl if using the OH-Cl Kd). To determine the H2O content of the parental melt, the F or Cl abundance of the parental melt is needed in place of the F or Cl abundance of the melt at the time of apatite crystallization. Importantly, however, exchange coefficients may vary as a function of temperature, pressure, melt composition, apatite composition, and/or oxygen fugacity, so the combined effects of these parameters must be investigated further before exchange coefficients are applied broadly to determine volatile abundances of coexisting melt from apatite volatile abundances.

Understanding the origin and evolution of water in the Moon through lunar sample studies

A paradigm shift has recently occurred in our knowledge and understanding of water in the lunar interior. This has transpired principally through continued analysis of returned lunar samples using modern analytical instrumentation. While these recent studies have undoubtedly measured indigenous water in lunar samples they have also highlighted our current limitations and some future challenges that need to be overcome in order to fully understand the origin, distribution and evolution of water in the lunar interior. Another exciting recent development in the field of lunar science has been the unambiguous detection of water or water ice on the surface of the Moon through instruments flown on a number of orbiting spacecraft missions. Considered together, sample-based studies and those from orbit strongly suggest that the Moon is not an anhydrous planetary body, as previously believed. New observations and measurements support the possibility of a wet lunar interior and the presence of distinct reservoirs of water on the lunar surface. Furthermore, an approach combining measurements of water abundance in lunar samples and its hydrogen isotopic composition has proved to be of vital importance to fingerprint and elucidate processes and source(s) involved in giving rise to the lunar water inventory. A number of sources are likely to have contributed to the water inventory of the Moon ranging from primordial water to meteorite-derived water ice through to the water formed during the reaction of solar wind hydrogen with the lunar soil. Perhaps two of the most striking findings from these recent studies are the revelation that at least some portions of the lunar interior are as water-rich as some Mid-Ocean Ridge Basalt source regions on Earth and that the water in the Earth and the Moon probably share a common origin.

Giant quartz vein formation and high-elevation meteoric fluid infiltration into the South Armorican Shear Zone: geological, fluid inclusion and stable isotope evidence

Giant quartz veins associated with the South Armorican Shear Zone record important fluid circulation during the Hercynian period. Regional-scale mapping of veins allows two groups of veins to be identified, on the basis of their geometric relationship with the South Armorican Shear Zone. Veins in the first group are parallel to the shear zone, whereas those in the second group developed in a direction oblique to it. The former probably record infiltration of fluids along permeable pathways in highly deformed zones; the latter may represent crustal-scale tension gashes in the regional context. Most quartz veins have δ18O values between 10 and 16‰ indicating a mid-crustal origin for the fluids. Microthermometry on fluid inclusions from euhedral quartz indicates that late fluids were mostly aqueous with very low salinity (0–1.7 wt% eq.) and with homogenization temperatures ranging between 150 and 270 °C. Together with very low δ18O values of some euhedral quartz, down to −2‰, these features argue for a surface origin. Corresponding δ18Ofluid values estimated near −11‰ are probably related to the high palaeo-elevation of meteoric precipitation. Scarce, but significant, H2O–CO2 fluid inclusions in euhedral quartz indicate a metamorphic contribution. These were probably sourced from the exhumed metamorphic basement in the southern part of the region.

NanoSIMS Pb/Pb dating of tranquillityite in high-Ti lunar basalts: Implications for the chronology of high-Ti volcanism on the Moon

Abstract In this study, we carried out Pb/Pb dating of tranquillityite in high-Ti mare basalts 10044, 75055, and 74255, using a Cameca NanoSIMS 50 at a spatial resolution of ~3 μm. The analyses yielded 207Pb/206Pb dates of 3722 ± 11 Ma for sample 10044, 3772 ± 9 Ma for sample 75055, and 3739 ± 10 Ma for sample 74255, at 95% confidence level. These dates are consistent with previously determined crystallization and emplacement ages of these samples using different radiogenic systems. These high-precision ages allow refinement of the timing of some of the high-Ti basaltic volcanism on the Moon. Crystallization ages of three different high-Ti basalt units, integrating these new Pb/Pb ages with previous Rb-Sr and Sm-Nd age determinations, are consistent with previous estimates but associated with uncertainties 3 to 5 times lower. In addition, the data obtained in this study confirm that tranquillityite contains very low amounts of initial common Pb and has a high-Pb ionization efficiency, making it an excellent candidate for Pb/Pb dating by ion microprobe. The higher spatial resolution afforded by NanoSIMS 50 and the recent discovery of tranquillityite in several terrestrial mafic rocks opens up a new area of research allowing an independent and rapid age dating of basaltic rocks in polished sections.