Chlorine belongs to a group of elements known as the halogens, which are important tracers to investigate questions related to planet formation processes or habitability, for example (more on that in this previous blog entry on halogens in chondrite meteorites). Chlorine also has two stable isotopes, 35Cl and 37Cl. Because these two isotopes have slightly different masses, their relative abundances can be fractionated in response to some geological processes such as degassing of Cl-bearing gases from a magma.
In the Solar System, the 37Cl/35Cl ratios measured in chondritic meteorites, which represent samples from a large range of small bodies within the asteroid belt between the orbits of Mars and Jupiter, are very similar to the 37Cl/35Cl ratio measured in rocks derived from the Earth mantle (see this study involving group member Rhian Jones). Meteorites from Mars are characterised by slightly larger 37Cl/35Cl variations of around +/- 0.5% compared to the average 37Cl/35Cl ratio of the Earth’s ocean (see figure below). However, nothing in the Solar System comes anywhere close to the range of 37Cl/35Cl ratios in rocks from the Moon, which can be up to ~4% higher than the ratio of the Earth’s ocean (see figure below – 4% is incredibly large in the Cl isotope world)! In a paper published a couple of years ago, we have interpreted the large 37Cl enrichment displayed by KREEPy lunar rocks (those enriched in incompatible elements such as potassium [K], the rare earth elements [REE], and phosphorous [P] – see this blog by Emily Lakdawalla) as resulting from degassing of Cl-bearing gases from the Lunar Magma Ocean towards the end of its solidification, possibly triggered by a huge impact that exposed it at the Moon’s surface.
In a new study led by Dr Nicola Potts from the University of Edinburgh and colleagues at the Open University published in Geochimica et Cosmochimica Acta, we present the results of a detailed study of the Cl isotope composition of the mineral apatite in rocks samples collected in 1971 by astronauts Alan Shepard and Edgar Mitchell during the Apollo 14 mission.
Our new results are consistent with the large range of Cl isotope variations observed in previous studies of lunar samples, apatite in Apollo 14 rocks having 37Cl/35Cl ratios ~1.5 to 4% higher than the ratio of the Earth’s ocean. In the the studied samples, Cl isotope ratios in apatite display a rather large intra-sample variability that is hard to explain by magmatic processes only, but which may have involved interaction of the rocks with a vapour phase after their formation around 4 billion years ago. Similar observations have been made for samples collected at the Apollo 16 and 17 landing sites, pointing to the intriguing possibility of large scale vapour-rock interaction around 4 billion years ago on the nearside of the Moon.
Full paper citation:
Potts N.J., Barnes J.J., Tartese R., Franchi I.A. and Anand M. (2018) Chlorine isotopic compositions of apatite in Apollo 14 rocks: Evidence for widespread vapor-phase metasomatism on the lunar nearside 4 billion years ago. Geochimica Et Cosmochimica Acta 230, 46-59. doi.org/10.1016/j.gca.2018.03.022