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The study, published in Nature Geoscience, reveals a key step in the genesis of these distinctive magmas. A combination of high temperature laboratory experiments using molten rocks, together with sophisticated isotopic analyses of lunar samples, identify a critical reaction that controls their composition.
This reaction took place in the deep lunar interior some three and a half billion years ago, involving exchange of the element iron (Fe) in the magma with the element magnesium (Mg) in the surrounding rocks, modifying the chemical and physical properties of the melt.
Co-lead author Tim Elliott, Professor of Earth Sciences at the University of Bristol, said: "The origin of volcanic lunar rocks is a fascinating tale involving an 'avalanche' of an unstable, planetary-scale crystal pile created by the cooling of a primordial magma ocean.
"Central to constraining this epic history is the presence of a magma type unique to the Moon, but explaining how such magmas could even have got to the surface, to be sampled by Space missions, has been a troublesome problem. It is great to have resolved this dilemma."
Surprisingly high concentrations of the element titanium (Ti) in parts of the lunar surface have been known since the NASA Apollo missions, back in the 1960s and 1970s, which successfully returned solidified, ancient lava samples from the Moon's crust. More recent mapping by orbiting satellite shows these magmas, known as 'high-Ti basalts', to be widespread on the Moon.
"Until now models have been unable to recreate magma compositions that match essential chemical and physical characteristics of the high-Ti basalts. It has proven particularly hard to explain their low density, which allowed them to be erupted some three and a half billion years ago," added co-lead author Dr Martijn Klaver, Research Fellow at the University of Munster Institute of Mineralogy.
The international team of scientists, led by the Universities of Bristol in the UK and Munster in Germany managed to mimic the high-Ti basalts in the process in the lab using high-temperature experiments. Measurements of the high-Ti basalts also revealed a distinctive isotopic composition that provides a fingerprint of the reactions reproduced by the experiments.
Both results clearly demonstrate how the melt-solid reaction is integral in understanding the formation of these unique magmas.
Research Report:'Titanium-rich basaltic melts on the Moon modulated by reactive flow processes'
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