The Apollo 15 mission landed astronauts Dave Scott and Jim Irvine onto the surface of the Moon on the 30th July 1971. The mission visited an area on the nearside of the Moon, to the SW corner of the Imbrium impact basin. The landing site was located between Hadley Rille – a sinuous lava channel – and the Apennine mountains, which are part of Imbrium basin’s uplifted rim. You can explore the landing site further using the LRO mission LROC Featured Site tool.
Our team, led by Dr Sam Bell, has been investigating the volcanic history of the Apollo 15 landing site using basaltic samples collected by the mission. We borrowed several geological thin sections (~30 micron thin slice of highly polished rock glued to a glass slide) from NASA and have determined the texture and chemistry of the minerals found in the samples. We looked at samples from two different lava flows – one has more amounts of the mineral olivine, the other more quartz. We were interested in understanding if the lava flows originated from the same magma system, and how these magmas cooled as they were erupted from volcanoes at the lunar surface some 3.2 billion years ago.
In the first of the two papers (Bell et al., 2023a) we report on the shape of the crystals in the sample. Sam did an amazing amount of work to both count and measure the length of different minerals in the sample (see figure below). We could use these data to say if the crystals in the lavas cooled in one or more different events, suggesting that some crystals formed at depth within the Moon before the lava erupted and then cooled forming more minerals. Our study found that the two different lava flows (the olivine-rich and quartz-rich ones) had similar cooling histories.
In the second paper (Bell et al., 2023b) we present a detailed study of the chemistry of olivine minerals in the same basaltic samples. The element budget of olivine can be used to understand how the mineral has grown within a magma. This can tell us the rate of growth, helping us to calculate the number of days over which the erupted lava cooled. We used these records to show that average timescale including all crystals from both Apollo 15 mare basalt suites was 31 days and that the lava flows were likely to have been between 1.5 to 6 metres in thickness as they were cooling.
We can compare our estimates to observations of ‘field outcrops’ of lava flows at the Apollo 15 landing site, which were seen in the walls of Hadley Rille lava tube (see photo below). We postulate that olivine-richer basalt samples (erupted at 3287 ± 21 Ma) could have originated from the photographed upper massive units, and the quartz-rich basalts samples (erupted at 3371 ± 21 Ma) could correspond to the lower more thinly bedded units. Hopefully if astronauts ever return to the Apollo 15 landing site and scramble down the walls of Hadley Rille our hypothesis could be tested.
The new research papers can be accessed at:
Bell S. K., K. H. Joy, J. F. Pernet-Fisher and M. E. Hartley (2023a) Investigating the crystallisation history of Apollo 15 mare basalts using quantitative textural analysis Meteoritics and Planetary Science doi.org/10.1111/maps.14032
Bell, S.K., Morgan, D.J., Joy, K.H., Pernet-Fisher, J.F. and Hartley, M.E. (2023b) Determining the thermal histories of Apollo 15 mare basalts using diffusion modelling in olivine. Geochimica et Cosmochimica Acta. doi.org/10.1016/j.gca.2023.08.009
Funding was possible from an STFC PhD studentship to S.Bell, and STFC and Royal Society funding to K.Joy. We thank NASA and the Apollo curatorial team for the loan of materials.
More resources
Virtual Microscope – if you want to look at thin sections of the Apollo 15 samples these are presented at https://www.virtualmicroscope.org/content/apollo-15
Explore the Apollo 15 landing site at http://lroc.sese.asu.edu/featured_sites/view_site/4
The Apollo 15 Lunar Surface Journal has an amazing amount of resources including astronaut transcripts, photos and more from the mission https://history.nasa.gov/alsj/a15/a15.html
Earth & Solar System 