This blog has been written by group member Dr Rhian Jones
Ordinary chondrites are the most common type of meteorite that lands on the Earth. They are pieces of asteroids that were knocked free by collisions, and then wandered through the solar system for a few million years before hitting the Earth. There are three different groups of ordinary chondrites, named H, L and LL, which have varying amounts of iron,nickel metal. Therefore, these meteorites must be sampling at least three different asteroids, although that number is probably much larger because the original three bodies have been broken up by numerous collisions.
Group member Rhian Jones has been studying ordinary chondrites (OCs) to find out about the geological processes that took place on their parent asteroids in the early days of solar system history, soon after the asteroids formed. It has been recognized for many years that OC parent bodies were heated, many to high temperatures, around 950 °C, causing metamorphism in the rocks. Metamorphism changed the appearance of the rocks, blurring out the original texture of chondrules and matrix, and it changed the chemical composition of mineral grains. In addition, new minerals grew during the time the rocks were heated. Two important groups of new minerals are feldspar and phosphate minerals. Rhian has been studying what these groups of minerals tell us about conditions on the OC asteroids, in particular whether fluids were present when metamorphism took place. Fluids are an important part of understanding the geology of OC asteroids, but previously their presence has been overlooked.
Rhian has recently published two papers that discuss these topics. The first is about phosphate minerals in the H group of ordinary chondrites, published in American Mineralogist as part of a thematic issue on the phosphate mineral, apatite. This paper is co-authored by Francis McCubbin of NASA Johnson Space Center and Yunbin Guan of Caltech. We showed that the chemical composition of apatite varies from one meteorite to another, which means that the fluids from which apatite grew had variable compositions throughout the asteroid. Also, we think that because of the variable compositions, apatite grew as the asteroid cooled. It is important to understand when apatite grew because apatite is used to date rocks such as OCs, and we need to know what such an age means in relationship to the history of the asteroid. Another part of the work reported in this paper was our study of the H chondrite, Zag, which is a very interesting meteorite because it contains halite, or common rock salt. In this meteorite, we found that apatite compositions vary hugely, with a range of different ratios of chlorine to fluorine. Previously it has been proposed that halite might have been added to the asteroid from space, but we argue that it could be part of the fluid history within the asteroid that is also recorded in apatite.
The second paper is about feldspar and phosphate minerals in the L group of ordinary chondrites. This paper, published in Meteoritics and Planetary Science, was written by Rhian’s PhD student at the University of New Mexico, Jonathan Lewis. We showed that the way feldspar grows in response to metamorphism is similar in L and LL chondrites, but these two groups are different from H chondrites. However, in all cases we show that fluids play a very important role in feldspar growth, something that has not been recognized before. In contrast to feldspar, apatite compositions are different in the H, L and LL groups. This makes sense if feldspar formed as the asteroid heated up, whereas apatite formed much later as the asteroid cooled, as mentioned above.
Overall, these two papers show the importance of considering fluids during metamorphism of the OC parent asteroids, a process called metasomatism. Fluids include water and other volatile elements such as the halogens (chlorine, fluorine, etc). There is much interest in determining how volatile compounds behaved during the early history of the solar system, not least because it is important to understand the way that the Earth acquired its volatile elements if we are to understand the conditions under which life evolved.
The two papers discussed in this blog can be seen at:
Jones R. H., McCubbin F. M. and Guan Y. (2016) Phosphate minerals in the H group of ordinary chondrites, and fluid activity recorded by apatite heterogeneity in the Zag H3-6 regolith breccia. American Mineralogist 101, 2452-2467. doi:10.2138/am-2016-5728.
Lewis J. A. and Jones R. H. (2016) Phosphate and feldspar mineralogy of equilibrated L ordinary chondrites: The record of metasomatism during metamorphism in ordinary chondrite parent bodies. Meteoritics and Planetary Science 51, 1886-1913. doi:10.1111/maps.12719.