Northwest Africa (NWA) 15189
Classification led by Jen Mitchell
This meteorite belongs to Gemma Thomas, AKA Meteorite Girl UK.
The type specimen (22g) will be on deposit at the University of Plymouth, along with a single, polished block. The main mass is held by Gemma Thomas.
Meteorites often break into several pieces as they hurtle through Earth's atmosphere and land on the ground. Sometimes they land close together, or they can be scattered across the landscape in a "strewn field". NWA 15189 was brought to us as several chunks of rock, so we had to find out if they were all the same meteorite.
To do this, we analysed two different stones of NWA 15189 and found their minerals were a perfect match! This meteorite is a breccia made up of chunks of rock and mineral fragments but the proportions of the different minerals and the types of rock we could identify were also the same in both pieces, so we're confident that both stones are the same.
Naming a Breccia...
There are lots of naming conventions in geoscience, and as NWA 15189 is a HED meteorite there are some specific ones that we need to keep track of. In this case, we have to classify the meteorite as a polymict eucrite or a howardite. Polymict eucrites are made from chunks of many different types of eucrite (Vesta's basaltic lavas) which can be identified by composition and texture. Howardites are also polymict breccias, but they contain more than 10% diogenite (orthopyroxene cumulate rocks) material. By collecting element maps of across the sample, we then calculate how much diogenite material is mixed into our rock. In this case, there's not very much which means that NWA 15189 is a polymict eucrite and not a howardite.
Combined Mg-Ca-Fe element maps of the two analysed stones of NWA 15189, overlain with accessory phases. Bright pink = diogenite pyroxene; dark pink & purple = low-Ca pyroxene; orange = high-Ca pyroxene; dark green = plagioclase; bright green = terrestrial weathering products; turquoise = silica; white = ilmenite; yellow = troilite.
Backscattered electron (BSE) image of a melt pocket in NWA 15189. These crystals are only several tens of microns in length, suggesting that they were cooled really quickly.
Very, very hot...
There's evidence of thermal metamorphism everywhere you look in NWA 15189, but there are also areas where we can see evidence of temperatures getting high enough to actually cause melting. This also explains why MGUK 04 looks very dark in colour compared to other HED meteorites.
It's possible to calculate the maximum metamorphic temperature experienced by a rock using a two-pyroxene thermometer - a fancy bit of maths that uses pyroxene compositions. Our calculations show that MGUK 04 reached almost 1250 °C! This is hot enough to cause a eucrite to melt and is likely the result of impacts on the parent body.