Exciting extraterrestrial eclogites

Eclogites are beautiful rocks that on Earth are associated with the process of subduction – where pieces of crust sink into the deep mantle region. A recent paper by Makoto Kimura and 5 other Japanese authors, describes the first ever evidence of eclogitic rocks found beyond Earth, formed within an unusually large asteroid now found only as tiny pieces.

Our authors were studying samples of a meteorite1 called a chondrite. It contains numerous small fragments (“clasts”) of different types. The paper focuses on a single set of clasts that contain distinctive eclogitic minerals – omphacite and pyrope-rich garnet. The sample also contains more typical minerals such as olivine and orthopyroxene. All of the Sodium and Aluminium within the sample is found within the garnet and omphacite – indicative of formation under high pressure. Based on the black arts of geothermobarometry our authors estimate formation under conditions of 2.8–4.2 GPa and 940–1080 °C.

On earth, eclogitic minerals are associated with subduction because this is a process that makes rocks experience high pressures and provides mechanisms for getting them back to the surface where we can enjoy then. This meteorite sample formed in a different world 2 – so there is no need to infer subduction on another body, but it is still remarkable (comment added later). To quote the paper: “It is believed that meteorites formed in small asteroidal bodies under very low-pressure conditions, except for the high pressures produced during secondary impact events, as recorded in features such as shock veins.” But these high-pressure minerals do not appear to have been formed in a shock vein, but within the interior of an unusually large asteroidal body (that was later smashed into pieces).

How large a body would this need to be? On earth, pressures like this are found at 100km depth. Does this mean the asteroid could have been 100km in radius? No. The pressure is caused by the weight of the rocks above and so relates to the gravitational pull of the entire body. The smaller the body, the lower the force of gravity. Back of the envelope calculations suggest that in order to achieve these pressures, the asteroid would need to have a radius of 1000s of kilometres – getting into planet territory. By comparison, the pressure at the core of our (unusually large) moon has been estimated to be 4.5GPa 3, which is only slightly higher than the upper pressure estimate from these samples.

This study is based on a tiny fragment of rock – only three thin sections. But from this, we can infer there once existed a huge piece of rock, now smashed into countless fragments. All thanks to our understanding the way minerals behave under different conditions.

Update: I do like Twitter. Various geotweeps found this story as interesting as I did. @lockwooddewit has long suspected that some types of meteorite (such as kamacite Fe-Ni ones) “suggest major differentiated body existed“. Pieces of eclogitic mantle would be consistent with this. Ryan Brown (@glacialtill) pointed out that “we know planets were differentiating w/in the first few million years of the soar system- few survived though“.

One thing that struck me my untutored eye was how remarkable it would be that a large body could form and be destroyed and the only trace be a tiny fragment in one meteorite. Andrew Alden (@aboutgeology) points out in a post that there is an obvious candidate – Theia – “the “Mars-sized object” that is thought to have collided with Earth, way back in the Hadean Eon, to create the mess that formed the Moon.”

One striking thing about the paper is the lack of speculation  about the source of this material – the guessing all comes from me and the folks mentioned above. The last paragraph suggests there is more to come from Kimura et al. – “The precursor materials of the clasts, and the genetic relationships between the clasts and the host CR chondrite, are not yet clear. We are now measuring the isotopic and trace element compositions of the clasts, which will shed light on this issue.”  Studies like this have a great record of tracing events from the early solar system. I look forward to their next paper.

References

Many thanks to @TriclinicFlow (Konstantinos) for alerting me to this paper:

Kimura M., Sugiura N., Mikouchi T., Hirajima T., Hiyagon H. & Takehana Y. (2013). Eclogitic clasts with omphacite and pyrope-rich garnet in the NWA 801 CR2 chondrite, American Mineralogist, 98 (2-3) 387-393. DOI: 10.2138/am.2013.4192