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.

ZS Unit Aosta valley, glaucophane bearing eclogite (scanned thin section 50x30mm) showing that glaucophane may be stable under eclogite facies, together with omphcite + grt _ rutile

A terrestrial eclogite in thin-section

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:

Notes:
1. the North Africa 801 CR chondrite meteorite
2. both literally and metaphorically
Categories: eclogites, geochemistry, subduction

Comments (3)

  1. ABDUL MUTTALIB says:

    Dear Sir,
    I have read about your research on eclogite. I am doutbfull about mode of formation of eclogite .The condition of formation of eclogite in both location is different .
    1. Eclogite in meteorite
    2. Eclogite in subduction Zone
    If your research is accepted. Then, please give me my answer.
    1. Carbonatite have carbon
    2. Coal have carbon.
    I think both carbon content have lot of difference in their origin.

    • Metageologist says:

      Thanks for your remarks.
      This is not my research: I am merely writing about it.

      You are correct in saying that Eclogite in the meteorite formed in a different location from eclogite on earth (usually found in subduction zones).
      I’m not sure what you mean by your remarks about carbon. The eclogites discussed here are not rich in carbon.

  2. morganism says:

    Theia is not the answer. Latest papers on Luna geo show surface isotopic comp is exactly the same as Earth. More likely the Earth slammed into Luna, and that is how it captured all the water it now has. Would also explain the shocked core, mares and the farside highlands and cratering rates. If Luna was re-sweeping the orbit.

    I originally stopped by because of this exact search. Spinels and other inclusions in meteorites led me to eclogites. The real problem out there, is that most of these rocks are glued together with hydrates, commonly referred to as mud.
    How can something deep inside large bodies mix with shocked and fused and tumbled hi temp materials, coalesce with low temp muds that haven’t been above 200k ?

    And it is possible that the perovskites and olivines shadings could actually result from core destruction depressurizing , and causing freeze outs that would resemble mixing.
    http://pubs.rsc.org/en/Content/ArticleLanding/2013/CP/c3cp50678a

    About the best reference site out there is at
    http://www.psrd.hawaii.edu/July08/H-chondrite-parent.html

    They have pretty good mineralogy , Better than the Flagstaff astrogeo site !
    Springer has an astrominerology print, thin, and almost entirely on dust formation and clumping.

    A lot of papers get reshown at the LPI meets, and they have some older astromin books up for download on the site
    http://www.lpi.usra.edu/meetings/lpsc2013/pdf/program.pdf

    This guy is about the only one keeping updated astrochemistry info on the web
    http://astrochymist.org/

    Mining guys are very interested in astrogeo, but they don’t share info.
    http://www.ustream.tv/channel/space-manufacturing-14

    https://www.youtube.com/channel/UCQ4pUVq3euPwUNX00If0FTw?feature=watch

    Was working with the folks at the Yahoo group NEAmines, but we have faltered in the face of Planetary Resources. That said, i still find the astromin very interesting, and still doing research on it.

    Pretty tough without a journal sub

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