Enstatite and the Heliophile

Abee Enstatite Chondrite
Meteorlab photograph, R Kempton

Type: ENSTATITE CHONDRITE
Name: Abee
Found: Alberta, Canada
Composition: Orthopyroxene, sulfides
Texture: Impact Breccia

 

The first orbits of Mercury by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft revealed a crust that should not exist. Earlier measurements had shown that not only was Mercury dominated by a relatively huge iron core, but that its crust was thin and iron-poor. The lack of iron in the crust was also recognized from telescope observations. It was believed that something had to have been done to remove the early Mercurial crust and upper mantle soon after it had differentiated. All subsequent theories of Mercury’s evolution, therefore, had to include some mechanism to strip off the crust. Maybe it evaporated in the hot plasma environment of the early inner solar system. Perhaps a huge planetesimal whacked it and blew off the crust. Something must have happened to account for the large iron core, and the iron depleted crust.

When MESSENGER had some time to finally measure the surface composition of this tiny body, scientists found something they hadn’t expected: lots and lots of sulfur. Based on X-Ray fluorescence observations of the surface, Nittler et al. (2011) reported that Mercury’s crust appeared to hold at least 10 times more sulfur than the crust of either the Moon or the Earth. Sulfur is one of those elements that gets lost as a volatile during volcanic eruptions. In fact, Nittler et al. (2014) were able to show that in the pyroclastic deposits of some large volcanic eruptions on Mercury, there was very little sulfur. Sulfur and other volatiles would also have been stripped out if some violent or energetic event removed the early crust. However, here was all this sulfur in Mercury’s crust.

Enter the Enstatite Chondrite. These meteorites are very rare, comprising less than 2% of the meteorites found on Earth. They have a very low oxygen abundance. Actually, they are some of the most reduced (oxygen-starved) rocks known to Man. On Earth, oxygen is the most abundant element in rocks at the crust, making up almost 50% of the mass. Just about everything here is an oxide, except for comparatively rare deposits of carbonates (eg. limestone) or sulfides (think: Yellowstone Park). Enstatite chondrites, on the other hand, must have formed where there was very little oxygen.

Interestingly, even though there is proportionally very little oxygen, the rock is dominated by the magnesium end member of the orthopyroxene family by which it gets its name, enstatite (MgSiO3). The other minerals that comprise the rock are exotic sulfides, like oldhamite (CaS), niningerite (MgS) and other wonderful wierdos. It also happens to have very little iron.

Now, being a chondrite, this enstatite meteorite is thought to be composed of bits of material that cobbled together within the early protoplanetary disk. In other words, it is not something that floated to the top of a differentiating planetoid, but a primary material that hasn’t undergone much if any melting. This is one of the early building blocks of the solar system.

It may be that the highly reducing environment in which the enstatite chondrites assembled themselves was deep in the interior of the early solar system. It may also be that the enstatite chondrites were the original building blocks of planet Mercury. This is the new, going hypothesis of those who are trying to interpret the new data from MESSENGER.

559635main_messenger_news20110616_image2_4by3_946-710
NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Planets are thought to assemble themselves like snowballs, gathering up all the rubble in the protoplanetary disk. As the planet gets larger, the heat from compaction and radioactive decay begins to melt the interior. In general, the rock does not completely melt, but forms a partial melt. In this partial melt, different constituents can go different ways, depending on density and other factors. What is left at the surface is always of a different composition than the original rock that melted. On Earth, these partial melts go from more basaltic at depth to more granitic at the surface. The basaltic ocean floor is formed from extremely basaltic, or ultramafic, mantle rocks that have partially melted and then differentiated.

MESSENGER found that the composition of Mercury’s surface fell between the ultramafic Earth mantle rock, and the basaltic ocean floor, as seen in the figure above. Therefore, Mercury’s surface must have formed by partial melting of a super-ultramafic rock – one just like the enstatite chondrite. Experiments have already been performed to see what would’ve happened if an enstatite chondrite were partially melted. We end up with a composition similar to another type of meteorite, the enstatite achondrites! This is another type of enstatite-dominated meteorite which has a ton of exotic sulfide minerals, including troillite (FeS), but no oxides save enstatite. And it looks a lot like what MESSENGER saw at the surface of Mercury.

So, maybe enstatite chondrites, like that which fell at Abee, Alberta, Canada, and the enstatite achondrites are pieces blown off of Mercury. Or, maybe they are some of the original building blocks left over after all their siblings accreted onto a young Mercury. Either way, we have them in hand. We will probably never be absolutely sure until we are able to pluck some rock off of Mercury and bring it back, or at least study it in situ, but such a mission is still in the concept phase. The next mission to the tiny world launches in 2016: ESA’s BepiColumbo. Following that, the current plan is to send a lander to collect a sample, but nobody has officially proposed how to do that yet. That would be one tough job.

References

Nittler, L. R, Starr, R. D., Weider, S. Z., McCoy, T. J., Boynton, W. V., Ebel, D. S., Ernst, C. M., Evans, L. G., Goldsten, J. O., Hmara, D. K., Lawrence, D. J., McNutt, R. L., Schlemm, C. E., Solomon, S. C., Sprague, A. L. (2011), The major-element composition of Mercury’s surface from MESSENGER X-ray spectrometry, Science, v. 333, p. 1847-1850.

Nittler, L. R., Weider, S. Z., Starr, R. D., Chabot, N., Denevi, B. W., Ernst, C. M., Goudge, T. A., Head, J. W., Helbert, J., Klima, R. L., McCoy, T. J., Solomon, S. C. (2014), Sulfur-depleted composition of Mercury’s largest pyroclastic deposit: implications for explosive volcanism and surface reflectance on the innermost planet, 45th Lunar and Planetary Science Conference.

McCoy, T. J., Dickinson, T. L., Lofgren, G. E. (1999), Partial melting of the Indarch (EH4) meteorite: A textural, chemical, and phase relations view of melting and melt migration, Meteoritics & Planetary Science, v. 34, p. 735-746.

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