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3.67 grams.   Martian (shergottite)

TKW 1056 grams. Fall not observed. Found 24 January 2000 in Oman.

 

Steve writes:

The 1056 gram Dhofar 019 meteorite was found in the desert of Oman on January 24, 2000. Highly weathered and lacking any fusion crust, its texture, mineralogy, major element chemistry, and trace elements resulted in its classification as the 17th Martian meteorite. Dhofar 019 is a shergottite (an igneous rock of plutonic or volcanic origin that resembles its terrestrial cousins more than they do other achondrite meteorites). Less than half a percent of known meteorites come from Mars, but of those, the shergottites are the most common of the SNCs and are further subdivided into three distinct subgroups - basaltic, lherzolitic, and olivine-phyric (Dhofar 109 is an olivine bearing doleritic basalt). Alternatively, shergottites can be categorized into as many as four unique groups based on their rare-earth element content, but these two classification systems do not align with each other, suggesting complex relationships between the various source rocks and magmas that the shergottites formed from.

When I first acquired Dhofar 019 several years ago, it was rather unique: not only was its bulk chemistry similar to India's 1865 Shergotty meteorite - the type specimen for shergotittes - but it's crystallization age of more than 550 million years was, like its famous cousin ALH84001, quite different than the 150 to 250 million year ages of the majority of Martian meteorites known at that time¹. Like EETA79001A and DaG 476/489, Dhofar 019 was unusual in its lack of orthopyroxenes. And while alteration from terrestrial weathering was assumed to explain the older carbonate radiogenic dating results in the younger shergottites, Raman spectroscopic analysis suggested Martian alteration in Dhofar 019's (as did testing of ALH84001 - whose unusual carbonate globules are similar to Dhofar 019's smectite-calcium-gypsum "orangettes").

However, more recent analyses of subsequent Martian meteorites suggest two Martian reservoirs - one far less radiogenic than the other - mixed, causing ambiguous trends in Pb isotope variation that result in inaccurate estimates of Martian meteorites' ages. In addition, distinctly different oxygen reservoirs, because of the lack of plate tectonics, continue to exist, thus providing a highly bimodal pattern of surface ages on Mars and allowing for a broader range of origins in the plains of the northern hemisphere (though such exceptions as the 4.4 billion year "Black Beauty" and ALH84001 still likely came from the more ancient southern upland regions). These new findings also allow for a Martian origin for the previously assumed terrestrially weathered carbonates found in a large number of Martian meteorites.

While Dhofar 019 was no longer necessarily unique in its indications of Martian weathering or having one of the oldest crystallization ages of any Martian meteorite, it was still unusual in exhibiting excessive cosmic ray exposure and having a heterogeneous interior with apparent layering that gives it a texture visually quite different from coarse-grained shergottites like Zagami. This 3.67 gram slice of Dhofar 019 also exhibits a distinctive red/green bicolor appearance that makes it one of the favorite Martian meteorites in my collection.

Footnote

¹ With the notable exception of a very few unique examples like Dhofar 019 and ALH 84001, the majority of the first twenty or so Martian meteorites shared similar bulk chemistries and extremely young crystallization ages - ages younger than most of the Martian surface. At the time, these observations suggested a most unlikely explanation: most Martian meteorites came from an area that covered only a few percent of the total surface on Mars. The prevailing explanation for this 1990's "Martian age paradox" suggested the bulk of the shergottites were ejected in a single event from the northern plains of Mars rather than in multiple events, with the few more ancient exceptions coming from separate events in the much older southern uplands. Even as late as the mid 2000's, scientists were suggesting new reasons to support a limited source area for most Martian meteorites.