THE XENOLITH MÉLANGE OF MARTIAN METEORITE EET 79001A: HIDING IN PLAIN SIGHT FOR THIRTY-FIVE YEARS

The first meteorite to be confirmed as martian was EET 79001. This sample contains two lithologies, A and B, which are in igneous contact. The finer-grained Lithology A also contains entrained harzburgitic xenoliths and olivine megacrysts, we collectively term Lithology X (Lith X). There has been some controversy as to whether Lith X is truly xenolithic or whether it might be “cognate” and relate directly to the crystallization of the EET 79001A parent liquid. We show here that the variation in time-temperature (t-T) histories of individual clasts is so large as to effectively preclude Lith X being cognate.

Observations: (I) The MgO content of the Lith A groundmass constrains its liquidus T to be ~1280°C. (II) There are large variations in olivine chemistry: Fe/Mg ranges Fo₇₉-Fo₄₁ and CaO ranges 0.15-0.30 wt.%, a factor of two in both cases. (III) The groundmass contains no olivine, so presumably olivine is in reaction relationship with the liquid and olivine compositional variations are not due to overgrowth. (IV) one xenolith has rim zoning from Fo₇₇ to Fo₆₇ over a 150 µ special scale. (V) If olivine were stable, the first olivine to crystallize should be ~Fo₈₀, but rims on Lith X olivines tend to be ≤ Fo₇₀, in apparent disequilibrium with the groundmass.

The Lith X suite cannot have spent sustained time at the liquidus temperatures of the Lithology A groundmass without substantial modification of observed zoning. Consequently, we view the Lith X suite as entrained fragments of the martian lithosphere. We make two bounding calculations: (i) observed 20µ scale compositional gradients should be erased within hours at 1350°C; and (ii) even at granulite temperatures of 900°C, they should be erased within years. Zoning at the 150µ scale should be subdued within 300 years at 900°C. Therefore, even in the lithosphere the xenoliths cannot have existed at great depths.

All these observations lead us to the conclusion that Lith X is a martian mélange. Not only do the various Lith X components not relate to the Lithology A basalt, they also do not relate to each other.

We emphasize that we have only studied six thin sections in detail. Effectively, therefore, our present results are only reconnaissance mapping. A more detailed study might reveal even greater diversity. But until we have returned samples from Mars, we will still be mapping on float.