ary heritage is the oxygen isotopic composition. (Oxygen has three isotopes with masses of 16, 17, and 18.) Although some types of meteorites overlap in oxygen isotopic composition, most do not. The mix of oxygen isotopes in NWA 7034 is different from all other meteorite types and the Earth (and Moon, which is indistinguishable from Earth). NWA 7034 is slightly different from the other Martian meteorites (see diagram below), but not so different to lead Carl Agee and his colleagues to conclude that the rock did not come from Mars. Instead, they suspect that the slight difference reflects some inherent variation in the oxygen isotopic composition of the Martian interior. Another possibility is that oxygen isotopic abundances were modified by interaction with the Martian atmosphere, which is thought to be higher than the meteorites in Δ 17O (the vertical displacement of any point from the terrestrial fractionation line on the plot shown below). A third, more speculative, possibility is that an exotic cometary or carbonaceous chondrite component was added to the rock through impact processes on Mars.
|Oxygen isotopic abundances in NWA 7034 compared to Martian meteorites and the line representing Earth rocks (TF, for Terrestrial Fractionation line). The plot shows the 17O/16O and 18O/16O ratios in the meteorite samples in parts per thousand, also called per mil (‰). Data have been standardized to standard mean ocean water (SMOW) and plotted as deviations from that value. The important points are that the NWA 7034 data are similar to, yet slightly different from, those of Martian meteorites and they are distinctly different from the terrestrial line and (though not shown) from any other meteorite type. Note that the oxygen isotope measurements were made in two different laboratories, one at the University of California, San Diego (UCSD) and one at the University of New Mexico (UNM).|
Any expert on Martian meteorites would not immediately shout, "Hey, this is a Martian meteorite!" NWA 7034 does not look like any of the Martian meteorites we already know about. It is a breccia, with rock and mineral fragments distributed in a fine-grained, glassy matrix. The texture can be seen even when looking at a freshly cut surface and in both optical and scanning electron microscopes. But unlike familiar impact melt breccias found in the lunar highlands, Agee and his colleagues conclude that this one is probably a volcanic breccia. Such rocks form in forceful eruptions that bust up pre-existing rock beneath or in the volcano, and spew out a mixture of magma and rock fragments.
|[LEFT] Photograph of gloved hand holding a piece of NWA 7034 showing the interior after a slice was taken for analysis. Note the light and dark fragments in a medium gray matrix, the typical texture of a breccia. The exterior has a black fusion crust. [RIGHT] This backscatter electron image of NWA 7034 shows compositional variations and texture of a region about 1 millimeter across. The large dark fragments are feldspar crystals, lighter ones are pyroxene. Fragments with both light and dark areas are rock fragments. (Backscatter images are bright when the average atomic number is high for a crystal, as in minerals containing iron, such as pyroxene, and dark when the atomic number is low, as in feldspar, which contains little iron.)|
NWA 7034 is compositionally distinct from other Martian meteorites. This is exemplified by its abundances of the rare earth elements. These elements, the series in the periodic table from lanthanum (La) on the left to lutetium (Lu) on the right, are useful indicators of the geochemical history of a rock. In particular, they help us discover differences in the compositions of the regions of the mantle in which different magmas form. This knowledge is important to understanding the bulk chemical composition of a planet (in this case Mars), how it differentiated into core, mantle, and primary crust, and subsequent magmatic activity that produce most of the crust we observe today. The rare earth element pattern (see diagram below) of NWA 07