 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 0734 is distinct from those of other Martian meteorites. This almost certainly indicates that it formed in a region of the mantle different from those giving rise to the other Martian meteorites.
|Concentrations of rare earth elements in distinctive Martian rock types. The concentrations are divided by those in carbonaceous chondrites (thought to represent primitive solar system materials) in order to make the patterns smoother. Without this normalization, the pattern for each rock would have zigs and zags because elements with even atomic numbers are more abundant than those with odd atomic numbers. Note that the three representatives of the previously-known Martian meteorites plotted, Shergotty, Tissint, and Nakhla, are quite distinct from each other. NWA 7034 has higher concentrations of the rare earths (and other elements, too) and has a pattern that slopes downward from La to Lu. WR stands for "whole rock," meaning that these analyses were done on representative chips of the meteorites.|
The oxygen isotope diagram shown above reinforces the idea that distinctive chemical reservoirs exist inside Mars. Agee and colleagues point out that both the enrichment in 18O (NWA 7034 data are shifted to the right on the diagram compared to other Martian meteorites) and the slight elevation of the data above the line defined by the other Martian meteorites suggest fundamental differences in the source region in the Martian mantle that produced the NWA 7034 magma compared to those that produced the other Martian meteorites.
So, this rock is not like the other Martian meteorites. But it is a lot like rocks on the surface of Mars as analyzed from orbit and on the ground by the series of rovers, such as Spirit and Opportunity. The diagram below is a standard way to classify terrestrial igneous rocks. Note that the analysis for NWA 7034 plots with the cluster of rocks and soils from the Gusev landing site and within the box defined by orbital chemical analyses. It appears to be typical Martian crust. The rest of the Martian mete