34 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 meteorites, though extremely valuable, must represent less common areas of the crust.
|Standard classification diagram for terrestrial igneous rocks. It plots the sum of the main alkali elements (potassium and sodium, expressed as oxides) against the silicon concentrations (expressed as silicon dioxide, SiO2). The names (some quite unusual!) encompass different ranges of alkali and silica abundances. Note that the Martian meteorites ("SNC" meteorites in the diagram) are distinctly lower than not only the mean value for NWA 7034 (cyan circle with uncertainties shown as a cross), but also almost all soil and rock analyses done by the Spirit rover (red dots) and the mean surface analysis done by the orbiting Mars Odyssey Gamma Ray Spectrometer (GRS, the yellowish rectangle). NWA 7034 is clearly like typical Martian crustal materials, in contrast to other Martian meteorites. See the paper by Hap McSween and coworkers for more details about the Mars orbiter and rover data.|
The Martian meteorites are also atypical of the general surface of Mars in that, with the exception of one sample, they are 1.3 billion years old or younger, some down to about 180 million years old. The one exception is ALH 84001, an igneous rock that formed 4.1 billion years ago (see PSRD article: A Younger Age for the Oldest Martian Meteorite). Geologists have known from careful mapping and crater counting that most of the Martian surface must be older than around 3 billion years, so all those meteorites being so young made everyone wonder "Why?" Was it just luck that an impact or a few impacts that liberated rocks from the surface hit only the youngest terrane? Or did water so alter the older surface areas that the rocks were too weathered to remain coherent when launched by an impact?
NWA 7034 partially fills in the age gap. Rubidium-strontium dating of mineral separates from the rock gives an age of about 2.1 billion years, in the first half of the Amazonian era of Martian history (about 3 billion years ago to the present). This is still a relati