orites, 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 relatively dry period in Martian history, but the volcanic-breccia nature of NWA 7034 indicates that water played an important role in its eruption.
|Rubidium (Rb)-Strontium (Sr) isotope plot of a sample of the whole rock (WR) and materials separated magnetically from NWA 7034. The important point is that these different separates (labeled Drk-1 etc. and Light-1) have different Rb/Sr ratios. If they fall on a line on this plot, they define a unique age for the sample based on the known rate of radioactive decay. This unique age for NWA 7034 is 2089 ± 81 million years.|
NWA 7034 is one wet rock. It contains about 0.6 wt% H2O, at least ten times more than any other Martian meteorite. Orbital gamma ray and neutron spectrometer data indicate that the upper half meter or so of the Martian surface between about 50 degrees north and 50 degrees south latitudes contains at least 1 wt% water, so NWA 7034 is more like those regions than are typical Martian meteorites, further emphasizing that NWA 7034 is a lot like the typical crust of Mars.
Big questions remain about the source of the water. It might have been in the magma when it ascended from the interior, in which case it contains important information about the source of water to Mars and perhaps explains why the rock is the product of a strong eruption on Mars. (At even modest depths in planets, magma can have considerable water dissolved in it. At the low pressure near the surface the water leaves solution in the magma, forms bubbles, often so many that the magma shoots out of the volcanic vent explosively. Alternatively, the water might represent liquid water (or ice) already in the crust that was mobilized at some point and altered the pre-existing volcanic rock. Even the younger Martian meteorites have a rich record of such alteration. Additional research is needed to determine whether the water was in the magma or added later.
NWA7034 contains igneous rock fragments containing abundant apatite. This mineral contains water in the form of OH in its crystal structure and 