Unique Meteorite from Early Amazonian Mars: Water-Rich Basaltic Breccia Northwest Africa 7034 (original) (raw)

The only tangible samples of the planet Mars that are available for study in Earth-based laboratories, have up to now, been limited to the so-called SNC (1) meteorites and a single cumulate orthopyroxenite (Allan Hills 84001). The SNCs currently number 110 named stones and have provided a treasure trove for elucidating the geologic history of Mars (2). But because of their unknown field context and geographic origin on Mars, their fairly narrow range of igneous rock types and formation ages (3), it is uncertain to what extent SNC meteorites sample the crustal diversity of Mars. In fact, geochemical data from NASA's orbiter and lander missions suggest that the SNC meteorites are a mismatch for much of the martian crust exposed at the surface (4). For example, the basalts analyzed by the Mars Exploration Rover Spirit at Gusev Crater (5, 6) are distinctly different from SNC meteorites, and the Odyssey Orbiter gamma ray spectrometer (7) (GRS) data show that the average martian crust composition does not closely resemble SNC. NWA 7034, on deposit at the Institute of Meteoritics, purchased by Jay Piatek from Aziz Habibi, a Moroccan meteorite dealer, in 2011, is a 319.8 g single stone, porphyritic basaltic monomict breccia, with a few euhedral phenocrysts up to several millimeters and many phenocryst fragments of dominant andesine, low-Ca pyroxene, pigeonite, and augite set in a very fine-grained, clastic to plumose, groundmass with abundant magnetite and maghemite; accessory sanidine, anorthoclase, Cl-rich apatite, ilmenite, rutile, chromite, pyrite, a ferric oxide hydroxide phase, and a calcium carbonate identified by electron microprobe analyses on eight different sections at the University of New Mexico (UNM). X-ray diffraction analyses conducted at UNM on a powdered sample and on a polished surface show that plagioclase feldspar is the most abundant phase (38.0 ± 1.2%), followed by low-Ca pyroxene (25.4 ± 8.1%), clinopyroxenes (18.2 ± 4.0%), iron-oxides (9.7 ± 1.3%), alkali feldspars (4.9 ± 1.3%), and apatite (3.7 ± 2.6%). The x-ray data also indicate a minor amount of iron-sulfide and chromite. The data are also consistent with magnetite and maghemite making up ~70% and ~30%, respectively, of the iron oxide detected (8). Numerous clasts and textural varieties are present in NWA 7034 that include gabbros, quenched melts, and iron oxide-ilmenite-rich reaction spherules (figs. S1 to S4) (8), however the dominant textural type is a finegrained basaltic porphyry with feldspar and pyroxene phenocrysts. NWA 7034 is a monomict brecciated porphyritic basalt that is texturally unlike any SNC meteorite. Basaltic breccias are common in Apollo samples, lunar meteorites, and HED meteorites, but wholly absent in the world's collection of SNC meteorites (9). Absence of shockedproduced SNC breccias seems curious at face value, since nearly all of them show evidence of being subjected to high shock pressures, with feldspar commonly converted to maskelynite. Martian volcanic breccias are probably not rare given the observed widespread occurrence of volcanism on Mars. However launch and delivery of such materials to Earth as meteorites has not been observed (9). Although NWA 7034 is texturally heterogeneous both in hand sample and microscopically (Fig. 1), it can be considered a monomict breccia because it shows a continuous range of feldspar and pyroxene compositions that are consistent with a common petrologic origin (figs. S5 and S6). We find no outlier minerals or compositions that would indicate the existence of multiple lithologies or exotic components. We also see no evidence for polymict lithologies in either the radiogenic or stable isotope ratios of NWA 7034 solids. However, many clasts and some of the fine-grained groundmass have phases that appear to have been affected by secondary processes to form reaction zones. We observed numerous reaction textures, some with a ferric oxide hydroxide phase, which along with apatite, are the main hosts of the water in NWA 7034 (fig. S2). Impact processes are likely to have affected NWA 7034 by virtue of the fact that this meteorite was launched off of Mars, exceeding the escape velocity-presumably by an impact-although the shock pressures did not produce maskelynite. One large (1-cm) quench melt clast that was found could originate from shock processes (fig. S3). On the other hand, the very fine groundmass with the large phenocrystic feldspars and pyroxenes strongly suggests an eruptive volcanic origin for NWA 7034, thus it is likely that volcanic processes are a source of the brecciation.