Northwest Africa 7034: New Unique Water-rich Martian Meteorite from the Early Amazonian Epoch

Northwest Africa (NWA) 7034 is a porphyritic basaltic breccia that shares some geochemical characterstics with known martian meteorites (SNC), but also possesses some unique characteristics that would exclude it from the current SNC grouping. Instead, it has a major and minor element composition that is a remarkably good match with the geochemistry of the rocks and soil at Gusev Crater measured by the Spirit rover and the average martian crust composition from the Odyssey Orbiter gamma ray spectrometer. The mismatch of orbiter and rover data with SNC meteorites has been a perplexing enigma, however with the discovery of NWA 7034 we may now have found a "missing link" between martian meteorites and space craft data. A five-point isochon gives an Rb-Sr age for NWA 7034 of 2.089±0.081 Ga (2σ) (MSWD=6.6) and an initial 87Sr/86Sr ratio of 0.71359±54. The Sm-Nd data for the same samples show more scatter, with an isochron of 2.19±1.4 Ga (2σ). NWA 7034 is REE enriched crustal rock (La x58 CI) and strongly light REE over heavy REE enriched (La/Yb)N=2.3, with negative-Eu anomaly (Eu/Eu*=0.67). The whole rock has 143Nd/144Nd=0.511756 and 147Sm/144Nd=0.1664, giving a calculated initial (source value) 143Nd/144Nd=0.509467 (initial ɛNd=-9.1) which requires that it be derived from an enriched martian reservoir, with an inferred time-integrated 147Sm/144Nd=0.1689, assuming separation from a chondrite-like martian mantle 4.5 Ga. An age of ~2.1 Ga for NWA 7034 would make it the first meteorite sample from the early Amazonian or late Hesperian epoch in Mars geologic history. Oxygen isotope analyses of NWA 7034 were performed by laser fluorination at UNM on acid-washed bulk sample and at UCSD on vacuum pre-heated (1000°C) bulk sample and give mean values Δ17O=0.57±0.05‰ n=10 and Δ17O=0.50±0.03‰ n=2, respectively. These interlab values are in good agreement, but are significantly higher than literature values for SNC meteorites (Δ17O range 0.15-0.45‰). There may be several possible explanations for the relatively high oxygen isotope values of NWA 7034. One possibility is that two or more distinct oxygen isotope reservoirs exist on Mars - for example a reservoir represented by the SNC meteorites and another martian reservoir with higher δ18O, δ17O, and Δ17O that we see in NWA 7034. The existence of multiple oxygen isotopic reservoirs within the silicate portion of Mars, as indicated by the new data from NWA 7034, is difficult to reconcile in the context of a global magma ocean and may indicate incomplete planetary differentiation on Mars. NWA 7034 has an order of magnitude more indigenous water than most SNC meteorites. Six whole-rock combustion measurements yielded a bulk water content for NWA 7034 of 6190±620 ppm with δD=+46.3±8.6. The maximum δD value in three separate stepwise heating experiments was +276, +327, and +319‰ reached at 1050°C, 1014°C, and 804°C respectively. The mean amount of water released at high temperature (>320°C) is 3280±720 ppm. The comparatively high water content of NWA 7034 may be giving a glimpse of conditions in its source on Mars 2.1 billion years ago.