Geochemistry of Yamato-82192, -86032 and -793274 lunar meteorites

The major and trace element compositions of lunar meteorites Yamato (Y)-82192,Y-86032 and Y-793274 were determined by neutron activation analysis. Y-82192 and Y-86032 are anorthositic lunar meteorites rich in Al_2O_3 and CaO and poor in FeO, MgO and incompatible elements. Although these meteorites are similar in composition to each other and other anorthositic lunar meteorites, they are distinct in several key compositional characteristics. Y-793274 is a basaltic lunar meteorite rich in FeO, MgO, Sc, Cr, Co, and incompatible elements and poor in Al_2O_3 and CaO compared to anorthositic lunar meteorites. It is similar in many ways to lunar meteorite EET87521 which is also a basaltic breccia. It is distinct from EET87521 in its higher proportion of highland material, its meteoritic contamination and regolith glass, and in the composition of its dominant basalt component. Y-793274 contains 65-75% magnesian VLT basalt, while EET87521 consists of ferroan VLT basalt. The eleven lunar meteorites probably represent eight distinct falls. Four are anorthositic and four are basaltic. This 50-50 proportion of highlands-mare material contrasts strongly with the 83-17 proportion derived from photogeologic mapping. The dominance of VLT basalt among lunar meteorites contrasts with its scarcity among Apollo samples. The resolution of these discrepancies awaits further studies of basaltic lunar meteorites and further discoveries of new lunar meteorites

Properties and distribution of paired candidate stony meteorites at Meridiani Planum, Mars

The Mars Exploration Rover Opportunity investigated four rocks, informally dubbed Barberton, Santa Catarina, Santorini, and Kasos, that are possible stony meteorites. Their chemical and mineralogical composition is similar to the howardite, eucrite, and diogenite group but with additional metal, similar to mesosiderite silicate clasts. Because of their virtually identical composition and because they appear to represent a relatively rare group of meteorites, they are probably paired. The four rocks were investigated serendipitously several kilometers apart, suggesting that Opportunity is driving across a larger population of similar rock fragments, maybe a meteorite strewn field. Small amounts of ferric Fe are a result of weathering. We did not observe evidence for fusion crusts. Four iron meteorites were found across the same area. Although mesosiderites are stony irons, a genetic link to these irons is unlikely. The stony meteorites probably fell later than the irons. The current atmosphere is sufficiently dense to land such meteorites at shallow entry angles, and it would disperse fragments over several kilometers upon atmospheric breakup. Alternatively, dispersion by spallation from an impacting meteoroid may have occurred. Santa Catarina and a large accumulation of similar rocks were found at the rim of Victoria crater. It is possible that they are associated with the impactor that created Victoria crater, but our limited knowledge about their distribution cannot exclude mere coincidence

Evidence for mechanical and chemical alteration of iron‐nickel meteorites on Mars: Process insights for Meridiani Planum

The weathering of meteorites found on Mars involves chemical and physical processes that can provide clues to climate conditions at the location of their discovery. Beginning on sol 1961, the Opportunity rover encountered three large iron meteorites within a few hundred meters of each other. In order of discovery, these rocks have been assigned the unofficial names Block Island, Shelter Island, and Mackinac Island. Each rock presents a unique but complimentary set of features that increase our understanding of weathering processes at Meridiani Planum. Significant morphologic characteristics interpretable as weathering features include (1) a large pit in Block Island, lined with delicate iron protrusions suggestive of inclusion removal by corrosive interaction; (2) differentially eroded kamacite and taenite lamellae in Block Island and Shelter Island, providing relative timing through crosscutting relationships with deposition of (3) an iron oxide–rich dark coating; (4) regmaglypted surfaces testifying to regions of minimal surface modification, with other regions in the same meteorites exhibiting (5) large‐scale, cavernous weathering (in Shelter Island and Mackinac Island). We conclude that the current size of the rocks is approximate to their original postfall contours. Their morphology thus likely results from a combination of atmospheric interaction and postfall weathering effects. Among our specific findings is evidence supporting (1) at least one possible episode of aqueous acidic exposure for Block Island; (2) ripple migration over portions of the meteorites; (3) a minimum of two separate episodes of wind abrasion; alternating with (4) at least one episode of coating‐forming chemical alteration, most likely at subzero temperatures