Mathematics – Logic
Scientific paper
Jul 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004psrd.repte..85c&link_type=abstract
Planetary Science Research Discoveries
Mathematics
Logic
Meteorites, Alh84001, Martian Meteorites, Carbonates, Water On Mars, Mars
Scientific paper
Carbonate-rich regions in ALH 84001 are complicated. There are familiar forms of carbonate as well as fascinating textural forms previously unreported including carbonate rosettes, planiform "slab" carbonates, distinct "post-slab" magnesium carbonates (magnesite), and carbonates interstitial to feldspathic glass and orthopyroxene. Slab carbonates reveal portions of the carbonate growth sequence not seen in the rosettes and suggest that initial nucleating compositions were rich in calcium. They formed in two major stages. The first stage involved growth of the rosettes and slab carbonates. This step was controlled by the rate of crystal nucleation, how fast the ingredients were delivered to the growing crystals, and how much fluid was available. Cosmochemists call this type of growth "kinetically controlled." Next, an alteration event formed the magnesite-siderite (iron carbonate) layers on the exterior surfaces of the carbonate. Post-slab magnesite, intimately associated with silica glass, is compositionally similar to the magnesite in these secondary exterior layers, but represents a later generation of carbonate growth. Formation of feldspathic glasses had little or no thermal effect on carbonates, as indicated by the lack of thermal decomposition or any compositional changes associated with glass/carbonate contacts.
The carbonates tell an important story about water in the ancient crust of Mars. The presence of numerous, distinct generations of carbonate formation and relatively clear fracture chronology within carbonate further suggest that interactions between ALH 84001 and the crustal fluids of Mars were discontinuous and occurred only a few times over its 4.5 Ga history. The reactivation and remobilization of fluids (causing events such as formation of magnesite-siderite-magnesite layers and precipitation of post-slab magnesite) and the fracturing within the rock were almost certainly driven by impacts. The evidence for punctuated, impact-driven interaction between rocks and fluids supports scenarios describing temporary hydrous environments as opposed to those including large-scale, long-term hydrologic systems including oceans. Therefore, unless ALH 84001 is a particularly rare sample that escaped intense alteration, the hydrosphere of Mars may not have interacted with the rocks as thoroughly as planetary geologists have inferred from the presence of river networks and other features formed by flowing water.
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