Isotope geochemistry of caliche developed on basalt

摘要

Enormous variations in oxygen and carbon isotopes occur in caliche developed on < 3 Ma basalts in 3 volcanic fields in Arizona, significantly extending the range of delta(18)O and delta(13)C observed in terrestrial caliche. Within each volcanic field, delta(18)O is broadly co-variant with VC and increases as VC increases. The most O-18 and C-13 enriched samples are for subaerial calcite developed on pinnacles, knobs, and flow lobes that protrude above tephra and soil. The most O-18 and C-13 depleted samples are for pedogenic carbonate developed in soil atmospheres. The pedogenic caliche has delta(18)O fixed by normal precipitation in local meteoric waters at ambient temperatures and has low delta(13)C characteristic of microbial soil CO2. Subaerial caliche has formed from O-18-rich evapoconcentrated meteoric waters that dried out on surfaces after local rains. The associated C-13 enrichment is due either to removal of C-12 by photosynthesizers in the evaporating drops or to kinetic isotope effects associated with evaporation. Caliche on basalt lava flows thus initially forms with the isotopic signature of evaporation and is subsequently over-layered during burial by calcite carrying the isotopic signature of the soil environment. The large change in carbon isotope composition in subsequent soil calcite defines an isotopic biosignature that should have developed in martian examples if Mars had a "warm, wet" early period and photosynthesizing microbes were present in the early soils. The approach can be similarly applied to terrestrial Precambrian paleocaliche in the search for the earliest record of life on land. Large variations reported for delta(18)O of carbonate in Martian meteorite ALH84001 do not necessarily require high temperatures, playa lakes, or flood runoff if the carbonate is an example of altered martian caliche. Copyright (C) 2003 Elsevier Science Ltd. [References: 51]