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Project Progress

Carbonate Globules in Igneous Rocks (dm)

This project was inspired by the carbonate globules in martian meteorite ALH84001, which is an igneous rock in which carbonates have formed on Mars in cracks and holes within the rock. These carbonates have generated two major controversies: (1) did the carbonates form at high temperatures thereby excluding any possible biological role in their formation? (2) if the carbonates formed at low temperatures, did biology play any part?

We have begun detailed studies of carbonate globules in Columbia River basalt samples from Spokane, WA, and in basaltic rocks and ultrabasic igneous inclusions from Spitzbergen, Norway. Both samples contain Fe-carbonate (siderite) as well as other compositions of carbonate. In both samples the carbonates are closely associated with silica-rich veins and thin layers having somewhat different chemical compositions from layer to layer. Microstratigraphic relationships proves that some and possibly all of the silica in the CRB samples formed after the carbonate was crystallized. The lack of alteration or decomposition of the siderite adjacent carbonate shows that the silica was emplaced at relatively low temperatures, likely as an aqueous deposit of a gel.The close association of the silica gel with the carbonate globules suggests that the carbonate globules also formed at low temperatures, at least in the CRB samples. Some of both the CRB globules and the Spitzbergan globules are radially zoned in texture and composition, showing some similarity to the ALH84001 carbonate globules.

An unexpected result for the first two examples studied is the close association of carbonate globules with silica-rich glass or gel-like microlayers or veins. The glass or gel-like silica shows a complex surface morphology. Work is underway to search for organic particulates or residue in these two terrestrial samples and to use the results to influence additional work on the ALH84001 samples. Even without organic residue or microfossil morphology, the presence of secondary carbonates in igneous rocks has preserved a history of likely hydrothermal or low temperature alteration which adds a new dimension to the preserved volcanic history of the rocks; the rocks have recorded information about the hydrosphere and atmosphere. Preliminary literature search indicates that secondary carbonate globules in igneous rocks on Earth may be much more common than realized. For example, carbonates are often found in vesicles in suboceanic basalts. Carbonates readily form in cavities in desert surface rocks, including at least one documented meteorite.

With one example already from Mars and with multiple terrestrial examples, the possibility exists that many igneous rocks on Mars (mainly basalts and andesites) may contain secondary carbonate globules formed at relatively low temperatures. Such carbonates may record a history of near surface aqueous activity on Mars. The carbonates may also be potential locations of fossils or even extant life. Consequently they constitute a possible new environment for life in the Solar System, particularly for bodies that are dominated by igneous rocks.