Basal dolomitic sediments ofthe Tyrrhenian Sea

Basal dolomitic sediments were recovered at three drill sites in the Tyrrhenian Sea during Ocean Drilling Program (ODP) Leg 107 (Sites 650, 651, and 655). These sediments overlie the basaltic basement complex and are enriched in iron, and in some instances, also in manganese. The manganese enrichments, together with a very slight enrichment in trace transition elements, strongly suggest that the basal sediments have an affinity to deep-sea metalliferous deposits of hydrothermal origin. At Sites 651 and 655, the dolostones contain variable amounts of authigenic palygorskite, a Mgrich clay mineral. At Site 651, the basal sediments are 40 m thick and contain nonstoichiometric dolomite, sometimes Ca rich, but primarily Mg rich. The occurrence of Mg-rich dolomite with excess Mg up to 4% is unusual for the deep-sea environment; it may be associated with a hydrothermally driven flux of altered sea water through the directly underlying basement complex, which comprises basalt, dolerite, and serpentinized peridotite. Low-temperature alteration of the basement complex could produce solutions enriched in Mg. Oxygen-isotope equilibrium temperatures indicate that all of the studied dolomites formed under low-temperature conditions (i.e., < 70?C). The carbon-isotope compositions, together with the strong isotopic covariance, suggest that the Mg-rich dolomite precipitated more rapidly than the Carich dolomite. We suggest that the low-temperature, hydrothermal convection of Mg-rich solutions through the basal sediments in this back-arc basin environment (1) overcame kinetic problems related to the formation of massive dolostones, and (2) provided a mass-transport mechanism for dolomitization.