Kastens, K. A., Mascle, J., et al., 1990 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 107 10. BASAL DOLOMITIC SEDIMENTS, TYRRHENIAN SEA, OCEAN DRILLING PROGRAM LEG 1071 J. A. McKenzie,2 A. Isern,3 A. M. Karpoff,4 and P. K. Swart5 ABSTRACT 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 Mg-rich 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 under-lying 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 Ca-rich 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. INTRODUCTION The Tyrrhenian Sea in the western Mediterranean is a small oceanic back-arc basin that opened behind a volcanic arc and above a subduction zone. The areas surrounding this small tri-angular sea are quite well known for widespread active volcan-ism and frequent earthquake tremors, attesting to the continu-ing tectonic activity at the subduction plate boundary. The ther-mal regime in the central Tyrrhenian Basin is characterized by relatively high heat flow, with maximum values greater than 200 mW/m2 in the Vavilov and Marsili Basins. Regionally, in these two sub-basins, very low and extremely high heat flow anoma-lies have been observed. These have been attributed to hydro-thermal circulation (e.g., Rehault et al., 1987). A major objective of ODP Leg 107 was to investigate the evolution of the back-arc basin, in part by determining the age and geochemistry of the basaltic basement. In conjunction with this objective, the possible existence of metalliferous sediments overlying the basement was to be probed. To accomplish these goals, three sites in the two easterly sub-basins of the Tyrrhenian Sea were drilled to igneous basement, in water depths between 3330 and 3590 m: (1) the western rim of the Vavilov Basin near the crest of a north-south trending ridge (Site 655); (2) the axis of the Vavilov Basin on the eastern flank of a north-south trend-ing basement swell (Sites 651); and (3) the western rim of the Marsili Basin (Site 650) (Shipboard Scientific Party, 1987a, b, d; see also Fig. 1). 1 Kastens, K. A., Mascle, I, et al., 1990. Proc. ODP, Sci. Results, 107: Col-lege Station, TX (Ocean Drilling Program). 2 Geological Institute, ETH-Zentrum, 8092 Zurich, Switzerland. 3 Graduate School of Oceanography, University of Rhode Island, Narragan-sett, RI 02882. 4 Centre de Sedimentologie et Geochimie de la Surface-CNRS, 1, rue Blessig, 67084 Strasbourg, France. 5 Marine Geology and Geophysics, RSMAS, University of Miami, 4600 Rick-enbacker Causeway, Miami, FL 33149. The oldest biostratigraphically datable sediments above base-ment along this three-site transect become younger (latest early Pliocene to latest late Pliocene) from northwest to southeast, suggesting that the locus of oceanic accretion in the back-arc basin has migrated, through time, from northwest to southeast towards the subduction zone. The recovered basement rocks varied among the sites. At Site 650, 32 m of vesicular basalt were cored, compared with 110 m of basalt flows at Site 655; deeper coring at Site 651 revealed an unexpectedly complex basement assemblage containing highly serpentinized perido-tites overlain by 30 m of basalt and basaltic breccia, followed by 28 m of dolerite, metasediments and metadolerite, capped by 76 m of basalt flows (Shipboard Scientific Party, 1987a, b, d). At all three sites the basement rocks are directly overlain by dolomitic sediments. The presence of dolomitized pelagic sedi-ments overlying deep-sea basalt is not a new discovery, having been recognized with the early drilling results from the experi-mental Mohole site in the Pacific Ocean (Riedel et al., 1961; Murata and Erd, 1964). Stable isotope data suggested that the well-indurated dolomite recovered above the Mohole basalt was formed in isotopic equilibrium with normal marine water at an ambient bottom water temperature of approximately 5°C (De-gens and Epstein, 1964). Other occurrences of authigenic, deep-sea dolomite apparently have a hydrothermal origin (Bonatti, 1966). Subsequent drilling during the Deep Sea Drilling Project (DSDP) produced more examples of dolomitization of sedi-ments in close proximity to the oceanic basement (see Garrison, 1981, for a partial review of these occurrences). One such exam-ple particularly relevant to ODP Leg 107 was cored at DSDP Site 373 in the Vavilov sub-basin of the Tyrrhenian, where mod-erately well-lithified, ferruginous, calcium-rich dolomite was re-covered stratigraphically sandwiched between basaltic breccia and soft nannofossil marl (Bernoulli et al., 1978). Isotope data likewise indicated an origin in cold marine waters, while petro-graphic observations suggested that the dolomite had replaced or infilled molds of probable foraminifers. 141J. A. MCKENZIE, A. ISERN, A. M. KARPOFF, P. K. SWART Figure 1. Map of the Tyrrhenian Sea; basal dolomitic sediments overlying basalt were recovered at Sites 650, 651, and 655. The general bathymetry is simplified from the international bathymetric chart of the Mediterranean Intergovernmental