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A suite of serpentinized peridotites from Leg 153 was analyzed for major and trace elements. All the rocks are less-refractory peridotites. The mineral chemistry of olivines (Mg# = 90.3), orthopyroxenes (Mg# = 90.4), clinopyroxenes (Mg# = 91.6), and some Cr spinels (Mg# = 68.4; Cr# = 29.7) shows the characteristic features of minerals from less-depleted mantle peridotites. Some geothermometric calculations gave temperatures for the pyroxene system of about 1300°C, reflecting the prekinematic to synkinematic crystallization of the pyroxenes, and gave a range from 1180° to 1075°C for the mineral pair orthopyroxene and spinel. Oxygen-isotope determinations show δ 18 θ values of about 6%c for olivine, orthopyroxene, and clinopyroxene; δ 18 θ values from chromian spinel/magnetite vary between 2.9%e and 3.8%o, values from serpentine vary between 2.8‰ and 5.3%e, and whole-rock values vary between 2.9%e and 4.6‰. Two temperatures could be calculated from the oxygenisotope exchange for the mineral pair clinopyroxene and spinel: about 1100° and about 900°C. The 1100°C value nearly corresponds to the data obtained using the orthopyroxene-spinel mineral thermometer, and may indicate subsolidus deformation and recrystallization. The temperature range of the serpentinization could only be estimated. It depends upon the rock/water ratio, and may not have exceeded ~300°C. Finally, a calculation of possible rock/water ratios is given.

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Leg 153 represents an Ocean Drilling Program (ODP) offset drilling leg in the MARK area (Mid-Atlantic Ridge at the Kane Transform), around 22°33' N, 45° 1.9' W. Shipboard documentation has shown that the gabbroic rocks from Sites 921, 922, and 923 are part of a layered complex and are generally similar, although cumulus textures are particularly well developed in the rocks from Site 922. These rocks also exhibit some special geochemical features. Major-and trace-element chemistries were studied from a limited pool of 26 samples from nine holes. In some polished thin sections, the chemical compositions of olivines, orthopyroxenes, clinopyroxenes, amphiboles, and plagioclases were determined using an electron microprobe. These data are presented here as a data report; a detailed paper will be prepared, if additional rock samples are analyzed, and if fluid inclusions and oxygen isotope data become available. SAMPLE MATERIAL All gabbroic rock samples in this study are from Holes 153-921 A, 921B, 921C, 921E, 922A, 922B, and 923A. Two diabase vein rocks are from Holes 920B and 920D. The special designations correspond to shipboard documentation and to standard ODP sample nomenclature conventions, but, for clarity, single numbers are generally used in the data tables (Table 1). The individual rock names were determined after microscopic investigation and calculation of mineral norms in accordance with the CIPW system (after the authors Cross, Iddings, Pearson, and Washington). ANALYTICAL METHODS Whole-Rock Chemistry Major-element oxides and the trace elements Cl, Co, Cr, Cu, F, Ga, Nb, Ni, Pb, S, Se, V, Y, Zn, and Zr were measured by X-ray fluorescence spectroscopy using a Phillips PW 1480 sequential spectrometer. Glass discs were prepared by melting the powdered, but not ignited, whole-rock material with lithium tetraborate. International reference rocks, as well as two internal mafic standard samples, were used for calibration of the method. K, Li, Na, Rb, and Zn were analyzed using flame photometry, and Ba and Sr were analyzed using inductively coupled plasma-atomic emission spectroscopy using a Perkin-Elmer PLASMA 1000 spectrometer. B was determined by optical spectroscopy, with boron-free spectral carbons, using a Carl Zeiss PGS 2 spectrograph. Table 1. Sample information. Sample name 2D 7M-D

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