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 delta¹⁸O values of about 6‰ for olivine, orthopyroxene, and clinopyroxene; delta¹⁸O values from chromian spinel/magnetite vary between 2.9‰ and 3.8‰, values from serpentine vary between 2.8‰ and 5.3‰, and whole-rock values vary between 2.9‰ and 4.6‰. Two temperatures could be calculated from the oxygen-isotope 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.

Date of initial receipt: 24 August 1995
Date of acceptance: 27 February 1996

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