WHOLE-ROCK GEOCHEMISTRY AND Sr-, Nd-, AND Pb-ISOTOPIC CHARACTERISTICS OF UNDEFORMED, DEFORMED, AND RECRYSTALLIZED GABBROS FROM SITES 921, 922, AND 923 IN THE MARK AREA

Jane Barling, Jan Hertogen, and Dominique Weis

ABSTRACT

Gabbros and diabases from Layer 3 of the oceanic lithosphere were recovered from the western wall of the Mid-Atlantic Rift Valley near the Kane Fracture Zone (23.5N, 45W) during Ocean Drilling Program Leg 153. Thirty-five gabbroic samples from Sites 921 to 923 and three diabases from Sites 920 and 921 have been analyzed for major and trace elements. The gabbroic samples were chosen to be representative of range of composition and variation in degree of deformation. Sr-, Nd-, and Pb-isotope ratios were measured for 19 of the gabbros and a single diabase.

Trace-element variations, in particular, reflect the nature of the samples, which varies from troctolite to gabbro (sensu stricto) to Fe-Ti-oxide-rich gabbro. Interpretation of the rare-earth element (REE) and Sc characteristics of the gabbros leads to contrasting models for the origin of gabbros from Sites 921 and 923 as compared to those from Site 922. Site 921 and 923 troctolitic rocks and gabbros (sensu stricto) can be modeled as comprising a fairly primitive cumulate fraction plus trapped parental intercumulus liquid. Second-order trace-element variations are indicative of fractionation attending adcumulus growth and compaction. The Site 922 troctolitic gabbros (sensu lato) are remarkable because of the high REE abundances and high REE/Sc ratios, characteristics normally associated with evolved compositions. The REE geochemistry of these troctolites cannot be accounted for by mixing between primitive troctolites and diabase liquid as for gabbros from Sites 921 and 923. Instead, these troctolites are modeled as mixtures between barren troctolites and a strongly enriched liquid. Such a liquid is likely to have accompanied the formation of the Fe-Ti oxide gabbros with which these troctolites are associated.

Sr-, Nd-, and Pb-isotope ratios for undeformed and deformed and recrystallized gabbros from Sites 921 and 923 have a very restricted range of values: 87Sr/86Sr = 0.70234-0.70239; 143Nd/144Nd = 0.51324-0.51319; 206Pb/204Pb = 18.02-18.07; 207Pb/204Pb = 15.42-15.47; and 208Pb/204Pb = 37.37-37.54. Gabbros from Site 922 have a similar range of Nd- and Pb-isotope compositions to Site 921 and 923 gabbros, but at a given 206,/sup>Pb/204Pb value they have slightly lower 207Pb/204Pb and 208Pb/204Pb ratios. This may indicate the involvement of a component with isotopically distinct Pb in the generation of these gabbros. Site 922 gabbros also have more variable Sr-isotope compositions (87Sr/86Sr = 0.70235-0.70251) than Site 921 and 923 gabbros. However, this is probably caused by incomplete removal of secondary radiogenic Sr introduced during alteration. The Site 921 diabase has more radiogenic Sr- (87Sr/86Sr = 0.70243) and Pb- (e.g., 206Pb/204Pb = 18.12) isotope ratios than Site 921 and 923 gabbros, but its 143Nd/144Nd (0.513221) is within the range of values for all gabbros. The Sr-, Nd-, and Pb-isotope ratios of both the gabbros and the diabase are typical of mid-ocean ridge basalts (MORB) and lie toward the depleted extreme of values for Atlantic MORB.

Date of initial receipt: 4 August 1995
Date of acceptance: 29 December 1995


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