We report Sr, Nd, and Pb isotope ratios of 51 mid-ocean ridge basalts (MORB) from the Juan de Fuca Ridge, the Gorda Ridge, the East Pacific Rise (EPR) and the Galapagos Spreading Center (GSC). The results confirm the general isotopic homogeneity of Pacific MORB, though high Pb isotope ratios (206Pb/204Pb up to 19.24) occur in basalts from the Easter microplate, reflecting the influence of the Easter hotspot. There is no evidence in our data of a "Dupal" geochemical anomaly in the mantle beneath the EPR in the southern hemisphere. Sr, Nd, and Pb isotope ratios are all correlated with one another, but correlations involving 87Sr/86Sr are poorer than other correlations, indicating that Rb-Sr fractionation in the sub-Pacific mantle has to some degree been decoupled from Sm-Nd, U-Pb, and Th-Pb fractionation. Small but significant isotopic variations occur in MORB on regional scales of 1-100 km. These variations are, however, of lesser amplitude than those observed on larger scales. Small-scale isotopic variations indicate either that large magma chambers do not exist beneath the EPR or that they are not efficient homogenizers of melts passing through them. On regional scales, Pb isotope ratios are more highly correlated than in Pacific MORB overall. We interpret these correlations as isochrons which record the age of events that created the regional mantle heterogeneities. These ages range from 1.4 to 2.4 Ga. Regional coherence of isotopic compositions indicates that the MORB source region consists of integral volumes which may be related to convection patterns. A comparison of the mean isotopic compositions of Pacific spreading ridges with means of Indian and Atlantic MORB shows that Pacific MORB have generally lower 87Sr/86Sr than MORB from the Mid-Atlantic Ridge (MAR) and Indian Ocean ridges but have generally lower 143Nd/144Nd and higher 206Pb/204Pb than the MAR. Presently available data do not support the relationship of isotopic diversity and mean isotopic composition to spreading rate suggested by others. Differences between isotopic compositions and between isotopic diversity appear to be inherent features of the mantle beneath the Pacific, Indian, and Atlantic ocean basins. Zindler et al., 1984]. This paper focuses mainly on that geochemical variability inPacific MORB that is clearly unrelated to oceanic islands and mantle plumes. We particularly wished to consider questions of scale of heterogeneity in MORB sources and questions of the mechanisms which produce homogeneity and heterogeneity in mid-ocean ridge basalts. We also wished to test the hypothesis of a global geochemical anomaly centered around 30øS proposed by Hart [1984] and to examine the much debated question of correlations among various isotope ratios in Pacific MORB. Using results of analyses of 51 basalts from Pacific spreading centers and data in the literature, we argue that while significant isotopic heterogeneity occurs even on a scale of less than 10 km, basalts from ridge segments up to a few hundred kilometers in length are more closel...

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Authigenic pyrite grains from a section of the Lower Toarcian Posidonia Shale were analysed for their trace‐element contents and sulphur‐isotope compositions. The resulting data are used to evaluate the relationship between depositional conditions and pyrite trace‐element composition. By using factor analysis, trace‐elements in pyrite may be assigned to four groups: (i) heavy metals (including Cu, Ni, Co, Pb, Bi and Tl); (ii) oxyanionic elements (As, Mo and Sb); (iii) elements partitioned in sub‐microscopic sphalerite inclusions (Zn and Cd); and (iv) elements related to organic or silicate impurities (Ga and V). Results indicate that trace‐element contents in pyrite depend on the site and mechanism of pyrite formation, with characteristic features being observed for diagenetic and syngenetic pyrites. Diagenetic pyrite formed within anoxic sediments generally has a high heavy metals content, and the degree of pyritization of these elements increases with increasing oxygen deficiency, similar to the degree of pyritization of reactive Fe. The highest gradient in the increase of the degree of trace element pyritization with bottom‐water oxygenation was found for the elements Ni < Cu < Mo = As < Tl. In contrast, syngenetic pyrite formed within a euxinic water column typically is enriched in As, Mo and Sb, but is low in heavy metals, and the geochemical variation reflects changes in sea water composition.

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