Composition of chromian diopside megacryst in DSDP Leg 37 basalts (Table 2, 3)

Refractory megacrysts of olivine, plagioclase, chromian diopside and Cr-Al spinel, which were not in equilibrium with the host oceanic tholeiite on eruption, are present in samples from several dredge sites and DSDP drill sites in the Atlantic and Pacific Oceans. They have multiple origins: (1) cognate or accidental mantle fragments; (2) relict fragments from fractional crystallization of parental liquids considerably more primitive than oceanic tholeiite; and most commonly (3) the fractional crystallization products of such liquids mixed with oceanic tholeiite magma. Melt inclusions in chrome-spinel phenocrysts provide evidence for this postulated Mg- and Ca-rich magma which has counterparts in the Scottish Tertiary Province and in west Greenland.

Chemical composition of ocean-floor basalts and separated magnetites DSDP Legs 37, 45 and 46

Titanomagnetites separated from 15 different rock samples (including ocean-floor basalts from DSDP Legs 37, 45 and 46) were analyzed together with whole-rock samples by instrumental neutron-activation analysis for Sc, Cr, Co, Zn, Hf, Ta, Th and the REE La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Tm, Yb and Lu. In titanomagnetities from ocean-floor basalts and some other rocks, REE are enriched with respect to the whole-rock composition by factors of between 1.5 and 3 for light REE and between 1.0 and 1.9 for heavy REE; that is, REE with larger ionic radii are preferentially incorporated into the magnetite lattice. Three magnetite samples are REE depleted. Their whole-rock samples contain P in appreciable amounts, so apatite, an important REE-accumulating mineral, could have captured REE to some extent. All titanomagnetites show a marked negative Eu anomaly, this is most probably caused by discrimination of Eu(2+) from the magnetite lattice. Co, Zn, Hf and Ta are significantly enriched in magnetites. The distribution behaviour of Sc and Cr is masked chiefly by the crystallization of clinopyroxene and therefore is not easy to estimate. Ulvöspinel contents of about 70% for the titanomagnetites from ocean-floor basalts were estimated from qualitative microprobe analysis. Ulvöspinel contents of all other samples varied in a wide range from 20% to about 90%. No correlation could be observed between this and the REE contents of the magnetites. Ilmenite exsolution lamellae could only be observed in titanomagnetites from a doleritic basalt from Leg 45.

Lead isotope ratios of basalts from DSDP Leg 37 (Table 1)

Analyses of Pb from mid-ocean ridge basalts obtained from DSDP Leg 37 lie on a line of near-zero-age slope. The data seem consistent with an evolutionary model of three stages, the beginning of the second stage being at about 600 m.y. - the model age obtained for the least radiogenic Pb. The beginning of the third stage of evolution at the time of formation of the rocks apparently did not change the average Th/U ratio since measured values are consistent with the ratio deduced from Pb isotope ratios. Ratios of U/Pb are not consistent with Pb isotope ratios, however, thus ruling out a simple two-stage evolutionary model.

Strontium isotope and rare earth element geochemistry of basalts from DSDP Leg 37

87Sr/S6Sr ratios have been determined on eleven whole rock basalt samples from DSDP Leg 37. The 87Sr/S6Sr ratios range from 0.70305 +/- 4 to 0.70451 +/- 4 due to alteration and contamination with seawater Sr. Leaching with 5% HF has only a small effect on the 87Sr/86Sr of the samples. However, treatment with 6M HCl in acid digestion bombs at 130°C removes the contaminant more effectively. Altered plagioclase and olivine are dissolved during this process. The mean 87Sr/86Sr of four HCl-treated samples from hole 332A is 0.70299 and that for five samples from hole 332B is 0.70297. The 87Sr/86Sr ratios of treated samples from holes 333A and 335 are 0.70304 +/- 4 and 0.70316 +/- 4, respectively. These 87Sr/86Sr ratios are within the range observed for other basalts elsewhere along the Mid-Atlantic Ridge in the North Atlantic. REE distribution patterns have been determined for four samples, three from hole 332B and one from hole 335. CeN/YbN ratios range from 0.58 to 1.30 and do not correlate with 87Sr/86Sr ratios. The source regions of these basalts appear to have been variable in REE abundances.

The distribution of sulfur in oceanic crust with respect to oxidative diagenesis (Table 1)

A detailed geochemical-petrological examination of layer 2 basalts recovered during Leg 37 of the DSDP has revealed that the original distribution, form and abundance of igneous sulfide have been profoundly altered during low-grade oxidative diagenesis. The net result appears to have been a rather pervasive remobilization of igneous sulfide to form secondary pyrite accompanied by a bulk loss of sulfur equivalent to about 50-60% of the original igneous value, assuming initial saturation. It is suggested that during infiltration of seawater into the massive crystalline rock, igneous sulfide has experienced pervasive oxidation, under conditions of limited oxidation potential, to form a series of unstable, soluble sulfur species, primarily in the form of SO3[2-] and S2O3[2-]. Spontaneous decomposition of these intermediate compounds through disproportionation has resulted in partial reconstitution of the sulfur as secondary pyrite and the generation of SO4[2-] ion, which, due to its kinetic stability, has been lost from the basalt system and ultimately transferred to the ocean. This model not only satisfies the geochemical and petrological observations but also provides a suitable explanation for the highly variable delta34S values which characterize secondary sulfides in deep ocean floor basalts.

Amino acid in Deep Sea Drilling Project cores

Several amino acid diagenetic reactions, which take place in the deep-sea sedimentary environment, were investigated, using various Deep Sea Drilling Project (DSDP) cores. Initially it was found that essentially all the amino acids in sediments are bound in peptide linkages; but, with increasing age, the peptide bonds undergo slow hydrolysis that results in an increasingly larger fraction of amino acids in the free state. The hydrolysis half-life in calcareous sediments was estimated to be ~1-2 million years, while in non-carbonate sediment the hydrolysis rate may be considerably slower. The amino acid compositions and the extent of racemization of several amino acids were determined in various fractions isolated from the sediments. These analyses demonstrated that the mechanism, kinetics, and rate of amino acid diagenesis are highly dependent upon the physical state (i.e., free, bound, etc.) in which the amino acids exist in the sedimentary environment. In the free state, serine and threonine were found to decompose primarily by a dehydration reaction, while in the bound state (residue or HCl-insoluble fraction) a reversible aldol-cleavage reaction is the main decomposition pathway of these amino acids. The change in amino acid composition of the residue fraction with time was suggested to be due to the hydrolysis of peptide bonds, while in foraminiferal tests the compositional changes over geological time are the result of various decomposition reactions. Reversible first-order racemization kinetics are not observed for free amino acids in sediments. The explanation for these anomalous kinetics involves a complex reaction series which includes the hydrolysis of peptide bonds and the very rapid racemization of free amino acids. The racemization rates of free amino acids in sediments were found to be many orders of magnitude faster than those predicted from elevated temperature experiments using free amino acids in aqueous solution. The racemization rate enhancement of free amino acids in sediments may be due to the catalysis of the reaction by trace metals. Reversible first-order kinetics are followed for amino acids in the residue fraction isolated from sediments; the rate of racemization in this fraction is slower than that predicted for protein-bound amino acids. Various applications of amino acid diagenetic reactions are discussed. Racemization and the decomposition reaction of serine and threonine can both be used, with certain limitations, to make rough age estimates of deep-sea sediments back to several million years. The extent of racemization in foraminiferal tests which have been dated by some other independent technique can be used to estimate geothermal gradients, and thus heat flows, and to evaluate the bottom water temperature history in certain oceanic areas.