(Table 1) Geochemistry of realatively fresh rocks of ODP Sites 127-794 and 127-797

The flows and sills drilled at Sites 794 and 797 in the Yamato Basin of the Japan Sea are subalkalic, olivine, and/or plagioclase phyric basalts. Compositionally, the rocks can be divided into a depleted, low-K type and an enriched, relatively high-K type. In addition, two contrasting evolution trends are reflected in the rock compositions, which allow four different magmatic suites to be identified. It is suggested that the depleted or enriched nature of these suites represent primary characteristics, while the different evolution trends are related to fractionation processes in crustal magma chambers. A tholeiitic evolution trend, with increasing FeO and TiO2 and decreasing Al2O3, can be modelled by fractional crystallization of 40%-50% plagioclase, olivine, and augite. A mildly calc-alkalic evolution trend, with decreasing FeO, increasing Al2O3, and nearly constant TiO2, can be modelled by 8%-12% olivine fractionation. Mineralogical evidence suggests that these differences may be related to the effect of small amounts of water during crystallization of the calc-alkalic suites. The tholeiitic suites occur in the lower parts of the drill cores, while the calc-alkalic suites occur in the upper parts. This suggests a complex tectonic and magmatic evolution, perhaps reflecting a transition between calc-alkalic magmatism related to subduction zone activity and tholeiitic magmatism related to back-arc spreading. Furthermore, any magmatic model must be able to account for the range in parental magmas from depleted to enriched throughout the tectonic history of the Yamato Basin.

Geochemical analyses of secondary minerals from ODP Leg 115 basalts (Tables 1+2)

Basalts recovered along the Reunion hotspot track on Ocean Drilling Program (ODP) Leg 115 range in age from 34 Ma at Site 706 to 64 Ma at Site 707. They have undergone various degrees of secondary alteration. Within single holes the amount of alteration can vary from a few percent to near complete replacement of phenocrysts and groundmass by secondary minerals. Olivine appears to be the most susceptible to alteration and in some sections it is the only mineral altered. In other sections, olivine, pyroxene and plagioclase phenocrysts, and groundmass have been completely replaced by secondary minerals. Clays are the predominant form of secondary mineralization. In addition to replacing olivine, pyroxene, glass, and groundmass, clays have filled veins, vesicles, and voids. Minor amounts of calcite, zeolites, and K-feldspar were also detected. The clays that filled vesicles and veins often show color zonations of dark, opaque bands near the edges that grade into tan or green transparent regions in the centers of the veins. The electron microprobe was used to obtain chemical analyses of these veins as well as to characterize isolated clays that replaced specific minerals and filled voids and vesicles.

Rb, Sr, Sm and Nd concentrations and isotopic composition of basalts from DSDP Leg 74 (Table 1)

Basement intersected in DSDP holes 525A, 528 and 527 on the Walvis Ridge consists of submarine basalt flows and pillows with minor intercalated sediments. These holes are situated on the crest and mid and lower northwest flank of a NNW-SSE-trending ridge block which would have closely paralleled the paleo mid-ocean ridge (Rabinowitz and LaBrecque, 1979 doi:10.1029/JB084iB11p05973, Moore et al. (1983 doi:10.1130/0016-7606(1983)94<907:TWRTDS>2.0.CO;2). The basalts were erupted approximately 70 m.y. ago, an age equivalent to that of immediately adjacent oceanic crust in the Angola Basin and coraistent with formation at the paleo mid-ocean ridge (Moore et al., 1983). The basalt types vary from aphyric quartz tholeiites on the ridge crest to highly plagioclase phyric olivine tholeiites on the ridge flank. These show systematic differences in incompatible trace element and isotopic composition. Many element and isotope ratio pairs form systematic trends with the ridge crest basalts at one end and the highly phyric ridge flank basalts at the other. The low 143Nd/144Nd (0.51238), 206Pb/204Pb (17.54), 207Pb/204Pb (15.47), 208Pb/204Pb (38.14) and high 87Sr/86Sr (0.70512) ratios of the ridge crest basalts suggest derivation from an old Nd/Sm-, Rb/Sr- and Pb/U-enriched mantle source. This isotopic signature is similar to that of alkaline basalts on Tristan da Cunha but offset to significantly lower Nd and Pb isotopic ratios. The isotopic ratio trends may be extrapolated beyond the ridge flank basalts with higher 143Nd/144Nd (0.51270), 206Pb/204Pb (18.32), 207Pb/204Pb (15.52), 208Pb/204Pb (38.77) and lower 87Sr/86Sr (0.70417) ratios in the direction of increasingly Nd/Sm-, Rb/Sr- and Pb/U-depleted source compositions. These isotopic correlations are equally consistent with mixing of depleted and enriched end member melts or partial melting of an inhomogeneous, variably enriched mantle source. However, observed Zr-Ba-Nb-Y interelement relationships are inconsistent with any simple two-component model of magma mixing, as might result from the rise of a lower mantle plume through the upper mantle. Incompatible element and Pb isotopic systematics also preclude extensive involvement of depleted (N-type) MORB material or its mantle sources. In our preferred petrogenetic model the Walvis Ridge basalts were derived by partial melting of mantle similar to an enriched (E-type) MORB source which had become heterogeneous on a small scale due to the introduction of small-volume melts and metasomatic fluids.

Chemical analysis and argon geochronology from dive SAR_1 on Cornacya seamount of the Sardinia Channel

Sarcya 1 dive explored a previously unknown 12 My old submerged volcano, labelled Cornacya. A well developed fracturation is characterised by the following directions: N 170 to N-S, N 20 to N 40, N 90 to N 120, N 50 to N 70, which corresponds to the fracturation pattern of the Sardinian margin. The sampled lavas exhibit features of shoshonitic suites of intermediate composition and include amphibole-and mica-bearing lamprophyric xenoliths which are geochemically similar to Ti-poor lamproites. Mica compositions reflect chemical exchanges between the lamprophyre and its shoshonitic host rock suggesting their simultaneous emplacement. Nd compositions of the Cornacya K-rich suite indicate that continental crust was largely involved in the genesis of these rocks. The spatial association of the lamprophyre with the shoshonitic rocks is geochemically similar to K-rich and TiO2-poor igneous suites, emplaced in post-collisional settings. Among shoshonitic rocks, sample SAR 1-01 has been dated at 12.6±0.3 My using the 40Ar/39Ar method with a laser microprobe on single grains. The age of the Cornacya shoshonitic suite is similar to that of the Sisco lamprophyre from Corsica, which similarly is located on the western margin of the Tyrrhenian Sea. Thus, the Cornacya shoshonitic rocks and their lamprophyric xenolith and the Sisco lamprophyre could represent post-collisional suites emplaced during the lithospheric extension of the Corsica-Sardinia block, just after its rotation and before the Tyrrhenian sea opening. Drilling on the Sardinia margin (ODP Leg 107) shows that the upper levels of the present day margin (Hole 654) suffered tectonic subsidence before the lower part (Hole 652). The structure of this lower part is interpreted as the result of an eastward migration of the extension during Late Miocene and Early Pliocene times. Data of Cornacya volcano are in good agreement with this model and provide good chronological constraints for the beginning of the phenomenon.

Geochemistry of Cretaceous sills and lava flows of ODP Holes 129-800A and 129-802A

On the basis of their respective eruptive environments and chemical characteristics, alkalic dolerite sills from the northern Pigafetta Basin (Site 800) and tholeiitic pillow lavas from the Mariana Basin (Site 802) sampled during Ocean Drilling Program Leg 129 are considered to represent examples of the widespread mid-Cretaceous volcanic event in the western Pacific. Both groups of basic rocks feature mild, low-grade, anoxic smectite-celadonite-carbonate-pyrite alteration; late-stage oxidation is very limited in extent, with the exception of the uppermost sill unit at Site 800. The aphyric and nonvesicular Site 800 alkalic dolerite sills are all well-evolved mineralogically and chemically, being mainly of hawaiite composition, and are similar to ocean island basalts. They are characterized by high contents of incompatible elements (for example, 300-400 ppm Zr), well-fractionated rare earth element patterns ([La/Yb]N 18-21) and HIMU isotopic characters. They probably represent deep-sea, lateral, intrusive off-shoots from nearby seamounts of similar age. The olivine-plagioclase +/- clinopyroxene phyric tholeiitic pillow lavas and thin flows of Site 802 are nonvesicular and quench-textured throughout. Relative to normal-type mid-ocean ridge basalt, they are enriched in large-ion-lithophile elements, exhibit flat (unfractionated) rare earth element patterns and have distinctive (lower) Zr/Nb, Zr/Ta, La/Ta, and Hf/Th ratios. Overall they are compositionally and isotopically similar to the mid-Cretaceous tholeiites of the Nauru basin and the Ontong-Java and Manihiki plateaus. The Site 802 tholeiites differ from the thickened crustal segments of the oceanic plateaus, however, in apparently representing only a thin veneer over the local basement in an off-axis environment.

(Table 1) Sr, Nd, and Pb isotopic composition of ODP Leg 129 basalts

Basaltic rocks recovered from three drill sites in the western Pacific during Ocean Drilling Program Leg 129 have fairly distinct Sr, Nd, and Pb isotopic compositions. The Cretaceous alkali olivine dolerites from Site 800 in the northern part of Pigafetta Basin have fairly low 87Sr/86Sri (0.70292-0.70320) and 143Nd/144Ndi (0.51277-0.51281) and high present-day Pb isotopic ratios (206Pb/204Pb = 20.53-21.45; 207Pb/204Pb = 15.70-15.77; 208Pb/204Pb = 40.02-40.68). The Middle Jurassic tholeiites from Site 801 in the southern part of the basin have low 87Sr/86Sri (0.70237-0.70248), high 143Nd/144Ndi (0.51298-0.51322), and moderate present-day Pb isotopic ratios (206Pb/204Pb = 18.20-19.12; 207Pb/204Pb = 15.47-15.60; 208Pb/204Pb = 37.56-38.18); isotopic compositions of the alkali olivine basalts overlying the tholeiites fall between those of the tholeiites and Site 800 dolerites. The Cretaceous tholeiites from Site 802 in the East Mariana Basin have high 87Sr/86Sri (0.70360-0.70372), fairly low 143Nd/144Ndi (0.51277-0.51280), and fairly low and homogeneous present-day Pb isotopic ratios (206Pb/204Pb = 18.37-18.39; 207Pb/204Pb = 15.49-15.51; 208Pb/204Pb = 38.34-38.39). Isotopic compositions of Site 801 tholeiites are indistinguishable from those of modern mid-ocean ridge basalts, consistent with the proposal that these tholeiites are a part of the oldest Pacific crust. The diverse isotopic compositions of the younger basalts appear to be the result of Jurassic Pacific plate migration over the geologically anomalous south-central Pacific region, wherein they acquired their distinct isotopic compositions. The anomalous region was volcanically more active during the Cretaceous than at present.

Geochemistry and mineralogy at DSDP Holes 61-462, 61-462A and 33-315A

Sedimentary rocks of Barremian through early Maestrichtian age recovered on Deep Sea Drilling Project Leg 61 had their principal source in the complex of igneous rocks with which they are interlayered in the Nauru Basin. Relict textures and primary sedimentary structures show these Cretaceous sediments to be of hyaloclastic origin, in part reworked and redeposited by slumps and currents. The dominant composition now is smectite, but locally iron, titanium, and manganese oxides, plagioclase, pyroxene, analcime, clinoptilolite, chalcedonic quartz, cristobalite, amphibole, nontronite, celadonite, and pyrite are also present. The mineral assemblages and the geochemistry reflect the original basaltic composition and its subsequent alteration by one or more processes of submarine weathering, authigenesis, hydrothermal circulation, and contact metamorphism. Hyaloclastitic sandstone, siltstone, and breccia within the sheet flows below 729 meters sub-bottom depth have Barremian fossils, thus establishing the age of the lower, or extrusive, complex of post-ridge-crest volcanism. Similar hyaloclastites between 564 and 729 meters are invaded by hypabyssal sills of the upper igneous complex, and fossil ages of Albian or Cenomanian set an older limit to the age of that second post-ridge-crest episode. Cenomanian to early Campanian sedimentary rocks between 490 and 564 meters have a substantial contribution of clays of submarine-weathered-basalt origin, as well as hydrothermal and pelagic components. The interval of reworked hyaloclastitic siltstone, sandstone, and breccias between 450 and 490 meters is of late Campanian and early Maestrichtian age. These sediments probably formed from glassy basalt that fragmented upon eruption nearby, when sills were being emplaced. In addition to pelagic elements, these Upper Cretaceous volcanogenic sediments include redeposited material of shallow-water origin, apparently derived from the Marshall Islands.

Major and trace element analyses of ODP Leg 120 volcanic rocks (Table 1)

Widespread Lower Cretaceous magmatism occurred along the Indian-Australian/Antarctic margins, and in the juvenile Indian Ocean, during the rifting of eastern Gondwana. The formation of this magmatic province probably began around 120-130 Ma with the eruption of basalts on the Naturaliste Plateau and at Bunbury, western Australia. On the northeast margin of India, activity began around 117 Ma with the Rajmahal continental basalts and associated lamprophyre intrusions. The formation of the Kerguelen Plateau in the Indian Ocean began no later than 114 Ma. Ultramafic lamprophyres (alnoites) were emplaced in the Prince Charles Mountains near the Antarctic continental margin at ~ 110 Ma. These events are considered to be related to a major mantle plume, the remnant of which is situated beneath the region of Kerguelen and Heard islands at the present day. Geochemical data are presented for each of these volcanic suites and are indicative of complex interactions between asthenosphere-derived magmas and the continental lithosphere. Kerguelen Plateau basalts have Sr and Nd isotopic compositions lying outside the field for Indian Ocean mid-ocean ridge basalts (MORB) but, with the exception of Site 738 at the southern end of the plateau, within the range of more recent hotspot basalts from Kerguelen and Heard Islands. However, a number of the plateau tholeiites are characterized by lower 206Pb/204Pb ratios than are basalts from Kerguelen Island, and many also have anomalously high La/Nb ratios. These features suggest that the source of the Kerguelen Plateau basalts suffered contamination by components derived from the Gondwana continental lithosphere. An extreme expression of this lithospheric signature is shown by a tholeiite from Site 738, suggesting that the southernmost part of the Kerguelen Plateau may be underlain by continental crust. The Rajmahal tholeiites mostly fall into two distinct geochemical groups. Some Group I tholeiites have Sr and Nd isotopic compositions and incompatible element abundances, similar to Kerguelen Plateau tholeiites from Sites 749 and 750, indicating that the Kerguelen-Heard mantle plume may have directly furnished Rajmahal volcanism. However, their elevated 207Pb/204Pb ratios indicate that these magmas did not totally escape contamination by continental lithosphere. In contrast to the Group I tholeiites, significant contamination is suggested for Group II Rajmahal tholeiites, on the basis of incompatible element abundances and isotopic compositions. The Naturaliste Plateau and the Bunbury Basalt samples show varying degrees of enrichment in incompatible elements over normal MORB. The Naturaliste Plateau samples (and Bunbury Basalt) have high La/Nb ratios, a feature not inconsistent with the notion that the plateau may consist of stretched continental lithosphere, near the ocean-continent divide.

Geochemistry and oxygen and iron isotope ratios of mafic rocks

Recycling of oceanic crust into the deep mantle via subduction is a widely accepted mechanism for creating compositional heterogeneity in the upper mantle and for explaining the distinct geochemistry of mantle plumes. The oxygen isotope ratios (d18O) of some ocean island basalts (OIB) span values both above and below that of unmetasomatised upper mantle (5.5 ± 0.4 per mil) and provide support for this hypothesis, as it is widely assumed that most variations in d18O are produced by near-surface low-temperature processes. Here we show a significant linear relationship between d18O and stable iron isotope ratios (d57Fe) in a suite of pristine eclogite xenoliths. The d18O values of both bulk samples and garnets range from values within error of normal mantle to significantly lighter values. The observed range and correlation between d18O and d57Fe is unlikely to be inherited from oceanic crust, as d57Fe values determined for samples of hydrothermally altered oceanic crust do not differ significantly from the mantle value and show no correlation with d18O. It is proposed that the correlated d57Fe and d18O variations in this particular eclogite suite are predominantly related to isotopic fractionation by disequilibrium partial melting although modification by melt percolation processes cannot be ruled out. Fractionation of Fe and O isotopes by removal of partial melt enriched in isotopically heavy Fe and O is supported by negative correlations between bulk sample d57Fe and Cr content and bulk sample and garnet d18O and Sc contents, as Cr and Sc are elements that become enriched in garnet- and pyroxene-bearing melt residues. Melt extraction could take place either during subduction, where the eclogites represent the residues of melted oceanic lithosphere, or could take place during long-term residence within the lithospheric mantle, in which case the protoliths of the eclogites could be of either crustal or mantle origin. This modification of both d57Fe and d18O by melting processes and specifically the production of low-d18O signatures in mafic rocks implies that some of the isotopically light d18O values observed in OIB and eclogite xenoliths may not necessarily reflect near-surface processes or components.