Geochemistry of lavas from the Lau-Tonga arc and back-arc system

Detailed comparison of mineralogy, and major and trace geochemistry are presented for the modern Lau Basin spreading centers, the Sites 834-839 lavas, the modern Tonga-Kermadec arc volcanics, the northern Tongan boninites, and the Lau Ridge volcanics. The data clearly confirm the variations from near normal mid-ocean-ridge basalt (N-MORB) chemistries (e.g., Site 834, Central Lau Spreading Center) to strongly arc-like (e.g., Site 839, Valu Fa), the latter closely comparable to the modern arc volcanoes. Sites 835 and 836 and the East Lau Spreading Center represent transitional chemistries. Bulk compositions range from andesitic to basaltic, but lavas from Sites 834 and 836 and the Central Lau Spreading Center extend toward more silica-undersaturated compositions. The Valu Fa and modern Tonga-Kermadec arc lavas, in contrast, are dominated by basaltic andesites. The phenocryst and groundmass mineralogies show the strong arc-like affinities of the Site 839 lavas, which are also characterized by the existence of very magnesian olivines (up to Fo90-92) and Cr-rich spinels in Units 3 and 6, and highly anorthitic plagioclases in Units 2 and 9. The regional patterns of mineralogical and geochemical variations are interpreted in terms of two competing processes affecting the inferred magma sources: (1) mantle depletion processes, caused by previous melt extractions linked to backarc magmatism, and (2) enrichment in large-ion-lithophile elements, caused by a subduction contribution. A general trend of increasing depletion is inferred both eastward across the Lau Basin toward the modern arc, and northward along the Tongan (and Kermadec) Arc. Numerical modeling suggests that multistage magma extraction can explain the low abundances (relative to N-MORB) of elements such as Nb, Ta, and Ti, known to be characteristic of island arc magmas. It is further suggested that a subduction jump following prolonged slab rollback could account for the initiation of the Lau Basin opening, plausibly allowing a later influx of new mantle, as required by the recognition of a two-stage opening of the Lau Basin.

Geochemistry at DSDP Leg 75 Holes

Samples of sediments and rocks collected at DSDP Sites 530 and 532 were analyzed for 44 major, minor, and trace elements for the following purposes: (1) to document the downhole variability in geochemistry within and between lithologic units; (2) to document trace-element enrichment, if any, in Cretaceous organic-carbon-rich black shales at Site 530; (3) to document trace-element enrichment, if any, in Neogene organic-carbon-rich sediments at Site 532; (4) to document trace-element enrichment, if any, in red claystone above basalt basement at Site 530 that might be attributed to hydrothermal activity or weathering of basalt. Results of the geochemical analyses showed that there are no significant enrichments of elements in the organic-carbon-rich sediments at Site 532, but a number of elements, notably Cd, Co, Cr, Cu, Mo, Ni, Pb, V, and Zn, are enriched in the Cretaceous black shales. These elements have different concentration gradients within the black-shale section, however, which suggests that there was differential mobility of trace elements during diagenesis of interbedded more-oxidized and less-oxidized sediments. There is little or no enrichment of elements from hydrothermal activity in the red claystone immediately overlying basalt basement at Site 530, but slight enrichments of several elements in the lowest meter of sediment may be related to subsea weathering of basalt

Geochemistry, isotopic ratios and mineral composition of samples from ODP Leg 163 sites

Thirty-five samples from the drill core of the three Leg 163 sites (Sites 988, 989, and 990) off the southeast coast of Greenland were analyzed for 27 major, minor, and trace elements by X-ray fluorescence (XRF) and for 25 trace elements, including 14 rare-earth elements (REEs), by an inductively coupled plasma source mass spectrometer (ICP/MS). Sr- and Nd-isotope data are reported for seven samples and oxygen-isotope data are reported for 19 plagioclase separates. In addition, a reconnaissance survey of the composition of the main mineral phases, plagioclase, pyroxene, and oxides was determined on an electron microprobe to provide the basic information required for petrogenetic modeling. Olivine pseudomorphs are present in many of the samples, but in no case was an olivine grain found that was fresh enough to give a reliable analysis. The chemical and isotopic data recorded here were determined to provide a comparison with the larger data sets acquired by the Edinburgh, Copenhagen, and Leicester laboratories from both Legs 152 and 163 drill cores. This will permit a detailed comparison of the North Atlantic flood basalt province as a whole with the better known Columbia River, Deccan, and Karoo continental flood basalt provinces, for which substantial chemical data sets are already available at Washington State University.

Geochemistry of the volcanic basement of ODP Hole 134-831B

The basement of Bougainville Guyot drilled at Site 831 consists of andesitic hyalobreccias derived from a submarine arc volcano. The volcanic sequence has been dated by K/Ar at approximately 37 Ma. The 121 m of andesitic hyalobreccias drilled in Hole 831B have been divided into five subunits of two types: one appears to be primary, and the other contains evidence of reworking and a subaerial clastic input. Variations are attributed to fluctuations in water depth. The distinctive hyalobreccias consist of andesitic blebs with chilled margins and peripheral fractures set in a chaotic greenish matrix that is mainly altered glass, with crystals similar to those in the blebs or clasts. Their formation is attributed to violent reaction of andesitic magma discharged into seawater, in perhaps the submarine equivalent of fire-fountaining. There was limited reworking by currents and debris flows on the flanks of the submarine volcano. The andesite shows no significant compositional variation in phenocryst phases throughout the drilled sequence and contains phenocrysts of plagioclase (An88-43), clinopyroxene (Ca44Mg46Fe10-Ca41Mg40Fe19), orthopyroxene (Ca4Mg79Fe17-Ca3Mg58Fe39), and titanomagnetite. There is a systematic change in volcanic composition with height in the section, from more mafic andesites at the base, to overlying more acid andesites, and strong evidence exists that magma mixing may have played a significant role in the genesis of these lavas. The andesites have affinities with the low-K arc tholeiite series. Trace element and isotopic systematics for these rocks indicate very minor involvement of a LILE- and 87Sr-enriched slab-derived fluid in their petrogenesis. This accords with the previous suggestion that Bougainville Guyot forms part of an Eocene proto-island arc developed along the southern side of the d'Entrecasteaux Zone, above a southward-dipping subduction zone.

Geochemistry from DSDP Holes 67-497 and 67-496

Mineralogical identification, glass chemistry, and instrumental neutron activation analyses of Quaternary volcanic ash layers from Leg 67 Holes 496, 497, and 499 are used to correlate the drill holes and on-land sources. We have identified two units at Hole 496 that correspond to the 23,000-yr.-old Pinos Altos ash (Samples 496-3-4, 55-57 cm and 496-3-5, 74-76 cm); the 84,000-yr.-old Los Chocoyos ash corresponds with Sample 496-5-4, 134-146 cm, but this latter correlation is less certain.

Major and trace element abundances of igneous rocks from DSDP Site 37-334

REE abundances in gabbros and peridotites from Site 334 of DSDP Leg 37 show that these rocks are cumulates produced by fractional crystallization of a primitive oceanic tholeiite magma. They may be part of a layered oceanic complex. The REE distributions in the residual liquids left after such a fractionation are similar to those of incompatible element-depleted oceanic tholeiites. The REE data indicate that the basalts which overlie the gabbro-peridotite complex, are not genetically related to plutonic rocks.