Geochemistry of Indian Ocean tephra and Afro-Arabian lavas

Widespread silicic pyroclastic eruptions of the Oligocene Afro-Arabian flood volcanic province (ignimbrites and airfall tuffs) produced up to 20% of the total flood volcanic stratigraphy (>6*10**4 km**3). Volumes of individual ignimbrites and tuffs exposed on land range from ~150 to >2000 km**3 and eight major units (15-100 m thick) were erupted in <2 Myr, placing these amongst the largest-magnitude silicic pyroclastic eruptions on Earth. They are compositionally distinctive time-stratigraphic markers which were deposited as co-ignimbrite ashfall deposits on a near-global scale around the time of the Oi2 cooling anomaly at ~30 Ma. Two ignimbrites from the lower part of the flood volcanic succession in Yemen have been correlated to: (a) the conjugate rifted margin of Ethiopia (>500 km distant); and (b) to two deep sea ash layers sampled by ODP Leg 115 in the Indian Ocean ~2700 km to the southeast. This correlation is based on whole rock analyses of silicic units for isotope ratios (Pb, Nd) and rare earth element compositions, in conjunction with novel in situ Pb isotope laser ablation multicollector inductively coupled plasma mass spectroscopy analysis of groundmass and glass shards. Compositional diversity preserved on the scale of individual ash shards in these deep sea tephra layers record chemical heterogeneity present in the silicic magma chambers that is not evident in the welded on-land deposits. Ages of the ash layers can be established by correlation to precisely dated on-land ignimbrites, and current evidence suggests that while these eruptions may have exacerbated already changing climatic conditions, they both marginally post-date the Oi2 global cooling anomaly.

Geochemistry of highly depleted peridotites of the Mid-Atlantic Ridge

During ODP Leg 209, a magma-starved area of the Mid-Atlantic Ridge (MAR) was drilled in the vicinity of the Fifteen-Twenty Fracture Zone (FZ) that offsets one of the slowest portions of the spreading ridge. We present here the results of a bulk rock multi-elemental study of 27 peridotites drilled at Sites 1272 and 1274 (to the south and the north of the FZ, respectively). The peridotites comprise mainly of harzburgites with minor dunites. Clinopyroxene (Cpx), which is interstitial and interpreted as secondary, is observed in Site 1274 peridotites. Sites 1272 and 1274 peridotites have low Al2O3 contents (<1 anhydrous wt.%), high Mg# (>91.5), and bulk rock trace elements compositions mostly below 0.1X primitive mantle (PM). These peridotites, and in particular Site 1272 peridotites, represent the most depleted peridotites yet sampled at a slow spreading ridge. Their compositions indicate high degrees of partial melting and melt extraction. A single open-system melting event (melting plus percolation of melts produced within upwelling mantle) can explain their highly depleted yet linear chondrite-normalized REE patterns, characterized by a steady depletion from HREE to LREE. Late melt-rock reactions and precipitation of Cpx explains the slightly less depleted compositions of Site 1274 peridotites. Hence, the differences in composition between Sites 1272 and 1274 peridotites do not provide evidence for regional variations in the degrees of partial melting from the south to the north of the FZ. The occurrence of highly refractory peridotites in the Fifteen-Twenty area suggests we sampled a more actively convecting mantle than generally supposed below slow spreading centers.

Geochemistry of the gabbroic lithosphere in IODP Holes 304-U1309D and 305-U1309D

IODP Hole U1309D (Atlantis Massif, Mid-Atlantic Ridge 30°N) is the second deepest hole drilled into slow spread gabbroic lithosphere. It comprises 5.4% of olivine-rich troctolites (~ > 70% olivine), possibly the most primitive gabbroic rocks ever drilled at mid-ocean ridges. We present the result of an in situ trace element study carried out on a series of olivine-rich troctolites, and neighbouring troctolites and gabbros, from olivine-rich intervals in Hole U1309D. Olivine-rich troctolites display poikilitic textures; coarse-grained subhedral to medium-grained rounded olivine crystals are included into large undeformed clinopyroxene and plagioclase poikiloblasts. In contrast, gabbros and troctolites have irregularly seriate textures, with highly variable grain sizes, and locally poikilitic clinopyroxene oikocrysts in troctolites. Clinopyroxene is high Mg# augite (Mg# 87 in olivine-rich troctolites to 82 in gabbros), and plagioclase has anorthite contents ranging from 77 in olivine-rich troctolites to 68 in gabbros. Olivine has high forsterite contents (82-88 in olivine-rich troctolites, to 78-83 in gabbros) and is in Mg-Fe equilibrium with clinopyroxene. Clinopyroxene cores and plagioclase are depleted in trace elements (e.g., Ybcpx ~ 5-11 * Chondrite), they are in equilibrium with the same MORB-type melt in all studied rock-types. These compositions are not consistent with the progressively more trace element enriched (evolved) compositions expected from olivine rich primitive products to gabbros in a MORB cumulate sequence. They indicate that clinopyroxene and plagioclase crystallized concurrently, after melts having the same trace element composition, consistent with crystallization in an open system with a buffered magma composition. The slight trace element enrichments and lower Cr contents observed in clinopyroxene rims and interstitial grains results from crystallization of late-stage differentiated melts, probably indicating the closure of the magmatic system. In contrast to clinopyroxene and plagioclase, olivine is not in equilibrium with MORB, but with a highly fractionated depleted melt, similar to that in equilibrium with refractory oceanic peridotites, thus possibly indicating a mantle origin. In addition, textural relationships suggest that olivine was in part assimilated by the basaltic melts after which clinopyroxene and plagioclase crystallized (impregnation). These observations suggest a complex crystallization history in an open system involving impregnation by MORB-type melt(s) of an olivine-rich rock or mush. The documented magmatic processes suggest that olivine-rich troctolites were formed in a zone with large magmatic transfer and accumulation, similar to the mantle-crust transition zone documented in ophiolites and at fast spreading ridges.

Geochemistry and isotopic composition of volcanic rocks of ODP Hole 128-794D

The Yamato Basin basement in the Sea of Japan was drilled below the sediment pile during Legs 127 and 128. Two superposed volcanic complexes are distinguished. The upper complex consists of continental tholeiite sills dated around 20-18 Ma and attributed to the rifting stage of the backarc basin. The lower complex consists of backarc basin basalts probably intruded below the upper complex during the spreading stage. Trace-element compositions and Sr and Nd isotopic signatures may be explained by mixing of at least two end members with a very small addition of crustal and subducted sediment component. Thus, upwelling of mantle diapir occurred during the rifting stage. Contribution of the depleted mantle increased in the spreading stage. The Neogene magmatic history of the Japan Sea is reviewed in the light of the ODP new data.

Geochemistry of samples from Valtournenche, Western Italian Alps

Slices of polycyclic metasediments (marbles and meta-cherts) are tectonically amalgamated with the polydeformed basement of the Dent Blanche tectonic system along a major Alpine shear zone in the Western Alps (Becca di Salé area, Valtournenche Valley). A combination of techniques (structural analysis at various scales, metamorphic petrology, geochronology and trace element geochemistry) was applied to determine the age and composition of accessory phases (titanite, allanite and zircon) and their relation to major minerals. The results are used to reconstruct the polyphase structural and metamorphic history, comprising both pre-Alpine and Alpine cycles. The pre-Alpine evolution is associated with low-pressure high-temperature metamorphism related to Permo-Triassic lithospheric thinning. In meta-cherts, microtextural relations indicate coeval growth of allanite and garnet during this stage, at ~ 300 Ma. Textures of zircon also indicate crystallization at HT conditions; ages scatter from 263-294 Ma, with a major cluster of data at ~ 276 Ma. In impure marble, U-Pb analyses of titanite domains (with variable Al and F contents) yield apparent 206Pb/238U dates range from Permian to Jurassic. Chemical and isotopic data suggest that titanite formed at Permian times and was then affected by (extension-related?) fluid circulation during the Triassic and Jurassic, which redistributed major elements (Al and F) and partially opened the U-Pb system. The Alpine cycle lead to early blueschist facies assemblages, which were partly overprinted under greenschist facies conditions. The strong Alpine compressional overprint disrupted the pre-Alpine structural imprint and/or reactivated earlier structures. The pre-Alpine metamorphic record, preserved in these slices of metasediments, reflects the onset of the Permo-Triassic lithospheric extension to Jurassic rifting.

Geochemistry and mineralogy of variably altered dacite volcanic rocks from ODP Leg 193 sites, PACMANUS field

A geochemical, mineralogical, and isotopic database comprising 75 analyses of Ocean Drilling Program (ODP) Leg 193 samples has been prepared, representing the variable dacitic volcanic facies and alteration types observed in drill core from the subsurface of the PACMANUS hydrothermal system (Table T1. The data set comprises major elements, trace and rare earth elements (REE), various volatiles (S, F, Cl, S, SO4, CO2, and H2O), and analyses of 18O and 86Sr/87Sr for bulk rock and mineral separates (anhydrite). Furthermore, normative mineral proportions have been calculated based on the results of X-ray diffraction (XRD) analysis (Table T2) using the SOLVER function of the Microsoft Excel program. Several of the samples analyzed consist of mesoscopically distinctive domains, and separate powders were generated to investigate these hand specimen-scale heterogeneities. Images of all the samples are collated in Figure F1, illustrating the location of each powder analyzed and documenting which measurements were performed.

Chemical composition of bottom sediments and rocks from DSDP Holes 3-14 through 64-477

Data on analyses of chemical composition of DSDP samples of bottom sediments and rocks carried out in P.P. Shirshov Institute of Oceanology are reported. Basal sediments and sedimentary rocks prevail in the sample set.