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.

Elements and mineralogical compostions from different sediment cores off the southern China Sea

The mineralogy, major and trace elements, and neodymium and strontium isotopes of surface sediments in the South China Sea (SCS) are documented with the aim of investigating their applicability in provenance tracing. The results indicate that mineralogical compositions alone do not clearly identify the sources for the bulk sediments in the SCS. The Nd isotopic compositions of the SCS sediments show a clear zonal distribution. The most negative epsilon-Neodymium values were obtained for sediments from offshore South China (-13.0 to -10.7), while those from offshore Indochina are slightly more positive (-10.7 to -9.4). The Nd isotopic compositions of the sediments from offshore Borneo are even higher, with epsilon-Neodymium ranging from -8.8 to -7.0, and the sediments offshore from the southern Philippine Arc have the most positive epsilon-Neodymium values, from -3.7 to +5.3. This zonal distribution in epsilon-Neodymium is in good agreement with the Nd isotopic compositions of the sediments supplied by river systems that drain into the corresponding regions, indicating that Nd isotopic compositions are an adequate proxy for provenance tracing of SCS sediments. Sr isotopic compositions, in contrast, can only be used to identify the sediments from offshore South China and offshore from the southern Philippine Arc, as the 87Sr/86Sr ratios of sediments from other regions overlapped. Similar zonal distributions are also apparent in a La-Th-Sc discrimination diagram. Sediments fromthewestmargin of the SCS, such as those fromBeibuwan Bay, offshore fromHainan Island, offshore from Indochina, and from the Sunda Shelf plot in the same field, while those offshore from the northeastern SCS, offshore from Borneo, and offshore from the southern Philippine Arc plot in distinct fields. Thus, the La-Th-Sc discrimination diagram, coupledwith Nd isotopes, can be used to trace the provenance of SCS sediments. Using this method, we re-assessed the provenance changes of sediments at Ocean Drilling Program (ODP) Site 1148 since the late Oligocene. The results indicate that sediments deposited after 23.8 Ma (above 455 mcd: meters composite depth) were supplied mainly from the eastern South China Block, with a negligible contribution from the interior of the South China Block. Sediments deposited before 26 Ma (beneath 477 mcd) were supplied mainly from the North Palawan Continental Terrane, which may retain the geochemical characteristics of the materials covered on the late Mesozoic granitoids along the coastal South China. For that the North Palawan Continental Terrane is presently located within the southern Philippine Arc but was located close to ODP Site 1148 in the late Oligocene. The weathering products of volcanic material associated with the extension of the SCS ocean crust also contributed to these sediments. The rapid change in sediment source at 26-23.8 Ma probably resulted from a sudden cessation of sediment supply from the North Palawan Continental Terrane. Wesuggest that the North Palawan Continental Terrane drifted southwards alongwith the extension of the SCS ocean crust during that time, and when the basin was large enough, the supply of sediment from the south to ODP Site 1148 at the north slope may have ceased.

Sediment, isotopic and geochemical record during Marine Isotope Stages 29 to 34 of IODP Site 339-U1387

Centennial-to-millennial scale records from IODP Site U1387, drilled during IODP Expedition 339 into the Faro Drift at 558 m water depth, now allow evaluating the climatic history of the upper core of the Mediterranean Outflow (MOW) and of the surface waters in the northern Gulf of Cadiz during the early Pleistocene. This study focuses on the period from Marine Isotope Stage (MIS) 29 to 34, i.e. the interval surrounding extreme interglacial MIS 31. Conditions in the upper MOW reflect obliquity, precession and millennial-scale variations. The benthic d18O signal follows obliquity with the exception of an additional, smaller d18O peak that marks the MIS 32/31 transition. Insolation maxima (precession minima) led to poor ventilation and a sluggish upper MOW core, whereas insolation minima were associated with enhanced ventilation and often also increased bottom current velocity. Millennial-scale periods of colder sea-surface temperatures (SST) were associated with short-term maxima in flow velocity and better ventilation, reminiscent of conditions known from MIS 3. A prominent contourite layer, coinciding with insolation cycle 100, was formed during MIS 31 and represents one of the few contourites developing within an interglacial period. MIS 31 surface water conditions were characterized by an extended period (1065-1091 ka) of warm SST, but SST were not much warmer than during MIS 33. Interglacial to glacial transitions experienced 2 to 3 stadial/interstadial cycles, just like their mid-to-late Pleistocene counterparts. Glacial MIS 30 and 32 recorded periods of extremely cold (< 12°C) SST that in their climatic impact were comparable to the Heinrich events of the mid and late Pleistocene. Glacial MIS 34, on the other hand, was a relative warm glacial period off southern Portugal. Overall, surface water and MOW conditions at Site U1387 show strong congruence with Mediterranean climate, whereas millennial-scale variations are closely linked to North Atlantic circulation changes.

Geochemistry of Early Cretaceous basaltic rocks from ODP Sites 123-765 and 123-766

The Early Cretaceous basaltic rocks obtained from Sites 765 and 766 in the eastern Indian Ocean floor were mostly iron-rich normal mid-ocean ridge basalts (N-MORB), which were derived from a depleted mantle source having strongly light rare earth element (LREE)-depleted rare-earth patterns and a high titanium/zirconium (Ti/Zr) ratio. Basaltic rocks in the upper part of the Site 765 basement section include megacrysts and gabbroic fragments of widely varying mineral chemistry. These megacrysts range from An90 plagioclase, including highly magnesian basaltic glass coexisting with augite of Mg# (= 100 Mg/[Fe+Mg]) at 85, to An50 plagioclase coexisting with hypersthene. This varying mineralogy of megacrysts and gabbroic fragments indicates that a considerable degree of fractional crystallization took place in the magma chamber. The unusual negative correlation between incompatible elements (e.g., TiO2) and FeO*/MgO observed among Site 765 basement basalts and fresh volcanic glasses suggest source-mantle heterogeneity in terms of FeO*/MgO. Strontium isotope ratios (87Sr/86Sr) of the basaltic rocks from both sites are between 0.7027 and 0.7033 and are comparable to those of mid-Indian Ocean ridge basalts (MIORB). The basalt pebbles encountered in the sedimentary section may have been transported from the basement highs nearer the Australian continent and include basaltic compositions ranging from primitive N-MORBs to evolved enriched (E)-MORBs. Their mantle source was not as depleted as that of the basement basalts. These rocks may be the products of earlier volcanism that took place during the rifting of the Australian continent.

Geochemical and mineralogical analyses of clay-rich intervals in ODP Sites 166-1006 and 166-1007

The western flank of the Great Bahama Bank, drilled during ODP Leg 166 at seven sites, represents a prograding carbonate sequence from late Oligocene to Holocene [Eberli et al., Proc. ODP Init. Reports 166 (1997)]. The signatures of the detrital input and of diagenetic alteration are evident in clay enriched intervals from the most distal Sites 1006 and 1007 in the Straits of Florida. Mineralogical and chemical investigations (XRD, TEM, SEM, ICP-MS) run on bulk rocks and on the clay fractions enable the origin and evolution of silicate parageneses to be characterized. Plio-Pleistocene silt and clay interbeds contain detrital clay assemblages comprising chlorite, illite, interstratified illite smectite, smectite, kaolinite and palygorskite. The greater smectite input within late Pliocene units than in Pleistocene oozes may relate either varying source areas or change in paleoclimatic conditions and weathering intensity. The clay intervals from Miocene-upper Oligocene wackestone sections are fairly different, with prevalent smectite in the fine fraction, whose high crystallinity and Mg contents that point towards an authigenic origin. The lower Miocene section, below 1104 mbsf, at depths where compaction features are well developed, is particularly characterized by abundant authigenic Na-K-clinoptilolite filling foraminifer tests. The authigenic smectite and clinoptilolite paragenesis is recorded by the chemical trends, both of the sediment and the interstitial fluid. This diagenetic evolution implies Si- and Mg rich fluids circulating in deeper and older sequences. For lack of any local volcaniclastic input, the genesis of zeolite and the terms of water rock interaction are discussed. The location of the diagenetic front correlates with that of the seismic sequence boundary P2 dated as 23.2 Ma. This correspondence may allow the chronostratigraphic significance of some specific seismic reflections to be reassessed.