Geochemical analysis of minerals from sediment core CRP-2 in the Ross Sea, Antarctica

Distribution patterns, petrography, whole-rock and mineral chemistry, and shape and fabric data are described for the most representative basement lithologies occurring as clasts (granule to bolder grain-size class) from the 625 m deep CRP-2/2A drillcore. A major change in the distribution pattern of the clast types occurs at c. 310 mbsf., with granitoid-dominated clasts above and mainly dolerite clasts below; moreover, compositional and modal data suggest a further division into seven main detrital assemblages or petrofacies. In spite of this variability, most granitoid pebbles consist of either pink or grey biotite±hornblende monzogranites. Other less common and ubiquitous lithologies include biotite syenogranite, biotite-hornblende granodiorite, tonalite, monzogranitic porphyries (very common below 310 mbsf), microgranite, and subordinately, monzogabbro, Ca-silicate rocks, biotite-clinozoisite schist and biotite orthogneiss (restricted to the pre-Pliocene strata). The ubiquitous occurrence of biotite±hornblende monzogranite pebbles in both the Quaternary-Pliocene and Miocene-Oligocene sections, apparently reflects the dominance of these lithologies in the onshore basement, and particularly in the Cambro-Ordovician Granite Harbour Igneous Complex which forms the most extensive outcrop in southern Victoria Land. The petrographical features of the other CRP-2/2A pebble lithologies are consistent with a supply dominantly from areas of the Transantarctic Mountains facing the CRP-2/2A site, and they thus provide further evidence of a local provenance for the supply of basement clasts to the CRP-2/2A sedimentary strata.

Geochemistry of subducted volcaniclastic sediments from Mariana arc magmas

The phase relations of natural volcaniclastic sediments from the west Pacific Ocean were investigated experimentally at conditions of 3-6 GPa and 800-900 °C with 10 wt.% added H2O (in addition to ~ 10 wt.% structurally-bound H2O) to induce hydrous melting. Volcaniclastic sediments are shown to produce a sub-solidus assemblage of garnet, clinopyroxene, biotite, quartz/coesite and the accessory phases rutile ± Fe-Ti oxide ± apatite ± monazite ± zircon. Hydrous melt appears at temperatures exceeding 800-850 °C, irrespective of pressure. The melt-producing reaction consumes clinopyroxene, biotite and quartz/coesite and produces orthopyroxene. These phase relations differ from those of pelagic clays and K-bearing mid ocean ridge basalts (e.g. altered oceanic crust) that contain phengite, rather than biotite, as a sub-solidus phase. Despite their relatively high melt productivity, the wet solidus for volcaniclastic sediments is found to be higher (825-850 °C) than other marine sediments (700-750 °C) at 3 GPa. This trend is reversed at high-pressure conditions (6 GPa) where the biotite melting reaction occurs at lower temperatures (800-850 °C) than the phengite melting reaction (900-1000 °C). Trace element data was obtained from the 3 GPa run products, showing that partial melts are depleted in heavy rare earth elements (REE) and high field strength elements (HFSE), due to the presence of residual garnet and rutile, and are enriched in large ion lithophile elements (LILE), except for Sr and Ba. This is in contrast to previous experimental studies on pelagic sediments at sub-arc depths, where Sr and Ba are among the most enriched trace elements in glasses. This behavior can be partly attributed to the presence of residual apatite, which also host some light REE in our supra-solidus residues. Our new experimental results account for a wide range of trace element and U-series geochemical features of the sedimentary component of the Mariana arc magmas, including imparting a substantial Nb anomaly to melts from an anomaly-free protolith.

Composition of suspended matter and bottom sediments from the Atlantic Ocean and its seas

The book summarizes data on distribution and composition of sedimentary material suspended in waters of the Atlantic Ocean and its seas. Results of observations of Soviet and foreign expeditions are given. Distribution of suspended matter in sections across the ocean, as well as in the most studied seas are shown. New data on grain size, mineral and chemical composition of suspended matter are published. Summary of history of investigation of bottom sediments from the Atlantic Ocean from the first scientific cruises to the present is done. A brief description of sediment types in the ocean and a detailed description of Mediterranean Sea sediments are given.

Bulk chemical analysis of sediments ODP Hole 110-671B and 110-672A

Chemical analyses for calcium carbonate, organic carbon, and major constituents (Al, Ti, Ca, Mg, K, Fe) of bulk sediments collected in Hole 671B have been carried out. Organic carbon contents in Pleistocene through middle Miocene sediments above the zone of decollement are very low (<0.1%); below the decollement considerably higher concentrations of organic carbon occur (up to 1 %). Changes in Ti/Al and Fe/Al ratios are minor, but K/Al and Mg/Al ratios show clear trends with the age of the sediments. Preliminary comparisons of these ratios with mineralogic information on clays indicate good correspondence with clay abundances. Calculations of the mass flux of magnesium from the overlying ocean into the pore fluids suggest that addition of magnesium to the sediments is difficult to detect, especially in the absence of a background reference concentration.

Grain size, mineral and chemical compositions of modern deposits fron the northern Caspian Sea and the Volga River delta

Comprehensive investigations revealed that modern deposits in the northern Caspian Sea involve terrigenous sands and aleurites with admixture of detritus and intact bivalve shells, including coquina. Generally, these deposits overlay dark grayish viscous clays. Similar geological situation occurs in the Volga River delta; however, local deposits are much poorer in biogenic constituents. Illite prevails among clay minerals. In coarse aleurite fraction (0.100-0.050 mm) heavy transparent minerals are represented mostly by epidotes, while light minerals - mostly by quartz and feldspars. Sedimentary material in the Volga River delta is far from completely differentiated into fractions due to abundant terrigenous inflows. Comparatively better grading of sediments from the northern Caspian Sea is due to additional factors such as bottom currents and storms. When passing from the Volga River delta to the northern Caspian Sea, sediments are enriched in rare earth elements (except Eu), Ca, Au, Ni, Se, Ag, As, and Sr, but depleted in Na, Rb, Cs, K, Ba, Fe, Cr, Co, Sc, Br, Zr, ??, U, and Th. Concentrations of Zn remain almost unchanged. Sedimentation rates and types of recent deposits in the northern Caspian Sea are governed mainly by abundant runoff of the Volga River.

Zircons, isotope ratios and element analyses from tonalite and oxide gabbro of the mid-ocean ridge from ODP Hole 176-735B

A limiting factor in the accuracy and precision of U/Pb zircon dates is accurate correction for initial disequilibrium in the 238U and 235U decay chains. The longest-lived-and therefore most abundant-intermediate daughter product in the 235U isotopic decay chain is 231Pa (T1/2 = 32.71 ka), and the partitioning behavior of Pa in zircon is not well constrained. Here we report high-precision thermal ionization mass spectrometry (TIMS) U-Pb zircon data from two samples from Ocean Drilling Program (ODP) Hole 735B, which show evidence for incorporation of excess 231Pa during zircon crystallization. The most precise analyses from the two samples have consistent Th-corrected 206Pb/238U dates with weighted means of 11.9325 ± 0.0039 Ma (n = 9) and 11.920 ± 0.011 Ma (n = 4), but distinctly older 207Pb/235U dates that vary from 12.330 ± 0.048 Ma to 12.140 ± 0.044 Ma and 12.03 ± 0.24 to 12.40 ± 0.27 Ma, respectively. If the excess 207Pb is due to variable initial excess 231Pa, calculated initial (231Pa)/(235U) activity ratios for the two samples range from 5.6 ± 1.0 to 9.6 ± 1.1 and 3.5 ± 5.2 to 11.4 ± 5.8. The data from the more precisely dated sample yields estimated DPazircon/DUzircon from 2.2-3.8 and 5.6-9.6, assuming (231Pa)/(235U) of the melt equal to the global average of recently erupted mid-ocean ridge basaltic glasses or secular equilibrium, respectively. High precision ID-TIMS analyses from nine additional samples from Hole 735B and nearby Hole 1105A suggest similar partitioning. The lower range of DPazircon/DUzircon is consistent with ion microprobe measurements of 231Pa in zircons from Holocene and Pleistocene rhyolitic eruptions (Schmitt (2007; doi:10.2138/am.2007.2449) and Schmitt (2011; doi:10.1146/annurev-earth-040610-133330)). The data suggest that 231Pa is preferentially incorporated during zircon crystallization over a range of magmatic compositions, and excess initial 231Pa may be more common in zircons than acknowledged. The degree of initial disequilibrium in the 235U decay chain suggested by the data from this study, and other recent high precision datasets, leads to resolvable discordance in high precision dates of Cenozoic to Mesozoic zircons. Minor discordance in zircons of this age may therefore reflect initial excess 231Pa and does not require either inheritance or Pb loss.

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.

Geochemical composition of the Trincheira Complex in the southwestern Amazon craton, Brazil

We document the first-known Mesoproterozoic ophiolite from the southwestern part of the Amazon craton, corresponding to the Trincheira Complex of Calymmian age, and propose a tectonic model that explains many previously enigmatic features of the Precambrian history of this key craton, and discuss its role in the reconstruction of the Columbia supercontinent. The complex comprises extrusive rocks (fine-grained amphibolites derived from massive and pillowed basalts), mafic-ultramafic intrusive rocks, chert, banded iron formation (BIFs), pelites, psammitic and a smaller proportion of calc-silicate rocks. This sequence was deformed, metasomatized and metamorphosed during the development of the Alto Guaporé Belt, a Mesoproterozoic accretionary orogen. The rocks were deformed by a single tectonic event, which included isoclinal folding and metamorphism of the granulite-amphibolite facies. Layered magmatic structures were preserved in areas of low strain, including amygdaloidal and cumulate structures. Metamorphism was pervasive and reached temperatures of 780-853°C in mafic granulites and 680-720°C in amphibolites under an overall pressure of 6.8 kbar. The geochemical composition of the extrusive and intrusive rocks indicates that all noncumulus mafic-ultramafic rocks are tholeiitic basalts. The mafic-ultramafic rocks display moderately to strongly fractionation of light rare earth elements (LREE), near-flat heavy rare earth elements (HREE) patterns and moderate to strong negative high field strength elements (HFSE) anomalies (especially Nb), a geochemical signature typical of subduction zones. The lowest units of mafic granulites and porphyroblastic amphibolites in the Trincheira ophiolite are similar to the modern mid-ocean ridge basalt (MORB), although they locally display small Ta, Ti and Nb negative anomalies, indicating a small subduction influence. This behavior changes to an island arc tholeiites (IAT) signature in the upper units of fine-grained amphibolites and amphibole rich-amphibolites, characterized by progressive depletion in the incompatible elements and more pronounced negative Ta and Nb anomalies, as well as common Ti and Zr negative anomalies. Tectono-magmatic variation diagrams and chondrite-normalized REE and primitive mantle normalized patterns suggest a back-arc to intra-oceanic island arc tectonic regime for the eruption of these rocks. Therefore, the Trincheira ophiolite appears to have originated in an intraoceanic supra-subduction setting composed of an arc-back-arc system. Accordingly, the Trincheira Complex is a record of oceanic crust relics obducted during the collision of the Amazon craton and the Paraguá block during the Middle Mesoproterozoic. Thus, the recognition of the Trincheira ophiolite and suture significantly changes views on the evolution of the southern margin of the Amazon craton, and how it can influence the global tectonics and the reconstruction of the continents.