Petrology at DSDP Leg 66 Holes

The sandstone petrology of Leg 66 samples provides insights into changes through time in the geology of the source regions along the Guerrero portion of the Middle America continental margin. This in turn constrains possible models of the evolution of the Middle America Trench (e.g., de Czerna, 1971; Malfait and Dinkleman, 1972; Karig, 1974). Primarily medium-grained sands and sandstones, representing the widest variety available of trench/trench slope settings and ages, were analyzed in both light and heavy mineral studies. Standard techniques were used as much as possible in order to compare results from other margins and from ancient rocks.

Representative and whole rock analyses of rocks from Filchnerfjella, Antarctica

The geological history of Filchnerfjella and surrounding areas (2°E to 8°E) in central Dronning Maud Land, East Antarctica, is constructed from metamorphic and igneous petrology, and structural investigations. The geology of Filchner-fjella consists mainly of metamorphic rocks accompanied by intrusive rocks. Two stages of metamorphism can be recognized in this area. The earlier stage metamorphism is defined as a porphyroblast stage (garnet, hornblende, and sillimanite stable), and the later one is recognized as a symplectic stage (orthopyroxene and cordieritestable). Taking metamorphic textures and geothermobarometries into account, the rocks experienced an early high-P/medium-T followed by a low-P and high-T stage. Partial melting took place during the low-P/high-T stage, because probable melt of leucocratic gneiss contains cordierite. The field relationships and petrography of the syenite at Filchnerfjella are similar to those of post-tectonic plutons from central Dronning Maud Land, and most of the post-tectonic intrusive rocks have within-plate geochemical features. The structural history in Filchnerfjella and surrounding areas can be divided into the Pan-African stage and the Meso to Cenozoic stage that relates to the break-up of Gondwana.

(Appendix 1-4) Geochemistry, desorption methods, TOC, TC, and mineral content of different settings

Sorption of volatile hydrocarbon gases (VHCs) to marine sediments is a recognized phenomenon that has been investigated in the context of petroleum exploration. However, little is known about the biogeochemistry of sorbed methane and higher VHCs in environments that are not influenced by thermogenic processes. This study evaluated two different extraction protocols for sorbed VHCs, used high pressure equipment to investigate the sorption of methane to pure clay mineral phases, and conducted a geochemical and mineralogical survey of sediment samples from different oceanographic settings and geochemical regimes that are not significantly influenced by thermogenic gas. Extraction of sediments under alkaline conditions yielded higher concentrations of sorbed methane than the established protocol for acidic extraction. Application of alkaline extraction in the environmental survey revealed the presence of substantial amounts of sorbed methane in 374 out of 411 samples (91%). Particularly high amounts, up to 2.1 mmol kg**-1 dry sediment, were recovered from methanogenic sediments. Carbon isotopic compositions of sorbed methane suggested substantial contributions from biogenic sources, both in sulfate-depleted and sulfate-reducing sediments. Carbon isotopic relationships between sorbed and dissolved methane indicate a coupling of the two pools. While our sorption experiments and extraction conditions point to an important role for clay minerals as sorbents, mineralogical analyses of marine sediments suggest that variations in mineral composition are not controlling variations in quantities of sorbed methane. We conclude that the distribution of sorbed methane in sediments is strongly influenced by in situ production.

Mineral composition of 0.1-0.05 mm grain size fraction from the surface layer of bottom sediments in the West European province of the Atlantic Ocean

Rock material sampled from the Mir manned deep-sea submersibles and by dradges, grabs, and sediment cores over a vast area of the North Atlantic was analyzed to show that this material is of continental origin, unlike original rocks of the ocean floor. It is proved to be related to iceberg rafting during Quaternary glaciations. Independent data on distribution and composition of sandy and silty grains in sediment cores also support this relation to the recent glaciation. New criteria for identification of iceberg rock matter in pelagic sediments are presented on the base of analysis of all available data.

(Table T1) Mineral composition of turbidite sands and sandstones from ODP Leg 190 sites

During Ocean Drilling Program Leg 190 several turbidite successions in the Nankai Trough were drilled through including Pleistocene trench fill (Sites 1173 and 1174), Pleistocene-Pliocene slope basin deposits and underlying trench fill (Sites 1175 and 1176), Miocene Shikoku Basin deposits (Site 1177), and upper Miocene trench fill (Site 1178). Sands from the Pleistocene trench-fill succession of the Nankai Trough are of mixed derivation with significant monomineralic components (quartz and feldspar) and mafic to intermediate volcanic rock fragments, in addition to sedimentary and less abundant metamorphic detritus. They have a source in the Izu collision zone in central Honshu. Sands from the slope and accreted trench fill at Sites 1175 and 1176 are dominated by quartz with less abundant feldspar, sedimentary rock fragments, and only minor volcanic and metamorphic rock fragments. In contrast to the trench turbidites of Sites 1173 and 1174, these sands are very quartzose with characteristic radiolarian chert fragments. Volcanic rock fragments are mainly of silicic composition. Potential sources of these sands are uplifted subduction complexes of southwest Japan. Sands from the accreted trench turbidites at Site 1178 have clast types similar to those at Sites 1175 and 1176. In contrast, however, framework detrital modes are distinctive, with Site 1178 sands having substantially lower total quartz contents and more abundant fine-grained sedimentary rock fragments. These sands were also probably derived from the island of Shikoku, but their composition indicates that sedimentary rocks were abundant in the source area and these may have been Miocene forearc basin successions that were largely removed by erosion. Erosional remnants of Miocene forearc basin deposits are present on the Kii Peninsula east-northeast of Shikoku. Erosion followed a phase of exhumation of the Shimanto Belt indicated by apatite fission track ages at ~10 Ma. Sand in the lower-upper Miocene turbidites of the lower Shikoku Basin section at Site 1177 is more varied in composition, with the upper part of the unit similar to Site 1178 (i.e., rich in sedimentary rock fragments) and the lower part similar to those at Sites 1175 and 1176 (i.e., rich in quartz with some silicic volcanic rock fragments). Sands from the lower part of the Miocene turbidite unit were derived from a continental source with plutonic and volcanic rocks, possibly the inner zone of southwest Japan.

Lithospheric thickness of Pangea

Surface wave tomography, using the fundamental Rayleigh wave velocities and those of higher modes between 1 and 4 and periods between 50 and 160 s, is used to image structures with a horizontal resolution of ~250 km and a vertical resolution of ~50 km to depths of ~300 km in the mantle. A new model, PM_v2_2012, obtained from 3×10**6 seismograms, agrees well with earlier lower resolution models. It is combined with temperature estimates from oceanic plate models and with pressure and temperature estimates from the mineral compositions of garnet peridotite nodules to generate a number of estimates of SV(P,T) based on geophysical and petrological observations alone. These are then used to estimate the unrelaxed shear modulus and its derivatives with respect to pressure and temperature, which agree reasonably with values from laboratory experiments. At high temperatures relaxation occurs, causing the shear wave velocity to depend on frequency. This behaviour is parameterised using a viscosity to obtain a Maxwell relaxation time. The relaxation behaviour is described using a dimensionless frequency, which depends on an activation energy E and volume Va. The values of E and Va obtained from the geophysical models agree with those from laboratory experiments on high temperature creep. The resulting expressions are then used to determine the lithospheric thickness from the shear wave velocity variations. The resolution is improved by about a factor of two with respect to earlier models, and clearly resolves the thick lithosphere beneath active intracontinental belts that are now being shortened. The same expressions allow the three dimensional variations of the shear wave attenuation and viscosity to be estimated.

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.

(Table 1) H2O and CO2 contents in submarine basaltic glasses from Ontong Java Plateau

The Ontong Java Plateau in the western Pacific is anomalous compared to other oceanic large igneous provinces in that it appears to have never formed a large subaerial plateau. Paleoeruption depths (at 122 Ma) estimated from dissolved H2O and CO2 in submarine basaltic glass pillow rims vary from ~1100 m below sea level (mbsl) on the central part of the plateau to 2200-3000 mbsl on the northeastern edge. Our results suggest maximum initial uplift for the plateau of 2500-3600 m above the surrounding seafloor and 1500+/-400 m of postemplacement subsidence since 122 Ma. Our estimates of uplift and subsidence for the plateau are significantly less than predictions from thermal models of oceanic lithosphere, and thus our results are inconsistent with formation of the plateau by a high-temperature mantle plume. Two controversial possibilities to explain the anomalous uplift and subsidence are that the plateau (1) formed as a result of a giant bolide impact, or (2) formed from a mantle plume but has a lower crust of dense garnet granulite and/or eclogite; neither of these possibilities is fully consistent with all available geological, geophysical, and geochemical data. The origin of the largest magmatic event on Earth in the past 200 m.y. thus remains an enigma.

Composition of bottom sediments and rocks from the Philippine Trough

New data on bottom sediments and igneous rocks of the Philippine Trench are under consideration. They show differences in geological structures of the island slope and the ocean slope of the trench. The island slope is comparable to the accretionary prism formations on the Philippines; there processes of gravitational re-deposition of sediments occur. The ocean slope is an edge of the Philippine Plate sinking into the trough, where basalts of the oceanic crust are exposed.

Chemical and mineral compositions of recent and ancient sediments and sedimentary rocks from the Mariana Trench

On the bed and on the ocean slope of the southern latitudinal part of the Mariana Trench ancient sediments, as well as sedimentary and igneous rocks are exposed. In the lower part of the sampled part of the studied section Late Oligocene to Early Miocene chalk-like limestones and marls occur. Upward marly tuffites and tuffs (apparently alternating with carbonate rocks) occur. These rocks are overlain by Early Miocene tuffaceous clays and siliceous-clayey muds. In the upper part of the section there are Pleistocene pelagic clays and ethmodiscus oozes.