Geochemistry and mineralogy of ODP Leg 107 basalts

During ODP Leg 107, the basement of the Tyrrhenian Sea was drilled at Site 650, located in the Marsili basin, and at Sites 651 and 655, both located in the Vavilov basin. In addition, a lava flow was drilled at Site 654 on the Sardinia rifted margin. Mineral and whole rock major and trace element chemistry, including rare earth element (REE) and Sr and Nd isotopic ratios, were determined in samples of these rocks. Site 654 lava was sampled within uppermost Pliocene postrift sediments. This lava is a basaltic andesite of intraplate affinity, and is analogous to some Plio-Pleistocene tholeiitic lavas from Sardinia. Site 650 basalts, drilled beneath 1.7-1.9-Ma-old basal sediment, are strongly altered and vesicular suggesting a rapid subsidence of the Marsili basin. Based on incompatible trace elements, these basalts show calc-alkaline affinity like some products of the Marsili Seamount and the Eolian arc. The basement of the two sites drilled within Vavilov basin shows contrasting petrologies. Site 655, located along the Gortani ridge in the western part of the basin, drilled a 116-m-thick sequence of basalt flows beneath 3.4-3.6-Ma-old basal sediments. These basalts are chemically relatively homogeneous and show affinity to transitional MORB. Four units consisting of slightly differentiated basaltic lavas, have been identified. Site 655 basalts are geochemically similar to the high Ti lavas from DSDP Leg 42, Site 373 (Vavilov Basin). The basement at Site 651, overlain by 40 m of metalliferous dolostone covered by fossiliferous sediments with an age of 2 Ma, consists of two basalt units separated by a dolerite-albitite intrusive body; serpentinized harzburgites were drilled for 30 m at the base of the hole. The two basalt units of Site 651 are distinct petrochemically, though both show incompatible elements affinity with high-K calc-alkaline/calc-alkaline magmas from Eolian arc. The cpx chemistry and high K/Na ratio of the lower unit lavas suggest a weak alkaline tendency of potassic lineage. Leg 107 basement rock data, together with data from DSDP Site 373 and from dredged samples, indicate that the deepest basins of the central Tyrrhenian Sea are underlain by a complex back-arc basin crust produced by magmas with incompatible element affinities to transitional MORB (Site 655 and DSDP Site 373), and to calc-alkaline and high-K calc-alkaline converging plate margin basalts (Sites 650 and 651). This petrogenetic complexity is in accordance with the back-arc setting of the Vavilov and Marsili basins. Other back-arc basin basalts, particularly those from ensialic basins such as the Bransfield Strait (Antarctica), show a comparable petrogenetic complexity (cf., Sounders and Tarney, 1984).

Mineral and chemical compositions of sedimentary and volcano-sedimentary rocks drilled in DSDP Legs 43 and 44

Sedimentary cover on the bottom of the Northwest Atlantic Ocean is underlain by Late Jurassic - Cretaceous tholeiite-basalt formation. It consists of come sedimentary formations with different lithologic features and age. Their composition, stratigraphic position and, distribution are described on materials of deep-sea drilling. Mineralogical and geochemical studies of DSDP Leg 43 and Leg 44 holes lead to new ideas about composition and genesis of some sediment types of and their associations. High metal contents in the chalk formation of black clays on the Bermuda Rise probably result from exhalations. Connection of red-colored and speckled deposits with hiatuses in sedimentation is shown. Main stages of geological history of the North American Basin are reflected in accumulation of the followed formations: ancient carbonate formation (Late Jurassic - Early Cretaceous), formation of black clays rich in organic matter (Cretaceous), formation of speckled clays (Late Cretaceous), siliceous-clayey turbidite formation (Eocene), hemipelagic and pelagic clayey formation (Neogene), and terrigenous turbidite formation (Pleistocene).

Compositions of Ni-rich magnesiowüstite and coexisting magnesioferrite spinel from sediments of DSDP Holes 86-577 and 91-596

We have found trace inclusions of Ni-rich magnesiowüstite within grains of magnesioferrite spinel recovered from Cretaceous/Tertiary boundary sediments from DSDP Site 596, South Pacific (23°51.20'S, 169°39.27'W) and DSDP Site 577, North Pacific (3°6.51'N, 157°43.40'E). Measured compositions of these inclusions range from (Mg_0.85Ni_0.74Fe_0.17)O to (Mg_0.74Ni_0.09Fe_0.17)O. Coexisting magnesioferrite and magnesiowüstite can only crystallize from ultramafic, refractory, Mg-rich liquids with Mg/Si > 2 (atom ratio). Such liquid compositions cannot form as a result of fractional crystallization and are unknown to occur as a result of terrestrial igneous processes or meteoroid ablation. We infer that these minerals crystallized from liquid droplets that equilibrated with silicate vapor at high temperatures (probably >2300°C), resulting in fractionation of volatile SiO2 from more refractory MgO. The most plausible source of this high-temperature vapor is in the fireball of the major impact event that terminated the Cretaceous.

(Table 1) Chemical composition of Middle Cenozoic magmatic rocks from the Schmidt Peninsula, North Sakhalin

Middle Cenozoic evolution of magmatism in the Schmidt Peninsula between 37 and 25 Ma began with eruptions of subalkaline and moderately alkaline andesite, latite, trachyandesite, and trachyrhyolite lavas and ended with subvolcanic intrusions of highly alkaline strongly undersaturated essexites. According to trace element data magmatism evolved from melting of a mantle source in the zone of ocean-continent plate convergence to small degree partial melting in the lithospheric mantle at the final stage. This succession is generally typical for Late Cenozoic continental-margin magmatism in the Southeast Russia. Similarity in the Middle and Late Cenozoic stages of magmatism is an evidence for their individual significance.

Major element concentrations in spinel, olivine and pyroxene from peridotites of ODP Hole 210-1277 of the Newfoundland margin

Serpentinized spinel peridotites of the Newfoundland margin drilled during ODP Leg 210 at Site 1277 have preserved, relic mineral compositions similar to the most depleted abyssal peridotites worldwide and different from those of the conjugate Iberian margin. The samples are derived from mass flows containing clasts of peridotite and gabbro and from in-situ basement, and are mostly mylonitic cpx-poor spinel harzburgites with Cr-rich spinels (Cr#0.35-0.66). Melting of the Newfoundland mantle occurred in the spinel peridotite field and probably exceeded the cpx-out phase boundary for some samples. Using proposed spinel peridotite melting models and experimentally derived phase diagrams, the Newfoundland harzburgites can be modeled as a residue after extraction of 14 to 20-25% melting. Basalts that are interleaved with mass flow deposits on top of the peridotite basement resemble normal to transitional mid-ocean ridge basalt. This, together with the unusually high Cr# of some spinel harzburgites suggest that the formation of basalts and partial melting of the underlying peridotite are not cogenetic. Among mantle samples some of the Newfoundland harzburgites approach mineral compositions of the Bay of island ophiolite and ophiolites from Japan that represent peridotites formed in an arc-setting. Thus, the peridotites drilled at Site 1277 may represent inherited (Caledonian or older) subarc mantle that was exhumed close to the ocean floor during the rifting evolution of the Atlantic. Compared to the spinel harzburgites from Newfoundland, the peridotites from the conjugate Iberian margin are, on average, less depleted and provide evidence for local equilibration in the plagioclase stability field. This can either be explained by an inherited, primary, Ca-richer composition of the Iberia peridotite, or, alternatively, by local melt impregnation and stagnation during continental rifting, and thus refertilizing previously depleted (arc-related) peridotite.

SHRIMP zircon geochronology and geochemistry of Phanerozoic granitoids in southeastern Korea

Phanerozoic granitoids are widespread in the Korean Peninsula and form a part of the East Asian Cordilleran-type granitoid belt extending from southeastern China to Far East Russia. Here we present SHRIMP zircon U-Pb ages and geochemical and Nd isotopic compositions of Late Paleozoic to Early Jurassic granitoid plutons in the northern Gyeongsang basin, southeastern Korea; namely the Jangsari, Yeongdeok, Yeonghae, and Satkatbong plutons. The granite and associated gabbroic rocks from the Jangsari pluton were coeval and respectively dated at 257.3 ± 2.0 Ma and 255.7 ± 1.4 Ma. This result represents the first finding of a Late Paleozoic pluton in South Korea. Three granite samples from the Yeongdeok pluton yielded a slightly younger age span ranging from 252.9 ± 2.5 Ma to 246.7 ± 2.1 Ma. Two diorite samples from the Yeonghae pluton gave much younger ages of 195.1 ± 1.9 Ma and 196.3 ± 1.6 Ma. An Early Jurassic age of 192.4 ± 1.6 Ma was also obtained from a diorite sample from the Satkatbong pluton. The mineral assemblage and Al2O3/(Na2O + K2O) versus Al2O3/(CaO + Na2O + K2O) relationship indicate that all the analyzed plutons are subduction zone granitoids. Enrichments in large-ion-lithophile-elements and depletions in high-field-strength-elements of these plutons are also concordant with geochemical characteristics typical for the subduction zone magma. The presence of Late Permian to Early Triassic arc system is in contrast with the conventional idea that the arc magmatism along the continental margin of the Korean Peninsula has commenced from Early Jurassic after the termination of Triassic collisional orogenesis. The epsilon-Nd(t) values of the granitoid plutons are consistently positive (2.4-4.6), suggesting that crustal residence time of the basement beneath the Gyeongsang basin is relatively short. Moreover, the reevaluation of previously-published data reveals that geochemical compositions of the Yeongdeok pluton are compatible with those of high-silica adakites; La/Yb = 37.5-114.6, Sr/Y = 138.2-214.0, SiO2 = 62.9-72.0 wt. %, Al2O3 = 15.5-17.0 wt. %, Sr = 562-1173 ppm, MgO = 0.4-1.6 wt. %, Y = 3-6 ppm, Yb = 0.18-0.45 ppm, and Eu/Eu* = 0.92-1.31. The occurrence of adakites in southeastern Korea, and presumably in the Hida belt of central-western Japan, is indicative of a hot subduction regime developing at least partly along the East Asian continental margin during the Permian-Triassic transition period.

Physical properties, mineral and chemical compositions, and accumulation rates of Holocene and Upper Pleistocene bottom sediments from the Sea of Okhotsk

Mineral and chemical compositions and physical properties of diatomaceous clayey-siliceous sediments from the Sea of Okhotsk are studied. Accumulation rates of silica are determined. Their compositional model based on silica content is similar to that of Late Jurassic and Olenekian-Middle Anisian cherts from the Sikhote Alin region. Thickness of Holocene siliceous unit and accumulation rates of siliceous deposits depended on bioproductivity in the upper water layer and seafloor topography. Accumulation rates of amorphous SiO2 (0.05-5.7 g/cm**2/ka) and free SiO2 (0.5-11.6 g/cm**2/ka) are minimal on seamounts and maximal in depressions near foothills. These values match accumulation rates of free SiO2 in Triassic and Late Jurassic basins of the Sikhote Alin region (0.33-3 g/cm**2/ka). Comparison of composition and accumulation rates of silica shows that Triassic and Late Jurassic siliceous sequences of Sikhote Alin could accumulate in a marginal marine basin near a continent.

Representative analyses of pre-tectonic metabasic rocks and metapelitic gneiss from Sverdrupfjella, East Antarctica

Extensive high-grade polydeformed metamorphic provinces surrounding Archaean cratonic nuclei in the East Antarctic Shield record two tectono-thermal episodes in late Mesoproterozoic and late Neoproterozoic-Cambrian times. In Western Dronning Maud Land, the high-grade Mesoproterozoic Maud Belt is juxtaposed against the Archaean Grunehogna Province and has traditionally been interpreted as a Grenvillian mobile belt that was thermally overprinted during the Early Palaeozoic. Integration of new U-Pb sensitive high-resolution ion microprobe and conventional single zircon and monazite age data, and Ar-Ar data on hornblende and biotite, with thermobarometric calculations on rocks from the H.U. Sverdrupfjella, northern Maud Belt, resulted in a more complex P-T-t evolution than previously assumed. A c. 540?Ma monazite, hosted by an upper ampibolite-facies mineral assemblage defining a regionally dominant top-to-NW shear fabric, provides strong evidence for the penetrative deformation in the area being of Pan-African age and not of Grenvillian age as previously reported. Relics of an eclogite-facies garnet-omphacite assemblage within strain-protected mafic boudins indicate that the peak metamorphic conditions recorded by most rocks in the area (T = 687-758°C, P = 9·4-11·3?kbar) were attained subsequent to decompression from P > 12·9?kbar. By analogy with limited U-Pb single zircon age data and on circumstantial textural grounds, this earlier eclogite-facies metamorphism is ascribed to subduction and accretion around 565?Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions is ascribed to the intrusion of post-orogenic granite at c. 480?Ma. The recognition of extensive Pan-African tectonism in the Maud Belt casts doubts on previous Rodinia reconstructions, in which this belt takes a pivotal position between East Antarctica, the Kalahari Craton and Laurentia. Evidence of late Mesoproterozoic high-grade metamorphism during the formation of the Maud Belt exists in the form of c. 1035?Ma zircon overgrowths that are probably related to relics of granulite-facies metamorphism recorded from other parts of the Maud Belt. The polymetamorphic rocks are largely derived from a c. 1140?Ma volcanic arc and 1072 ± 10?Ma granite.

Geochemistry of volcanic rocks of ODP Holes 126-792E and 126-793B

The Izu-Bonin forearc basement volcanic rocks recovered from Holes 792E and 793B show the same phenocrystic assemblage (i.e., plagioclase, two pyroxenes, and Fe-Ti oxides ±olivine), but they differ in the crystallization sequence and their phenocryst chemistry. All the igneous rocks have suffered low-grade hydrothermal alteration caused by interaction with seawater. As a result, only clinopyroxenes, plagioclases, and oxides have preserved their primary igneous compositions. The Neogene olivine-clinopyroxene diabasic intrusion (Unit II) recovered from Hole 793B differs from the basement basaltic andesites because it lacks Cr-spinels and contains abundant titanomagnetites (Usp38.5-46.4) and uncommon FeO-rich (FeO = 29%) spinels. It displays petrological and geochemical similarities to the Izu Arc volcanoes and, thus, can be considered as related to Izu-Bonin Arc magmatic activity. The titanomagnetites (Usp28.5-33) in the calc-alkaline andesitic fragments of the Oligocene volcaniclastic breccia in Hole 793B (Unit VI) represent an early crystallization phase. The Plagioclase phenocrysts enclosed in these rocks show oscillatory zoning and are less Ca-rich (An78.6-67.8) than the plagioclase phenocrysts of the diabase sill and the basement basaltic andesites. Their clinopyroxenes are Fe-rich augites (Fs ? 19.4; FeO = 12%) and thus, differ significantly from the clinopyroxenes of the Hole 793B arc-tholeiitic igneous rocks. The 30-32 Ma porphyritic, two-pyroxene andesites recovered from Hole 792E are very similar to the andesitic clasts of the Neogene breccia recovered in Hole 793B (Unit VI). Both rocks have the same crystallization sequence, and similar chemistry of the Fe-Ti oxides, clinopyroxenes, and plagioclases: that is, Ti-rich (Usp25.5-30.4) magnetites, Fe-rich augites, and intensely oscillatory zoned plagioclases with bytownitic cores (An86-63) and labradorite rims (An73-68). They display a calc-alkaline differentiation trend (Taylor et al., this volume). So, the basement highly porphyritic andesites recovered at Hole 792E, and the Hole 793B andesitic clasts of Unit VI show the same petrological and geochemical characteristics, which are that of calc-alkaline suites. These Oligocene volcanic rocks represent likely the remnants of the Izu-Bonin normal arc magmatic activity, before the forearc rifting and extension. The crystallization sequence in the basaltic andesites recovered from Hole 793B is olivine-orthopyroxene-clinopyroxene-plagioclase-Fe-Ti oxides, indicating a tholeiitic differentiation trend for these volcanic rocks. Type i is an olivine-and Cr-spinel bearing basaltic andesite whereas Type ii is a porphyritic pyroxene-rich basaltic andesite. The porphyritic plagioclase-rich basaltic andesite (Type iii) is similar, in most respects, to Type ii lavas but contains plagioclase phenocrysts. The last, and least common lava is an aphyric to sparsely phyric andesite (Type iv). Cr-spinels, included either in the olivine pseudomorphs of Type i lavas or in the groundmass of Type ii lavas, are Cr-rich and Mg-rich. In contrast, Cr-spinels included in clinopyroxenes and orthopyroxenes (Types i and ii lavas) show lower Cr* and Mg* ratios and higher aluminium contents. Orthopyroxenes from all rock types are Mg-rich enstatites. Clinopyroxenes display endiopsidic to augitic compositions and are TiO2 and Al2O3 depleted. All the crystals exhibit strong zoning patterns, usually normal, although, reverse zoning patterns are not uncommon. The plagioclases show compositions within the range of An90-64. The Fe-Ti oxides of the groundmass are TiO2-poor (Usp16-17). The Hole 793B basaltic andesites show, like the Site 458 bronzites from the Mariana forearc, intermediate features between arc tholeiites and boninites: (1) Cr-spinel in olivine, (2) presence of Mg-rich bronzite, Ca-Mg-rich clinopyroxenes, and Ca-plagioclase phenocrysts, and (3) transitional trace element depletion and epsioln-Nd ratios between arc tholeiites and boninites. Thus, the forearc magmatism of the Izu-Bonin and Mariana arcs, linked to rifting and extension, is represented by a depleted tholeiitic suite that displays boninitic affinities.

Chemical and isotopic compositions of volcanics from ODP Hole 210-1276

Site 1276, Leg 210 of the Ocean Drilling Program, was located on the Newfoundland margin in a seismically-defined ~128 Ma "transitional" crust just west of the presumed oceanic crust, and the M3 magnetic anomaly. The goal of drilling on this non-volcanic margin was to study the rifting, nature of basement, and post-rift sedimentation in the Newfoundland-Iberia rift. Drilling of this 1739 m hole was terminated 90-160 m above basement, in the lower of a doublet of alkaline diabase sills. We have carried out geochemical studies of the sill complex, in the hopes that they will provide proxy information regarding the nature of the underlying basement. Excellent 40Ar/39Ar plateau ages were obtained for the two sills: upper sill ~105.3 Ma; lower sill ~97.8 Ma. Thus the sills are substantially younger than the presumed age of the seafloor at site 1276 (~128 Ma), and were intruded beneath substantial sediment overburden (250 m for the upper, older sill, and 575 m for the lower younger sill). While some of the geochemistry of the sills has been compromised by alteration, the "immobile" trace elements show these sills to be hawaiites, differentiated from an enriched alkaline or basanitic parentage. Sr, Nd and Pb isotopes are suggestive of an enriched hotspot/plume mantle source, with a possible "added" component of continental material. These sills unequivocally were not derived from typical MORB (asthenospheric) upper mantle.

Geochemical composition of surface snow, ice and debris from glaciers in Norway, Nepal and New Zealand

Alpine glacier samples were collected in four contrasting regions to measure supraglacial dust and debris geochemical composition. A total of 70 surface glacier ice, snow and debris samples were collected in 2009 and 2010 in Svalbard, Norway, Nepal and New Zealand. Trace elemental abundances in snow and ice samples were measured via inductively coupled plasma mass spectrometry (ICP-MS). Supraglacial debris mineral, bulk oxide and trace element composition were determined via X-ray diffraction (XRD) and X-ray fluorescence spectroscopy (XRF). A total of 45 elements and 10 oxide compound abundances are reported. The uniform data collection procedure, analytical measurement methods and geochemical comparison techniques are used to evaluate supraglacial dust and debris composition variability in the contrasting glacier study regions. Elemental abundances revealed sea salt aerosol and metal enrichment in Svalbard, low levels of crustal dust and marine influences to southern Norway, high crustal dust and anthropogenic enrichment in the Khumbu Himalayas, and sulfur and metals attributed to quiescent degassing and volcanic activity in northern New Zealand. Rare earth element and Al/Ti elemental ratios demonstrated distinct provenance of particulates in each study region. Ca/S elemental ratio data showed seasonal denudation in Svalbard and Norway. Ablation season atmospheric particulate transport trajectories were mapped in each of the study regions and suggest provenance pathways. The in situ data presented provides first order glacier surface geochemical variability as measured from four diverse alpine glacier regions. This geochemical surface glacier data is relevant to glaciologic ablation rate understanding as well as satellite atmospheric and land-surface mapping techniques currently in development.