Geochemistry and isotopic ratios of samples from the Tiger gabbroic complex, Northern Victoria Land, Antarctica

Subduction related mafic/ultramafic complexes marking the suture between the Wilson Terrane and the Bowers Terrane in northern Victoria Land (Antarctica) are well-suited for evaluating the magmatic and structural evolu- tion at the Palaeo-Pacific continental margin of Gondwana. One of these intru- sions is the "Tiger Gabbro Complex" (TGC), which is located at the southern end of the island-arc type Bowers Terrane. The TGC is an early Palaeozoic island-arc related layered igneous complex characterized by extraordinarly fresh sequences of ultramafic, mafic and evolved lithologies and extensive development of high-temperature high-strain zones. The goal of the present study is to establish the kinematic, petrogenetic and temporal development of the TGC in order to evaluate the magmatic and structural evolution of the deep crustal roots of this Cambrian-aged island-arc. Fieldwork during GANOVEX X was carried out to provide insight into: (i) the spatial relations between the different igneous lithologies of the TGC, (ii) the nature of the contact between the TGC and Bowers Terrane, and (iii) the high-temperature shear zones exposed in parts of the TGC. Here, we report the results of detailed field and petrological observations combined with new geochronological data. Based on these new data, we tentatively propose a petrogenetic-kinematic model for the TGC, which involves a two-phase evolution during the Ross orogeny. These phases can be summarized as: (i) an early phase (maximum age c. 530 Ma) involving tectono-magmatic processes that were active at the deep crustal level represented by the TGC within the Bowers island arc and within a general NE-SW directed contractional regime and (ii) a late phase (maximum age c. 490 Ma) attributed to the late Ross orogenic intrusion of the TGC into the higher-crustal metasedimentary country rocks of the Bowers Terrane under NE-SW directed horizontal maximum stress and subsequent cooling.

Geochemistry at DSDP Leg 82 Holes

The nine holes (556-564) drilled during DSDP Leg 82 in a region west and southwest of the Azores Platform (Fig. 1) exhibit a wide variety of chemical compositions that indicate a complex petrogenetic history involving crystal fractionation, magma mixing, complex melting, and mantle heterogeneity. The major element chemistry of each hole except Hole 557 is typical of mid-ocean ridge basalts (MORBs), whereas the trace element and rare earth element (REE) abundances and ratios are more variable, and show that both depleted Type I and enriched Type II basalts have been erupted in the region. Hole 556 (30-34 Ma), located near a flow line through the Azores Triple Junction, contains typically depleted basalts, whereas Hole 557 (18 Ma), located near the same flow line but closer to the Azores Platform, is a highly enriched FeTi basalt, indicating that the Azores hot-spot anomaly has existed in its present configuration for at least 18 Ma, but less than 30-34 Ma. Hole 558 (34-37 Ma), located near a flow line through the FAMOUS and Leg 37 sites, includes both Type I and II basalts. Although the differences in Zr/Nb and light REE/heavy REE ratios imply different mantle sources, the (La/Ce)ch (>1) and Nd isotopic ratios are almost the same, suggesting that the complex melting and pervasive, small-scale mantle heterogeneity may account for the variations in trace element and REE ratios observed in Hole 558 (and FAMOUS sites). Farther south, Hole 559 (34-37 Ma), contains enriched Type II basalts, whereas Hole 561 (14-17 Ma), located further east near the same flow line, contains Type I and II basalts. In this case, the (La/Ce)ch and Nd isotopic ratios are different, indicating two distinct mantle sources. Again, the existence along the same flow line of two holes exhibiting such different chemistry suggests that mantle heterogeneity may exist on a more pervasive and transient smaller scale. (Hole 560 was not sampled for this study because the single basalt clast recovered was used for shipboard analysis.) All of the remaining three holes (562, 563, 564), located along a flow line about 100 km south of the Hayes Fracture Zone (33°N), contain only depleted Type I basalts. The contrast in chemical compositions suggests that the Hayes Fracture Zone may act as a "domain" boundary between an area of fairly homogeneous, depleted Type I basalts to the south (Holes 562-564) and a region of complex, highly variable basalts to the north near the Azores hot-spot anomaly (Holes 556-561).

Geochemistry of basalts from ODP Hole 113-690C

Basalts from Maud Rise, Weddell Sea, are vesicular and olivine-phyric. Major, trace, and rare earth element concentrations are similar to those of alkali basalts from ocean islands and seamounts. The rocks are low in MgO, Cr, Ni, and Sc, and high in TiO2, K2O, P2O5, Zr, and LREE contents. The abundance of "primary" biotite and apatite in the matrix indicates the melting of a hydrous mantle. Prevalence of olivine and absence of plagioclase in the rocks suggests that the volatile in the melt was an H2O-CO2 mixture, where H2O was <0.5. Mantle derived xenocrysts in the basalt include corroded orthopyroxene, chromite, apatite, and olivine. Olivine (Fo90) is too magnesian to be in equilibrium with the basalts, as they contain only 5-6 wt% MgO. Based on the presence of mantle xenocrysts, the high concentration of incompatible elements, the spatial and chemical affinity with other ocean island basalts from the area, and the relative age of the basalt (overlain by late Campanian sediments), it is suggested that Maud Rise was probably generated by hot-spot activity, possible during a ridge crest jump prior to 84 Ma (anomaly 34 time). Iddingsite, a complex intergrowth of montmorillonite and goethite, is the major alteration product of second generation olivine. It is suggested that iddingsite crystallized at low temperatures (<200°C) from an oxidized fluid during deuteric alteration. Vesicles are commonly filled by zeolites which have been replaced by K-feldspars.

Chemical and isotopic compositions of lavas from the Broken Ridge and Southern Kerguelen Plateau

Lavas from several major bathymetric highs in the eastern Indian Ocean that are likely to have formed as Early to Middle Cretaceous manifestations of the Kerguelen hotspot are predominantly tholeiitic; so too are glass shards from Eocene to Paleocene volcanic ash layers on Broken Ridge, which are believed to have come from eruptions on the Ninetyeast Ridge. The early dominance of tholeiitic compositions contrasts with the more recent intraplate, alkalic volcanism of the Kerguelen Archipelago. Isotopic and incompatible-element ratios of the plateau lavas are distinct from those of Indian mid-ocean ridge basalts; their Nd, Sr, 207Pb/204Pb and 2078b/204Pb isotopic ratios overlap with but cover a much wider range than measured for more recent oceanic products of the Kerguelen hotspot (including the Ninetyeast Ridge) or, indeed, oceanic lavas from any other hotspot in the world. Samples from the Naturaliste Plateau and ODP Site 738 on the southern tip of the Kerguelen Plateau are particularly noteworthy, with e-Nd(T) = -13 to -7, (87Sr/86Sr)T=0.7090 to 0.7130 and high 207Pb/204Pb relative to 206Pb/204Pb. In addition, the low-e-Nd(T) Naturaliste Plateau samples are elevated in SiO2 (>54 wt%). In contrast to "DUPAL" oceanic islands such as the Kerguelen Archipelago, Pitcairn and Tristan da Cunha, the plateau lavas with extreme isotopic characteristics also have relative depletions in Nb and Ta (e.g., Th/Ta, La Nb > primitive mantle values); the lowest e-Nd(T) and highest Th/Ta and La Nb values occur at sites located closest to rifted continental margins. Accepting a Kerguelen plume origin for the plateau lavas, these characteristics probably reflect the shallow-level incorporation of continental lithosphere in either the head of the early Kerguelen plume or in plume-derived magmas, and suggest that the influence of such material diminished after the period of plateau construction. Contamination of asthenosphere with the type of material affecting Naturaliste Plateau and Site 738 magmatism appears unlikely to be the cause of low-206Pb/204Pb Indian mid-ocean ridge basalts. Finally, because isotopic data for the plateaus do not cluster or form converging arrays in isotope-ratio plots, they provide no evidence for either a quickly evolving, positive ?Nd, relatively high-206Pb/204Pb plume composition, or a plume source dominated by mantle with e-Nd of -3 to ~0.

(Table 1) Chemical composition of volcanic rocks from the Mariisky sequence and extrusive vent rocks of the Mary Cape, Schmidt Peninsula (North Sakhalin)

Early Cretaceous volcanic rocks of the Mariisky sequence and Early Cenozoic extrusive-vent rocks of the Mary Cape are exposed at the most northwest of the Schmidt Peninsula, North Sakhalin. In chemical composition, all the rocks are subdivided into four groups. Three groups include volcanic rocks of the Mariisky sequence, which consists, from bottom to top, of calc-alkaline rocks, transitional calc-alkaline-tholeiite rocks, and incompatible element-depleted tholeiites. These rocks show subduction geochemical signatures and are considered as a fragment of the Moneron-Samarga island arc system. Trace-element modeling indicates their derivation through successive melting of a garnet-bearing mantle and garnet-free shallower mantle sources containing amphibole; pyroxene; and, possibly, spinel. The mixed subduction and intra-plate characteristics of the extrusive vent rocks of the Mary Cape attest to their formation in a transform continental margin setting.

Description, geochemistry and mineral composition of ODP Site 193-1189 jasperoids, PACMANUS field

Bright red "jasperoids" were recovered at three positions during Leg 193 drilling below Roman Ruins (Site 1189) in the PACMANUS hydrothermal field. These do not represent fossil exhalative oxide deposits equivalent to those associated with sulfide chimneys at the Roman Ruins seafloor. Rather, they constitute an integral, relatively early stage involving oxidized fluids in the development of veins and breccias that characterize the mostly sulfidic stockwork zone intersected below Roman Ruins in Hole 1189B. They formed by growth of quartz in open spaces created by hydrofracturing, the characteristic feature being mostly euhedral cores dusted by tiny hematite flakes. In one occurrence there are also frondlike aggregates and possible earlier cavity linings of hematite, overgrown by quartz, that potentially formed by maturation of ferruginous gels first deposited in the openings. The trace element geochemistry of the jasperoids, apart from minor enrichment in uranium, provides no indication that they represent subsurface conduits for fluids that deposit Fe-Mn-Si at the seafloor, though this remains a possibility for some such deposits.

Chemical composition of bottom sediments at DSDP Sites 91-595 and 91-596 and ODP Sites 114-701 and 129-801

Subducted sediments play an important role in arc magmatism and crust-mantle recycling. Models of continental growth, continental composition, convergent margin magmatism and mantle heterogeneity all require a better understanding of the mass and chemical fluxes associated with subducting sediments. We have evaluated subducting sediments on a global basis in order to better define their chemical systematics and to determine both regional and global average compositions. We then use these compositions to assess the importance of sediments to arc volcanism and crust-mantle recycling, and to re-evaluate the chemical composition of the continental crust. The large variations in the chemical composition of marine sediments are for the most part linked to the main lithological constituents. The alkali elements (K, Rb and Cs) and high field strength elements (Ti, Nb, Hf, Zr) are closely linked to the detrital phase in marine sediments; Th is largely detrital but may be enriched in the hydrogenous Fe-Mn component of sediments; REE patterns are largely continental, but abundances are closely linked to fish debris phosphate; U is mostly detrital, but also dependent on the supply and burial rate of organic matter; Ba is linked to both biogenic barite and hydrothermal components; Sr is linked to carbonate phases. Thus, the important geochemical tracers follow the lithology of the sediments. Sediment lithologies are controlled in turn by a small number of factors: proximity of detrital sources (volcanic and continental); biological productivity and preservation of carbonate and opal; and sedimentation rate. Because of the link with lithology and the wealth of lithological data routinely collected for ODP and DSDP drill cores, bulk geochemical averages can be calculated to better than 30% for most elements from fewer than ten chemical analyses for a typical drill core (100-1000 m). Combining the geochemical systematics with convergence rate and other parameters permits calculation of regional compositional fluxes for subducting sediment. These regional fluxes can be compared to the compositions of arc volcanics to asses the importance of sediment subduction to arc volcanism. For the 70% of the trenches worldwide where estimates can be made, the regional fluxes also provide the basis for a global subducting sediment (GLOSS) composition and flux. GLOSS is dominated by terrigenous material (76 wt% terrigenous, 7 wt% calcium carbonate, 10 wt% opal, 7 wt% mineral-bound H2O+), and therefore similar to upper continental crust (UCC) in composition. Exceptions include enrichment in Ba, Mn and the middle and heavy REE, and depletions in detrital elements diluted by biogenic material (alkalis, Th, Zr, Hf). Sr and Pb are identical in GLOSS and UCC as a result of a balance between dilution and enrichment by marine phases. GLOSS and the systematics of marine sediments provide an independent approach to the composition of the upper continental crust for detrital elements. Significant discrepancies of up to a factor of two exist between the marine sediment data and current upper crustal estimates for Cs, Nb, Ta and Ti. Suggested revisions to UCC include Cs (7.3 ppm), Nb (13.7 ppm), Ta (0.96 ppm) and TiO2 (0.76 wt%). These revisions affect recent bulk continental crust estimates for La/Nb and U/Nb, and lead to an even greater contrast between the continents and mantle for these important trace element ratios. GLOSS and the regional sediment data also provide new insights into the mantle sources of oceanic basalts. The classical geochemical distinction between 'pelagic' and 'terrigenous' sediment sources is not valid and needs to be replaced by a more comprehensive understanding of the compositional variations in complete sedimentary columns. In addition, isotopic arguments based on surface sediments alone can lead to erroneous conclusions. Specifically, the Nd/Hf ratio of GLOSS relaxes considerably the severe constraints on the amount of sediment recycling into the mantle based on earlier estimates from surface sediment compositions.

Geochemistry of fallout tephra from ODP Hole 125-782A

Very rare, halogen-rich andesite melt inclusions (HRA) in bytownitic plagioclase phenocrysts (An89-90) from tephra fallout of the Izu arc volcanic front (Izu VF) provide new insights into the processes of fluid release from slab trenchward to the volcanic front in a cool subduction zone. These HRA are markedly enriched in Cl, F and Li - by factors of up to 8 (Cl, F) and 1.5 (Li) - but indistinguishable with respect to the fluid-mobile large-ion lithophile elements (LILE; K, Sr, Rb, Cs, Ba, Pb, U), rare earths (REE) or high field strength elements (HFSE) from the low-K tholeiitic magmas of the Izu VF. We suggest that the chemical signature of the HRA reflects the presence of a fluid in the mantle source that originated from the serpentinized mantle peridotite above the metacrust. This "wedge serpentinite" presumably formed by fluid infiltration beneath the forearc and was subsequently down-dragged with the slab to arc front depths. The combined evidence from the Izu VF (?110 km above slab) and the outer forearc serpentinite seamounts (~25 to 30 km above slab) suggests that the slab flux of B and Cl is highest beneath the forearc, and decreases with increasing slab depths. In contrast, the slab flux of Li is minor beneath the forearc, but increases with depth. Fluorine may behave similarly to Li, whereas the fluid-mobile LILE appear to be largely retained in the slab trenchward from the Izu VF. Consequently, the chemical signatures of both Izu trench sediments and basaltic rocks appear preserved until arc front depths.

Geochemistry and isotopic ratios of samples obtained during R/V Akademik Nikolaj Strakhov cruise ANS24

New petrological and geochemical data were obtained for basalts recovered during cruise 24 of the R/V "Akademik Nikolay Strakhov" in 2006. These results significantly contributed to the understanding of the formation of tholeiitic magmatism at the northern end of the Knipovich Ridge of the Polar Atlantic. Dredging was performed for the first time both in the rift valley and on the flanks of the ridge. It showed that the conditions of magmatism have not changed since at least 10 Ma. The basalts correspond to slightly enriched tholeiites, whose primary melts were derived at the shallowest levels and were enriched in Na and depleted in Fe (Na-TOR type). The most enriched basalts are typical of the earlier stages of the opening and were found on the flanks of the ridge in its northernmost part. Variations in the ratios of Sr, Nd, and Pb isotopes and lithophile elements allowed us to conclude that the primary melts generated beneath the spreading zone of the Knipovich Ridge were modified by the addition of the enriched component that was present both in the Neogene and Quaternary basalts of Spitsbergen Island. Compared with the primitive mantle, the extruding magmas were characterized by positive Nb and Zr anomalies and a negative Th anomaly. The formation of primary melts involved melting of the metasomatized depleted mantle reservoir that appeared during the early stages of opening of the Norwegian-Greenland Basin and transformation of the paleo-Spitsbergen Fault into the Knipovich spreading ridge, which was accompanied by magmatism in western Spitsbergen during its separation from the northern part of Greenland.

Chemical composition of bulk sediments from ODP Hole 199-1221C

A record of inorganic geochemical variability was produced from a contiguous sequence of 35 samples, with 1 cm spacing, recovered from Hole 1221C. This record covers from 153.91 to 154.27 meters below seafloor and spans the Carbon Isotope Excursion (CIE) associated with the Paleocene/Eocene boundary interval. Elemental concentrations were determined for Al, As, Ba, Ca, Fe, K, Mg, Mn, P, Si, Sr, Ti, Cd, Co, Cr, Cu, Hf, Mo, Nb, Ni, Pb, Pt, Re, Sc, V, Y, Zn, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Most concentration profiles exhibit a marked peak coincident with or just prior to the CIE. In addition, the rare earth element pattern exhibits a significant flattening of the typical, prominent negative Ce anomaly across the same interval.

Chemical composition of altered volcanic rocks from ODP Sites 193-1188 and 193-1189

The conventional model of leaching volcanic rocks as a source of metals in a seafloor hydrothermal systems has been tested by examining the behavior of Pb and other trace elements during hydrothermal alteration. ODP Leg 193 drill sites 1188 (Snowcap) and 1189 (Roman Ruins) on Pual Ridge in the eastern Manus Basin offshore eastern Papua New Guinea provide a unique three-dimensional window into an active back-arc hydrothermal system. We investigate by means of a LA-ICP-MS microbeam technique the capacity of Pb to be leached from a host volcanic rock exposed to various types and intensities of alteration. Our results are in general agreement with previous studies that utilized bulk analytical techniques but provide a more detailed explanation of the processes. Fresh representative dacitic lavas from the Pual Ridge have an average whole rock Pb content of 5.2 ppm, an average interstitial glass Pb content of 5.6 ppm and an average plagioclase Pb content of 1.0 ppm. Altered matrix samples have highly variable Pb values ranging from 0 to 52.4 ppm. High Pb values in altered samples are associated with a low temperature chlorite and clay mineral assemblage, in some cases overprinted by a high temperature (up to 350°C) silica-rich "bleaching" alteration. Only the most highly altered matrix samples have REE patterns that differ from the fresh Pual Ridge dacite. This may represent either different lava histories or alteration characteristics that have affected normally immobile REEs. Altered samples with the highest Pb values have similar REE patterns to those of the local unaltered lavas. They are compositionally similar to typical Pual Ridge dacites indicating a genetic relationship between the main regional volcanic suite and the subseafloor hydrothermally altered, Pb-enriched material. Relative loss/gain for Pb between the analyzed altered samples and a calculated precursor show a maximum relative gain of 901%. Samples with relative Pb gain from both drill sites are associated with lower temperature alteration mineral assemblages characterized by pervasive chloritization. The related lower temperature (220-250°C) neutral to slightly acidic fluids have been ascribed by others to return circulation of hydrothermal fluids that did not interact with seawater. Because altered samples have a higher Pb content than the fresh precursor, leaching of fresh volcanic rocks cannot be the source of Pb in the hydrothermal systems.

Chemial and isotope compositions of rocks from the West Antarctic

The paper is devoted to a marine geophysical-geological research in the West Antarctic. This researche contributed to establishing the base geodesic network of the West Antarctic and supplemented geokinematic monitoring based on this network with geophysical and geologic information on structure and features of geomorphological and tectonic development of the South Ocean floor. Collected materials allow to conclude about the inhomogeneity of the Scotia Sea floor and about combination of fragments of a continental massif with young rift structures in conditions of the upwelling mantle. The ancient continental bridge, faunal connections between the South America and the West Antarctic has been destroyed by processes of destruction, taphrogeny and sea floor spreading. Structures of the Scotia and Caribbean Seas, North Fiji and Arctic Basins are similar.

Chemistry of volcanic ash from Celebes and Sulu Sea Basins

During Leg 124, off the Philippines, volcanic material was recovered in deep-sea sediments dating from the late Oligocene in the Celebes Sea Basin, and from the early Miocene in the Sulu Sea Basin. Chemical and petrological studies of fallout ash deposits are used to characterize volcanic pulses and to determine their possible origin. All of the glass and mineral compositions belong to medium-K and high-K calc-alkaline arc-related magmatic suites including high-Al basalts, pyroxene-hornblende andesites, dacites, and rhyolites. Late Oligocene and early Miocene products may have originated from the Sunda arc or from the Sabah-Zamboanga old Sulu arc. Late early Miocene Sulu Sea tuffs originated from the Cagayan arc, whereas early late Miocene fallout ashes are attributed to the Sulu arc. A complex magmatic production is distinguished in the Plio-Quaternary with three sequences of basic to acidic lava suites. Early Pliocene strata registered an important activity in both Celebes Sea and Sulu Sea areas, from the newly born Sangihe arc (low-alumina andesite series) and from the Sulu, Zamboanga, and Negros arcs (high-alumina basalt series and high-K andesite series). In the late Pliocene and the early Pleistocene, renewal of activity affects the Sangihe-Cotobato arc as well as the Sulu and Negros arcs (same magmatic distinctions). The last volcanic pulse took place in the late Pleistocene with revival of all the present arc systems.