Geochemistry of ODP Leg 125 peridotites

Trace element analyses (first-series transition elements, Ti, Rb, Sr, Zr, Y, Nb, and REE) were carried out on whole rocks and minerals from 10 peridotite samples from both Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu-Bonin forearc using a combination of XRF, ID-MS, ICP-MS, and ion microprobe. The concentrations of incompatible trace elements are generally low, reflecting the highly residual nature of the peridotites and their low clinopyroxene content (<2%). Chondrite-normalized REE patterns show extreme U shapes with (La/Sm)n ratios in the range of 5.03-250.0 and (Sm/Yb)n ratios in the range of 0.05-0.25; several samples show possible small positive Eu anomalies. LREE enrichment is common to both seamounts, although the peridotites from Conical Seamount have higher (La/Ce)n ratios on extended chondrite-normalized plots, in which both REEs and other trace elements are organized according to their incompatibility with respect to a harzburgitic mantle. Comparison with abyssal peridotite patterns suggests that the LREEs, Rb, Nb, Sr, Sm, and Eu are all enriched in the Leg 125 peridotites, but Ti and the HREEs exhibit no obvious enrichment. The peridotites also give positive anomalies for Zr and Sr relative to their neighboring REEs. Covariation diagrams based on clinopyroxene data show that Ti and the HREEs plot on an extension of an abyssal peridotite trend to more residual compositions. However, the LREEs, Rb, Sr, Sm, and Eu are displaced off this trend toward higher values, suggesting that these elements were introduced during an enrichment event. The axis of dispersion on these plots further suggests that enrichment took place during or after melting and thus was not a characteristic of the lithosphere before subduction. Compared with boninites sampled from the Izu-Bonin-Mariana forearc, the peridotites are significantly more enriched in LREEs. Modeling of the melting process indicates that if they represent the most depleted residues of the melting events that generated forearc boninites they must have experienced subsolidus enrichment in these elements, as well as in Rb, Sr, Zr, Nb, Sm, and Eu. The lack of any correlation with the degree of serpentinization suggests that low-temperature fluids were not the prime cause of enrichment. The enrichment in the high-field-strength elements also suggests that at least some of this enrichment may have involved melts rather than aqueous fluids. Moreover, the presence of the hydrous minerals magnesio-hornblende and tremolite and the common resorption of orthopyroxene indicate that this high-temperature peridotite-fluid interaction may have taken place in a water-rich environment in the forearc following the melting event that produced the boninites. The peridotites from Leg 125 may therefore contain a record of an important flux of elements into the mantle wedge during the initial formation of forearc lithosphere. Ophiolitic peridotites with these characteristics have not yet been reported, perhaps because the precise equivalents to the serpentinite seamounts have not been analyzed.

Isotopic geochemistry of granitoids in the Jinshajiang suture zone, SW China

The Jinshajiang suture zone, located in the eastern part of the Tethyan tectonic domain, is noticeable for a large-scale distribution of Late Jurassic to Triassic granitoids. These granitoids were genetically related to the evolution of the Paleo-Tethys Ocean. The Beiwu, Linong and Lunong granitoids occur in the middle zone of the Jinshajiang Suture Zone, and possess similar geochemical features, indicating they share a common magma source. SIMS zircon U-Pb dating reveals the Beiwu, Linong and Lunong granitic intrusions were emplaced at 233.9±1.4 Ma (2 sigma), 233.1 ±1.4 Ma (2 sigma) and 231.0±1.6 Ma (2 sigma), respectively. All of these granitoids are enriched in abundances of Si (SiO2 =65.2-73.5 wt.%), and large-ion-lithophile-elements (LILEs), but depleted in high-field-strength-elements contents (HFSEs, e.g., Nb, Ta, Ti). In addition, they have low P2O5 contents (0.06-0.11 wt.%), A/CNK values ([molecular Al2O3/(CaO+Na2O+K2O)], mostly<1.1) and 10000Ga/Al ratios (1.7-2.2), consistent with the characteristics of I-type granites. In terms of isotopic compositions, these granitoids have high initial 87Sr/86Sr ratios (0.7078-0.7148), Pb isotopic compositions [(206Pb/204Pb)t=18.213-18.598, (207Pb/204Pb)t=15.637-15.730 and (208Pb/204Pb)t=38.323-38.791], zircon d18O values (7. per mil-9.3 per mil) and negative eNd(t) values (-5.1 to -6.7), suggesting they were predominantly derived from the continental crust. Their Nb/Ta ratios (average value=8.6) are consistent with those of the lower continental crust (LCC). However, variable ?Hf(t) values (-8.6 to +2.8) and the occurrences of mafic microgranular enclaves (MMEs) suggest that mantle-derived melts and lower crustal magmas were involved in the generation of these granitoids. Moreover, the high Pb isotopic ratios and elevated zircon d18O values of these rocks indicate a significant contribution of the upper crustal composition. We propose a model in which the Beiwu, Linong and Lunong granitoids were generated under a late collisional or post-collisional setting. It is possible that this collision was completed before Late Triassic. Decompression induced mantle-derived magmas underplated and provided the heat for the anatexis of the crust. Hybrid melts including mantle-derived and the lower crustal magmas were then generated. The hybrid melts thereafter ascended to a shallow depth and resulted in some degree of sedimentary rocks assimilation. Such three-component mixing magmas source and subsequent fractional crystallization could be responsible for the formation of the Beiwu, Linong and Lunong granitoids.

Paleomagnetic analysis of ODP Site 133-820

Paleomagnetic analysis of sediment samples from Ocean Drilling Program (ODP) Leg 133, Site 820, 10 km from the outer edge of the Great Barrier Reef, is undertaken to investigate the mineral magnetic response to environmental (sea level) changes. Viscous remanent magnetization (VRM) of both multidomain and near-superparamagnetic origin is prevalent and largely obscures the primary remanence, except in isolated high-magnetization zones. The Brunhes/Matuyama boundary cannot be identified, but is expected to be below 120 mbsf. The only evidence that exists for a geomagnetic excursion occurs at about 33 mbsf (-135 k.y.). Only one-half the cores were oriented, and many suffered from internal rotation about the core axis, caused by coring and/or slicing. The decay of magnetic remanence below the surface layer (0-2 mbsf) is attributed to sulfate reduction processes. The magnetic susceptibility (K) record is central for describing and understanding the magnetic properties of the sediments, and their relationship to glacio-eustatic fluctuations in sea level. Three prominent magnetic susceptibility peaks, at about 7, 32, and 64 mbsf, are superimposed on a background of smaller susceptibility oscillations. Fluctuations in susceptibility and remanence in the ôbackgroundö zone are controlled predominantly by variations in the concentration, rather than the composition of ferrimagnetics, with carbonate dilution playing an important role (type-A properties). The sharp susceptibility maxima occur at the start of the marine transgressions following low stands in sea level (high d18O, glacial maxima), and are characterized by a stable single-domain remanence, with a significant contribution from ultra-fine, superparamagnetic grains (type-C properties). During the later marine transgression, the susceptibility gradually returns to low values and the remanence is carried by stable single-domain magnetite (type-B properties). The A, B, and C types of sediment have distinctive ARM/K ratios. Throughout most of the sequence a strong inverse correlation exists between magnetic susceptibility and both CaCO3 and d18O variations. However, in the sharp susceptibility peaks (early transgression), more complex phase relationships are apparent among these parameters. In particular, the K-d18O correlation switches to positive, then reverts to negative during the course of the late transgression, indicating that two distinct mechanisms are responsible for the K-d18O correlation. Lower in the sequence, where sea-level-controlled cycles of upward-coarsening sediments, we find that the initial, mud phase of each cycle has been enriched in high-coercivity magnetic material, which is indicative of more oxic conditions. The main magnetic characteristics of the sediments are thought to reflect sea-level-controlled variations in the sediment source regions and related run-off conditions. Some preliminary evidence is seen that biogenic magnetite may play a significant role in the magnetization of these sediments.

Petrology and geochemistry of basalts from the Vavilov Basin

During ODP Leg 107, two holes were drilled in the basement of Vavilov Basin, a central oceanic area of the Tyrrhenian sea. Hole 655B is located near the Gortani ridge in off-axis position at the western rim of the basin; Hole 651A is located on a basement swell at the axis of the basin. This paper deals with mineral chemistry, major and trace element geochemistry, and petrogenesis of the basalts recovered in the two holes. The mineralogy of the basalts is broadly homogeneous, but all of them have suffered important seawater alteration. Their major-element compositions are similar to both normal-mid-ocean-ridge-basalts (N-MORB) and back-arc-basalts (BAB) except for Na2O contents (BAB-like), and K2O which is somewhat enriched in upper unit of Hole 651 A. Their affinity with N-MORB and BAB is confirmed by using immobile trace elements such as Zr, Y, and Nb. However, basalts from the two sites present contrasting geochemical characteristics on spidergrams using incompatible elements. Hole 655B basalts are homogeneous enriched tholeiites, similar to those from DSDP Hole 373 (located on the opposite side of the basin near the eastern rim), and show affinities with enriched MORB (E-MORB). At Hole 651 A, the two basalt units are chemically distinct. One sample recovered in lower unit is rather similar to those from Hole 655B, but basalts from upper unit display calc-alkaline characteristic evidenced by the increase of light-ion-lithophile-element (LILE)/high-field-strength-element (HFSE) ratio, and appearance of a negative Nb-anomaly, making them comparable with orogenic lavas from the adjacent Eolian arc. The observed chemical compositions of the basalts are consistent with a derivation of the magmas from a N-MORB type source progressively contaminated by LILE-enriched fluids released from dehydration of the bordering subducted plate. Implications for evolution of the Tyrrhenian basin are tentatively proposed taking into consideration geochemical and chronological relationships between basalts from Leg 107 Holes 655B and 651 A, together with data from Leg 42 Site 373 and Vavilov Seamount. These data illustrate back-arc spreading in ensialic basin closely associated with the maturation of the adjacent subduction, followed by the growth of late off-axis central volcano, whereas the active subduction retreats southeastward.

Analysis of the iron oxide assemblages in the ANDRILL AND-1B drill core, Ross Sea, Antarctica

The AND-1B drill core recovered a 13.57 million year Miocene through Pleistocene record from beneath the McMurdo Ice Shelf in Antarctica (77.9°S, 167.1°E). Varying sedimentary facies in the 1285 m core indicate glacial-interglacial cyclicity with the proximity of ice at the site ranging from grounding of ice in 917 m of water to ice free marine conditions. Broader interpretation of climatic conditions of the wider Ross Sea Embayment is deduced from provenance studies. Here we present an analysis of the iron oxide assemblages in the AND-1B core and interpret their variability with respect to wider paleoclimatic conditions. The core is naturally divided into an upper and lower succession by an expanded 170 m thick volcanic interval between 590 and 760 m. Above 590 m the Plio-Pleistocene glacial cycles are diatom rich and below 760 m late Miocene glacial cycles are terrigenous. Electron microscopy and rock magnetic parameters confirm the subdivision with biogenic silica diluting the terrigenous input (fine pseudo-single domain and stable single domain titanomagnetite from the McMurdo Volcanic Group with a variety of textures and compositions) above 590 m. Below 760 m, the Miocene section consists of coarse-grained ilmenite and multidomain magnetite derived from Transantarctic Mountain lithologies. This may reflect ice flow patterns and the absence of McMurdo Volcanic Group volcanic centers or indicate that volcanic centers had not yet grown to a significant size. The combined rock magnetic and electron microscopy signatures of magnetic minerals serve as provenance tracers in both ice proximal and distal sedimentary units, aiding in the study of ice sheet extent and dynamics, and the identification of ice rafted debris sources and dispersal patterns in the Ross Sea sector of Antarctica.

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.

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.

X-ray fluorescence scanning of discrete samples on the Schwalbenberg II loess-paleosol sequence, raw data, magnetic susceptibility, organic carbon and U-ratio as grainsize index

The Schwalbenberg II loess-paleosol sequence (LPS) denotes a key site for Marine Isotope Stage (MIS 3) in Western Europe owing to eight succeeding cambisols, which primarily constitute the Ahrgau Subformation. Therefore, this LPS qualifies as a test candidate for the potential of temporal high-resolution geochemical data obtained X-ray fluorescence (XRF) scanning of discrete samplesproviding a fast and non-destructive tool for determining the element composition. The geochemical data is first contextualized to existing proxy data such as magnetic susceptibility (MS) and organic carbon (Corg) and then aggregated to element log ratios characteristic for weathering intensity [LOG (Ca/Sr), LOG (Rb/Sr), LOG (Ba/Sr), LOG (Rb/K)] and dust provenance [LOG (Ti/Zr), LOG (Ti/Al), LOG (Si/Al)]. Generally, an interpretation of rock magnetic particles is challenged in western Europe, where not only magnetic enhancement but also depletion plays a role. Our data indicates leaching and top-soil erosion induced MS depletion at the Schwalbenberg II LPS. Besides weathering, LOG (Ca/Sr) is susceptible for secondary calcification. Thus, also LOG (Rb/Sr) and LOG (Ba/Sr) are shown to be influenced by calcification dynamics. Consequently, LOG (Rb/K) seems to be the most suitable weathering index identifying the Sinzig Soils S1 and S2 as the most pronounced paleosols for this site. Sinzig Soil S3 is enclosed by gelic gleysols and in contrast to S1 and S2 only initially weathered pointing to colder climate conditions. Also the Remagen Soils are characterized by subtle to moderate positive excursions in the weathering indices. Comparing the Schwalbenberg II LPS with the nearby Eifel Lake Sediment Archive (ELSA) and other more distant German, Austrian and Czech LPS while discussing time and climate as limiting factors for pedogenesis, we suggest that the lithologically determined paleosols are in-situ soil formations. The provenance indices document a Zr-enrichment at the transition from the Ahrgau to the Hesbaye Subformation. This is explained by a conceptual model incorporating multiple sediment recycling and sorting effects in eolian and fluvial domains.

Geochemistry on rocks from Shackleton Range

During the GEISHA expedition (Geologische Expedition in die Shackleton Range 1987/88), the Pioneers Escarpment was visited and sampled extensively for the first time. Most of the rock types encountered represent amphibolite facies metamorphics, but evidence for granulite facies conditions was found in cores of garnet. These conditions must have been at least partly reached during the peak of metamorphism. For the Pioneers Escarpment a varicolored succession of sedimentary and bimodal volcanic origin is typical. It comprises: quartzites muscovite quartzite, sericite quartzite, fuchsite quartzite, garnet-quartz schists etc.; pelites: mica schists and plagioclase or plagioclase-microcline gneisses, aluminous schists; marls and carbonates: grey meta-limestones, carbonaceous quartzites, but also pure white, often fine-grained, saccharoidal marble, or a variety of tremolite marble, olivine (forsterite) marble, diopside-clinopyroxene-tremolite marble, etc.; basic volcanic rocks: amphibole fels, amphibolite schist, garnet amphibolite, and acidic to intermediate volcanic rocks: garnet-biotite schist, epidote-biotite-plagioclase gneiss, microcline gneiss. These rocks are considered to be a supracrustal unit, called the Pioneers Group. In the easternmost parts of the Pioneers Escarpment, e.g. at Vindberget, nonmetamorphic shales, sandstones and greywackes crop out, which are cover rocks of possibly Jurassic age. These metasediments, which represent a quartz-pelite-carbonate (QPC) association, indicate that deposition took place on a stable shelf, i.e. on the submerged rim of a craton. Marine shallow-water sedimentation including marls and aluminous clays form the protoliths. The volcanics may be part of a bimodal volcanics-arkose-conglomerate (BVAC) association. Geochemical analyses support the assumption of volcanic protoliths. This is demonstrated especially by the elevated amounts of the immobile, incompatible high-field-strength elements (HFSE) Nb, Ta, Ti, Y, and Zr encountered in some of the gneisses. Microscopic investigation suggests the existence of ortho-amphibolites. This is confirmed by the geochemistry. A bimodal volcanic association is evident. The amphibolites plot in both the tholeiite and calc-alkaline fields. The acidic volcanics are mainly rhyolitic. The sediments and volcanics were subjected to conditions of 10-11 kbar and 600°C during the peak of metamorphism, i.e. granulite facies metamorphism, which can be deduced from the Fe mole ratios of 0.71-0.73 in the garnet cores. Due to the relatively low temperatures, no anatectic melting took placc. The rims of the garnets show a Fe mole ratio of 0.84-0.86, and the coexisting mineral association garnet-biotite-staurolite-kyanite indicate amphibolite facies. The thermobarometry shows P-T conditions of 5-6 kbar and 570-580°C for this stage. The metamorphic history indicates deep burial at depths down to 35 km (subduction?) i.e. high pressure metamorphism, followed by pressure release due to uplift associated with retrograde metamorphism. This may have happened during a pre-Ross metamorphic event or orogeny. The Ross Orogeny at about 500 Ma probably just led to the weak greenschist facies overprint that is evident in the rocks of the Pioneers Group. Finally, sedimentation resumed in the area of the present Shackleton Range, or at least in the eastern part of the Pioneers Escarpment, probably when detritus from erosion of the basement (Read Group and Pioneers Group) was deposited, forming sandstones and greywackes of possibly Jurassic age. There is no indication that these sediments belong to the former Turnpike Bluff Group.

Geochemistry of ODP Leg 115 basalts

Melting-phase relations at high pressures and Sr-Nd isotopic compositions are reported for basalts collected from the western Indian Ocean during Ocean Drilling Program Leg 115. Based on the concentrations of high-field-strength elements, we have subdivided the basalts into eight groups. A tholeiitic primary magma estimated using an olivine maximum fractionation model is representative of depleted lavas. This melt is in equilibrium with lherzolite minerals at 1.3 GPa and 1330°C under dry conditions. Also, an alkaline primary magma, representative of enriched lavas, is not saturated with orthopyroxene under dry conditions, but it is saturated with lherzolite minerals under CO2-saturated conditions at 1.7 GPa and 1350°C. These results imply that the tholeiitic magmas were segregated from mantle diapirs at shallower levels than the alkaline magmas. The highest 143Nd/144Nd value is obtained for the most depleted tholeiitic basalts, and the lowest value corresponds to the enriched alkaline basalt. The Sr isotopes of the basalts range from 0.70378 to 0.70449 and are inversely correlated with the Nd isotopic values. The present experimental and geochemical data suggest that depleted mantle material is underlain by the enriched material in the upper mantle beneath the region.

Geochemistry of ODP Leg 127 igneous rocks

Legs 127 and 128 of the Ocean Drilling Program cored basement samples from two sites in the Yamato Basin (Sites 794 and 797) and one site in the Japan Basin (Site 795) of the Japan Sea. These samples represent sills and lava flows erupted or shallowly intruded in a marine environment during backarc extension and spreading in the middle Miocene. In this paper, we describe the geochemical characteristics of these igneous units using 52 new instrumental neutron activation analyses (INAA), 8 new X-ray fluorescence (XRF) analyses, and previous shipboard XRF analyses. The sills intruded into soft sediment at Sites 794 and 797 were subject to extensive hydrothermal activity, estimated at <230° C under subgreenschist facies conditions, which heavily to totally altered the fine-grained unit margins and moderately to heavily altered the coarse-grained unit interiors. Diagenesis further altered the composition of these igneous bodies and lava flows at Sites 794, 795, and 797, most intensely at unit margins. Our study of two well-sampled units shows that Mg, Ca, Sr, and the large-ion lithophile elements (LILE) mobilized during alteration, and that the concentrations of Y, Yb, and Lu decreased and Ce increased in the most severely altered samples. Nevertheless, our study shows that the rare-earth elements (REE) were relatively immobile in the majority of the samples, even where secondary mixed-layer clays comprised the great majority of the rock. Fresher Yamato Basin samples are compositionally heterogenous tholeiitic basalts and dolerites. At Site 794 in the north-central portion of the basin, Units 1 to 5 (upper basement) comprise mildly light rare-earth element (LREE) enriched basalts and dolerites (chondrite-normalized La/Sm of 1.4-1.8), while the stratigraphically lower Units 6 to 9 are less enriched dolerites with (La/Sm)N of 0.7-1.3. All Site 794 samples lack Nb and Ta depletions and LILE enrichments, lacking a strong subduction-related incompatible element geochemical signature. At Site 797 in the western margin of the basin, two stratigraphically-definable unit groups also occur. The upper nine units are incompatible-element depleted tholeiitic sills and flows with strong depletions of Nb and Ta relative to normal mid-ocean ridge basalt (N-MORB). The lower twelve sills represent LREE-enriched tholeiites (normalized La/Sm ranges from 1.1 to 1.8), with distinctly higher LILE and high field-strength element (HFSE) contents. At Site 795 at the northern margin of the Japan Sea, three eruptive units consist of basaltic andesite to calc-alkaline basalt (normalized La/Sm of 1.1 to 1.5) containing moderate depletions of the HFSE relative to N-MORB. The LILE-depleted nature of these samples precludes their origin in a continental arc, indicating that they more likely erupted within a rifting oceanic arc system. The heterogenous nature of the Japan Sea rocks indicate that they were derived at each site from multiple parental magmas generated from a compositionally heterogenous mantle source. Their chemistry is intermediate in character between arc basalts, MORB, and intraplate basalts, and implies little involvement of continental crust at any point in their genesis. Their flat chondrite-normalized, medium-to-heavy rare earth patterns indicate that the primary magmas which produced them last equilibrated with and segregated from spinel lherzolite at shallow depths (<30 kbar). In strong contrast to their isotopic compositional arrays, subduction-related geochemical signatures are usually poorly defined. No basin-wide temporal or geographic systematics of rock chemistry may be confidently detailed; instead, the data show both intimate (site-specific) and widespread backarc mantle heterogeneity over a narrow (2 Ma or so) range in time, with mantle heterogeneity most closely resembling a "plum-pudding" model.

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.

Chemical and boron isotopic compositions of volcanics from ODP Sites 135-834 to 135-840

The influence of fluid flux on petrogenesis in the Tonga-Kermadec Arc was investigated using ion microprobe measurements of B/Be and boron isotope ratios (11B/10B) to document the source and relative volumes of the fluids released from the subducting oceanic plate. We analyzed young lavas from eight different islands along the Tonga-Kermadec Arc, as well as glass shards in volcanic sediments from Ocean Drilling Program (ODP) Site 840, which record the variations in the chemistry of Tonga magmatism since 7 Ma. B/Be is variable (5.8-122), in young Tonga-Kermadec Arc lavas. In contrast, glass shards from around 3 to 4 Ma old volcanic sediments at Site 840 have the highest B/Be values yet reported for arc lavas (18-607). These values are too high to be related simply to a sediment influence on petrogenesis. Together with very high d11B values (-11.6 to +37.5) for the same shards and lavas these data indicate that most of the B is derived from fluid escaped from the subducting altered Pacific oceanic crust, rather than from sediment. High d11B values also reflect large degrees of isotopic fractionation in this cold fast subduction zone. Lower d11B values noted in the Kermadec Arc (17 to -4.4) are related to the influence of sediment eroded from New Zealand and slower convergence. High fluid flux (B/Be) is synchronous in Tonga and the Marianas at 3 to 4 Ma and may be related to acceleration of the Pacific Plate just prior to this time. The timing of maximum B/Be at 3 to 4 Ma correlates with maximum light rare earth (LREE) and high field strength element depletion. This suggests maximum degrees of partial melting at this time. Although thinning of the arc lithosphere during rifting to form the Lau Basin is expected to influence the arc geochemistry, variable aqueous fluid flux from the subducting plate alone appears capable of explaining boron and other trace element systematics in the Tonga-Kermadec Arc with no indication of slab melting.

Geochemistry of Lesser Antilles forearc sediments

Oceanic sediments contain the products of erosion of continental crust, biologic activity and chemical precipitation. These processes create a large diversity of their chemical and isotopic compositions. Here we focus on the influence of the distance from a continental platform on the trace element and isotopic compositions of sediments deposited on the ocean floor and highlight the role of zircons in decoupling high-field strength elements and Hf isotopic compositions from other trace elements and Nd isotopic compositions. We report major and trace element concentrations as well as Sr and Hf isotopic data for 80 sediments from the Lesser Antilles forearc region. The trace-element characteristics and the Sr and Hf isotopic compositions are generally dominated by detrital material from the continental crust but are also variably influenced by chemical or biogenic carbonate and pure biogenic silica. Next to the South American continent, at DSDP Site 144 and on Barbados Island, sediments, coarse quartz arenites, exhibit marked Zr and Hf excesses that we attribute to the presence of zircon. In contrast, the sediments from DSDP Site 543, which were deposited farther away from the continental platform, consist of fine clay and they show strong deficiencies in Zr and Hf. The enrichment or depletion of Zr-Hf is coupled to large changes in Hf isotopic compositions (-30 < epsilon-Hf < +4) that vary independently from the Nd isotopes. We interpret this feature as a clear expression of the "zircon effect" suggested by Patchett and coauthors in 1984. Zircon-rich sediments deposited next to the South American continent have very low epsilon-Hf values inherited from old zircons. In contrast, in detrital clay-rich sediments deposited a few hundred kilometers farther north, the mineral fraction is devoid of zircon and they have drastically higher epsilon-Hf values inherited from finer, clay-rich continental material. In the two DSDP sites, average Hf isotopes are very unradiogenic relative to other oceanic sediments worldwide (epsilon-Hf = -14.4 and -7.4) and they define the low Hf end member of the sedimentary field in Hf-Nd space. Their compositions correspond to end members that, when mixed with mantle, are able to reproduce the pattern of volcanic rocks from the Lesser Antilles. More generally, we find a relationship between Nb/Zr ratios and the vertical deviation of Hf isotope ratios from the Nd-Hf terrestrial array and we suggest that this relationship can be used as a tool to distinguish sediment input from fractionation during melting during the formation of arc lavas.