Chemical composition of basement rocks from ODP Leg 121, Ninetyeast Ridge

Data and observation from Drifting Program Leg 121 and plate-tectonic reconstructions indicate that the Ninetyeast Ridge (Indian Ocean) was derived from the interaction of a deep-seated Dupal hotspot and a nearby spreading-ridge axis. The 5000-km-long ridge, from lat 34°S to lat 10°N, was drilled at three sites during Leg 121. About 178 m of basalt, >38 to >80 Ma, were recovered from a total penetration of ~310 m. Shipboard petrographic and geochemical studies showed that each site has distinctive characteristics. Most of the cored lavas have a tholeiitic basalt composition. Incompatible-element abundanes and ratios show systematic trends, consistent with an origin for the Ninetyeast Ridge lavas by mixing between a depleted component-Indian Ocean mid-ocean ridge basalt-and an enriched component-oceanic-island basalt similar to that observed in the youngest alkalic basalts from the Kerguelen archipelago. Preliminary shore-based trace element abundance and isotopic data are compatible with this hypothesis, although Pb isotopes indicate the involvement of another component. The long-lasting and more or less continuous activity of the Kerguelen-Heard plume (ca. 115 Ma), now located under Heard Island, south of the Southeast Indian Ridge, provides evidence that the source of the Dupal anomaly is deep seated.

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.

Chemical element concentrations in sediments from DSDP Holes 60-458 and 60-459

The development of laser ablation-inductively coupled plasma-mass spectrometry has revolutionized the analysis of tephras by providing (1) an efficient and precise method for determining abundances of a wide variety of trace elements at low concentrations in individual glass shards and (2) assessment of geochemical heterogeneities within individual ash horizons. This development is important for petrogenetic studies of intraoceanic arc systems, where tephras provide the most complete temporal record of magmatism. Results from the Izu-Bonin and Mariana arc systems indicate that despite close geographical proximity and similar tectonic evolution, they contrast strongly in terms of geochemical evolution since 35 Ma. Whereas the Mariana tephras have exceptional compositional diversity, ranging from low-K (Oligocene), to high-K (Miocene), and subsequently medium-K compositions (Pliocene-Quaternary), the Izu-Bonin arc has been dominated by low-K compositions throughout. The Mariana increases in K are paralleled by increases in abundances of incompatible trace elements and by increased values of diagnostic ratios (e.g., Nb/yb and Th/yb) regarded as monitors of potential mantle-source fertility. The relative uniformity of Nb/yb and Nb/Zr ratios in Izu-Bonin tephras indicates that cyclic processes of backarc basin development and mantle depletion do not necessarily induce large-scale temporal geochemical variations in the associated arc. Temporal variability within the Mariana arc, and its divergence from the Izu-Bonin arc ca. 13 Ma, can be traced to a major injection of subducted sediment in the Mariana system at this time.

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.

Composition of dikes and lavas from the Pito Deep Rift

The northwest trending walls of the Pito Deep Rift (PDR), a tectonic window in the southeast Pacific, expose in situ oceanic crust generated ?3 Ma at the superfast spreading southern East Pacific Rise (SEPR). Whole rock analyses were performed on over 200 samples of dikes and lavas recovered from two ~8 km**2 study areas. Most of the PDR samples are incompatible-element-depleted normal mid-ocean ridge basalts (NMORB; (La/Sm)N < 1.0) that show typical tholeiitic fractionation trends. Correlated variations in Pb isotope ratios, rare earth element patterns, and ratios of incompatible elements (e.g., (Ce/Yb)N) are best explained by mixing curves between two enriched and one depleted mantle sources. Pb isotope compositions of most PDR NMORB are offset from SEPR data toward higher values of 207Pb/204Pb, suggesting that an enriched component of the mantle was present in this region in the past ?3 Ma but is not evident today. Overall, the PDR crust is highly variable in composition over long and short spatial scales, demonstrating that chemically distinct lavas and dikes can be emplaced within the same segment over short timescales. However, the limited spatial distribution of high 206Pb/204Pb samples and the occurrence of relatively homogeneous MgO compositions (ranging <2.5 wt %) within a few of the individual dive transects (over distances of ~1 km) suggests that the mantle source composition evolved and magmatic temperatures persisted over timescales of tens of thousands of years. The high degree of chemical variability between pairs of adjacent dikes is interpreted as evidence for along-axis transport of magma from chemically distinct portions of the melt lens. Our findings suggest that lateral dike propagation occurs to a significant degree at superfast spreading centers.

Chemical composition of rocks and minerals from the Karymsky volcanic center, Eastern Kamchatka

The powerful eruption in the Akademii Nauk caldera on January 2, 1996 marked a new activity phase of the Karymsky volcano and became a noticeable event in the history of modern volcanism in Kamchatka. The paper reports data obtained by studying more than 200 glassy melt inclusions in phenocrysts of olivine (Fo82-72), plagioclase (An92-73), and clinopyroxene (Mg# 83-70) in basalts of the 1996 eruption. The data were used to estimate composition of the parental melt and physicochemical parameters of the magma evolution. According to our data, the parental melt corresponded to low magnesium, high aluminum basalt (SiO2 = 50.2%, MgO = 5.6%, Al2O3 = 17%) of the mildly potassium type (K2O = 0.56%) and contained much dissolved volatile components (H2O = 2.8%, S = 0.17%, and Cl = 0.11%). Melt inclusions in the minerals are similar in chemical composition, a fact testifying that the minerals crystallized simultaneously with one another. Their crystallization started at pressure ~1.5 kbar, proceeded within a narrow temperature range of 1040+/-20°C, and continued until near-surface pressure ~100 bar was reached. Degree of crystallization of the parental melt during its eruption was close to 55%. Massive crystallization was triggered by H2O degassing under pressure <1 kbar. Magma degassing in an open system resulted in escape of 82% H2O, 93% S, and 24% Cl (of their initial contents in the parental melt) to the fluid phase. Release of volatile compounds to the atmosphere during the eruption that lasted for 18 h was estimated as 1.7x10**6 t H2O, 1.4x10**5 t S, and 1.5x10**4 t Cl. Concentrations of most incompatible trace elements in the melt inclusions are close to those in the rocks and to the expected fractional differentiation trend. Melt inclusions in plagioclase were found to be selectively enriched in Li. The Li-enriched plagioclase with melt inclusions thought to originate from cumulate layers in the feeding system beneath Karymsky volcano, in which plagioclase interacted with Li-rich melts/brines and was subsequently entrapped and entrained by the magma during the 1996 eruption.

(Table 1) Osmium concentrations and isotopic ratios for ODP Leg 125 peridotites

Mantle peridotites drilled from the Izu-Bonin-Mariana forearc have unradiogenic 187Os/188Os ratios (0.1193 to 0.1273), which give Proterozoic model ages of 820 to 1230 million years ago. If these peridotites are residues from magmatism during the initiation of subduction 40 to 48 million years ago, then the mantle that melted was much more depleted in incompatible elements than the source of mid-ocean ridge basalts (MORB). This result indicates that osmium isotopes record information about ancient melting events in the convecting upper mantle not recorded by incompatible lithophile isotope tracers. Subduction zones may be a graveyard for ancient depleted mantle material, and portions of the convecting upper mantle may be less radiogenic in osmium isotopes than previously recognized.

U-Pb, trace element, and Hf isotope analysis for Macquarie Island detrital zircon

Oceanic zircon trace element and Hf-isotope geochemistry offers a means to assess the magmatic evolution of a dying spreading ridge and provides an independent evaluation of the reliability of oceanic zircon as an indicator of mantle melting conditions. The Macquarie Island ophiolite in the Southern Ocean provides a unique testing ground for this approach due to its formation within a mid-ocean ridge that gradually changed into a transform plate boundary. Detrital zircon recovered from the island records this change through a progressive enrichment in incompatible trace elements. Oligocene age (33-27 Ma) paleo-detrital zircon in ophiolitic sandstones and breccias interbedded with pillow basalt have trace element compositions akin to a MORB crustal source, whereas Late Miocene age (8.5 Ma) modern-detrital zircon collected from gabbroic colluvium on the island have highly enriched compositions unlike typical oceanic zircon. This compositional disparity between age populations is not complimented by analytically equivalent eHf data that primarily ranges from 14 to 13 for sandstone and modern-detrital populations. A wider compositional range for the sandstone population reflects a multiple pluton source provenance and is augmented by a single cobble clast with eHf equivalent to the maximum observed composition in the sandstone (~17). Similar sandstone and colluvium Hf-isotope signatures indicate inheritance from a similar mantle reservoir that was enriched from the depleted MORB mantle average. The continuity in Hf-isotope signature relative to trace element enrichment in Macquarie Island zircon populations, suggests the latter formed by reduced partial melting linked to spreading-segment shortening and transform lengthening along the dying spreading ridge.

Isotopic and chemical compositions of lower crust rocks and minerals from ODP Hole 176-735B

The composition of gabbroic rocks from the drill core of Hole 735B (ODP Leg 176) at the 11 Ma Atlantis II bank close to the slow spreading Southwest Indian Ridge (SWIR) has been analyzed for major and trace elements and Sr, Nd and Pb isotopic composition. The samples are thought to represent much of the mineralogical and geochemical variation in a vertical 1-km section (500-1500 m below the sea floor) of the lower ocean crust. Primitive troctolitic gabbros, olivine gabbros and gabbros that have Mg#=84-70, Ca#>61 and low Na# (Na/(Na+Al)) (8-17) are intruded by patches or veins of more evolved FeTi-oxide rich gabbroic and dioritic rocks with Mg# to 20, Ca# to 32, Na#=14-23, TiO2<7 wt.% and FeOtotal<18 wt.%. All rocks are acdcumulates, and incompatible element concentrations are low, e.g. Pb=0.1-0.7 ppm and U

Geochemistry of ODP Hole 135-841B

Two igneous rock units were recovered at Site 841. More than 200 m of island-arc rhyolites, rhyolitic tuffs, lapilli tuffs, and pumice breccias, divided into five units, compose the basement at the site. These rhyolitic volcanics are late middle Eocene or older and formed part of a subaerial rhyolitic volcano. These low-K rhyolites were produced by fractional crystallization of a more mafic arc-tholeiitic lava or by dehydration melting of lower crustal arc tholeiites. The Site 841 basement rocks are similar in composition to high-SiO2 lavas in the Eocene basement on 'Eua and crystallized from depleted island-arc-tholeiitic basalts like those exposed on 'Eua. No evidence is present in the rhyolites, or in the clasts enclosed within them, for boninite series magmas at Site 841. The Site 841 rhyolitic complex bears no resemblance to Cretaceous rhyolites from the Lord Howe Rise, which are enriched in K and incompatible elements. The volcanic rocks at Site 841 are part of a widely distributed Eocene volcanic episode that marked the earliest phases of subduction in the Tonga region; they are not part of an older crustal fragment. The second igneous sequence is a series of basaltic dikes and sills that intruded Miocene sediments. These basalts have trace element abundances and ratios identical to upper Miocene lavas from the Lau Ridge. The Site 841 basalts do not have any geochemical characteristics that suggest they were generated by unusual thermal conditions in the shallow sub-forearc mantle. They are most reasonably interpreted as intrusions fed by basement dikes propagated from the associated active arc. No evidence for local serpentinite exposures, like those that are common in the Mariana forearc, was found at Site 841. The results from Site 841 provide strong support for hypotheses of forearc evolution that have been advanced for the Izu-Bonin-Mariana system.

Geochemistry of basaltic rocks from ODP Site 129-801

Middle Jurassic basaltic lavas obtained from Site 801 in the western Pacific Pigafetta Basin represent ocean crust from the oldest segment of the present-day Pacific Ocean. A composite 131 m section shows the basement to be composed of an upper alkalic basalt sequence (about 157 Ma) with ocean island basalt chemical features and a lower tholeiitic basalt sequence (about 167 Ma) with typical normal-type mid-ocean ridge basalt features. The basalt sequences are separated by a quartz-cemented, yellow goethite hydrothermal deposit. Most basalts are altered to some degree and exhibit variable, low-grade smectite-celadonite-pyrite-carbonate-zeolite assemblages developed under a mainly hydrated anoxic environment. Oxidation is very minor, later in development than the hydration assemblages, and largely associated with the hydrothermal deposit. The tholeiitic normal-type mid-ocean ridge basalt has characteristically depleted incompatible element patterns and all compositions are encompassed by recent mid-ocean ridge basalt from the East Pacific Rise. Chemically, the normal-type mid-ocean ridge basalt is divided into a primitive plagioclase-olivine +/- spinel phyric group (Mg* = 72-60) and an evolved (largely) aphyric group of olivine tholeiites (Mg* = 62-40). Both groups form a single comagmatic suite related via open-system fractionation of initial olivine-spinel followed by olivine-plagioclase-clinopyroxene. The alkalic ocean island basalt are largely aphyric and display enriched incompatible element abundances within both relatively primitive olivine-rich basalts and evolved olivine-poor hawaiites related via mafic fractionation. In gross terms, the basement lithostratigraphy is a typical mid-ocean ridge basalt crust, generated at a spreading center, overlain by an off-axis seamount with ocean island basalt chemical characters.

Isotopic composition of rocks from the Lau/Tonga back-arc basin, data of DSDP Leg 21 and ODP Legs 91 an 135

New trace element, Sr-, Nd-, Pb- and Hf isotope data provide insights into the evolution of the Tonga-Lau Basin subduction system. The involvement of two separate mantle domains, namely Pacific MORB mantle in the pre-rift and early stages of back-arc basin formation, and Indian MORB mantle in the later stages, is confirmed by these results. Contrary to models proposed in recent studies on the basis of Pb isotope and other compositional data, this change in mantle wedge character best explains the shift in the isotopic composition, particularly 143Nd/144Nd ratios, of modern Tofua Arc magmas relative to all other arc products from this region. Nevertheless, significant changes in the slab-derived flux during the evolution of the arc system are also required to explain second order variations in magma chemistry. In this region, the slab-derived flux is dominated by fluid; however, these fluids carry Pb with sediment-influenced isotopic signatures, indicating that their source is not restricted to the subducting altered mafic oceanic crust. This has been the case from the earliest magmatic activity in the arc (Eocene) until the present time, with the exception of two periods of magmatic activity recorded in samples from the Lau Islands. Both the Lau Volcanic Group, and Korobasaga Volcanic Group lavas preserve trace element and isotope evidence for a contribution from subducted sediment that was not transported as a fluid, but possibly in the form of a melt. This component shares similarities with that influencing the chemistry of the northern Tofua Arc magmas, suggesting some caution may be required in the adoption of constraints for the latter dependent upon the involvement of sediments from the Louisville Ridge. A key outcome of this study is to demonstrate that the models proposed to explain subduction zone magmatism cannot afford to ignore the small but important contributions made by the mantle wedge to the incompatible trace element inventory of arc magmas.

Geochemistry of Early Cretaceous basaltic rocks from ODP Sites 123-765 and 123-766

The Early Cretaceous basaltic rocks obtained from Sites 765 and 766 in the eastern Indian Ocean floor were mostly iron-rich normal mid-ocean ridge basalts (N-MORB), which were derived from a depleted mantle source having strongly light rare earth element (LREE)-depleted rare-earth patterns and a high titanium/zirconium (Ti/Zr) ratio. Basaltic rocks in the upper part of the Site 765 basement section include megacrysts and gabbroic fragments of widely varying mineral chemistry. These megacrysts range from An90 plagioclase, including highly magnesian basaltic glass coexisting with augite of Mg# (= 100 Mg/[Fe+Mg]) at 85, to An50 plagioclase coexisting with hypersthene. This varying mineralogy of megacrysts and gabbroic fragments indicates that a considerable degree of fractional crystallization took place in the magma chamber. The unusual negative correlation between incompatible elements (e.g., TiO2) and FeO*/MgO observed among Site 765 basement basalts and fresh volcanic glasses suggest source-mantle heterogeneity in terms of FeO*/MgO. Strontium isotope ratios (87Sr/86Sr) of the basaltic rocks from both sites are between 0.7027 and 0.7033 and are comparable to those of mid-Indian Ocean ridge basalts (MIORB). The basalt pebbles encountered in the sedimentary section may have been transported from the basement highs nearer the Australian continent and include basaltic compositions ranging from primitive N-MORBs to evolved enriched (E)-MORBs. Their mantle source was not as depleted as that of the basement basalts. These rocks may be the products of earlier volcanism that took place during the rifting of the Australian continent.

Iodine and boron concentrations in pore fluids, ultramafic clasts and interstitial serpentinite mud of ODP Hole 125-779A and Site 195-1200

Iodine and boron were analyzed in pore fluids, serpentinized ultramafic clasts, and the serpentinized mud matrix of the South Chamorro Seamount mud volcano (Ocean Drilling Program Leg 195 Site 1200) to determine the distribution of these elements in deep forearc settings. Similar analyses of clasts and muds from the Conical Seamount mud volcano (Leg 125 Site 779) were also carried out. Interstitial pore fluids are enriched in boron and iodine without appreciable change in chloride concentration relative to seawater. Both the ultramafic clasts and the associated serpentinized mud present the highest documented iodine concentrations for all types of nonsedimentary rocks (6.3-101.7 µmol/kg). Such high iodine concentrations, if commonplace in marine forearc settings, may constitute a significant, previously unknown reservoir of iodine. This serpentinized forearc mantle reservoir may potentially contribute to the total crustal iodine budget and provide a mechanism for its recycling at convergent plate margins. Both clasts and mud show concurrent enrichments in boron and iodine, and the similarity in pore fluid profiles also suggests that these two incompatible, fluid-mobile elements behave similarly at convergent plate margins.