Whole-rock and mineral geochemistry of ODP Hole 176-735B gabbros

We report mineral chemistry, whole-rock major element compositions, and trace element analyses on Hole 735B samples drilled and selected during Leg 176. We discuss these data, together with Leg 176 shipboard data and Leg 118 sample data from the literature, in terms of primary igneous petrogenesis. Despite mineral compositional variation in a given sample, major constituent minerals in Hole 735B gabbroic rocks display good chemical equilibrium as shown by significant correlations among Mg# (= Mg/[Mg + Fe2+]) of olivine, clinopyroxene, and orthopyroxene and An (=Ca/[Ca + Na]) of plagioclase. This indicates that the mineral assemblages olivine + plagioclase in troctolite, plagioclase + clinopyroxene in gabbro, plagioclases + clinopyroxene + olivine in olivine gabbro, and plagioclase + clinopyroxene + olivine + orthopyroxene in gabbronorite, and so on, have all coprecipitated from their respective parental melts. Fe-Ti oxides (ilmenite and titanomagnetite), which are ubiquitous in most of these rocks, are not in chemical equilibrium with olivine, clinopyroxene, and plagioclase, but precipitated later at lower temperatures. Disseminated oxides in some samples may have precipitated from trapped Fe-Ti-rich melts. Oxides that concentrate along shear bands/zones may mark zones of melt coalescence/transport expelled from the cumulate sequence as a result of compaction or filter pressing. Bulk Hole 735B is of cumulate composition. The most primitive olivine, with Fo = 0.842, in Hole 735B suggests that the most primitive melt parental to Hole 735B lithologies must have Mg# 0.637, which is significantly less than Mg# = 0.714 of bulk Hole 735B. This suggests that a significant mass fraction of more evolved products is needed to balance the high Mg# of the bulk hole. Calculations show that 25%-45% of average Eastern Atlantis II Fracture Zone basalt is needed to combine with 55%-75% of bulk Hole 735B rocks to give a melt of Mg# 0.637, parental to the most primitive Hole 735B cumulate. On the other hand, the parental melt with Mg# 0.637 is far too evolved to be in equilibrium with residual mantle olivine of Fo > 0.89. Therefore, a significant mass fraction of more primitive cumulate (e.g., high Mg# dunite and troctolite) is yet to be sampled. This hidden cumulate could well be deep in the lower crust or simply in the mantle section. We favor the latter because of the thickened cold thermal boundary layer atop the mantle beneath slow-spreading ridges, where cooling and crystallization of ascending mantle melts is inevitable. These observations and data interpretation require reconsideration of the popular concept of primary mantle melts and relationships among the extent of mantle melting, melt production, and the composition and thickness of igneous crust.

Geochemistry of sediment samples from ODP Leg 135 holes

The results of inductively coupled argon plasma (ICAP) chemical analyses carried out on some 300 core samples from Ocean Drilling Program Sites 834, 835, 838, and 839 are presented. These sites were drilled during Leg 135 in the Lau Basin. The data are compared with total gamma (SGR) wireline logs at Sites 834 and 835. Pliocene (Piacenzian) nannofossil Zone CN12, which has been identified at Sites 834 and 835, is examined in detail using spectral analyses on core and wireline logs. The potassium and calcium concentrations from the core material were used to calculate an objective depth-to-geological time stretching function, which improved the stratigraphic correlation between sites. The integrated use of chemical analyses, wireline-log data and paleomagnetic results improved confidence in the correlations obtained. Although no significant sedimentation periodicities were obtained from the two sites, a common concentration of energy between 30 and 60 k.y. was recorded.

Nd and Mn concentration in planktonic foraminifera

Neodymium isotopes are becoming widely used as a palaeoceanographic tool for reconstructing the source and flow direction of water masses. A new method using planktonic foraminifera which have not been chemically cleaned has proven to be a promising means of avoiding contamination of the deep ocean palaeoceanographic signal by detrital material. However, the exact mechanism by which the Nd isotope signal from bottom waters becomes associated with planktonic foraminifera, the spatial distribution of rare earth element (REE) concentrations within the shell, and the possible mobility of REE ions during changing redox conditions, have not been fully investigated. Here we present REE concentration and Nd isotope data from mixed species of planktonic foraminifera taken from plankton tows, sediment traps and a sediment core from the NW Atlantic. We used multiple geochemical techniques to evaluate how, where and when REEs become associated with planktonic foraminifera as they settle through the water column, reside at the surface and are buried in the sediment. Analyses of foraminifera shells from plankton tows and sediment traps between 200 and 2938 m water depth indicate that only ~20% of their associated Nd is biogenically incorporated into the calcite structure. The remaining 80% is associated with authigenic metal oxides and organic matter, which form in the water column, and remain extraneous to the carbonate structure. Remineralisation of these organic and authigenic phases releases ions back into solution and creates new binding sites, allowing the Nd isotope ratio to undergo partial equilibration with the ambient seawater, as the foraminifera fall through the water column. Analyses of fossil foraminifera shells from sediment cores show that their REE concentrations increase by up to 10-fold at the sediment-water interface, and acquire an isotopic signature of bottom water. Adsorption and complexation of REE3+ ions between the inner layers of calcite contributes significantly to elevated REE concentrations in foraminifera. The most likely source of REE ions at this stage of enrichment is from bottom waters and from the remineralisation of oxide phases which are in chemical equilibrium with the bottom waters. As planktonic foraminifera are buried below the sediment-water interface redox-sensitive ion concentrations are adjusted within the shells depending on the pore-water oxygen concentration. The concentration of ions which are passively redox sensitive, such as REE3+ ions, is also controlled to some extent by this process. We infer that (a) the Nd isotope signature of bottom water is preserved in planktonic foraminifera and (b) that it relies on the limited mobility of particle reactive REE3+ ions, aided in some environments by micron-scale precipitation of MnCO3. This study indicates that there may be sedimentary environments under which the bottom water Nd isotope signature is not preserved by planktonic foraminifera. Tests to validate other core sites must be carried out before downcore records can be used to interpret palaeoceanographic changes.

Strontium and neodymium concentrations and isotope compositions in fossil fish teeth and pore waters at ODP Sites 130-807 and 125-786

We analyzed Nd and Sr isotopic compositions of Neogene fossil fish teeth from two sites in the Pacific in order to determine the effect of cleaning protocols and burial diagenesis on the preservation of seawater isotopic values. Sr is incorporated into the teeth at the time of growth; thus Sr isotopes are potentially valuable for chemostratigraphy. Nd isotopes are potential conservative tracers of paleocirculation; however, Nd is incorporated post-mortem, and may record diagenetic pore waters rather than seawater. We evaluated samples from two sites (Site 807A, Ontong Java Plateau and Site 786A, Izu-Bonin Arc) that were exposed to similar bottom waters, but have distinct lithologies and pore water chemistries. The Sr isotopic values of the fish teeth appear to accurately reflect contemporaneous seawater at both sites. The excellent correlation between the Nd isotopic values of teeth from the two sites suggests that the Nd is incorporated while the teeth are in chemical equilibrium with seawater, and that the signal is preserved over geologic timescales and subsequent burial. These data also corroborate paleoseawater Nd isotopic compositions derived from Pacific ferromanganese crusts that were recovered from similar water depths (Ling et al., 1997; doi:10.1016/S0012-821X(96)00224-5). This corroboration strongly suggests that both materials preserve seawater Nd isotope values. Variations in Pacific deepwater e-Nd values are consistent with predictions for the shoaling of the Isthmus of Panama and the subsequent initiation of nonradiogenic North Atlantic Deep Water that entered the Pacific via the Antarctic Circumpolar Current.

(Table 1) Li, B and Sr isotopic composition and concentration in ODP Leg 209 samples

Spinel harzburgites from ODP Leg 209 (Sites 1272A, 1274A) drilled at the Mid-Atlantic ridge between 14°N and 16°N are highly serpentinized (50-100%), but still preserve relics of primary phases (olivine >= orthopyroxene >> clinopyroxene). We determined whole-rock B and Li isotope compositions in order to constrain the effect of serpentinization on d11B and d7Li. Our data indicate that during serpentinization Li is leached from the rock, while B is added. The samples from ODP Leg 209 show the heaviest d11B (+29.6 to +40.52 per mil) and lightest d7Li (-28.46 to +7.17 per mil) found so far in oceanic mantle. High 87Sr/86Sr ratios (0.708536 to 0.709130) indicate moderate water/rock ratios (3 to 273, on the average 39), in line with the high degree of serpentinization observed. Applying the known fractionation factors for 11B/10B and 7Li/6Li between seawater and silicates, serpentinized peridotite in equilibrium with seawater at conditions corresponding to those of the studied drill holes (pH: 8.2; temperature: 200 °C) should have d11B of +21.52 per mil and d7Li of +9.7 per mil. As the data from ODP Leg 209 are clearly not in line with this, we modelled a process of seawater-rock interaction where d11B and d7Li of seawater evolve during penetration into the oceanic plate. Assuming chemical equilibrium between fluid and a rock with d11B and d7Li of ODP Leg 209 samples, we obtain d11B and d7Li values of +50 to +60 per mil, -2 to +12 per mil, respectively, for the coexisting fluid. In the oceanic domain, no hydrothermal fluids with such high d11B have yet been found, but are predicted by theoretical calculations. Combining the calculated water/rock ratios with the d7Li and d11B evolution in the fluid, shows that modification of d7Li during serpentinization requires higher water/rock ratios than modification of d11B. Extremely heavy d11B in serpentinized oceanic mantle can potentially be transported into subduction zones, as the B budget of the oceanic plate is dominated by serpentinites. Extremely light d7Li is unlikely to survive as the Li budget is dominated by the oceanic crust, even at small fractions.

Chemical and Sr isotopic compositions of 3 step leachates and residues of bulk samples, zeolites, and smectites from Lower Miocene sediments at ODP Site 166-1007

Massive clinoptilolite authigenesis was observed at about 1105 meters below sea floor (mbsf) in lower Miocene wellcompacted carbonate periplatform sediments from the Great Bahama Bank [Ocean Drilling Program, ODP Leg 166, Site 1007]. The diagenetic assemblage comprises abundant zeolite crystallized within foraminifer tests and sedimentary matrix, as well as Mg smectites. In carbonate-rich deposits, the formation of the zeolite requires a supply of silica. Thus, the objective of the study is to determine the origin of the silica supply, its diagenetic evolution, and consequently the related implications on interpretation of the sedimentary record, in terms of local or global paleoceanographic change. For lack of evidence for any volcaniclastic input or traces of Si-enriched deep fluids circulation, an in situ biogenic source of silica is validated by isotopic data and chemical modeling for the formation of such secondary minerals in shallow-water carbonate sequences. Geochemical and strontium isotopic data clearly establish the marine signature of the diagenetic zeolite, as well as its contemporaneous formation with the carbonate deposition (Sr model ages of 19.6-23.2 Ma). The test of saturation for the pore fluids specifies the equilibrium state of the present mineralogical assemblage. Seawater-rock modeling specifies that clinoptilolite precipitates from the dissolution of biogenic silica, which reacts with clay minerals. The amount of silica (opal-A) involved in the reaction has to be significant enough, at least 10 wt.%, to account for the observed content of clinoptilolite occurring at the most zeolite-rich level. Modeling also shows that the observed amount of clinoptilolite (~19%) reflects an in situ and short-term reaction due to the high reactivity of primary biogenic silica (opal-A) until its complete depletion. The episodic occurrence of these well-lithified zeolite-rich levels is consistent with the occurrence of seismic reflectors, particularly the P2 seismic sequence boundary located at 1115 mbsf depth and dated as 23.2 Ma. The age range of most zeolitic sedimentary levels (biostratigraphic ages of 21.5-22 Ma) correlates well with that of the early Miocene glaciation Mi-1 and Mi-1a global events. Thus, the clinoptilolite occurrence in the shallow carbonate platform environment far from volcanogenic supply, or in other sensitive marine areas, is potentially a significant new proxy for paleoproductivity and oceanic global events, such as the Miocene events, which are usually recognized in deep-sea pelagic sediments and high latitude deposits.

Chemical, isotope and mineral composition of hydrothermally altered upper oceanic crust drilled at ODP Site 129-801C

ODP Hole 801C penetrates >400 m into 170-Ma oceanic basement formed at a fast-spreading ridge. Most basalts are slightly (10-20%) recrystallized to saponite, calcite, minor celadonite and iron oxyhydroxides, and trace pyrite. Temperatures estimated from oxygen isotope data for secondary minerals are 5-100°C, increasing downward. At the earliest stage, dark celadonitic alteration halos formed along fractures and celadonite, and quartz and chalcedony formed in veins from low-temperature (<100°C) hydrothermal fluids. Iron oxyhydroxides subsequently formed in alteration halos along fractures where seawater circulated, and saponite and pyrite developed in the host rock and in zones of restricted seawater flow under more reducing conditions. Chemical changes include variably elevated K, Rb, Cs, and H2O; local increases in FeT, Ba, Th, and U; and local losses of Mg and Ni. Secondary carbonate veins have 87Sr/86Sr = 0.706337 - 0.707046, and a negative correlation with d18O results from seawater-basalt interaction. Carbonates could have formed at any time since the formation of Site 801 crust. Variable d13C values (-11.2? to 2.9?) reflect the incorporation of oxidized organic carbon from intercalated sediments and changes in the d13C of seawater over time. Compared to other oceanic basements, a major difference at Site 801 is the presence of two hydrothermal silica-iron deposits that formed from low-temperature hydrothermal fluids at the spreading axis. Basalts associated with these horizons are intensely altered (60-100%) to phyllosilicates, calcite, K-feldspar, and titanite; and exhibit large increases in K, Rb, Cs, Ba, H2O, and CO2, and losses of FeT, Mn, Mg, Ca, Na, and Sr. These effects may be common in crust formed at fast-spreading rates, but are not ubiquitous. A second important difference is that the abundance of brown oxidation halos along fractures at Site 801 is an order of magnitude less than at some other sites (2% vs. 20-30%). Relatively smooth basement topography (<100 m) and high sedimentation rate (8 m/Ma) probably restricted the access of oxygenated seawater. Basement lithostratigraphy and early low-temperature hydrothermal alteration and mineral precipitation in fractures at the spreading axis controlled permeability and limited later flow of oxygenated seawater to restricted depth intervals.

(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.

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-0.70299, average 0.70287+/-0.00005 (1 S.D., N=30 samples) (except one felsic vein with 87Sr/86Sr=0.7045), 143Nd/144Nd=0.51304-0.51314, average 0.51310+/-0.00002 (1 S.D., N=28), 206Pb/204Pb=17.43-18.55, 207Pb/204Pb=15.40-15.61 and 208Pb/204Pb=37.19-38.28. The range of Sr and the almost constant Nd isotopic composition resemble that found in the upper 500 m of Hole 735B, while Pb ranges to more radiogenic compositions. In general, there is a decrease in isotopic variation of Sr and Pb as well as ? (238U/204Pb), U and Pb with depth, with a trend towards relatively unradiogenic compositions. This correlates with a decrease in alteration and frequency of evolved rock-types in the core. Leached samples generally have less radiogenic Pb with values trending towards 206Pb/204Pb=17.35, 207Pb/204Pb=15.35 and 208Pb/204Pb=37.0, while their 87Sr/86Sr ratios deviate less systematically from unleached rocks and reach both higher, 0.70307, and lower values, 0.70276. Separated clinopyroxene has elevated 87Sr/86Sr up to 0.7035, while plagioclase generally has close to whole rock Sr. Leaching reduced 87Sr/86Sr in clinopyroxene and in two (out of nine) cases leached separates and whole rock display isotopic equilibrium. Relatively minor hydrothermal seawater alteration is thought to have increased 87Sr/86Sr in the rocks, while a secondary high temperature percolation of a mantle-derived agent is thought to be the cause for the trend towards radiogenic Pb. This material had intermediate 87Sr/86Sr and may have originated from non-MORB off axis mantle. The main primary igneous isotopic variation of the gabbros is suggested to have been derived from the MORB-mantle and is defined mainly by leached samples from both ODP Leg 176 and Leg 118 and can be explained by two-component mixing of an end-member with composition like Central Indian Ridge basalts and an end-member with composition unlike any MORB. The latter is characterized by very unradiogenic Pb, in particular 207Pb/204Pb, and may have an origin with affinity to old depleted mantle (DM). The isotopic composition of the magmas parental to the FeTi-oxide rich rocks cannot be distinguished from the magmas parental to the primitive gabbros and an intimate relationship is indicated. The small-scale inhomogeneity indicated for the SWIR MORB-mantle at the Atlantis II Fracture Zone was probably inherited by the lower crustal rocks due to small-scale melting and monogenetic magma chambers at this slow spreading ridge.

Chemical and isotopic compositions of altered MORB from ODP Holes 187-1154A, 187-1155B, and 187-1160B

Boron and Pb isotopic compositions together with B-U-Th-Pb concentrations were determined for Pacific and Indian mantle-type mid-ocean ridge basalts (MORB) obtained from shallow drill holes near the Australian Antarctic Discordance (AAD). Boron contents in the altered samples range from 29.7 to 69.6 ppm and are extremely enriched relative to fresh MORB glass with 0.4-0.6 ppm B. Similarly the d11B values range from 5.5? to 15.9? in the altered basalts and require interaction with a d11B enriched fluid similar to seawater ~39.5? and/or boron isotope fractionation during the formation of secondary clays. Positive correlations between B concentrations and other chemical indices of alteration such as H2O CO2, K2O, P2O5, U and 87Sr/86Sr indicate that B is progressively enriched in the basalts as they become more altered. Interestingly, d11B shows the largest isotopic shift to +16? in the least altered basalts, followed by a continual decrease to +5-6? in the most altered basalts. These observations may indicate a change from an early seawater dominated fluid towards a sediment-dominated fluid as a result of an increase in sediment cover with increasing age of the seafloor. The progression from heavy d11B towards lighter values with increasing degrees of alteration may also reflect increased formation of clay minerals (e.g., saponite). A comparison of 238U/204Pb and 206Pb/204Pb in fresh glass and variably altered basalt from Site 1160B shows extreme variations that are caused by secondary U enrichment during low temperature alteration. Modeling of the U-Pb isotope system confirms that some alteration events occurred early in the 21.5 Ma history of these rocks, even though a significant second pulse of alteration happened at ~12 Ma after formation of the crust. The U-Pb systematics of co-genetic basaltic glass and variably low temperature altered basaltic whole rocks are thus a potential tool to place age constraints on the timing of alteration and fluid flow in the ocean crust.

Nitrogen isotopic composition and chemical composition of altered oceanic crust from ODP Legs 129 and 185

Knowledge of the subduction input flux of nitrogen (N) in altered oceanic crust (AOC) is critical in any attempt to mass-balance N across arc-trench systems on a global or individual-margin basis. We have employed sealed-tube, carrier-gas-based methods to examine the N concentrations and isotopic compositions of AOC. Analyses of 53 AOC samples recovered on DSDP/ODP legs from the North and South Pacific, the North Atlantic, and the Antarctic oceans (with larger numbers of samples from Site 801 outboard of the Mariana trench and Site 1149 outboard of the Izu trench), and 14 composites for the AOC sections at Site 801, give N concentrations of 1.3 to 18.2 ppm and d15N_air of -11.6? to +8.3?, indicating significant N enrichment probably during the early stages of hydrothermal alteration of the oceanic basalts. The N-d15N modeling for samples from Sites 801 and 1149 (n=39) shows that the secondary N may come from (1) the sedimentary N in the intercalated sediments and possibly overlying sediments via fluid-sediment/rock interaction, and (2) degassed mantle N2 in seawater via alteration-related abiotic reduction processes. For all Site 801 samples, weak correlation of N and K2O contents indicates that the siting of N in potassic alteration phases strongly depends on N availability and is possibly influenced by highly heterogeneous temperature and redox conditions during hydrothermal alteration. The upper 470-m AOC recovered by ODP Legs 129 and 185 delivers approximately 800 kg/km N annually into the Mariana margin. If the remaining less-altered oceanic crust (assuming 6.5 km, mostly dikes and gabbros) has MORB-like N of 1.5 ppm, the entire oceanic crust transfers 5100 kg/km N annually into that trench. This N input flux is twice as large as the annual N input of 2500 kg/km in seafloor sediments subducting into the same margin, demonstrating that the N input in oceanic crust, and its isotopic consequences, must be considered in any assessment of convergent margin N flux.

Chemical analysis and argon geochronology from dive SAR_1 on Cornacya seamount of the Sardinia Channel

Sarcya 1 dive explored a previously unknown 12 My old submerged volcano, labelled Cornacya. A well developed fracturation is characterised by the following directions: N 170 to N-S, N 20 to N 40, N 90 to N 120, N 50 to N 70, which corresponds to the fracturation pattern of the Sardinian margin. The sampled lavas exhibit features of shoshonitic suites of intermediate composition and include amphibole-and mica-bearing lamprophyric xenoliths which are geochemically similar to Ti-poor lamproites. Mica compositions reflect chemical exchanges between the lamprophyre and its shoshonitic host rock suggesting their simultaneous emplacement. Nd compositions of the Cornacya K-rich suite indicate that continental crust was largely involved in the genesis of these rocks. The spatial association of the lamprophyre with the shoshonitic rocks is geochemically similar to K-rich and TiO2-poor igneous suites, emplaced in post-collisional settings. Among shoshonitic rocks, sample SAR 1-01 has been dated at 12.6±0.3 My using the 40Ar/39Ar method with a laser microprobe on single grains. The age of the Cornacya shoshonitic suite is similar to that of the Sisco lamprophyre from Corsica, which similarly is located on the western margin of the Tyrrhenian Sea. Thus, the Cornacya shoshonitic rocks and their lamprophyric xenolith and the Sisco lamprophyre could represent post-collisional suites emplaced during the lithospheric extension of the Corsica-Sardinia block, just after its rotation and before the Tyrrhenian sea opening. Drilling on the Sardinia margin (ODP Leg 107) shows that the upper levels of the present day margin (Hole 654) suffered tectonic subsidence before the lower part (Hole 652). The structure of this lower part is interpreted as the result of an eastward migration of the extension during Late Miocene and Early Pliocene times. Data of Cornacya volcano are in good agreement with this model and provide good chronological constraints for the beginning of the phenomenon.