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.

Physical properties nd chemistry of igneous rocks from the Pigafetta and East Mariana Basins

Physical properties of basalts from Ocean Drilling Program Sites 800 and 801 in the Pigafetta Basin and Site 802 in the East Mariana Basin, including porosity, wet-bulk density, grain density, compressional wave velocity, and thermal conductivity, were measured aboard JOIDES Resolution during Leg 129. The ranges for the properties are large, as typified by the velocity, which varies from 3.46 to 6.59 km/s. Extensively altered basalts immediately above and below a silicified hydrothermal deposit (60-69 m sub-basement depth) at Site 801 display the highest porosity, and lowest bulk density, velocity, and thermal conductivity, whereas the slightly altered rocks from Site 802 and the lowermost part of Site 801 represent the other extreme in physical properties variations. In order to better establish the relationship between physical properties and alteration of the rocks, the compressional wave velocities were compared with results from major and trace elemental analyses and petrographic examination of select samples. For the Leg 129 basalts, velocity displays a generally consistent decrease with increasing K2O, H2O+, loss on ignition, and Rb contents and the value of Fe3+/FeT and decreasing concentrations of SiO2, FeOT, CaO, MgO, and MnO. These trends are consistent with trends documented for the progressive alteration of oceanic crust and indicate that on a laboratory sample scale, basalt alteration is largely responsible for the variation of the physical properties of basalts sampled at Sites 800, 801, and 802.

Chemical and mineral compositions of altered peridotites from ODP Leg 209

Abyssal peridotite from the 15°20'N area of the Mid-Atlantic Ridge show complex geochemical variations among the different sites drilled during ODP Leg 209. Major element compositions indicate variable degrees of melt depletion and refertilization as well as local hydrothermal metasomatism. Strongest evidence for melt-rock interactions are correlated Light Rare Earth Element (LREE) and High Field Strength Element (HFSE) additions at Sites 1270 and 1271. In contrast, hydrothermal alteration at Sites 1274, 1272, and 1268 causes LREE mobility associated with minor HFSE variability, reflecting the low solubility of HFSE in aqueous solutions. Site 1274 contains the least-altered, highly refractory, peridotite with strong depletion in LREE and shows a gradual increase in the intensity of isochemical serpentinization; except for the addition of H2O which causes a mass gain of up to 20 g/100 g. The formation of magnetite is reflected in decreasing Fe(2+)/Fe(3+) ratios. This style of alteration is referred to as rock-dominated serpentinization. In contrast, fluid-dominated serpentinization at Site 1268 is characterized by gains in sulfur and development of U-shaped REE pattern with strong positive Eu anomalies which are also characteristic for hot (350 to 400°C) vent-type fluids discharging from black smoker fields. Serpentinites at Site 1268 were overprinted by talc alteration under static conditions due to interaction with high a_SiO2 fluids causing the development of smooth, LREE enriched patterns with pronounced negative Eu anomalies. These results show that hydrothermal fluid-peridotite and fluid-serpentinite interaction processes are an important factor regarding the budget of exchange processes between the lithosphere and the hydrosphere in slow spreading environments.

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.

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.

Secondary minerals and geochemistry at DSDP Legs 68 and 69

Basalt samples recovered during DSDP Legs 68, 69, and 70 from a 550-meter-thick section in two holes near the Costa Rica Rift (Holes 501 and 504B) were found to contain the following secondary minerals: trioctahedral and dioctahedral smectite, chlorite, mixed-layer clays, talc, hematite, pyrite, foujasite, phillipsite, analcime, natrolite, thomsonite, gyrolite, aragonite, calcite, anhydrite, chalcocite, Fe-hydrosilicate, okenite, apophyllite, actinolite, cristobalite, quartz, and magnesite. A less positive identification of bismutite was made. A mineral rich in Mn and minerals with strong reflections at 12.9 Å and 3.20 Å remain unidentified. Trioctahedral smectite replaces glass and olivine in the basalt groundmass. The other secondary minerals occur in veins. The distribution of the secondary minerals in the basalt section shows both hydrothermal and oxidizing-nonoxidizing zonation. Most of the secondary minerals formed under alkaline, nonoxidizing conditions at temperatures up to 120° C. An acidic regime probably existed in the lowest portion of basalt. Oxidative diagenesis followed nonoxidative diagenesis in the upper part of the section. Oxidative diagenesis is characterized by the absence of celadonite, rare occurrences of dioctahedral smectite, and widespread hematite and phillipsite.

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.

Geochemistry of last glacial sediments of Isla de los Estados, southeastern Tierra del Fuego

The island of Isla de los Estados is situated at 54.5°S, 64°W, east of Argentinian Tierra del Fuego, and is located in a sensitive geographic position in relation to the zonal circulation between Antarctica and South America. Its terrestrial records of the last deglaciation, recording atmospheric conditions but within an oceanic setting, can help to clarify changes of regional circulation patterns, both atmospheric and marine. Here, we present geochemical analyses from 16-10 ka cal BP of a peat core from Lago Galvarne Bog at the northern coast of the island, and a lake sediment core from Laguna Cascada 3 km further south. The data comprise TC, TN, loss on ignition analyses and continuous XRF scanning on both cores as well as age-depth modeling based on AMS-14C dating. Deglaciation and onset of peat formation in the coastal areas began before 16 ka cal BP followed by a rapid glacial retreat and the start of lacustrine sedimentation further inland. Data suggest initially windy conditions with permafrost succeeded by gradually warmer and wetter conditions until ca 14.5 ka cal BP. The warming trend slows down until ca 13.5 ka cal BP, followed by arid conditions culminating around 12.8 ka cal BP. Our data suggest fairly warm conditions and the establishment of denser peat and forest vegetation ca 10.6 ka cal BP, contemporaneous with the onset of the Antarctic thermal optimum. This indicates large-scale shifts in the placement of zonal flow and the Westerlies at the beginning of the Holocene.

Geochemistry of highly depleted peridotites of the Mid-Atlantic Ridge

During ODP Leg 209, a magma-starved area of the Mid-Atlantic Ridge (MAR) was drilled in the vicinity of the Fifteen-Twenty Fracture Zone (FZ) that offsets one of the slowest portions of the spreading ridge. We present here the results of a bulk rock multi-elemental study of 27 peridotites drilled at Sites 1272 and 1274 (to the south and the north of the FZ, respectively). The peridotites comprise mainly of harzburgites with minor dunites. Clinopyroxene (Cpx), which is interstitial and interpreted as secondary, is observed in Site 1274 peridotites. Sites 1272 and 1274 peridotites have low Al2O3 contents (<1 anhydrous wt.%), high Mg# (>91.5), and bulk rock trace elements compositions mostly below 0.1X primitive mantle (PM). These peridotites, and in particular Site 1272 peridotites, represent the most depleted peridotites yet sampled at a slow spreading ridge. Their compositions indicate high degrees of partial melting and melt extraction. A single open-system melting event (melting plus percolation of melts produced within upwelling mantle) can explain their highly depleted yet linear chondrite-normalized REE patterns, characterized by a steady depletion from HREE to LREE. Late melt-rock reactions and precipitation of Cpx explains the slightly less depleted compositions of Site 1274 peridotites. Hence, the differences in composition between Sites 1272 and 1274 peridotites do not provide evidence for regional variations in the degrees of partial melting from the south to the north of the FZ. The occurrence of highly refractory peridotites in the Fifteen-Twenty area suggests we sampled a more actively convecting mantle than generally supposed below slow spreading centers.

Composition of rocks and minerals from the transition zone of the Reykjanes Ridge

Investigations of petrography, mineralogy, and chemical composition of gases and fluids in tuffs and lavas were carried out on samples dredged in the transition zone from the shelf and slope of Iceland to the Reykjanes Ridge. The samples were collected from the depths of 950-720 m during different expeditions of R/V Akademik Kurchatov and Mikhail Lomonosov. Mantle ultrabasite inclusions were first recognized in the region of Iceland. It can be assumed that they are related to eruptive structures formed on the ocean floor during Pliocene and are associated with the Iceland hot spot.

Geochemistry of the gabbroic lithosphere in IODP Holes 304-U1309D and 305-U1309D

IODP Hole U1309D (Atlantis Massif, Mid-Atlantic Ridge 30°N) is the second deepest hole drilled into slow spread gabbroic lithosphere. It comprises 5.4% of olivine-rich troctolites (~ > 70% olivine), possibly the most primitive gabbroic rocks ever drilled at mid-ocean ridges. We present the result of an in situ trace element study carried out on a series of olivine-rich troctolites, and neighbouring troctolites and gabbros, from olivine-rich intervals in Hole U1309D. Olivine-rich troctolites display poikilitic textures; coarse-grained subhedral to medium-grained rounded olivine crystals are included into large undeformed clinopyroxene and plagioclase poikiloblasts. In contrast, gabbros and troctolites have irregularly seriate textures, with highly variable grain sizes, and locally poikilitic clinopyroxene oikocrysts in troctolites. Clinopyroxene is high Mg# augite (Mg# 87 in olivine-rich troctolites to 82 in gabbros), and plagioclase has anorthite contents ranging from 77 in olivine-rich troctolites to 68 in gabbros. Olivine has high forsterite contents (82-88 in olivine-rich troctolites, to 78-83 in gabbros) and is in Mg-Fe equilibrium with clinopyroxene. Clinopyroxene cores and plagioclase are depleted in trace elements (e.g., Ybcpx ~ 5-11 * Chondrite), they are in equilibrium with the same MORB-type melt in all studied rock-types. These compositions are not consistent with the progressively more trace element enriched (evolved) compositions expected from olivine rich primitive products to gabbros in a MORB cumulate sequence. They indicate that clinopyroxene and plagioclase crystallized concurrently, after melts having the same trace element composition, consistent with crystallization in an open system with a buffered magma composition. The slight trace element enrichments and lower Cr contents observed in clinopyroxene rims and interstitial grains results from crystallization of late-stage differentiated melts, probably indicating the closure of the magmatic system. In contrast to clinopyroxene and plagioclase, olivine is not in equilibrium with MORB, but with a highly fractionated depleted melt, similar to that in equilibrium with refractory oceanic peridotites, thus possibly indicating a mantle origin. In addition, textural relationships suggest that olivine was in part assimilated by the basaltic melts after which clinopyroxene and plagioclase crystallized (impregnation). These observations suggest a complex crystallization history in an open system involving impregnation by MORB-type melt(s) of an olivine-rich rock or mush. The documented magmatic processes suggest that olivine-rich troctolites were formed in a zone with large magmatic transfer and accumulation, similar to the mantle-crust transition zone documented in ophiolites and at fast spreading ridges.

Geochemistry of Cretaceous sediments from Argo and Gascoyne Abyssal Plains

The accumulation of organic matter, ferrous and pyrite iron, and the ratios of organic carbon/total sulfur and organic carbon/total phosphorus in the Lower Cretaceous sediments from the Argo and Gascoyne abyssal plains have been used as indicators of both the source and reactivity of organic matter in the sediments and the depositional environment. Total sulfur, used as an indicator of pyrite sulfur, is more abundant in sediments from the Gascoyne Abyssal Plain than in those from the Argo Abyssal Plain. Sulfur positively correlates with TOC at both sites (although poorly at the Argo Abyssal Plain site, R = 0.48), with an extension of the line of best-fit through the origin, indicating that pyrite (TOC <2 wt%) is diagenetic and deposited from normal marine conditions. The average ratio of C/S for samples of TOC <2 wt% is 5.4 at Argo Abyssal Plain (compared to the modern normal marine value of 2.8) indicating deposition of organic matter probably of mixed terrestrial and oxidized marine sources that is unreactive to the sulfate-reducing bacteria. One sample from the Aptian sediments is rich in TOC (5.1 wt%) and has a C/S ratio of 0.5. The average C/S ratio in Gascoyne Abyssal Plain sediments is 0.8 (R = 0.97), which indicates the formation of abundant pyrite in addition to burial and preservation of relatively fresh organic matter that is reactive to the sulfate-reducing bacteria. Organic carbon to phosphorus ratios (C/P) in the sediments indicate preferential remobilization of organic carbon over phosphorus with increasing water depth. Estimates of the degree of pyritization (DOP) increase with increasing TOC at both sites, indicating iron is not limiting and pyrite is formed diagenetically. The one sample with a TOC content of 5.1 wt%, from the Argo Abyssal Plain near the Barremian-Aptian boundary, is composed mostly of framboidal pyrite, finely laminated and not bioturbated, and hence may have been deposited during a brief period of anoxia in the overlying waters.

Composition of bottom sediments from the Central Basin of the Indian Ocean

A study of petrographic and mineral compositions of 26 sediment cores from the western part of the Central Basin of the Indian Ocean has identified biogenic, terrigenous, volcanogenic, and authigenic sediment types formed in certain facies conditions. On the basis of bio- and paleomagnetic stratigraphy data from the cores sedimentation rates of different sediment types have been calculated. Modern and Pliocene-Pleistocene positions of the main facies boundaries (the critical depth of carbonate accumulation, the geochemical boundary between hemi- and miopelagic zones, the frontal boundaries of the equatorial belt of biogenic silica accumulation) have been determined. It has been shown that the sedimentary process during Pliocene-Quaternary had cycle variations characterized by successive changes of different sedimentation types - hemipelagic, miopelagic, and biogenic.

Geochemical and mineralogical analyses of clay-rich intervals in ODP Sites 166-1006 and 166-1007

The western flank of the Great Bahama Bank, drilled during ODP Leg 166 at seven sites, represents a prograding carbonate sequence from late Oligocene to Holocene [Eberli et al., Proc. ODP Init. Reports 166 (1997)]. The signatures of the detrital input and of diagenetic alteration are evident in clay enriched intervals from the most distal Sites 1006 and 1007 in the Straits of Florida. Mineralogical and chemical investigations (XRD, TEM, SEM, ICP-MS) run on bulk rocks and on the clay fractions enable the origin and evolution of silicate parageneses to be characterized. Plio-Pleistocene silt and clay interbeds contain detrital clay assemblages comprising chlorite, illite, interstratified illite smectite, smectite, kaolinite and palygorskite. The greater smectite input within late Pliocene units than in Pleistocene oozes may relate either varying source areas or change in paleoclimatic conditions and weathering intensity. The clay intervals from Miocene-upper Oligocene wackestone sections are fairly different, with prevalent smectite in the fine fraction, whose high crystallinity and Mg contents that point towards an authigenic origin. The lower Miocene section, below 1104 mbsf, at depths where compaction features are well developed, is particularly characterized by abundant authigenic Na-K-clinoptilolite filling foraminifer tests. The authigenic smectite and clinoptilolite paragenesis is recorded by the chemical trends, both of the sediment and the interstitial fluid. This diagenetic evolution implies Si- and Mg rich fluids circulating in deeper and older sequences. For lack of any local volcaniclastic input, the genesis of zeolite and the terms of water rock interaction are discussed. The location of the diagenetic front correlates with that of the seismic sequence boundary P2 dated as 23.2 Ma. This correspondence may allow the chronostratigraphic significance of some specific seismic reflections to be reassessed.

Composition of rocks and minerals from the Cape Verde Fault Zone

Results of comprehensive geological, geophysical and geochemical studies carried out in the Cape Verde Fracture Zone (Central Atlantic) during Cruise 9 of R/V ''Antares'' (1990-1991) are published in the book. Detailed characterization of various bedrock complexes (ultrabasites, gabbroids, dolerites, basalts, metamorphic rocks) is given. Geological conditions of newly found hydrothermal mineralization in the area are described. Problems of ore melts are under consideration. New data on hydrochemical anomalies and heat flow are given. The book contains original materials on sedimentary formations of the area.