Silicate mineralogy of basalts at DSDP Leg 54 Holes

Basalts drilled from the East Pacific Rise, OCP Ridge, and Siqueiros fracture zone during Leg 54 are texturally diverse. Dolerites are equigranular at Sites 422 and 428 and porphyritic, with phenocrysts of plagioclase (An69.73) and Ca-rich clinopyroxene (Ca42Mg48Fe10) at Site 427. The East Pacific Rise lavas and some of those from the OCP Ridge are fine-grained and porphyritic. The majority of the large crystals are clustered skeletal glomerocrysts of plagioclase An64-77), together with olivine (Fo80-87), Ca-rich clinopyroxene, or both. Euhedral phenocrysts of plagioclase, together with olivine, Carich clinopyroxene, and Cr-Al spinel in some cases, occur in most of the fine-grained lavas. These phenocrysts are small (maximum dimension <1 mm in all but one sample), sparse (combined modal amount <1% in all samples), and distinctive from the megacrysts which characterize many ocean-floor lavas. In two East Pacific Rise lavas, zoned plagioclase (An83 cores) is the sole phenocryst phase. In other porphyritic lavas from all the main East Pacific Rise and OCP Ridge units drilled during Leg 54, the plagioclase phenocrysts contain cores of bytownite (An79-87) surrounded by more-sodic feldspar (An67-77). Core/rim relationships vary from continuous normal zoning, through discontinuous zoning, to extensive resorption of the calcic cores in some samples. The compositions of the plagioclase calcic cores are systematically related to those of the glomerophyric plagioclase and olivine in the lavas containing them. Furthermore, only one compositional population of calcic cores occurs in each rock. The possible causes of these relationships are far from clear. Magma mixing, although superficially applicable, is inconsistent with important aspects of the phenocryst mineralogy of these particular lavas. A more satisfactory model to explain both phenocryst zoning and rapid glomerocryst growth immediately before extrusion may be constructed by postulating influx of water into the upwelling magmas within Layer 3 of the oceanic crust beneath the East Pacific Rise, and subsequent loss of part of this water during effervescence within feeder dykes between Layer 3 and the ocean floor. It is shown that this model is fully consistent with published data on water and carbon dioxide contents and ratios in the pillow-margin glasses, vesicles, and phenocryst inclusions of ocean-floor basalts. The evidence for the precipitation of plagioclase- dominated crystalline assemblages from these magmas in the upper part of Layer 3 is concordant with recent geophysically based modeling of the structure of the East Pacific Rise. Calcium-rich clinopyroxenes in dolerites from the OCP Ridge and Siqueiros fracture zone show radial, oscillatory, and sector-zoning. In Sample 428A-5-2 (Piece 5a), the compositional trends resulting from this zoning closely resemble those of the pyroxenes in some lunar lavas. The controls on crystallization of interstitial pigeonite - epitaxial upon augite - in this rock are discussed. Both sector-zoning of the augite and nucleation of pigeonite within microvolumes of magma with a low Ca(Mg + Fe) ratio appear to be important factors.

Geochemical and rock magnetic investigation of sediment cores of the western South Atlantic

Geochemical and rock magnetic investigations of sediments from three sites on the continental margin off Argentina and Uruguay were carried out to study diagenetic alteration of iron minerals driven by anaerobic oxidation of methane (AOM). The western Argentine Basin represents a suitable sedimentary environment to study nonsteady-state processes because it is characterized by highly dynamic depositional conditions. Mineralogic and bulk solid phase data document that the sediment mainly consists of terrigenous material with high contents of iron minerals. As a typical feature of these deposits, distinct minima in magnetic susceptibility (k) are observed. Pore water data reveal that these minima in susceptibility coincide with the current depth of the sulfate/methane transition (SMT) where HS- is generated by the process of AOM. The released HS- reacts with the abundant iron (oxyhydr)oxides resulting in the precipitation of iron sulfides accompanied by a nearly complete loss of magnetic susceptibility. Modeling of geochemical data suggest that the magnetic record in this area is highly influenced by a drastic change in mean sedimentation rate (SR) which occurred during the Pleistocene/Holocene transition. We assume that the strong decrease in mean SR encountered during this glacial/interglacial transition induced a fixation of the SMT at a specific depth. The stagnation has obviously enhanced diagenetic dissolution of iron (oxyhydr)oxides within a distinct sediment interval. This assumption was further substantiated by numerical modeling in which the mean SR was decreased from 100 cm/kyr during glacial times to 5 cm/kyr in the Holocene and the methane flux from below was fixed to a constant value. To obtain the observed geochemical and magnetic patterns, the SMT must remain at a fixed position for ~9000 yrs. This calculated value closely correlates to the timing of the Pleistocene/Holocene transition. The results of the model show additionally that a constant high mean SR would cause a concave-up profile of pore water sulfate under steady state conditions.

Mineralogy and geochemistry of Mg-phyllosilicates from Middle Valley, northern Juan de Fuca Ridge

We present results of a detailed mineralogical and geochemical study of the progressive hydrothermal alteration of clastic sediments recovered at ODP Site 858 in an area of active hydrothermal venting at the sedimented, axial rift valley of Middle Valley (northern Juan de Fuca Ridge). These results allow a characterization of newly formed phyllosilicates and provide constraints on the mechanisms of clay formation and controls of mineral reactions on the chemical and isotopic composition of hydrothermal fluids. Hydrothermal alteration at Site 858 is characterized by a progressive change in phyllosilicate assemblages with depth. In the immediate vent area, at Hole 858B, detrital layers are intercalated with pure hydrothermal precipitates at the top of the section, with a predominance of hydrothermal phases at depth. Sequentially downhole in Hole 858B, the clay fraction of the pure hydrothermal layers changes from smectite to corrensite to swelling chlorite and finally to chlorite. In three pure hydrothermal layers in the deepest part of Hole 858B, the clay minerals coexist with neoformed quartz. Neoformed and detrital components are clearly distinguished on the basis of morphology, as seen by SEM and TEM, and by their chemical and stable isotope compositions. Corrensite is characterized by a 24 Å stacking sequence and high Si- and Mg-contents, with Fe/(Fe+Mg) ratio of = 0.08. We propose that corrensite is a unique, possibly metastable, mineralogical phase and was precipitated directly from seawater-dominated hydrothermal fluids. Hydrothermal chlorite in Hole 858B has a stacking sequence of 14 Å with Fe/(Fe+Mg) ratios of ? 0.35. The chemistry and structure of swelling chlorite suggest that it is a corrensiteychlorite mixed-layer phase. The mineralogical zonation in Hole 858B is accompanied by a systematic decrease in d18O, reflecting both the high thermal gradients that prevail at Site 858 and extensive sediment-fluid interaction. Precipitation of the Mg-phyllosilicates in the vent region directly controls the chemical and isotopic compositions of the pore fluids. This is particularly evident by decreases in Mg and enrichments in deuterium and salinity in the pore fluids at depths at which corrensite and chlorite are formed. Structural formulae calculated from TEM-EDX analyses were used to construct clay-H2O oxygen isotope fractionation curves based on oxygen bond models. Our results suggest isotopic disequilibrium conditions for corrensite-quartz and swelling chlorite-quartz precipitation, but yield an equilibrium temperature of 300° C ± 30° for chlorite-quartz at 32 m below the surface. This estimate is consistent with independent estimates and indicates steep thermal gradients of 10-11°/m in the vent region.

Chemistry of hydrothermal clasy and sulfides of ODP Site 106-649

Several meters of unconsolidated hydrothermal sediment were recovered from the Snake Pit hydrothermal field during ODP Leg 106. Polymetallic sulfides comprise most of the sediment with minor fragments of massive sulfide, organic debris, clay minerals, and fresh glass shards. Trace element and Sr-isotope contents of hydrothermal clays and sulfides from Holes 649B and 649G indicate that these minerals precipitated from a mixed hydrothermal fluid-seawater solution. Evaluation of the REE mineral data and the Snake Pit hydrothermal fluids shows that the REE distribution coefficients between the hydrothermal fluids and clay-sulfide mixes range from 100-500. This indicates that hydrothermal fluids originating in the root-zone of the Snake Pit hydrothermal system may be modified by the precipitation of hydrothermal minerals, either in the shallow subsurface or within chimney structures. Contrasting REE profiles of clay-sulfide aggregates and massive sulfides from Holes 649B and 649G may be accounted for by spatial and/or temporal variations in redox conditions in the plumbing system.

Petrographic description and mineral composition of ODP Site 161-975 sediments

Ocean Drilling Program Site 975 is located near the base of the Menorca Rise in the South Balearic Basin of the western Mediterranean Sea. Coring at this site penetrated the Pliocene/Miocene boundary and recovered a sequence of sediments that represent the final stages of salt deposition and the transition from evaporitic to open marine conditions at the end of the Miocene (Messinian). Detailed petrographic observations and bulk mineralogical analyses by X-ray diffraction form the basis for preliminary interpretations of depositional environments for this section. Gypsum is thought to have been deposited in an evaporating basin below wave base. Cycles consisting of a clay layer overlain by gypsiferous chalk, laminated gypsum, and finally pinch-and-swell gypsum suggest upsection increases in salinity. The gypsum section is overlain by two exotic sand layers thought to mark events of fresher water (marine or meteoric) inflow to the basin. Gypsum deposition terminated and was replaced by inorganic precipitation of micritic calcite with periodic, variable dilution by fine-grained terrigenous sediment. The micritic sediments have fine, slightly wavy, laminations indicating either an algal/microbial mat origin, or varve-like fluctuations in deposition, perhaps in a deep basin. The Pliocene/Miocene boundary falls within an interval of banded micritic silty clays that reflect the final environmental fluctuations during the transition to the open marine conditions of the Pliocene.

The alkali element and boron geochemistry of ODP Site 169-1038 in the Escanaba Trough, East Pacific

A suite of conjugate pore fluid and sediment samples were collected during Leg 169 of the ODP from within the clastic sedimentary sequences which host massive sulphides at Central Hill, Escanaba Trough (ODP Site 1038). We report the alkali element and boron, and Li and B isotope data for these samples. Relative to a reference site (Site 1037) located outside the zone of high heat flow, pore fluids from Site 1038 show a wide variation in Cl (300-800 mM), and have far higher concentrations of Li (up to 6.2 mM), B (up to 9.7 mM), Cs (up to 5.0 mM), and Rb (up to 97 mM). We show that the pore fluids are derived from hydrothermal circulation that has extended into the basement oceanic crust, with input of the alkali elements and B as the rising hydrothermal fluids interact geochemically with the overlying clastic sediments. There is, however, no marked depletion of these elements in the conjugate sediments, suggesting that there has been advective transport of fluids away from the primary hydrothermal reaction site. This is supported by modelling of the Li and B isotope systematics of the pore fluids, which shows that they record extensive formation of secondary minerals during cooling of the fluids from ~350 to ~20ºC. Precipitation of metal-rich sulphides would have occurred prior to the formation of these minerals, thus, the pore fluid Li and B isotope data can place important constraints on the locus of sulphide deposition beneath the seafloor at Escanaba.

Pore water (SO4, CH4, alkalinity, HS-, Ca, Sr, Mg, Ba) and solid phase (Ca, Sr, Mg, Ba) data measured in pockmark sediments of the Northern Congo Fan

This study focuses on the vertical distribution of authigenic carbonates (aragonite and high Mg-calcite) in the form of finely disseminated precipitates as well as massive carbonate concretions present in and above gas hydrate bearing sediments of the Northern Congo Fan. Analyses of Ca, Mg, Sr and Ba in pore water, bulk sediments and authigenic carbonates were carried out on gravity cores taken from three pockmark structures (Hydrate Hole, Black Hole and Worm Hole). In addition, a background core was retrieved from an area not influenced by fluid seepage. Pore water Sr/Ca and Mg/Ca ratios are used to reveal the current depths of carbonate formation as well as the mineralogy of the authigenic precipitates. The Sr/Ca and Mg/Ca ratios of bulk sediments and massive carbonate concretions were applied to infer the presence and depth distribution of authigenic aragonite and high Mg-calcite, based on the approach presented by Bayon et al. [Bayon et al. (2007). Sr/Ca and Mg/Ca ratios in Niger Delta sediments: Implications for authigenic carbonate genesis in cold seep environments. Marine Geology 241(1-4), 93-109, doi:10.1016/j.margeo.2007.03.007]. We show that the approach developed by Bayon et al. (2007) for sediments of cold seeps of the Niger Delta is also suitable to identify the mineralogy of authigenic carbonates in pockmark sediments of the Congo Deep-Sea Fan. We expand this approach by combining interstitial with solid phase Sr/Ca and Mg/Ca ratios, which demonstrate that high Mg-calcite is the predominant authigenic carbonate that currently forms at the sulfate/methane reaction zone (SMRZ). This is the first study which investigates both solid phase and pore water signatures typical for either aragonite or high Mg-calcite precipitation for the same sediment cores and thus is able to identify active and fossil carbonate precipitation events. At all investigated pockmark sites fossil horizons of the SMRZ were deduced from high Mg-calcite located above and below the current depths of the SMRZ. Additionally, aragonite enrichments typical for high seepage rates were detected close to the sediment surface at these sites. However, active precipitation of aragonite as indicated by pore water characteristics only occurs at the Black Hole site. Dissolved and solid phase Ba concentrations were used to estimate the time the SMRZ was fixed at the current depths of the diagenetic barite fronts. The combined pore water and solid phase elemental ratios (Mg/Ca, Sr/Ca) and Ba concentrations allow the reconstruction of past changes in methane seepage at the investigated pockmark sites. At the Hydrate Hole and Worm Hole sites the time of high methane seepage was estimated to have ceased at least 600 yr BP. In contrast, a more recent change from a high flux to a more dormant stage must have occurred at the Black Hole site as evidenced by active aragonite precipitation at the sediment surface and a lack of diagenetic Ba enrichments.

Composition of ODP Leg 126 sandstones

Early diagenesis in Leg 126 forearc and backarc sands/sandstones is characterized by the dissolution of intermediate to mafic brown glass, the alteration of colorless rhyolitic glass to clay minerals, precipitation of thin clay-mineral rim cements, and minor precipitation of clinoptilolite cements. Later, more intense diagenesis is restricted to Oligocene forearc basin sediments at Sites 787,792, and 793. In these sections, the effects of early diagenesis have been intensified and overprinted by later diagenetic effects including (1) large-scale dissolution of feldspar and pyroxene crystals, (2) further dissolution of vitric components, (3) precipitation of minor carbonate cements, and (4) pervasive, multiple-staged zeolite cementation. Zeolite minerals present include analcite, mordenite, natrolite, heulandite, wairakite, chabazite, erionite, herschelite, and phillipsite. The latest diagenetic events appear to be the minor dissolution of zeolite cements and the precipitation of minor carbonate and potassium feldspar(?) cements. Observed porosity types include primary interparticles; primary intraparticles in vesicular glass and foraminifers; primary interparticles reduced by compaction and cementation; secondary intraparticles produced by dissolution of feldspar, nonopaque heavy minerals, volcanic glass, and foraminifer tests; and secondary interparticles produced by the dissolution of zeolite cements. Within forearc Oligocene sections at Sites 787 and 792, diagenetic effects appear to decrease with depth in the Oligocene section; however, at Site 793 the majority of samples are intensely altered.