Chemistry of minerals from amphibole-chlorite veins of ODP Site 209-1270

We examined small-scale shear zones in drillcore samples of abyssal peridotites from the Mid-Atlantic Ridge. These shear zones are associated with veins consisting of chlorite + actinolite/tremolite assemblages, with accessory phases zircon and apatite, and they are interpreted as altered plagiogranite melt impregnations, which originate from hydrous partial melting of gabbroic intrusion in an oceanic detachment fault. Ti-in-zircon thermometry yields temperatures around 820°C for the crystallization of the evolved melt. Reaction path modeling indicates that the alteration assemblage includes serpentine of the adjacent altered peridotites. Based on the model results, we propose that formation of chlorite occurred at higher temperatures than serpentinization, thus leading to strain localization around former plagiogranites during alteration. The detachment fault represents a major pathway for fluids through the oceanic crust, as evidenced by extremely low d18O of altered plagiogranite veins (+3.0-4.2 per mil) and adjacent serpentinites (+ 2.6-3.7 per mil). The uniform oxygen isotope data indicate that fluid flow in the detachment fault system affected veins and adjacent host serpentinites likewise.

Concentration of trace elements in samples of sea water determined by use of two methods of concentrating

A device and a specific procedure for simultaneous concentration of trace ele¬ments from sea water by co-precipitating them in precipitate of magnesium hydroxide obtained by electrolytic alkalization of samples in a two-chamber electrolyzer are described. Analyses of sea water samples for zinc, iron, copper, nickel and lead demonstrate that the method produces a thousand-fold concentration and gives results that agree well with those obtained by extractive concentration using diethyldithiocarbamate and 8-hydroxyquinoline in chloroform. Extracts were analyzed by flame atomic absorption. Correlation coefficients of results obtained with use of these two methods of concentration were 0.76-0.87 for zinc, iron, and copper at confidence levels of 0.05-0.07. Average zinc and iron concentrations determined by the method differed by less than 10%.

Mineralogy and geochemistry of Miocene tuff from ODP Hole 129-802A

Drilling at Ocean Drilling Program Site 802 in the central Mariana Basin, northwest Pacific Ocean, revealed an unexpected 222-m-thick sequence of well-cemented tuff of Miocene age. The deposits are unusual in that their source is presumably an unmapped seamount and they exhibit several peculiar petrological and mineralogical features. The well-developed secondary mineral sequence which includes analcime is rare in such relatively young, unburied deposits, in an area where there is little other evidence of hydrothermal activity. The massive tuff section also contains abundant fissure veins made of a rare silicate carbonate sulfate hydroxide hydrate of calcium, called thaumasite, which has not before been described in deep submarine deposits. The smectite-zeolite-thaumasite paragenesis coincides with the presence of chloride and calcium-enriched interstitial waters. The diagenetic evolution of the deposit appears to have been largely controlled by the depositional mode. The discharges of disaggregated and rejuvenated volcaniclasts seem to have been abrupt and repeated. The Miocene tuff at Site 802 thus provides new insights on the interactions between basaltic glass, biogenic phases, and seawater, in a specific deep-sea environment.

Petrography and geochemistry of lithic clasts in sediments obtained from ODP Leg 180 sites

This report includes the petrographic description and reviews the distribution of lithic clasts in sediments drilled during Leg 180 in the Woodlark Basin (southwest Pacific). The lithic clasts include (1) metamorphic rocks; (2) granites; (3) serpentinites, gabbros, dolerites, and basalts likely derived from the Papuan ophiolite belt; (4) rare alkaline volcanites reworked in middle Miocene sediments; (5) medium- to high-K calc-alkaline island arc volcanites, in part as reworked clasts, and explosive products deposited by fallout or reworked by turbiditic currents; and (6) rare sedimentary fragments. At the footwall sites the clast assemblage evidences the association of dolerites and evolved gabbroic rocks; the serpentinite likely pertaining to the same ophiolitic complex are likely derived from onland outcrops and transported by means of turbidity currents. On the whole, extensional tectonics active at least since the middle Pliocene can be inferred. The calc-alkaline volcanism is in continuity with the arc-related products from the Papua Peninsula and D'Entrecasteaux Islands and with the latest volcanics of the Miocene Trobrian arc. However, the medium- to high-K and shoshonitic products do not display a significant temporal evolution within the stratigraphic setting. Lava clasts, volcanogenic grains, and glass shards are associated with turbidity currents, whereas in the Pliocene of northern margin the increasing frequency of tephra (glass shards and vesicular silicic fragments) suggests more explosive activity and increasing contribution to the sediments from aerial fallout materials. Evidence of localized alkalic volcanism of presumable early to middle Miocene age is a new finding. It could represent a rift phase earlier than or coeval to the first opening of the Woodlark Basin or, less probably, could derive from depositional trajectories diverted from an adjacent basin.

Mineralogy and chemistry of basalts and secondary minerals from DSDP Site 417, Atlantic Ocean

Basalts from DSDP Site 417 (109 Ma) exhibit the effects of several stages of alteration reflecting the evolution of seawater-derived solution compositions and control by the structure and permeability of the crust. Characteristic secondary mineral assemblages occur in often superimposed alteration zones within individual basalt fragments. By combining bulk rock and single phase chemical analyses with detailed mineralogic and petrographic studies, chemical changes have been determined for most of the alteration stages identified in the basalts. 1) Minor amounts of saponite, chlorite, and pyrite formed locally in coarse grained portions of massive units, possibly at high temperatures during initial cooling of the basalts. No chemical changes could be determined for this stage. 2) Possible mixing of cooled hydrothermal fluids with seawater resulted in the formation of celadonite-nontronite and Fe-hydroxide-rich black halos around cracks and pillow rims. Gains of K, Rb, H20, increase of Fe 3 +/FeT and possibly some losses of Ca and Mg occurred during this stage. 3a) Extensive circulation of oxygenated seawater resulted in the formation of various smectites, K-feldspar, and Fe-hydroxides in brown and light grey alteration zones around formerly exposed surfaces. K, Rb, H20, and occasionally P were added to the rocks, Fe3+/FeT increased, and Ca, Mg, Si and occasionally Al and Na were lost. 3 b) Anoxic alteration occurred during reaction of basalt with seawater at low water-rock ratios, or with seawater that had previously reacted with basalt. Saponite-rich dark grey alteration zones formed which exhibit very little chemical change: generally only slight increases in Fe 3 +/FeT and H20 occurred. 4) Zeolites and calcite formed from seawater-derived fluids modified by previous reactions with basalt. Chemical changes involved increases of Ca, Na, H20 , and CO2 in the rocks. 5) A late stage of anoxic conditions resulted in the formation of minor amounts of Mn-calcites and secondary sulfides in previously oxidized rocks. No chemical changes were determined for this stage. Recognition of such alteration sequences is important in understanding the evolution of submarine hydrothermal systems and in interpreting chemical exchange due to seawater-basalt reactions.

Chemical composition of sediments from DSDP Site 223, Indian Ocean

Detailed major- and trace-element chemistry is presented for 41 sediment samples from DSDP Site-223 borehole cores. A marked change in chemical (and mineralogical) character is shown at the end of the Early Miocene Epoch which relates to tectonic events and associated changes in sedimentary regime. Enrichment in the contents of such elements as Mg, Cr and Ni compared with average values for fine-grained sediments occurs throughout the sequence and is particularly marked in the upper group of samples. A basic-ultrabasic provenance is suggested - the Oman ophiolites. Leaching with combined acid-reducing agent indicated typical lithogenous-character ordering for the elements and emphasised the enrichment of Mg, Cr, Ni (and Li, Cu, Zn, Pb, Fe and Ti) over values for near-shore muds and terrigenous material. Factor analysis on the bulk chemical data identifies the main lithogenous and biogenous components, subdividing the latter. It separates the upper and lower group of chemically dissimilar sediments and delineates a Mn-hydroxide phase. It also shows the essentially independent roles of Na, Ba and P.

Concentrations and accumulation rates of chemical elements and sediment components in sediments of central equatorial Pacific, DSDP data

Accumulation rates of Mg, Al, Si, Mn, Fe, Ni, Cu, Zn, opal, and calcium carbonate have been calculated from their concentrations in samples from equatorial Deep Sea Drilling Project sites. Maps of element accumulation rates and of Q-mode factors derived from raw data indicate that the flux of trace metals to equatorial Pacific sediments has varied markedly through time and space in response to changes in the relative and absolute influence of several depositional influences: biogenic, detrital, authigenic, and hydrothermal sedimentation. Biologically derived material dominates the sediment of the equatorial Pacific. The distributions of Cu and Zn are most influenced by surface-water biological activity, but Ni, Al, Fe, and Mn are also incorporated into biological material. All of these elements have equatorial accumulation maxima similar to those of opal and calcium carbonate at times during the past 50 m.y. Detritus distributed by trade winds and equatorial surface circulation contributes Al, non-biogenic Si, Fe, and Mg to the region. Detrital sediment is most important in areas with a small supply of biogenic debris and low bulk-accumulation rates. Al accumulation generally increases toward the north and east, indicating its continental source and distribution by the northeast trade winds. Maxima in biological productivity during middle Eocene and latest Miocene to early Pliocene time and concomitant well-developed surface circulation contributed toward temporal maxima in the accumulation rates of Cu, Zn, Ni, and Al in sediments of those ages. Authigenic material is also important only where bulk-sediment accumulation rates are low. Ni, Cu, Zn, and sometimes Mn are associated with this sediment. Fe is almost entirely of hydrothermal origin. Mn is primarily hydrothermal, but some is probably scavenged from sea water by amorphous iron hydroxide floes along with other elements concentrated in hydrothermal sediments, Ni, Cu, and Zn. During the past 50 m.y. all of these elements accumulated over the East Pacific Rise at rates nearly an order of magnitude higher than those at non-rise-crest sites. In addition, factor analysis indicates that some of this material is carried substantial distances to the west of the rise crest. Accumulation rates of Fe in basal metalliferous sediments indicate that the hydrothermal activity that supplied amorphous Fe oxides to the East Pacific Rise areas was most intense during middle Eocene and late Miocene to early Pliocene time.

Highly dealkylated copper and nickel etioporpyfrins in marine sediments

Copper porphyrins have been recognized as natural constituents of marine sediments only within the past 5 years (Palmer and Baker, 1978, Science201, 49-51). In that report it was suggested that these pigments may derive from and be markers for oxidized terrestrial organic matter redeposited in the marine environment. In the present study we describe the distribution of copper porphyrins in sediments from several north Pacific and Gulf of California DSDP/IPQD sites (Legs 56,63,64). These allochthonous pigments have now been found to be accompanied by identical arrays of highly dealkylated nickel etioporphyrins. Evaluation of data from this and past studies clearly reveals that there is a strong carbon-number distribution similarity betweeen coincident Cu and Ni etioporphyrins. This homology match is taken as reflecting a common source for the tetrapyrrole ligands of this population of Cu and Ni chelates. Predepositional generation of these highly dealkylated etioporphyrins is concluded from the occurrence of these pigments in sediments continuing essentially all stages of in situ chlorophyll diagenesis (cf. Baker and Louda, 1983). That is, their presence is not regulated by the in situ diagenetic continuum. Thus, the highly dealkylated Cu and Ni etioporphyrins represent an 'allochthonous' background over which 'autochthonous' (viz. marine produced) chlorophyll derivatives are deposited and are undergoing in situ diagenesis.

Geochemistry of primary and secondary mineral phases in ODP Leg 209 sediments

Primary and secondary mineral phases from Holes 1268A (11 samples), 1272A (9 samples), and 1274A (12 samples) were analyzed by electron microprobe in Bonn and Cologne (Germany). Bulk rock powders of these samples were also analyzed geochemically, including major and trace elements (Paulick et al., 2006, doi:10.1016/j.chemgeo.2006.04.011). Ocean Drilling Program (ODP) Leg 209 Holes 1268A, 1272A, and 1274A differ remarkably in alteration intensity and mineralogy, and details regarding their lithologic characteristics are presented in Bach et al. (2004, doi:10.1029/2004GC000744) and Shipboard Scientific Party (2004, doi:10.2973/odp.proc.ir.209.101.2004). Because of the least altered character of peridotite in Hole 1274A, abundant clinopyroxene, orthopyroxene, olivine, and spinel were analyzed at this site. In Hole 1272A, primary silicates are rare and analyses were restricted to some samples that contain traces of olivine and orthopyroxene. Because of the intensity of alteration, Hole 1268A is devoid of primary phases except spinel. Commonly, alteration is pseudomorphic and serpentinization of olivine and orthopyroxene can be distinguished. Accordingly, compositional variations of the alteration minerals with regard to the precursor minerals are one of the issues investigated in this data report.

Petrology of mafic and ultramafic intrusions from the Portneuf-Mauricie Domain, Grenville Province, Canada

The Portneuf-Mauricie Domain (PMD), located in the south-central part of the Grenville Province, comprises several mafic and ultramafic intrusions hosting Ni-Cu ± platinum-group element (PGE) prospects and a former small mining operation (Lac Édouard mine). These meter- to kilometer-scale, sulfide-bearing intrusions display diverse forms, such as layered and tabular bodies with no particular internal structure, and zoned plutons. They were injected ~ 1.40 Ga into a mature oceanic arc, before and during accretion of the arc to the Laurentian margin. The pressure-temperature conditions of the magmas at the beginning of their emplacement were 3 kbar and 1319-1200 °C (according to the petrologic modeling results from this study). The PMD mineralized intrusions are interpreted to represent former magma chambers or magma conduits in the roots of the oceanic arc. The parent magmas of the mineralized intrusions resulted mainly from the partial melting of a mantle source composed of spinel-bearing lherzolite. Petrologic modeling and the occurrence of primary amphibole in the plutonic rocks indicate that these parent melts were basaltic and hydrous. In addition, fractional crystallization modeling and Mg/Fe ratios suggest that most of the intrusions may have formed from evolved magmas, with Mg# = 60, resulting from the fractionation of more primitive magmas (primary magmas, with Mg# = 68). Petrologic modeling demonstrates that 30% fractional crystallization resulted in the primitive to evolved characteristics of the studied intrusive rocks (as indicated by the crystallization sequences and mineral chemistry). Exceptions are the Réservoir Blanc, Boivin, and Rochette West parent magmas, which may have undergone more extensive fractional crystallization, since these intrusions contain pyroxenes that are more iron rich and have lower Mg numbers than pyroxenes in the other PMD intrusions. The PMD mafic and ultramafic intrusions were intruded into an island arc located offshore from the Laurentian continent. Thus, their presence confirms the existence of a well-developed magmatic network (responsible of the fractionation processes) beneath the Proterozoic arc, which resulted in the wide range of compositions observed in the various plutons.