Geochemistry of felsic rocks and minerals of ODP Hole 135-841B

A felsic volcanic series (605-825 mbsf) overlain by upper Eocene shallow-water sediments (500-605 mbsf) and basalticandesitic sills that intruded into sediments of Holocene to Miocene age (0-500 mbsf) was drilled in the forearc region of the Lau Basin at a water depth of 4810 m. The volcanic sequence at Site 841 includes altered and mineralized calc-alkaline rhyolites and dacites, dacitic tuffs, lapilli tuffs, flow breccias, and welded tuffs. These rocks formed subaerially or in a very shallow-water environment suffering a subsidence of >5000 m since Eocene times. Calculations of gains and losses of the major components during alteration show most pronounced changes in the uppermost 70 m of the volcanic sequence. Here, Al, Fe, Mg, and K are enriched, whereas Si and Na are strongly depleted. Illite, vermiculite, chlorite, and hematite predominate in this part of the hole. Throughout the section, quartz, plagioclase, kaolinite, and calcite are present. Sulfide mineralization (up to 10 vol%) consisting mainly of disseminated pyrite (with minor pyrrhotite inclusions) and marcasite together with minor amounts of chalcopyrite is pervasive throughout. Locally, a few sulfide-bearing quartz-carbonate veins as well as Ti-amphibole replacement by rutile and then by pyrite were observed. Strong variations in the As content of sulfides (from 0 to 0.69 wt%) from the same depth interval and local enrichments of Co, Ni, and Cu in pyrite are interpreted to result from fluctuations in fluid composition. Calculations of oxygen and sulfur fugacities indicate that fO2 and fS2 were high at the top and lower at the bottom of the sequence. Sulfur isotope determinations on separated pyrite grains from two samples give d34S values of +6.4ë and +8.4ë, which are close to those reported from Kuroko and Okinawa Trough massive sulfide deposits and calc-alkaline volcanic rocks of the Japanese Ryukyu Island Arc. Calculated chlorite formation temperatures of 265°-290°C at the top of the sequence are consistent with minimum formation temperatures of fluid inclusions in secondary quartz, revealing a narrow range of 270°-297°C. Chlorite formation temperatures are constant downhole and do not exceed 300°C. The presence of marcasite and 4C-type pyrrhotite indicates a formation temperature of <= 250°C. At a later stage, illite was formed at the top of the volcanic series at temperatures well below 200°C.

Magnetic properties of basalts and sediments of ODP Leg 135 sites

Paleomagnetic and rock-magnetic investigations of basalts from Hole 834B in the Lau backarc basin and of sediments from Holes 841A and 841B at the Tonga Ridge are reported. Three groups of blocking temperatures in the basalts suggest the presence of at least three magnetic phases: pure magnetite, a Ti-poor titanomagnetite, and a Ti-rich phase. The drill-string-induced remanence in the basalts is typically between three and six times the original normal remanent magnetization intensity, but it is mostly removed by alternating-field (AF) cleaning in 5 mT. Volume susceptibility values range from 0.04 * 10**-3 to 4 * 10**-3 cgs. The modified Q-ratio J5/sus ranges from 0.5 to 10. The drill-string-induced remanence behaves different in the two sediment cores from Holes 841A and 841B, which may be the result of differences in the sediment or caused by the different drilling equipment used. The AF-cleaned inclinations of the sediment in Holes 841A and 841B suggest a slight flattening with increasing depth (up to 6° under a load of 400 m of sediment) to be present. This flattening is likely to be caused by the differential rotation of detrital particles under compaction during diagenesis.

Late Neogene biostratigraphy of ODP Leg 135 sites

Integration of biostratigraphic and magnetostratigraphic results from Leg 135 sites has given additional information as to the position and reliability of various bioevents compared with previously published results. Two sites (834: Gilbert to Brunhes; and 836: Brunhes) provided excellent magnetic and biostratigraphic data. From these it is suggested that some bioevents are older than previously recorded: the first appearances (FAs) of Emiliania huxleyi (within the Brunhes Chron, at the same level as the FA of Helicosphaera inversa) and Globorotalia (Truncorotalia) truncatulinoides (within the upper Gauss Chron), and the last appearance (LA) of Gr. (Tr.) tosaensis (upper Matuyama Chron). The FA of Gr. (Tr.) crassaformis hessi is variable, but the oldest occurrence is just below the Cobb Mountain Subchron. Other key bioevents, such as the LAs of Discoaster pentaradiatus (just above the Réunion Subchron), D. tamalis (within the lower reversed part of the Matuyama Chron), Sphenolithus (lower Gauss Chron), and Amaurolithus primus (topmost Gilbert Chron) appear higher than previously recorded. Some key biostratigraphic taxa, such as Globigerinoides quadrilobatus fistulosus, Pulleniatina finalis, P. primalis, and Sphaeroidinella dehiscens, are either rare or their distribution is sporadic to the extent that they are unsuitable for biostratigraphic use in the area studied. Because of the rarity of P. primalis, the FA of Globorotalia (Globorotalia) multicamerata has been used to mark the base of Zone N17B. Though levels are present at most sites in which populations of Pulleniatina are sinistrally coiled, it is difficult to equate these coiling changes with previous records.

Organic geochemistry of ODP Leg 135 sediment samples

The average total organic carbon (TOC) content obtained after Rock-Eval/TOC analysis of 156 sediment samples from the eight sites cored during Leg 135 is 0.05%. Hence, the TOC content of Leg 135 sediments is extremely low. The organic matter that is present in these samples is probably mostly reworked and oxidized material. Ten sediment samples were selected for extraction and analysis by gas chromatography and gas chromatography-mass spectrometry. Very low amounts of extractable hydrocarbons were obtained and some aspects of the biomarker distributions suggest that these hydrocarbons are not representative of the organic matter indigenous to the samples. A sample of an oil seep from Pili, Tongatapu was also analyzed. The seep is a biodegraded, mature oil that shows many characteristics in common with previously published analyses of oil seeps from Tongatapu. Biomarker evidence indicates that its source is a mature, marine carbonate of probable Late Cretaceous-Early Tertiary age. The source rock responsible for the Tongatapu oil seeps remains unknown.

Geochemistry of reduction haloes in ODP Holes 135-834A and 135-837A

The sediment column overlying basement in the Lau Basin consists of a sequence of volcaniclastic turbidites interbedded with hemipelagic clayey nannofossil mixed sediments, overlain in turn by a sequence of hemipelagic clayey nannofossil oozes containing sporadic calcareous turbidites. The clayey nannofossil oozes and mixed sediments are pervasively stained by hydrothermally derived iron and manganese oxyhydroxides. Sharply defined, lighter colored bands occur in the hemipelagic sediments, immediately beneath some (but by no means all) volcaniclastic and calcareous turbidites. These are identified as reduction haloes, of a type previously identified in quite different turbidite/pelagic sequences. The haloes are attributed to the burial of labile surficial Corg by turbidites, followed by the remineralization of this Corg with Mn and Fe oxyhydroxides as electron acceptors. The resultant characteristic Mn and Fe concentration/depth profiles are described, and a model is proposed for their development. The color alteration of the halo is ascribed to the removal of Mn oxyhydroxides, because, although the Fe content fluctuates through the haloes, this does not appear to affect their color. Other elements (Co, Cu, and Ni) are also at low concentration levels in the haloes like Mn, consistent with remobilization and migration out of the halo section, although the profile shapes are not identical with those of Mn. The behavior of V is distinctive in that it appears to have migrated into the haloes to be enriched there. Haloes are unlikely to form if turbidite emplacement is erosive and removes the near-surface layer, which generally is the most fluid part of the sediment and contains the highest levels of reactive Corg to drive the reduction process. Conversely, the presence of a halo implies that emplacement of the overlying turbidite did not significantly erode the pre-existing sediment/water interface.