(Table T1) Strontium isotopic composition and ages of limestone samples from ODP Hole 180-1118A

Limestone from Unit VI (857.1-859.15 meters below seafloor) collected at Site 1118 contains a planktonic foraminiferal fauna indicating a latest Miocene to early Pliocene age. Globorotalia tumida is recorded in Sample 180-1118A-68R-4, 46-48 cm, indicating an early Pliocene (N18) age at this level (J. Resig, pers. comm., 2000). Based on their known range in the western Papuan Basin, the presence of (rare) Lepidocyclina and common Amphistegina in some samples suggests that abundant shallow-water bioclastic debris present in these limestones may be reworked from older, possibly middle or early late Miocene sediments. Four samples were selected for whole-rock strontium isotopic analysis to further investigate this possibility. A petrographic examination of samples analyzed was also conducted.

Mineral, chemical and isotopic compositions of altered rocks at ODP Sites 193-1188 and 193-1189

During ODP Leg 193, 4 sites were drilled in the active PACMANUS hydrothermal field on the crest of the felsic Pual Ridge to examine the vertical and lateral variations in mineralization and alteration patterns. We present new data on clay mineral assemblages, clay and whole rock chemistry and clay mineral strontium and oxygen isotopic compositions of altered rocks from a site of diffuse low-temperature venting (Snowcap, Site 1188) and a site of high-temperature venting (Roman Ruins, Site 1189) in order to investigate the water-rock reactions and associated elemental exchanges. The volcanic succession at Snowcap has been hydrothermally altered, producing five alteration zones: (1) chlorite+/-illite-cristobalite-plagioclase alteration apparently overprinted locally by pyrophyllite bleaching at temperatures of 260-310°C; (2) chlorite+/-mixed-layer clay alteration at temperatures of 230°C; (3) chlorite and illite alteration; (4) illite and chlorite+/-illite mixed-layer alteration at temperatures of 250-260°C; and (5) illite+/-chlorite alteration at 290-300°C. Felsic rocks recovered from two holes (1189A and 1189B) at Roman Ruins, although very close together, show differing alteration features. Hole 1189A is characterized by a uniform chlorite-illite alteration formed at ~250°C, overprinted by quartz veining at 350°C. In contrast, four alteration zones occur in Hole 1189B: (1) illite+/-chlorite alteration formed at ~300°C; (2) chlorite+/-illite alteration at 235°C; (3) chlorite+/-illite and mixed layer clay alteration; and (4) chlorite+/-illite alteration at 220°C. Mass balance calculations indicate that the chloritization, illitization and bleaching (silica-pyrophyllite assemblages) alteration stages are accompanied by different chemical changes relative to a calculated pristine precursor lava. The element Cr appears to have a general enrichment in the altered samples from PACMANUS. The clay concentrate data show that Cr and Cu are predominantly present in the pyrophyllites. Illite shows a significant enrichment for Cs and Cu relative to the bulk altered samples. Considerations of mineral stability allow us to place some constraints on fluid chemistry. Hydrothermal fluid pH for the chloritization and illitization was neutral to slightly acidic and relatively acidic for the pyrophyllite alteration. In general the fluids, especially from Roman Ruins and at intermediate depths below Snowcap, show only a small proportion of seawater mixing (<10%). Fluids in shallow and deep parts of the Snowcap holes, in contrast, show stronger seawater influence.

Mineral and chemical composition of hydrothermally altered rocks and sediments from the Middle Valley, ODP Leg 139 data

The Middle Valley segment at the northern end of the Juan de Fuca Ridge is a deep extensional rift blanketed with 200-500 m of Pleistocene turbiditic sediment. Sites 857 and 858 were drilled during Ocean Drilling Program Leg 139 to determine whether these two sites were hydrologically linked end members of an active hydrothermal circulation system. Site 858 was placed in an area of active hydrothermal discharge with fluids up to 270°C venting through anhydrite-bearing mounds on top of altered sediment. The shallow basement of fine-grained basalt that underlies the vents at Site 858 is interpreted as a seamount that was subsequently buried by turbidites. Site 857 was placed 1.6 km south of the Site 858 vents in a zone of high heat flow and numerous seismically imaged ridge-parallel faults. Drilling at Site 857 encountered sediments that are increasingly altered with depth and that overlie a series of mafic sills at depths of 460-940 m below sea floor. Sill margins and adjacent baked sediment are highly altered to magnesian chlorite and crosscut with veins filled with quartz, chlorite, sulfides, epidote, and wairakite. The sill interiors vary from slightly altered, with unaltered plagioclase and clinopyroxene in a mesostasis replaced by chlorite, to local zones of intense alteration and brecciation. In these latter zones, the sill interiors are pervasively replaced by chlorite, epidote, quartz, pyrite, titanite, and rare actinolite. The most complete replacement is associated with brecciated horizons with low recovery and slickensides on fracture surfaces, which we interpret as intersections between faults and the sills. Geochemically, the alteration of the sill complex is reflected in significant whole-rock depletions in Ca, Sr, and Na with corresponding enrichments in Mg, Al, and most metals. The latter results from the formation of conspicuous sulfide poikiloblasts. In contrast, metamorphism of the Site 858 seamount includes incomplete albitization of plagioclase phenocrysts and replacement of sparse mafic phenocrysts. Much of the basement alteration at Site 858 is confined to crosscutting veins except for a highly altered and veined horizon at the contact between basaltic basement and the overlying sediment. The sill complex at Site 857 is more highly depleted in 18O (d18O = 2.4 per mil - 4.7 per mil) and more pervasively replaced by secondary minerals relative to the extrusives at Site 858 (d18O = 4.5 per mil - 5.5 per mil). There is no evidence of significant albitization of the plagioclase at Site 857, suggesting high Ca/Na in the pore fluids. Fluid-inclusion data from hydrothermal minerals in altered mafic rocks and veins at Sites 857 and 858 show a consistency of homogenization temperatures, varying from 245 to 270°C, which is within the range of temperatures observed for the fluids venting at Site 858. The consistency of the fluid inclusion temperatures, the lack of albitization within the Site 857 sills, and the apparently low water/rock ratio collectively suggest that the sill complex at Site 857 is in thermal equilibrium and being altered by a highly evolved Ca-rich fluid similar to the fluids now venting at Site 858. The alteration evident in these two deep crustal drillsites is a result of the ongoing hydrothermal circulation and is consistent with downhole logging results, instrumented borehole results, and hydrothermal fluid chemistry. The pervasive alteration of the laterally extensive sill-sediment complex at Site 857 determines the chemistry of the fluids that are venting at Site 858. The limited alteration of the Site 858 lavas suggests that this basement edifice acts as a penetrator or ventilator for the regional hydrothermal reservoir with much of the flow focussed at the highly altered and veined sediment-basalt contact.

CaCO3, organic carbon, and nitrogen at DSDP Hole 93-603B

Contents of organic carbon and carbonate carbon were determined on the same set of Cretaceous samples from DSDP Hole 603B in three different laboratories in order to assess the degree of comparability of organic carbon and carbonate values obtained by different labs using the same or different methods. We report the results of analyses for organic carbon using two different CHN analyzers, LECO, and Rock-Eval II and for carbonate carbon by CHN (total C minus C after acidification), the carbonate bomb technique, and CaCO3 calculated on the basis of total calcium obtained from X-ray fluorescence and induction-coupled plasma techniques. In addition, total nitrogen was obtained by two different labs using a CHN analyzer, but different bases for calculation were used. The various techniques for organic carbon analysis yielded comparable results, with the exception of those obtained by one of the CHN analyses of acid-treated samples. The calculation of organic carbon values and comparison on a whole-rock basis is very sensitive to errors in determination of carbonate contents, and this factor explains most, but not all, of the disparities between the data sets. The carbonate bomb technique gives CaCO3 values that correspond well with those calculated from total calcium concentrations (XRF and ICP analyses), whereas the CaCO3 calculated from CHN total carbon minus acid-soluble carbon consistently overestimated CaCO3. Total nitrogen and C/N results from the two different CHN analyses are not comparable and are subject to more error than the factor related to error in estimation of CaCO3.

Lithologic, mineral, chemical and isotopic compositions of altered rocks with increasing distance across the eastern flank of the Juan de Fuca Ridge, ODP Leg 168 data

During ODP Leg 168, 10 sites were drilled across the eastern flank of the Juan de Fuca Ridge (JdFR), to examine the conditions of fluid-rock interaction in three distinct hydrothermal regimes (referred to as the Hydrothermal Transition (HT), Buried Basement (BB) and Rough Basement (RB) transects), extending over a ~120 km linear transect perpendicular to the spreading ridge. This was carried out in an attempt to constrain the conditions and processes that control the location, style and magnitude of low temperature (<150°C) fluid-rock interaction within this setting. This paper presents new data on the petrology, mineral chemistry and whole rock strontium and oxygen isotopic compositions of basalts from the eastern flank of the JdFR, in order to investigate the extent, style and sequence of low-temperature hydrothermal alteration and to establish how the hydrothermal regime evolved with time. Throughout the flank, a progressive sequence of low-temperature hydrothermal alteration has been identified, marked by changes in the dominant secondary mineral assemblage, changing from: chlorite+chlorite/smectite; to iron oyxhydroxide+celadonite; to saponite+/-pyrite; culminating at present with Ca- to CaMg(+/-Fe,Mn)-carbonate. The changes in secondary mineralogy have been used to infer a series of systematic shifts in the conditions of alteration that occurred as the basement moved off-axis and was progressively buried by sediment. In general, hydrothermal alteration of the uppermost oceanic crust commenced under open, oxidative conditions, with interaction between unmodified to slightly modified seawater and basaltic crust, to a regime in which circulation of a strongly modified seawater-derived fluid was more restricted, and alteration occurred under non-oxidative conditions. Across the flank, petrological observations and microprobe analyses indicate that the observed ranges in secondary mineral composition are directly related to changes in the geochemical and textural characteristics of the basement, as well as to interaction between fluids and phases from the four stages of alteration. This is suggestive of an increase in fluid-rock increased with time. Whole rock 87Sr/86Sr and d18O analyses of basalts from across the eastern flank of the JdFR reinforce petrological observations, with 87Sr/86Sr and d18O values slightly elevated above accepted pristine MORB values for this region. These results are consistent with an increase in the amount of fluid-rock interaction with time. Across the flank, enrichment in the 87Sr/86Sr and d18O relative to MORB, is influenced by a number of factors, including: local and regional variations in the crustal lithology and structure; the age of the crust; the extent of bulk rock alteration; and theoretically, the relative abundance of different isotopically-enriched secondary mineral phases in the crust.

Mineral an chemical composition of basalts from DSDP Sites 417 and 418, West Atlantic Ocean

Major element composition ranges of closely associated basalt glass-whole rock pairs from individual small cooling units approach the total known range of basalt glass and whole rock compositions at IPOD sites 417 and 418. The whole rock samples fall into two groups: one is depleted in MgO and distinctly enriched in plagioclase but has lost some olivine and/or pyroxene relative to its corresponding glass; and the other is enriched in MgO and in phenocrysts of olivine and pyroxene as well as plagioclase compared to its corresponding glass. By analogy with observed phenocryst distributions in lava pillows, tubes, and dikes, and with some theoretical studies, we infer that bulk rock compositions are strongly affected by phenocryst redistribution due to gravity settling, flotation, and dynamic sorting after eruption, although specific models are not well constrained by the one-dimensional geometry of drill core. Compositional trends or groupings in whole rock data resulting from such late-stage processes should not be confused with more fundamental compositional effects produced in deep chambers or during partial melting.