Chemistry of fluid inclusions in rock samples from Dronning Maud Land

Dense, CO2-rich fluid inclusions hosted by plagioclases, An45 to An54, of the O.-v.-Gruber- Anorthosite body, central Dronning Maud Land, East Antarctica, contain varying amounts of small calcite, paragonite and pyrophyllite crystals detected by Raman microspectroscopy. These crystals are reaction products that have formed during cooling of the host and the original CO2-rich H2O-bearing enclosed fluid. Variable amounts of these reaction products illustrates that the reaction did not take place uniformly in all fluid inclusions, possibly due to differences in kinetics as caused by differences in shape and size, or due to compositional variation in the originally trapped fluid. The reaction albite + 2anorthite + 2H2O + 2CO2 = pyrophyllite + paragonite + 2calcite was thermodynamically modelled with consideration of different original fluid compositions. Although free H2O is not detectable in most fluid inclusions, the occurrence of OH-bearing sheet silicates indicates that the original fluid was not pure CO2, but contained significant amounts of H2O. Compared to an actual fluid inclusion it is obvious, that volume estimations of solid phases can be used as a starting point to reverse the retrograde reaction and recalculate the compositional and volumetrical properties of the original fluid. Isochores for an unmodified inclusion can thus be reconstructed, leading to a more realistic estimation of P-T conditions during earlier metamorphic stages or fluid capturing.

Chemistry and fluid inclusions of the sheeted dike comples of ODP Holes 137-504B and 140-504B

Fluid inclusions in variably altered diabase recovered from Ocean Drilling Program Legs 137 and 140 at Hole 504B, Costa Rica Rift, exhibit fluid salinities up to 3.7 times that of seawater values (11.7 wt% NaCl equivalent) and exhibit uncorrected homogenization temperatures of 125°C to 202°C. The liquid-dominated inclusions commonly are entrapped in zones of secondary plagioclase and may be primary in origin. Fluid salinities are similar to compositions of fluids venting on the seafloor (0.4-7.0 wt% NaCl) and overlap with those measured in metabasalt samples recovered from near the Kane Fracture Zone on the Mid-Atlantic Ridge and from the Troodos ophiolite, Cyprus. The salinity variations may reflect hydration reactions involving formation of secondary mineral assemblages under rock-dominated conditions, which modify the ionic strength of hydrothermal fluids by consuming or liberating water and chloride ion. Rare CO2-CH4-bearing inclusions, subjacent to zones where talc after olivine becomes an important secondary mineral phase (1700 mbsf), may have formed due to local interaction of seawater and olivine at low water to rock ratios. Corrected average fluid inclusion homogenization temperatures exhibit a gradient from 159°C at a depth of 1370 mbsf to 183°C at a depth of 1992 mbsf and are in apparent equilibrium with the present conductive downhole temperatures. These data indicate that fluid inclusions may be used to estimate downhole temperatures if logging data are unavailable. The compositional and thermal evolution of the diabase-hosted fluids may reflect late-stage, off-axis circulation and conductive heating of compositionally modified seawater in the sheeted dike complex at Hole 504B.

Geochemical composition and crystallization temperatures of ODP Hole 176-735B gabbroic rocks

The transition from magmatic crystallization to high-temperature metamorphism in deep magma chambers (or lenses) beneath spreading ridges has not been fully described. High-temperature microscopic veins found in olivine gabbros, recovered from Ocean Drilling Program Hole 735B on the Southwest Indian Ridge during Leg 176, yield information on the magmatic-hydrothermal transition beneath spreading ridges. The microscopic veins are composed of high-temperature minerals, (i.e., clinopyroxene, orthopyroxene, brown amphibole, and plagioclase). An important feature of these veins is the 'along-vein variation' in mineralogy, which is correlated with the magmatic minerals that they penetrate. Within grains of magmatic plagioclase, the veins are composed of less calcic plagioclase. In grains of olivine, the veins are composed of orthopyroxene + brown amphibole + plagioclase. In clinopyroxene grains, the veins consist of plagioclase + brown amphibole and are accompanied by an intergrowth of brown amphibole + orthopyroxene. The mode of occurrence of the veins cannot be explained if these veins were crystallized from silicate melts. Consequently, these veins and nearby intergrowths were most likely formed by the reaction of magmatic minerals with fluid phases under the conditions of low fluid/rock ratios. Very similar intergrowths of brown amphibole + orthopyroxene are observed in clinopyroxene grains with 'interfingering' textures. It is believed, in general, that the penetration of seawater does not predate the ductile deformation within Layer 3 gabbros of the slow-spreading ridges. If this is the case, the fluid responsible for the veins did not originate from seawater because the formation of the veins and the interfingering textures preceded ductile deformation and, perhaps, complete solidification of the gabbroic crystal mush. It has been proposed, based on fluid inclusion data, that the exsolution of fluid from the latest-stage magma took place at temperatures >700°C in the slow-spreading Mid-Atlantic Ridge at the Kane Fracture Zone (MARK) area. No obvious mineralogical evidence, however, has been found for these magmatic fluids. The calculated temperatures for the veins and nearby intergrowths found in Hole 735B gabbros are up to 1000°C, and these veins are the most plausible candidate for the mineralogical expression of the migrating magmatic fluids.

Characterization of diagenetic fluids at ODP Site 103-639

Site 639, drilled during Leg 103 of the Ocean Drilling Program, penetrated an Upper Jurassic to Lower Cretaceous carbonate platform on a tilted fault block along the Galicia margin off the northwest Iberian Peninsula. The carbonate platform is composed primarily of a sequence of dolomite overlying limestone. Samples were analyzed for mineral chemistry, stable isotope geochemistry, fluid inclusion microthermometry, and volatile contents and by dolomite pyrolysis mass spectrometry for trace sulfate minerals. The dolomite recovered from the Galicia margin at Site 639 formed during shallow burial from sulfate-bearing, hypersaline brines at slightly elevated temperatures. The light oxygen isotopic signatures of the dolomite are interpreted as the result of the evaporative loop and slightly elevated temperatures during dolomite formation or from reequilibration at higher temperatures during deeper burial. The hypersalinity is interpreted to be associated with a nearby, shallow restricted basin that formed during rifting of the Iberian margin from Newfoundland. The dolomitization of the platform is therefore a by-product of the rifting.

(Table 1) Geochemical and geothermometry data from fibrous calcite vein fills in the Dabashan Fold Belt, China

Fibrous calcite veins with organic inclusions have been widely considered as indicators of oil and gas generation and migration under overpressure. Abundant fibrous calcite veins containing organic-bearing inclusions occur in faulted Lower Paleozoic through Triassic hydrocarbon source rocks in the Dabashan Foreland Belt (DBF). d13CPDB and d18OPDB values of the fibrous calcite range from - 4.8 to -1.9 to per mil and - 12.8 to - 8.4 per mil respectively, which is lighter than that of associated carbonate host rocks ranging from - 1.7 to + 3.1 per mil and - 8.7 to - 4.5 per mil. A linear relationship between d13CPDB and d18OPDB indicates that the calcite veins were precipitated from a mixture of basinal and surface fluids. The fibrous calcite contains a variety of inclusions, such as solid bitumen, methane bearing all-liquid inclusions, and vapor-liquid aqueous inclusions. Homogenization temperatures of aqueous inclusions range from 140 to 196° with an average of 179°. Salinities of aqueous inclusions average 9.7 wt% NaCl. Independent temperatures from bitumen reflectance and inclusion phase relationships of aqueous and methane inclusions were used to determine fluid pressures. Results indicate high pressures, elevated above typical lithostatic confining pressure, from 150 to 200 MPa. The elevated salinity and high temperature and pressure conditions of the fibrous calcite veins argue against an origin solely from burial overpressure resulting from clay transformation and dehydration reactions. Instead fluid inclusion P-T data and geochemistry results and regional geology indicate abnormally high pressures during fluid migration. These findings indicate that tectonic stress generated fracture and fault fluid pathways and caused migration of organic bearing fluids from the DBF during the Yanshan orogeny.

Geochemistry of vein rocks of ODP Hole 118-735B

Rock samples from Hole 735B, Southwest Indian Ridge, were examined to determine the principal vein-related types of alteration that occurred, the nature of fluids that were present, and the temperatures and pressures of these fluids. Samples studied included veined metagabbro, veined mylonitic metagabbro, felsic trondhjemite, and late-stage leucocratic diopside-bearing veins. The methods used were standard petrographic analysis, mineral chemical analysis by electron microprobe, fluid inclusion petrography and analysis by heating/freezing techniques and laser Raman microspectroscopy, and oxygen isotopic analyses of mineral separates. Alteration in lithologic Units I and II (above the level of Core 118-735B-3OR; approximately 140 meters below the seafloor) is dominated by hydration by seawater-derived fluids at high temperature, up to about 700°C, and low water/rock ratio, during and immediately after pervasive ductile deformation. Below Core 118-735B-30R, pervasive deformation is less common, and brittle veining and brecciation are the major alteration styles. Leucocratic centimeter-scale veins, often containing diopside and plagioclase, were produced by interaction of hot (about 500°C) seawater-derived fluid and gabbro. The water/rock ratio was locally high at the veins and breccia zones, but the integrated water/rock ratio for the lower part of the hole is probably low. Accessory hydrous magmatic or deuteric phases formed from magmatic volatiles in some gabbro and in trondhjemite. Most subsequent alteration was affected by fluids that were seawater-derived, based on isotopic and chemical analyses of minerals and analyses of fluid inclusions. Many early-generation fluid inclusions, associated with high-temperature veining, contain appreciable methane as well as saline water. The source of methane is unclear, but it may have formed as seawater was reduced during low water/rock interaction with ultramafic upper mantle or ultramafic and mafic layer 3. Temperatures of alteration were calculated on the basis of coexisting mineral chemistry and isotopic values. Hydrothermal metamorphism commenced at about 720°C and continued to about 550°C. Leucocratic veining took place at about 500°C. Alteration within brecciated horizons was also at about 500° to less than 400°C, and the trondhjemite was altered at about 550° to below 490°C. Pressures calculated from a diopside-bearing vein, based on a combination of fluid inclusion and isotopic analysis, were 90 to 100 MPa. This pressure places the sample, from Core 118-735B-70R in Unit V, at about 2 km below the seafloor.

Strontium, oxygen and hydrogen isotope ratios of basalts from DSDP Hole 504B

DSDP Hole 504B was drilled into 6 Ma crust, about 200 km south of the Costa Rica Rift, Galapagos Spreading Center, penetrating 1.35 km into a section that can be divided into four zones-Zone I: oxic submarine weathering; Zone II: anoxic alteration; Zones III and IV: hydrothermal alteration to greenschist facies. In Zone III there is intense veining of pillow basalts. Zone IV consists of altered sheeted dikes. Isotopic geochemical signatures in relation to the alteration zones are recorded in Hole 504B, as follows: Zone Depth(m) Average87Sr/86Sr Average delta18O (?) Average deltaD (?) I 275-550 0.7032 7.3 -63 II 550-890 0.7029 6.5 -45 III 890-1050 0.7035 5.6 -31 IV 1050-1350 0.7032 5.5 -36 Alteration temperatures are as low as 10°C in Zones I and II based on oxygen isotope fractionation. Strontium isotopic data indicate that a circulation of seawater is much more restricted in Zone II than in Zone I. Fluid inclusion measurements of vein quartz indicate the alteration temperature was mainly 300 +/- 20°C in Zones III and IV, which is consistent with secondary mineral assemblages. The strontium, oxygen, and hydrogen isotopic compositions of hydrothermal fluids which were responsible for the greenschist facies alteration in Zones III and IV are estimated to be 0.7037, 2?, and 3?, respectively. Strontium and oxygen isotope data indicate that completely altered portions of greenstones and vein minerals were in equilibrium with modified seawater under low water/rock ratios (in weight) of about 1.6. This value is close to that of the end-member hydrothermal fluids issuing at 21°N EPR. Basement rocks are not completely hydrothermally altered. About 32% of the greenstones in Zones III and IV have escaped alteration. Thus 1 g of fresh basalt including the 32% unaltered portion are required in order to make 1 g of end-member solution from fresh seawater in water-rock reactions.

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.

Helium and oxygen isotope ratios of ODP Sites 193-1188 and 193-1189

Fluid mixing processes and thermal regimes within the Snowcap and Roman Ruins vent sites of the PACMANUS hydrothermal system, Papua New Guinea, were investigated using 3He/4He ratios from fluid inclusions within pyrite and anhydrite and the d18O signature of anhydrite. Depressed 3He/4He ratios of 0.2-6.91RA appear to be caused by significant atmospheric diffusive exchange, whilst He-Ne diffusive fractionation precludes correction using 20Ne. 40Ar/36Ar ratios of 295-310 are elevated above seawater, indicating the majority of argon is seawater derived but with a magmatic component. d18O anhydrite ratios are 6.5 per mil to 11 per mil for Snowcap and 6.4 per mil to 11.9 per mil for Roman Ruins. Using oxygen isotope fractionation factors for the anhydrite-water system, the temperatures calculated assuming isotopic equilibrium at depth are up to 100 °C cooler than fluid inclusion trapping temperatures. It is likely that anhydrite is precipitated rapidly, preventing d18O equilibration. By comparing new d18O values for anhydrite with corresponding published 87Sr/86Sr ratios, seawater is inferred to penetrate deep into the Snowcap system with little conductive heating. A simple fluid mixing model has been constructed whereby the differing venting styles can be explained by a plumbing system at depth which favors delivery of end-member hydrothermal fluid to the high temperature sites.