Chemistry of minerals from ODP Leg 127/128 basalts

Suites of basalts drilled during Legs 127 and 128 can be distinguished by their mineral assemblages and compositions of phenocrysts and groundmass phases. An upper suite of plagioclase phyric basaltic sills with a groundmass composed of plagioclase, augite, and magnetite was recovered from Site 794. The upper, evolved part of this suite is highly plagioclase phyric, including calcic plagioclases (~An90). The most primitive, lower part of this upper suite, in addition, contains olivine, but lacks calcic plagioclase. A lower suite at Site 794 is plagioclase and olivine phyric to aphyric basaltic sills and flows with a groundmass of plagioclase, augite, olivine (~Fo75-83), and magnetite. At Site 795, plagioclase and augite phyric basalts and andesites were recovered. The relatively low Ti and Cr contents of augite of these basalts suggest typical arc tholeiitic parental magmas. Two suites of basalt were recovered from Site 797, an upper suite of plagioclase and olivine phyric to aphyric olivine basalts, and a lower suite of evolved plagioclase phyric basaltic sills. The most evolved sills at both sites lack olivine as phenocryst and groundmass phases, while this phase is present in the relatively primitive sills. The olivine-bearing suites contain plagioclase with relatively low potassium content and augite with relatively high sodium content. An exception is the olivine-bearing sills of the upper suite at Site 794 that contains plagioclase with relatively high potassium content similar to the associated olivine-free sills. The olivine-free suites contain plagioclase with high potassium content and augite with low sodium content and have the most evolved compositions of any of the Japan Sea rocks.

Petrology and geochemistry of sedimentary Unit III of ODP Hole 135-840B

The sedimentary sequence recovered at Site 840, on the Tonga frontal-arc platform, is 597.3 m thick and is subdivided into three lithostratigraphic units. The lowermost, late Miocene Unit III is 336.8 m thick and consists of a sequence of volcaniclastic mass-flow deposits (predominantly turbidites) interbedded with pelagic/hemipelagic deposits. Unit III was deposited in the forearc basin of the Lau volcanic arc, probably on a slope dominated by mass flows that built eastward from the ridge front and across the forearc. Upward through the unit a thinning and fining of individual turbidites takes place, interpreted to reflect a reduced sediment supply and a change from large to smaller flows. Decreasing volcanic activity with time is inferred from a decrease in coarse-grained volcaniclastic content in the upper part of the unit. The majority of the turbidites show the typical Bouma-type divisions, although both high- and low-density turbidity currents are inferred. High-density turbidity currents were especially common in the lower part of the unit. Geochemical analyses of detrital glass lie mainly in the low-K tholeiite field with a compositional range from basalt to rhyolite. A coherent igneous trend indicates derivation from a single volcanic source. This source was probably situated on the rifted part of the Lau-Tonga Ridge, within the present Lau backarc basin. The initial opening of the Lau Basin may have been around 6.0 m.y. ago. The onset of more extensive rifting, approximately 5.6 m.y. ago, is reflected in an increase in the silica content of volcanic glass. At the boundary toward Unit II, at approximately 5.25 Ma, an influx of thicker bedded and coarser grained volcaniclastic material is interpreted to reflect increasing volcanism and tectonism during the final breakup of the Lau-Tonga Ridge.

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.

Chemistry of igneous rocks from, the Celebes Sea basement

Major and trace elements, mineral chemistry, and Sr-Nd isotope ratios are reported for representative igneous rocks of Ocean Drilling Program Sites 767 and 770. The basaltic basement underlying middle Eocene radiolarianbearing red clays was reached at 786.7 mbsf and about 421 mbsf at Sites 767 and 770, respectively. At Site 770 the basement was drilled for about 106 m. Eight basaltic units were identified on the basis of mineralogical, petrographical, and geochemical data. They mainly consist of pillow lavas and pillow breccias (Units A, B, D, and H), intercalated with massive amygdaloidal lavas (Units Cl and C2) or relatively thin massive flows (Unit E). Two dolerite sills were also recognized (Units F and G). All the rocks studied show the effect of low-temperature seafloor alteration, causing almost total replacement of olivine and glass. Calcite, clays, and Fe-hydroxides are the most abundant secondary phases. Chemical mobilization due to the alteration processes has been evaluated by comparing elements that are widely considered mobile during halmyrolysis (such as low-field strength elements) with those insensitive to seafloor alteration (such as Nb). In general, MgO is removed and P2O5 occasionally enriched during the alteration of pillow lavas. Ti, Cs, Li, Rb, and K, which are the most sensitive indicators of rock/seawater interaction, are generally enriched. The most crystalline samples appear the least affected by chemical changes. Plagioclase and olivine are continuously present as phenocrysts, and clinopyroxene is confined in the groundmass. Textural and mineralogical features as well as crystallization sequences of Site 770 rocks are, in all, analogous to typical mid-ocean-ridge basalts (MORBs). Relatively high content of compatible trace elements, such as Ni and Cr, indicate that these rocks represent nearly primitive or weakly fractionated MORBs. All the studied rocks are geochemically within the spectrum of normal MORB compositional variation. Their Sr/Nd isotopic ratios plot on the mantle array (87Sr/87Sr 0.70324-0.70348 with 143Nd/144Nd 0.51298-0.51291) outside the field of Atlantic and Pacific MORBs. However, Sr and Nd isotopes are typical of both Indian Ocean MORBs and of some back-arc basalts, such as those of Lau Basin. The mantle source of Celebes basement basalts does not show a detectable influence of a subduction-related component. The geochemical and isotopic data so far obtained on the Celebes basement rocks do not allow a clear discrimination between mid-ocean ridge and back-arc settings.

Iridium contents and microprobe analyses of DSDP Leg 77 samples

Restudy of Deep Sea Drilling Project Sites 536 and 540 in the southeast Gulf of Mexico gives evidence for a giant wave at Cretaceous-Tertiary boundary time. Five units are recognized: (1) Cenomanian limestone underlies a hiatus in which the five highest Cretaceous stages are missing, possibly because of catastrophic K-T erosion. (2) Pebbly mudstone, 45 m thick, represents a submarine landslide possibly of K-T age. (3) Current-bedded sandstone, more than 2.5 m thick, contains anomalous iridium, tektite glass, and shocked quartz; it is interpreted as ejecta from a nearby impact crater, reworked on the deep-sea floor by the resulting tsunami. (4) A 50-cm interval of calcareous mudstone containing small Cretaceous planktic foraminifera and the Ir peak is interpreted as the silt-size fraction of the Cretaceous material suspended by the impact-generated wave. (5) Calcareous mudstone with basal Tertiary forams and the uppermost tail of the Ir anomaly overlies the disturbed interval, dating the impact and wave event as K-T boundary age. Like Beloc in Haiti and Mimbral in Mexico, Sites 536 and 540 are consistent with a large K-T age impact at the nearby Chicxulub crater.

Chemistry of gabbroic rocks of ODP Hole 118-735B

We present results of a microprobe investigation of fresh and least-deformed and metamorphosed gabbroic rocks from Leg 118, Hole 735B, drilled on the east side of the Atlantis II Fracture Zone, Southwest Indian Ridge. This rock collection comprises cumulates ranging from troctolites to olivine-gabbro and olivine-gabbronorite to ilmenite-rich ferrogabbros and ferrogabbronorites. As expected, the mineral chemistry is variable and considerably expands the usual oceanic reference spectrum. Olivine, plagioclase, and clinopyroxene are present in all the studied samples. Orthopyroxene and ilmenite, although not rare, are not ubiquitous. Olivine compositions range from Fo85 to Fo30, while plagioclase compositions vary from An70 to An27. Mg/(Mg + Fe2+) of clinopyroxene (mostly diopside to augite) varies from 0.88 to 0.54. Mg/(Mg + Fe2+) of orthopyroxene varies from 0.84 to 0.50. These minerals are not significantly zoned. All mineralogical data indicate that fractional crystallization is an important factor for the formation of cumulates. However, sharp contacts, interpreted as layering boundaries or intrusion margins, suggest polycyclic fractionation of several magma batches of limited volumes. Calculated compositions of magmas in equilibrium with the most magnesian mineral samples at the bottom of the hole represent fractionated liquids through separation of olivine, plagioclase, and clinopyroxene at moderate to low pressures (less than 9 kb). Crystallization of orthopyroxene and ilmenite occurs in the most differentiated liquids. Mixing of magmas having various compositions before entering the cumulate zone is another mechanism necessary to explain extremely differentiated iron-rich gabbros formed in this slow-spreading ridge environment.

Composition of minerals and rocks from the Markov Deep, Central Atlantic

The paper presents materials on composition and texture of weakly serpentinized ultrabasic rocks from the western and eastern walls of the Markov Deep (5°30.6'-5°32.4'N) in the rift valley of the Mid-Atlantic Ridge. Predominant harzburgites with protogranular and porphyroclastic textures contain two major generations of minerals: the first generation composes the bulk of rocks and consists of Ol_89.8-90.4 + En_90.2-90.8 + Di_91.8 + Chr (Cr#32.3-36.6, Mg#67.2-70.0), while the second generation composes very thin branching veinlets and consists of PlAn_32-47 + Ol_74.3-77.1 + Opx_55.7-71.9 + Cpx_67.5 + Amph_53.7-74.2 + Ilm. Syndeformational olivine neoblasts in recrystallization zones are highly magnesian. Concentrations and covariations of major elements in harzburgites indicate that these rocks are depleted in mantle residues (high Mg# of minerals and whole-rock samples and low in CaO, Al2O3, and TiO2) that are significantly enriched in trace HFSE and REE (Zr, Hf, Y, LREE, and all REE). Mineralogy and geochemistry of harzburgites were formed by interaction of mantle residues with hydrous, strongly fractionated melts that impregnated them. Mineral composition of veinlets in harzburgites and mineralogical-geochemical characteristics of related plagiogranites and gabbronorites suggest that these plagiogranites were produced by melt residuals after crystallization of gabbronorites. Modern characteristics of harzburgites were shaped by the following processes: (i) partial melting of mantle material simultaneously with its subsolidus deformations, (ii) brittle-plastic deformations associated with cataclastic flow and recrystallization, and (iii) melt percolation along zones of maximal stress relief and interaction of this melt with magnesian mantle residue.

Chemistry of orthopyroxenes, clinopyroxenes, and spinels of ODP Hole 103-637A peridotites

Mineral compositions of the plagioclase-bearing ultramafic tectonites dredged and cored seaward of the continental slope of the Galicia margin (Leg 103, Site 637) were compared to mineral compositions from onshore low-pressure ultramafic bodies (southeastern Ronda, western Pyrenees, and Lizard Point), on the basis of standardized (30-s counting time) probe analyses. The comparison was extended to some plagioclase-free harzburgites related to ophiolites (Santa Elena in Costa Rica, north Oman, and the Humboldt body in New Caledonia) on the basis of new analytical data and data from the literature. The behavior of Cr, Na, Al, Mg, Fe, Ni, and Ti in olivine, pyroxenes, and spinel was examined in order to distinguish between the effects of partial melting and mineral facies change, from the spinel to plagioclase stability fields. The peridotite from the Galicia margin appears slightly depleted in major incompatible elements and experienced a minor partial melting. However, it experienced large scale but heterogeneous recrystallization in the plagioclase field. These features are very similar to those observed in Ronda, whereas in the western Pyrenees the minerals exemplify a very minor partial-melting event (or none at all) and have retained compositions corresponding to those of the relatively high-pressure Seiland sub facies. The minerals from the Lizard Point peridotite have characteristics (low Mg/(Mg + Fe) ratio; high Cr/(Cr + Al) ratio in spinel) more related to cumulate from a differentiated tholeiitic melt than related to ophiolitic tectonite. Diffusion profiles of Al and Cr across pyroxenes and spinel show that recrystallization features occurred at different speeds or temperatures in the different bodies. The pyroxenes from Ronda would have experienced recrystallization about 14 times faster than the peridotite from the Galicia margin. The western Pyrenean lherzolites also experienced rapid recrystallization; nevertheless, because they are of a different mineral facies, the data are not directly comparable to that from Ronda and Galicia. The harzburgite at Santa Elena as well as a xenolith from alkali basalt exemplify rapid cooling characterized by very weak re-equilibration. Recrystallization speed is related to emplacement speed in the present geological environment. The slow-rising Galicia margin peridotite was emplaced by thinning of the lithospheric subcontinental mantle near an incipient mid-oceanic ridge. The fast-rising peridotites from Ronda and the western Pyrenees were hot diapirs emplaced from the asthenosphere along transcurrent faults, possibly related to the opening of the Atlantic Ocean.

Magnetic mineralogy and geochemistry of ODP Holes 137-504B and 140-504B samples

Leg 140 of the Ocean Drilling Program deepened Hole 504B to a total depth of 2000.4 m below seafloor (mbsf), making it the deepest hole drilled into ocean crust. Site 504, south of the Costa Rica Rift, is considered the most important in-situ reference section for the structure of shallow ocean crust. We present the results of studies of magnetic mineralogy and magnetic properties of Hole 504B upper crustal rocks recovered during Legs 137 and 140. Results from this sample set are consistent with those discussed in Pariso et al. (this volume) from Legs 111, 137, and 140. Coercivity (Hc) ranges from 5.3 to 27.7 mT (mean 12 mT), coercivity of remanence (HCR) ranges from 13.3 to 50.6 mT (mean 26 mT), and the ratio HCR/HC ranges from 1.6 to 3.19 (mean 2.13). Saturation magnetization (JS) ranges from 0.03 to 5.94 * 10**-6 Am**2, (mean 2.52 * 10**-6 Am**2), saturation remanence (JR) ranges from 0.01 to 0.58 * 10**-6 Am2 (mean 0.37 * 10**-6 Am**2), and the ratio JR/JS ranges from 0.08 to 0.29 (mean 0.16), consistent with pseudo-single-domain behavior. Natural remanent magnetization (NRM) intensity ranges from 0.029 to 7.18 A/m (mean 2.95 A/m), whereas RM10 intensity varies only from 0.006 to 4.8 A/m and has a mean of only 1.02 A/m. Anhysteretic remanent magnetization (ARM) intensity ranges from 0.04 to 6.0 A/m, with a mean of 2.46 A/m, and isothermal remanent magnetization (IRM) intensity ranges from 0.5 to 1683 A/m, with a mean of 430.7 A/m. Volume susceptibility ranges from 0.0003 to 0.043 SI (mean 0.011 SI). In all samples examined, high-temperature oxidation of primary titanomagnetite has produced lamellae or pods of magnetite and ilmenite. Hydrothermal alteration has further altered the minerals in some samples to a mixture of magnetite, ilmenite, titanite, and a high-titanium mineral (either rutile or anatase). Electron microprobe analyses show that magnetite lamellae are enriched in the trivalent oxides Cr2O3, Al2O3, and V2O5, whereas divalent oxides (MnO and MgO) are concentrated in ilmenite lamellae.

Investigations on volcanic rocks in sediment core CRP-3 from the Ross Sea, Antarctica

The petrography, mineralogy and geochemistry of volcanic and subvolcanic rocks in CRP-3 core have been examined in detail in order to characterise and to compare them with volcanic and subvolcanic rocks cropping out in the Victoria Land area, and to define the clast provenance or to establish possible volcanic activity coeval with deposition. Clasts with sizes ranging from granule to boulder show geochemical and mineralogical features comparable with those of Ferrar Supergroup rocks. They display a subalkaline affinity and compositions ranging from basalts to dacite. Three different petrographic groups with distinct textural and grain size features (subophitic, intergranular-intersertal, and glassy-hyalopilitic) are recognised and are related to the emplacement/cooling mechanism. In the sand to silt fraction, the few glass shards that have been recognised are strongly altered: however chemical analyses show they have subalkalic magmatic affinity. Mineral compositions of the abundant free clinopyroxene grains found in the core, are less affected by alteration processes, and indicate an origin from subalkaline magmas. This excludes the presence, during the deposition of CRP-3 rocks of alkaline volcanic activity comparable with the McMurdo Volcanic Group. Strong alteration of the magmatic body intruded the Beacon sandstones obliterates the original mineral assemblage. Geochemical investigations confirm that intrusion is part of the Ferar Large Igneous Province.

Chemistry of igneous rocks of DSDP Leg 30 holes

Igneous rock units were encountered at four of the five sites drilled on Leg 30 of the Deep Sea Drilling Project. These units uncluded a diabase sill at Site 285, a basalt underlain by a gabbro at 286, two basalt flows at 287, and a basalt flow at 289. Site 285 is located approximately in the center of the South Fiji Basin, Site 286 is adjacent to a filled portion of the New Hebrides Trench, Site 287 is adjacent to a basement high in the Coral Sea Basin, and Sites 288 and 289 are located on the Ontong-Java Plateau north of the Solomon Islands (Figure 1). Figure 2 presents generalized lithologic columns for the igneous rock units found at these sites. When a unit number is given, e.g., Site 286, Unit 4 basalt, this number conforms with the unit number assigned to it in the overall stratigraphic sequence of that hole as defined in the individual Site Reports in this volume. Unless otherwise stated, depths are given as measured from the sediment-igneous rock contact rather than the mudline.

Geochemistry and stable isotopes of peridotites of ODP Hole 103-637A

A ridge of strongly serpentinized, plagioclase-bearing peridotite crops out at the boundary between the Atlantic oceanic crust and the Galicia continental margin (western Spain). These peridotites, cored at Hole 637A (ODP Leg 103) have been mylonitized at high-temperature, low-pressure conditions and under large deviatoric stress during their uplift (Girardeau et al., 1988, doi:10.2973/odp.proc.sr.103.135.1988). After this main ductile deformation event, the peridotite underwent a polyphase metamorphic static episode in the presence of water, with the crystallization of Ti- and Cr-rich pargasites at high-temperature (800°-900°C) interaction with a metasomatic fluid or alkaline magma. Introduction of water produced destabilization of the pyroxenes and the subsequent development of hornblendes and tremolite at temperatures decreasing from 750° to 350°C. The main serpentinization of the peridotite occurred at a temperature below 300°C, and possibly around 50°C, as a consequence of the introduction of a large amount of seawater, which is suggested by stable isotope (d18O and SD) data. Finally, calcite derived from seawater precipitated in late-formed fractures or locally pervasively impregnated the peridotite at low temperature (~10°C).

Primary mineralogy of ODP Leg 115 basalts

Basement rocks were recovered at four sites on Leg 115 along the Reunion hotspot track in the western Indian Ocean. Plate tectonic reconstructions indicate that the drilled structures formed in three different volcanic environments. Sites 706 and 713 from the eastern side of the Saya de Malha Bank and the northern end of the Chagos Bank, respectively, are on a large volcanic platform analogous to Iceland on the Mid-Atlantic Ridge. Lavas at Site 707 on the northwestern side of the Saya de Malha Bank erupted during the early stages of rifting of the Seychelles from India. Basalts from Site 715 were erupted onto an isolated oceanic island that was distant from ocean ridges and continents much as Reunion Island is today. Many of the rocks were examined in thin section and found to be primarily augite-plagioclase basalts with minor olivine and rare opaque oxides. Site 715 is unusual in that it contains a variety of basalts including olivine-rich and aphyric Fe-Ti basalts. At each of the four sites the rocks were grouped into chemical types (units) on the basis of ship- board bulk-rock analyses and at least one thin section from each chemical unit was analyzed by electron microprobe. The plagioclase and augite chemistry reflects the bulk-rock chemistry and, in general, these minerals were in equilibrium with their host magmas at the time the basalts were quenched. Olivine was rarely preserved, but where it is still present it also appears to have crystallized in equilibrium with the host magma. At three of the drill sites plagioclase phenocrysts or megacrysts that crystallized from a primitive magma are also present. The one site (715) that does not contain these primitive plagioclase phenocrysts is also the site that appears to have been influenced the least by ocean- ridge or Deccan-type magmas. Site 715, furthermore, has a mineralogy that is dominated by olivine as compared with the plagioclase-rich lavas of the other sites.

Rocks and minerals from the Strakhov Fracture Zone, 4°N Mid-Atlantic Ridge

Geological-geophysical data obtained during Cruises 7, 11, and 12 of R/V Akademic Nikolay Strakhov (1989-1991) within the international project EQUARIDGE in the Strakhov Fracture Zone region (4°N) are presented. The trough of the fracture is interpreted as an open extension joint, a graben produced by stretching along the axis of the Mid-Atlantic Ridge. Bedrock studies showed that typical mid-ocean tholeiitic basalts occur within the narrow (60 nm wide) axial rift zone, whereas igneous rocks not typical for the ocean were found on the eastern and western flank plateaus. This allows to suppose that a reworked relict continental-type basement of pre-Upper Jurassic age possibly exists beneath the flank plateaus, within the segment under discussion. The above data correspond to the hypothesis of E. Bonatti about nonspreading nature of the basement of Mid-Atlantic Ridge within the equatorial segment and the Strakhov Fracture Zone.

Chemical and isotopic compositions of rocks and minerals from the Palenyi Island, Por'ya Guba (Bay), White Sea

Hypersthene-garnet-sillimanite-quartz enclaves were studied in orthopyroxene-plagioclase and orthopyroxene-clinopyroxene crystalline schists and gneisses from shear zones exposed in the Palenyi Island within the Early Proterozoic Belomorian Mobile Belt. Qualitative analysis of mineral assemblages indicates that these rocks were metamorphosed to the granulite facies (approximately 900°C and 10-11 kbar). Oxygen isotopic composition was determined in rock-forming minerals composing zones of the enclaves of various mineral and chemical composition. Closure temperatures of the isotopic systems obtained by methods of oxygen isotopic thermometry are close to values obtained with mineralogical geothermometers (garnet-orthopyroxene and garnet-biotite) and correspond to the high-temperature granulite facies (860-900°C). Identified systematic variations in d18O values were determined in the same minerals from zones of different mineral composition. Inasmuch as these zones are practically in contact with one another, these variations in d18O cannot be explained by primary isotopic heterogeneity of the protolith. Model calculations of the extent and trend of d18O variations in minerals suggest that fluid-rock interaction at various integral fluid/rock ratios in discrete zones was the only mechanism that could generate the zoning. This demonstrates that focused fluid flux could occur in lower crustal shear zones. Preservation of high-temperature isotopic equilibria of minerals testifies that the episode of fluid activity at the peak of metamorphism was very brief.