Physical properties and minerals of Leg 65 Holes basalts

Velocities and densities of submarine basalts obtained during Leg 65 are distinguished by their relatively high values (about 6.3 km/s and 2.9 g/cm/**3, respectively). This is consistent with their low degree of alteration. The range of velocities covered by these samples correlates well with porosity, and a comparison with in situ velocities from refraction data suggests maximum porosities ih the upper 100 meters of the crust of about 10%.

Petrology and diagenesis of sandstones at DSDP Hole 58-445

The sandstone succession in the lower 240 meters of DSDP Site 445, on the Daito Ridge, provided an opportunity to evaluate the effect of burial diagenesis of sandstones in a deep hole in a tectonic environment (remnant arc) characterized by a history of high heat flow. This report provides preliminary data concerning the petrology and diagenesis of these sandstones and records diagenetic changes which have occurred with increasing depth of burial. Methods used for this study included grain-size analysis (measured from thin sections using the method of Friedman, 1958), polarizing microscopy, X-ray diffraction, and scanning electron microscopy. A JEOL scanning electron microscope fitted with an energydispersive- X-ray detector was used for obtaining qualitative chemical data on certain minerals to aid in identification.

Geochemistry and petrology of ODP Site 136-843 basalts

About 13 m of Cretaceous, tholeiitic basalt, ranging from normal (N-MORB) to transitional (T-MORB) mid-ocean-ridge basalts, was recovered at Ocean Drilling Program Site 843 west of the island of Hawaii. These moderately fractionated, aphyric lavas are probably representative of the oceanic basement on which the Hawaiian Islands were built. Whole-rock samples from parts of the cores exhibiting only slight, low-temperature, seawater alteration were analyzed for major element, trace element, and isotopic composition. The basalts are characterized by enrichment in the high field strength elements relative to N-MORB, by a distinct positive Eu anomaly, and by Ba/Nb and La/Nb ratios that are much lower than those of other crustal or mantle-derived rocks, but their isotope ratios are similar to those of present-day N-MORB from the East Pacific Rise. Hole 843A lavas are isotopically indistinguishable from Hole 843B lavas and are probably derived from the same source at a lower degree of partial melting, as indicated by lower Y/Nb and Zr/Nb ratios and by higher concentrations of light and middle rare earth elements and other incompatible elements relative to Hole 843B lavas. Petrographic and trace-element evidence indicates that the Eu anomaly was the result of neither plagioclase assimilation nor seawater alteration. The Eu anomaly and the enrichments in Ta, Nb, and possibly U and K relative to N-MORB apparently are characteristic of the mantle source. Age-corrected Nd and Sr isotopic ratios indicate that the source for the lavas recovered at ODP Site 843 was similar to the source for Southeast Pacific MORB. An enriched component within the Cretaceous mantle source of these basalts is suggested by their initial 208Pb/204Pb-206Pb/204Pb and epsilon-Nd-206Pb/204Pb ratios. The Sr-Pb isotopic trend of Hawaiian post-shield and post-erosional lavas cannot be explained by assimilation of oceanic crust with the isotopic composition of the Site 843 basalts.

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.

Petrology and geochemistry of oceanic intraplate sheet-flow basalts at DSDP Hole 89-462A

Reentry of Hole 462A during Leg 89 resulted in the penetration of a further 140 m of basalt sheet-flows similar to those found during Leg 61 at the same site. Twelve volcanic units (45 to 56) were recognized, comprising a series of rapidly extruded, interlayered aphyric and poorly clinopyroxene-plagioclase-olivine phyric, nonvesicular basalts. All exhibit variable, mild hydration and oxidation, relative to fresh oceanic basalts, produced under reducing, low-CO2-activity conditions within the zeolite facies. Secondary assemblages are dominated by smectites, zeolites, and pyrite, produced by low-temperature reaction with poorly oxygenated seawater. No systematic mineralogical or chemical changes are observed with depth, although thin quenched units and more massive hypocrystalline units exhibit slightly different alteration parageneses. Chemically, the basalts are olivine- and quartz-normative tholeiites, characterized by low incompatible-element abundances, similar to mildly enriched MORB (approaching T-type), with moderate, chrondite-normalized, large-ionlithophile- element depletion patterns and generally lower or near-chrondritic ratios for many low-distribution-coefficient (KD) element pairs. In general, relative to cyclic MORB chemical variation, they are uniform throughout, although 3 chemical megagroups and 22 subgroups are recognized. It is considered that the megagroups represent separate low-pressure-fractionated systems (olivine + Plagioclase ± clinopyroxene), whereas minor variations within them (subgroups) indicate magma mixing and generation of near-steady-state conditions. Overall, relatively minor fractionation coupled with magma mixing produced a series of compositionally uniform lavas. Parental melts were produced by similar degrees of partial melting, although the source may have varied slightly in LIL-element content.

Geochemistry at DSDP Leg 82 Holes

DSDP Leg 82 drilled nine sites to the southwest of the Azores Islands on the west flank of the Mid-Atlantic Ridge (MAR) in an attempt to determine the temporal and spatial evolution of the Azores "hot-spot" activity. The chemistry of the basalts recovered during Leg 82 is extremely varied: in Holes 558 and 561, both enriched (E-type: CeN/YbN = 1.5 to 2.7; Zr/Nb = 4.5 to 9.6) and depleted (or normal-N-type: CeN/YbN = 0.6 to 0.8; Zr/Nb > 20) mid-ocean ridge basalts (MORB) occur as intercalated lava flows. To the north of the Hayes Fracture Zone, there is little apparent systematic relationship between basalt chemistry and geographic position. However, to the south of the Hayes Fracture Zone, the chemical character of the basalts (N-type MORB) is more uniform. The coexistence of both E-type and N-type MORB in one hole may be explicable in terms of either complex melting/ fractionation processes during basalt genesis or chemically heterogeneous mantle sources. Significant variation in the ratios of strongly incompatible trace elements (e.g., La/Ta; Th/Ta) in the basalts of Holes 558 and 561 are not easily explicable by processes such as dynamic partial melting or open system crystal fractionation. Rather, the trace element data require that the basalts are ultimately derived from at least two chemically distinct mantle sources. The results from Leg 82 are equivocal in terms of the evolution of the Azores "hot spot," but would appear not to be compatible with a simple model of E-type MORB magmatism associated with upwelling mantle "blobs." Models that invoke a locally chemically heterogeneous mantle are best able to account for the small-scale variation in basalt chemistry.

Composition of phyllosilicates from hydrothermally altered basalts of DSDP Hole 83-504B, Costa Rica Rift, Pacific Ocean

Phyllosilicates occurring as replacements of olivine, clinopyroxene and interstitial materials and as veins or fracture-fillings in hydrothermally altered basalts from DSDP Hole 504B, Leg 83 have been studied using transmission and analytical electron microscopy. The parageneses of phyllosilicates generally change systematically with depth and with the degree of alteration, which in turn is related to permeability of basalts. Saponite and some mixed-layer chlorite/smectite are the dominant phyllosilicates at the top of the transition zone. Chlorite, corrensite, and mixed-layer chlorite/corrensite occur mainly in the lower transition zone and upper levels of the sheeted dike zone. Chlorite, talc, and mixed-layer talc/chlorite are the major phyllosilicates in the sheeted dike zone, although replacement of talc or olivine by saponite is observed. The phyllosilicates consist of parallel or subparallel discrete packets of coherent layers with packet thicknesses generally ranging from < 100 A to a few hundred A. The packets of saponite layers are much smaller or less well defined than those of chlorite, corrensite and talc, indicating poorer crystallinity of saponite. By contrast, chlorite and talc from the lower transition zone and the sheeted dike zone occur in packets up to thousands of A thick. The Si/(Si + A1) ratio of these trioctahedral phyllosilicates increases and Fe/(Fe + Mg) decreases in the order chlorite, corrensite, saponite, and talc. These relations reflect optimal solid solution consistent with minimum misfit of articulated octahedral and tetrahedral sheets. Variations in composition of hydrothermal fluids and precursor minerals, especially in Si/(Si+A1) and Fe/(Fe+Mg) ratios, are thus important factors in controlling the parageneses of phyllosilicates. The phyllosilicates are generally well crystallized discrete phases, rather than mixed-layered phases, where they have been affected by relatively high fluid/rock ratios as in high-permeability basalts, in veins, or areas adjacent to veins. Intense alteration in basalts with high permeability (indicating high fluid/rock ratios) is characterized by pervasive albitization and zeolitization. Minimal alteration in the basalts without significant albitization and zeolitization is characterized by the occurrence of saponite ± mixed-layer chlorite/smectite in the low-temperature alteration zone, and mixed-layer chlorite/corrensite or mixed-layer talc/chlorite in the high-temperature alteration zone. Textural non-equilibrium for phyllosilicates is represented by mixed layering and poorly defined packets of partially incoherent layers. The approach to textural equilibrium was controlled largely by the availability of fluid or permeability.

Preliminary results and documentation of sediment cores CRP2 and CRP-2A from the Ross Sea off Cape Roberts, Antarctica

The site for CRP-2, 14 km east of Cape Roberts (77.006°S; 163.719°E), was selected to overlap the early Miocene strata cored in nearby CRP-1, and to sample deeper into the east-dipping strata near the western margin ofe he Victoria Land Basin to investigate Palaeogene climatic and tectonic history. CRP-2 was cored from 5 to 57 mbsf (metres below the sea floor) (core recovery 91 %), with a deviation resulting in CRP-2A being cored at the same site. CRP-2A reached down to 624mbsf (recovery 95%), and to strata with an age of c. 33-35 Ma. Drilling took place from 16 October to 25 November 1998, on 2.0-2.2 m of sea ice and through 178 m of water. Core fractures and other physical properties, such as sonic velocity, density and magnetic susceptibility, were measured throughout the core. Down-hole logs for these and other properties were run from 63 to 167 mbsf and subsequently from 200 to 623 mbsf, although density and velocity data could be obtained only to 440 mbsf because of hole collapse. Sonic velocity averages c. 2.0 km S-1 for the upper part of the hole, but there is an sharp increase to c. 3.0 km s-1 and also a slight angular unconformity, at 306 mbsf, corresponding most likely to the early/late Oligocene boundary (c. 28-30 Ma). Velocity then increases irregularly to around 3.6 km s-1 at the bottom of the hole, which is estimated to lie 120 m above the V4/V5 boundary. The higher velocities below 306 mbsf probably reflect more extensive carbonate and common pyrite cementation, in patches, nodules, bedding-parallel masses and as vein infills. Dip of the strata also increases down-hole from 3° in the upper 300 in to over 10° at the bottom. Temperature gradient is 21° k-1. Over 2 000 fractures were logged through the hole. Borehole televiewer imagery was obtained for the interval from 200 to 440 mbsf to orient the fractures for stress field analysis. Lithostratigraphical descriptions on a scale of 1:20 are presented for the full length of the core, along with core box images, as a 200 page supplement to this issue. The hole initially passed through a layer of muddy gravel to 5.5 mbsf (Lithological Sub-Unit or LSU 1.1), and then into a Quaternary diatom-bearing clast-rich diamicton to 21 mbsf (LSU 2. l), with an interval of alternating compact diamicton and loose sand, and containing a rich Pliocene foraminiferal fauna, to 27 mbsf (LSU 2.2). The unit beneath this (LSU 3.1) has similar physical properties (sonic velocity, porosity, magnetic susceptibility) and includes diamictites of similar character to those of LSU 2.1 and 2.2, but an early Miocene (c. 19 Ma) diatom assemblage at 28 mbsf (top of LSU 3.1) shows that this sub-unit is part of the older section. The strata beneath 27 mbsf, primary target for the project, extend from early Miocene to perhaps latest Eocene age, and are largely cyclic glacimarine nearshore to offshore sediments. They are described as 41 lithological sub-units and interpreted in terms of 12 recurrent lithofacies. These are 1) mudstone, 2) inter-stratified mudstone and sandstone, 3) muddy very fine to coarse sandstone, 4) well-sorted stratified fine sandstone, 5) moderately to well-sorted, medium-grained sandstone, 6) stratified diamictite, 7) massive diamictite, 8) rhythmically inter-stratified sandstone and mudstone, 9) clast-supported conglomerate, 10) matrix-supported conglomerate, 11) mudstone breccia and 12) volcaniclastic sediment. Sequence stratigraphical analysis has identified 22 unconformity-bounded depositional sequences in pre- Pliocene strata. They typically comprise a four-part architecture involving, in ascending order, 1) a sharp-based coarse-grained unit (Facies 6,7,9 or 10), 2) a fining-upward succession of sandstones (Facies 3 and 4), 3) a mudstone interval (Facies l), in some cases coarsening upward to muddy sandstones (Facies 3), and 4) a sharp-based sandstone dominated succession (mainly Facies 4). The cyclicity recorded by the strata is interpreted in terms of a glacier ice margin retreating and advancing from land to the west, and of rises and falls in sea level. Analysis of sequence periodicity awaits afirmer chronology. However, apreliminary spectral analysis of magnetic susceptibility for a deepwater mudstone within one of the sequences (from 339 to 347 mbsf) reveals ratios between hierarchical levels that are similar to those of the three Milankovitch orbital forcing periodicities. The strata contain a wide range of fossils, the most abundant being marine diatoms. These commonly form up to 5% of the sediment, though in places the core is barren (notably between 300 and 412 mbsf). Fifty samples out of 250 reviewed were studied in detail. The assemblages define ten biostratigraphical zones, some of them based on local or as yet undescribed forms. The assemblages are neritic, and largely planktonic, suggesting that the sea floor was mostly below the photic zone throughout deposition of the corcd sequence. Calcareous nannofossils, representing incursions of ocean surface waters, are much less common (72 out of 183 samples examined) and restricted to mudstone intervals a few tens of metres thick, but are important for dating. Foraminifera are also sparse (73 out of 135 samples) and represented only by calcareous benthic species. Changing assemblages indicate a shift from inshore environments in the early Oligocenc to outer shelf in the late Oligocenc, returning to inshore in the early Miocene. Marine palynomorplis yielded large numbers of well-preserved forms from most of the 116 samples examined. The new in situ assemblagc found last year in CRP-1 is extended down into the late Oligocene and a further new assemblage is found in the early Oligoccnc. Many taxa are new, and cannot us yet contribute to an improved understanding of chronology or ecology. Marine invertebrate macrofossils, mostly molluscs and serpulid tubes, are scattered throughout the core. Preservation is good in mudstones but poor in other lithologies. Climate on land is reflected in the content of terrestrial palynomorphs, which are extremely scarce down to c. 300 mbsf. Some forms are reworked, and others represent a low growing sparse tundra with at least one species of Nothofagus. Beneath this level, a significantly greater diversity and abundance suggests a milder climate and a low diversity woody vegetation in the early Oligocene, but still far short of the richness found in known Eocene strata of the region. Sedimentary facies in the oldest strata also suggest a milder climate in the oldest strata cored, with indications of substantial glacial melt-water discharges, but are typical of a coldcr climate in late Oligocene and early Miocene times. Clast analyses from diamictites reveal weak to random fabrics, suggesting either lack of ice-contact deposition or post-depositional modification, but periods when ice grounded at the drill site are inferred from thin zones of in-situ brecciated rock and soft-sediment folding. These are more common above c. 300 mbsf, perhaps reflecting more extensive glacial advances during deposition of those strata. Erosion of the adjacent Transantarctic Mountains through Jurassic basalt and dolerite-intruded Beacon strata into basement rocks beneath is recorded by petrographical studies of clast and sand grain assemblages. Core below 310 mbsf contains a dominance of fine-grained Jurassic dolerite and basalt fragments along with Beacon-derived coal debris and rounded quartz grains, whereas the strata above this level have a much higher proportion of basement derived granitoids, implying that the large areas of the adjacent mountains had been eroded to basement by the end of the early Oligocene. There is little indication of rift-related volcanism below 310 mbsf. Above this, however, basaltic and trachytic tephras are common, especially from 280 to 200 mbsf, from 150 to 46 mbsf, and in Pliocene LSU 2.2 from 21 to 27 mbsf. The largest volcanic eruptions generated layers of coarse (up to 1 cm) trachytic pumice lapilli between 97 and 114 mbsf. The thickest of these (1.2 m at 112 mbsf) may have produced an eruptive column extending tens of km into the stratosphere. A source within a few tens of km of the drill site is considered most likely. Present age estimates for the pre-Pliocene sequence are based mainly on biostratigraphy (using mainly marine diatoms and to a lesser extent calcareous nannofossils), with the age of the tephra from 112 to 114 mbsf (21.44k0.05 Ma from 84 crystals by Ar-Ar) as a key reference point. Although there are varied and well-preserved microfossil assemblages through most of the sequence (notably of diatoms and marine palynomorphs), they comprise largely taxa either known only locally or as yet undescribed. In addition, sequence stratigraphical analysis and features in the core itself indicate numerous disconformities. The present estimate from diatom assemblages is that the interval from 27 to 130 mbsf is early Miocene in age (c. 19 to 23.5 Ma), consistent with the Ar-Ar age from 112 to 114 mbsf. Diatom assemblages also indicate that the late Oligocene epoch extends from c. 130 to 307 mbsf, which is supported by late Oligocene nannofossils from 130 to 185 mbsf. Strata from 307 to 412 mbsf have no age-diagnostic assemblages, but below this early Oligocene diatoms and nannofossils have been recovered. A nannoflora at the bottom of the hole is consistent with an earliest Oligocene or latest Eocene age. Magnetostratigraphical studies based on about 1000 samples, 700 of which have so far undergone demagnetisation treatment, have provided a polarity stratigraphy of 12 pre-Pliocene magnetozones. Samples above 270 mbsf are of consistently high quality. Below this, magnetic behaviour is more variable. A preliminary age-depth plot using the Magnetic Polarity Time Scale (MPTS) and constrained by biostratigraphical data suggests that episodes of relatively rapid sedimentation took place at CRP-2 during Oligocene times (c. 100 m/My), but that more than half of the record was lost in a few major and many minor disconformities. Age estimates from Sr isotopes in shell debris and further tephra dating are expected to lead to a better comparison with the MPTS. CRP-2/2A has recorded a history of subsidence of the Victoria Land Basin margin that is similar to that found in CIROS-170 km to the south, reflecting stability in both basin and the adjacent mountains in late Cenozoic times, but with slow net accumulation in the middle Cenozoic. The climatic indicators from both drill holes show a similar correspondence, indicating polar conditions for the Quaternary but with sub-polar conditions in the early Miocene-late Oligocene and indications of warmer conditions still in the early Oligocene. Correlation between the CRP-2A core and seismic records shows that seismic units V3 and V4, both widespread in the Victoria Land Basin, represent a period of fluctuating ice margins and glacimarine sedimentation. The next drill hole, CRP-3, is expected to core deep into V5 and extend this record of climate and tectonics still further back in time.