(Table 2) Mineralogy and carbon, oxygen, and hydrogen isotope ratios for secondary mineral samples from igneous rocks from ODP Hole 128-794D

The basaltic rocks of Hole 794D drilled during Leg 128 are strongly altered. Microprobe analyses and XRD spectra on small quantities of matter extracted from thin sections show that primary minerals and glassy zones of the groundmass are totally or partially replaced by clay minerals with chlorite/saponite mixed-layer composition whatever the rock sample considered. This mixed-layer was also identified in veins and vesicles where it crystallizes in spheroidal aggregates. The largest veins and vesicles are filled by a zoned deposit: the chlorite/saponite mixed-layer always occupies the central part and is rimmed by pure saponite. Calcite crystallizes in secondary fractures which crosscut the clayey veins and vesicles. Chemographic analysis based on the M+-4Si-3R2+ projection shows that the chemical composition of the saponite component in the mixed-layer is identical to that of the free saponite. This indicates that the clay mineral crystallization was controlled by the chemical composition of the alteration fluids. From petrographic evidence, it is suggested that both chlorite/saponite mixed-layer and free saponite belong to the same hydrothermal event and are produced by a temperature decrease. This is supported by the stable isotopic data. The isotopic data show very little variation: d18O saponite ranges from 13.1 per mil to 13.5 per mil, and dD saponite from -73.6 per mil to -70.0 per mil. d18O calcite varies from +19.7 per mil to +21.9 per mil vs SMOW and d13C from -3.2 per mil to +0.4 per mil vs. PDB. These values are consistent with seawater alteration of the basalt. The formation of saponite took place at 150°-180°C and the formation of calcite at about 65°C.

Geochemistry of mafic clasts of ODP Holes 125-778A and 125-779A

Clasts of metamorphosed mafic igneous rock of diverse composition were recovered in two drill sites on a serpentine mud volcano in the outer Mariana forearc during Ocean Drilling Program Leg 125. These clasts are xenolithic fragments that have been entrained in the rising serpentine mud, and make up less that 9% of the total rock recovered at Sites 778 and 779. Most samples are metabasalt or metadiabase, although one clast of possible boninite and one cumulate gabbro were recovered. On the basis of trace element signatures, samples are interpreted to represent both arc-derived and mid-ocean ridge-derived compositions. Rocks with extremely low TiO2 (<0.3 wt%) and Zr (<30 ppm) are similar to boninite series rocks. Samples with low TiO2 (<0.9 wt%) and Zr (<50 ppm) and extreme potassium enrichment (K2O/Na2O >3.9) may represent island arc rocks similar to shoshonites. However, the K2O/Na2O ratios are much higher than those reported for shoshonites from modem or ancient arcs and may be the result of metamorphism. Samples with moderate TiO2 (1.4 to 1.5 wt%) and Zr (72 to 85 ppm) are similar to rocks from mid-ocean ridges. A few samples have TiO2 and Zr intermediate between island arc and mid-ocean ridge basalt-like rocks. Two samples have high iron (Fe2O3* = >12.8 to 18.5 wt%) (Fe2O3* = total iron calculated as Fe2O3) and TiO2 (>2.3 wt%) and resemble FeTi basalt recovered from mid-ocean ridges. Metamorphism in most samples ranges from low-temperature zeolite, typical of ocean floor weathering, to prehnite-pumpellyite facies and perhaps lower greenschist. Blue amphibole and lawsonite minerals are present in several samples. One diabase clast (Sample 9) exhibits Ca enrichment, similar to rodingite metamorphism, typical of mafic blocks in serpentinized masses. The presence of both low-grade (clays and zeolites) and higher grade (lawsonite) metamorphism indicates retrograde processes in these clasts. These clasts are fragments of the forearc crust and possibly of the subducting plate that have been entrained in the rising serpentine and may represent the deepest mafic rocks ever recovered from the Mariana forearc. The variable compositions and degree of metamorphism of these clasts requires at least two tectonic origins. The recovery of clasts with mid-ocean ridge and arc chemical affinities in a single drill hole requires these clasts to have been "mixed" on a small scale either (1) in the forearc crustal sequence, or (2) after inclusion in the rising serpentine mud. The source of the MORB-like samples and an explanation for the presence of both MORB-like and arc-like rocks in close proximity is critical to any model of the evolution of the Mariana forearc. The source of the MORB-like samples likely will be one (or more) of the following: (1) accretion of Pacific plate lithosphere, (2) remnants of original forearc crust (trapped plate), (3) volcanism in the supra-subduction zone (arc or forearc) environment, or (4) derivation from the subducting slab by faulting along the dÈcollement.

Granulometry, mineralogy and geochemistry of sediments from ODP Hole 169-1037B and Site 169-1038

Central Hill is in the northern part of the Escanaba Trough, which is a sediment-filled rift of southern Gorda Ridge. Central Hill is oriented north-south and is associated with extensive sulfide deposits. Hydrothermal alteration of sediment from Site 1038 was studied through analyses of mineralogy and the chemistry and oxygen isotopic compositions of one nearly pure clay sample. In addition, Site 1037 was drilled to establish the character of the unaltered sedimentary sequence away from the hydrothermal centers of the Northern Escanaba Trough Study Area (NESCA). Mineralogy of the clay-size fraction of turbiditic and hemipelagic sediments of Hole 1037B are predominantly quartz, feldspar, pyroxene, illite, chlorite, and smectite, representing continental-derived material. Cores from Hole 1038I, located within the area of Central Hill but away from known active vent areas, recovered minor amounts of chlorite/smectite mixed-layer clay in the fine fraction, indicating a low-temperature hydrothermal alteration. The 137.4-m-thick sediment section of Hole 1038G is located in an area of low-temperature venting. The uppermost sample is classified as chlorite/smectite mixed layer, which is underlain by chlorite as the dominant mineral. The lowermost deposits of Hole 1038G are also characterized by chlorite/smectite mixed-layer clay. In comparison to Hole 1038I, the mineralogic sequence of Hole 1038G reflects increased chloritization. Intensely altered sediment is almost completely replaced by hydrothermal chlorite in subsurface sediments of Hole 1038H. Alteration to chlorite is characterized by depletion in Na, K, Ti, Ca, Sr, Cs, and Tl and enrichment in Ba. Further, Eu depletion reflects a high-temperature plagioclase alteration. A chlorite 18O value of 2.6 indicates formation at a temperature of ~190°C. It is concluded that the authigenic chlorite in Hole 1038H formed by an active high-temperature fluid flow in the shallow subsurface.

(Table 1) Mineralogy at DSDP Leg 79 Holes

Four sites in the region of the Mazagan Plateau off northwest Africa were drilled during Leg 79 of the Deep Sea Drilling Project. Bulk mineralogy and clay mineralogy were analyzed from the Cenozoic sediments recovered from the four sites.

Mineralogy at DSDP Sites 68-502 and 68-503

Eighty-four sediment samples from four holes at Site 502 and 54 samples from three holes at Site 503 were analyzed for mineral content by semiquantitative X-ray diffraction methods. Site 502 is located in the Western Caribbean, whereas Site 503 lies in the Eastern Pacific (probably on the north flank of the Galapagos Spreading Center). Both sites were chosen to yield continuous core sections for investigations of late Neogene and Quaternary biostratigraphy and magnetostratigraphy and to study events such as the closing of the Isthmus of Panama. Our X-ray diffraction work should provide a framework for further investigations - for example, determination of climatic changes in relationship to clay mineral composition or the influx of terrigeneous sediment components from South America before and after development of the Panama landbridge.

Insoluble residues of the fine-grained sediments from DSDP Leg 67 Holes

Insoluble residues of Late Cretaceous to Quaternary deep-sea samples from slope, trench, and oceanic plate sites south of Guatemala were examined, specifically for the distribution of clay minerals in the <2-µm fraction and of silt grains in the 20-63-µm fraction. Widespread "oceanic" particles (biogenic opal, rhyolitic glass) and their diagenetic products (smectite, clinoptilolite, heulandite) were distinguished from terrigenous material - illite, kaolinite, chlorite, plagioclase, quartz, and heavy minerals. The main results of this investigation are: (1) At Site 494 on the slope immediately adjacent to the trench, terrigenous supplies testify to a slope position of the whole sequence back to the Late Cretaceous. (2) At Site 495 on the Pacific Cocos Plate, "oceanic" and terrigenous sedimentation are clearly separated. Whereas the pelagic sedimentation prevailed in the early Miocene, terrigenous minerals appeared in the middle Miocene in the clay fraction, and in the early Pliocene in the coarse silt fraction. These terrigenous supplies are interpreted as having been transported by suspension clouds crossing the slope and even the trench. The alternative, however, an eolian transport, cannot be excluded.

Mineralogy at DSDP Legs 58-60

Sediments recovered by drilling during Legs 58, 59, and 60 in the North and South Philippine Sea have been analyzed by X-ray diffractometry. The CaCO3 content was measured separately. The sites encompass several volcanic ridges and intervening inter-arc basin troughs as well as sites on the Mariana arc fore-arc sediment prism and the Mariana Trench. The sediments at all sites received major volcanogenic input from the various arcs; they tend to be rich in volcanic glass, with associated quartz, feldspar, pyroxenes and amphibole. Carbonate is a major component only at Site 445 at the southern end of the Daito Ridge, and at Site 448 on the Palau-Kyushu Ridge. All other sites were either deep relative to the carbonate compensation depth or had very high non-carbonate sedimentation rates. Clay minerals are mainly smectite and illite with lesser variable proportions of chlorite and kaolinite. Smectite predominates over illite except at sites in the Shikoku Basin and the Daito Ridge, and at one site in the Mariana Trench. At several sites, smectite increases and illite decreases with depth. Principal zeolites are phillipsite and clinoptilolite. Analcime occurs in some samples.

Sedimentology and mineralogy of surface sediments from the Chilean continental margin

The regional patterns of texture and composition of modern continental slope and pelagic sediments off Chile between 25°S and 43°S reflect the latitudinal segmentation of geological, morphological, and climatic features of the continental hinterland. Grain-size characteristics are controlled by the grain-size of source rocks, the weathering regime, and mode of sediment input (eolian off northern Chile vs fluvial further south). Bulk-mineral assemblages reveal a low grade of maturity. Regional variations are governed by the source-rock composition of the different geological terranes and the relative source-rock contribution of the Coastal Range and Andes, as controlled by the continental hydrology. The relative abundance of clay minerals is also predominantly influenced by the source-rock composition and partly by continental smectite neoformation. Latitudinal variations of illite crystallinities along the Chilean continental slope (and west of the Peru-Chile trench) clearly reflect modifications of the weathering regime which correspond to the strong climatic zonation of Chile.