988 resultados para Ocean Island basalts


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Cr-spinels in cores drilled during Ocean Drilling Program Leg 135 exhibit wide variations in composition and morphology that reflect complex petrogenetic histories. These Cr-spinels are found within basaltic lava flows that erupted in north-trending sub-basins within the Lau Basin backarc. Cr-spinels from Sites 834 and 836 occur as euhedral groundmass grains and inclusions in plagioclase, and range up to 300 ?m in size. These Cr-spinels are similar in composition, morphology, and mode of occurrence to Cr-spinels found within depleted, N-type mid-ocean-ridge basalts (N-MORB), reflecting similar crystallization conditions and host lava composition to N-MORB. Their compositional range is relatively narrow, with Cr/(Cr + Al + Fe3+) (Cr#) and Mg/(Mg + Fe2+) (Mg#) varying from 0.38 to 0.48 and 0.56 to 0.72, respectively; like Cr-spinels from N-MORB, they contain low amounts of TiO2 (0.37%-1.05%) and Fe3+/(Cr + Al + Fe3+) (Fe3+#; <0.11). In contrast, Cr-spinels from Site 839 have much higher Cr# at a given Mg#, with Cr# varying from 0.52 to 0.76 and Mg# varying from 0.27 to 0.75. These Cr-spinels are similar in composition to those from primitive, boninitic or low-Al2O3 arc basalts, sharing their low TiO2 and Fe3+# (typically below 0.35% and 0.1, respectively for spinel grain interiors). Site 839 Cr-spinels occur as small (to 50 µm) euhedra within strongly zoned olivine or as unusually large (to 3 mm), euhedral to subhedral megacrysts. These megacrysts are strongly zoned in Mg#, but they display little zoning in Cr#, providing evidence of strong compositional disequilibria with the host melt. The magnesian cores of the megacrysts crystallized from primitive, near-primary melts derived from harzburgitic or highly depleted lherzolitic sources, and they provide evidence that the Site 839 spinel-bearing lavas were derived by the mixing of melt with a Mg# of 0.75-0.80 and evolved, Cr-spinel barren melt with a Mg# < 0.6 shortly before eruption.

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Leg 65 of the Deep Sea Drilling Project successfully recovered basalts from four sites in the mouth of the Gulf of California, thus completing a transect begun during Leg 64 from the continental margin of Baja California to the east side of the East Pacific Rise (EPR). Sixty-three whole-rock samples from Sites 482, 483, and 485 have been analyzed by X-ray fluorescence techniques, and a further eleven samples by instrumental neutron-activation techniques, in order to assess magma variability within and between sites. Although the major element compositions and absolute hygromagmatophile (HYG) element abundances are quite variable, all of the basalts are subalkaline tholeiites exhibiting strong more-HYG element (e.g., Rb, La, Nb, Ta) depletion (LaN/YbN ~ 0.4; Nb/Zr ~ 0.02; Ba/Zr ~ 0.23; Th/Hf ~ 0.05). These ratios, together with La/Ta ratios of 20 and Th/Ta ratios of 1.25, demonstrate that the Leg 65 basalts resemble the depleted "N-type" ocean ridge basalts recovered from the Mid-Atlantic Ridge (MAR) at 22 °N and other sections of the EPR. Zr/Ti, Zr/Y, and La/Yb ratios increase with increasing fractionation. It is clear that the basalts recovered from Sites 482, 483, and 485 were all derived from a compositionally similar source and that the compositional differences observed between lithological units can be explained by varying degrees of open system fractional crystallization (magma mixing) in subridge magma chambers. The basaltic rocks recovered from Site 474 near the margin of Baja California, and Sites 477, 478, and 481 in the Guaymas Basin, all drilled during Leg 64, have consistently higher Th/Hf, La/Sm, Zr/Ti, and Zr/Y ratios and higher absolute Sr contents than the Leg 65 basalts. While some of these variations may be explained by different conditions of partial melting, it is considered more likely that the mantle source underlying the Guaymas Basin is chemically distinct from that feeding the EPR at the mouth of the Gulf. These source variations probably reflect the complex tectonic setting of the Gulf of California, the magmas formed at the inception of spreading and in the central part of the Gulf containing a minor but significant component of sub-continental (calc-alkaline) material.

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Leg 58 successfully recovered basalt at Sites 442, 443, and 444, in the Shikoku Basin, and at Site 446 in the Daito Basin. Only at Site 442 did penetration reach unequivocal oceanic layer 2; at the other sites, only off-axis sills and flows were sampled. Petrographic observations indicate that back-arc basalts from the Shikoku Basin, with the exception of the kaersutite-bearing upper sill at Site 444, are mineralogically similar to basalts being erupted at normal mid-ocean ridges. However, the Shikoku Basin basalts are commonly very vesicular, indicating a high volatile content in the magmas. Site 446 in the Daito Basin penetrated a succession of 23 sills which include both kaersutite-bearing and kaersutite-free basalt varieties. A total of 187 samples from the four sites has been analyzed for major and trace elements using X-ray-fluorescence techniques. Chemically, the basalts from Sites 442 and 443 and the lower sill of Site 444 are subalkaline tholeiites and resemble N-type ocean-ridge basalts found along the East Pacific Rise and at 22° N on the Mid-Atlantic Ridge (MAR), although they are not quite as depleted in certain hygromagmatophile (HYG) elements. They do not show any chemical affinities with island-arc tholeiites. The basalts from Site 446 and from the upper sill at Site 444 show alkaline and tholeiitic tendencies, and are enriched in the more-HYG elements; they chemically resemble enriched or E-type basalts and their differentiates found along sections of the MAR (e.g., 45°N) and on ocean islands (e.g., Iceland and the Azores). Most of the intra-site variation may be attributed to crystal settling within individual massive flows and sills, to high-level fractional crystallization in sub-ridge magma chambers, or, where there is evidence of a long period of magmatic quiescence between units, to batch partial melting. However, the basalts from Sites 442 and 443 and from the lower sill at Site 444 cannot easily be related to those from Site 446 and the upper sill at Site 444, and it is possible that the different basalt types were derived from chemically distinct mantle sources. From comparison of the Leg 58 data with those already available for other intra-oceanic back-arc basins, it appears that the mantle sources giving rise to back-arc-basin basalts are chemically as diverse as those for mid-ocean ridges. In addition, the high vesicularity of the Shikoku Basin basalts supports previous observations that the mantle source of back-arc-basin basalts may be contaminated by a hydrous component from the adjacent subduction zone.

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Seventeen whole-rock samples, generally taken at 25- to 50-meter intervals from 5 to 560 meters sub-basement in Deep Sea Drilling Project Hole 504B, were analyzed for 87Sr/86Sr ratios, and rubidium and strontium concentrations. Ten of these samples also were analyzed for Pb-isotope composition. Strontium-isotope ratios for eight samples from the upper 260 meters of the hole range from 0.70287 to 0.70377, with a mean of 0.70320. In the interval 330 to 560 meters, five samples have a restricted range of 0.70259 to 0.70279, with a mean of 0.70266, almost identical to the average value of fresh mid-ocean-ridge basalts. In the interval 260 to 330 meters, approximately intermediate strontium- isotope ratios are found. The higher 87Sr/86Sr ratios in the upper part of the hole can be interpreted in terms of strontium-isotope alteration during basalt-sea-water interaction. Relative to average fresh mid-ocean ridge basalts, the upper 260 meters of basalts are enriched by an average of about 9% in sea-water strontium 87Sr/86Sr = 0.7091). This Sr presumably is located in the smectites, which, as the main secondary minerals throughout the hole, replace olivine and matrix glass and locally fill vesicles (analyzed samples contained no veins). The strontium-isotope data strongly suggest that the integrated flux of sea water through the upper part of the Hole 504B crust has been greater than through the lower part. This is also suggested by (1) the common occurrence of Feoxide- hydroxide minerals as alteration products above 270 meters, but their near absence below 320 meters, (2) the presence of vein calcite above 320 meters, but its near absence below this level, and (3) the occurrence of vein pyrite only below a depth of 270 meters. Sea-water circulation in the lower basalts may have been partly restricted by the greater number of relatively impermeable massive lava flows below 230 meters sub-basement. Although sufficient sea water was present within the lower part of the hole to produce smectitic alteration products, the overall water /rock ratio was low enough to prevent significant modification of strontium-isotope ratios. Lead-isotope ratios of Hole 504B basalts form approximately linear arrays in plots of 208Pb/204Pb and 207Pb/204Pb versus 206Pb/204Pb. The arrays are similar to those reported for basalts from other mid-ocean ridges. There is no trend in Hole 504B lead-isotope ratios with vertical position in the basement. The arrays indicate that the lead-isotope composition of the upper mantle from which the Hole 504B basaltic melts were derived was inhomogeneous.

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Volcaniclastic rocks of Late Cretaceous age occur in four out of five sites (525, 527, 528, 529) drilled on the crest and the northwest flank of the Walvis Ridge during Leg 74. They are mostly interlayered with and overlie basement in the lowermost 10-100 m of the sedimentary section. Rocks from Holes 525A and 528 were studied megascopically and microscopically, by XRD, and XRF chemical analyses of whole-rock major and trace elements were undertaken. The dominant rock of Hole 528 volcaniclastics is a fine-grained (silt to fine sand), mostly matrix-bearing (partly matrix-rich) vitric "tuff," occurring as 5-110 cm thick, partly graded layers, some of which are distinctly bedded. Volcaniclastics of Hole 525A are generally richer in sanidine crystals. Most rocks contain some nonvolcanic clasts, chiefly foraminifers and lesser amounts of shallow-water fossil debris. Scoria shards, clasts of tachylite, and fine-grained basalts as well as chemical analyses suggest a basaltic to intermediate composition for most rocks of Hole 528, whereas volcaniclastics of Hole 525A are more silicic. The occurrence of tachylite and epiclastic, coarse-grained, basaltic clasts throughout the volcaniclastic sequence at Site 528 indicates shallow-water eruptions and perhaps even ocean island volcanism. The minor occurrence in Hole 528 of trachytic? pumice shards with phenocrysts of K-feldspar and the abundance of such shards in rocks from Hole 525A indicate Plinian eruptions characteristic of more mature stages of ocean island evolution. The sedimentary structures of volcaniclastic layers and their occurrence within deep sea calcareous oozes indicate a mass flow origin. Diagenetic alteration of the volcaniclastic rocks is pronounced, and four major stages of glass shard alteration are distinguished. Despite the effects of alteration and small-scale redistribution of elements and the admixture of nonvolcanic components, there were no drastic changes in the chemical composition of the rocks, except for pronounced increases in K and Rb and decreases in Ca and Fe. The basaltic volcaniclastic rocks very much resemble basement basalts in that they are moderately evolved tholeiites derived from an LIL-enriched mantle source with Zr/Nb ratios (Hole 528) of 5 to 6. This, in conjunction with the interbedding of volcaniclastic rocks and basement lavas, indicates contemporaneous seamount or island and basement volcanic activity involving magmas derived from similar sources.

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Middle Jurassic basaltic lavas obtained from Site 801 in the western Pacific Pigafetta Basin represent ocean crust from the oldest segment of the present-day Pacific Ocean. A composite 131 m section shows the basement to be composed of an upper alkalic basalt sequence (about 157 Ma) with ocean island basalt chemical features and a lower tholeiitic basalt sequence (about 167 Ma) with typical normal-type mid-ocean ridge basalt features. The basalt sequences are separated by a quartz-cemented, yellow goethite hydrothermal deposit. Most basalts are altered to some degree and exhibit variable, low-grade smectite-celadonite-pyrite-carbonate-zeolite assemblages developed under a mainly hydrated anoxic environment. Oxidation is very minor, later in development than the hydration assemblages, and largely associated with the hydrothermal deposit. The tholeiitic normal-type mid-ocean ridge basalt has characteristically depleted incompatible element patterns and all compositions are encompassed by recent mid-ocean ridge basalt from the East Pacific Rise. Chemically, the normal-type mid-ocean ridge basalt is divided into a primitive plagioclase-olivine +/- spinel phyric group (Mg* = 72-60) and an evolved (largely) aphyric group of olivine tholeiites (Mg* = 62-40). Both groups form a single comagmatic suite related via open-system fractionation of initial olivine-spinel followed by olivine-plagioclase-clinopyroxene. The alkalic ocean island basalt are largely aphyric and display enriched incompatible element abundances within both relatively primitive olivine-rich basalts and evolved olivine-poor hawaiites related via mafic fractionation. In gross terms, the basement lithostratigraphy is a typical mid-ocean ridge basalt crust, generated at a spreading center, overlain by an off-axis seamount with ocean island basalt chemical characters.

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We report S concentrations and relative proportions of (SO4)2- and S2- in OL- and CPX-hosted glass inclusions and in host glassy lapilli from Miocene basaltic hyaloclastites drilled north and south of Gran Canaria during ODP Leg 157. Compositions of glass inclusions and lapilli resemble those of subaerial Miocene shield basalts on Gran Canaria and comprise mafic to more evolved tholeiitic to alkali basalt and basanite (10.3-3.7 wt.% MgO, 44.5-56.9 wt.% SiO2). Glass inclusions fall into three groups based on their S concentrations: a high-sulfur group (1050 to 5810 ppm S), an intermediate-sulfur group (510 to 1740 ppm S), and a low-sulfur group (<500 ppm S). The most S-rich inclusions have the highest and nearly constant proportion of sulfur dissolved as sulfate determined by electron microprobe measurements of SKa peak shift. Their average S6+/S_total value is 0.75+/-0.09, unusually high for ocean island basalt magmas. The low-sulfur group inclusions have low S6+/S_total ratios (0.08+/-0.05), whereas intermediate sulfur group inclusions show a wide range of S6+/S_total (0.05-0.83). Glassy lapilli and their crystal-hosted glass inclusions with S concentrations of 50 to 1140 ppm S have very similar S6+/S_total ratios of 0.36+/-0.06 implying that sulfur degassing does not affect the proportion of (SO4)2- and S2- in the magma. The oxygen fugacities estimated from S6+/S_total ratios and from Fe3+/Fe2+ ratios in spinel inclusions range from NNO-1.1 to NNO+1.8. The origin of S-rich magmas is unclear. We discuss (1) partial melting of a mantle source at relatively oxidized fO2 conditions, and (2) magma contamination by seawater either directly or through magma interaction with seawater-altered Jurassic oceanic crust. The intermediate sulfur group inclusions represent undegassed or slightly degassed magmas similar to submarine OIB glasses, whereas the low-sulfur group inclusions are likely to have formed from magmas significantly degassed in near-surface reservoirs. Mixing of these degassed magmas with stored volatile-rich ones or volatile-rich magma replenishing the chamber filled by partially degassed magmas may produce hybrid melts with strongly varying S concentrations and S6+/S_total ratios.

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Basalts drilled from the East Pacific Rise, OCP Ridge, and Siqueiros fracture zone during Leg 54 are texturally diverse. Dolerites are equigranular at Sites 422 and 428 and porphyritic, with phenocrysts of plagioclase (An69.73) and Ca-rich clinopyroxene (Ca42Mg48Fe10) at Site 427. The East Pacific Rise lavas and some of those from the OCP Ridge are fine-grained and porphyritic. The majority of the large crystals are clustered skeletal glomerocrysts of plagioclase An64-77), together with olivine (Fo80-87), Ca-rich clinopyroxene, or both. Euhedral phenocrysts of plagioclase, together with olivine, Carich clinopyroxene, and Cr-Al spinel in some cases, occur in most of the fine-grained lavas. These phenocrysts are small (maximum dimension <1 mm in all but one sample), sparse (combined modal amount <1% in all samples), and distinctive from the megacrysts which characterize many ocean-floor lavas. In two East Pacific Rise lavas, zoned plagioclase (An83 cores) is the sole phenocryst phase. In other porphyritic lavas from all the main East Pacific Rise and OCP Ridge units drilled during Leg 54, the plagioclase phenocrysts contain cores of bytownite (An79-87) surrounded by more-sodic feldspar (An67-77). Core/rim relationships vary from continuous normal zoning, through discontinuous zoning, to extensive resorption of the calcic cores in some samples. The compositions of the plagioclase calcic cores are systematically related to those of the glomerophyric plagioclase and olivine in the lavas containing them. Furthermore, only one compositional population of calcic cores occurs in each rock. The possible causes of these relationships are far from clear. Magma mixing, although superficially applicable, is inconsistent with important aspects of the phenocryst mineralogy of these particular lavas. A more satisfactory model to explain both phenocryst zoning and rapid glomerocryst growth immediately before extrusion may be constructed by postulating influx of water into the upwelling magmas within Layer 3 of the oceanic crust beneath the East Pacific Rise, and subsequent loss of part of this water during effervescence within feeder dykes between Layer 3 and the ocean floor. It is shown that this model is fully consistent with published data on water and carbon dioxide contents and ratios in the pillow-margin glasses, vesicles, and phenocryst inclusions of ocean-floor basalts. The evidence for the precipitation of plagioclase- dominated crystalline assemblages from these magmas in the upper part of Layer 3 is concordant with recent geophysically based modeling of the structure of the East Pacific Rise. Calcium-rich clinopyroxenes in dolerites from the OCP Ridge and Siqueiros fracture zone show radial, oscillatory, and sector-zoning. In Sample 428A-5-2 (Piece 5a), the compositional trends resulting from this zoning closely resemble those of the pyroxenes in some lunar lavas. The controls on crystallization of interstitial pigeonite - epitaxial upon augite - in this rock are discussed. Both sector-zoning of the augite and nucleation of pigeonite within microvolumes of magma with a low Ca(Mg + Fe) ratio appear to be important factors.

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During Deep Sea Drilling Project Leg 73 (South Atlantic), basaltic pillow lava, flows, and sills were encountered in Holes 519A, 520, 522B, and 524. Paleomagnetic data indicate that the basalts from Holes 519A (magnetic Anomaly 51) and 522B (Anomaly 16) have ages of about 12 m.y. and about 38 m.y., respectively. The major- and trace- (including rare-earth-) element characteristics of the Hole 519A basalts (a total of 27 m) demonstrate that these basalts are typical normal-type mid-ocean-ridge basalts (N-type MORB). In composition the basalts overlap olivine tholeiites from other normal Mid-Atlantic Ridge segments. Both the spectra of incompatible, or less-hygromagmatophile elements (such as Ti, V, Y, and Zr) and REE abundances indicate that these basalts are the result of a low-pressure fractionation of olivine, spinel, and Plagioclase prior to eruption. In Hole 520 only 1.7 m of basalt were recovered from a total drilling depth of 10.5 m. These pillow basalts crystallized from fairly evolved (N-type MORB) tholeiitic melts. In total, 19 m of basaltic pillow lavas and flows were penetrated in Hole 522B. Thirteen cooling units were distinguished on the basis of glassy margins and fine quench textures. In contrast to Holes 519A and 520, the basalts of the Hole 522B ridge section can be divided into two major groups of tholeiites: (1) Cooling Units 1 through 12 and (2) Cooling Unit 13. The basalts in this ridge section are also N-type MORBs but are generally more differentiated than those of Holes 519A and 520. The lowermost basalts (Cooling Unit 13) have the most primitive composition and make up a compositional group distinct from the more evolved basalts in the twelve units above it. Hole 524 was drilled on the south flank of the Walvis Ridge and thus provided samples from a more complex part of the South Atlantic seafloor. Three different basaltic rock suites, interlayered with volcanic detrital sediments, were encountered. The rock suites are, from top to bottom, an alkali basaltic pillow lava; a 16-m-thick alkaline diabase sill with an age of about 65 m.y. (according to K-Ar dating and planktonic foraminifers); and a second sill that is approximately 9 m thick, about 74 m.y. in age, and tholeiitic in composition, thus contrasting strongly with the overlying alkaline rocks. The alkali basalts of Hole 524 show chemical characteristics that are very similar to the basaltic lavas of the Tristan da Cunha group volcanoes, which are located approximately 400 km east of the Mid-Atlantic Ridge crest. Thus, the Walvis Ridge may plausibly be interpreted as a line of hot-spot alkaline volcanoes.

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Chemical compositions and 1-atm. phase relations were determined for basalts drilled from Holes 501, 504A, 504B, 505, and 505B on Legs 68, 69, and 70 of the Deep Sea Drilling Project. Chemical, experimental, and petrographic data indicate that these basalts are moderately evolved (Mg' values from 0.60 to 0.70), with olivine plus Plagioclase and often clinopyroxene on the liquidus. Chemical stratigraphy was used to infer that sequential influxes of magma into a differentiating magma chamber or separate flows from different magma chambers or both had occurred. Two major types of basalt were found to be inter layered: Group M, a rarely occurring type with major element chemistry and magmaphile element abundances within the range of the majority of ocean-floor basalts (TiO2 = 1.3%, Na2O 2.5%, Zr = 103 ppm, Nb = 2.5 ppm, and Y = 31 ppm); and Group D, a highly unusual series of basalt compositions that exhibit much lower magmaphile element abundances (TiO2 = 0.75-1.2%, Na2O = 1.7-2.3%, Zr = 34-60 ppm, Nb = 0.5-1.2 ppm, and Y = 16-27 ppm). The liquidus temperatures of the Group D basalts are high (1230- 1260°C) compared with those of other ocean-floor basalts of similar Mg' values. They have high CaO/Na2O ratios (5-8) and are calculated to be in equilibrium with unusually calcic Plagioclase (An78-84). The two basalt groups cannot be related by fractionation processes. However, constant Zr/Nb ratios (>40) for the two groups suggest a single mantle source, with differences in magmaphile element abundances and other element ratios (e.g., Zr/Ti, Zr/Y, Ce/Yb) arising through sequential melting of the same source. Magmas similar to Group D, if mixed with more typical mid-ocean-ridge basalt (MORB) magmas in shallow magma chambers, could provide a source for the highly calcic Plagioclase phenocrysts that appear in more common (i.e., less depleted) phyric ocean-floor basalts.

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Abundances of rare earth elements (REE), Ba, and Sr and isotopic ratios of Sr, Nd, and Ce were determined for six samples of basalts drilled at Hole 504B on Leg 111 of the Ocean Drilling Program. Analyses found that these basalts are the most depleted in Sr, Ba, and light REE among mid-ocean ridge basalts (MORB); Ba depletion is especially notable. On the other hand, Sr, Nd, and Ce isotopic ratios for basalts from Hole 504B are within the range of typical MORB values.

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Basalts from Hole 504B, Leg 83, exhibit remarkable uniformity in major and trace element composition throughout the 1075.5 m of basement drilled. The majority of the basalts, Group D', have unusual compositions relative to normal (Type I) mid-ocean ridge basalts (MORB). These basalts have relatively high mg values (0.60-0.70) and CaO abundances (11.7-13.7%; Ca/Al = 0.78-0.89), but exhibit a marked depletion in compatible trace elements (Cr and Ni); moderately incompatible trace elements (Zr, Y, Ti, etc.); and highly incompatible trace elements (Nb, LREE, etc.). Petrographic and compositional data indicate that most of these basalts are evolved, having fractionated significant amounts of plagioclase, olivine, and clinopyroxene. Melting experiments on similar basalt compositions from the upper portion of Hole 504B (Leg 70; Autio and Rhodes, 1983) indicate that the basalts are co-saturated with olivine and plagioclase and often clinopyroxene on the 1-atm. liquidus. Two rarely occurring groups, M' and T, are compositionally distinct from Group D' basalts. Group T is strongly depleted in all magmaphile elements except the highly incompatible ones (Nb, La, etc.), while Group M' has moderate concentrations of both moderately and highly incompatible trace elements and is similar to Type I MORB. Groups M' and T cannot be related to Group D' nor to each other by crystal fractionation, crystal accumulation, or magma mixing. The large differences in magmaphile element ratios (Zr/Nb, La/Yb) among these three chemical groups may be accounted for by complex melting models and/or local heterogeneity of the mantle beneath the Costa Rica Ridge. Xenocrysts and xenoliths of plagioclase and clinopyroxene similar in texture and mineral composition to crystals in coarse-grained basalts from the lower portion of the hole are common in Hole 504B basalts. These suggest that addition of solid components either from conduit or magma chamber walls has occurred and may be a common source of disequilibrium crystals in these basalts. However, mixing of plagioclase-laden depleted melts (similar to the Costa Rica Ridge Zone basalts) with normal MORB magmas could provide an alternate source for some refractory plagioclase crystals found out of equilibrium in many phyric MORB. The uniformity of major element compositions in Hole 504B basalts affords an ideal situation for investigating the effects of alteration on some major and trace elements in oceanic basalts. Alteration observed in whole-rock samples records primarily two events - a high-temperature and a low-temperature phase. High-temperature phases include: chlorite, talc, albite, actinolite, sphene, quartz, and pyrite. The low-temperature phases include smectite (saponite), epistilbite or laumontite, and minor calcite. Laumontite may actually straddle the gap between the low- and high-temperature mineral assemblages. Alteration is restricted primarily to partial replacement of primary phases. Metamorphic grade, in general, increases from the top to the bottom of Hole 504B (Legs 69, 70, and 83) as seen in the change from a smectiteto- chlorite-dominated secondary mineral assemblage. However, a systematic progression for the interval recovered during Leg 83 is not apparent. Rather, the extent of alteration appears to be a function of the initial texture and fracture density. Variations in whole-rock major and trace element concentrations cannot be attributed convincingly to any differences in alteration observed. Compositional characteristics of the secondary minerals indicated that extensive remobilization of elements has not occurred; local redistribution is suggested in most cases. Thus, the major and trace element signature of these basalts remains effectively the same as the original composition prior to alteration.

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Ocean Drilling Program Leg 135 backarc basin lavas are characterized by anomalously high Au contents (1.0-11.4 ppb) and strongly fractionated relative platinum group element (PGE) abundances (Pd/Ir ratio, approximately 100). The Rh and Ir contents are very low, ranging from below detection (approximately 0.02 ppb) to 0.08 ppb. The Pd and Pt contents range from <0.3 to 4 ppb. Rh, Pd, and Pt values are consistently and significantly higher in Site 836 and 839 samples relative to those from Sites 834 and 835. Major, trace, and rare earth element (REE) data suggest Sites 836 and 839 have a more pronounced arc signature than Sites 834 and 835. No correlation exists between noble metal abundance and indices of alteration or fractionation (e.g., loss on ignition (LOI), Mg#, and Cr or Ni contents), suggesting that measured values and ratios are primary and reflect characteristics of the mantle source. The evaluation of Leg 135 noble metal data with respect to potential mantle-source components is hindered by the lack of data on magmas derived from such sources. However, analyses of the limited available data for the different magma types suggest that the characteristic enrichment of Leg 135 lavas in Au, relative to Pd and Cu, cannot be derived solely from simple MORB-type or ocean-island-type mantle, or mantle depleted by a previous melt extraction event. The Au-enriched signature of the Lau basin lavas could, however, be produced through the addition of a sedimentary component from the downgoing slab. Separation of Au from the PGE occurs within oceanic hydrothermal systems and gold values of the resultant precipitates are 2-3 orders of magnitude higher than other oceanic crustal components. Even small additions of this component from the downgoing oceanic crust to a supra-subduction zone mantle melt could account for the high mean Au/Pd ratios of the Leg 135 samples (Sites 834 and 835, Au/Pd = 5.04; Sites 836 and 839, Au/Pd = 2.26).

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Selected basalts from a suite of dredged and drilled samples (IPOD sites 525, 527, 528 and 530) from the Walvis Ridge have been analysed to determine their rare earth element (REE) contents in order to investigate the origin and evolution of this major structural feature in the South Atlantic Ocean. All of the samples show a high degree of light rare earth element (LREE) enrichment, quite unlike the flat or depleted patterns normally observed for normal mid-ocean ridge basalts (MORBs). Basalts from Sites 527, 528 and 530 show REE patterns characterised by an arcuate shape and relatively low (Ce/Yb)N ratios (1.46-5.22), and the ratios show a positive linear relationship to Nb content. A different trend is exhibited by the dredged basalts and the basalts from Site 525, and their REE patterns have a fairly constant slope, and higher (Ce/Yb)N ratios (4.31-8.50). These differences are further reflected in the ratios of incompatible trace elements, which also indicate considerable variations within the groups. Mixing hyperbolae for these ratios suggest that simple magma mixing between a 'hot spot' type of magma, similar to present-day volcanics of Tristan da Cunha, and a depleted source, possibly similar to that for magmas being erupted at the Mid-Atlantic Ridge, was an important process in the origin of parts of the Walvis Ridge, as exemplified by Sites 527, 528 and 530. Site 525 and dredged basalts cannot be explained by this mixing process, and their incompatible element ratios suggest either a mantle source of a different composition or some complexity to the mixing process. In addition, the occurrence of different types of basalt at the same location suggests there is vertical zonation within the volcanic pile, with the later erupted basalts becoming more alkaline arid more enriched in incompatible elements. The model proposed for the origin and evolution of the Walvis Ridge involves an initial stage of eruption in which the magma was essentially a mixture of enriched and depleted end-member sources, with the N-MORB component being small. The dredged basalts and Site 525, which represent either later-stage eruptives or those close to the hot spot plume, probably result from mixing of the enriched mantle source with variable amounts and variable low degrees of partial melting of the depleted mantle source. As the volcano leaves the hot spot, these late-stage eruptives continue for some time. The change from tholeiitic to alkalic volcanism is probably related either to evolution in the plumbing system and magma chamber of the individual volcano, or to changes in the depth of origin of the enriched mantle source melt, similar to processes in Hawaiian volcanoes.

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Major-, trace-, and rare-earth element analyses are presented from a suite of basaltic rocks from the basement of the Celebes Sea. The major elements and trace-elements were determined by X-ray fluorescence techniques, and the rare-earth elements were analyzed by instrumental neutron activation analysis. Compositionally the Celebes Sea basalts are very similar to typical normal mid-ocean ridge basalts, such as those described from the Indian Ocean triple junction. Petrogenetic modeling shows that all of the basalts analyzed can be formed by 10% to 20% partial melting of a light rare-earth element-depleted spinel lherzolite followed by fractional crystallization of mixtures of olivine, Plagioclase, and iron oxide. The Celebes Sea is interpreted as a fragment of the basement of the Jurassic Argo abyssal plain trapped during the Eocene to the north of Australia.