Geochemistry and Nd-Pb isotope ratios of sediments from the Lesser Antilles arc

The Lesser Antilles arc is a particularly interesting island arc because it is presently very active, it is located perpendicular to the South American continent and its chemical and isotopic compositions display a strong north-south gradient. While the presence in the south of a thick pile of sedimentary material coming from the old South American continent has long been suspected to explain the geochemical gradient, previous studies failed to demonstrate unambiguously a direct link between the arc lava compositions and the subducted sediment compositions. Here, we present new Nd, Sm, Th, U and Pb concentrations and Nd-Pb isotopic data for over 60 sediments from three sites located in the fore arc region of the Lesser Antilles arc. New data for DSDP Site 543 drill core located east of Dominica Island complement the data published by White et al. (1985, doi:10.1016/0016-7037(85)90082-1) and confirm their relatively uniform isotopic compositions (i.e., 206Pb/204Pb between 19.13 and 19.53). In contrast, data obtained on DSDP Site 144 located further south, on the edge of the South American Rise and on sediments from Barbados Island are much more variable (206Pb/204Pb ranges from 18.81 to 27.69). The very radiogenic Pb isotopic compositions are found in a 60 m thick black shale unit, which has no age equivalent in the Site 543 drill core. We interpret the peculiar composition of the southern sediments as being due to two factors, (a) the proximity of the South American craton, which contributes coarse grain old detrital material that does not travel far from the continental shelf, and (b) the presence of older sediments including the thick black shale unit formed during Oceanic Anoxic events 2 and 3. The north-south isotopic change known along the Lesser Antilles arc can be explained by the observed geographical changes in the composition of the subducted sediments. About 1% contamination of the mantle wedge by Site 543 sediments explains the composition of the northern islands while up to 10% sediments like those of Site 144 is required in the source of the southern island lavas. The presence of black shales in the subducted pile provides a satisfactory explanation for the very low Delta8/4 values that characterize the Lesser Antilles arc.

(Table 1) Stratigraphic position, abundance, shape, color, and refractive indices of glass shards of selected samples from DSDP Leg 60 Holes vitric muds

Precisely determined refractive indices of glass shards from 32 ash-rich, volcaniclastic sediments, mostly turbidites interbedded with nonvolcanic sediments in the Mariana Trough, range from 1.480 to 1.585 (corresponding to SiO2 ca. 75 to 49%), with most in the range 1.500 to 1.540 (SiO2 ca. 70-62%) and a second, smaller mode between ca. 1.560 and 1.585 (57 to 49% SiO2). Shards are almost exclusively colorless from 1.480 to ca. 1.530, light brown with minor colorless and green tones between 1.530 and 1.560, and dominantly brown at higher refractive indices. Tubular pumice shards are more common at higher silica percentages and non- to poorly-vesicular cuniform shards at low SiO2 values, but there is no clear correlation between shape and composition of shards. About half of the samples have bimodal shard populations with silica differences ranging up to 20 percent; unimodal layers have a range of up to about 7 percent SiO2. Of 21 samples in which one type of shard dominates, seven have the main mode in the rhyolitic composition (>69% SiO2), eight in the intermediate range (56 to 69% SiO2), and five in mafic composition (SiO2 <53%). These unusually abundant mafic shards occur mainly in site survey piston cores, SP-IA and 4E, and in Holes 454, 456, 458, and 459B. These are the sites closest to the present arc. Hole 453, containing by far the most vitric tuff turbidites, shows a gradual increase in silica content of ash layers upward to the hole from Cores 36 to 19 (about 4.6 to 3.0 Ma). A drastic decrease in ash-rich beds in the younger (Pleistocene) part of this hole was noted by the shipboard party (see site chapter, Site 453) and was interpreted by them as indicating increasing distance from the arc volcanoes as the trough opened. The increase in silica in ashes from the early to the late Pliocene at Site 453 could be interpreted in the same way and might indicate that the trough started to open in early Pliocene time.

Petrography and geochemistry of basement rocks from five DSDP Leg 60 Sites

Bulk chemistry and trace elements data were measured in 72 samples, selected from 5 basement sections, which have been recovered by Leg 60 drilling (Sites 453, 454, 456, 458, and 459). According to analytical results a metagabbro- metabasalt breccia, deposited about 5 Ma at the westernmost flank of the Mariana Trough (Site 453), was derived from an island arc source. Basalts from the Mariana Trough (Sites 454 and 456) are similar in many respects to midoceanic ridge basalts (MORB). Yet rocks of unusual geochemistry, reflecting the possible influence of arc volcanism, were found among the pillow lavas at the easternmost trough (Site 456). The acoustic basement in the Mariana fore-arc region was formed by submarine eruptions of arc tholeiites (Sites 458 and 459) and peculiar high-MgO andesites related to the boninite suite.

Crystal morphologies and pyroxene compositions in boninites and tholeiitic basalts at DSDP Holes 60-458 and 60-459B

Not all boninites are glassy lavas. Those of Hole 458 in the Mariana fore-arc region are submarine pillow lavas and more massive flows in which glass occurs only in quenched margins. Pillow and flow interiors have abundant Plagioclase spherulites, microlites, or even larger crystals but can be recognized as boninites by (1) occurrence of bronzite, (2) presence of augite-bronzite microphenocryst intergrowths, and (3) reversal of the usual basaltic groundmass crystallization sequence of plagioclase-augite to augite-plagioclase. The latter is accentuated by sharply contrasting augite and Plagioclase crystal morphologies near pillow margins, a consequence of rapid cooling rates. This crystallization sequence appears to be a consequence of boninites having higher SiO2 and Mg/Mg + Fe than basalts but lower CaO/Al2O3. Microprobe data are used to illustrate the effects of rapid cooling on the compositions of pyroxene and microphenocrysts in a glassy boninite sample and to estimate temperatures of crystallization of coexisting bronzite and augite. A range from 1320°C to 1200°C is calculated with an average of 1250°C. This is higher by 120°-230° than the known range for western Pacific arc tholeiites and by over 300° than for calc-alkalic andesites. Boninites of Hole 458 lack olivine and clinoenstatite but are otherwise chemically and petrographically similar to boninites that have these minerals. In order to distinguish the two types, the Hole 458 lavas are here termed boninites and the others are termed olivine boninites. Arc tholeiite pillow lavas from Holes 458 and 459B are briefly described and their textures compared to fractionated, moderately iron-enriched, abyssal tholeiites. Massive tholeiite flows contain striking quartz-alkali feldspar micrographic intergrowths with coarsely spherulitic textures resulting from in situ magmatic differentiation. Such intergrowths are rare in massive abyssal tholeiites cored by DSDP and probably occur here because arc tholeiites have higher normative quartz at comparable degrees of iron enrichment - a result of higher oxygen fugacities and earlier separation of titanomagnetite - than abyssal tholeiites.

Seismic velocities at elevated pressures of igneous rocks at DSDP Holes 60-454 and 60-458

The Deep Sea Drilling Project, in addition to providing valuable information on the history and processes of development of the ocean, has significantly contributed to our knowledge of the chemical and physical nature of the upper oceanic crust. Among the important physical properties of the crust are its seismic velocity and structure, the interpretation of which requires laboratory studies of seismic velocities in oceanic rocks.

40Ar-39Ar dating of rocks drilled at sites 60-458 and 60-459

40Ar-39Ar dating of a high-MgO bronzite andesite from near the top of basement drilled at Site 458 shows the characteristic symptoms of artificially disturbed samples - i.e., an inverse staircase-type age spectrum, approximate linearity on an isochron plot, and concordance between total fusion age and isochron age. From conclusions based on other artificially disturbed samples (Ozima et al., 1979), we suggest that the reference isochron age (33.6 Ma) approximates the age of the sample. A basalt from deeper in Hole 458 gives an isochron age of 19.1 ± 0.2 Ma, which is slightly younger than the plateau age of 21.4 ± 1.0 Ma. Both ages are, however, considerably younger than the age of fossils in the overlying sediments (30 - 34 Ma). The age discrepancy may be explained if the 40Ar-39Ar age represents the age of secondary minerals, which formed later. No useful age data were obtained from a basalt sample recovered from Hole 459B.

Minerals, geochemistry and x-ray power characteristics at DSDP Sites 54-424, 60-458 and 60-459

Secondary minerals filling veins and vesicles in volcanic basement at Deep Sea Drilling Project Sites 458 and 459 indicate that there were two stages of alteration at each site: an early oxidative, probably hydrothermal, stage and a later, low-temperature, less oxidative stage, probably contemporaneous with faulting in the tectonically active Mariana forearc region. The initial stage is most evident in Hole 459B, where low-Al, high Fe smectites and iron hydroxides formed in vesicles in pillow basalts and low-Al palygorskite formed in fractures. Iron hydroxides and celadonite formed in massive basalts next to quartz-oligoclase micrographic intergrowths. Palygorskite was found in only one sample near the top of basement in Hole 458, but it too is associated with iron hydroxides. Palygorskite has previously been reported only in marine sediments in DSDP and other occurrences. It evidently formed here as a precipitate from fluids in which Si, Mg, Fe, and even some Al were concentrated. Experimental data suggest that the solutions probably had high pH and somewhat elevated temperatures. The compositions of associated smectites resemble those in hydrothermal sediments and in basalts at the Galapagos mounds geothermal field. The second stage of alteration was large-scale replacement of basalt by dioctahedral, trioctahedral, or mixed-layer clays and phillipsite along zones of intense fracturing, especially near the bottom of Holes 458 and 459B. The basalts are commonly slickensided, and there are recemented microfault offsets in overlying sediments. Native copper occurs in one core of Hole 458, but associated smectites are dominantly dioctahedral, unlike Hole 459B, where they are mainly trioctahedral, indicating nonoxidative alteration. The alteration in both holes is more intense than at most DSDP ocean crust sites and may have been augmented by water derived from subducting ocean crust beneath the fore-arc region.