Composition of bioclastic sands, carbonates and pyroclastic rocks of the Great Meteor and Josephine Seamounts, eastern North Atlantic

1. Great Meteor Seamount (GMS) is a very large (24,000 km**3) guyot with a flat summit plateau at 330-275 m; it has a volcanic core, capped by 150-600 m of post-Middle-Miocene carbonate and pyroclastic rocks, and is covered by bioclastic sands. The much smaller Josephine Seamount (JS, summit 170- 500 m w. d.) consists mainly of basalt which is only locally covered by limestones and bioclastic sands. 2. The bioclastic sands are almost free of terrigenous components, and are well sorted, unimodal medium sands. (1) "Recent pelagic sands" are typical of water depths > 600 m (JS) or > 1000 m (GMS). (2) "Sands of mixed relict-recent origin" (10-40% relict) and (3) "relict sands" (> 40% relict) are highly reworked, coarse lag deposits from the upper flanks and summit tops in which recent constituents are mixed with Pleistocene or older relict material. 3. From the carbonate rocks of both seamounts, 12 "microfacies" (MF-)types were distinguished. The 4 major types are: (1) Bio(pel)sparites (MF 1) occur on the summit plateaus and consist of magnesian calcite cementing small pellets and either redeposited planktonic bioclasts or mixed benthonic-planktonic skeletal debris ; (2) Porous biomicrites (MF 2) are typical of the marginal parts of the summit plateaus and contain mostly planktonic foraminifera (and pteropods), sometimes with redeposited bioclasts and/or coated grains; (3) Dense, ferruginous coralline-algal biomicrudites with Amphistegina sp. (MF 3.1), or with tuffaceous components (MF 3.2); (4) Dense, pelagic foraminiferal nannomicrite (MF 4) with scattered siderite rhombs. Corresponding to the proportion and mineralogical composition of the bioclasts and of the (Mgcalcitic) peloids, micrite, and cement, magnesian calcite (13-17 mol-% MgCO3) is much more abundant than low-Mg calcite and aragonite in rock types (1) and (2). Type (3) contains an "intermediate" Mg-calcite (7-9 mol-X), possibly due to an original Mg deficiency or to partial exsolution of Mg during diagenesis. The nannomicrite (4) consists of low-Mg calcite only. 4. Three textural types of volcanic and associated gyroclastic rocks were distinguished: (1) holohyaline, rapidly chilled and granulated lava flows and tuffs (palagonite tuff breccia and hyaloclastic top breccia); (2) tachylitic basalts (less rapidly chilled; with opaque glass); and (3) "slowly" crystallized, holocrystalline alkali olivine basalts. The carbonate in most mixed pyroclastic-carbonate sediments at the basalt contact is of "post-eruptive" origin (micritic crusts etc.); "pre-eruptive" limestone is recrystallized or altered at the basalt contact. A deuteric (?hydrothermal) "mineralX", filling vesicles in basalt and cementing pyroclastic breccias is described for the first time. 5. Origin and development of GMS andJS: From its origin, some 85 m. y. ago, the volcano of GMS remained active until about 10 m. y. B. P. with an average lava discharge of 320 km**3/m. y. The volcanic origin of JS is much younger (?Middle Tertiary), but the volcanic activity ended also about 9 m. y. ago. During L a t e Miocene to Pliocene times both volcanoes were eroded (wave-rounded cobbles). The oldest pyroclastics and carbonates (MF 3.1, 3.2) were originally deposited in shallow-water (?algal reef hardground). The Plio (-Pleisto) cene foraminiferal nannomicrites (MF 4) suggest a meso- to bathypelagic environment along the flanks of GMS. During the Quaternary (?Pleistocene) bioclastic sands were deposited in water depths beyond wave base on the summit tops, repeatedly reworked, and lithified into loosely consolidated biopelsparites and biomicrites (MF 1 and 2; Fig. 15). Intermediate steps were a first intragranular filling by micrite, reworking, oncoidal coating, weak consolidation with Mg-calcite cemented "peloids" in intergranular voids and local compaction of the peloids into cryptocrystalline micrite with interlocking Mg-calcite crystals up to 4p. The submarine lithification process was frequently interrupted by long intervals of nondeposition, dissolution, boring, and later infilling. The limestones were probably never subaerially exposed. Presently, the carbonate rocks undergo biogenic incrustation and partial dissolution into bioclastic sands. The irregular distribution pattern of the sands reflects (a) the patchy distribution of living benthonic organisms, (b) the steady rain of planktonic organism onto the seamount top, (c) the composition of disintegrating subrecent limestones, and (d) the intensity of winnowing and reworking bottom current

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.

40Ar-39Ar investigation of volcanic clasts in glaciogenic sediments at ODP Sites 1097 and 1103

Three selected diamictite samples recovered within sequence group S3 at Sites 1097 (Sample 178-1097A-27R-1, 35-58 cm) and 1103 (Samples 178-1103A-31R-2, 0-4 cm, and 36R-3, 4-8 cm) of Ocean Drilling Program Leg 178 have been investigated by scanning electron microscope, electron microprobe, and 40Ar-39Ar laser-heating techniques. They contain variable proportions of fragments of volcanic rock groundmass (mostly in the range of 100-150 µm) with textures ranging from microcrystalline to ipocrystalline. Their rounded shapes indicate mechanical reworking. Fresh groundmass glasses, recognized only in grains from samples of Site 1103, show mainly a subalkaline affinity on the basis of total alkali-silica variations. However, they are characterized by relatively high TiO2 and P2O5 contents (1.4-2.8 and 0.1-0.9 wt%, respectively). Because of the small size of homogeneous grains (100-150 µm), they were not suitable for single-grain total fusion 40Ar-39Ar analyses. The incremental laser-heating technique was applied to milligram-sized samples (only for Samples 178-1097A-27R-1, 35-58 cm, and 178-1103A-36R-3, 4-8 cm) and to various small fractions (each consisting of 10 grains for the sample from Site 1097 and 30 grains for samples from Site 1103). The latter approach resulted in more effective resolution of sample heterogeneity. Argon ages from the small fractions show significantly different ranges in the three samples: 75-173 Ma for Sample 178-1097A-27R-1, 35-58 cm, 18-57 Ma for Sample 178-1103A-31R-2, 0-4 cm, and 7.6-50 Ma for Sample 178-1103A-36R-3, 4-8 cm. Ca/K ratios derived from argon isotopes at Site 1103 suggest that the data mainly refer to outgassing of groundmass glass. At Site 1103, we observe an overall apparent age increase with decreasing sample depth. This is compatible with glacial erosion that affected with time deeper levels of a volcanic sequence previously deposited on the continent. The youngest apparent age of 7.6 ± 0.7 Ma detected close to the bottom of Hole 1103A (340 meters below seafloor [mbsf]) is compatible with the age range of the diatom Actinocyclus ingens v. ovalis Zone (6.3-8.0 Ma) determined for the interval 320-355 mbsf and with the maximum ages derived from strontium isotope composition of barnacle fragments obtained at 262-263 mbsf at the same site. Nevertheless, this age cannot be taken as the maximum youngest age of the volcanic sequence sampled by glacial erosion or as the maximum age for the deposition of the Sequence S3 at 340 mbsf unless validated by further research.

Geochemistry of basalts at DSDP Leg 73 Holes

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.

Geochemistry of Cenozoic ash layers from the Kerguelen Plateau

Geochemical investigations were conducted on 10 discrete ash layers and 22 samples of dispersed ash accumulations from Sites 747, 749, and 751 of Ocean Drilling Program (ODP) Leg 120 to the Kerguelen Plateau in the southern Indian Ocean. The chemical data obtained from some 400 single-grain glass analyses allow the characterization of two rock series. The first consists of transitional to alkali basalts; the second, mainly of trachytes with subordinated rhyolites, all reflecting the characteristic magmatological evolution of the Kerguelen Plateau as a hotspot-related volcanism. Chemical correlation with possible source areas indicates that the ashes were most probably erupted from the Kerguelen Islands. The investigated ash layers clearly reflect the Oligocene to Quaternary changes in the composition of the volcanic material recorded from the Kerguelen Islands. In addition to the Kerguelen Islands, Heard Island, Crozet Island, and other sources may have contributed to deposition of the tephras. Pleistocene tephras of "exotic" calc-alkaline composition are most probably derived from enhanced magmatic activity during that time span at the South Sandwich island arc. When using data obtained from tephras of the ODP Leg 119 Kerguelen sites, several eruptive periods can be correlated through the composition of the deposited ashes. Some of them are widely distributed over the Kerguelen Plateau and are seen as a first step toward a southern Indian Ocean tephrostratigraphy.

(Table 1) Percentage of non-opaque heavy minerals, heavy residue, magnetite, and opaque heavy minerals, DSDP Sites 58-445 and 58-446

Heavy-mineral analyses were made for 39 samples, 27 from DSDP Site 445 and 12 from Site 446. About one-fourth of the samples were so loose that they were easily disaggregated in water. The amount of heavy residue and the magnetite content of the heavy fraction were very high, 0.2 to 44 per cent and (on the average) more than 20 per cent, respectively. Among the non-opaque heavy minerals, common hornblende (0 to 80%) and augite (0 to 98%) are most abundant. Pale-green and bluish-green amphiboles (around 10%) and the epidote group (a few to 48%) are next in abundance. Euhedral apatite and biotite and irregularly shaped chromite are not abundant, but are present throughout the sequence. Hacksaw structure is developed in pale-green amphibole and augite. At Site 445, a fair amount of chlorite and a few glauconite(?) grains are present from Core 445-81 downward. The content of common hornblende and opaque minerals also changes from Core 445-81 downward. A geological boundary may exist between Cores 445-77 and 445-81. Source rocks of the sediments at both sites were basaltic volcanic rocks (possibly alkali suite), schists, and ultramafic rocks. The degree of lithification and amount of heavy residue, and the content of magnetite, non-opaque heavy minerals (excluding mafic minerals), and mafic minerals in the cores were compared with Eocene, Oligocene, and Miocene sandstones of southwest Japan. In many respects, the sediments at Sites 445 and 446 are quite different from those of southwest Japan. From the early Eocene to the early Miocene, the area of these sites belonged to a different geologic province than southwest Japan.

Isotope ratios of smectites, calcites and whole rocks from DSDP Legs 51 and 52, Atlantic Ocean

Delta180 and 87Sr/86Sr isotopic data from smectites, calcites, and whole rocks, together with published isotopic age determinations, alkali element concentration data and petrographic observations suggest a sequential model of ocean floor alteration. The early stage lasts about 3 m.y. and is characterized by palagonite and smectite formation, and solutions with a large basaltic component, increasing with temperature which varies from 15° to 80° C at DSDP site 418A. Most carbonates are deposited after this stage from solutions with a negligible basaltic Sr component and temperatures of 15° to 40° C. Water of seawater Sr and O isotopic composition is shown to percolate to at least 500 m into the basaltic basement. No evidence was found for continuing exchange of strontium or oxygen after 3 m.y.

Geochemistry of tephra layers in ODP Site 121-758

A tephrochronology of the past 5 Ma is constructed with ash layers recovered from Neogene sediments during drilling at ODP Leg 121 Site 758 on northern Ninetyeast Ridge. The several hundred tephra layers observed in the first 80 m of cores range in thickness from a few millimeters to 34 cm. Seventeen tephra layers, at least 1 cm thick, were sampled and analyzed for major elements. Relative ages for the ash layers are estimated from the paleomagnetic and d18O chronostratigraphy. The ash layers comprise about 1.7% by volume of the sediments recovered in the first 72 m. The median grain size of the ashes is about 75 ?m, with a maximum of 150 ?m. The ash consists of rhyolitic bubble junction and pumice glass shards. Blocky and platy shards are in even proportion (10%-30%) and are dominated by bubble wall shards (70%-90%). The crystal content of the layers is always less than 2%, with Plagioclase and alkali feldspar present in nearly every layer. Biotite was observed only in the thickest layers. The major element compositions of glass and feldspar reflect fractionation trends. Three groupings of ash layers suggest different provenances with distinct magmatic systems. Dating by d18O and paleomagnetic reversals suggests major marine ash-layer-producing eruptions (marine tephra layers > 1 cm in thickness) occur roughly every approximately 414,000 yr. This value correlates well with landbased studies and dates of Pleistocene Sumatran tuffs (average 375,000-yr eruptive interval). Residence times of the magmatic systems defined by geochemical trends are 1.583, 2.524, and 1.399 Ma. The longest time interval starts with the least differentiated magma. The Sunda Arc, specifically Sumatra, is inferred to be the source region for the ashes. Four of the youngest five ash layers recovered correlate in time and in major element chemistry to ashes observed on land at the Toba caldera.

Geochemistry of basaltic rocks from ODP Hole 183-1137A on the Kerguelen Plateau

Cretaceous basalts recovered during Ocean Drilling Program Leg 183 at Site 1137 on the Kerguelen Plateau show remarkable geochemical similarities to Cretaceous continental tholeiites located on the continental margins of eastern India (Rajmahal Traps) and southwestern Australia (Bunbury basalt). Major and trace element and Sr-Nd-Pb isotopic compositions of the Site 1137 basalts are consistent with assimilation of Gondwanan continental crust (from 5 to 7%) by Kerguelen plume-derived magmas. In light of the requirement for crustal contamination of the Kerguelen Plateau basalts, we re-examine the early tectonic environment of the initial Kerguelen plume head. Although a causal role of the Kerguelen plume in the breakup of Eastern Gondwana cannot be ascertained, we demonstrate the need for the presence of the Kerguelen plume early during continental rifting. Activity resulting from interactions by the newly formed Indian and Australian continental margins and the Kerguelen plume may have resulted in stranded fragments of continental crust, isolated at shallow levels in the Indian Ocean lithosphere.

Geochemistry of glass shards in the volcanic ash layers of the Izu-Bonin Arc

To examine the processes and histories of arc volcanism and of volcanism associated with backarc rifting. 130 samples containing igneous glass shards were taken from the Plioccne-Quatemai^ succession on the rift Hank (Site 788) and the Quaternary fill in the basin fill of the Sumisu Rift (Sites 790 and 791). These samples were subsequently analyzed at the University of Illinois at Chicago and Shizuoka University. The oxides determined by electron probe do not account for the total weight of the material; differences between summed oxides and 100% arise from the water contents, probably augmented by minor losses thai result from alkali vaporization during analysis. Weight losses in colorless glasses are up to 9%; those in brown glasses (dacitcs to basalts) arc no more than 4.5%; shards from the rift-flank (possibly caused by prolonged proximity to ihc scafloor) generally have higher values than those from the rift-basin fill How much of the lost water is magmatic, and how much is hydrated is uncertain; however, although the shards absorb potassium, calcium, and magnesium during hydration in the deep sea, they do so only to a minor extent that does not significantly alter their major element compositions. Therefore, the electron-probe results are useful in evaluating the magmatism recorded by the shards. Pre- and syn-rift Izu-Bonin volcanism were overwhelmingly dominated by rhyolile explosions, demonstrating that island arcs may experience significant silicic volcanism in addition to the extensive basaltic and basaltic andestic activity, documented in many arcs since the 1970s, that occurs in conjunction with the andesitic volcanism formerly thought to be dominant. Andesitic eruptions also occurred before rifting, but the andesitic component in our samples is minor. All the pre- and syn-rift rhyolites and andesites belong to the low-alkali island-arc tholeiitic suite, and contrast markedly with the alkali products of Holocene volcanism on the northernmost Mariana Arc that have been attributed to nascent rifting. The Quaternary dacites and andesites atop the rift flank and in the rift-basin fill are more potassic than those of Pliocene age, as a result of assimilation from the upper arc crust, or from variations in degrees of partial melting of the source magmas, or from metasomatic fluids. All the glass layers from the rift-flank samples belong to low-K arc-tholeiitic suites. Half of those in the Pliocene succession are exclusively rhyolitic: the others contain minor admixtures of dacite and andesite, or andesite and either basaltic andesite or basalt. In Contrast, the Quaternary (syn-rift) volcaniclastics atop the rift-flank lack basalt and basaltic andesite shards. These youngest sediments of the rift flank show close compositional affinities with five thick layers of coarse, rhyolitic pumice deposits in the basin fill, the two oldest more silicic than the younger ones. The coarse layers, and most thin ash layers that occur in hemipelagites below and intercalated between them, are low-K rhyolites and therefore probably came from sources in the arc. However, several thin rhyolitic ash beds in the hemipelagites are abnormally enriched in potassium and must have been provided by more distal sources, most likely to the west in Japan. Remarkably, the Pliocene-Pleistocene geochemistry of the volcanic front does not appear to have been influenced by the syn-rift basaltic volcanism only a few kilometers away. Rare, thin layers of basaltic ash near the bases of the rift-basin successions are not derived from the arc. They deviate strongly from trends that the arc-derived glasses display on oxide-oxide plots, and show close affinities to the basalts empted all over the Sumisu Rift during rifting. These basalts, and the basaltic ashes in the basal rift-basin fill, arc compositionally similar to those erupted from mature backarc basins elsewhere.

Geochemistry of ash layers from the Bonin Forearc

Many ash-rich layers, varying from a few millimeters to several centimeters thick, were identified in the sedimentary sequences penetrated during Ocean Drilling Program Leg 125 at Sites 782, 784, and 786, located about 400 to 500 km south of Tokyo in the Bonin forearc. The total age range of the ash layers is from Eocene to Pleistocene, although not all sites cover this full span. The ashes consist of vitric, microlite-bearing, and crystal-rich components; the glassy shards are typically highly vesicular, with elongate, flattened bubbles. The dominant crystalline phases are orthopyroxene, clinopyroxene, and plagioclase. The major-element compositions of individual vitric shards collected from selected layers of Holes 782A, 784A, and 786A were determined by electron microprobe analyses; particular care was taken to ensure that the analytical results were not compromised by electron beam damage to the glasses. Compositions range from basalt through andesite and dacite to rhyolite and generally belong to a tholeiitic, low-K suite. There is no indication of any regular secular change during the evolution of the Bonin arc from tholeiitic through calc-alkalic to alkali compositions with time. In Holes 782A and 784A, some high-K rhyolite compositions of late Miocene and Pleistocene age are present. A clear chemical distinction has existed since arc inception between the source(s) of these ashes and the upper mantle source(s) tapped during construction of the igneous basement that formed the forearc.

Geochemistry and petrographic characters at DSDP Leg 58 Holes

Refractive index and chemical composition were determined for glass shards contained in more than 100 tephra layers in DSDP Leg 58 sediment cores collected in the Shikoku Basin, North Philippine Sea. The refractive index is consistent with chemical composition. Refractive index and total iron show a linear relationship. Tephra in Pleistocene and Pliocene sediments is mostly rhyolitic and dacitic (non-alkali), whereas tephra in the Miocene shows wide composition variations in the eastern part of the basin. Basaltic tephra is recognized in Miocene sediments at Sites 443 and 444, but not at Site 442, west of the other two sites. This indicates that the basaltic tephra came from eruption relatively close to those drill sites (perhaps the Kinan Seamounts and the Shichito-Iwo Jima volcanic arc), although the exact source has not been identified.