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

Isotopic composition and major and trace element concentrations of mafic rocks obtained from western Dronning Maud Land

On the basis of new bulk major and trace element (including REE) as well as Sm-Nd and Rb-Sr isotope data, used in conjunction with available geochronological data, a post-tectonic mafic igneous province and four groups of pre- to syntectonic amphibolite are distinguished in the polymetamorphic Maud Belt of western Dronning Maud Land, East Antarctica. Protoliths of the Group 1 amphibolites are interpreted as volcanic arc mafic intrusions with Archaean to Palaeoproterozoic Nd model ages and depletion in Nb and Ta. Isotopic and lithogeochemical characteristics of this earliest group of amphibolite indicate that the Maud Belt was once an active continental volcanic arc. The most likely position of this arc, for which a late Mesoproterozoic age (c. 1140 Ma) is indicated by available U-Pb single-zircon age data, was on the southeastern margin of the Kaapvaal-Grunehogna Craton. The protoliths of Group 2 amphibolites are attributed to the 1110 Ma Borgmassivet-Umkondo thermal event on the basis of comparable Nd model ages and trace element distributions. Group 3 amphibolite protoliths are characterized by mid-ocean ridge basalt-type REE patterns and low Th/Yb ratios, and they are related to Neoproterozoic extension. Group 4 amphibolite protoliths are distinguished by high Dy/Yb ratios and are attributed to a phase of syntectonic Pan-African magmatism as indicated by Rb-Sr isotope data.

Composition of matrix glasses and melt inclusions of fallout tephra from ODP Hole 125-782A

We studied the systematics of Cl, F and H2O in Izu arc front volcanic rocks using basaltic through rhyolitic glass shards and melt inclusions (Izu glasses) from Oligocene to Quaternary distal fallout tephra. These glasses are low-K basalts to rhyolites that are equivalent to the Quaternary lavas of the Izu arc front (Izu VF). Most of the Izu glasses have Cl ~400-4000 ppm and F ~70-400 ppm (normal-group glasses). Rare andesitic melt inclusions (halogen-rich andesites; HRA) have very high abundances of Cl (~6600-8600 ppm) and F (~780-910 ppm), but their contents of incompatible large ion lithophile elements (LILE) are similar to the normal-group glasses. The preeruptive H2O of basalt to andesite melt inclusions in plagioclase is estimated to range from ~2 to ~10 wt% H2O. The Izu magmas should be undersaturated in H2O and the halogens at their preferred levels of crystallization in the middle to lower crust (~3 to ~11 kbar, ~820° to ~1200°C). A substantial portion of the original H2O is lost due to degassing during the final ascent to surface. By contrast, halogen loss is minor, except for loss of Cl from siliceous dacitic and rhyolitic compositions. The behavior of Cl, F and H2O in undegassed melts resembles the fluid mobile LILE (e.g.; K, Rb, Cs, Ba, U, Pb, Li). Most of the Cl (>99%), H2O (>95%) and F (>53%) in the Izu VF melts appear to originate from the subducting slab. At arc front depths, the slab fluid contains Cl = 0.94+/-0.25 wt%, F = 990+/-270 ppm and H2O = 25+/-7 wt%. If the subducting sediment and the altered basaltic crust were the only slab sources, then the subducted Cl appears to be almost entirely recycled at the Izu arc (~77-129%). Conversely, H2O (~13-22% recycled at arc) and F (~4-6% recycled) must be either lost during shallow subduction or retained in the slab to greater depths. If a seawater-impregnated serpentinite layer below the basaltic crust were an additional source of Cl and H2O, the calculated percentage of Cl and H2O recycled at arc would be lower. Extrapolating the Izu data to the total length of global arcs (~37000 km), the global arc outflux of fluid-recycled Cl and H2O at subduction zones amounts to Cl ~2.9-3.8 mln ton/yr and H2O ~70-100 mln ton/yr, respectively - comparable to previous estimates. Further, we obtain a first estimate of global arc outflux of fluid-recycled F of ~0.3-0.4 mln ton/yr. Despite the inherent uncertainties, our results support models suggesting that the slab becomes strongly depleted in Cl and H2O in subduction zones. In contrast, much of the subducted F appears to be returned to the deep mantle, implying efficient fractionation of Cl and H2O from F during the subduction process. However, if slab devolatilization produces slab fluids with high Cl/F (~9.5), slab melting will still produce components with low Cl/F ratios (~0.9), similar to those characteristic of the upper continental crust (Cl/F ~0.3-0.9).

Paleomagnetic of Cretaceous basalts from ODP Hole 129-802A

Cretaceous basalts have been recovered at several Ocean Drilling Program and Deep Sea Drilling Project sites where basement of Jurassic age was predicted. Sites 800 and 802, Leg 129, both fall in this category. We have examined the paleomagnetic properties of 25 basalt samples from Site 802 in order to establish a paleolatitude for the site at the time of basalt emplacement and to compare the results to those from Deep Sea Drilling Project Site 462. Mean natural remanent magnetization intensity for the Site 802 basalts was found to be approximately 12 A/m consistent with typical oceanic basalts. Mean stable inclination is -34.7° ± 2.2 which implies a paleolatitude of approximately 19.4°S. This is very similar to the paleolatitudes calculated for Site 462 basalts and suggests - along with similarities in geochemistry, magnetic properties, and projected age of Site 802 basalt emplacement - both contemporaneity of and a possible source link between the two sites.

Geochemistry of pore fluids and bulk solids of ODP Site 195-1200, South Chamorro Seamount

At the South Chamorro Seamount in the Mariana subduction zone, geochemical data of pore fluids recovered from Ocean Drilling Program Leg 195 Site 1200 indicate that these fluids evolved from dehydration of the underthrusting Pacific plate and upwelling of fluids to the surface through serpentinite mud volcanoes as cold springs at their summits. Physical conditions of the fluid source at 27 km were inferred to be at 100°-250°C and 0.8 GPa. The upwelling of fluid is more active near the spring in Holes 1200E and 1200A and becomes less so with increasing distance toward Hole 1200D. These pore fluids are depleted in Cl and Br, enriched in F (except in Hole 1200D) and B (up to 3500 µM), have low 11B (16-21), and have lower than seawater Br/Cl ratios. The mixing ratios between seawater and pore fluids is calculated to be ~2:1 at shallow depth. The F, Cl, and Br concentrations, together with B concentrations and B isotope ratios in the serpentinized igneous rocks and serpentine muds that include ultramafic clasts from Holes 1200A, 1200B, 1200D, 1200E, and 1200F, support the conclusion that the fluids involved in serpentinization originated from great depths; the dehydration of sediments and altered basalt at the top of the subducting Pacific plate released Cl, H2O, and B with enriched 10B. Calculation from B concentrations and upwelling rates indicate that B is efficiently recycled through this nonaccretionary subduction zone, as through others, and may contribute the critical missing B of the oceanic cycle.

Element and isotope compostion of basalts and sediments from the Japan Sea

Ocean Drilling Program Legs 127 and 128 in the Yamato Basin of the Japan Sea, a Miocene-age back-arc basin in the western Pacific Ocean, recovered incompatible-element-depleted and enriched tholeiitic dolerites and basalts from the basin floor, which provide evidence of a significant sedimentary component in their mantle source. Isotopically, the volcanic rocks cover a wide range of compositions (e.g., 87Sr/86Sr = 0.70369 - 0.70503, 206Pb/204Pb = 17.65 - 18.36) and define a mixing trend between a depleted mantle (DM) component and an enriched component with the composition of EM II. At Site 797, the combined isotope and trace element systematics support a model of two component mixing between depleted, MORB-like mantle and Pacific pelagic sediments. A best estimate of the composition of the sedimentary component has been determined by analyzing samples of differing lithology from DSDP Sites 579 and 581 in the western Pacific, east of the Japan arc. The sediments have large depletions in the high field strength elements and are relatively enriched in the large-ion-lithophile elements, including Pb. These characteristics are mirrored, with reduced amplitudes, in Japan Sea enriched tholeiites and northeast Japan arc lavas, which strengthens the link between source enrichment and subducted sediments. However, Site 579/581 sediments have higher LILE/REE and lower HFSE/REE than the enriched component inferred fiom mixing trends at Site 797. Sub-arc devolatilization of the sediments is a process that will lower LILE/REE and raise HFSE/REE in the residual sediment, and thus this residual sediment may serve as the enriched component in the back-arc basalt source. Samples from other potential sources of an enriched, EM II-like component beneath Japan, such as the subcontinental lithosphere or crust, have isotopic compositions which overlap those of the Japan Sea tholeiites and are not "enriched" enough to be the EM II end-member.

K-Ag age of volcanites from underwater rises of the Sea of Okhotsk and the Sea of Japan

New K-Ar datings of Meso-Cenozoic volcanites from the Sea of Japan and the Sea of Okhotsk were obtained. They enabled to reason age of different volcanic complexes. Basalts from volcanic edifices of the Sea of Japan Basin were determined as Middle Miocene - Pliocene (13.1-4.5 Ma) in age, which correlates well with geological evolution of the Sea of Japan. New datings for basalts from the continental slope of the South Primorye (11.1 Ma) confirm their age being similar to volcanites from Neogene basalt plateaus of the South Primorye; they are very similar not only in age but also in mineral and chemical compositions. Datings for rocks from the andesite series of the Northern Yamato Rise (24.7, 21.5 Ma) show that they are coeval with volcanites of the trachyandesite complex; this allows to combine them into one Oligocene - Early Miocene complex. In the Sea of Okhotsk datings of volcanite samples from three complexes were obtained: Cretaceous, Paleogene, and Pliocene-Pleistocene. Cretaceous magmatic rocks make part of basements of large rises in the Sea of Okhotsk, and Paleogene and Pliocene - Pleistocene complexes illustrate stages of Cenozoic tectono-magmatic activation of the region.

Nd isotope composition of fossil fish teeth of ODP Hole 119-738B

We report an optimized method for extracting neodymium (Nd) from fossil fish teeth with a single-stage column (125 µl stem volume; LN Resin, Eichrom Industries, Darien Illinois) for isotopic analysis by multi-collector inductively coupled mass spectrometry (MC-ICMPS). Three reference materials (basalt: BCR-2, BHVO-2; phosphate: fossil bone composite) and splits of fossil fish teeth samples previously processed with existing two-stage column methods were processed using the single-stage column method. 143Nd/144Nd values of reference materials agree within error with published values, and the values for fish teeth correspond with sample splits processed with two-stage columns. Precision to ± ~0.23 epsilon-Nd was achieved for 30 ng Nd samples of reference materials, and Nd isotope measurements of fossil fish tooth sample replicates as small as 7 ng Nd were reproducible within long term instrumental uncertainty. We demonstrate the utility of the new method with the first high resolution Nd isotope record spanning the ~40.0 Ma middle Eocene Climatic Optimum, which shows an excursion of 0.65 epsilon-Nd during the peak warming at the study site (Ocean Drilling Program Leg 119, Site 738; 30 kyr sample spacing from 40.3 to 39.6 Ma). LN Resin is already used in standard methods for separating Nd, and Nd isotopes are routinely measured by MC-ICPMS with high efficiency inlet systems. Our innovation is a single, small volume LN Resin column for Nd separation. The streamlined approach results in a 10X increase in sample throughput.

Geochemistry of sands and sandstones in ODP Leg 126 sites

During Ocean Drilling Program Leg 126, six sites were cored in a young backarc rift basin and its flanks (rift onset 1.1-3.56 Ma) and in the forearc basin of the Izu-Bonin Arc. In the backarc area, strata are younger than about 4.5 Ma, whereas in the forearc, ages are about 0-31 Ma in sections punctuated by important Miocene unconformities. Bulk chemical analyses of volcaniclastic turbidite sands and sandstones, derived directly from the arc, were obtained from 271 atomic absorption analyses (major elements), 253 XRF analyses (trace elements) and 16 ICP-MS analyses (trace and rare-earth elements). Of the 271 samples, 78 come from the backarc area and the remainder from the forearc. The sands and sandstones reflect the igneous compositions of their sources. Most are formed of materials derived from subalkaline, low-K andesites, and dacites, although compositions range from basalt to rhyolite. Basic and acid andesites are predominant in Oligocene rocks; in contrast, Pliocene-Pleistocene sediments were derived from acid andesitic to rhyolitic sources. The oldest sandstones, estimated to have an age of about 31 Ma, were derived from an arc tholeiitic, not boninitic, source. The 26-31 Ma sandstones furthest to the north, at Sites 787 and 792, have higher relative concentrations of Ti, Zr, and Y than do those at southern Site 793. Data from younger samples indicate that, for more than 30 m.y., the average composition of volcaniclastic sediments and volcanism near Aoga Shima was more basic than to the south, near Sumisu Jima. Using the sandstones as igneous proxies, we conclude that magmas erupted along the arc have become more depleted in light-rare-earth elements (LREE) with time. There was a major change in rare-earth-element (REE) concentrations in the late Oligocene, from essentially flat patterns (normalized La/Yb about 1-1.5) to LREE-depleted patterns (normalized La/Yb about 0.5). At the same time, Zr/Y ratios decreased from about 2-4 to about 1.5-2.5. These changes may reflect a shift in provenance, or changes in the composition of the mantle wedge beneath the arc. In the backarc area, lithic clasts and glass shards of rift-facies basalt are present in sediments as old as 2.35-3.15 Ma. Two samples of mafic sand from the backarc basin have flat REE patterns (normalized La/Yb about 1.0), like some of the <1-Ma rift lavas and unlike pre-rift sand and sandstone samples. These possibly represent the local effects of sedimentary mixing of detritus from arc and backarc eruptions because no evidence from the arc itself exists to suggest a recent change in the REE content of magmas.

Grain shape and mineralogy of loess on New Zealands South Island

Infilled fissures are described from the interface between two loess deposits on Banks Peninsula, South Island, New Zealand. Both loesses differ from the other loesses by having a solifluction deposit at their base consisting of angular basalt fragments of the same angularity as fresh frost shattered basalt mixed with the loess. Typically, the fissures are narrow and up to 160 cm deep while the infilling of the overlying loess shows no obvious structure. They occur mainly at higher elevations on south (poleward) facing slopes, and the host loess forms a fragipan of high density. They are most readily explained as being seasonal frost fissures or more probably ice-wedge casts, which would have required either permafrost or deep seasonal frost for their formation. If permafrost had existed, this would imply a cooling of the mean annual temperatures by at least 16 to 18°C.

Geochemistry of oceanic flood basalts from the Caribbean plate

The thick oceanic crust of the Caribbean plate appears to be the tectonized remnant of an eastern Pacific oceanic plateau that has been inserted between North and South America. The emplacement of the plateau into its present position has resulted in the obduction and exposure of its margins, providing an opportunity to study the age relations, internal structure and compositional features of the plateau. We present the results of 40Ar-39Ar radiometric dating, major-, trace-element, and isotopic compositions of basalts from some of the exposed sections as well as drill core basalt samples from Leg 15 of the Deep Sea Drilling Project. Five widely spaced, margin sections yielded ages ranging from 91 to 88 Ma. Less well-constrained radiometric ages from the drill cores, combined with the biostratigraphic age of surrounding sediments indicate a minimum crystallization age of ~90 Ma in the Venezuelan Basin. The synchroneity of ages across the region is consistent with a flood basalt origin for the bulk of the Caribbean plateau i.e., large volume, rapidly erupted, regionally extensive volcanism.. The ages and compositions are also consistent with plate reconstructions that place the Caribbean plateau in the vicinity of the Galápagos hotspot at its inception. The trace-element and isotopic compositions of the ~90 Ma rocks indicate a depleted mantle and an enriched, plume-like mantle were involved in melting to varying degrees across the plateau. Within the same region, a volumetrically secondary, but widespread magmatic event occurred at 76 Ma, as is evident in Curacao, western Colombia, Haiti, and at DSDP Site 152/ODP Site 1001 near the Hess Escarpment. Limited trace-element data indicate that this phase of magmatism was generally more depleted than the first. We speculate that magmatism may have resulted from upwelling of mantle, still hot from the 90 Ma event, during lithospheric extension attending gravitational collapse of the plateau, andror tectonic emplacement of the plateau between North and South America. Still younger volcanics are found in the Dominican Republic (69 Ma) and the Quepos Peninsula of Costa Rica (63 Ma). The latter occurrence conceivably formed over the Galápagos hotspot and subsequently accreted to the western edge of the plateau during subduction of the Farallon plate.

Table I. Composition of till in north Greenland

From 1950 through 1900 studies on the glacial geology of northern Greenland have been made in cooperation with the U.S. Air Force Cambridge Research Laboratories. As a result of these studies four distinct phases of the latest glaciation have been recognized. The last glaciation extended over most of the land and removed traces of previous anes. Retreat of the ice mass began some time previous to 6000 years ago. This was followed by a rtse in sea level which deposited clay-silt succeeded by karne gravels around stagnant ice lobes in the large valleys. Marine terraces, up to 129 meters above present sea level, developed as readjustment occurred in the land free of ice. About 3700 years ago an advance of glaciers down major fjords took place followed by retreat to approximately the present position of the ice. Till in Peary Land, north of Frederick E. Hyde Fjord, contains only locally derived matertals indicating that the central Greenland ice cap did not cover the area.

(Table 1) Minerals found in vesicles and fractures from DSDP Leg 54 Holes basalts

A variety of secondary minerals, formed in response to different oxidation and hydration states, are found in vugs and on fracture surfaces of the basalt cores from DSDP Leg 54. The minerals are smectite (blue to grey), high-magnesium calcite, manganoan calcite, aragonite, iron oxides, phillipsite, todorokite, marcasite, and hydrobiotite. The relationship of the mineral assemblages to four depositional modes of the basalts are delineated. A definite sequence and genetic link exists between mineral type and host rock which is dependent upon the origin and subsequent cooling history of the basalt.

Composition of palagonitized basalts, their alteration products, and palagonitized basaltic glasses

Detailed petrochemical and geochemical studies of two samples of palagonitized basalts collected from depths 3060 and 4800 m have shown that palagonitization of tholeiitic basalt is accompanied by intensive removal of Ca and Mg and some removal of SiO2 from rocks. Appreciable amount of K is added to rocks in this process. Behavior of Fe, Al, Ti, Cr, and Na is inert. Palagonitization of alkalic basalt is accompanied by loss of SiO2, Ca, and Na from rocks. Contents of K and Mg are not changed. Four stages can be discerned in alteration of basalts under deep-sea conditions: syngenetic and diffusional palagonitization, hydrothermal leaching, and underwater weathering. Crusts of Fe-Mn ores are formed through removal of Fe, Mn, Ni, Co, Sn, and Mo from rocks and sorption of Pb, Hg, Yb, La, Bi, W, and Be from sea water.