Sediments and tephra layers from the southern Tyrrhenian abyssal plain near Marsili Seamount

From the south-eastern Tyrrhenian deep-sea floor, four sediment cores of "Meteor" cruise 22 (1971) are described. These cores were taken in the basin between the Aeolian Islands and the Marsili Seamount, an elevation of more tha 3000 m above the sea floor. The sedimentation of the deep-sea basin is distinguished by a sequence of turbidites with a high sedimentation rate. The composition of the clastic material and the position of the cores in the mouth area of the morphologically very pronounced Stromboli Canyon suggest an interpretation of the turbidite sequence as fan of this canyon onto the deep-sea floor. A white rhyolitic pumice-tephra at the base of the 4 m thick sequence of turbidites in core M22-102 has been correlated with the Pelato eruption of the island of Liparo in the 6th century A.D. At the foot of the Marsili Seamount - apparently in morphologically elevated positions - the influence of the turbidite sedimentation increases, the rate of sedimentation is lower and stratigraphic omissions are probable. Here, rather compacted globigerina marls have been found in only 15 -25 cm depth. In addition, volcanic material in the form of lapilli layers, palagonitized ashes and detrital volcanic sands of the Marsili Seamount have been encountered in this area. An up to 3 cm thick layer of completely palagonitized basaltic ash intercalates with the marls at the base of two cores. Layers of very fresh olivine basaltic lapilli in core 103 and palagonitized lapilli of latitic composition in core 104 testify to an explosive submarine volcanism of the Marsili Seamount. According to the stratigraphy of core 103, the latest manifestations of this basaltic volcanism belong to the late Pleistocene (Emiliana huxleyi-zone of Nannoplankton stratigraphy) The basaltic lapilli are glassy to perhyaline with phenocrysts or microphenocrysts predominantely of olivine. The petrological character of the basaltic volcanites with high MgO, Ni, Cr and high MgO/FeO- and Ni/Co-ratios exhibits primitive basaltic features. These basalts clearly differ from basalts of the ocean floors, mid-ocean ridges and marginal basins. Prominent features are a missing iron-enrichment trend and low TiO2. Al2O3 tends to be high, as well as K2O and related trace elements (Ba, Sr). In spite of silica undrsaturation and high color index, the Marsili basalt exhibit some analogies with the calcalkaline basalts of the Aeolian arc, as well as the undersaturated basalts of some other circumoceanic areas.

Mineralogy and geochemistry of Miocene tuff from ODP Hole 129-802A

Drilling at Ocean Drilling Program Site 802 in the central Mariana Basin, northwest Pacific Ocean, revealed an unexpected 222-m-thick sequence of well-cemented tuff of Miocene age. The deposits are unusual in that their source is presumably an unmapped seamount and they exhibit several peculiar petrological and mineralogical features. The well-developed secondary mineral sequence which includes analcime is rare in such relatively young, unburied deposits, in an area where there is little other evidence of hydrothermal activity. The massive tuff section also contains abundant fissure veins made of a rare silicate carbonate sulfate hydroxide hydrate of calcium, called thaumasite, which has not before been described in deep submarine deposits. The smectite-zeolite-thaumasite paragenesis coincides with the presence of chloride and calcium-enriched interstitial waters. The diagenetic evolution of the deposit appears to have been largely controlled by the depositional mode. The discharges of disaggregated and rejuvenated volcaniclasts seem to have been abrupt and repeated. The Miocene tuff at Site 802 thus provides new insights on the interactions between basaltic glass, biogenic phases, and seawater, in a specific deep-sea environment.

Geochemistry of sediments from the mid-oceanic Kolbeinsey Ridge

In order to assess recent submarine volcanic contributions to the sediments from the active Kolbeinsey Ridge, surface samples were analyzed chemically. The contribution of major and trace elements studied differ within the study area. A statistical analysis of the geochemical variables using factor analysis and cluster method allows to distinguish possible sample groups. Cluster method identifies three distinct sediment groups located in different areas of sedimentation. Group 1 is characterized by highest contents of Fe2O3, V, Co, Ni, Cu and Zn demonstrating the input of volcaniclastic material. Group 2 comprises high values of CaCO3, CaO and Sr representing biogenic carbonate. Group 3 is characterized by the elements K, Rb, Cs, La and Pb indicating the terrigenous component. The absolute percentage of the volcanic, biogenic and terrigenous components in the bulk sediments was calculated by using a normative sediment method. The highest volcanic component (> 60% on a carbonate free basis) is found on the ridge crest. The biogenic component is highest (10-30%) in the eastern part of the Spar Fracture Zone influenced by the East Iceland Current. Samples from the western and southeastern region of the study area contain more than 90% of terrigenous component which appears to be mainly controlled by input of ice-rafted debris.

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.

Mineralogy and geochemistry at DSDP Hole 62-465A

Core recovered from Hess Rise contains concentrations of pyrite, marcasite, and barite in the lowermost meter of limestone (Unit II) and in the brecciated upper part of the underlying volcanic basement (Unit HI). Petrographic and chemical data indicate that the sulfide-barite assemblage in the limestone is mainly a product of low-temperature diagenetic processes. The iron-sulfide phases are biogenic and their concentrations mark the diffusion of sea water sulfate through sedimentary horizons containing abundant organic matter and mafic, glassy volcanogenic detritus. There is some evidence, however, that elevated temperatures augmented or intensified the synsedimentary diagenetic process.

Petrology and geochemistry of silicified upper Miocene chalk at DSDP Site 69-504

Chert, Porcellanite, and other silicified rocks formed in response to high heat flow in the lower 50 meters of 275 meters of sediments at Deep Sea Drilling Project Site 504, Costa Rica Rift. Chert and Porcellanite partly or completely replaced upper Miocene chalk and limestone. Silicified rock occurs as nodules, laminae, stringers, and casts of burrows, and consists of quartz and opal-CT in varying amounts, associated with secondary calcite. The secondary silica was derived from dissolution of opal-A (biogenic silica), mostly diatom frustules and radiolarian tests. Temperature data obtained at the site indicate that transformation of opal-A to opal-CT began at about 50°C, and transformation from opal-CT to quartz at about 55°C. Quartz is most abundant close to basement basalts. These silica transformations occurred over the past 1 m.y., and took place so rapidly that there was incomplete ordering of opal-CT before transformation to quartz; opal-CT formed initially with an uncommonly wide d spacing. Quartz shows poor crystallinity. Chemical data show that the extensively silicified rocks consist of over 96% SiO2; in these rocks, minor and trace elements decreased greatly, except for boron, which increased. Low Al2O3 and TiO2 contents in all studied rocks preclude the presence of significant volcanic or terrigenous detritus. Mn content increases with depth, perhaps reflecting contributions from basalts or hydrothermal solutions. Comparisons with cherts from oceanic plateaus in the central Pacific point to a more purely biogenic host sediment for the Costa Rica Rift cherts, more rapid precipitation of quartz, and formation nearer a spreading center. Despite being closer to continental sources of ash and terrigenous detritus, Costa Rica Rift cherts have lower Al2O3, Fe2O3, and Mn concentrations.

Chemical composition of Ninetyeast Ridge basalts

On Leg 121 of the Ocean Drilling Program, we recovered basaltic rocks from a total of three basement sites in the southern, central, and northern regions of Ninetyeast Ridge. These new sites complement the previous four basement holes drilled during Legs 22 and 26 of the Deep Sea Drilling Project, and confirm the predominantly tholeiitic, light rare earth element-enriched character of the basalts that cap the ridge. The basalts show marked iron enrichment; ferrobasalts occur at Sites 214 and 216 and oceanic andesites at Site 253. All of the basalts recovered during Leg 121 are altered, and range from aphyric olivine tholeiites (Site 756), to strongly plagioclase-phyric basalts (Site 757). Basalts from Site 758, which were clearly erupted in a submarine environment (pillow basalts are present in the section), are sparsely to strongly plagioclase-phyric. The basalts recovered at any one hole are isotopically homogeneous (except for the basalts from Site 758, which show a range of Pb isotopes), and it is possible to relate the magmas at any one site by high-level fractionation processes. However, there are significant variations in isotope ratios and highly incompatible element ratios between sites, which suggest that the mantle source for the ridge basalts was compositionally variable. Such variation, in view of the large volume of magmatic products that form the ridge system, is not surprising. There is not, however, a systematic variation in basalt composition along the ridge. We agree with previous models that relate Ninetyeast Ridge to a mantle plume in the southern Indian Ocean. The tholeiitic, iron-enriched, and voluminous character of the ridge basalts is typical of oceanic islands associated with plumes on or near a mid-ocean ridge (e.g., Iceland, Galapagos Islands, and St. Paul/Amsterdam islands). The absence of recovered alkalic suites is inconsistent with an intraplate setting, such as the Hawaiian Islands or Kerguelen Island. Thus, the major element data, like the gravity data, strongly suggest that the ridge was erupted on or very close to an active spreading center. Isotopically, the most likely plume that created the excess magmatism on the Ridge is the Kerguelen-Heard plume system, but the Ninetyeast Ridge basalts do not represent a simple mixing of the Kerguelen plume and mid-ocean Ninetyeast Ridge basalt mantle.

Geochemistry and mineralogy of interstitial waters and clays in sediments of ODP Leg 180 sites

Oxygen and strontium isotopes and Rb and Ba were determined in interstitial water (IW) collected from Sites 1109, 1115, and 1118 drilled on the Woodlark Rise during Ocean Drilling Program Leg 180. The trace element and mineralogical composition of the clay fraction of sediments isolated from the squeeze cakes corresponding to IW samples from Site 1109 was also determined.

Geochemistry of volcanivlastic sediments of ODP Sites 134-832 and 134-833

Volcaniclastic sediments of North Aoba Basin (Vanuatu) recovered during Ocean Drilling Program (ODP) Leg 134 show a mineralogical and chemical overprint of low grade hydrothermal alteration superimposed on the primary magmatic source compositions. The purpose of this study was to identify authigenic mineral phases incorporated in the volcaniclastic sediments, to distinguish authigenic chemical and mineralogical signals from the original volcaniclastic mineralogical and chemical compositions, and to determine the mechanism of authigenic minerals formation. Mineralogical, micro-chemical and bulk chemical analyses were utilized to identify and characterize authigenic phases and determine the original unaltered ash compositions. 117 volcaniclastic sediment samples from North Aoba Basin Sites 832 and 833 were analyzed. Primary volcaniclastic materials accumulated in North Aoba Basin can be divided into three types. The older basin-filling sequences show three different magmatic trends: high K, calc-alkaline, and low K series. The most recent accumulations are rhyodacitic composition and can be attributed to Santa Maria or Aoba volcanic emissions. Original depositional porosity of volcaniclastic sediments is an important factor in influencing distribution of authigenic phases. Finer-grained units are less altered and retain a bulk mineralogical and chemical composition close to the original pyroclastic rock composition. Coarser grained units (microbreccia and sandstones) are the major hosts of authigenic minerals. At both sites, authigenic minerals (including zeolites, clay minerals, Mg-carbonates, and quartz) exhibit complex zonation with depth that crosses original ash depositional boundaries and stratigraphic limits. The zeolite minerals phillipsite and analcime are ubiquitous throughout the altered intervals. At Site 832, the first zeolite minerals (phillipsite) occur in Pleistocene deposits as shallow as 146 meters below seafloor (mbsf). At Site 833 the first zeolite minerals (analcime) occur in Pleistocene deposits as shallow as 224 mbsf. The assemblage phillipsite + analcime + chabazite appears at 635 mbsf (Site 832) and at 376 mbsf (Site 833). Phillipsite + analcime + chabazite + thomsonite + heulandite are observed between 443 and 732 mbsf at Site 833. Thomsonite is no longer observed below 732 mbsf at Site 833. Heulandite is present to the base of the sections cored. The zeolite assemblages are associated with authigenic clay minerals (nontronite and saponite), calcite, and quartz. Chlorite is noticeable at Site 832 as deep as 851 mbsf. Zeolite zones are present but are less well defined at Site 832. Dolomite and rare magnesite are present below 940 m at Site 832. The coarse-grained authigenic mineral host intervals exhibit geochemical signatures that can be attributed to low grade hydrothermal alteration. The altered intervals show evidence of K2O, CaO, and rare earth elements mobilization. When compared to fine-grained, unaltered units, and to Santa Maria Island volcanics rocks, the altered zones are relatively depleted in rare earth elements, with light rare earth elements-heavy rare earth elements fractionation. Drilling at Site 833 penetrated a sill complex below 840 m. No sill was encountered at Site 832. Complex zonation of zeolite facies, authigenic smectites, carbonates and quartz, and associated geochemical signatures are present at both sites. The mineralogical and chemical alteration overprint is most pronounced in the deeper sections at Site 832. Based on mineralogical and chemical evidence at two locations less than 50 km apart, there is vertical and lateral variation in alteration of the volcaniclastic sediments of North Aoba Basin. The alteration observed may be activated by sill intrusion and associated expulsion of heated fluids into intervals of greater porosity. Such spatial variation in alteration could be attributed to the evolution of the basin axis associated with subduction processes along the New Hebrides Trench.

Chemical and isotopic composition of rocks and minerals at ODP Site 209-1275

This paper reports results of an investigation of a representative collection of samples recovered by deep-sea drilling from the oceanic basement 10 miles west of the rift valley axis in the crest zone of the Mid- Atlantic Ridge at 15°44'N (Sites 1275B and 1275D). Drilling operations were carried out during Leg 209 of the Drilling Vessel JOIDES Resolution within the framework of the Ocean Drilling Program (ODP). The oceanic crust was penetrated to depth of 108.7 m at Site 1275B and 209 m at Site 1275D. We reconstructed the following sequence of magmatic and metamorphic events resulting in the formation of a typical oceanic core complex of slow-spreading ridges: (1) formation of strongly fractionated (enriched in iron and titanium) tholeiitic magmatic melt parental to gabbroids under investigation in a large magma chamber located in a shallow mantle and operating for a long time under steady-state conditions; (2) transfer of the parental magmatic melt of the gabbroids to the base of the oceanic crust, its interaction with host mantle peridotites, and formation of troctolites and plagioclase peridotites; (3) intrusion of enriched trondhjemite melts as veins and dikes in the early formed plutonic complex, contact recrystallization of the gabbro, and development in the peridotite-gabbro complex of enriched geochemical signatures owing to influence of trondhjemite injections; (4) emplacement of dolerite dikes (transformed to diabases); (5) metamorphism of upper epidoteamphibolite facies with participation of marine fluids; and (6) rapid exhumation of the plutonic complex to the seafloor accompanied by greenschist-facies metamorphism. Distribution patterns of Sr and Nd isotopes and strongly incompatible elements in the rocks suggest contributions from two melt sources to the magmatic evolution of the MAR crest at 15°44'N: a depleted reservoir responsible for formation of the gabbros and diabases and an enriched reservoir, from which trondhjemites (granophyres) were derived.

(Table 2) Major and trace element content of ODP Leg 130 basalts

Estimated relative errors on major and minor elements are 1%. For trace elements, errors (% standard deviation at levels measured) are estimated at 1 % for Cr, 3% for Ni, 3% for Rb at 30 ppm, and >20% at < 10 ppm; 2% for Sr and V, and 4% for Y and Zr.