Chemical and isotopic compositions of sediments from DSDP Hole 21-204

The highly depleted intra-oceanic Tonga-Kermadec island arc forms an endmember of arc systems and a unique location in which to isolate the effects of the slab flux. High precision TIMS uranium, thorium, strontium, neodymium, and lead isotopes, along with complete major and trace element data, have been obtained on an extensive sample set comprising fifty-eight lavas along the arc as well as nineteen samples of the subducting sediments at DSDP site 204 just to the east of the Tonga-Kermadec trench. Ca/Ti and Al/Ti ratios extend from values appropriate to an N-MORB source in the southern Kermadecs to very high ratios in Tonga interpreted to reflect increasing degrees of depletion of the mantle wedge due to backarc basalt extraction. The isotope data emphasize the need for four components in the petrogenesis of the lavas: (1) the mantle wedge; (2) a component with elevated 207Pb/204Pb towards which the Kermadec and southern Tongan lavas extend; (3) a component characterised by high 206Pb/204Pb, Ta/Nd, and low 143Nd/144Nd observed only in the northernmost Tongan islands of Tafahi and Niuatoputapu; (4) a fluid component characterised by strong enrichments of Rb, Ba, U, K, Ph, and Sr, relative to Th, Zr, and the REE and producing large 238U excesses ((230Th/238U) = 0.8-0.5) in the more depleted lavas. The mantle wedge (Component 1) is isotopically similar to the source of the Lau BABB. Component 2 is average pelagic sediment on the downgoing Pacific plate as observed at DSDP sites 595/596 and in the upper sections of the sediment pile at DSDP site 204. Mass balance calculations indicate that less than 0.5% is recycled into the arc lavas; essentially all the subducted sediment is returned to the upper mantle (~0.03 km**3/yr). Exceptionally low concentrations of Ta and Nb relative to Th and the LREE requires that this sediment component is added as a partial melt which was in equilibrium with residual rutile or ilmenite. Component 3 is identified as volcaniclastics from the Louisville Ridge which comprise the lower 44 m of the sediment section intersected at DSDP site 204. These volcaniclastics are spatially restricted to the vicinity of the Louisville Ridge and provide a unique sediment tracer which can be used to show that it takes 4 Myr from the time of subduction to its first appearance in the arc lava signature. Component 4, the fluid contribution to the lava source is inferred to contribute ~1 ppm Rb, 10 ppm Ba, 0.02 ppm U, 600 ppm K, 0.2 ppm Ph, and 30 ppm Sr. It has 87Sr/86Sr = 0.7035 and 206Pb/204Pb = 18.5 and thus it is inferred to have been derived from dehydration of the subducting altered oceanic crust. U-Th isotope disequilibria reflect the time since fluid release from the subducting slab and a reference line through the lowest (230Th/232Th) lavas constrains this to be 30000-50000 yr. The U-Th and Th-Ra isotope systematics are decoupled, and it is suggested that Th-Ra isotope disequilibria record the time since partial melting and thus indicate rapid channelled magma ascent. Olivine gabbro xenoliths from Raoul are interpreted as cumulates to their host lavas with which they form zero age U-Th isochrons indicating that minimal time was spent in magma chambers. The subduction signature is not observed in lavas from the backarc island of Niuafo'ou. These were derived from partial melting of fertile peridotite at 130-160 km depth with melt rates around 0.0002 kg/m**3/yr.

Chemical composition and age of metamorphic rocks from the Khavyven Highland, Eastern Kamchatka

Metamorphic rocks of the Khavyven Highland in eastern Kamchatka were determined to comprise two complexes of metavolcanic rocks that have different ages and are associated with subordinate amounts of metasediments. The complex composing the lower part of the visible vertical section of the highland is dominated by leucocratic amphibole-mica (+/-garnet) and epidote-mica (+/-garnet) crystalline schists, whose protoliths were andesites and dacites and their high-K varieties of island-arc calc-alkaline series. The other complex composing the upper part of the vertical section consists of spilitized basaltoids transformed into epidote-amphibole and phengite-epidote-amphibole green schists, which form (together with quartzites, serpentinized peridotites, serpentinites, and gabbroids) a sea-margin ophiolitic association. High LILE concentrations, high K/La, Ba/Th, Th/Ta, and La/Nb ratios, deep Ta-Nb minima, and low (La/Yb)_N and high 87Sr/86Sr ratios of the crystalline schists of the lower unit are demonstrated to testify to their subduction nature and suggest that their protolithic volcanics were produced in the suprasubduction environment of the Ozernoi-Valaginskii (Achaivayam-Valaginskii) island volcanic arc of Campanian-Paleogene age. The green schists of the upper unit show features of depleted MOR tholeiitic melts and subduction melts, which cause the deep Ta-Nb minima, and low K/La and 87Sr/86Sr ratios suggesting that the green schists formed in a marginal basin in front of the Ozernoi-Valaginskaya island arc. Recently obtained K-Ar ages in the Khavyven Highland vary from 32.4 to 39.3 Ma and indicate that metamorphism of the protolithic rocks occurred in Eocene under effect of collision and accretion processes of the arc complexes of the Ozernoi-Valaginskii and Kronotskii island arcs with the Asian continent and the closure of forearc oceanic basins in front of them. The modern position of the collision suture that marks the fossil subduction zone of the Ozernoi-Valaginskii arc and is spatially restricted to the buried Khavyven uplift in the Central Kamchatka Depression characterized by well-pronounced linear gravity anomalies.

Major and trace element composition of sepentine muds and metabasic rocks of ODP Leg 195 and Leg 125 sites

New geochemical data on serpentinite muds and metamorphic clasts recovered during Ocean Drilling Program Legs 195 (Holes 1200A-1200E) and 125 (Holes 778A and 779A) provide insights into the proportions of rock types of various sources that compose the serpentinite mudflows and the fluid-rock interactions that predominate in these muds. We interpret the metamorphic rock fragments as derivatives of mostly metamorphosed mafic rocks from the descending Pacific oceanic crust. Based on their mid-ocean-ridge basalt (MORB)-like Al2O3, TiO2, CaO, Si/Mg, and rare earth element (REE) systematics, these metamorphic rocks are classified as metabasalts/metagabbros and, therefore, ~30-km depths represent an active subduction zone setting. The serpentinite muds from Holes 1200A and 1200E have slightly lower REE when compared to Hole 1200D, but overall the REE abundance levels range between 0.1-1 x chondrite (CI) levels. The chondrite-normalized patterns have [La/Sm]N ~ 2.3 and [Sm/Yb]N ~ 2. With the exception of one sample, the analyzed metamorphic clasts show flat to slightly depleted light REE patterns with 1.0-15 x CI levels, resembling MORBs. Visually, ~6 vol% of the serpentinized muds are composed of 'exotic' materials (metamorphic clasts [schists]). Our mixing calculations confirm this result and show that the serpentinite muds are produced by additions of ~5% metamafic materials (with flat and up to 10 x CI REE levels) to serpentinized peridotite clast material (with very low REE abundances and U-shaped chondrite-normalized patterns). The preferential incorporation of B, Cs, Rb, Li, As, Sb, and Ba into the structure of H2O-bearing sheet silicates (different than serpentine) in the Leg 125 and Leg 195 metamorphic clasts (chlorite, amphibole, and micas) have little effect on the overall fluid-mobile element (FME) enrichments in the serpentinite muds (average B = ~13 ppm; average Cs = ~0.05 ppm; average As = ~1.25 ppm). The extent of FME enrichment in the serpentinized muds is similar to that described for the serpentinized peridotites, both recording interaction with fluids very rich in B, Cs, and As originating from the subducting Pacific slab.

Petrology and geochemistry of basalts from the Vavilov Basin

During ODP Leg 107, two holes were drilled in the basement of Vavilov Basin, a central oceanic area of the Tyrrhenian sea. Hole 655B is located near the Gortani ridge in off-axis position at the western rim of the basin; Hole 651A is located on a basement swell at the axis of the basin. This paper deals with mineral chemistry, major and trace element geochemistry, and petrogenesis of the basalts recovered in the two holes. The mineralogy of the basalts is broadly homogeneous, but all of them have suffered important seawater alteration. Their major-element compositions are similar to both normal-mid-ocean-ridge-basalts (N-MORB) and back-arc-basalts (BAB) except for Na2O contents (BAB-like), and K2O which is somewhat enriched in upper unit of Hole 651 A. Their affinity with N-MORB and BAB is confirmed by using immobile trace elements such as Zr, Y, and Nb. However, basalts from the two sites present contrasting geochemical characteristics on spidergrams using incompatible elements. Hole 655B basalts are homogeneous enriched tholeiites, similar to those from DSDP Hole 373 (located on the opposite side of the basin near the eastern rim), and show affinities with enriched MORB (E-MORB). At Hole 651 A, the two basalt units are chemically distinct. One sample recovered in lower unit is rather similar to those from Hole 655B, but basalts from upper unit display calc-alkaline characteristic evidenced by the increase of light-ion-lithophile-element (LILE)/high-field-strength-element (HFSE) ratio, and appearance of a negative Nb-anomaly, making them comparable with orogenic lavas from the adjacent Eolian arc. The observed chemical compositions of the basalts are consistent with a derivation of the magmas from a N-MORB type source progressively contaminated by LILE-enriched fluids released from dehydration of the bordering subducted plate. Implications for evolution of the Tyrrhenian basin are tentatively proposed taking into consideration geochemical and chronological relationships between basalts from Leg 107 Holes 655B and 651 A, together with data from Leg 42 Site 373 and Vavilov Seamount. These data illustrate back-arc spreading in ensialic basin closely associated with the maturation of the adjacent subduction, followed by the growth of late off-axis central volcano, whereas the active subduction retreats southeastward.

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.

Minerals and geochemistry at DSDP Hole 62-466

Alkali-basalt clasts in Upper Cretaceous sediments from Site 466 on southern Hess Rise suggest that parts of Hess Rise were constructed by off-ridge volcanic activity. Apparently, tectonic adjustments at Hess Rise occurred during the Late Cretaceous (Campanian-Maastrichtian), when parts of the original volcanic pedestal were uplifted and provided source rocks for the clasts. Synchronous volcanism may have occurred. Causes for the Late Cretaceous tectonic adjustments (and volcanism?) are not known, but they may be related to intraplate movement along the Mendocino Fracture Zone.

Age and petrographic characterization of volcanic ash from ODP Leg 198 and Leg 126 sites

In this study of volcanic ash retrieved from Shatsky Rise during Ocean Drilling Program Leg 198, the texture and composition of the volcanic components (glass and crystals) were used to fingerprint ash layers for detailed correlation. Correlations among ash layers in holes drilled at the same site as well as between sites, including sites on different parts (highs) of the rise, were tested. Although high-to-high correlations failed, intrahigh correlations were more successful. Our data suggest a significantly different source for some pyroclastic debris, especially at Site 1208, possibly associated with pumice rafts carried northward from the Izu-Bonin arc by the Kuroshio Current. Other ashes are consistent with rhyolitic to dacitic air fall ash from Asian arc volcanoes. We were not able to texturally distinguish between air fall ash and pumice-raft fallout but suspect that the latter is associated with higher percentages of vesiculated ash components, as we demonstrate occur in more proximal Izu-Bonin pyroclastic deposits.

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 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.

Geochemistry and structural formulas of heavy minerals from sands and sandstones of ODP Leg 126 samples

Lower Oligocene to Pleistocene volcaniclastic sands and sandstones recovered around the Izu-Bonin Arc during Ocean Drilling Program Leg 126 were derived entirely from Izu-Bonin Arc volcanism. Individual grains consist of volcanic glass, pumice, scoria, basaltic or andesitic fragments, plagioclase, pyroxene, and minor olivine and hornblende. In Pliocene-Pleistocene samples plagioclase and heavy minerals in the volcaniclastic sands and sandstones are present in the following abundances: plagioclase > orthopyroxene > clinopyroxene > pigeonite > olivine. In contrast, plagioclase and heavy minerals found in Oligocene-Miocene samples occur in the following order: plagioclase > clinopyroxene > orthopyroxene > hornblende. The low concentration of Al, Ti, and Cr in calcium-rich clinopyroxenes in Oligocene to Holocene sediments suggests that the sources of the volcaniclastic detritus were nonalkalic igneous rocks. There are, however, some distinctive differences in the chemical composition of pyroxene between the Pliocene-Pleistocene and Oligocene-Miocene volcaniclastic sands and sandstones. Orthopyroxene belongs to the hypersthene-ferrohypersthene series (Fe-rich) in Pliocene-Pleistocene sediments, and the bronzitehypersthene series (Mg-rich) in Oligocene-Miocene sediments. Clinopyroxene is characterized by augite and pigeonite in Pliocene-Pleistocene sediments, and by the diopside-augite series in Oligocene-Miocene sediments. Mineral assemblages and mineral chemistry of the volcaniclastic sands and sandstones reflect those of the volcanic source rocks. Therefore, the observed changes in mineralogy record the historical change in volcanism of the Izu-Bonin Arc. The mineralogy is consistent with the geochemistry of the volcaniclastic sands and sandstones and the geochemistry of forearc volcanic rocks of the Izu-Bonin Arc since the Oligocene.

Heavy mineral abundance of ODP Leg 174A sites

The purpose of this paper is to report the heavy mineral content of Miocene to Pleistocene sequences drilled during Ocean Drilling Program Leg 174A on the New Jersey Shelf. Sandy intervals recovered from Holes 1071A, 1071F, 1072A, and 1073A were sampled for heavy mineral analysis. Because of the low core recovery of the sandy parts of the succession, sampling has been incomplete. In spite of the resulting restriction and because of major variations in heavy mineral assemblages, eight distinct heavy mineral associations could be defined. The data presented thus considerably extend the present knowledge on the lithology of the stratigraphic record as described by Austin, Christie-Blick, Malone, et al. (1998). In this chapter the heavy mineral associations and their assignment to particular sequences are described.