Geochemistry and ages at DSDP Hole 19-192A

Conventional K-Ar and 40Ar/39Ar age data on altered basalts from DSDP Hole 192A on Meiji Guyot, Emperor Seamount chain, indicate a minimum age for the volcano of 61.9 ± 5.0 m.y. The K-Ar data are consistent with the early Maestrichtian date of the overlying sediments, but do not provide either a positive or negative test of the hypothesis that Meiji is older than Emperor volcanoes to the south. The most prominent alteration affecting the age measurements is potassium metasomatism, particularly of feldspar phenocrysts. The K-Ar apparent ages of feldspar separates from the Meiji basalts show that more than half of the potassium metasomatism occurred within the last 25 m.y. or so, and that if the potassium replacement rate has been constant, then the alteration of the Meiji basalts did not begin for 10 to 20 m.y. after the volcano formed.

Chemical composition of DSDP and ODP cherts

Rare earth element (REE), major, and trace element abundances and relative fractionations in forty nodular cherts sampled by the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) indicate that the REE composition of chert records the interplay between terrigenous sources and scavenging from the local seawater. Major and (non-REE) trace element ratios indicate that the aluminosilicate fraction within the chert is similar to NASC (North American Shale Composite), with average Pacific chert including ~7% NASC-like particles, Indian chert ~11% NASC, Atlantic chert ~17% NASC, and southern high latitude (SHL) chert 53% NASC. Using La as a proxy for sum REE, approximations of excessive La (the amount of La in excess of that supplied by the detrital aluminosilicate fraction) indicate that Pacific chert contains the greatest excessive La (85% of total La) and SHL chert the least (38% of total La). As shown by interelement associations, this excessive La is most likely an adsorbed component onto aluminosilicate and phosphatic phases. Accordingly, chert from the large Pacific Ocean, where deposition occurs relatively removed from significant terrigenous input, records a depositional REE signal dominated by adsorption of dissolved REEs from seawater. Pacific chert Ce/Ce* <<1 and normative La/Yb ~ 0.8-1, resulting from adsorption of local Ce-depleted seawater and preferential adsorption of LREEs from seawater (e.g., normative La/Yb ~0.4), which increases the normative La/Yb ratio recorded in chert. Chert from the Atlantic basin, a moderately sized ocean basin lined by passive margins and with more terrigenous input than the Pacific, records a mix of adsorptive and terrigenous REE signals, with moderately negative Ce anomalies and normative La/Yb ratios intermediate to those of the Pacific and those of terrigenous input. Chert from the SHL region is dominated by the large terrigenous input on the Antarctic passive margin, with inherited Ce/Ce* ~1 and inherited normative La/Yb values of ~1.2-1.4. Ce/Ce* does not vary with age, either throughout the entire data base or within a particular basin. Overall, Ce/Ce* does not correlate with P2O5 concentrations, even though phosphatic phases may be an important REE carrier.

Geochemistry of peridotites from ODP Leg 125 diapiric serpentinites

Refractory spinel peridotites were drilled during Leg 125 from two diapiric serpentinite seamounts: Conical Seamount in the Mariana forearc (Sites 778-780) and Torishima Forearc Seamount (Sites 783-784) in the Izu-Ogasawara forearc. Harzburgite is the predominant rock type in the recovered samples, with subordinate dunite; no lherzolite was found. The harzburgite is diopside-free to sparsely diopside-bearing, with modal percentages of diopside that range from 0% to 2%. Spinels in the harzburgites are chrome-rich (Cr/[Cr + Al] = 0.38-0.83; Fe3+/[Fe3+ + Cr + Al] = 0.01-0.07). Olivine and orthopyroxene are magnesian (Mg# = 0.92). Discrete diopsides reveal extreme depletion of light rare earth elements. Primary hornblende is rare. The bulk major-element chemistry shows low average values of TiO2 (trace), Al2O3 (0.55%) and CaO (0.60%), but high Mg# (0.90). These rocks are more depleted than the abyssal peridotites from the mid-oceanic ridge. They are interpreted as residues of extensive partial melting (= 30%), of which the last episode was in the mantle wedge, probably associated with the generation of incipient island-arc magma, including boninite and/or arc-tholeiite. These depleted peridotites probably represent the residues of melting within mantle diapirs that developed within the mantle wedge.

Mineralogical and geochemical analyses of serpentines from ODP Hole 125-778A

Thirty-five samples from Hole 778A were prepared for X-ray diffraction (XRD) mineralogical analyses and for chemical analyses of major and trace elements. Most of the selected samples were silt- and sand-sized sedimentary serpentinites or microbreccias except for a soft clast of mafic rock, a hard clast of massive serpentinized peridotite, and a pebble of consolidated, undeformed serpentine microbreccia that contained planktonic foraminifers. Both mineralogical and geochemical analyses allow discrimination of three groups among the analyzed samples. These groups correspond to three stratigraphic intervals present along the drilled section. Group A contains the upper samples (lithologic Unit I). These consist of poorly consolidated serpentine muds carrying hard-rock clasts (serpentinized peridotites, metabasalts). They are characterized by the following mineralogical assemblage: serpentine, Fe-oxides and hydroxides, aragonite, and halite. They exhibit variable SiO2, MgO contents, but are characterized by a SiO2/MgO ratio near 1. CaO content is high in relation to development of aragonite. Al2O3 content is low. Relatively high K2O, Na2O, and Sr contents are present, presumably in relation to interactions with seawater. Group B (30-77 mbsf) contains samples exhibiting very homogeneous chemical and mineralogical compositions. They consist of serpentinite microbreccias exhibiting frequent shear structures. Hard-rock clasts are also present (serpentinized peridotites, metabasalts, one possible chert fragment). The mineralogy of the Group B samples is characterized by the presence of serpentine and authigenic minerals: hydroxycarbonates and hydrogrossular. Calcite and chlorite are also present, but all the samples lack aragonite. Their chemical compositions are remarkably similar to compositions of their parent rocks. Group C contains silt- and sand-sized serpentine and serpentine microbreccias, which are locally rich in red clasts, probably strongly altered (oxidized?) mafic fragments. Intervals having clasts of more diverse origin than those higher in the section were recovered. Clast lithology includes serpentinized peridotites, metabasalts, metavolcaniclastite, meta-olivine gabbro, and amphibolite sandstone. Mineralogy and geochemistry reflect these compositions. Serpentine content of the samples is less than in previous groups. Correlatively, sepiolite, palygorskite, and chlorite-smectite are mineral phases present in the analyzed samples. Accessory igneous minerals (amphiboles, pyroxenes, hematite) also were found. The chemical compositions of most of Group C samples differ from that of massive serpentinized peridotites. The main differences are (1) higher SiO2, CaO, TiO2 and Al2O3 contents, (2) a SiO2/MgO ratio greater than 1, and (3) a negative correlation between Al2O3, and MgO, Cr, and Ni. These characteristics suggest new constraints relative to the flow structure of the flank of Conical Seamount.

Major, trace and rare earth element concentration of ODP Site 210-1277 basalts, Newfoundland margin

Drilling of the distal Newfoundland margin at Ocean Drilling Program Site 1277 recovered part of the transition between exhumed sub-continental mantle lithosphere and normal mid-ocean-ridge basalt (N-MORB) volcanism perhaps related to the initiation of seafloor spreading, which may have occurred near the Aptian/Albian boundary, coincident with the final separation of subcontinental mantle lithosphere. Subcontinental mantle lithosphere was recovered near the crest of a basement high, the Mauzy Ridge. This ridge lies near magnetic Anomaly M1 and is inferred to be of Barremian age. The recovered section is dominated by serpentinized spinel harzburgite, with subordinate dunite and minor gabbroic intrusives, and it includes inferred high-temperature ductile shear zones. The serpentinite is capped by foliated gabbro cataclasite that is interpreted as the product of a major seafloor extensional detachment. The serpentinized harzburgite beneath is highly depleted subcontinental mantle lithosphere that was exhumed to create new seafloor within the ocean-continent transition zone. After inferred removal of overlying brittle crust, the detachment was eroded, producing multiple mass flows that were dominated by clasts of serpentinite and gabbro in a lithoclastic and calcareous matrix. Basaltic lavas were erupted spasmodically, mainly as sheet flows, with subordinate lava breccia, hyaloclastite, and possible pillow lava. The sedimentary-volcanic succession and the exhumed mantle lithosphere experienced later high-angle extensional fracturing and probably faulting. Extensional fissures opened incrementally and were filled with silt-sized carbonate, basalt-derived clastic sediment, and hyaloclastite, forming neptunian dykes and geopetal structures. Chemical analysis of representative basalts for major elements and trace elements were made using a high-precision, high-accuracy X-ray fluorescence method (utilizing increased count times) and by whole-rock inductively coupled plasma-mass spectrometry that yielded additional evidence for rare earth elements. The analyses indicate N-MORB to slightly enriched compositions. The MORB was produced by relatively high degree melting of a fertile mantle source that differed strongly from the cored serpentinized peridotites. The basalts exhibit a distinct negative Nb anomaly on MORB-normalized plots that can be explained by prior extraction of melt from upper mantle that had previously been affected by subduction, possibly during closure of the Iapetus or Rheic oceans. In the proposed interpretation, mantle lithosphere was exhumed to the seafloor and experienced mass wasting to form serpentinite-rich mass flows. The interbedded MORB records the beginning of a transition to "normal" seafloor spreading. This interpretation takes into account drilling results from the Iberia-Galicia margin and the Jurassic Alps-Apennines.

Whole-rock geochemical analyses of serpentinized harzburgites from ODP Site 209-1270

Small-scale shear zones are present in drillcore samples of abyssal peridotites from the Mid-Atlantic ridge at 15°20'N (Ocean Drilling Program Leg 209). The shear zones act as pathways for both evolved melts and hydrothermal fluids. We examined serpentinites directly adjacent to such zones to evaluate chemical changes resulting from melt-rock and fluid-rock interaction and their influence on the mineralogy. Compared to fresh harzburgite and melt-unaffected serpentinites, serpentinites adjacent to melt-bearing veins show a marked enrichment in rare earth elements (REE), strontium and high field strength elements (HFSE) zirconium and niobium. From comparison with published chemical data of variably serpentinized and melt-unaffected harzburgites, one possible interpretation is that interaction with the adjacent melt veins caused the enrichment in HFSE, whereas the REE contents might also be enriched due to hydrothermal processes. Enrichment in alumina during serpentinization is corroborated by reaction path models for interaction of seawater with harzburgite-plagiogranite mixtures. These models explain both increased amounts of alumina in the serpentinizing fluid for increasing amounts of plagiogranitic material mixed with harzburgite, and the absence of brucite from the secondary mineralogy due to elevated silica activity. By destabilizing brucite, nearby melt veins might fundamentally influence the low-temperature alteration behaviour of serpentinites. Although observations and model results are in general agreement, due to absence of any unaltered protolith a quantification of element transport during serpentinization is not straightforward.

Ages, sedimentation rates and geochemical composition of ODP Site 162-983 sediments

A geochemical study of sediments from Ocean Drilling Program Site 983 was conducted to examine low-frequency variations in carbonate content as expressed by blue-band reflectance (450-500 nm) over the last 1.2 Ma. Sedimentary percent organic carbon, percent carbonate, and excess barium (Ba[ex]) were used as the primary tools to evaluate the factors responsible for these long-term changes. We observe positive correlation between the mass-accumulation rate of various biogenic components and the mass-accumulation rate of Ba(ex), especially in sediments younger than ~600 ka. Deeper in the section (~600-1200 ka), the correlation between Ba(ex) and the other biogenic tracers is weak. The lack of correlation between Ba(ex) and biogenic carbonate likely results either from a higher supply of terrigenous material at that time (which confounds Ba[ex] estimation), or remobilization of Ba resulting from low pore-water sulfate ion concentrations, or both. Nonbiogenic sediments at Site 983, represented by Th, K2O, and the molar Ti/Al ratio, exhibit cyclic variations that represent mixing between continental and oceanic (i.e., basaltic) terrigenous sources. The timing of these cycles matches that of the major glacial-interglacial cycles, which suggests that they result from the supply of continental material as ice-rafted debris during glacial periods and fine-grained basaltic material by bottom currents during interglacial periods. Given these observations, the most likely causes for the low-frequency carbonate variations observed in the Site 983 sediments are shifts in surface productivity and, to a lesser extent, dilution by the input of terrigenous material.

Geochemistry and K-Ar dates of the Mazagan granodiorite at V30-225, VA79-96KD and DSDP Hole 79-544A, off Moroccan coast

Gneissic granodiorite was recovered by drilling at the base of the Mazagan escarpment, 100 km west of the Casablanca, Morocco, at 4000 m water depth. Coarse, predeformative muscovite yielded dates of -515 Ma, fine-grained muscovite of -455 Ma, biotite -360 and 335 Ma, and feldspar -315 Ma. These dates are tentatively correlated with the microscopic results. We assume a minimum age of middle Cambrian for the granodiorite, an Ordovician deformation and mylonitization, and a Late Carboniferous overprint under upper greenschist facies conditions.

Chemical composition of ultrabasites and gabbro from rift zones of the Indian Ocean

Results of petrographic studies of ultrabasites and gabbro from rift zones of the Indian Ocean are discussed using materials of Cruise 36 of R/V Vityaz. Rocks sampled from two sites 2700 km apart are close to each other in their composition. Petrographically ultrabasic rocks are divided into four subgroups: I - dunite; II - harzburgite, serpentinite; III - plagioclase lherzolite; and IV - metamorphically altered rocks. Petrographic description and chemical composition of basic rock varieties are presented as well as description of rock-forming minerals and their optical properties. Formation of pyroxene and plagioclase is shown to be related to autometasomatosis. Formation of ultrabasite in rift zones is related to complicated processes.

Mineral composition of mid-ocean-ridge basalts from ODP Leg 187 sites

The chemical compositions of olivine, plagioclase, pyroxene, and spinel in lavas collected during Ocean Drilling Program Leg 187 in the Australian Antarctic Discordance, Southeast Indian Ridge (41°-46°S, 126°-135°E) were analyzed, and modeling of the theoretical equilibrium petrogenetic conditions between olivine and melt was conducted. The cores of larger olivine phenocrysts, particularly in the isotopic Indian-type mid-ocean-ridge basalt (MORB), are not equilibrated with melt compositions and are considered to be xenocrystic. Larger plagioclase phenocrysts with compositionally reversed zonation are also xenocrystic. The compositions of primary magma were calculated using a "maximum olivine fractionation" model for primitive MORB that should fractionate only olivine. Olivine compositions equilibrated with calculated primary magma and compositions of calculated primary magma suggest that (1) isotopic Pacific-type MORB is more fractionated than Indian-type MORB, (2) Pacific-type MORB was produced by higher degrees of partial melting than Indian-type MORB, and (3) primary magma for Indian-type MORB was segregated from mantle at 10 kbar (~30 km depth), whereas that for Pacific-type MORB was segregated at 15 kbar (~45 km depth).

Geochemistry of basalts from ODP Hole 109-648B

Legs 106-109 achieved the first basaltic bare-rock drill hole, on a small volcano (Serocki volcano) located on the flanks of the rift valley in the MAR about 70 km south of the Kane fracture zone. Because of severe technical difficulties only 50.5 m of basalt below seafloor was recovered. Geochemical analysis shows that the recovered basalts display typical N-MORB characteristics as expected in this segment of the Mid-Atlantic ridge. The lava flows display rather equivalent geochemical characteristics all over the drilled section and show the imprint of a previous magmatic differentiation suffered by the magmas before their emission, indicative of a fractional crystallization of plagioclase-bearing cumulates. The incompatible and alkali element content of these 648B magmas is very low, a feature which resembles those of other N-MORB. The geochemical characteristics of these basalts look closely similar to those of basalts from the same flow line emitted respectively 10 m.y. (Hole 395, Legs 45-46), and 110 m.y. (Hole 417A, Legs 51-53) ago, supporting the persistence in this ridge segment of a mantle source with depleted characteristics over the last 110 m.y., but with some variations in the degree of depletion of the source along this period. Although these rocks appear fresh, the imprint of an incipient low temperature alteration can be noticed in a few samples, as evidenced by slight increases of alkali, U elements, and 87Sr/86Sr isotopic compositions.

Volcanic rocks from the submarine Vityaz Ridge: their age, chemical and isotopic compositions

Results of geological studies at the submarine Vityaz Ridge carried out during cruises 37 and 41 of R/V Akademik Lavrent'ev in 2005 and 2006 are reported. The studied area is located at an near-island trench of the slope in the central part of the Kuril Island arc. Morphologically it consists of two parts: an inner volcanic arc represented by the Great Kuril Range and an outer arc corresponding to the submarine Vityaz Ridge. Diverse rocks composing the basement and the sedimentary cover of the ridge were recovered by dredging. Based on K-Ar dating and geochemistry, volcanics were divided into Paleocene, Eocene, late Oligocene, and Pliocene-Pleistocene complexes. Each of the complexes reflects a tectonomagmatic stage in the ridge evolution. Geochemical and isotope data on the volcanics indicate contribution of ancient crustal material in the magma source and, correspondingly, formation of this structure on the continental basement. Two-stage model ages (TDM2) vary in a wide range from zero values in mafic rocks to 0.77 Ga in felsic varieties, pointing to presence of Precambrian protolith in the source of the felsic rocks of the Vityaz Ridge. The Pliocene-Pleistocene volcanics are classed with tholeiitic, calc-alkaline, and subalkaline series, which differ in alkali contents and REE fractionation. Values of (La/Sm)_n and (La/Yb)_n ratios vary from 0.74 and 0.84 in the tholeiitic varieties to 1.19 and 1.44 in the calc-alkaline and 2.32 and 3.73 in the subalkaline rocks. All three varieties occur within the same volcanic edifices and formed during differentiation of magmatic melts that were channeled along fault zones from the mantle source slightly enriched in crustal component.

Geochemistry of Neogene-Quaternary sediments in ODP Site 105-645

Subcontinuously cored early(?)-middle Miocene to recently deposited sediments from ODP Site 645 were studied texturally, mineralogically, and geochemically. The entire sequence contains minerals and associated chemical elements that are chiefly of detrital origin. In particular, the clay minerals, which include smectite, kaolinite, chlorite, and illite, are detrital. No obvious evidence of diagenesis with depth, of burial, of volcanism, or of hydrothermal alteration was observed. The sedimentary textures, clay mineralogy, and <2-µm fraction geochemistry of the early middle Miocene sediments (630 to 1147 mbsf) suggest the pronounced but variable influence of a southward bottom current. Two clay facies are defined. The lower one, Cj (780 to 1147 mbsf), is characterized by the great abundance of discrete smectite (with less than 15% illite interlayers), probably detrital in origin, and reworked older, discrete, smectite-rich sediments. The upper clay facies, C2 (630 to 780 mbsf), shows a net decrease of the fully expandable clay abundances, with a great abundance of mixed-layer, illite-smectite clays (60 to 80% of illite interlayers). Such clay assemblages can be inherited from paleosoils or older sedimentary rocks. An important change occurs at 630 mbsf (clay fraction) or 600 mbsf (sedimentary texture), which may be explained by the beginning of continental glaciation (630 mbsf, ~9 Ma) and the onset of ice rafting in Baffin Bay (600 mbsf, ~8 Ma). Above this level, the characteristics and modifications of the clay assemblages are controlled climatically and can be explained by the fluctuations of (1) ice-rafting, (2) speed of weak bottom currents, and (3) some supply by mud turbiditic currents. Three clay facies (C3, C4, and C5) can be defined by the abrupt increases of the inherited chlorite and illite clays.

Inorganic and organic geochemistry and sediment descriptions at DSDP Hole 93-603B

About one hundred samples of sediments and rocks recovered in Hole 603B were analyzed for type, abundance, and isotopic composition of organic matter, using a combination of Rock-Eval pyrolysis, C-H-N-S elemental analysis, and isotope-ratio mass spectrometry. Concentrations of major, minor, and trace inorganic elements were determined with a combination of X-ray fluorescence and induction-coupled plasma spectrometry. The oldest strata recovered in Hole 603B (lithologic Unit V) consist of interbedded light-colored limestones and marlstones, and black calcareous claystones of Neocomian age. The inorganic and organic geochemical results suggest a very terrigenous aspect to the black claystones. The organic geochemical results indicate that the limestones and marlstones contain a mixture of highly degraded marine and terrestrial organic matter. Comparison of the Neocomian carbonates at Site 603 with those on the other side of the North Atlantic, off Northwest Africa at Site 367, shows that the organic matter at Site 367 contains more marine organic matter, as indicated by higher pyrolysis hydrogen indices and lighter values of d13C. Comparison of inorganic geochemical results for the carbonate lithologies at Site 603 with those for carbonate lithologies at Site 367 suggests that the Site 603 carbonates may contain clastic material from both North American and African sources. The black claystones at Site 603, on the other hand, probably were derived almost entirely from North American clastic sources. Lithologic Unit IV overlying the Neocomian carbonates, consists of interbedded red, green, and black claystones. The black claystones at Site 603 contain more than ten times the organic carbon concentration of the interbedded green claystones. The average concentration of organic carbon in the black claystones (2.8%), however, is low relative to most mid-Cretaceous black claystones and shales in the Atlantic, particularly those found off Northwest Africa. The geochemical data all suggest that the organic matter in the black claystones is more abundant but generally more degraded than the organic matter in the green claystones, and that it was derived mainly from terrestrial sources and deposited in oxygenated bottom waters. The increased percentage of black claystone beds in the upper Cenomanian section, and the presence of more hydrogen-rich organic matter in this part of the section, probably resulted from the increased production and accumulation of marine organic matter that is represented worldwide near the Cenomanian/Turonian boundary in deep-sea and land sections. A few upper Cenomanian black claystone samples that have hydrogen indices > 150 also contain particularly high concentrations of V and Zn. Most samples of black claystone, however, are not particularly metal-rich compared with other black claystones and shales. Compared with red claystones from lithologic Unit IV, the green and black claystones are enriched in many trace transition elements, especially V, Zn, Cu, Co, and Pb. The main difference between the "carbonaceous" claystones of lithologic Unit IV and "variegated" or "multicolored" claystones of the overlying Upper Cretaceous to lower Tertiary Unit III is the absence of black claystone beds. As observed at several other sites (105 and 386), the multicolored claystones at Site 603 are somewhat enriched in several trace transition elements-especially Cu, Ni, and Cr-relative to most deep-sea clays. The multicolored claystones are not enriched in Fe and Mn, and therefore are not "metalliferous" sediments in the sense of those found at several locations in the eastern Pacific. The source of the slightly elevated concentrations of transition metals in the multicolored claystones probably is upward advection and diffusion of metals from the black claystones of the underlying Hatteras Formation. The red, orange, and green claystone beds of lithologic Unit II (Eocene), like those of Unit III, really represent a continuation of deposition of multicolored claystone that began after the deposition of the Neocomian carbonates. The color of the few black beds that occur within this unit results from high concentrations of manganese oxide rather than high concentrations of organic matter.