Geochemistry of ODP Leg 135 vitrophyric rhyolite samples

In-situ proton-microprobe analyses are presented for glasses, plagioclases, pyroxenes, olivines, and spinels in eleven samples from Sites 834-836, 839, and 841 (vitrophyric rhyolite), plus a Tongan dacite. Elements analyzed are Mn, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Pb, and Sn (in spinels only). The data are used to calculate two sets of partition coefficients, one set based on the ratio of element in mineral/element in coexisting glass. The second set of coefficients, thought to be more robust, is corrected by application of the Rayleigh fractionation equations, which requires additional use of modal data. Data are presented for phenocryst core-rim phases and microphenocryst-groundmass phases from a few samples. Comparison with published coefficients reveals an overall consistency with those presented here, but with some notable anomalies. Examples are relatively high Zr values for pyroxenes and abnormally low Mn values in olivines and clinopyroxenes from Site 839 lavas. Some anomalies may reflect kinetic effects, but interpretation of the coefficients is complicated, especially in olivines from Sites 836 and 839, by possible crystal-liquid disequilibrium resulting from mixing processes.

Annotated record and chemical composition of manganese nodules from station VITYAZ 4575, Indian Ocean

Analyses are given for the core and outer colliform shell of a manganese nodule collected at a depth of 5000 m in the Indian Ocean, and for the red clay that encloses the nodules. Trace elements determined include rare earths, Nb, Ta, Th, and V. The cores of the nodules were once composed of basaltic rock, but now are phillipsite and nontronite. The outer shell is composed of manganite, with admixed quartz, phillipsite, and some geothite. The correlations established between the redox potentials and the concentration coefficients for 12 elements indicate that Eh plays a greater role in the formation of the manganiferous shells than coprecipitation properties.

Chemical composition of basalts from the Atlantic Ocean

Basalt recovered beneath Jurassic sediments in the western Atlantic at Deep Sea Drilling Project sites 100 and 105 of leg 11 has petrographic features characteristic of water-quenched basalt extruded along modern ocean ridges. Site 100 basalt appears to represent two or three massive cooling units, and an extrusive emplacement is probable. Site 105 basalt is less altered and appears to be a compositionally homogeneous pillow lava sequence related to a single eruptive episode. Although the leg 11 basalts are much more closely related in time to the Triassic lavas and intrusives of eastern continental North America, their geochemical features are closely comparable to those of modern Mid-Atlantic Ridge basalts unrelated to postulated "mantle plume" activity. Projection of leg 11 sites back along accepted spreading "flow lines" to their presumed points of origin shows that these origins are also outside the influence of modern" plume" activity. Thus, these oldest Atlantic seafloor basalts provide no information on the time of initiation of these "plumes". The Triassic continental diabases show north to south compositional variations in Rb, Ba, La, and Sr which lie within the range of " plume "-related basalt on the Mid-Atlantic Ridge (20° - 40° N) This suggests that these diabases had mantle sources similar in composition to those beneath the present Mid-Atlantic Ridge. "Plumes" related to deep mantle sources may have contributed to the LIL-element enrichment in the Triassic diabase and may also have been instrumental in initiating the rifting of the North Atlantic. Systematically high values for K and Sr87/Sr86 in the Triassic diabases may reflect superimposed effects of crustal contamination in the Triassic magmas.

Geochemistry of mafic clasts of ODP Holes 125-778A and 125-779A

Clasts of metamorphosed mafic igneous rock of diverse composition were recovered in two drill sites on a serpentine mud volcano in the outer Mariana forearc during Ocean Drilling Program Leg 125. These clasts are xenolithic fragments that have been entrained in the rising serpentine mud, and make up less that 9% of the total rock recovered at Sites 778 and 779. Most samples are metabasalt or metadiabase, although one clast of possible boninite and one cumulate gabbro were recovered. On the basis of trace element signatures, samples are interpreted to represent both arc-derived and mid-ocean ridge-derived compositions. Rocks with extremely low TiO2 (<0.3 wt%) and Zr (<30 ppm) are similar to boninite series rocks. Samples with low TiO2 (<0.9 wt%) and Zr (<50 ppm) and extreme potassium enrichment (K2O/Na2O >3.9) may represent island arc rocks similar to shoshonites. However, the K2O/Na2O ratios are much higher than those reported for shoshonites from modem or ancient arcs and may be the result of metamorphism. Samples with moderate TiO2 (1.4 to 1.5 wt%) and Zr (72 to 85 ppm) are similar to rocks from mid-ocean ridges. A few samples have TiO2 and Zr intermediate between island arc and mid-ocean ridge basalt-like rocks. Two samples have high iron (Fe2O3* = >12.8 to 18.5 wt%) (Fe2O3* = total iron calculated as Fe2O3) and TiO2 (>2.3 wt%) and resemble FeTi basalt recovered from mid-ocean ridges. Metamorphism in most samples ranges from low-temperature zeolite, typical of ocean floor weathering, to prehnite-pumpellyite facies and perhaps lower greenschist. Blue amphibole and lawsonite minerals are present in several samples. One diabase clast (Sample 9) exhibits Ca enrichment, similar to rodingite metamorphism, typical of mafic blocks in serpentinized masses. The presence of both low-grade (clays and zeolites) and higher grade (lawsonite) metamorphism indicates retrograde processes in these clasts. These clasts are fragments of the forearc crust and possibly of the subducting plate that have been entrained in the rising serpentine and may represent the deepest mafic rocks ever recovered from the Mariana forearc. The variable compositions and degree of metamorphism of these clasts requires at least two tectonic origins. The recovery of clasts with mid-ocean ridge and arc chemical affinities in a single drill hole requires these clasts to have been "mixed" on a small scale either (1) in the forearc crustal sequence, or (2) after inclusion in the rising serpentine mud. The source of the MORB-like samples and an explanation for the presence of both MORB-like and arc-like rocks in close proximity is critical to any model of the evolution of the Mariana forearc. The source of the MORB-like samples likely will be one (or more) of the following: (1) accretion of Pacific plate lithosphere, (2) remnants of original forearc crust (trapped plate), (3) volcanism in the supra-subduction zone (arc or forearc) environment, or (4) derivation from the subducting slab by faulting along the dÈcollement.

X-ray fluorescence (XRF) scannings and grain size analysis at sites in the Gulf of Cádiz

Contourites in the Gulf of Cádiz preserve a unique archive of Mediterranean Outflow Water (MOW) variability over the past 5.3 Ma. In our study we investigate the potential of geochemical data obtained by XRF scanning to decipher bottom current processes and paleoclimatic evolution at two different sites drilled through contourite deposits in the northern Gulf of Cadiz: Site U1387, which is bathed by the upper MOW core, and Site U1389, located more proximal to the Straits of Gibraltar. The lack of major downslope transport at both locations during the Pleistocene makes them ideal locations for the purpose of our study. The results indicate that the Zr/Al ratio, representing the relative enrichment of heavy minerals (zircon) over less dense alumosilicates under strong bottom current flow, is the most useful indicator for a semi-quantitative assessment of current strength. While most elements are biased by current-related processes, the bromine record, representing organic content, preserves the most pristine climate signal rather independent of grain size changes. Hence, Br can be used for chronostratigraphy and site-to-site correlation in addition to stable isotope stratigraphy. Based on these findings we reconstructed MOW variability for Marine Isotope Stages 1-5 using the Zr/Al ratio from Site U1387. The results reveal abrupt, millennial-scale variations of MOW strength during Greenland Stadials (GS) and Interstadials (GI) with strong MOW during GS and glacial Terminations and a complex behavior during Heinrich Stadials. Millennial-scale variability persisting during periods of poorly expressed GS/GI cyclicities implies a strong internal oscillation of the Mediterranean/North Atlantic climate system.

Analyses of lake La Thuile sediment core

Lake La Thuile, in the Northern French Prealps (874 m a.s.l.), provides an 18 m long sedimentary sequence spanning the entire Lateglacial/Holocene period. The high resolution multi-proxy (sedimentological, palynological, geochemical) analysis of the uppermost 6.2 meters reveals the Holocene dynamics of erosion in the catchment in response to landscape modifications. The mountain belt is at relevant altitude to study past human activities and the watershed is sufficiently disconnected from large valleys to capture a local sedimentary signal. From 12,000 to 10,000 cal. BP (10 to 8 ka cal. BC), the onset of hardwood species triggered a drop in erosion following the Lateglacial/Holocene transition. From 10,000 to 4500 cal. BP (8 to 2.5 ka cal. BC), the forest became denser and favored slope stabilization while erosion processes were very weak. A first erosive phase was initiated at ca . 4500 cal. BP without evidence of human presence in the catchment. Then, the forest declined at approximately 3000 cal. BP, suggesting the first human influence on the landscape. Two other erosive phases are related to anthropic activities: approximately 2500 cal. BP (550 cal. BC) during the Roman period and after 1600 cal. BP (350 cal. AD) with a substantial accentuation in the Middle Ages. In contrast, the lower erosion produced during the Little Ice Age, when climate deteriorations are generally considered to result in an increased erosion signal in this region, suggests that anthropic activities dominated the erosive processes and completely masked the natural effects of climate on erosion in the late Holocene.

Geochemistry on the Kekesai composite intrusion

The late Carboniferous to Permian is a critical period for final amalgamation of the Central Asian Orogenic Belt (CAOB), which is characterized by voluminous igneous rocks, particularly granitoids. The Kekesai composite granitoid porphyry intrusion, situated in the Chinese western Tianshan (southwest part of CAOB) includes two intrusive phases, a monzogranite phase, intruded by a granodiorite phase. LA-ICPMS U-Pb zircon analyses suggest that the monzogranitic rocks formed at 305.5±1.1 Ma, with a wide age range of inherited zircons (358-488 Ma and 1208-1391 Ma), whereas the granodioritic rocks formed at 288.7±1.5 Ma. The monzogranitic and granodioritic phases have similar geochemical features and Sr-Nd-Hf isotopic compositions. They exhibit high and variable SiO2 (66-71 wt.%) and MgO (0.41-2.14 wt.%) contents with some arc-like geochemical characteristics (e.g., enrichment of large ion lithophile elements and negative anomalies of Nb, Ta and Ti) and relatively high initial 87Sr/86Sr ratios (ISr=0.7055-0.7059), low positive eNd(t) (+0.84 to +1.03) as well as a large variation in Hf isotopic compositions with eHf(t) between +3.43 to +14.8, implying both of them were derived from similar source materials. These geochemical characteristics suggest that they might be mainly derived from the partial melting of arc-derived Mesoproterozoic mafic lower crust with involvement of a mantle-derived component in variable proportions by mantle-derived magma underplating. The presence of late-Ordovician to earliest early Carboniferous inherited zircons and the Hf isotopic compositions in the monzogranitic sample, similar to that of the widespread juvenile arc rocks, indicates some crust contamination during magma emplacement. Our new data, combined with previous studies, imply that extensive post-collisional magmatism due to underplating of mantle-derived magma, could plausibly be explained by slab break-off regime.

Chemistry of igneous rocks from, the Celebes Sea basement

Major and trace elements, mineral chemistry, and Sr-Nd isotope ratios are reported for representative igneous rocks of Ocean Drilling Program Sites 767 and 770. The basaltic basement underlying middle Eocene radiolarianbearing red clays was reached at 786.7 mbsf and about 421 mbsf at Sites 767 and 770, respectively. At Site 770 the basement was drilled for about 106 m. Eight basaltic units were identified on the basis of mineralogical, petrographical, and geochemical data. They mainly consist of pillow lavas and pillow breccias (Units A, B, D, and H), intercalated with massive amygdaloidal lavas (Units Cl and C2) or relatively thin massive flows (Unit E). Two dolerite sills were also recognized (Units F and G). All the rocks studied show the effect of low-temperature seafloor alteration, causing almost total replacement of olivine and glass. Calcite, clays, and Fe-hydroxides are the most abundant secondary phases. Chemical mobilization due to the alteration processes has been evaluated by comparing elements that are widely considered mobile during halmyrolysis (such as low-field strength elements) with those insensitive to seafloor alteration (such as Nb). In general, MgO is removed and P2O5 occasionally enriched during the alteration of pillow lavas. Ti, Cs, Li, Rb, and K, which are the most sensitive indicators of rock/seawater interaction, are generally enriched. The most crystalline samples appear the least affected by chemical changes. Plagioclase and olivine are continuously present as phenocrysts, and clinopyroxene is confined in the groundmass. Textural and mineralogical features as well as crystallization sequences of Site 770 rocks are, in all, analogous to typical mid-ocean-ridge basalts (MORBs). Relatively high content of compatible trace elements, such as Ni and Cr, indicate that these rocks represent nearly primitive or weakly fractionated MORBs. All the studied rocks are geochemically within the spectrum of normal MORB compositional variation. Their Sr/Nd isotopic ratios plot on the mantle array (87Sr/87Sr 0.70324-0.70348 with 143Nd/144Nd 0.51298-0.51291) outside the field of Atlantic and Pacific MORBs. However, Sr and Nd isotopes are typical of both Indian Ocean MORBs and of some back-arc basalts, such as those of Lau Basin. The mantle source of Celebes basement basalts does not show a detectable influence of a subduction-related component. The geochemical and isotopic data so far obtained on the Celebes basement rocks do not allow a clear discrimination between mid-ocean ridge and back-arc settings.

Physics, geochemistry and bulk sedimentology on cores from the Peru Basin

Empirical relationships between physical properties determined non-destructively by core logging devices and calibrated by carbonate and opal measurements determined on discrete samples allow extraction of carbonate and opal records from the non-destructive measurements in biogenic settings. Contents of detrital material can be calculated as a residual. For carbonate and opal the correlation coefficients (r) are 0.954 and ?0.916 for sediment density, ?0.816 and 0.845 for compressional-wave velocity, 0.908 and ?0.942 for acoustic impedance, and 0.886 and ?0.865 for sediment color (lightness). Carbonate contents increase in concert with increasing density and acoustic impedance, decreasing velocity and lighter sediment color. The opposite is true for opal. The advantages of deriving the sediment composition quantitatively from core logging are: (i) sampling resolution is increased significantly, (ii) non-destructive data can be gathered rapidly, and (iii) laboratory work on discrete samples can be reduced. Applied to paleoceanographic problems, this method offers the opportunity of precise stratigraphic correlations and of studying processes related to biogenic sedimentation in more detail. Density is most promising because it is most strongly affected by changes in composition.

Geochemistry at DSDP Hole 76-534A

At Site 534 in the Blake-Bahama Basin, western North Atlantic, an interval of 68 m of Maestrichtian (Upper Cretaceous) and upper middle to upper Eocene sediments consists of terrigenous siltstones, mudstones, and varicolored zeolitic claystones; minor recovery of micritic limestones, porcellanites, and quartzitic chert was made at this site as well. Comparisons with other Deep Sea Drilling Project (DSDP) sites in the western North Atlantic suggest that the following formations are present in this interval: Hatteras (Maestrichtian), Plantagenet (Maestrichtian and upper Eocene), Bermuda Rise (upper middle to upper Eocene), and the basal Blake Ridge Formation (upper middle to upper Eocene). Recognition of a Tertiary interval of the Plantagenet allows that formation to be divided into lower and upper informal units. Condensation makes this formal lithostratigraphic subdivision difficult. Together the formations record marked net condensed sedimentation (average rate ca. 2.5 m/m.y.) in strongly oxidizing bottom waters. From sedimentary structures and petrography, it is inferred that the terrigenous siltstones and micritic limestones were redeposited from the continental margin by turbidity currents. Chemical data plus petrography confirm relatively high plankton productivity during the upper Eocene. Much of the nonrecovered Eocene interval may represent chert and porcellanite. Fragments recovered were formed by replacement of relatively porous calciturbidites by opal-CT and quartz. Radiolarians in interbedded claystones rich in clinoptilolite show extensive dissolution. Relative to typical hemipelagic sediments, the claystones are enriched in many metals (Cu, Ni, Zn, Pb), particularly within manganese micronodules. The metal accumulation is related to a 30-m.y. period of slow net sediment accumulation, rather than to hydrothermal enrichment or to upward mobilization of metals from the underlying reduced Hatteras black shale facies. Elsewhere in the Blake-Bahama Basin, at Site 391, 22 km to the northwest, upper Eocene facies are missing, reportedly due to deep seafloor erosion of up to 800 m of the sedimentary succession. By contrast, the discovery that this interval is preserved at nearby Site 534 points to much less extensive seafloor erosion, possibly mostly in the Oligocene, which is missing at both DSDP Sites.

Chemical composition of DSDP cherts and associated host sediments

Chert and associated host sediments from Monterey Formation and Deep Sea Drilling Project (DSDP) sequences were analyzed in order to assess chemical behavior during diagenesis of biogenic sediments. The primary compositional contrast between chert and host sediment is a greater absolute SiO2 concentration in chert, often with final SiO2 >=98 wt%. This contrast in SiO2 (and Si/Al) potentially reflects precursor sediment heterogeneity, diagenetic chemical fractionation, or both. SiO2 concentrations and Si/Al ratios in chert are far greater than in modern siliceous oozes, however and often exceed values in acid-cleaned diatom tests. Compositional contrasts between chert and host sediment are also orders-of-magnitude greater than between multiple samples of the host sediment. Calculations based on the initial composition of adjacent host, observed porosity reductions from host to chert and a postulated influx of pure SiO2, construct a chert composition which is essentially identical to observed SiO2 values in chert. Thus, precursor heterogeneity does not seem to be the dominant factor influencing the current chert composition for the key elements of interest. In order to assess the extent of chemical fractionation during diagenesis, we approximate the precursor composition by analyzing host sediments adjacent to the chert. The SiO2 concentration contrast seems caused by biogenic SiO2 dissolution and transport from the local adjacent host sediment and subsequent SiO2 reprecipitation in the chert. Along with SiO2, other elements are often added (with respect to Al) to Monterey and DSDP chert during silicification, although absolute concentrations decrease. The two Monterey quartz chert nodules investigated, in contrast to the opal-CT and quartz chert lenses, formed primarily by extreme removal of carbonate and phosphate, thereby increasing relative SiO2 concentrations. DSDP chert formed by both carbonate/phosphate dissolution and SiO2 addition from the host. Manganese is fractionated during chert formation, resulting in MnO/Al2O3 ratios that no longer record the depositional signal of the precursor sediment. REE data indicate only subtle diagenetic fractionation across the rare earth series. Ce/Ce* values do not change significantly during diagenesis of either Monterey or DSDP chert. Eu/Eu* decreases slightly during formation of DSDP chert. Normative La/Yb is affected only minimally as well. During formation of one Monterey opal-CT chert lens, REE/Al ratios show subtle distribution changes at Gd and to a lesser extent near Nd and Ho. REE compositional contrasts between diagenetic states of siliceous sediment and chert are of a vastly smaller scale than has been noted between different depositional environments of marine sediment, indicating that the paleoenvironmental REE signature is not obscured by diagenetic overprinting.

Chemical composition of basalts from ODP holes at the Kerguelen Plateau

Petrographic and geochemical study of basalts in the Kerguelen Plateau basement revealed changes in composition and character of volcanism during development of this tectonovolcanic structure. The Kerguelen Plateau is one of the largest intraplate rises in the World Ocean. It started to form about 120 Ma ago. Age of basalts and overlying sediments shows that plateau formation was in the northwest direction. Basalts of the Kerguelen Plateau basement are products of tholeiitic melts in terms of geochemistry, but differ from mid-ocean ridge basalt (MORB). They are enriched in incompatible trace elements and rare earth elements (REE) relative to MORB, and degree of enrichment varies in basalts from different segments of the plateau. Composition of basalts does not directly depend on their age. Specific features of plateau magmatism are commonly explained in terms of a long-living deep magma plume, which variously interacted with a depleted upper mantle source at different stages of plateau formation. However, taking into account block morphology and deep structure of the plateau, one can suggest that plateau volcanism was initiated by a large fault. As the volcanism prograded to the northwest, depth of fault penetration into the mantle changed. Composition of basalts in the plateau basement was also governed by formation depth of primary melts.