Geochemistry of tephra from the Taupo Volcanic Zone

The Taupo Volcanic Zone (TVZ), central North Island, New Zealand, is the most frequently active Quaternary rhyolitic system in the world. Silicic tephras recovered from Ocean Drilling Programme Site 1123 (41°47.16'S, 171°29.94'W; 3290 m water depth) in the southwest Pacific Ocean provide a well-dated record of explosive TVZ volcanism since ~1.65 Ma. We present major, minor and trace element data for 70 Quaternary tephra layers from Site 1123 determined by electron probe microanalysis (1314 analyses) and laser ablation inductively coupled plasma mass spectrometry (654 analyses). Trace element data allow for the discrimination of different tephras with similar major element chemistries and the establishment of isochronous tie-lines between three sediment cores (1123A, 1123B and 1123C) recovered from Site 1123. These tephra tie-lines are used to evaluate the stratigraphy and orbitally tuned stable isotope age model of the Site 1123 composite record. Trace element fingerprinting of tephras identifies ~4.5 m and ~7.9 m thick sections of repeated sediments in 1123A (49.0-53.5 mbsf [metres below seafloor]) and 1123C (48.1-56.0 mbsf), respectively. These previously unrecognised repeated sections have resulted in significant errors in the Site 1123 composite stratigraphy and age model for the interval 1.15-1.38 Ma and can explain the poor correspondence between d18O profiles for Site 1123 and Site 849 (equatorial Pacific) during this interval. The revised composite stratigraphy for Site 1123 shows that the 70 tephra layers, when correlated between cores, correspond to ~37-38 individual eruptive events (tephras), 7 of which can be correlated to onshore TVZ deposits. The frequency of large-volume TVZ-derived silicic eruptions, as recorded by the deposition of tephras at Site 1123, has not been uniform through time. Rather it has been typified by short periods (25-50 ka) of intense activity bracketed by longer periods (100-130 ka) of quiescence. The most active period (at least 1 event per 7 ka) occurred between ~1.53 and 1.66 Ma, corresponding to the first ~130 ka of TVZ rhyolitic magmatism. Since 1.2 Ma, ~80% of tephras preserved at Site 1123 and the more proximal Site 1124 were erupted and deposited during glacial periods. This feature may reflect either enhanced atmospheric transport of volcanic ash to these sites (up to 1000 km from source) during glacial conditions or, more speculatively, that these events are triggered by changes in crustal stress accumulation associated with large amplitude sea-level changes. Only 8 of the ~37-38 Site 1123 tephra units (~20%) can be found in all three cores, and 22 tephra units (~60%) are only present in one of the three cores. Whether a tephra is preserved in all three cores does not have any direct relationship to eruptive volume. Instead it is postulated that tephra preservation at Site 1123 is 'patchy' and influenced by the vigorous nature of their deposition to the deep ocean floor as vertical density currents. At this site, at least 5 cores would need to have been drilled within a proximity of 10's to 100's of metres of each other to yield a >99% chance of recovering all the silicic tephras deposited on the ocean surface above it in the past 1.65 Ma.

(Table 1) Redox potential and chemical element contents in CaCO3 and amorphous silica free matter in surface layer bottom sediments of the Pacific Ocean from Hawaiian Islands to Mexico coast

Distribution of Fe, Mn, P, Ti, Cu, Ni, Co, V, Cr, W, Mo, and As in the surface sediment layer on the section from the Hawaiian Islands to the coast of Mexico (Mexico section) is studied. Contents of all studied elements increase from biogenic-terrigenous sediments off the coast of Mexico to pelagic red clays of the Northeast Basin, and more sharply for mobile elements - Mn, Mo, Cu, Ni, Co, and As. In near Hawaii sediments rich in coarsely fragmented volcanic-terrigenous and pyroclastic material of basaltic composition with high contents of Ti, Fe, V, Cr, W, and P, contents of these elements increase sharply, and contents of Mn, Mo, Ni, Co, and Cu for the same reason decrease sharply in comparison with red clay. Abnormally high contents of Mn, Mo, Cu, Ni, Co, and As in the upper layer of hemipelagic and transition sediments of the Mexico section result from diagenetic redistribution and their accumulation on the surface. Processes of diagenetic redistribution in hemipelagic and transition sediment mass of the Mexico section are more rapid than in similar sediments of the Japan section due lower sedimentation rates and higher initial concentrations of Mn. Basic similarity of element distribution regularities in sediments of Japan and Mexico sections is shown.

Geochemistry of bulk sediment from the Cape Basin

A technique for onsite application of X-ray fluorescence (XRF) spectrometry to samples from sediment cores aboard a research vessel was developed and tested. The method is sufficiently simple, precise, and fast to be used routinely for high-resolution analyses of depth profiles as well as surface samples. Analyses were performed with the compact high-performance energy-dispersive polarisation X-ray fluorescence (EDPXRF) analyser Spectro Xepos. Contents of the elements Si, Ti, Al, Fe, Mn, Mg, Ca, K, Sr, Ba, Rb, Cu, Ni, Zn, P, S, Cl and Br were simultaneously determined on 200-225 samples of each core within 24 h of recovery. This study presents a description of the employed shipboard preparation and analysis technique, along with some example data. We show land-based datasets that support our decisions to use powder samples and to reduce the original measuring time for onboard analyses. We demonstrate how well the results from shipboard measurements for the various elements compare with the land-based findings. The onboard geochemical data enabled us to establish an element stratigraphy already during the cruise. Correlation of iron, calcium and silicon enrichment trends with an older reference core provided an age model for the newly retrieved cores. The Spectro Xepos instrument performed without any analytical and technical difficulties which could have been caused by rougher weather conditions or continuous movement and vibration of the research vessel. By now, this XRF technique has been applied during three RV Meteor cruises to approximately 5,000 Late Quaternary sediment samples from altogether 23 gravity cores, 25 multicorer cores and two box cores from the eastern South Atlantic off South Africa/Namibia and the eastern Atlantic off NW Africa.

Geochemistry of ocean floor and forearc serpentinites

We provide new insights into the geochemistry of serpentinites from mid-ocean ridges (Mid-Atlantic Ridge and Hess Deep), passive margins (Iberia Abyssal Plain and Newfoundland) and fore-arcs (Mariana and Guatemala) based on bulk-rock and in situ mineral major and trace element compositional data collected on drill cores from the Deep Sea Drilling Project and Ocean Drilling Program. These data are important for constraining the serpentinite-hosted trace element inventory of subduction zones. Bulk serpentinites show up to several orders of magnitude enrichments in Cl, B, Sr, U, Sb, Pb, Rb, Cs and Li relative to elements of similar compatibility during mantle melting, which correspond to the highest primitive mantle-normalized B/Nb, B/Th, U/Th, Sb/Ce, Sr/Nd and Li/Y among subducted lithologies of the oceanic lithosphere (serpentinites, sediments and altered igneous oceanic crust). Among the elements showing relative enrichment, Cl and B are by far the most abundant with bulk concentrations mostly above 1000 µg/g and 30 µg/g, respectively. All other trace elements showing relative enrichments are generally present in low concentrations (µg/g level), except Sr in carbonate-bearing serpentinites (thousands of µg/g). In situ data indicate that concentrations of Cl, B, Sr, U, Sb, Rb and Cs are, and that of Li can be, increased by serpentinization. These elements are largely hosted in serpentine (lizardite and chrysotile, but not antigorite). Aragonite precipitation leads to significant enrichments in Sr, U and B, whereas calcite is important only as an Sr host. Commonly observed brucite is trace element-poor. The overall enrichment patterns are comparable among serpentinites from mid-ocean ridges, passive margins and fore-arcs, whereas the extents of enrichments are often specific to the geodynamic setting. Variability in relative trace element enrichments within a specific setting (and locality) can be several orders of magnitude. Mid-ocean ridge serpentinites often show pronounced bulk-rock U enrichment in addition to ubiquitous Cl, B and Sr enrichment. They also exhibit positive Eu anomalies on chondrite-normalized rare earth element plots. Passive margin serpentinites tend to have higher overall incompatible trace element contents than mid-ocean ridge and fore-arc serpentinites and show the highest B enrichment among all the studied serpentinites. Fore-arc serpentinites are characterized by low overall trace element contents and show the lowest Cl, but the highest Rb, Cs and Sr enrichments. Based on our data, subducted dehydrating serpentinites are likely to release fluids with high B/Nb, B/Th, U/Th, Sb/Ce and Sr/Nd, rendering them one of the potential sources of some of the characteristic trace element fingerprints of arc magmas (e.g. high B/Nb, high Sr/Nd, high Sb/Ce). However, although serpentinites are a substantial part of global subduction zone chemical cycling, owing to their low overall trace element contents (except for B and Cl) their geochemical imprint on arc magma sources (apart from addition of H2O, B and Cl) can be masked considerably by the trace element signal from subducted crustal components.

Aerosols in the Arctic

In this monograph on the basis of materials obtained by the author and his colleagues in Arctic expeditions of 1991-2005 and of published data results of studies effect of aerosols on environmental conditions and marine sedimentation in the Arctic are summarizes. Processes of aeolian transport and transformation of sedimentary material from sources to places of its accumulation in bottom sediments are described. Results of this study can be used to assess current state of ecosystem of Arctic seas and as a background for evaluation of possible human impact on nature during exploration of mineral resources of the Arctic shelf. For oceanographers, geochemists, geoecologists.

Description, geochemistry, mineralogy and isotopes of sulfides from ODP Site 193-1189, PACMANUS field

Chemical and isotopic data for rare massive and semimassive sulfide samples cored at Site 1189 (Roman Ruins, PACMANUS) suggest their genetic relationship with sulfide chimneys at the seafloor. Sand collected from the hammer drill after commencement of Hole 1189B indicates that at least the lower section of the cased interval was occupied by material similar to the stockwork zone cored from 31 to ~100 meters below seafloor (mbsf) in this hole, but with increased content of barite, sphalerite, and lead-bearing minerals. Fractional crystallization of ascending hydrothermal fluid involving early precipitation of pyrite may explain vertical mineralogical and chemical zoning within the stockwork conduit and the high base and precious metal contents of Roman Ruins chimneys. A mineralized volcaniclastic unit cored deep in Hole 1189A possibly represents the lateral fringe of the conduit system. Lead isotope ratios in the sulfides differ slightly but significantly from those of fresh lavas from Pual Ridge, implying that at least some of the Pb within the Roman Ruins hydrothermal system derived from a deeper, more radiogenic source than the enclosing altered volcanic rocks.

Chemical composition of ferromanganese concretions and host bottom sediments from the Black Sea

Ferromanganese concretions spread out on the bottom of the shallow northwest part of the Black Sea are mainly represented by Fe and Mn nodules on shells and substituted worm tubes. Element composition of these formations was measured by methods of chemical, atomic absorbtion, neutron activation, and ICP-MS analyses. It was established that Fe and Mn contents and Mn/Fe ratio in the concretions varied considerably and which controlled occurrence of several associated metals and minor elements; some of them have not been studied in Black Sea concretions before.

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.

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.