Rocks of the Western Kamchatka-Koryak continental margin volcanogenic belt: age, chemical and mineral compositions

An isotope-geochemical study of Eocene-Oligocene magmatic rocks from the Western Kamchatka-Koryak volcanogenic belt revealed lateral heterogeneity of mantle magma sources in its segments: Western Kamchatka, Central Koryak, and Northern Koryak ones. In the Western Kamchatka segment magmatic melts were generated from isotopically heterogeneous (depleted and/or insignificantly enriched) mantle sources significantly contaminated by quartz-feldspathic sialic sediments; higher 87Sr/86Sr (0.70429-0.70564) and lower 143Nd/144Nd [eNd(T) = 0.06-2.9] ratios in volcanic rocks from the Central Koryak segment presumably reflect contribution of an enriched mantle source; high positive eNd(T) and low 87Sr/86Sr ratios in magmatic rocks from the Northern Koryak segment area indicate their derivation from an isotopically depleted mantle source without significant contamination by sialic or mantle material enriched in radiogenic Sr and Nd. Significantly different contamination histories of Eocene-Oligocene mantle magmas in Kamchatka and Koryakia are related to their different thermal regimes: higher heat flow beneath Kamchatka led to crustal melting and contamination of mantle suprasubduction magmas by crustal melts. Cessation of suprasubduction volcanism in the Western Kamchatka segment of the continental margin belt was possibly related to accretion of the Achaivayam-Valagin terrane 40 Ma ago, whereas suprasubduction activity in the Koryak segment stopped due to closure of the Ukelayat basin in Oligocene.

Osmium isotope composition of ODP Hole 121-758A

This study presents osmium (Os) isotope and elemental data for cleaned planktic foraminifera, authigenic Fe-Mn oxyhydroxides and pelagic carbonate host sediments from ODP site 758 in the southernmost reaches of the Bay of Bengal. The Os in the bulk sediments appears to be dominantly hydrogeneous (sourced by carbonate and Fe-Mn oxyhydroxide), but variations in this particular core are controlled by the presence of volcanic ash. Fe-Mn oxyhydroxide leachates (of the bulk sediments) from Holocene samples also yield an Os isotope composition close to that of seawater, but the record diverges from that of foraminifera at a depth corresponding to the oxic/post-oxic boundary, suggesting diagenetic mobilization of Os at depths below this. Holocene planktic foraminifera, cleaned using oxidative-reductive techniques, also give Os isotope compositions indistinguishable from modern seawater, but the record obtained for the past 150 kyr shows strong covaraitions of 187Os/188Os with both the local and global oxygen isotope record, with less radiogenic Os isotope compositions during glacial intervals. These results indicate that foraminifera provide a robust record of seawater Os isotope compositions, and comparison of the data obtained here with records from the other major oceans demonstrate global changes in 187Os/188Os over this time interval, while the covariation with oxygen isotopes suggest a process controlling the Os isotope composition that is in phase with global climate cycles. Global excursions to relatively unradiogenic 187Os/188Os during glacial intervals are consistent with decreased input of radiogenic continental material, reflecting cooler temperatures and reduced continental runoff. Modelling indicates that the shift to unradiogenic values during glacial intervals could be caused by an ~30% decrease in the global river flux, with an ~5% change in river composition. If the residence time of Os in the oceans is ~5 ka then the post-glacial recovery to present-day seawater values is consistent with a corresponding increase in the river flux of around 30%. However, if the residence time of Os is closer to 40 ka, as is suggested by the global river flux, then this demands either significant changes in both the riverine Os flux and composition of around 40% and 30%, respectively, that closely follow the oxygen isotope record, or else a short-lived post-glacial pulse of weathering some 75% greater than the steady-state flux. In either case, these results clearly indicate that climatic changes affect both the flux and composition of weathered material delivered to the oceans on glacial-interglacial timescales.

Neodymium isotopic data, radiocarbon dates and calibrated ages of sediment samples off the La Plata Estuary, southeastern South America

The continental margin off the La Plata Estuary (SE South America) is characterized by high fluvial sediment supply and strong ocean currents. High-resolution sediment-acoustic data combined with sedimentary facies analysis, AMS-14C ages, and neodymium isotopic data allowed us to reconstruct late Quaternary sedimentary dynamics in relation to the two major sediment sources, the La Plata Estuary and the Argentine margin. Sediments from these two provinces show completely different dispersal patterns. We show that the northward-trending La Plata paleo-valley was the sole transit path for the huge volumes of fluvial material during lower sea levels. In contrast, material from the Argentine margin sector was transported northwards by the strong current system. Despite the large sediment volumes supplied by both sources, wide parts of the shelf were characterized by either persistent non-deposition or local short-term depocenter formation. The location and formation history of these depocenters were primarily controlled by the interplay of sea level with current strength and local morphology. The high sediment supply was of secondary importance to the stratigraphic construction, though locally resulting in high sedimentation rates. Thus, the shelf system off the La Plata Estuary can be considered as a hydrodynamic-controlled end-member.

Geochemistry and provenance of basaltic clasts within volcaniclastic debris flows at DSDP Site 89-585

Pebble-sized basaltic and glassy clasts were extracted from seamount-derived volcaniclastic debris flows and analyzed for various trace elements, including the rare earths, to determine their genetic relationships and provenance. All the clasts were originally derived from relatively shallow submarine lava flows prior to sedimentary reworking, and have undergone minor low-grade alteration. They are classified into three petrographic groups (A, B, and C) characterized by different phenocryst assemblages and variable abundances and ratios of incompatible elements. Group A (clast from Hole 585) is a hyaloclastite fragment which is olivine-normative and distinct from the other clasts, with incompatibleelement ratios characteristic of transitional or alkali basalts. Groups B and C (clasts from Hole 585A) are quartz-normative, variably plagioclase-clinopyroxene-olivine phyric tholeiites, all with essentially similar ratios of highly incompatible elements and patterns of enrichment in light rare earth elements (chrondrite-normalized). Variation within Groups B and C was governed by low-pressure fractionation of the observed phenocryst phases, whereas the most primitive compositions of each group may be related by variable partial melting of a common source. The clasts have intraplate chemical characteristics, although relative to oceanic hot-spot-related volcanics (e.g., Hawaiian tholeiites) they are marginally depleted in most incompatible elements. The source region was enriched in all incompatible elements, compared with a depleted mid-ocean-ridge basalt source.

Geochemistry of North Atlantic Igneous Province basalts

New Sr-Nd-Pb-Hf data require the existence of at least four mantle components in the genesis of basalts from the the North Atlantic Igneous Province (NAIP): (1) one (or more likely a small range of) enriched component(s) within the Iceland plume, (2) a depleted component within the Iceland plume (distinct from the shallow N-MORB source), (3) a depleted sheath surrounding the plume and (4) shallow N-MORB source mantle. These components have been available since the major phase of igneous activity associated with plume head impact during Paleogene times. In Hf-Nd isotope space, samples from Iceland, DSDP Leg 49 (Sites 407, 408 and 409), ODP Legs 152 and 163 (southeast Greenland margin), the Reykjanes Ridge, Kolbeinsey Ridge and DSDP Leg 38 (Site 348) define fields that are oblique to the main ocean island basalt array and extend toward a component with higher 176Hf/177Hf than the N-MORB source available prior to arrival of the plume, as indicated by the compositions of Cretaceous basalts from Goban Spur (~95 Ma). Aside from Goban Spur, only basalts from Hatton Bank on the oceanward side of the Rockall Plateau (DSDP Leg 81) lie consistently within the field of N-MORB, which indicates that the compositional influence of the plume did not reach this far south and east ~55 Ma ago. Thus, Hf-Nd isotope systematics are consistent with previous studies which indicate that shallow MORB-source mantle does not represent the depleted component within the Iceland plume (Thirlwall, J. Geol. Soc. London 152 (1995) 991-996; Hards et al., J. Geol. Soc. London 152 (1995) 1003-1009; Fitton et al., 1997 doi:10.1016/S0012-821X(97)00170-2). They also indicate that the depleted component is a long-lived and intrinsic feature of the Iceland plume, generated during an ancient melting event in which a mineral (such as garnet) with a high Lu/Hf was a residual phase. Collectively, these data suggest a model for the Iceland plume in which a heterogeneous core, derived from the lower mantle, consists of 'enriched' streaks or blobs dispersed in a more depleted matrix. A distinguishing feature of both the enriched and depleted components is high Nb/Y for a given Zr/Y (i.e. positive DeltaNb), but the enriched component has higher Sr and Pb isotope ratios, combined with lower epsilon-Nd and epsilon-Hf. This heterogeneous core is surrounded by a sheath of depleted material, similar to the depleted component of the Iceland plume in its epsilon-Nd and epsilon-Hf, but with lower 87Sr/86Sr, 208Pb/204Pb and negative DeltaNb; this material was probably entrained from near the 670 km discontinuity when the plume stalled at the boundary between the upper and lower mantle. The plume sheath displaced more normal MORB asthenosphere (distinguished by its lower epsilon-Hf for a given epsilon-Nd or Zr/Nb ratio), which existed in the North Atlantic prior to plume impact. Preliminary data on MORBs from near the Azores plume suggest that much of the North Atlantic may be 'polluted' not only by enriched plume material but also by depleted material similar to the Iceland plume sheath. If this hypothesis is correct, it may provide a general explanation for some of the compositional diversity and variations in inferred depth of melting (Klein and Langmuir, 1987 doi:10.1029/JB092iB08p08089) along the MAR in the North Atlantic.

Geochemistry of ODP Site 131-808 muds and mudstones

The principal aims of undertaking a shore-based bulk inorganic geochemical analysis of muds and mudstones from Site 808 were as follows: 1. Characterize the geochemical signature of the muds and mudstones at regular intervals downhole to sample and identify any changes in sediment type and provenance. 2. Integrate the inorganic geochemistry with the shipboard and more detailed land-based laboratory studies of the clay minerals. 3. Investigate any possible inorganic geochemical anomalies associated with the décollement.

Contents and accumulation rates of the rare earth and some other elements in metalliferous sediments from DSDP Hole 92-598

The sediments recovered during DSDP Leg 92 (Site 598) include a complete 16 m.y. record of hydrothermal sedimentation along the western flank of the East Pacific Rise at 19°S. Fifty samples from this sediment column were analyzed to test the hypothesis that the REE composition of the hydrothermal component is primarily acquired via scavenging from seawater. Site 598 provides an ideal sample suite for this purpose: the sediments are lithologically "simple," primarily consisting of a mixture of hydrothermal materials and biogenous carbonates; the composition of the hydrothermal component is essentially constant through space and time; and the sediments have undergone minimal diagenetic alteration. The following observations suggest the above-stated hypothesis is true. The Ce anomaly as well as key indices of light and heavy REE behavior all show that the REE pattern of hydrothermal sediments approaches that of seawater with increasing paleodistance from the rise crest. Moreover, shale-normalized REE patterns are similar to that of seawater, varying only in absolute REE content: the REE content increases with distance from the paleo-rise crest and exhibits a pronounced increase in sediments deposited below the paleolysocline. Based on significant correlative relationships between paleodistance from the rise crest and both the concentration and mass accumulation rates (MARs) of REEs and Fe, we conclude the REEs in the hydrothermal component are derived from the interaction of seawater and Fe in the hydrothermal plume.

Major and trace element composition of melts from the Gorely volcanic center, southern Kamchatka

Melt inclusions in olivine and plagioclase phenocrysts from rocks (magnesian basalt, basaltic andesite, andesite, ignimbrite, and dacite) of various age from the Gorely volcanic center, southern Kamchatka, were studied by means of their homogenization and by analyzing the glasses in 100 melt inclusions on an electron microprobe and 24 inclusions on an ion probe. The SiO2 concentrations of the melts vary within a broad range of 45-74 wt%, as also are the concentrations of other major components. According to their SiO2, Na2O, K2O, TiO2, and P2O5 concentrations, the melts are classified into seven groups. The mafic melts (45-53 wt% SiO2) comprise the following varieties: potassic (on average 4.2 wt% K2O, 1.7 wt% Na2O, 1.0 wt% TiO2, and 0.20 wt% P2O5), sodic (3.2% Na2O, 1.1% K2O, 1.1% TiO2, and 0.40% P2O5), and titaniferous with high P2O5 concentrations (2.2% TiO2, 1.1% P2O5, 3.8% Na2O, and 3.0% K2O). The melts of intermediate composition (53-64% SiO2) also include potassic (5.6% K2O, 3.4% Na2O, 1.0% TiO2, and 0.4% P2O5) and sodic (4.3% Na2O, 2.8% K2O, 1.3% TiO2, and 0.4% P2O5) varieties. The acid melts (64-74% SiO2) are either potassic (4.5% K2O, 3.6% Na2O, 0.7% TiO2, and 0.15% P2O5) or sodic (4.5% Na2O, 3.1% K2O, 0.7% TiO2, and 0.13% P2O5). A distinctive feature of the Gorely volcanic center is the pervasive occurrence of K-rich compositions throughout the whole compositional range (silicity) of the melts. Melt inclusions of various types were sometimes found not only in a single sample but also in the same phenocrysts. The sodic and potassic types of the melts contain different Cl and F concentrations: the sodic melts are richer in Cl, whereas the potassic melts are enriched in F. We are the first to discover potassic melts with very high F concentrations (up to 2.7 wt%, 1.19 wt% on average, 17 analyses) in the Kuriles and Kamchatka. The average F concentration in the sodic melts is 0.16 wt% (37 analyses). The melts are distinguished for their richness in various groups of trace elements: LILE, REE (particularly HREE), and HFSE (except Nb). All of the melts share certain geochemical features. The concentrations of elements systematically increase from the mafic to acid melts (except only for the Sr and Eu concentrations, because of active plagioclase fractionation, and Ti, an element contained in ore minerals). The paper presents a review of literature data on volcanic rocks in the Kurile-Kamchatka area in which melt inclusions with high K2O concentrations (K2O/Na2O > 1) were found. K-rich melts are proved to be extremely widespread in the area and were found on such volcanoes as Avachinskii, Bezymyannyi, Bol'shoi Semyachek, Dikii Greben', Karymskii, Kekuknaiskii, Kudryavyi, and Shiveluch and in the Valaginskii and Tumrok Ranges.

Geochemistry and mineralogy of ODP Leg 107 basalts

During ODP Leg 107, the basement of the Tyrrhenian Sea was drilled at Site 650, located in the Marsili basin, and at Sites 651 and 655, both located in the Vavilov basin. In addition, a lava flow was drilled at Site 654 on the Sardinia rifted margin. Mineral and whole rock major and trace element chemistry, including rare earth element (REE) and Sr and Nd isotopic ratios, were determined in samples of these rocks. Site 654 lava was sampled within uppermost Pliocene postrift sediments. This lava is a basaltic andesite of intraplate affinity, and is analogous to some Plio-Pleistocene tholeiitic lavas from Sardinia. Site 650 basalts, drilled beneath 1.7-1.9-Ma-old basal sediment, are strongly altered and vesicular suggesting a rapid subsidence of the Marsili basin. Based on incompatible trace elements, these basalts show calc-alkaline affinity like some products of the Marsili Seamount and the Eolian arc. The basement of the two sites drilled within Vavilov basin shows contrasting petrologies. Site 655, located along the Gortani ridge in the western part of the basin, drilled a 116-m-thick sequence of basalt flows beneath 3.4-3.6-Ma-old basal sediments. These basalts are chemically relatively homogeneous and show affinity to transitional MORB. Four units consisting of slightly differentiated basaltic lavas, have been identified. Site 655 basalts are geochemically similar to the high Ti lavas from DSDP Leg 42, Site 373 (Vavilov Basin). The basement at Site 651, overlain by 40 m of metalliferous dolostone covered by fossiliferous sediments with an age of 2 Ma, consists of two basalt units separated by a dolerite-albitite intrusive body; serpentinized harzburgites were drilled for 30 m at the base of the hole. The two basalt units of Site 651 are distinct petrochemically, though both show incompatible elements affinity with high-K calc-alkaline/calc-alkaline magmas from Eolian arc. The cpx chemistry and high K/Na ratio of the lower unit lavas suggest a weak alkaline tendency of potassic lineage. Leg 107 basement rock data, together with data from DSDP Site 373 and from dredged samples, indicate that the deepest basins of the central Tyrrhenian Sea are underlain by a complex back-arc basin crust produced by magmas with incompatible element affinities to transitional MORB (Site 655 and DSDP Site 373), and to calc-alkaline and high-K calc-alkaline converging plate margin basalts (Sites 650 and 651). This petrogenetic complexity is in accordance with the back-arc setting of the Vavilov and Marsili basins. Other back-arc basin basalts, particularly those from ensialic basins such as the Bransfield Strait (Antarctica), show a comparable petrogenetic complexity (cf., Sounders and Tarney, 1984).

(Table 1) Chemical composition of Middle Cenozoic magmatic rocks from the Schmidt Peninsula, North Sakhalin

Middle Cenozoic evolution of magmatism in the Schmidt Peninsula between 37 and 25 Ma began with eruptions of subalkaline and moderately alkaline andesite, latite, trachyandesite, and trachyrhyolite lavas and ended with subvolcanic intrusions of highly alkaline strongly undersaturated essexites. According to trace element data magmatism evolved from melting of a mantle source in the zone of ocean-continent plate convergence to small degree partial melting in the lithospheric mantle at the final stage. This succession is generally typical for Late Cenozoic continental-margin magmatism in the Southeast Russia. Similarity in the Middle and Late Cenozoic stages of magmatism is an evidence for their individual significance.