J. Blusztajn

Development of the Indus Fan and its significance for the erosional history of the Western Himalaya and Karakoram

Correlation of new multichannel seismic profiles across the upper Indus Fan and Murray Ridge with a dated industrial well on the Pakistan shelf demonstrates that ;40% of the Indus Fan predates the middle Miocene, and ;35% predates uplift of the Murray Ridge (early Miocene, ;22 Ma). The Arabian Sea, in addition to the Makran accretionary complex, was therefore an important repository of sediment from the Indus River system during the Paleogene. Channel and levee complexes are most pronounced after the early Miocene, coincident with an increase in sedimentation rates. Middle Eocene sandstones from Deep Sea Drilling Project Site 224 on the Owen Ridge yield K-feldspars whose Pb isotopic composition, measured by in situ ion microprobe methods, indicates an origin in, or north of, the Indus suture zone. This observation requires that India-Asia collision had occurred by this time and that an Indus River system, feeding material from the suture zone into the basin, was active soon after collision. Pleistocene provenance was similar to that during the Eocene, albeit with greater contribution from the Karakoram. A mass balance of the erosional record on land with deposition in the fan and associated basins suggests that only ;40% of the Neogene sediment in the fan is derived from the Indian plate.

Magmatic development of an intra-oceanic arc: High-precision U-Pb zircon and whole-rock isotopic analyses from the accreted Talkeetna arc, south-central Alaska

The accreted Talkeetna arc, south-central Alaska, is an archetypal example of an intra-oceanic arc crustal section. Arc-related units include all levels of a lithospheric column, from residual mantle harzburgites to sub-aerial volcanic rocks, and provide a rare opportunity to study intrusive arc processes directly. We present the first high-precision U-Pb zircon ages and an extensive new data set of 143Nd/144Nd and 87Sr/86Sr isotopic analyses from Talkeetna arc plutonic rocks. These data provide new insight into the timing and extent of Talkeetna arc magmatism, the tectonic development of the arc, and the role of preexisting crustal material in the generation of arc magmas. New analyses from the exposed arc crustal section in the Chugach Mountains indicate that the Talkeetna arc began to develop as a juvenile [ϵNd(t) = 6.0–7.8 and 87Sr/86Srint = 0.703379–0.703951] intra-oceanic arc between 202.1 and 181.4 Ma. This initial arc plutonism was followed ca. 180 Ma by a northward shift in the arc magmatic axis and generation of a large plutonic suite in the Talkeetna Mountains. Plutons from the eastern Talkeetna Mountains yield U-Pb zircon ages of 177.5–168.9 Ma and are isotopically similar to the Chugach Mountains intrusions [ϵNd(t) = 5.6–7.2 and 87Sr/86Srint = 0.703383–0.703624]. However, plutons from the western Talkeetna Mountains batholith have more evolved initial isotopic ratios [ϵNd(t) = 4.0–5.5 and 87Sr/86Srint = 0.703656–0.706252] and contain inherited xenocrystic Carboniferous–Triassic zircons. These data are interpreted to represent assimilation of adjacent Wrangellia crust into arc magmas and require amalgamation of the Talkeetna arc with the Wrangellia terrane by ca. 153 Ma. As a whole, the combined U-Pb zircon and isotopic data from the Chugach and Talkeetna Mountains indicate that the main volume of Talkeetna arc magmas formed with little or no involvement of preexisting crustal material. These observations justify the use of the Talkeetna arc as a type section for intrusive intra-oceanic arc crust.

Helium and lead isotopes reveal the geochemical geometry of the Samoan plume.

Hotspot lavas erupted at ocean islands exhibit tremendous isotopic variability, indicating that there are numerous mantle components hosted in upwelling mantle plumes that generate volcanism at hotspots like Hawaii and Samoa. However, it is not known how the surface expression of the various geochemical components observed in hotspot volcanoes relates to their spatial distribution within the plume. Here we present a relationship between He and Pb isotopes in Samoan lavas that places severe constraints on the distribution of geochemical species within the plume. The Pb-isotopic compositions of the Samoan lavas reveal several distinct geochemical groups, each corresponding to a different geographic lineament of volcanoes. Each group has a signature associated with one of four mantle endmembers with low 3He/4He: EMII (enriched mantle 2), EMI (enriched mantle 1), HIMU (high µ = 238U/204Pb) and DM (depleted mantle). Critically, these four geochemical groups trend towards a common region of Pb-isotopic space with high 3He/4He. This observation is consistent with several low-3He/4He components in the plume mixing with a common high-3He/4He component, but not mixing much with each other. The mixing relationships inferred from the new He and Pb isotopic data provide the clearest picture yet of the geochemical geometry of a mantle plume, and are best explained by a high-3He/4He plume matrix that hosts, and mixes with, several distinct low-3He/4He components.

Re-Os systematics and platinum-group element distribution in metalliferous sediments from the Troodos ophiolite

Umbers from 8 of 11 localities studied yield 187Os/188Os ratios that range from 0.51 to 0.57. We interpret this range as reflecting the Os isotopic composition of mid-Cretaceous seawater. 187Os/188Os measured in samples from the remaining three localities fall above and below this range and are believed to result from post-depositional redistribution of Os. Extremely large 187Os/188Os ratios occur in hydrothermal sulfides (>200) and in their oxidation products, ochres (3.6–13.2). Measured Re/Os ratios in these samples require open system behavior. Model age calculations indicate that ochre formation occurred over a 1–5 million year interval after sulfide precipitation. In most samples >99% Re loss occurred during sulfide oxidation. Comparison of element ratios in Cyprus umbers to those of modern seawater suggest the following sequence of scavenging efficiency by hydrothermal Fe-oxides: Ir>Pt>Os>Re. Cyprus sulfides exhibit the opposite pattern of element enrichment. These patterns of aqueous platinum-group element (PGE) fractionation are potentially relevant to study of recycled Os in mantle-derived rocks because they give rise to the following phenomena: (1) ephemeral Re enrichment within the basal sediments of the oceanic crust, (2) PGE concentrations in metalliferous sediments (Os up to 1 ppb and Pt up to 22 ppb) that are comparable to those in the mantle, and (3) Pt/Re ratios in Cyprus umbers that exceed those required to explain the coupled 186Os–187Os variations in Hawaiian basalts.

Sedimentary and geochemical evolution of the Dras forearc basin, Indus suture, Ladakh Himalaya, India

The Nindam Formation of the Indus suture zone represents the forearc basin to the Dras-Kohistan volcanic arc, which was generated in an intraoceanic setting during mid-Cretaceous time, and accreted to the margin of Eurasia during Late Cretaceous time. The Nindam Formation comprises volcaniclastic sediment and pelagic carbonate deposited in cyclic influxes of high-density, sheet-like turbidites, with locally well-defined channel morphologies. Debris-flow deposits as much as 5 m thick that rework older material are volumetrically <5% of the sequence. A fine-grained basal section is succeeded by several hundred meters of cyclic sandy turbidites, and finally a more shaley top. This large-scale variation may be linked to tectonic controls on arc activity, while individual sandy packages 50–100 m thick are inferred to have a glacioeustatic origin. Sedimentation mirrors similar patterns observed in the modern Mariana and Tonga forearcs. The lack of major unconformities and large-scale reworking suggests that the volcaniclastic sedimentary rocks should form a relatively complete record of geochemical change in the arc source. The sediment trace- and rare earth element (REE) signature is similar to that of the Dras and Chalt volcanic rocks erupted at the volcanic front during the intraoceanic phase of activity and is distinct from the Kardung Volcanic Group erupted during the continental arc phase, following collision with the Eurasian margin. Nd isotopes also support an intraoceanic origin. The sedimentary rocks show either no or slight light REE enrichment, similar to modern sediments from the Mariana arc, as well as the volcanic and intrusive core of the Dras-Kohistan Arc. The overall chemistry of the Dras-Kohistan Arc is thus more depleted in light REE than continental crust. Our data do not support models that propose continental crust being formed along intraoceanic active margins.

Post-breakup basaltic magmatism along the East Greenland Tertiary rifted margin

Mafic and ultramafic intrusions in East Greenland adjacent to the offshore Greenland‐Iceland ridge were emplaced 5‐9 My after continental breakup at 55 Ma [1]. Rare earth element (REE) concentrations determined by secondary ion mass spectrometry are reported for cumulus clinopyroxene from these intrusions, and the data are used to estimate REE abundance in equilibrium melts using available partitioning data. Estimated equilibrium melts from intrusions have strongly fractionated REE patterns with Nd=Dy(N) in the range 2 to 5.6 and Yb=Dy(N) 0.55 to 0.92, similar to values for coeval basalts. These melts have markedly higher Nd=Dy(N) than earlier breakup related flood basalts. The moderately low Yb=Dy(N) for the post-breakup volcanism is indicative of residual garnet in the source, while their high Nd=Dy(N) ratios can best be explained by aggregating low degree melts from a light-REE-enriched garnet- and spinel-bearing mantle source. We also report He, Sr, and Nd isotopic data for the intrusions. The highest 3 He= 4 He ratios (>10 R=Ra) are found in the samples whose REE data reflect the largest proportion of melts from a garnet-bearing source, and having Sr and Nd isotopic compositions identical with the radiogenic Sr and unradiogenic Nd isotope end of the Iceland compositional field. There is no indication of a MORB-type mantle in the source of the intrusions. We postulate that post-breakup volcanism along the East Greenland coastline reflects the increasing proximity of the mantle plume to the East Greenland continental margin. The low degree of melting at high mean pressure inferred for the parental melts for the intrusions may reflect re-thickening of the lithosphere, which in turn was caused by the vigorous volcanism during breakup, with accompanying depletion of upper mantle and underplating of the crust at the continental margin.

The trace-element variations in clinopyroxenes from spinel peridotite xenoliths from southwest Poland

Abstract Major- and trace-element content, and Sr- and Nd-isotopic composition are reported for clinopyroxenes from anhydrous spinel peridotites (SW Poland). LREE enrichment which occurs in all samples is decoupled from relatively high 143 Nd 144 Nd ratios (0.51286–0.51291), indicating that mantle metasomatism took place relatively recently. REE and HFSE (high-field strength element) abundances show marked variation but, interestingly, correlate with each other: samples with U-shaped REE patterns have Ti Zr > 700 ; samples with a REE concave-downward pattern have Ti/Zr of 20–200 and samples with a REE concave-upward pattern are characterized by Ti/Zr of 2–40. We suggest that trace-element enrichment is the result of interaction between basaltic melt migrating through the veins and contiguous wall-rock peridotites. Modal composition and trace-element characteristics of samples with U-shaped REE patterns probably reflect interaction of depleted peridotite residue with carbonatite melt.

Platinum-group elements and Os isotopic characteristics of the lower oceanic crust

Abstract We present osmium isotopic compositions and Os, Re and platinum-group element (PGE) contents in the lower oceanic crust gabbros from ODP Site 735B on the Southwest Indian Ridge. The average gabbros are very depleted in PGEs, e.g. the Os concentrations are very low and range from 0.5 to 38 ppt. One troctolite sample is an exception, containing, e.g. 376 ppt Os, 396 ppt Ir and 8 ppb Pd. Re contents are extremely heterogeneous, ranging from 128 ppt to 28 ppb. The present day 187 Os/ 188 Os varies from 0.140 to 0.467, and correlates with Rb/Cs, suggesting that seawater alteration affected the Os isotopic system. The average Os content in the lower oceanic crust is about 9 ppt while median value for Re is about 487 ppt. Such very high Re/Os is relatively similar to the upper MORB-type oceanic crust, indicating that recycled ocean crust basalts and gabbros will rapidly develop very radiogenic Os isotopic signatures.

Osmium isotopic systematics of melilitites from the Tertiary Central European Volcanic Province in SW Germany

Abstract Combined Os–Pb–Sr–Nd isotopic compositions of melilitite samples from SW Germany provide new constraints on the origin of melts derived by low degrees of mantle melting in the course of Tertiary–Quaternary volcanism in Central Europe. The first Os isotopic compositions reported here show an unusually wide range in initial 187Os/188Os isotopic ratios of 0.119 to 0.225. The low ratios overlap those in the lithospheric mantle and the highly radiogenic ratios exceed those in ocean island basalts. Pb and Sr isotopic compositions show small variations (206Pb/204Pb=19.19–19.57; 87Sr/86Sr=0.7034–0.7039) whereas Nd isotopes (eNd 3.4–4.2) indicate melting of overall uniform sources. A positive correlation between initial 187Os/188Os and 207Pb/204Pb isotopic ratios is interpreted as a mixing trend between melts with low and lithospheric mantle-like 187Os/188Os isotopic ratios and an old crustal component. Thus, we interpret the low 187Os/188Os isotopic ratios in some of the samples as primary mantle source-derived signatures and propose that the melilitites originated from the lithospheric mantle. Independent evidence for an lithospheric mantle source is provided by earlier reported trace element data indicating melting of metasomatized sources with residual K-bearing phases whose stability is confined to the lithospheric mantle rather than thermal plume. A contribution from deeper mantle sources is not required to explain the data; however, the lack of Os data from other more typical basalts of the Central European Volcanic Province (CEVP) precludes an evaluation of the extent of plume involvement. Remarkable characteristics of the melilitites are their high Re/Os and U/Pb ratios. It follows that overprinting of the lithospheric mantle by melts derived by low degrees of melting of metasomatized sources will eventually produce domains with evolved Os and Pb isotopic compositions as required as source material for basalts of high 238U/204Pb (HIMU) affinity.

Sr isotope variations in vent fluids from 9°46′-9°54′N East Pacific Rise: evidence of a non-zero-Mg fluid component

Abstract Sr isotopic variations in vent fluids from six high-temperature hydrothermal vents from between 9°46′N and 9°54′N on the East Pacific Rise are reported. The Mg/Sr-87Sr/86Sr systematics of one vent, Biovent, require the presence of a fluid component that has experienced significant Mg depletion (≈50%), but has undergone very little Sr exchange with local basalts. This fluid component accounts for 3% to 10% of the Biovent samples by volume. Similarities between the composition of the inferred fluid component and that of bore-hole fluids sampled from Hole 504B suggest that this fluid component is modified seawater, partially reacted with basalt at temperatures less than 150°C, and subsequently entrained by high-temperature vent fluids. Previously reported vent fluid data from 12°50′N East Pacific Rise suggest that the presence of the additional lower temperature fluid component admixed in Biovent samples may not be unique among seafloor hydrothermal systems. If such a fluid component is common in seafloor hydrothermal systems it will need to be considered in estimates of axial hydrothermal fluxes. If not accounted for properly, the presence of an additional fluid component can bias calculated end-member 87Sr/86Sr ratios. After correction for the influence of the additional fluid component on the Biovent samples, the calculated end-member 87Sr/86Sr ratio of this vent is 0.7041. Other nearby vents, P vent and Bio9′, are slightly more radiogenic, 0.7042. A and L vents from the southern portion of the study area have end-member 87Sr/86Sr ratios that are less radiogenic, 0.7039 and 0.7037, respectively. The reason for the greater influence of seawater-derived Sr in the more northern vents is not known, but it may be related to entrainment of seawater in the upflow zone.