Element and isotope compostion of basalts and sediments from the Japan Sea

Ocean Drilling Program Legs 127 and 128 in the Yamato Basin of the Japan Sea, a Miocene-age back-arc basin in the western Pacific Ocean, recovered incompatible-element-depleted and enriched tholeiitic dolerites and basalts from the basin floor, which provide evidence of a significant sedimentary component in their mantle source. Isotopically, the volcanic rocks cover a wide range of compositions (e.g., 87Sr/86Sr = 0.70369 - 0.70503, 206Pb/204Pb = 17.65 - 18.36) and define a mixing trend between a depleted mantle (DM) component and an enriched component with the composition of EM II. At Site 797, the combined isotope and trace element systematics support a model of two component mixing between depleted, MORB-like mantle and Pacific pelagic sediments. A best estimate of the composition of the sedimentary component has been determined by analyzing samples of differing lithology from DSDP Sites 579 and 581 in the western Pacific, east of the Japan arc. The sediments have large depletions in the high field strength elements and are relatively enriched in the large-ion-lithophile elements, including Pb. These characteristics are mirrored, with reduced amplitudes, in Japan Sea enriched tholeiites and northeast Japan arc lavas, which strengthens the link between source enrichment and subducted sediments. However, Site 579/581 sediments have higher LILE/REE and lower HFSE/REE than the enriched component inferred fiom mixing trends at Site 797. Sub-arc devolatilization of the sediments is a process that will lower LILE/REE and raise HFSE/REE in the residual sediment, and thus this residual sediment may serve as the enriched component in the back-arc basalt source. Samples from other potential sources of an enriched, EM II-like component beneath Japan, such as the subcontinental lithosphere or crust, have isotopic compositions which overlap those of the Japan Sea tholeiites and are not "enriched" enough to be the EM II end-member.

Petrology of mafic and ultramafic intrusions from the Portneuf-Mauricie Domain, Grenville Province, Canada

The Portneuf-Mauricie Domain (PMD), located in the south-central part of the Grenville Province, comprises several mafic and ultramafic intrusions hosting Ni-Cu ± platinum-group element (PGE) prospects and a former small mining operation (Lac Édouard mine). These meter- to kilometer-scale, sulfide-bearing intrusions display diverse forms, such as layered and tabular bodies with no particular internal structure, and zoned plutons. They were injected ~ 1.40 Ga into a mature oceanic arc, before and during accretion of the arc to the Laurentian margin. The pressure-temperature conditions of the magmas at the beginning of their emplacement were 3 kbar and 1319-1200 °C (according to the petrologic modeling results from this study). The PMD mineralized intrusions are interpreted to represent former magma chambers or magma conduits in the roots of the oceanic arc. The parent magmas of the mineralized intrusions resulted mainly from the partial melting of a mantle source composed of spinel-bearing lherzolite. Petrologic modeling and the occurrence of primary amphibole in the plutonic rocks indicate that these parent melts were basaltic and hydrous. In addition, fractional crystallization modeling and Mg/Fe ratios suggest that most of the intrusions may have formed from evolved magmas, with Mg# = 60, resulting from the fractionation of more primitive magmas (primary magmas, with Mg# = 68). Petrologic modeling demonstrates that 30% fractional crystallization resulted in the primitive to evolved characteristics of the studied intrusive rocks (as indicated by the crystallization sequences and mineral chemistry). Exceptions are the Réservoir Blanc, Boivin, and Rochette West parent magmas, which may have undergone more extensive fractional crystallization, since these intrusions contain pyroxenes that are more iron rich and have lower Mg numbers than pyroxenes in the other PMD intrusions. The PMD mafic and ultramafic intrusions were intruded into an island arc located offshore from the Laurentian continent. Thus, their presence confirms the existence of a well-developed magmatic network (responsible of the fractionation processes) beneath the Proterozoic arc, which resulted in the wide range of compositions observed in the various plutons.

Lavas from the Elbrus volcano, Greater Caucasus

Comprehensive geochronological and isotope-geochemical studies showed that the Late Quaternary Elbrus Volcano (Greater Caucasus) experienced long (approximately 200 ka) discrete evolution with protracted periods of igneous quiescence (approximately 50 ka) between large-scale eruptions. Volcanic activity of Elbrus is subdivided into three phases: Middle Neopleistocene (225-170 ka), Late Neopleistocene (110-70 ka), and Late Neopleistocene - Holocene (earlier than 35 ka). Petrogeochemical and isotope (Sr-Nd-Pb) signatures of Elbrus lavas point to their mantle-crustal origin. It was shown that hybrid parental magmas of the volcano formed due to mixing and/or contamination of deep-seated mantle melts by Paleozoic upper crustal material of the Greater Caucasus. Mantle reservoir that participated in genesis of Elbrus lavas as well as most other Neogene-Quaternary magmatic rocks of Caucasus was represented by the lower mantle "Caucasus" source. Primary melts generated by this source in composition corresponded to K-Na subalkali basalts with the following isotopic characteristics: 87Sr/86Sr = 0.7041+/-0.0001, e-Nd = +4.1+/-0.2, 147Sm/144Nd = 0.105-0.114, 206Pb/204Pb = 18.72, 207Pb/204Pb = 15.62, and 208Pb/204Pb = 38.78. Temporal evolution of isotope characteristics for lavas of the Elbrus Volcano is well described by a Sr-Nd mixing hyperbole between "Caucasus" source and estimated average composition of the Paleozoic upper crust of the Greater Caucasus. It was shown that, with time, proportions of mantle material in parental magmas of Elbrus gently increased: from ~60% at the Middle-Neopleistocene phase of activity to ~80% at the Late Neopleistocene - Holocene phase, which indicates an increase of activity of a deep-seated source at decreasing input of crustal melts or contamination with time. Unraveled evolution of the volcano with discrete eruption events, lacking signs of cessation of the Late Neopleistocene - Holocene phase, increasing contribution of the deep-seated mantle source in genesis of Elbrus lavas with time as deduced from isotope-geochemical data, as well as numerous geophysical and geological evidence indicate that Elbrus is a potentially active volcano and its eruptions may be resumed. Possible scenarios were proposed for evolution of the volcano, if its eruptive activity continued.

Geochemistry of sands and sandstones in ODP Leg 126 sites

During Ocean Drilling Program Leg 126, six sites were cored in a young backarc rift basin and its flanks (rift onset 1.1-3.56 Ma) and in the forearc basin of the Izu-Bonin Arc. In the backarc area, strata are younger than about 4.5 Ma, whereas in the forearc, ages are about 0-31 Ma in sections punctuated by important Miocene unconformities. Bulk chemical analyses of volcaniclastic turbidite sands and sandstones, derived directly from the arc, were obtained from 271 atomic absorption analyses (major elements), 253 XRF analyses (trace elements) and 16 ICP-MS analyses (trace and rare-earth elements). Of the 271 samples, 78 come from the backarc area and the remainder from the forearc. The sands and sandstones reflect the igneous compositions of their sources. Most are formed of materials derived from subalkaline, low-K andesites, and dacites, although compositions range from basalt to rhyolite. Basic and acid andesites are predominant in Oligocene rocks; in contrast, Pliocene-Pleistocene sediments were derived from acid andesitic to rhyolitic sources. The oldest sandstones, estimated to have an age of about 31 Ma, were derived from an arc tholeiitic, not boninitic, source. The 26-31 Ma sandstones furthest to the north, at Sites 787 and 792, have higher relative concentrations of Ti, Zr, and Y than do those at southern Site 793. Data from younger samples indicate that, for more than 30 m.y., the average composition of volcaniclastic sediments and volcanism near Aoga Shima was more basic than to the south, near Sumisu Jima. Using the sandstones as igneous proxies, we conclude that magmas erupted along the arc have become more depleted in light-rare-earth elements (LREE) with time. There was a major change in rare-earth-element (REE) concentrations in the late Oligocene, from essentially flat patterns (normalized La/Yb about 1-1.5) to LREE-depleted patterns (normalized La/Yb about 0.5). At the same time, Zr/Y ratios decreased from about 2-4 to about 1.5-2.5. These changes may reflect a shift in provenance, or changes in the composition of the mantle wedge beneath the arc. In the backarc area, lithic clasts and glass shards of rift-facies basalt are present in sediments as old as 2.35-3.15 Ma. Two samples of mafic sand from the backarc basin have flat REE patterns (normalized La/Yb about 1.0), like some of the <1-Ma rift lavas and unlike pre-rift sand and sandstone samples. These possibly represent the local effects of sedimentary mixing of detritus from arc and backarc eruptions because no evidence from the arc itself exists to suggest a recent change in the REE content of magmas.