Nd and Sr Isotopic Study of Leg 127 Basalts: Implications for the Evolution of the Japan Sea Backarc Basin

The aspects of the development of isotopic methods that are pertinent to the origin of volcanic rocks are introduced. This paper deals with studies on strontium isotopes and related elements which have provide their usefulness in the field of isotope geology.In the Northeast Japan arc, a number of Quaternary volcanoes form a long, narrow belt, parallel to the Japan Trench. Decreasing 87Sr/86Sr ratios across the arc were confirmed over a wide area of Northeast Japan. In the southwest Japan, the 87Sr/86Sr ratios of Cenozoic basaltic rocks are clearly different between the San-in and the northwest Kyushu regions. The higher 87Sr/86Sr ratios of basalts from the San-in region than that of basalts from the northwest Kyushu region also reflect the different properties of the upper mantle, which means there is regional heterogeneity of Sr isotopic ratios under the southwest Japan arcs. Furthermore, the relatively high and variable 87Sr/86 ratios of volcanic rocks are partticularly concentrated in the southwestern Japan arcs which have probably more continental properties than the northeastern Japan arcs.Arc volcanism is characterized by prevalence of calc-alkaline rock suites as well as island-arc tholeiites. Various models have been proposed to interpret its genesis of the calcalkaline rocks. 87Sr/86Sr ratios vary remarkably within individual rock sultes as well as volcano and volcano. In some volcanic suites, the date points in 87Sr/86Sr ratios vs. SO2, SI and Rb/Sr plot accord with a model to produce more silisic differentiates by combined processes of crystal fractionation and wall rock assimilation in Mount St. Helens volcano. Adatara and Kirishima volcanoes in the Japanese island arcs, both the tholeiite series (pigeonitic rock series, P-series) and the calc-alkaline rock series (hypersthenic rock series, H-series) have been shown to have erupted from one volcano and difference in 87Sr/86Sr ratios shows that the H-series has lower 87Sr/86Sr rotios than the P-series, and also that each series had a separate crystal fractionation process. The results reveal that the nature of the primitive magmas of the H-series and the P-series seems to generate from individual source materials.Many Tertiary volcanic rocks from the northeastern Japan were determined for strontium isotopic ratios for the purpose of examining the genesis among the volcanic rocks. Two distinct suites of volcanic rocks occur in the northeastern Japan : the rocks older than 16 Ma of predominantly intermediate composition volcanics and the rocks younger than 16 Ma with bimodal suite of mafic and felsic rocks. Initial values of 87Sr/86Sr in the Tertiary volcanic rocks from the northeastern Japan, lie in the range from 0.7033 to 0.7068. High (87Sr/86Sr) I ratios are observed for the rocks older than 16 Ma from the Japan Sea side (H zone). It is noteworthly lower (87Sr/86Sr) I ratios in the Dewa Hill, Japan Sea coast and North Akita areas (L zone). The rocks younger than 16 Ma from the L zone can also be interpreted as having been originated as a mantle-diapir associated with the spreading of the Japan Sea basin.

The origin of basic rocks of the Korosten AMCG complex, Ukrainian shield: Implication of Nd and Sr isotope data

Isotopic variations with distance and time in the volcanic islands of the Cameroon line: evidence for a mantle plume origin

The radiogenic strontium (Sr) and neodymium (Nd) isotope compositions of the detrital fraction of surface and subsurface sediments have been determined to trace sediment provenance and contributions from Asian dust off the east coast of Luzon Islands in the western Philippine Sea. The Sr and Nd isotope compositions have been very homogenous near the east coast of the Luzon Islands during the latest Quaternary yielding relatively least radiogenic Sr (87Sr/86Sr = 0.70453 to 0.70491) and more radiogenic Nd isotope compositions (εNd(0) = +5.3 to +5.5). These isotope compositions are similar to Luzon rocks and show that these sediments were mainly derived from the Luzon Islands. In contrast, the Sr and Nd isotope compositions of sediments on the Benham Rise and in the Philippine Basin are markedly different in that they are characterized by overall more variable and more radiogenic Sr isotope compositions (87Sr/86Sr = 0.70452 to 0.70723) and less radiogenic Nd isotope compositions (εNd(0) = −5.3 to +2.4). The Sr isotope composition in the Huatung Basin is intermediate between those of the east coast of Luzon and Benham Rise, but shows the least radiogenic Nd isotope compositions. The data are consistent with a two end‐member mixing relationship between Luzon volcanic rocks and eolian dust from the Asian continent, which is characterized by highly radiogenic Sr and unradiogenic Nd isotope compositions. The results show that Asian continental dust contributes about 10–50% of the detrital fraction of the sediments on Benham Rise in the western Philippine Sea, which offers the potentials to reconstruct the climatic evolution of eastern Asia from these sediments and compare this information to the records from the central and northern Pacific.

The tectono-magmatic formation and submarine geohazards of the Japan Sea: Constraints from in-situ trace elements and Sr isotopes of plagioclase and clinopyroxene

To address the nature of mantle-derived melts and submarine geohazards of the Japan Sea, we present new in-situ compositions for clinopyroxene (trace elements) and plagioclase (trace elements and Sr isotopes) from Sites 794 and 797 tholeiitic basalts. Plagioclase and clinopyroxene from Upper Site 797 and Lower Site 794 are characterized by relatively lower incompatible elements relative to those of Lower Site 797 and Upper Site 794. Lower Site 794 bulk-rocks and minerals have slight enrichment in Sr isotope compositions but depletion in trace element ratios. The observed zoning textures together with in-situ geochemical and isotopic diversity within individual plagioclase indicate fractional crystallization and the contribution of heterogeneous melt. In-situ Sr isotopic disequilibria between plagioclase phenocrysts and groundmass likely have important ramifications for the enriched compositional heterogeneity with the significant influence of recycled subduction-related oceanic fluid/sediment beneath the Japan Sea. The initial extension of the Japan Sea was relatively slow, resulting in the slow replenishment of depleted MORB-type melts derived from the upper mantle, and the extensive mixing of the migrating depleted melt with the preformed surrounding enriched melts. It thus forms the slight depletion of the highly incompatible elements but the enriched invariability of the isotopic compositions of Lower Site 794 bulk- rocks and minerals due to the relatively long half-life of the radiogenic elements. In summary, the expansion velocity of the Japan Sea varies from slow to rapid, which leads to time-progressive variations in the mineral and bulk-rock compositions beneath the Japan Sea. The maximum seismic depth of the earthquake decreases with the increase of the expansion velocity of the Japan Sea.

Opening of the Sea of Japan back-arc basin by asthenospheric injection

Hafnium isotopic studies of the Cameroon line and new HIMU paradoxes

Detailed three-dimensional (3-D) P and S wave attenuation (Qp and Qs) models of the crust and upper mantle under the entire Northeast Japan (Tohoku) arc from the Japan Trench to the Japan Sea coast are determined, for the first time, using a large number of high-quality t* data measured precisely from P and S wave spectra of local earthquakes. The suboceanic earthquakes used in this work are relocated precisely using sP depth phases. Our results reveal a prominent landward dipping high-Q zone representing the subducting Pacific slab, a landward dipping intermediate- to high-Q zone in the mantle wedge between the Pacific coast and the volcanic front, and significant low-Q anomalies in the crust and mantle wedge between the volcanic front and the Japan Sea coast. Prominent high-Q patches surrounded by low-Q anomalies are revealed in the interplate megathrust zone under the Tohoku fore arc where the great 2011 Tohoku-oki earthquake (Mw 9.0) occurred. The high-Q patches in the megathrust zone generally exhibit large coseismic slips of megathrust earthquakes and large slip deficit on the plate interface. We think that these high-Q patches represent asperities in the megathrust zone, whereas the low-Q anomalies reflect weakly coupled areas. We also find that the hypocenters of the 2011 Tohoku-oki interplate earthquakes (Mw > 7.0) are located in areas where Qp, Qs, and Qp/Qs change abruptly. These results suggest that structural heterogeneities in the megathrust zone control the interplate seismic coupling and the nucleation of megathrust earthquakes.

Sr isotopic compositions have been determined for the Tertiary volcanic rocks from Okushiri Island, Northeast Japan Sea, together with the temporal variation of compositions of magma sources beneath Okushiri Island. The Cretaceous to Tertiary volcanics from Okushiri Island have the following SrI (initial 87Sr/86Sr) ratios; Cretaceous Monai volcanics: 0.70532 (Shuto et al., in preparation), Oligocene, Lower Matsue basalts: 0.70344-0.70346, Upper Matsue basalts: 0.70378-0.70419, Early Miocene andesites in the Aonaegawa Formation: 0.70448-0.70475, Early Miocene andesites in the Tsurikake Formation: 0.70432-0.70442, Middle Miocene dolerite: 0.70325, Pliocene volcanics: 0.70294-0.70331. Significant Sr isotopic variations recognized in the Upper Matsue basalts may be explained by assimilation of the Cretaceous granitic rocks from Okushiri Island. Thus the uncontaminated Upper Matsue basalts may yield SrI ratio around ?? 0.7037. Comparative examinations of SrI ratios reported by the previous and present workers for the volcanic rocks after Cretaceous from the back-arc side in the NE Japan arc and the Japan Sea suggest that the primary magmas for the Cretaceous and the early Miocene volcanics from Okushiri Island were generated from the sub-continental mantle enriched in Sr isotopic ratio, whereas those for the middle Miocene and Pliocene volcanics from the same island were derived from the depleted asthenosphere, probably injected into the sub-continental mantle during the opening of the Japan Sea. The data on the Matsue basalts indicate a possibility of the generation of basalt magmas from the depleted asthenosphere before the spreading of the Japan Sea.

Trace element distribution and isotopic composition of Archean Greenstones

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, 204Pb/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 from 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.

Tracking along-arc sediment inputs to the Aleutian arc using thallium isotopes

Crustal xenoliths occur in a variety of Carboniferous and Tertiary alkali basaltic rocks across Scotland and provide indications of the nature of the deep crust beneath the major lithotectonic zones from the Archean Lewisian of the far NW to the Paleozoic terranes in the SE. Sr, Nd, and Pb isotopic and representative chemical compositions are presented for 35 mafic xenoliths from the different age and tectonic zones of Scotland and for 21 felsic, gneissose xenoliths, from the Midland Valley and Southern Uplands in the south. The coexistence of mafic granulite and spinel lherzolite xenoliths at several of these localities, together with seismic velocity data, provide evidence that the mafic xenoliths, and possibly some of the felsic xenoliths, may be derived from 20–30 km depth. The absence of garnet from the mafic assemblages suggests that in most areas the crust was probably not much more than 30 km thick in the Permo‐Carboniferous. The extremely variable Al2O3 (up to 27 wt %) and MgO contents at any one location, together with positive Eu and Sr anomalies in chondrite‐ and mantle‐normalized diagrams, provide evidence that many of the mafic xenoliths represent cumulates from basaltic magmas. The Loch Roag mafic xenoliths, erupted through the depleted high‐grade metamorphic Lewisian Complex, are especially rich in K (average 1.3 % K2O) and Rb (average 54 ppm) with K/Rb extending as low as 100. The K/U ratios for the mafic xenoliths are high, ranging to over 100,000 at Loch Roag and in the SW Midland Valley. The relative concentrations of U, K, Rb, Pb, Nd, Sm, Eu and Sr in most of the mafic Xenoliths, while displaying regional characteristics, display intra‐site variations that are consistent with crystal‐liquid fractionation. There is little evidence of U or Rb depletion via fluid or melt extraction during metamorphism. The concentrations of U in the mafic xenoliths display as much intrasite uniformity as the rare earth elements and correlate with Zr concentrations. U/Zr ratios are similar to those found in ocean island basalts. The mafic xenoliths display a marked decrease in ϵNd and 206Pb/ 204Pb with increasing age of the upper crust from SE to NW, as also observed in the ∼400 Ma Caledonian granites. Sm‐Nd crustal residence ages for the Loch Roag suite are particularly variable ranging from 1.6 to 3.6 Ga. However, these isotopic compositions and model ages appear to be, in part, the product of crustal assimilation by the igneous precursors to the mafic xenoliths. Model ages, mineral ages and scattered whole rock isochrons indicate that many of the precursors to these Xenoliths were in fact formed and metamorphosed during the late Proterozoic and Paleozoic and provide evidence of widespread deep crustal magma reservoirs, underplating, crustal assimilation and arc accretion at these times. Nd and Sr isotopic data for coexisting pyroxenite xenoliths at Loch Roag indicate that they may represent contaminated ultramafic cumulates that were cogenetic with the precursors to the mafic xenoliths. The Pb isotopic compositions of the Midland Valley mafic and felsic xenoliths are similar and relatively uniform despite large variations in Nd and Sr isotopic compositions. This is explicable if the crust in this region was relatively uniform and similar in composition to the mantle and/or if crustal Pb dominated in arc environments. The average major element composition of the Scottish mafic xenolith suite is similar to that reported from other regions such as the Sierra Nevada. This is taken as evidence that the processes of crystal fractionation and formation of mafic and ultramafic cumulates during underplating are relatively consistent between different regions. Average U/Pb ratios of mafic and felsic xenoliths are effectively identical (0.068 and 0.064, respectively). The average Sm/Nd ratio (0.20) for the entire suite is lower and average K/U ratio (27,000) higher than estimates of average lower crust based on Queensland mafic xenoliths. These compositions as well as those of basement rocks, sedimentary supracrustal rocks and granites provide evidence of systematic chemical differences between the Scottish and Australian lithosphere. The compositional features of the Paleozoic Scottish crust such as low Sm/Nd and high K/U may have been sustained in part by recycling of Archean and early Proterozoic lithosphere through sedimentary processes, subduction and arc volcanism. The average Sm‐Nd crustal residence age and Pb isotopic composition of the mafic xenoliths at Loch Roag is very similar to that of the host Lewisian. Conservative mass balance models imply an average ratio of Lewisian‐derived Nd to parent magma Nd in the xenoliths of about 2. Such large degrees of contamination indicate that although underplating may be significant process in reconstituting Archean lithosphere, it does not necessarily result in substantial changes to the average age of the crust.

DUPAL anomaly in the Sea of Japan: Pb, Nd, and Sr isotopic variations at the eastern Eurasian continental margin