Yoshiyuki Tatsumi

Trace element transport during dehydration processes in the subducted oceanic crust: 1. Experiments and implications for the origin of ocean island basalts

Abstract Dehydration experiments on natural amphibolite have been carried out under upper mantle P/T conditions, in order to examine transportation of trace elements during dehydration processes in the subducted oceanic lithosphere. Pb, Nd, and Rb are more readily transported by aqueous fluids during amphibolite dehydration than U-Th, Sm, and Sr, respectively. The results indicates that the dehydration of subducted oceanic crust may result in large increases in U/Pb, Th/Pb and Sm/Nd ratios, and a decrease inRb/Sr ratios of subducted oceanic crust. This ultimately leads to higher Pb and Nd isotopic ratios, and lower Sr isotopic ratios in the subducted oceanic crust than the present MORB source mantle, given a sufficiently long periods of isolation in the mantle. It follows that the very high Pb isotopic ratios observed in some ocean island basalts, known as HIMU, can be readily achieved by incorporation of ancient subducted crust into their mantle source. However, Sr and Nd isotopic ratios cannot be explained by bulk mixture of the ancient subducted oceanic crust with depleted or primitive mantle, but require significant fractionation ofNd/Sr ratios in the subducted oceanic crust before mixing with mantle material. Possible processes to produce Sr and Nd isotopic compositions similar to that of HIMU may involve partial melting of recycled subducted basaltic crust under lower mantle conditions and refertilization of primitive mantle by the partial melt.

Setouchi high-Mg andesites revisited : geochemical evidence for melting of subducting sediments

Abstract In order to evaluate the mechanism of production of unusual high-Mg andesite (HMA) magmas, Pb–Nd–Sr isotopic compositions were determined for HMAs and basalts from the Miocene Setouchi volcanic belt in the SW Japan arc. The isotopic compositions of Setouchi rocks form mixing lines between local oceanic sediments and Japan Sea backarc basin basalts, suggesting a significant contribution of the subducting sediment component to the HMA magma generation. Mixing calculations using compositions of an inferred original mantle and local oceanic sediments suggest that a sediment-derived melt, neither an H 2 O-rich fluid nor an amphibolite/eclogite-derived melt, could have been produced first and served as a plausible metasomatic agent for the HMA magma source. The unusual tectonic setting, including subduction of a newly-borne hence hot plate, may be responsible for melting of subducting sediments.

Origin of high-magnesian andesites in the Setouchi volcanic belt, southwest Japan, II. Melting phase relations at high pressures

Melting phase relations of an augite-olivine high-magnesian andesite and an augite-olivine basalt from the Miocene Setouchi volcanic belt in southwest Japan have been studied under water-saturated, water-undersaturated and under anhydrous conditions. Both the andesite and the basalt are characterized by low FeO*/MgO ratios (0.86 and 0.76 in weight, respectively) and qualify as primary magmas derived from the upper mantle. n nThe andesite melt coexists with olivine, orthopyroxene and clinopyroxene at 15 kbar and 1030°C under water-saturated conditions, and at 10 kbar and 1070°C under water-undersaturated conditions (7 wt.% H2O in the melt). The basalt-melt also coexists with the above three phases at 11 kbar and 1305°C under anhydrous conditions, and at 15 kbar and 1205°C in the presence of 4 wt.% water. n nPresent studies indicate that high-magnesian andesite magmas may be produced even under water-undersaturated conditions by partial melting of mantle peridotite. It is suggested that two types of high-magnesian andesites in the Setouchi volcanic belt (augite-olivine and bronzite-olivine andesites) were produced by different degrees of partial melting; augite-olivine andesite magmas, whose mantle residual is lherzolite, were formed by lower degrees of partial melting than bronzite-olivine andesite magmas, which coexist with harzburgite. The basalt magmas, which were often extruded in close proximity to the high-magnesian andesite magmas, are not partial melting products of a mantle peridotite which had previously melted to yield high-magnesian andesite magmas.

Origin of high magnesian andesites in the Setouchi volcanic belt, southwest Japan, I. Petrographical and chemical characteristics.

High-magnesian andesites of middle Miocene age occur in southwest Japan, forming an obvious volcanic belt. These andesites have low FeO*/MgO ratios (0.546–0.931), and are rich in Ni (101–312 ppm), Co (30.0–45.1 ppm), and Cr (208–756 ppm). They are relatively aphyric (phenocrysts <10 vol.%), and the phenocrysts of magnesian olivine (∼Fo88) are in equilibrium with the host high-magnesian andesite magmas on the basis of the Fe-Mg exchange partitioning. These features suggest that the high-magnesian andesites are not differentiated or accumulative; they appear to represent primary andesites generated in the upper mantle. These southwest Japanese high-magnesian andesites are rich in incompatible elements, and show light rare earth enrichment relative to boninites, suggesting that the former is derived from a less depleted mantle source than the latter.

Melting experiments on a high-magnesian andesite

Abstract Melting experiments were conducted on a high-magnesian bronzite olivine andesite (Teraga-Ike andesite) which is considered to be a primary andesite. The high-magnesian andesite magma is in equilibrium with both olivine and orthopyroxene at about 15.5 kbar and 1080°C under H2O-saturated conditions and at lower pressure and higher temperature under H2O-undersaturated conditions. This suggests that high-magnesian andesites could be generated by the partial melting of upper mantle peridotite containing a small amount of H2O.

Geochemical evidence for a mid-Cretaceous superplume

Basalt lavas with a high Nb/Y ratio for a given Nb/Zr ratio occur in the Polynesian “superswell” region of the South Pacific, which probably formed by upwelling of a deep-mantle superplume. The distinctive geochemical characteristics of the Polynesian basalts may be attributed to melting of a mantle source that is more enriched in a basaltic (ancient mid-oceanic-ridge basalt) component. Basalts displaying such chemical signatures have been found on Shatsky Rise, the Ontong Java Plateau, and greenstones from subduction-zone complexes of Sakhalin Island. The occurrence of Polynesian-type basalts, together with an estimate of their ages, suggests that the South Pacific superplume was active as long ago as 90–150 Ma. The superplume activity preceded the onset of the superchron, supporting an idea that the superplume acted as a trigger for such a global event.

Existence of andesitic primary magma: An example from southwest Japan

A high-magnesian andesite (SiO2 58.50%, MgO 9.47%) occurs at Teraga-Ike in southwest Japan. It belongs to the Setouchi volcanic rocks of middle Miocene age and carries olivine and bronzite as phenocrysts (4.2 and 1.4 modal percent, respectively). This andesite is characterized by Mg-values as high as 75, suggesting that it may be a primary andesite. Olivine phenocrysts (Fo87–91) are in equilibrium with the groundmass (= liquid) on the basis of Fe-Mg exchange partitioning between olivine and liquid, and they have high NiO contents (up to 0.45%). Chromite inclusions in olivine and rarely bronzite have high Cr2O3 contents (max. 54.87%). These features strongly suggest that the Teraga-Ike andesite keeps the chemical composition of the primary magma generated in the upper mantle, and therefore verify the existence of primary andesite magmas.

Magnesian andesite and basalt from Shodo-Shima Island, southwest Japan, and their bearing on the genesis of calc-alkaline andesites

Abstract Calc-alkaline andesites and olivine tholeiitic basalts are widely distributed on Shodo-Shima island, southwest Japan. The Fo content of olivine phenocrysts in the andesite is higher than in the basalt. The primary magma of the andesite, estimated on the basis of the olivine fractional crystallization model, is not basaltic but andesitic. The basalt contains both chromite and titanomagnetite as inclusions in olivine phenocrysts, while only chromite appears in the andesite. The Cr content of chromite in the andesite is higher than in the basalt. These facts again indicate that the andesite cannot be a fractionation product of the basalt, and that andesitic and basaltic primary magmas were generated independently.

Melting of a subducting slab and production of high‐mg andesite magmas: Unusual magmatism in SW Japan at 13∼15 Ma

Characteristic high-Mg andesite magmas were produced in the SW Japan arc at 13∼15 Ma that was synchronous with the commencement of subduction of a very young (<11 m.y.) lithosphere of the Shikoku Basin. Numerical simulation suggests that temperature at the surface of such a young subducting plate is high enough for partial melting both of the subducting sediments and oceanic crust at the beginning of the subduction. High-Mg andesite magmas were likely to be produced by interaction between silicic slab melts and the overlying mantle wedge. HMA magmas may be commonly produced in the Archean subduction zones under relatively high mantle temperature conditions, contributing to making continental crusts.

Quantitative determination of gold and the platinum-group elements in geological samples using improved NiS fire-assay and tellurium coprecipitation with inductively coupled plasma-mass spectrometry (ICP-MS)

Abstract New fire-assay and tellurium coprecipitation methods were developed for precise determination of Ru, Rh, Pd, Ir, Pt, and Au in geological samples by using ICP-MS. High recovery (>97%) of those elements was obtained by technical improvements including (1) duplicate sample fusion under reduced conditions, and (2) duplicate Te-coprecipitation at an adequate temperature (210°C) and for adequate duration (75 min). Total blanks were less than 1 ng except for Ru and Au, and detection limits were within 2–53 ppt on the rock sample base for all elements. The present procedures are relatively simple and can be applied with high precision and reproducibility to the analysis of geological samples with wide ranges of concentrations.