Tetsu Kogiso

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.

MELTING EXPERIMENTS ON HOMOGENEOUS MIXTURES OF PERIDOTITE AND BASALT : APPLICATION TO THE GENESIS OF OCEAN ISLAND BASALTS

Melting experiments on synthesized homogeneous mixtures of a pyrolitic peridotite (KLB-1) and average mid-oceanic ridge basalt (MORB) were carried out at pressures between 1.5 and 3.0 GPa and temperatures between 1300° and 1525°C with a piston–cylinder apparatus. Melt compositions were determined using the diamond aggregate method. Partial melts produced from the peridotite–basalt mixtures have higher FeO, TiO2 and alkali elements and similar or lower SiO2 relative to those from pyrolitic peridotite. This is generally consistent with the results of previous experimental studies and prediction of melting model calculations. Our results indicate that peridotite–basalt mixtures tend to produce silica-undersaturated magmas enriched in Fe and Ti even at moderate degrees of melting. In particular, partial melts produced at 3.0 GPa coexist with garnet and are similar in composition to those of alkali basalts which commonly occur in shield-building stages of ocean island volcanoes. Melting of peridotite–basalt mixtures is consistent with the general characteristics of ocean island basalts (OIBs) such as enrichment in Fe and Ti relative to MORB, predominance of alkali basalts, and trace element features indicating garnet-present melting. This suggests that the source mantle of OIBs is generally more enriched in a basaltic component than pyrolitic mantle. It has been proposed that tholeiitic magmas in ocean islands and continental flood basalts can be produced by melting of heterogeneous plumes including recycled basaltic crust. Our results imply that alkali basaltic magmas can also be produced from such heterogeneous mantle plumes. Therefore partial melting of mantle plumes including recycled basaltic crust can produce the major element diversity in OIBs, although it is difficult to explain the high-CaO signature of HIMU basalts.

Trace element transport during dehydration processes in the subducted oceanic crust: 2. Origin of chemical and physical characteristics in arc magmatism

The trace elements and Sr-Nd-Pb isotope compositions of arc magmas have been calculated from our estimates of: (1) the chemical composition of aqueous fluids emanating from the subducting basaltic oceanic crust, based on experimental data for element mobilization by aqueous fluids during the amphibolite/eclogite transition [1]; (2) the modal and chemical composition of the hydrous peridotite layer, which is formed by addition of the fluid derived from amphibolite dehydration to the base of the mantle wedge and the change in the hydrous mantle mode with increasing pressure and temperature; (3) the chemical composition of the fluid phase released by amphibole breakdown within the down-dragged hydrous peridotite layer, beneath the volcanic front; and (4) the mixing ratio of the this fluid with the pre-existing mantle wedge, as constrained by the H2O content of arc magmas and appropriate degrees of partial melting. The resulting estimate for the chemical composition of primary arc basalt magma matches well with that of typical oceanic arc basalts. This supports the suggestion that aqueous fluids derived from the subducting crust play an essential role in generating arc magmatism and in producing their distinctive chemistry. Our calculations also indicate that more aqueous fluid enters the magma source region beneath the trench-side volcanic chain from amphibole breakdown than beneath sites of backarc-side volcanism due to phlogopite decomposition. This greater fluid flux beneath the volcanic front is consistent with the observed greater volume and/or number density of volcanoes at the volcanic front than behind the volcanic front — one of the most notable physical features of arc magmatism.

High μ (HIMU) ocean island basalts in southern Polynesia: New evidence for whole mantle scale recycling of subducted oceanic crust

Major elements, trace elements, and Pb isotopic compositions were determined for ocean island basalts (OIBs) from Polynesia in the southern Pacific in order to document the chemical characteristics of OIB sources and to understand their origin. High μ (HIMU: μ=238U/204Pb) basalts, which have distinctly high Pb isotopic ratios, have systematically different compositions from non-HIMU basalts; HIMU basalts are more enriched in Fe2O3*, MnO, and CaO and more depleted in SiO2, K2O, P2O5, Ni and incompatible trace elements than non-HIMU, except for Nb. Major element characteristics of HIMU basalts suggest that the HIMU source is more fertile, i.e., more enriched in a basaltic component, than non-HIMU sources. This is consistent with the suggestion that subducted oceanic crust may contribute to the formation of the HIMU reservoir. Relative depletion of incompatible trace elements in HIMU is consistent with involvement of sedimentary components in non-HIMU sources. However, enrichment of Nb relative to other incompatible elements in HIMU cannot be explained by simple addition of the crustal component nor partial melting processes in the upper mantle, implying that lower mantle processes may contribute to the formation of the HIMU source.

The subduction factory: its role in the evolution of the Earth’s crust and mantle

Abstract Subduction zones are major sites of magmatism on the Earth. Dehydration processes and associated element transport, which take place in both the subducting lithosphere and the down-dragged hydrated peridotite layer at the base of the mantle wedge, are largely responsible for the following characteristics common to most subduction zones: (1) the presence of dual volcanic chains within a single volcanic arc; (2) the negative correlation between the volcanic arc width and the subduction angle; (3) selective enrichment of particular incompatible trace elements; and (4) systematic across-arc variations in incompatible trace element concentrations. The occurrence of two types of andesites, calcalkalic and tholeiitic, typifies magmatism in subduction zones. Examination of geochemical characteristics of those andesites in the NE Japan arc and bulk continental crust reveals marked compositional similarity between calc-alkalic andesites and continental crust. One of the principal mechanisms of generation of calc-alkalic andesites, at least those on the NE Japan arc, is the mixing of two magmas, having basaltic and felsic compositions and being derived from partial melting of the mantle and the overriding basaltic crust, respectively. It may be thus suggested that this process would also have contributed greatly to continental crust formation. If this is the case, then the melting residue after extraction of felsic melts should be removed and delaminated from the initial crust into the mantle in order to form ‘andesitic’ crust compositions. These processes cause accumulation in the deep mantle of residual materials, such as delaminated crust materials and dehydrated, compositionally modified subducted oceanic crusts and sediments. Geochemical modelling suggests that such residual components have evolved to form enriched mantle reservoirs.

A third volcanic chain in Kamchatka : thermal anomaly at transform/convergence plate boundary

The Kamchatka volcanic arc, which is located at the northern edge of the Kurile arc, consists of three volcanic chains, all parallel to the trench axis. In contrast, most subduction zones have only two subparallel volcanic chains. The third chain in Kamchatka, which is farthest from the trench, is characterized by the occurrence of voluminous plateau lavas; volcanoes in the two chains closer to the trench are stratovolcanoes typical in arc magmatism. The third chain magmatism is also unusual in that lavas show concentrations of incompatible elements intermediate between those in the two trenchward chains. Both the unusual occurrence of the third volcanic chain and the unusual lava chemistry could be caused by partial melting of K-amphibole bearing peridotites in the downdragged hydrous layer at the base of the mantle wedge under anomalously high-temperature conditions associated with the characteristic tectonic setting of transform/convergence transition in the region.

Geomagnetic paleosecular variation for the past 5 Ma in the Society Islands, French Polynesia

We report a paleomagnetic secular variation in the Society Islands, French Polynesia for the past 5 Ma. Paleomagnetic measurements were performed on volcanic rocks applying thermal and alternating field demagnetizations and principal-component analysis. As a result, 82 normal, 48 reversed, and 10 intermediate directions were observed after the correction of the site localities for the absolute motion of the Pacific plate. A mean VGP of the combined data set of normal and reversed polarities is located at 87.5°N and 4.0°W (A95 = 2.3°), which is close to the geographic pole. The angular standard deviation (ASD) around the mean VGP resulted in 14.6° (+1.3°, −1.2°). There is no significant inclination anomaly for the time-averaged field. However, if the plate motion is not taken into account, a significant anomaly of −3.4° would be observed at a 95% confidence level. This suggests that correction for the absolute plate motion is needed for the precise paleodirectional analysis of the Pacific region. These features are also supported by an analysis combining the previous data set with our data.

Formation of a third volcanic chain in Kamchatka: generation of unusual subduction-related magmas

The unusual development of three volcanic chains, all parallel to the trend of the subduction trench, is observed in Kamchatka at the northern edge of the Kurile arc. Elsewhere on the Earth volcanic arcs dominantly consist of only two such chains. In the Kurile arc, magmatism in the third volcanic chain, which is farthest from the trench, is also unusual in that lavas show concentrations of incompatible elements intermediate between those of the two trenchward chains. This observation can be explained by relatively shallow segregation of primary magmas and high degrees of partial melting of magmas in the third chain, compared to the conditions of magma separation expected from a simple application of the general acrossarc variation. Initial magmas in such an atypical third chain may be produced by melting of K-amphibolebearing peridotite in the down-dragged layer at the base of the mantle wedge under anomalously hightemperature conditions. Such an unusual melting event may be associated with the particular tectonic setting of the Kamchatka region, i.e. the presence of subductiontransform boundary. Such a mechanism is consistent with the across-arc variation in Rb/K ratios in the Kamchatka lavas: lowest in the third chain rocks and highest in the second chain rocks.

Contrasting behavior of noble-metal elements during magmatic differentiation in basalts from the Cook Islands, Polynesia

Concentrations of noble metals (Ir, Ru, Rh, Pt, Pd, and Au) in ocean-island basalts from the Cook Islands, Polynesia, were determined by improved fire-assay and tellurium coprecipitation techniques with an inductively-coupled-plasma mass spectrometer. Isotope, major element, and trace element compositions of these basalts indicate that the present samples include distinctive HIMU (high µ = high 238U/204Pb) and normal non-HIMU basalts. Examination based on Ni-Mg-Fe partitioning between olivine and liquid suggests an only minor effect of accumulation of phenocrysts in governing the compositional variations of the present samples. The fractionation trends obtained show monotonic decrease and increase in noble-metal elements with decreasing MgO content in HIMU and non-HIMU basalts, respectively. These characteristic trends indicate that HIMU magmas are differentiated by fractional crystallization and have higher sulfide/silicate ratios than non-HIMU basalts.

New K-Ar ages of the Society Islands, French Polynesia, and implications for the Society hotspot feature

We report 52 new K-Ar age determinations of volcanic rocks from the Society Islands in French Polynesia. The newly obtained ages range from 0.51 to 4.61 Ma, which are consistent with previously reported ones. The magnetostratigraphy based on 45 data sets combining K-Ar ages and paleomagnetic polarity agrees with the geomagnetic polarity time scale at the 2s level, except for a few age data. Assuming the absolute motion of the Pacific plate, paleopositions of the dated rocks are estimated from the K-Ar ages, showing a narrow distribution in the western part of the easternmost volcano of the active hotspot region. This suggests that a hotspot of a few tens of kilometers in diameter might have been fixed nearly at the eastern margin of the presently active region in the Society archipelago and that the volcanism of each island might have persisted for about 1 myr.