Temporal Evolution of Boron Flux in the NE Japan and Izu Arcs Measured by Ion Microprobe from the Forearc Tephra Record

The enrichment of boron relative to similarly incompatible elements,such as Be, in arc volcanic rocks has been used as a proxy forthe involvement of slab flux in petrogenesis. New ion microprobeanalyses of single glass shards in tephra layers recovered bythe Ocean Drilling Program (ODP) in the Izu and NE Japan forearcbasins now allow the temporal variation in slab flux to be chartedsince 7 and 5 Ma, respectively. B/Be ratios are typically <70in NE Japan and <130 in Izu, with no single grain exceeding200. Although moderate to high for modern arcs, these valuesare much less than those recorded in the Marianas and Tongaat 3–4 Ma, shortly after the start of rifting of theirback-arc basins. This observation suggests that the peak B/Bevalues seen in Tonga and the Marianas are related to the tectonicsof slab roll-back and basin opening, rather than changes inthe dynamics of the Pacific Plate. There is no temporal trendto enrichment in the high field strength elements (HFSE) orrare earth elements (REE) in either Izu or NE Japan since 7Ma, although the two elemental groups do show clear positivecorrelation. A lack of correlation between REE, HFSE and B/Besuggests that slab flux is not the only control on melting inthese arcs.     

Analysis of Determinants for an Enhanced and Long-lasting Coastal Convective System by Means of a Case Study(26 July 2011)

A precipitation system developed continuously along the western coastline of the Korean Peninsula and created considerable precipitation both along the coast and inland on 26 July 2011. In this study, the causes for this nearshore convective system are investigated from observations and the results of model experiments. Three-dimensional radar fields clearly show that a change of wind at the surface border played an important role in the development of the nearshore convection system. The simulation results, which are very similar to the observations, show that the surface border generated and maintained the convergence zone. The roughness change enhanced the convergence, and the interaction between the deepening cold pool and downward flow maintained the convergence zone. The surface mechanical discontinuity affected by the roughness change between sea and land formed the convergence (gradient of wind stress),which induced momentum transfer to the upper layer. The cold pool created a steep gradient of potential temperature and provided the reason for the propagated convergence zone with the downward flow. The maximum value of the surface change factor, which comprises the influencing factors for the long-lasting convective system, reflects the enhancement of the system at the coast.     

Melt Generation and Movement beneath Theistareykir, NE Iceland

A detailed study of the volume and composition of all the lavasfrom the Theistareykir segment of the Northern Volcanic Zoneof Iceland was designed to study basaltic melt generation andmovement beneath a spreading ridge. The trace element compositionsof the lavas are variable, and those of melt inclusions in olivine,clinopyroxene and plagioclase phenocrysts even more so. We showthat this variability can be produced by mixing instantaneousmelts produced by isentropic decompression of mantle whose initialpotential temperature is 1480°C, and that the calculatedvolume and composition of the average melt is consistent withgeophysical and petrological observations. Pressure and temperatureestimates suggest that the phenocrysts form in the upper mantle,at depths of 30–40 km, and trap melts formed at greaterdepths. Some mixing of the instantaneous melts occurs beforethe melt is trapped, and more mixing occurs before the lavasare erupted. A similar model can account for the compositionof melt inclusions from the FAMOUS area of the Mid-AtlanticRidge, and from the Gorda and Juan de Fuca Ridges. KEY WORDS: basalt; Iceland; melt inclusions; melting; ridges     

Geochemical, mineralogical and sedimentological studies on the Taklimakan Desert sands

Major-element composition, mineral composition and grain-size distribution have been studied for Quaternary aeolian sediments from the Taklimakan Desert, north-western China, together with the variation of chemical and mineralogical compositions of different grain-size fractions. Aeolian sediments from the Taklimakan Desert have higher ratios of feldspar/quartz and calcite/quartz, finer grain size, poorer roundness of quartz and feldspar grains and lower abundances of frosted quartz, than found in aeolian sediments from other deserts such as the Saudi Arabian Desert. In spite of these immature mineralogical and sedimentological features, the aeolian sediments from the Taklimakan Desert show low regional variations in major-element and mineral compositions and are homogenized. These observations confirm that two processes, glacial activity within surrounding mountains and aeolian activity at the Tarim Basin, are important in the homogenization of the Taklimakan Desert sands. Taklimakan Desert sediments are constantly and effectively supplied from basement rocks in the surrounding mountains by glacial erosion. The supplied sediments are further homogenized by aeolian activity in the desert and are partly blown away, serving as the source of Chinese aeolian loess. Compositional differences are observed between loess (mainly 10–40 μm particles) and the <45 μm fraction of the Taklimakan Desert sediments, as well as between loess and whole rock of the Taklimakan Desert sediments. These observations provide constraints for precise modelling of loess formation, and for assessment of the chemical composition of the upper continental crust based on the chemical composition of aeolian loess.     

The Geochemistry of Ultramafic to Mafic Volcanics from the Belingwe Greenstone Belt, Zimbabwe: Magmatism in an Archean Continental Large Igneous Province

The evolution of the late Archean Belingwe greenstone belt,Zimbabwe, is discussed in relation to the geochemistry of theultramafic to mafic volcanic rocks. Four volcanic types (komatiite,komatiitic basalt, D-basalt and E-basalt) are distinguishedin the 2·7 Ga Ngezi volcanic sequence using a combinationof petrography and geochemistry. The komatiites and D-basaltsare rocks in which isotopic systems and trace elements are depleted.Chemical variations in komatiites and D-basalts can be explainedby fractional crystallization from the parental komatiite. Incontrast, komatiitic basalts and E-basalts are siliceous anddisplay enriched isotopic and trace element compositions. Theirchemical trends are best explained by assimilation with fractionalcrystallization (AFC) from the primary komatiite. AFC calculationsindicate that the komatiitic basalts and E-basalts are derivedfrom komatiites contaminated with 20% and 30% crustal material,respectively. The volcanic stratigraphy of the Ngezi sequence,which is based on field relationships and the trace elementcompositions of relict clinopyroxenes, shows that the leastcontaminated komatiite lies between highly contaminated komatiiticbasalt flows, and has limited exposure near the base of thesuccession. Above these flows, D- and E-basalts alternate. Thekomatiite appears to have erupted on the surface only in theearly stages, when plume activity was high. As activity decreasedwith time, komatiite magmas may have stagnated to form magmachambers within the continental crust. Subsequent komatiiticmagmas underwent fractional crystallization and were contaminatedwith crust to form D-basalts or E-basalts. KEY WORDS: komatiite; crustal assimilation; Belingwe greenstone belt; continental flood basalt; plume magmatism     

Hydrothermal alteration and K–Ar ages of Neogene‐Quaternary magmatic‐hydrothermal systems at Toyoha‐Muine area in southwest Hokkaido,Japan

This paper presents a review of hydrothermal alteration and K–Ar age data from the Toyoha‐Muine area (TMA), where the Toyoha polymetallic (Ag–Pb–Zn–Cu–In) deposit is located near the Pliocene andesitic volcano that formed Mt Muine. Systematic prospect‐scale mapping, sampling, X‐ray analysis and microscopic observation show that hydrothermal alteration is divided into two groups: acid‐pH and neutral‐pH alteration types. The former is further divided into mineral assemblages I, II and III, while the latter into mineral assemblages IV and V. Different mineralogical features in five mineral assemblages are summarized as follows: (I) Quartz (silicified rock); (II) Pyrophyllite or dickite; (III) Kaolinite or halloysite ± alunite; (IV) Sericite or K‐feldspar; and (V) Interstratified minerals (illite/smectite and chlorite/smectite) and/or smectite. K–Ar radiometric ages determined on twenty‐eight K‐bearing samples (whole volcanic rocks and separated hydrothermal minerals) mainly fall into one of three periods: Early Miocene (24.6–21.4 Ma), Middle–Late Miocene (12.5–8.4 Ma) and Pliocene–Pleistocene (3.2–0 Ma). These three periods are characterized as follows. Early Miocene: A minor hydrothermal activity, which might be genetically related to the intermediate or felsic magmatic activities, formed mineral assemblage IV at 24.6 Ma in the northern part of the TMA. Middle to Late Miocene: The basaltic intrusion, andesitic eruption, and granodiorite intrusions induced hydrothermal activities between 12.5 and 8.4 Ma, resulting in the formation of a mineral assemblage IV with some base metal mineralization. Pliocene–Pleistocene: An andesitic eruption formed Mt Muine between 3.2 and 2.9 Ma. The andesitic activity was associated with acid‐pH mineral assemblages I, II and III locally around the volcano. Latent magmatic intrusions subsequent to the andesitic eruption generated hydrothermal activities that formed mineral assemblages IV and V between 1.9 and 0 Ma in the southern and southeastern parts of Toyoha deposit at depth, overprinting the Middle to Late Miocene alteration. The hydrothermal activities also formed mineral assemblages I, II and III along the Yunosawa fault (east of the Toyoha deposit) and assemblage III in the south and southeast of the Toyoha deposit near the surface.     

Metasomatism-induced Melting in Mantle Xenoliths from Mongolia

Mantle xenoliths from two locations in Mongolia contain patchesof glass-phenocryst aggregates (‘melt pockets’)up to 1 cm in diameter, including one ‘composite’xenolith, which shows a complete transition from unaltered spinelIherzolite to a zone containing melt pockets surrounded by acpx and spinel-free peridotite matrix. We have analyzed majorelements by wet chemistry, X-ray fluorescence (XRF), and electronmicroprobe, trace elements by ion microprobe and inductivelycoupled plasma mass spectrometry (ICP-MS) techniques, and Srand Nd isotopes by mass spectrometry, to elucidate the originof these melt pockets. Petrographic and chemical evidence shows that the melt pocketswere formed neither by infiltration of the host basalt nor bydehydration melting of hydrous phases, such as amphibole. Instead,melting was induced by the interaction of a metasomatic fluidwith clinopyroxene and spinel. The reaction produced melts ofvariable composition, with SiO₂ ranging from 52 to 68% and MgOfrom 4.5 to 0.5%. The melts contain euhedral grains of olivine,clinopyroxene, and spinel, and a large number of (now empty)vugs. The melt shows no sign of having invaded the Iherzolitematrix surrounding the pockets. There is some evidence for fractionalcrystallization, but some of the major element chemical trends,such as the negative correlation between Na₂O and SiO₂, cannotbe accounted for by such a mechanism. The glasses and clinopyroxenephenocrysts are very rich in light rare earth elements (LREE)and Sr, and completely dominate the bulk contents of these andsome other incompatible elements in the rocks with melt pockets.The invading fluid introduced high concentrations of LREE, Th,U, Pb, and Sr, but was relatively depleted in Ba, Rb, Nb, Ta,Zr, Hf, and Ti, and had unusually high Zr/Hf and Nb/Ta ratios. Ion microprobe analyses of fresh glass directly adjacent toclinopyroxene microphenocrysts yield a series of cpx-melt partitioncoefficients for REE and several other trace elements. D_Yb (cpx-melt)varies between 0–3 and 1.6 and is positively correlatedwith the A1₂O₃+SiO₂ and Na₂O contents of the glass, and negativelycorrelated with MgO, FeO, and CaO contents. These correlationsare consistent with qualitative predictions from considerationsof silicate melt structure. The clinopyroxenes in the unaltered zones of the composite xenolithshow evidence of an earlier phase of metasomatism which enrichedCe, La, and Sr, but did not affect the other REE. Clinopyroxenesfrom these zones have high _Nd values of + 14 and +19, indicatinga history of low Nd/Sm ratios. At the same time, ⁸⁷Sr/⁸⁶Sr ratiosare high (>0.704), indicating infiltration of relativelyradiogenic Sr during the early stage of metasomatism. Ion microprobetraverses show no zoning of La/Nd ratios. Therefore, there wasenough time to equilibrate the metasomatic effects in the graininteriors, and we estimate the time required for this equilibrationto be of the order of 10⁵ years. In sharp contrast, the second, or main, metasomatic event thatcaused the formation of the melt pockets must have been extremelyshort-lived and probably lasted only hours or days before thexenolith was captured by the magma and erupted at the surface.This short duration is required by the preservation of freshglass and by the lack of equilibration of the melt pockets withtheir surrounding matrix. The isotopic compositions of Sr and Nd are identical betweenmelt pockets and host basalts in both localities. Therefore,we conclude that the metasomatic fluids were probably derivedfrom the same source rocks as the host basalts. We speculatethat the xenoliths originally resided in an upper-mantle regionwhich was intruded by a partially molten diapir. Volatiles wereexpelled from the unmelted margin of the diapir and invadedthe adjacent upper-mantle peridotites. The fluid infiltrationtriggered formation of the melt pockets, whereupon the materialwas picked up by rapidly ascending magma and erupted at thesurface. The fluids appear to have been poor in water, as nohydrous minerals are present among phenocryst or quench phasesin melt pockets. The major component of the fluid may have beenCO₂ or liquid carbonate.     

Major element distribution in Archean banded iron formation (BIF): influence of metamorphic differentiation

The Archean (2.8 Ga) Banded Iron Formation (BIF) of the Bell Lake region of Yellowknife greenstone belt, Canada is recrystallized to metamorphic assemblages of the amphibolite facies. This BIF is characterized by centimetre‐scale Fe‐rich and Si‐rich mesobands. In the Si‐rich mesobands, thin layers of magnetite microbands are developed in a quartz matrix. The Fe‐rich mesobands are composed mainly of Ca‐amphibole (hornblende), Fe–Mg amphibole (grunerite), and magnetite. The metamorphic foliation locally cuts across the mesoband boundaries, indicating the mesobanding was formed prior to peak metamorphism. Variations in mineral modal proportions between Fe‐rich mesobands and microbands are diagnostic of depositional compositional differences between beds. Micro‐X‐ray fluorescence imaging reveals metamorphic differentiation within Fe‐rich mesobands, with segregation of Fe–Mg amphibole, and the incompatible element Mn is concentrated at the margins of the Fe‐rich mesobands during the amphibole‐forming reactions. Ti was relatively immobile during metamorphic segregation and its distribution provides a record of the original structures in the Fe‐rich mesobands.     

Re-Os Isotopes in the Horoman Peridotite: Evidence for Refertilization?

Re–Os isotopic data for 20 samples from a well-characterized140 m section across a layered sequence, ranging from plagioclaselherzolite through lherzolite to harzburgite, of the Horomanperidotite show: (1) a range in ¹⁸⁷Os/¹⁸⁸Os ratios (from 0·1158to 0·1283) similar to that reported for other peridotiticmassifs, thereby suggesting that the processes responsible forthe Re–Os isotopic variation at the meter-scale and thewhole-massif scale are similar; (2) that the Os isotopic ratiois controlled by the Re content through radiogenic ingrowthover a period of