Systematic Variation of Sr-, Nd- and Pb-Isotopes with Time in Lavas of Mauritius, Reunion Hotspot

We report Sr-, Nd- and Pb-isotopic compositions for the lavasof Mauritius, the second youngest volcanic island in the Réunionhotspot. The lavas of the Older Series (7·8–5·5Ma) have identical isotopic compositions (⁸⁷Sr/⁸⁶Sr = 0·70411to 0·70422,¹⁴³Nd/¹⁴⁴Nd = 0·512865 to 0·512854,and ²⁰⁶Pb/²⁰⁴Pb = 19·016 to 19·041) to those ofRéunion, where the center of volcanic activity is currentlylocated. The lavas of the Intermediate Series (3·5–1·9Ma) and Younger Series (0·70–0·17 Ma) areshifted to lower Sr-isotopic compositions (0·70364–0·70394,with ¹⁴³Nd/¹⁴⁴Nd = 0·512813 to 0·512948 and ²⁰⁶Pb/²⁰⁴Pb= 18·794 to 18·984). The Intermediate Series lavashave similar trace-element characteristics (e.g. Zr–Nb,Ba–Y) to those of Rodrigues, in both cases requiring theinvolvement of an enriched mantle-like component in the mantlesource. During the volcanic history of Mauritius, the magmaslost the principal isotopic characteristics of the Réunionhotspot with time, and became gradually imprinted with the isotopicsignature of a shallower mantle source that produced the CentralIndian Ridge basalts. KEY WORDS: hotspot; isotopes; Mauritius; Réunion; trace element     

Assimilation and Fractional Crystallization Controlled by Transport Process of Crustal Melt: Implications from an Alkali Basalt-Dacite Suite from Rishiri Volcano, Japan

Mechanisms of fractional crystallization with simultaneous crustalassimilation (AFC) are examined for the Kutsugata and Tanetomilavas, an alkali basalt–dacite suite erupted sequentiallyfrom Rishiri Volcano, northern Japan. The major element variationswithin the suite can be explained by boundary layer fractionation;that is, mixing of a magma in the main part of the magma bodywith a fractionated interstitial melt transported from the mushyboundary layer at the floor. Systematic variations in SiO₂ correlatewith variations in the Pb, Sr and Nd isotopic compositions ofthe lavas. The geochemical variations of the lavas are explainedby a constant and relatively low ratio of assimilated mass tocrystallized mass (‘r value’). In the magma chamberin which the Kutsugata and Tanetomi magmas evolved, a strongthermal gradient was present and it is suggested that the marginalpart of the reservoir was completely solidified. The assimilantwas transported by crack flow from the partially fused floorcrust to the partially crystallized floor mush zone throughfractures in the solidified margin, formed mainly by thermalstresses resulting from cooling of the solidified margin andheating of the crust. The crustal melt was then mixed with thefractionated interstitial melt in the mushy zone, and the mixedmelt was further transported by compositional convection tothe main magma, causing its geochemical evolution to be characteristicof AFC. The volume flux of the assimilant from the crust tothe magma chamber is suggested to have decreased progressivelywith time (proportional to t^–1/2), and was about 3 x 10^–2m/year at t = 10 years and 1 x 10^–2 m/year at t = 100years. It has been commonly considered that the heat balancebetween magmas and the surrounding crust controls the couplingof assimilation and fractional crystallization processes (i.e.absolute value of r). However, it is inferred from this studythat the ratio of assimilated mass to crystallized mass canbe controlled by the transport process of the assimilant fromthe crust to magma chambers. KEY WORDS: assimilation and fractional crystallization; mass balance model; magma chamber; melt transport; Pb isotope     

Shift and Rotation of Composition Trends by Magma Mixing: 1983 Eruption at Miyake-jima Volcano, Japan

Pre-eruption processes are investigated for magmas erupted in1983 from Miyake-jima volcano, which is one of the most activevolcanoes in Japan. The whole-rock compositional trends of theeruptive products are principally smooth and linear. Magmaserupted from some fissures have compositions that deviate fromthe main linear trend. Phenocryst contents of samples displacedfrom the linear compositional trends are significantly lowerthan those of samples on the main trends. Anorthite-rich plagioclasephenocrysts, present throughout the 1983 products, are too calcicto have crystallized from the erupted magma composition, andwere derived from a basaltic magma through magma mixing. Althoughthe linear whole-rock composition trends favor simple two-componentmagma mixing, this cannot explain the presence of samples thatdeviate from the main trend. Instead, the observed compositiontrends were formed by mixing of a homogeneous basaltic magmawith andesitic magmas exhibiting compositional diversity. Theoriginal linear composition trends of the andesitic end-membermagma were rotated and shifted to the direction of the basalticend-member magma by magma mixing. The samples out of the maintrends represent magmas with less basaltic component than thoseon the trend. The density and viscosity of the basaltic end-membermagma were comparable with those of the andesitic end-membermagmas. The basaltic magma, discharged from one magma chamberat 2 kbar pressure, was injected into a magma chamber at lowerpressure occupied by the chemically zoned andesite magma (1kbar), and possibly as a fountain. To establish the characteristicmixing trend of the 1983 magma, the basaltic component musthave been distributed systematically in the zoned andesite magma.A requirement is that the basaltic magma spread laterally andmixed with the andesite magma at various levels of ascent ofthe fountain in the host andesite magma. Analysis of compositionalzoning in titanomagnetite crystals revealed that the eruptionof the 1983 magmas was initiated soon after the replenishmentof the basaltic magma in the 1 kbar magma chamber. KEY WORDS: compositional trend; liquid–liquid blending; magma chamber; magma mixing; Miyake-jima Volcano     

Hydrous and anhydrous alterations of chondrules in Kaba and Mokoia CV chondrites

Abstract— Chondrules in the Bali-like CV chondrite Kaba and the Allende-like portion of the Mokoia breccia have been studied to explore the relationship between hydrous alteration to form phyllosilicates and anhydrous alteration resulting in secondary olivine zonation, replacement of enstatite by ferroan olivine and formation of feldspathoids (nepheline and sodalite). All Kaba chondrules experienced extensive hydrous alteration; whereas, anhydrous alteration was minor and resulted only in the olivine zonation. On the other hand, all of the Mokoia chondrules experienced both extensive anhydrous and hydrous alteration. Bronzite rims formed between relic enstatite grains and phyllosilicates in both Kaba and Mokoia during the hydrous alteration. Petrographic observations indicate that phyllosilicates in Mokoia postdate formation of the secondary ferroan olivine and feldspathoids. We conclude that anhydrous alteration in Kaba and Mokoia predated hydrous alteration and took place before accretion of chondrules into the CV parent asteroid.     

Petrology of magmatic silicate inclusions in the Allan Hills 77005 lherzolitic shergottite

Abstract— Magmatic inclusions occur in both chadacrystic olivine and oikocrystic pigeonite in ALH 77005 but are different from each other. Magmatic inclusions in olivine consist mainly of aluminous pyroxenes, intergrowths of plagioclase and silica, silica-predominant glass, and rhyodacitic glass, with minor amounts of chromite, spinel, pyrrhotite, and whitlockite. Those in pigeonite consist mainly of aluminous pyroxenes, nonaluminous ferroan pyroxenes, kaersutite, spinel, and K-rich trachytic glass, with minor amounts of pyrrhotite and whitlockite. The magmatic inclusions in chadacrystic olivine formed from trapped melts that were basaltic, apparently dry and crystallized additional olivine metastably. The basaltic magma, with entrained olivine, experienced magma mixing with K-rich and wet magmas, or assimilation of such crustal rocks, in the early to middle stages of the crystallization sequence of ALH 77005 during crystallization of chadacrystic olivine prior to precipitation of oikocrystic pigeonite. However the amount of mixed magmas or assimilated rocks was minor in comparison to the basaltic magma. Crystallization of pigeonite, augite, and plagioclase in the host lithologies took place in a shallow magma reservoir under an open-system condition, and the pigeonite trapped basaltic andesite to trachyandesitic melts, which resulted in magmatic inclusions in oikocrystic pigeonite. The magmatic inclusions in both olivine and pigeonite were formed under a rapid-cooling condition, resulting in a variety of inclusions. Kaersutite in magmatic inclusions in oikocrystic pigeonite crystallized under a closed-system wet condition during the late-stage crystallization of the inclusions.     

Dense-media separation as a more efficient pollen extraction method for use with organic sediment/deposit samples: comparison with the conventional method

Dense-media separation, frequently adopted as a standard pollen extraction method for use with minerogenic sediment samples, was adapted for use with organic-rich sediment and peat samples. A total of 15 organic-rich sediment samples, obtained from sites in Corsica and the Southern European Alps, was treated using both dense-media separation and the conventional preparation method in order to compare the relative efficiencies of the two methods. The dense-media separation method consistently achieved a remarkably higher purity of extracted pollen grains. This was especially true for the Corsican peat samples, the dense-media separations being quite pure, whereas the classical method produced samples that were so severely polluted with organic fragments that it was impossible to carry out a satisfactory pollen analysis. The percentage values of each taxon were generally similar using both methods, showing that the two methods are compatible for pollen analytical studies. Total pollen concentrations were generally higher with the dense-media separation method (especially for gyttja samples). In conclusion, dense-media separation is an efficient method for pollen extraction from organic as well as from minerogenic sediments and deposits.     

Magmatic inclusions and felsic clasts in the Dar al Gani 319 polymict ureilite

Abstract— Magmatic inclusions occur in type II ureilite clasts (olivine‐orthopyroxene‐augite assemblages with essentially no carbon) and in a large isolated plagioclase clast in the Dar al Gani (DaG) 319 polymict ureilite. Type I ureilite clasts (olivine‐pigeonite assemblages with carbon), as well as other lithic and mineral clasts in this meteorite, are described in Ikeda et al.(2000). The magmatic inclusions in the type II ureilite clasts consist mainly of magnesian augite and glass. They metastably crystallized euhedral pyroxenes, resulting in feldspar component‐enriched glass. On the other hand, the magmatic inclusions in the large plagioclase clast consist mainly of pyroxene and plagioclase, with a mesostasis. They crystallized with a composition along the cotectic line between the pyroxene and plagioclase liquidus fields. DaG 319 also contains felsic lithic clasts that represent various types of igneous lithologies. These are the rare components not found in the common monomict ureilites. Porphyritic felsic clasts, the main type, contain phenocrysts of plagioclase and pyroxene, and their groundmass consists mainly of plagioclase, pyroxene, and minor phosphate, ilmenite, chromite, and/or glass. Crystallization of these porphyritic clasts took place along the cotectic line between the pyroxene and plagioclase fields. Pilotaxitic felsic clasts crystallized plagioclase laths and minor interstitial pyroxene under metastable conditions, and the mesostasis is extremely enriched in plagioclase component in spite of the ubiquitous crystallization of plagioclase laths in the clasts. We suggest that there are two crystallization trends, pyroxene‐metal and pyroxene‐plagioclase trends, for the magmatic inclusions and felsic lithic clasts in DaG 319. The pyroxene‐metal crystallization trend corresponds to the magmatic inclusions in the type II ureilite clasts and the pilotaxitic felsic clasts, where crystallization took place under reducing and metastable conditions, suppressing precipitation of plagioclase. The pyroxene‐plagioclase crystallization trend corresponds to the magmatic inclusions in the isolated plagioclase clast and the porphyritic felsic clasts. This trend developed under oxidizing conditions in magma chambers within the ureilite parent body. The felsic clasts may have formed mainly from albite component‐rich silicate melts produced by fractional partial melting of chondritic precursors. The common monomict ureilites, type I ureilites, may have formed by the fractional partial melting of alkali‐bearing chondritic precursors. However, type II ureilites may have formed as cumulates from a basaltic melt.     

Middle to Late Pleistocene vegetation history and climatic changes at Lake Kopais, Southeast Greece

This study presents a detailed vegetation history of Lake Kopais, Southeast Greece based on pollen analysis of a 120 m-long sediment core. The record extends from the Holocene back to Oxygen Isotope Stage 11, and it gives a detailed history of Mediterranean evergreen woodlands during the last 500 ka. Vegetation of glacials consisted of open steppe with Chenopodiaceae and Artemisia associated with Tubuliflorae and other herbs, while that of interglacials was dominated by temperate Quercus forest. The Last Interglacial (Substage 5e) shows a complete vegetation succession starting with Juniperus and Betula, followed by deciduous oak and ending with Pinus and Abies. Wild olive is abundant in Substages 11c and 5e, suggesting very warm climatic conditions in Stage 11. Stages 9 and 7 were significantly cooler than 11 and 5e. The Pterocarya occurrence near the bottom of the core provides a possible equivalent of the Holsteinian Interglacial.     

Petrology of the Asuka 881931 ureilite with special reference to crystallization of interstitial silicate melt

Abstract— The Asuka 881931 meteorite is an unbrecciated ferroan ureilite and consists mainly of equi—granular olivine and pigeonite grains, a metal—sulfide network, interstitial silicates, and glass. Peripheral portions of equigranular olivine grains are often replaced by fine-grained forsterite—metal aggregates and sometimes by fine-grained enstatite—metal aggregates. These aggregates may have been produced from the equigranular olivine by reduction. Peripheral portions of equigranular pigeonite grains also are sometimes replaced by fine-grained orthopyroxene aggregates with tiny patches of Si-rich glass and may have been produced from the pigeonite by reduction reaction with silicate melt. Interstitial silicates are mainly orthopyroxene, magnesian pigeonite, high-Ca pyroxene (diopside/fassaite), and CaO-poor enstatite; and they crystallized from interstitial silicate melt. Interstitial glass is classified into two types—-Si-poor and Si-rich. The Si-poor glass is always in contact with equigranular olivine, but the Si-rich glass never contacts equigranular olivine and is in contact with pyroxene and the metal—sulfide network. Both types of glass were produced from an original interstitial silicate melt, but the Si-poor glass formed mainly by fractional crystallization of pyroxenes, and the Si-rich glass may have formed by addition of Si mainly from nearby metal—sulfide melt, as well as crystallization of pyroxenes. The Si-poor and Si-rich melts were finally quenched as interstitial glasses under rapid cooling conditions.     

库木库里背斜活动性初步研究

库木库里盆地位于青藏高原北缘,与柴达木盆地一山之隔,是二者的过渡地带,也是高原主体部分向NE扩展的前缘地区;现今构造表现为被3条大型活动构造带(走滑的阿尔金断裂带、东昆仑断裂带和逆冲的祁漫塔格褶皱逆冲系)所夹持。因此,该盆地对于研究青藏高原北缘的构造活动性、活动历史,探讨高原的扩展模式具有十分重要的意义。虽然库木库里盆地南、北两侧均发育活动性很强的大型走滑断裂,但是在盆地中央发育1条大型背斜,走向NWW-SEE,与祁漫塔格褶皱逆冲系和柴达木盆地内的褶皱构造走向一致,说明盆地目前遭受NNE向的挤压。通过对盆地地形横、纵剖面和阶地展布形态的分析,得出背斜有自西向东扩展变形的特征;野外调查和测年结果显示,背斜东段冰川融水形成了大型冰水扇,形成年龄为(87.09±2.31)~(102.4±3.7)ka,进而获得背斜东段自晚更新世以来平均隆升速率的最大值为(2.78±0.28)~(3.28±0.28)mm/a。库木库里盆地整体的活动性很强,在构造上与其北边的柴达木盆地类似,都受控于阿尔金断裂南侧的NNE向的区域挤压作用。