Rare earth element distribution in lavas and ultramafic xenoliths from the Comores Archipelago,Western Indian Ocean

Lavas and included xenoliths from the Comores Archipelago have been analysed for the rare earth elements (REE) La-Lu. Among basaltic lava types fractionation of REE rock/chondrite distribution patterns is more extreme with greater SiO₂ undersaturation and contents of incompatible elements. Enrichment and slight fractionation of REE in the rock series basanite-phonolite is considered compatible with a model of fractional crystallisation at low pressures involving mainly olivine and clinopyroxene, and to a much lesser extent, plagioclase. Apatite is probably effective in curtailing further enrichment of REE. High level fractional crystallisation and eclogite fractionation at depth appear unlikely causes for the relative enrichment of light REE (La-Eu) in the undersaturated basalts. This effect is more probably due to mineralogical control during partial melting in the upper mantle. Lherzolite xenoliths are poor in REE, exhibiting a slight relative depletion in the light REE. These patterns are interpreted as those of possible mantle material subjected to small degrees of partial melting, although not necessarily related to those melts erupted as lava flows at the surface.     

Evolution of basaltic and differentiated lavas from Anjouan,Comores Archipelago

Anjouan is one of four volcanic islands comprising the Comores Archipelago. Three (arbitrarily defined) categories of basic magma are recognised on Anjouan: ‘hypersthenenormative’, ‘alkalic’ and ‘basanitic’, which appear in that order with an eruptive sequence involving 1) shield construction, 2) peripheral fissure-controlled activity, and 3) rejuvenescent (posterosional) eruptions. Differentiated magmas have evolved within the three chemical groupings, and trend mainly towards undersaturated trachyte and phonolite. These trends are considered to have developed by initial removal of olivine and clinopyroxene, followed by Fe-Ti oxides, apatite and amphibole from arrested liquid pools within and beneath the volcano. The appearance of feldspar on the liquidus was clearly inhibited by the high contents of normative diopside in most Anjouan magmas, although late stage plagioclase fractionation is probably responsible for development of peralkaline phonolites at shallow depths, assisted eventually by alkali feldspar. Lherzolite-xenolith-bearing lavas are likely to be directly mantle-derived liquids. Three analysed representatives with 100 · Mg/Mg + Fe²⁺ (atomic) ratios approaching 70 are characteristically rich in normative diopside and may confirm the suggestion of primitive ankaramitic melts in mantle regions. High pressure fractional crystallisation may involve fractionation of ‘eclogite’, orthopyroxene or clinopyroxene, with or without olivine. It is believed however that such processes do not adequately explain the compositional change from hypersthene-normative basalt towards basanite, as they imply unacceptable degrees of Fe-enrichment. Problems of large ion element enrichment and fractionation would also remain, while the high contents of Cr and Ni place limits on the extent of crystal fractionation. A partial melting model of magma genesis is able to explain the main features of basalt geochemistry, especially if the upper mantle low velocity zone is regarded as having been severely fractionated due to upward migration of large ion elements. An ‘open’ system of magma production in the context of regional plate movement, and the implied decoupling between lithosphere and asthenosphere, accomodates realistic degrees of partial melting and allows a greater potential volume of mantle available for melting than possible ‘closed’ systems. The model also accounts for the migratory pattern of Comores volcanism.     

Trace-element distribution in lavas from Anjouan and Grande Comore,western Indian Ocean

The elements Cr, Ni, Cu, Zn, Rb, Sr, Y, Zr and Ba have been determined by X-ray fluorescence for 65 basaltic and differentiated lavas from Anjouan, while Sc, V, Cr, Co, Ni, Cu, Ga, Rb, Sr, Y, Zr, Nb, Cs, Ba and Hf have been determined by spark-source mass spectrometry for selected lavas from Anjouan and Grande Comore, the most recently formed of the Comores Archipelago. Basaltic lavas studied range through nephelinite, basanite, alkali basalt and hypersthene-normative basalt, while differentiated lavas belong mainly to the trends: alkali basalt - trachyte and basanite - phonolite. The results indicate that during magmatic fractionation behaviour of large-ion elements such as Rb, Sr, Y, Zr, Ba and Nb is controlled by size/charge criteria, resulting in their exclusion from crystallising phases until the late trachytic and phonolitic stages. These elements are clearly fractionated by amphibole, plagioclase and alkali feldspar. Variation of transition elements due to crystal-liquid differentiation is largely in accord with the predictions of crystal field theory. The behaviour of Zn is not readily accounted for. Fractionation of K/Rb, K/Cs and probably Zr/Nb and discrepancies in abundance levels of large-ion elements between the main basaltic types are best accounted for in terms of high-pressure processes and probably also reflect inherent features of source-region geochemistry, coupled with the effects of variable partial melting.     

Geochemistry and origin of basaltic lavas from Marquesas Archipelago,French Polynesia

The Marquesas Archipelago, a volcanic chain in French Polynesia (south-central Pacific Ocean), is predominantly composed of alkalic, transitional and tholeiitic basalts. The variation trends in these intraplate basaltic rocks imply that the magmas were derived from different upper mantle sources. Model calculations using the total inverse method show that the peridotite source of most Marquesas basalts was enriched in incompatible elements compared to a primordial mantle and had higher than chondritic ratios of several elements such as La/Yb, Ti/V and P/Ce. A metasomatic enrichment event is suggested by the sequence of element enrichment in the source relative to the primordial mantle (Ba>Nb>La>Ce>Sr>Sm>Eu> Zr>Hf>Ti>Y>Yb). On the other hand, some lavas including tholeiites of Ua Pou and alkalic basalts of Hiva Oa, were probably derived from relatively depleted upper mantle. In some islands such as Hatutu, the different types of basalts were generated from sources with rather similar compositions. The residual phases of the Marquesas magmas included garnet. The sources of these magmas were similar in trace element chemistry to the oceanic mantle below Hawaii.     

The Geochemistry of Lavas from the Gomores Archipelago, Western Indian Ocean: Petrogenesis and Mantle Source Region Characteristics

New mineral and bulk-rock analyses, as well as Nd, Sr and Pbisotope compositions are presented for lavas from Grande Comore,Moheli and Mayotte, thru of the four main islands of the ComoresArchipelago in the western Indian Ocean, and these data an usedto evaluate the petrogenesis, evolution and mantle source regioncharacteristics of Comorean lavas. The typically silica-undersaturated,alkaline lavas from all three islands can be grouped into twodistinct types: La Grille-type (LGT) lavas, which display strongrelative depletions in K, and Karthala-type (KT) lavas, whichdo not. With the exception of the lavas erupted by La Grillevolcano on Grande Comore, which exhibit the petrographic andgeochemical characteristics expected of primary mantle-derivedmagmas, all Comorean lavas analysed have experienced compositionalmodifications after they segregated from their source regions.Much of this variation can be explained quantitatively by fractionalcrystallization processes dominated by the fractionation ofolivineand clinopyroxene. Semi-quantitative modelling shows that theconsistent and fundamental difference in composition betweenK-depleted LGT lavas and normal KT lavas can be attributed topartial melting processes, provided amphibole is a residualmantle phase after extraction of LGT magmas at low degrees ofmelting. Low absolute abundances of the heavy rare earth elementsin LGT magmas are interpreted to reflect partial melting withinthe garnet stability field In contrast, KT magmas, which donot show relative K depletions, are considered to be the productsof somewhat larger degrees of partial melting of an amphibolefreesource at comparatively shallower depths. Whereas the Nd andSr isotopic compositions of Comorean lavas (which show a significantrange: ⁸⁷Sr/⁸⁶Sr = 0.70319–0.70393; ¹⁴³Nd/Nd = 0.51263–0.51288)bear evidence for a time-averaged depletion in incompatibleelements, the high incompatible element abundances of the lavasare interpreted to reflect the effects of a recent mantle enrichmentevent. At depths well within the garnet stability field thismantle enrichment is interpreted to have taken the form of modalmetasomatism with the introduction of amphibole (giving riseto the source of LGT magmas), whereas cryptic metasomatism tookplace at shallower levels (giving rise to the source of KT lavas).The Nd, Sr and Pb isotope signature of the majority of Comoreanlavas (both LGT and KT) is proposed to be the result of predominant4contributions from a somewhat heterogeneous source4 4 4 presentativeof the ambient sub-Comorean mantle, comprising a mixture betweena HIMU component and a component on the depleted portion ofthe mantle array (possibly the source of Indian Ocean MORB),with only limited contributions from an EM I plume component.The lavas erupted by Karthala volcano (the youngest Comoreanlavas), however, have significantly different isotopic compositionsfrom all other Comorean lavas (lower ¹⁴³Nd/¹⁴⁴Nd and higher⁸⁷Sr/⁸⁶Sr), suggesting increased contributions from the EM Icomponent. KEY WORDS: basalt petrogenesis; Comores; mantle geochemistry; ocean island basalts ^*Telephone: 27-21-6502921. Fax: 27-21-6503781 e-mail: alr{at}geology.uct.ac.za.     

Experimental constraints on the origin and evolution of mildly alkalic basalts from the Kerguelen Archipelago, Southeast Indian Ocean

This experimental study examines the role of clinopyroxene fractionation on major element trends and alkalinity variations in mildly alkalic basalts from the Kerguelen Archipelago, Southeast Indian Ocean. Equilibrium crystallization experiments were carried out on a natural basalt (MgO=5 wt.%, alkalinity index=0.10) over a range of pressures (0–1.43 GPa) and water contents (nominally dry to hydrous, 1.2 wt.% H₂O) under relatively oxidizing conditions (Δlog FMQ=+1 to +2) at 0 GPa and relatively reducing conditions (Δlog FMQ=0 to –2) at all higher pressures. The hydrous experiments at 0.93 GPa closely reproduce most of the compositional variations in the 24–25 Ma mildly alkalic lavas from the archipelago, which supports a major role for high-Al clinopyroxene fractionation (5–9 wt.% Al₂O₃) at pressures corresponding to the base of the Northern Kerguelen Plateau (15–20 km). However, clinopyroxene fractionation at depth fails to produce important changes in the alkalinity of the residual melts. The transition from tholeiitic to mildly alkalic basalts on the Kerguelen Archipelago thus reflects primarily changes in melting conditions (lower extents of partial melting at higher pressures), which is related to crustal and lithospheric thickening as distance from the Southeast Indian Ridge increased over time from 43 to 24 Ma.     

The origin of the Naturaliste Plateau,SE Indian Ocean: Implications from dredged basalts

Although a 1972 dredging by USNS Eltanin from the submarine Naturaliste Plateau was reported to yield rocks of continental origin, a re‐examination of the dredge haul shows that the rock clasts are in fact altered tholeiitic basalts. They have affinities both with MOR basalts and, especially, within‐plate basalts. Petrographically they correlate most closely with the Bunbury Tholeiitic Suite on the Australian mainland to the east. The basalts are reworked cobbles in a manganiferous Quaternary slump mass, which contains a quartz‐rich, felsic, detrital mineral suite with a granite‐gneiss provenance. The basalt cobbles were part of a basal conglomerate, which covered large areas of the Plateau. It is suggested that this was laid down from nearby elevated volcanic structures formed during the inception of seafloor spreading and the separation of Greater India from Australia at about 122 Ma BP.     

The origin of spongy texture in minerals of mantle xenoliths from the Western Qinling, central China

Spongy textures are observed in mantle peridotite xenoliths hosted in Cenozoic kamafugites from the Western Qinling, central China. These textures are mainly developed in clinopyroxenes and spinels, and occur as spongy rims consisting of low-Na clinopyroxene, ilmenite, and bubbles, enclosing nonspongy cores. The ilmenites and bubbles exhibit shapes and sizes that vary with the width of the spongy rims. The spongy-textured minerals preserve primary shapes and well-defined grain boundaries and do not show apparent interaction with contact minerals or observed melts except the subsequent melts forming melt pockets. The xenocrysts display reactive zoning textures with host magmas rather than spongy textures. Compositionally, the spongy rims are enriched in Ca, Ti, and most trace elements, have high Cr#, and are depleted in Na, Al, Fe, Al^VI, and Al^IV/Al^VI compared with the cores. These observations suggest that melts/host magmas did not play any significant role in the formation of the spongy textures. We therefore propose that spongy-textured clinopyroxenes and spinels in Western Qinling peridotite xenoliths developed from a decompression-induced partial melting event prior to formation of melt pockets and xenolith entrainment in host magmas.     

Petrology of the prehistoric lavas and dyke of the Barren Island,Andaman Sea,Indian Ocean

Although Barren Island (Andaman Sea, Indian Ocean) witnessed several volcanic eruptions during historic times, the eruptions that led to the formation of this volcanic island occurred mainly during prehistoric times. It is still active and currently in the fumarolic stage. Its volcanic evolution appears to be characterized by a constructive phase with the piling up of lava flows and scoria deposits and Strombolian activities, followed by a sudden collapse of the main cone. Deposits of a possible caldera-forming eruption were not recognized earlier. After a period of peri-calderic hydromagmatic activity, whose deposits presently mantle inner and outer caldera walls, a new phase of intracalderic Vulcanian activities took place. A prominent dyke in the SE inner side of the caldera wall was recognized. Petrographically the lava flows and dyke are similar but they differ in their chemical composition (viz., SiO₂, MgO, Ni, Cr) significantly. Similarity in major, minor and trace element composition (viz., K/La, K/Nb, K/Rb, K/Ti ratios) of these rocks together with Chondrite normalized trace element (Rb, Ba, Sr, P, Zr, Ti and Nb) and REE (La, Ce, Nd and Y) patterns of the Barren Island prehistoric lava flows and dyke and low-K lavas of Sunda Arc indicates that Barren Island must have evolved from a source similar to that of Sunda Arc lavas during the Quaternary Period.     

辽西阜新中生代玄武岩中深源捕虏体的岩石学和矿物化学研究

辽西阜新中生代碱性玄武岩的Ｋ－Ａｒ同位素年龄为８４．７６±１．６７Ｍａ, 其中含有丰富的深源岩石捕虏体, 包括角闪尖晶二辉橄榄岩、尖晶石二辉橄榄岩、含长角闪二辉石岩、二辉麻粒岩和辉石斜长片麻岩等。与新生代玄武岩中的同类捕虏体相比, 橄榄岩类捕虏体中橄榄石的成分相似 （Ｆｏ＝ ８６～９１）, 透辉石具有富Ａｌ、贫Ｃｒ 和Ｃａ 的特点, 尖晶石的成分以ＭｇＡｌ２ Ｏ４ 为主, 有的橄榄岩捕虏体中含少量韭闪石。二辉麻粒岩捕虏体中的斜方辉石为紫苏辉石, 单斜辉石为普通辉石, 斜长石属于更长石。橄榄岩类捕虏体的平衡温度为１０１２．６℃～１２７２．１℃; 压力为１．８２５～２．９３５ＧＰａ, 二辉麻粒岩和片麻岩的平衡温度为８９２℃～１１５７℃。该类深源岩石捕虏体的发现对中国东部中生代上地幔和下地壳的研究均具有重要意义。     

Rare earth abundances and Rb-Sr systematics of basalts,gabbro, anorthosite and minor granitic rocks from the Indian Ocean Ridge System,Western Indian Ocean

Basalts dredged from the Mid-Indian Ocean Ridge System have rare earth, Rb, and Sr concentrations like those from other mid-ocean ridges, but have slightly higher Sr⁸⁷/Sr⁸⁶ ratios. Underlying gabbroic complexes are similar to the basalts in Sr⁸⁷/Sr⁸⁶, but are poorer K, Rb, and in rare earths. The chemical and isotopic data, as well as the geologic relations suggest a cumulate origin for the bulk of the gabbroic complexes.     

Minette lavas and associated leucitites from the western front of the Mexican Volcanic Belt: petrology,chemistry, and origin

During the late Pliocene, K-rich minette and leucitite lavas erupted in the western Mexican Volcanic Belt near the town of Los Volcanes, a region which is located much closer to the Middle America Trench than the main line of currently active andesite stratovolcanoes. During this period the tectonic regime in western Mexico was highly complex due to the simultaneous occurrence of active subduction of the young Rivera Plate, and rifting caused by crustal extension. Most of these basic lavas contain phenocrysts of phlogopite, augite and apatite, along with microphenocrysts of leucite and Fe-Ti oxide. Olivine is absent from all but two of the flows: one an olivine leucitite, and the other a felsic minette. The phlogopite phenocrysts and high whole rock Fe₂O₃/FeO ratios which are characteristic of this suite record evidence of high magmatic water contents and oxygen fugacities. All of these rock types are highly enriched in the large ion lithophile and light rare earth elements, with Sr5100 ppm, Ba4800 ppm and Ce330 ppm. They are also mildly enriched in the high field strength elements (e.g. Zr 260–700 ppm) and display the strong relative enrichment of the LILE over the HFSE that is characteristic of magmas erupted in convergent margins. Consideration of high pressure phase equilibria in the Mg₂SiO₄-CaMgSi₂O₆-KAlSiO₄-SiO₂ system shows that the minettes from this region are not related through fractional crystallization to the more MgO-rich, olivinebearing minettes which have erupted in other parts of the western Mexican Volcanic Belt during the Quaternary. This conclusion is consistent with both the trace element geochemistry of these lavas and with the results of fractional crystallization models. Instead, the data suggests that these high-K magmas were derived from a source region which consists predominantly of phlogopite, clinopyroxene and apatite, and which has formed through hydrous enrichment of the subarc mantle in response to subduction.     

Sulfide-impregnated and pure silica precipitates of hydrothermal origin from the Central Indian Ocean

This is the first report about silica-rich hydrothermal precipitates which were sampled together with hydrothermal sulfides (chimney fragments) in an extinct vent field in the Central Indian Ocean. There are two kinds of silica-rich rocks: a jasper which is impregnated and replaced to various degrees mainly by sphalerite, and to a lesser extent by barite, pyrite and traces of chalcopyrite, and an opalite which is an almost pure silica-phase without any sulfide or sulfate impregnations, but which is sometimes covered by manganese crusts.No internal concentric zoning indicating typical chimney structures can be recognized in the jasper and/or opalite samples, the textures rather suggest a sedimentary silica and/or iron deposition from diffuse, low-temperature (±60 °C) vent fluids, partly with still visible indications of former bacterial mats and synsedimentary deformation structures; the sphalerite- and barite-impregnations within the jasper, however, are considered to have precipitated from white-smoker-type fluids since they were deposited under intermediate temperatures between 155 and 265 °C, according to fluid inclusion studies.The sulfur isotopic composition (δ³⁴S) of our sulfide samples has mean values of 6.1% for sphalerite and 5.7% for pyrite indicating a mixture of predominantly basaltic sulfur with subordinate amounts of reduced seawater sulfur. The oxygen isotope signals of some pure jasper concentrate samples indicate that the mean formation temperature calculated from these values lies at 63.2 °C.The relationship between the massive pyrite- and chalcopyrite-ores from the extinct chimney structures and the silica-rich precipitates can be explained by different cycles of hydrothermal activity: one high-temperature (above 300 °C) cycle dominated by pyrite and chalcopyrite formation and one later epithermal (below 300 °C) cycle which resulted in sphalerite- and silica-dominated precipitates. Furthermore, zonation and zone-refining processes are part of the evolution of the mineralized field.     

Geodynamics of the Amirante Ridge and Trench Complex,Western Indian Ocean

The formation and evolution of the ～600 km long arcuate Amirante Ridge and Trench Complex (ARTC) is a significant geomorphic–structural feature in the Western Indian Ocean (WIO). The WIO contains evidence of at least two major magmatic episodes followed by continental rifting within the span of a little more than 20 million years. This involved the splitting of Madagascar from India at around 85 Ma and then separation between India and the Seychelles at 64–63 Ma as a possible consequence of two powerful volcanic eruptions from the Marion and Reunion hot spots, respectively. Formation and evolution of the ARTC represents this tumultuous period in the Indian Ocean, approximately between 85 and 60 Ma (Late Cretaceous–Early Tertiary).

We integrated geophysical, palaeomagnetical, and petrological data to examine three existing models that attempt to explain the formation of ARTC. In contrast, our study hints at several stages of extension and compression responsible for its formation. Our integrated data also suggest that the Carlsberg Ridge may have played a prominent role in the evolution of the ARTC that seems to have formed through a ridge-jump process after the conjugate spreading centres – Mascarene and Palitana ridges formed earlier during the India–Madagascar separation – ceased spreading because of violent eruption of the Reunion hot spot at around 65 Ma. The eruption disturbed the plumbing system of magma ascent, resulting in cessation of spreading along the conjugate spreading centres, forcing a ridge jump.

A collage of seismic refraction and reflection, free-air gravity, magnetic anomaly data, and Ar dating of rocks indicates that as the Carlsberg Ridge swept the Seychelles towards south, the crust between Madagascar and the Seychelles was increasingly compressed, with the abandoned northern Mascarene spreading centre absorbing the maximum stress. With continued compression, the western limb of the abandoned spreading ridge was thrust below the eastern limb to a limited degree. This partial subduction agrees with the gravity and seismic results. Our new study also accounts for the anomalous presence of 14 km-thick oceanic crust beneath the ARTC and its characteristic difference in petrology with other established subduction zones in the world.     

Petrogenesis of ultramafic and mafic xenoliths from Mesozoic basanites in southern Sweden: constraints from mineral chemistry

Jurassic basanite necks occurring at the junction of two major fault zones in Scania contain ultramafic (peridotites, pyroxenites) and mafic xenoliths, which together indicate a diversity of upper mantle and lower crustal assemblages beneath this region. The peridotites can be subdivided into lherzolites, dunites and harzburgites. Most lherzolites are porphyroclastic, containing orthopyroxene and olivine porphyroclasts. They consist of Mg-rich silicates (Mg# = Mg/(Mg + Fe_tot) × 100; 88–94) and vermicular spinel. Calculated equilibration temperatures are lower in porphyroclastic lherzolites (975–1,007°C) than in equigranular lherzolite (1,079°C), indicating an origin from different parts of the upper mantle. According to the spinel composition the lherzolites represent residues of 8–13% fractional melting. They are similar in texture, mineralogy and major element composition to mantle xenoliths from Cenozoic Central European volcanic fields. Dunitic and harzburgitic peridotites are equigranular and only slightly deformed. Silicate minerals have lower to similar Mg# (83–92) as lherzolites and lack primary spinel. Resorbed patches in dunite and harzburgite xenoliths might be the remnants of metasomatic processes that changed the upper mantle composition. Pyroxenites are coarse, undeformed and have silicate minerals with partly lower Mg# than peridotites (70–91). Pyroxenitic oxides are pleonaste spinels. According to two-pyroxene thermometry pyroxenites show a large range of equilibration temperatures (919–1,280°C). In contrast, mafic xenoliths, which are mostly layered gabbronorites with pyroxene- and plagioclase-rich layers, have a narrow range of equilibration temperatures (828–890°C). These temperature ranges, together with geochemical evidence, indicate that pyroxenites and gabbroic xenoliths represent mafic intrusions within the Fennoscandian crust.     

Magma evolution beneath Bequia,Lesser Antilles,deduced from petrology of lavas and plutonic xenoliths

Extrusive and intrusive igneous rocks represent different parts of a magmatic system and ultimately provide complementary information about the processes operating beneath volcanoes. To shed light on such processes, we have examined and quantified the textures and mineral compositions of plutonic and cumulate xenoliths and lavas from Bequia, Lesser Antilles arc. Both suites contain assemblages of iddingsitized olivine, plagioclase, clinopyroxene and spinel with rare orthopyroxene and ilmenite. Mineral zoning is widespread, but more protracted in lavas than xenoliths. Plagioclase cores and olivine have high anorthite (An?≤?98) and low forsterite (Fo?≤?84) compositions respectively, implying crystallisation from a hydrous mafic melt that was already fractionated. Xenolith textures range from adcumulate to orthocumulate with variable mineral crystallisation sequences. Textural criteria are used to organize the xenoliths into six groups. Amphibole, notably absent from lavas, is a common feature of xenoliths, together with minor biotite and apatite. Bulk compositions of xenoliths deviate from the liquid line of descent of lavas supporting a cumulate origin with varying degrees of reactive infiltration by evolved hydrous melts, preserved as melt inclusions in xenolith crystals. Volatile saturation pressures in melt inclusions indicate cumulate crystallization over a 162–571 MPa pressure range under conditions of high dissolved water contents (up to 7.8 wt% H₂O), consistent with a variety of other thermobarometric estimates. Phase assemblages of xenoliths are consistent with published experimental data on volatile-saturated low-magnesium and high-alumina basalts and basaltic andesite from the Lesser Antilles at pressures of 200–1000 MPa, temperatures of 950–1050 °C and dissolved H₂O contents of 4–7 wt%. Once extracted from mid-crustal mushes, residual melts ascend to higher levels and undergo H₂O-saturated crystallization in shallow, pre-eruptive reservoirs to form phenocrysts and glomerocrysts. The absence of amphibole from lavas reflects instability at low pressures, whereas its abundance in xenoliths testifies to its importance in mid-crustal differentiation processes. A complex, vertically extensive (6 to at least 21 km depth) magmatic system is inferred beneath Bequia. Xenoliths represent fragments of the mush incorporated into ascending magmas. The widespread occurrence of evolved melts in the mush, but the absence of erupted evolved magmas, in contrast to islands in the northern Lesser Antilles, may reflect the relative immaturity of the Bequia magmatic system.     

Petrography,mineral chemistry and origin of Type I enstatite chondrites

Optical and cathodoluminescence petrography were coupled with electron microprobe analysis to relate the textures and chemical compositions of minerals in the chondrules and matrix of the Indarch, Kota-Kota, Adhi-Kot and Abee Type I enstatite chondrites. Clinoenstatites fall into two distinct chemical groups with characteristic red or blue luminescence; red crystals are higher in Ti, Al, Cr, Mn and Ca, and lower in Na, than blue ones. Rare forsterites in Indarch and Kota-Kota show distinct compositions associated with orange or blue luminescence. The chemical ranges are indistinguishable for each color type in chondrules of all textural types, and the presence of both color types in a single chondrule or a metal fragment requires mechanical aggregation of both crystals and liquids of both color types. Porphyritic chondrules are ascribed mainly to aggregation of existing crystals because both types of pyroxene and olivine occur in the same chondrule. Large crystals of one color type are surrounded by fine-grained crystals of another type in some barred and radiating chondrules. All types of chondrules are surrounded by fine-grained rims rich in sulfide. The matrix contains many broken chondrules and individual silicate grains but is rich in sulfide and metal. Analyses are given of albite (minor elements and luminescence color vary between chondrites), kamacite, schreibersite, oldhamite and niningerite.Although the mineral assemblages do not fit theoretical condensation sequences in detail, the red pyroxene and orange olivine might result ultimately from near-equilibrium crystallization in which early reduced condensates reacted with a gas, while the blue crystals might result from fractional condensation in which early condensates were removed mechanically from a gas. Subsequent episodes involving mixing, melting, crystallization, condensation, fracturing, and mechanical aggregation would be needed to produce the complex textures.     

Petrology and origin of primitive lavas from the Troodos ophiolite,Cyprus

Parental magmas to the Troodos ophiolite are characterised by low TiO₂ and Al₂O₃ and high SiO₂. Extremely fresh and chemically primitive (high MgO) rocks are found within the Upper Pillow Lavas and along the Arakapas Fault Belt of Cyprus and contain forsteritic olivine±enstatite and groundmass clinopyroxene set in glass or plagioclase, with accessory magnesiochromite and sometimes hornblende. They are quartz-normative and may have originally contained up to 3 wt% H₂O. Geochemically, there are three distinct groups of primitive lavas, based on TiO₂ and Zr contents but also reflected by CaO, Na₂O and REE abundances. These groups cannot be related by crystal fractionation and are considered to have been generated by incremental melting of a variably depleted source region. The parental magma to the least depleted group (Group I) was that of the major portion of the Troodos plutonic complex and is similar to those postulated for other low-Ti ophiolites. Chemically it has close affinities with komatiitic basalts. The most depleted lavas (Group III) all have U-shaped REE profiles and variable ¹⁴³Nd/ ¹⁴⁴Nd ratios, interpreted in terms of metasomatism of the source region by an incompatible element-enriched component which was probably derived from a subducted slab. These lavas represent an intermediate step in the development of boninite series rocks.     

LREE distribution in perovskite,apatite and titanite from South West Ugandan xenoliths and kamafugite lavas

Summary In-situ microprobe LREE analyses of perovskite and titanite (La, Ce, Nd), and apatite (La, Ce), from SW Ugandan clinopyroxenite xenoliths and kamafugite lavas indicate that LREE distribution in these minerals is determined by a number of factors related to their different parageneses: In particular LREE content is affected by whether the LREE-bearing minerals have crystallised from metasomatic carbonate or from silicate (i.e. metasomatic or magmatic) melts in the mantle. In this situation LREE partition favours carbonate over silicate melts. Distribution of LREE in perovskite and apatite crystallised from magmatic mantle melts or mantle-derived lavas is chiefly determined by preference of LREE for perovskite > apatite > titanite. LREE zoning in perovskite is influenced by changes in melt structure: increasing melt polymerisation enhancing mineral_LREE/melt_LREE partition into perovskite rims in magmatic xenoliths; decreasing melt polymerisation depleting LREE in lava perovskite rims. This zoning is reinforced by perovskite competition with apatite for LREE: perovskite (cores/rims) co-crystallising with apatite is reduced in LREE. There are 37 instances of perovskitewith Ce below detection while La and Nd levels are normal. These occur in both xenoliths and lavas; in grain zones or whole grains. Likewise Ce alone of the LREE is below detection in six out of ten titanite analyses. These observations are interpreted as evidence for increased f_{O 2}, Ce⁴ + being excluded from these mineral structures. Recognition of these various processes can elucidate the interpretation of bulk rock and bulk mineral LREE signatures in kamafugite volcanism.

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LREE Verteilung in Perovskit, Apatit und Titanit aus Xenolithen und kamafugitischen Laven Südwest-Ugandas

Zusammenfassung In-situ LREE Analysen von Perovskit und Titanit (La, Ce, Nd) und Apatit (La, Ce) aus Klinopyroxenit-Xenolithen und kamafugitischen Laven Südwest-Ugandas zeigen, daß die LREE Verteilung in diesen Mineralen durch eine Vielzahl von Faktoren, die mit Unterschieden in den Paragenesen zusammenhängen, bestimmt wird: Der LREE-Gehalt wird im besonderen davon bestimmt, ob die LREE-führenden Minerale aus metasomatischen Karbonat- oder aus (metasomatischen oder magmatischen) Silikatschmelzen im Mantel auskristallisierten. Dabei erfolgt die LREE Fraktionierung zu Gunsten der Karbonatschmelzen. Die LREE-Verteilung von Perovskit und Apatit, die aus magmatischen Mantelschmelzen oder -laven kristallisierten, wird vorrangig durch den bevorzugten Einbau der LREE in Perovskit > Apatit > Titanit kontrolliert. Der LREE Zonarbau von Perovskit wird durch die Änderungen der Schmelzstruktur beinflußt: Verstärkte Schmelzpolymerisation führt zu verstärkter Mineral_LFEE/Schmelze_LREE Fraktionierung in den Perovskiträndern magmatischer Xenolithe, eine Abnahme der Schmelzpolymerisation hingegen resultiert in einer Abreicherung der LREE in den Perovskiträndern. Diese Art der Zonierung wird durch den Wettbewerb von Perovskit mit Apatit um die LREE verstärkt. Perovskit (Kerne/Ränder), der mit Apatit gemeinsam auskristallisierte, ist ärmer an LREE. 37 Fälle, in denenCe nicht nachweisbar war, La und Nd aber in normaler Konzentration auftreten, wurden sowohl in den Xenolithen als auch in den Laven gefunden; und zwar entweder in Kornbereichen oder in ganzen Körnern. Vergleichsweise liegt Ce nur in sechs von zehn Titanitproben unterhalb der Nachweisgrenze. Diese Beobachtungen werden als Hinweise auf erhöhte SauerstoffFugazitäten, bei denen Ce^4– aus der Mineralstruktur ausgeschlossen wird, angesehen.Ein Verständnis dieser verschiedenen Prozesse kann zur besseren Interpretation von LREE Gesamtgesteins- und Gesamtmineral-Signaturen in Kamafugiten beitragen.

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With 3 Figures