The geology and petrology of Bouvetøya,south Atlantic Ocean

The Quaternary island Bouvetøya is situated near the triple junction on the South Atlantic Ridge. It is the top of a volcano of about 2000 km⁸, evolved beyond the stage of caldera collapse. Basic, intermediate and peralkaline silicic rocks of a transitional rock series are recorded. Major and trace element chemistry, model calculations and petrographic observations do not favour a closed system fractional crystallization from one parent. The rocks may have evolved from slightly different parents by fractional crystallization under varying P,T and P_H2O conditions, or from one parent modified by crustal contamination and/or volatile transfer.     

Mantle peridotites from the Bouvet Triple Junction Region, South Atlantic

ABSTRACT The Bouvet Triple Junction (BTJ) region in the South Atlantic, where the African, South American and Antarctica plates meet, is affected by several topographic/melting anomalies. Causes of these anomalies were investigated through a study of mantle-derived serpentinized peridotites sampled from three sites in the BTJ region: (1) the Inner Corner High at the intersection of the America Antarctic Ridge (AAR) with the Conrad transform; (2) the south wall of the Bouvet transform (South West Indian Ridge, SWIR); and (3) the eastern Bouvet SWIR Transform Intersection. The degree of melting undergone by these rocks was estimated from relic mineral major- and trace-element composition. Geochemical profiles from residual peridotites and associated basalts show a > 1000-km-wide melting anomaly centred on the Bouvet and Spiess topographic anomalies.     

A microearthquake survey of Bouvet Island, South Atlantic

A microearthquake survey carried out during the 1978–1979 Norwegian Antarctic Research Expedition indicates that only minor seismic activity is associated with the Bouvet volcano.     

Geochemistry and Petrology of Emeishan Basalts and Subcontinental Mantle Evolution in Southwestern China

Three major volcanic rock sequences in the P2β formation(Emeishan basalts)were sampled dur-ing a comprehensive study of the Late Permian volcanics associated with the Panxi paleorift in southwestern China .Two of the three sections-Emei and Tangfang are composed of continental flood basalts(CFB) while the third-Ertan is an alkalic center.Multi-element chemical analyses indi-cate a predominance of low MgO transitional quartz tholeiites at Emei and Tangfang,whereas the Ertan suite ranges from high-MgO alkaline olivine basalts to rhombic porphyry trachytes and quartz-bearing aegerine-augite syenites.Consanguineity of the rocks from the three sections is sug-gested by consistently high TiO2 ,K2O,incompatible trace elements and uniformly fractionated REE patterns typical of alkalic compositions,but antypical of CFB.Sr isotope data for ten Emei basalt samples(^87Sr/^86Sr=0.7066-0.7082)which show no correla-tion with Rb/Sr ratios (0.02-0.12) and Nd isotopes for two of the samples(^143Nd/^144Nd=0.51171-0.51174)are interpreted as being related to the mantle evolution.The primary magmas re-sponsible for all the three sequences have been modeled in terms of a uniformly metasomatized man-tle source.Trace element models support the derivation of the Emei and Tangfang primary magmas from 10-15 percent partial melting of spinel lherzolite,followed by fractional crystallization of olivive and clinopyroxene.The primary alkaline olivine basalts at Ertan are generated by 7-10 percent par-tial melting of a chemically equivalent source in the garnet-peridodite stability region.The assumed mantle composition is characterixzed by Rb=3.8-5.5 ppm,Sr=62-83ppm,Ba=45-64 ppm,La=3.8-5.6ppm,and Yb=0.46-0.57ppm.The proposed mechanism of regional mantle enrichment requires metasomatic stabilization of phlogopite which becomes depleted later during par-tial melting.Such enrichment is consistent with the models proposed for alkalic systems in which a large mantle diaper acts as the agent for upward enrichment as well as uplift and extension of the crust.     

Geochemistry of the Taitao ophiolite and near-trench intrusions from the Chile margin triple junction

The Taitao ophiolite of southern Chile lies 10 km from the buried extension of the Peru-Chile trench, and less than 50 km from the present position of the Nazca/South America/Antarctica triple junction. Plio-Pleistocene radiometric and paleontologic ages indicate its formation during ridge subduction, and an ultramafic rock, gabbro, sheeted dike, volcanic and sedimentary rock psuedostratigraphy suggests formation by typical accretionary processes for oceanic lithosphere. Yet major and trace element data show that mafic dikes and volcanic units are transitional from MORB to IAT, and there are abundant silicic volcanic units of calc-alkaline character that have high LIL element and light REE concentrations relative to oceanic plagiogranites. Sr and Nd isotopic data are consistent with that of modern oceanic suites, even though having a greater internal variability. Silicic volcanic units show the more enriched Sr and depleted Nd isotopic ratios relative to dike and gabbro samples. In addition to chemical distinctions, paleobathymetric data support a shallow water origin for some of the upper volcanic units and, assuming local compensation, suggest crustal thicknesses of continental proportion. In the vicinity of the Taitao ophiolite, and extending some 40 km landward of the plate margin, are a series of silicic stocks, sills, and plutons that were intruded into the forearc at the time of ridge collision and ophiolite generation. These calc-alkaline I-type granitoids are light REE enriched and have Sr and Nd isotopic compositions similar to those of the main volcanic chain 200 km landward. Chemically, some of the silicic intrusions are indistinguishable from volcanic units of the ophiolite. In general, major, trace, REE, and isotopic variations of both the ophiolite and the distributed intrusions are atypical of simple fractionation trends for basaltic liquids. Intermediate to silicic units lie along mixing hyperbolae between Taitao gabbro and either forearc sediment or metamorphic basement on a Nd---Sr correlation diagram, and these two crustal components support, respectively, either a 10–25% or 5–10% assimilation. Shutdown of magmatism, and therefore probably partial melting as well, appears to occur within 40 km of the trench, roughly spanning the depth interval for the disappearance of the plagioclase-lherzolite stability field as the zone of mantle upwelling is overridden by an increasing thickness of continental lithosphere. A deeper and more landward absence of partial melt related to the subducted ridge is supported by the correlation of the shutoff and re-initiation of arc volcanism over the northern and southern trailing edges of the postulated subcontinental asthenospheric window. Here, as well as elsewhere in the circum-Pacific, the general restriction of magmatism related to ridge subduction to near-trench settings supports a shallow (0 to 15 km) shutoff mechanism for adiabatic decompressive melting and a rather abrupt return to single-phase (solid) convective rise of mantle into an evolving asthenospheric window.     

Geochemistry and petrology of the late cenozoic alkali basalts of the western Carpathians (Czechoslovakia)

Summary Miocene to Quaternary alkali volcanism in the Western Carpathians followed calcalkali volcanism in the same region. The alkali volcanics have restricted composition ranging from alkali basalts and basanites to rare trachyandesites. They contain ultramafic xenoliths, megacrysts and xenocrysts of olivine, pyroxene and amphibole. Pyroxene xenocryst cores differ in color compared to pyroxene phenocrysts. Alkali basaltic rocks are enriched in LILE, LREE (La_n/Yb_n = 10–19) and HFS elements. Based on major and trace element distribution and Nd, Sr and Pb isotopic composition, we suggest that metasomatically-enriched manlte peridotite was the source material of the alkali magmas. The range in composition of rocks and their minerals was probably controlled by (1) inhomogenities in the mantle-source and/or different degrees of melting, and (2) metasomatism associated with infiltration of alkali basalt magma through peridotite during ascent through the subcontinental lithosphere. The origin of fluids metasomatically enriching the mande is unclear. Their derivation from the subducted slab cannot be excluded.

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Geochemie und Petrologie jungkänozoischer Alkali-Basalte der Westkarpaten (Tschechoslowakei)

Zusammenfassung Der miozäne bis quartäre alkalische Vulkanismus der Westkarpaten folgt in Raum und Zeit auf den Kalk-Alkali-Vulkanismus. Die Zusammensetzung der alkalischen Vulkanite verändert sich nur in einem begrenzten Ausmaß von überwiegend alkalischen Basalten und Basaniten zu seltenen Trachyandesiten. Die Vulkanite enthalten ultramafische Xenolithe, Megakriste und Xenokriste von Olivin, Pyroxen und Amphibol. Die Pyroxen-Xenokriste bilden Kerne abweichender Farbe in einigen Pyroxen-Phenokristen. Die alkalischen Basaltgesteine sind an LILE, LREE (La_n/Yb_n = 10–19) und HFS Elementen angereichert. Aufgrund der Haupt- und Spurenelementverteilung, sowie der Isotopen-Zusammensetzung von Nd, Sr un Pb kann angenommen werden, daß das Ursprungsmaterial der alkalischen Magmen ein metasomatisch angereicherter Mantel-Protolith war. Die in der Zusammensetzung der Gesteine und Minerale beobachtete Variabilität ist wahrscheinlich das Resultat der (1) Inhomogenität des Ursprungsmaterials und/oder des unterschiedlichen Aufschmelzungsgrades und (2) metasomatischen Erscheinungen in Verbindung mit der Infiltration des alkalibasaltischen Magmas in Mantel-Peridotite während ihres Emporsteigens durch die subkontinentale Lithosphäre. Der Ursprung des Fluids, welches den Mantel metasomatisch angereichert hat, ist unklar. Eine Herkunft aus der subduzierenden Platte kann nicht ausgeschlossen werden.

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Magmatic and hydrothermal processes in the Bouvet Triple Junction Region (South Atlantic)

Results of electron microprobe and microthermometric studies of samples collected from the Bouvet Triple Junction Region (BTJR) during a joint Russian-Italian geological expedition on the R/V Academician Nikolaj Strakhov (1994) have revealed new data on the composition of basaltic magmas and oceanic hydrothermal fluids connected with magmatic processes. Detailed analysis of basaltic glasses shows that the modem Mid-Atlantic Ridge (MAR) rift valley is composed of normal mid-ocean ridge basalts with low concentrations of K₂ O and TiO_z (N-MORB), while its flanks are more enriched with these components approaching E-MORB. A marked influence of the Bouvet hot spot volcanism on magma generation on the South-West Indian Ridge (SWIR) near Bouvet Island is observed. Basaltic melts in this area belong to alkalic and transitional series and have maximum contents of K₂O, TiO₂, H₂O.
Microthermometric analyses of fluid inclusions in the samples from the BTJR have revealed major differences in the oceanic hydrothermal fluid systems on the MAR and near SWIR, which depends on the peculiarities of magma. In the area of the MAR (with dry melts) only H₂O solution inclusions in quartz were found; thus, seawater is probably the only primary source of hydrothermal fluids (NaCl + MgCl₂+ H₂O; T = 170–200°C). In the SWIR area (with the high content of water in melts) syngenetic liquid CO₂ and H₂O solution inclusions in quartz indicate the influence of the magmatic fluid component on the ore-forming water/carbon dioxide solutions (NaCl + CaC1₂+ H₂O + CO₂; T = 200–310 °C; P = 900–1700 bar).     

Geochemistry, Mineralogy and Magmatic Evolution of the Basaltic and Trachytic Lavas from Gough Island, South Atlantic

The Gough Island lavas range from picrite basalt through tosodalite-bearing aegirine-augite trachyte. The basaltic lavasare predominantly nepheline normative alkali basalts, althougha group of hypersthene normative tholeiitic basalts does occur.The oldest lavas on the island, represented by the Lower Basaltseries, are approximately 1?0 m.y. old and the youngest arethe Upper Basalts with an age of {small tilde} 0?13 m.y. Relatively coherent variations are described by the basalticand trachytic lavas with respect to both bulk rock major andtrace element geochemistry and mineral chemistry, and quantitativepetrogenetic modelling suggests that most of the variation canbe attributed to crystal fractionation/accumulation processesacting on a number of geochemically distinct parental magmas.The Upper Basalts and Lower Basalts have (within the limitsof sampling) a relatively restricted composition compared tothe Middle Basalt series lavas, with the latter ranging frompicrite basalt through to trachyandesite. The picrite basaltsand coarsely pyroxene-olivine phyric basalts represent partialcumulates with varying proportions (up to 40 wt. per cent) ofaccumulated olivine and clinopyroxene. In contrast, the moderatelyphyric and aphyric/finely porphyritic lavas represent the productsof crystal fractionation with the most evolved lavas havingexperienced at least 40 per cent fractional crystallizationof clinopyroxene, olivine, plagioclase and minor Fe-Ti oxidesand apatite. The detailed abundance variations in these lavasindicate that a number of parental magma compositions have fractionatedto produce the overall variations in basalt geochemistry, andsome of the magmas have interacted through mixing processes. The trachytic lavas show a large range in trace element abundance,but have only a limited major element variation. Most of thisvariation can be attributed to extensive (up to 70 per cent)fractional crystallization of predominantly alkali feldsparwith minor clinopyroxene, olivine, biotite, titano-magnetiteand apatite. A number of genetically distinct trachytes canbe recognized which are probably not related to each other byany simple fractional crystallization process. The compositionof the least evolved trachytes can be adequately accounted forby relatively extensive (up to 60 per cent) fractionation ofthe more evolved Middle Basalt series lavas. The trace element and isotopic characteristics of primitiveGough Island basalts support the concept that the source region(s)giving rise to these lavas is extremely enriched in highly incompatibleelements relative to primordial or ‘undepleted’mantle of bulk earth composition. It is unlikely that the lavashave sampled undepleted mantle as might be suggested by thesimilarity of the Sr and Nd isotopic ratios to ‘bulk earth’values. Rather, a model is favoured whereby the lavas are derivedfrom previously enriched sub-oceanic mantle which was subsequentlyinvaded and further enriched, at some time prior to partialmelting, by melts or fluids highly enriched in incompatibleelements. The enrichment could have occurred as veining by smalldegree partial melts or by infiltration of metasomatic fluids.     

Geochemistry and petrology of ultramafic lamprophyres from Schirmacher Oasis, East Antarctica

Summary Petrographic and geochemical data are presented for some ouachitites from the Schirmacher Oasis. The studied dike intruded into the metamorphic basement of the East Antarctic craton. The ouachitite samples contain substantial amounts of zoned phlogopite and diopsidic to salitic pyroxene as phenocrysts. K-rich nepheline is the predominant phase of the matrix. The rocks are characterized by low SiO₂ concentrations (up to 36.5 wt.%), low mg# and high abundances of Al²O³, alkalis and volatiles. Trace element compositions of the Schirmacher ouachitites differ from those of ultramafic lamprophyres from other regions in that they contain significantly lower Cr and Ni concentrations and relatively low enrichments in LILE and LREE. The studied rocks have low¹⁴³Nd/¹⁴⁴Nd and enhanced⁸⁷Sr/⁸⁶Sr ratios. Pb isotope compositions of the ouachitites closely resemble those of MORB. The ¹⁸O values range between 4.1 and 4.7⁰/₀₀ which may be due to interactions with meteoric water. Because the Schirmacher ouachitites are only represented by a few samples it is impossible to discuss the petrogenesis of this magma. However, according to trace element and isotopic characteristics, it can be ruled out that Schirmacher ouachitites and associated minettes (documented inHoch, 1997;Hoch andTobschall, 1998) were derived from the same mantle reservoir.

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Geochemie und Petrologie von Ouachititen aus der Schirmacher Oase, Ostantarktika

Zusammenfassung In dieser Arbeit werden petrologische und geochemische Daten von Ouachititen aus der Schirmacher Oase vorgestellt. Der beprobte Gang intrudierte in das metamorphe Basement des ostantarktischen Kratons. Zonierte Phlogopite und diopsidische bis salitische Pyroxene sind die häufigsten mafischen Einsprenglinge. K-reiche Nepheline bilden die Hauptmineralphase der Grundmasse. Charakteristisch für die Ouachititproben sind niedrige SiO₂-Konzentrationen (bis 36.5 Gew.%) und mg#, aber hohe Gehalte an Al₂O₃ Alkalien und volatilen Bestandteilen. Die Spurenelementzusammensetzung der Schirmacher Ouachitite unterscheidet sich von UML-Proben aus anderen Regionen durch ihre extrem geringen Cr- und Ni-Konzentrationen und die vergleichsweise niedrigen Anreicherungen an LILE und LREE. Die bearbeiteten Gesteine zeigen niedrige¹⁴³Nd/¹⁴⁴Nd- und erhöhte⁸⁷Sr/⁸⁶Sr-Verhältnisse. Die Pb-Isotopie der Ouachitite entspricht der von MORB. 6180-Werte variieren zwischen 4.1 und 4.7⁰/₀₀, was auf eine eventuelle Beeinflussung durch meteorische Wässer hindeutet. Da von den Schirmacher Ouachititen nur wenige Proben für eine Bearbeitung zur Verfügung standen, ist es nicht möglich, die Petrogenese des Magmas zu diskutieren. Aber aufgrund der Spurenelement- und Isotopensignaturen kann ein gemeinsames Mantelreservoir der Schirmacher Ouachitite und Minette (beschrieben inHoch, 1997;Hoch undTobschall, 1998) ausgeschlossen werden.

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Geochemistry and petrology of superpure quartzites from East Sayan Mountains,Russia

Acta Geochimica - Quartzites are widespread within Earth’s lithosphere, but their highly pure varieties occur quite infrequently. With the development of alternative energy sources, including...     

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.     

Petrology and mineralogy of dredged gabbro from Gettysburg Bank,eastern Atlantic

Gabbros have been dredged from Gettysburg Bank, 110 km west of Portugal on the Azores/Gibraltar fracture zone. Primary minerals in olivine, pyroxene and brown hornblende gabbros are partially replaced by metamorphic minerals. Igneous textures are inhomogeneously overprinted by a granular polyhedral deformation and a cataclastic deformation. Amphiboles show characteristics which indicate a transition from crystallisation in a magma chamber to formation of amphibole in solid gabbro under metasomatic conditions. Of the amphiboles analysed, chlorine was present in the green amphiboles but below 0.05% in the brown suggesting the penetration of sea water after the formation of the brown amphibole but during the formation of the green.     

Geochemistry and petrology of glaucophane-bearing eclogites and associated rocks from Sunnfjord, Western Norway

Country-rock eclogites and amphibolites in the Sunnfjord area occur as pods, lenses and layers within gneisses of granitic to tonalitic composition. The contacts towards the enclosing gneisses are mostly tectonic, although apparent primary intrusive contacts occur. Geochemical data show that central parts of eclogitic bodies represent fairly undisturbed magmatic compositions, while the margins of such bodies show varying degrees of contamination. Exchange of components with the enclosing gneisses occurred at both a pre-eclogitic and a post-eclogitic stage. Petrographical and petrological data show that the eclogites developed from garnet amphibolites to eclogites during a prograde stage, and that a subsequent uplift/decompression period took place at fairly constant temperatures. The data presented here are compatible with an in situ model for the genesis of Norwegian country-rock eclogites.     

Geochemistry and petrology of the Main Limestone Series (Lower Carboniferous), South Wales,U.K.

Dolomitized Main Limestone rocks of the Lower Carboniferous crop out in a narrow band of about three fourths of a mile along the south, east and northeast rims of the South Wales Coalfield Basin which encompasses the general regions of Miskin, Taffs Well and the Clydach, respectively. The thickness of these rocks varies from a maximum of 2,750 feet to nothing.The Ca/Mg ratio in the Main Limestone in general ranged from 1.7 : 1 to over 100 : 1 with less than 2 : 1 to 3.5 : 1 being the commonest. Magnesium content ranged from less than 5% to over 60% (mole percent MgCO₃), with 40–50 mole percent p.f. MgCO₃ being the commoner.Petrographic study of the Main Limestone led to recognition of six major microfacies. They are: (1) biosparite; (2) dolobiomicrite; (3) quartz-dolomicrite; (4) dolomicrite; (5) oosparite; and (6) dolorudite. In terms of their geographic distribution, dolobiomicrite predominates in the Taffs Well region, while quartz-dolobiomicrite is common in the Clydach region. In terms of their distribution in time, biosparite is common in the basal part (Lower ZC₁ zone), dolobiomicrites (including the quartz-bearing variety) in the middle part (ZC₁ and C₂S₁ zone) of the Taffs Well region and its corresponding Calcite-Mudstone Group in the Clydach region.The bulk of dolomitization in these rocks occurred in the Caninia-Seminula zone or the Calcite-Mudstone Group. In this study, it marks geochemical stage B: the sea-connected lagoonal phase similar to one observed today in South Australia. It is suggested that such a sea-connected Lagoon in the Main Limestone Sea formed largely in response to a reef barrier (or a ridge) - a remnant of which is found today between Miskin and the Taffs Well region where the transition from lime to dolomite facies occurs. The Miskin area represented the deeper-water reef flank towards the open sea whereas the Taffs Well region marked the shallow-water reef flank landward.It is concluded that the Main Limestone Series contains predominantly diagenetic dolostones whilst the syngenetic and epigenetic dolostones are restricted in time and space. The diagenetic dolostone dominates in the Taffs Well and Clydach regions.     

西南天山托云地区白垩纪-早第三纪玄武岩地球化学及其成因机制

托云玄武岩主要分布于托云盆地东侧，按形成时代可分为白垩纪玄武岩和早第三纪玄武岩。白垩纪玄武岩包括早白垩世玄武岩和晚白垩世玄武岩，以碱玄岩和碱性橄榄玄武岩为主，碱性程度高；早第三纪玄武岩包括早第三纪玄武岩及脉岩，以碱性橄榄玄武岩、碱性橄榄辉绿岩为主，碱性程度低。所有岩石稀土元素(REE)、微量元素分布模式相似，REE均为向右陡倾型，富集不相容元素。白垩纪玄武岩的∑REE、Rb、Ba、Th、K、Sr、Nb和Ta等元素富集程度均高于早第三纪玄武岩，相容元素富集程度大体较低。微量元素和同位素特征显示，玄武岩起源于与洋岛玄武岩源区相似的富集地幔源。玄武岩中赋存有交代地幔捕虏体，这表明玄武岩浆可能是交代地幔经不同程度部分熔融的产物。微量元素的特征同时显示，早白垩世玄武岩部分熔融程度较低，早第三纪玄武岩部分熔融程度较高，且在不断的部分熔融过程中，形成的岩浆又有结晶分异作用发生。托云玄武岩形成于大陆板内拉伸环境，在形成过程中经历了较弱的壳幔相互作用。     

Geochemistry and petrogenesis of Late Ladinian OIB-like basalts from Tabai, Yunnan Province, China

Major and trace elements analysis has been carried out on the Late Ladinian Tabai basalts from Yunnan Province with the aim of studying their petrogenesis. Their SiO2 contents range from 43.63 wt.% to 48.23 wt.%. The basalts belong to the weakly alkaline(average total alkalis Na2 O+K2O=3.59 wt.%), high-Ti(3.21 wt.% to 4.32 wt.%) magma series. The basalts are characterized by OIB-like trace elements patterns, which are enriched in large ion lithosphile elements(LILE) including Rb and Ba, and display negative K, Zr and Hf anomalies as shown on the spider diagrams. The Tabai basalts display light rare-earth elements(LREE) enrichment and are depleted in heavy rare-earth elements(HREE) on the REE pattern. Those dates indicate that the parental magma of the Tabai basalts was derived from low-degree(1%–5%) partial melting of garnet peridotite. The magma underwent olivine fractional crystallization and minor crustal contamination during their ascent. The Tabai basalts were related to a relaxation event which had triggered the Emeishan fossil plume head re-melting in the Middle Triassic.     

Petrology and geochemistry of basalts from the American-Antarctic Ridge,Southern Ocean: implications for the westward influence of the Bouvet mantle plume

Ridge segments and fracture zones from the American-Antarctic Ridge have been systematically dredge sampled from 4° W to 18° W. Petrographic studies of the dredged basalts show that the dominant basalt variety is olivine-plagioclase basalt, although olivine-plagioclase-clinopyroxene basalt is relatively common at some localities. Selected samples have been analysed for major and trace elements, rare earth elements and Sr and Nd isotopes. These data show that the majority of samples are slightly evolved (Mg#=69-35) N-type MORB, although a small group of samples from a number of localities have enriched geochemical characteristics (T- and P-type MORB).These different types of MORB are readily distinguished in terms of their incompatible trace element and isotopic characteristics: N-type MORB have high Zr/Nb (17–78), Y/Nb (4.6–23) and ¹⁴³Nd/¹⁴⁴Nd (0.51303–0.51308) ratios, low Zr/Y (2.2–4.2) and ⁸⁷Sr/⁸⁶Sr (0.70263–0.70295) ratios and have (La/Sm)_N<1.0; T-type MORB have lower than chondritic Zr/Nb ratios (8.8–15.5), relatively low Y/Nb (1.9–4.3) and ¹⁴³Nd/¹⁴⁴Nd (0.51296–0.51288) ratios and relatively high Zr/Y (3.1–4.7), ⁸⁷Sr/⁸⁶Sr (0.70307–0.70334) and (La/Sm)_N (1.1–1.5) ratios; the single sample of P-type MORB has low Zr/Nb (6.3), Y/Nb (0.9) and ¹⁴³Nd/¹⁴⁴Nd (0.51287) ratios and high Zr/Y (7.1), ⁸⁷Sr/⁸⁶Sr (0.70351) and (La/Sm)_N (2.4) ratios. The geochemical characteristics of this sample are essentially identical to those of the Bouvet Island lavas.Geochemically enriched MORB are less abundant on the American-Antarctic Ridge than on the Southwest Indian Ridge but their geochemical characteristics are identical. The compositions of T- and P-type MORB are consistent with a regional mixing model involving normal depleted mantle and Bouvet plume type magma. On a local scale the composition of T-type MORB is consistent with derivation from depleted mantle which contains 4% veins of P-type melt.We propose a model for the evolution of the American-Antarctic Ridge lavas in which N-type MORB is derived from mantle with negligible to low vein/mantle ratios, T-type MORB is derived from domains with moderate and variable vein/mantle ratios and P-type MORB from regions with very high vein/mantle ratios where vein material comprises the major portion of the melt. The sparse occurrence of enriched lavas and by implication enriched mantle beneath the American-Antarctic Ridge, some distance (500–1,200 km) from the Bouvet plume location, is interpreted to be the result of lateral dispersion of enriched mantle domains by asthenospheric flow away from the Bouvet mantle plume towards the American-Antarctic Ridge.     

Geochemistry and petrology of selected coal samples from Sumatra, Kalimantan, Sulawesi, and Papua, Indonesia

Indonesia has become the world's largest exporter of thermal coal and is a major supplier to the Asian coal market, particularly as the People's Republic of China is now (2007) and perhaps may remain a net importer of coal. Indonesia has had a long history of coal production, mainly in Sumatra and Kalimantan, but only in the last two decades have government and commercial forces resulted in a remarkable coal boom. A recent assessment of Indonesian coal-bed methane (CBM) potential has motivated active CBM exploration. Most of the coal is Paleogene and Neogene, low to moderate rank and has low ash yield and sulfur (generally < 10 and < 1 wt.%, respectively). Active tectonic and igneous activity has resulted in significant rank increase in some coal basins. Eight coal samples are described that represent the major export and/or resource potential of Sumatra, Kalimantan, Sulawesi, and Papua. Detailed geochemistry, including proximate and ultimate analysis, sulfur forms, and major, minor, and trace element determinations are presented. Organic petrology and vitrinite reflectance data reflect various precursor flora assemblages and rank variations, including sample composites from active igneous and tectonic areas. A comparison of Hazardous Air Pollutants (HAPs) elements abundance with world and US averages show that the Indonesian coals have low combustion pollution potential.     

Tristan da Cunha, South Atlantic: Geochemistry and Petrogenesis of a Basanite-Phonolite Lava Series

The island of Tristan da Cunha, located at 37?S, 12?W in theSouth Atlantic, is the largest of a group of three islands,the others being Nightingale and Inaccessible. Tristan da Cunhacomprises a continuous series of alkaline lavas ranging in compositionfrom ankaramitic basanite through phonotephnte and tephnphonoliteto phonolite. Moderately porphyritic basanite is the dominantrock type ({small tilde}on the island. Major and trace element variations in the lavas describe well-definedtrends with increasing differentiation which are generally consistentwith control by fractional crystallization of phenocryst phases.None of the lavas can be considered to be primary in composition,mg-numbers range from 40 to 62, and covariation of certain minorand trace elements (e.g., Sr, Ba, P) suggests the presence ofat least two distinct fractionation trends. Sr, Nd, and Pb isotopicanalyses of a subset of the lavas confirm previously publisheddata for the island, but show a slightly greater range: ⁸⁷Sr/⁸⁶Sr= 0{dot}70495–0{dot}70517; ¹⁴³Nd/¹⁴⁴Nd = 0{dot}51259–0{dot}51247;²⁰⁶Pb/²⁰⁴Pb = 18{dot}47–18-{dot}74. Quantitative modelling of the compositional variations suggeststhat the ankaramitic basanites are partial olivine + clinopyroxene+ titanomagnetite (?minor plagioclase) cumulates, with {smalltilde}40% crystal accumulation being required to account forthe most porphyritic varieties. The range in composition frombasanite to phonotephrite can be accounted for by up to 50%fractional crystallization of clinopyroxene, olivine, titanomagnetite,and plagioclase, with minor apatite and, in some models, amphibole.Average proportions of these phases in the fractionate are Cpx40, TiMgt 20, Plag 30, Oliv 10. As much as 20% amphibole fractionationis required in models involving the phonotephrites. The compositionsof the evolved tephriphonolites and phonolites are consistentwith extensive (up to 80%) fractional crystallization of aninitial basanitic magma with clinopyroxenc (1–4%), amphibole(17–23%), plagioclase (6–20%) alkali feldspar (0–13%),and titanomagnetite (4–6%)?minor apatite and sphene beingthe dominant fractionating phases. Inferred trace element and isotopic characteristics of the sourceregions of the Tristan lavas are distinct from those givingrise to the Walvis Ridge, Gough Island, or Discovery Seamountbasalts. Normalized trace element abundances of the Tristanlavas are more similar to those of Marion Island, whereas Sr-,Nd-, dnd Pb-isotopic ratios are most similar to nearby InaccessibleIsland lavas. If Tristan da Cunha is the present-day surfaceexpression of the upwelling mantle plume that previously gaverise to the Walvis Ridge, then the source material tapped bythe lavas is distinctly heterogeneous (or has changed with time)within the limits imposed by maintaining the general characteristicsof DUPAL-type mantle.