Isotopic composition of lead and strontium in lavas and coarse-grained blocks from Ascension Island,South Atlantic—an addendum

Natural marokite (CaMn₂O₄) has been studied at high pressures and temperatures using a diamond-anvil press coupled with laser heating in the pressure range 100–250 kbar. A mixture of marokite, CaMnO₃ (perovskite) and MnO (rocksalt) has been observed in all runs in the above pressure range by X-ray diffraction study of the quenched samples. It was interpreted that marokite disproportionates into the mixture CaMnO₃ (perovskite) + MnO (rocksalt) at pressures below 100 kbar. A general comparison of the molar volume for all known compounds having the marokite-related structures (including CaFe₂O₄ and CaTi₂O₄) with those for a mixture of perovskite plus rocksalt structures suggested that the mixture is more stable than the marokite-related structures at high pressures, as confirmed by the present experimental result. The CaFe₂O₄-modification of common nepheline (NaAlSiO₄) is also suggested to be unstable relative to the component oxides of α-NaAlO₂ + SiO₂ (stishovite) at high pressures.     

The isotopic composition of strontium and oxygen in lavas from St. Helena,South Atlantic

Strontium and oxygen isotope measurements on the alkali basalt-trachyte-phonolite suite of St. Helena show that some of the late-fractionated rocks are enriched in ⁸⁷Sr and depleted in ¹⁸O relative to the older basalts. The data rule out both the formation of the late-fractionated rocks by the partial melting of hydrothermally altered oceanic crust and the contamination of the volcanic rocks by oceanic sediment. It also appears to be incompatible with models based either on the melting of previously fractionated and crystallized liquids in the volcanic pile, or the long-term fractionation of lavas over several millions of years in a sub-volcanic magma chamber.It is concluded that hydrothermal interaction with meteoric water is the most important cause of the ¹⁸O depletion. If the interaction occurred at widely differing temperatures, and involved meteoric and seawaters, it might conceivably have caused both the oxygen and strontium isotope heterogeneities.     

Gravity field of Ascension Island,South Atlantic

A gravity survey of Ascension Island was carried out in 1964, a total of 210 stations being occupied. The results suggest that magma has reached the surface via three distinct conduits, the easternmost of which has been associated with extrusion of trachytic lavas. A regional decrease in gravity from west to east across the island is probably due to the proximity to the east of the crest of the Mid Atlantic Ridge, underlain by low density mantle.     

Cosmogenic 3He exposure ages and geochemistry of basalts from Ascension Island,Atlantic Ocean

The chronology and origin of volcanism of Ascension Island, South Atlantic Ocean, is poorly resolved. Here we use in situ produced cosmogenic ³He in olivine and pyroxene phenocrysts from well-preserved lava flows to date the main sub-aerial basalt volcanism on the island. Etching olivine separates in HF/HNO₃ appears to remove a significant proportion of the implanted radiogenic ⁴He contribution. Average exposure ages of each flow corrected for radiogenic He range from 328 ka to 186 ka and are used to refine the chronology and stratigraphy of the island. Magmatic ³He/⁴He ratios derived from in vacuo crushing are in the range of 6.3–7.3 R_A. This range is lower than the neighbouring Mid-Atlantic Ridge segment (6–8°S) but slightly higher than measured in regional ocean islands of St. Helena, Tristan da Cunha and Gough. Combining these data with new trace element data and published radiogenic isotope ratios it appears that the Ascension Island magmatism is a mix of HIMU mantle material, typified by basalts from St. Helena, and depleted MORB-source mantle.     

Sr, Nd, O and H isotopic ratios in Ascension Island lavas and plutonic inclusions; cogenetic origin

Eighteen basic rocks from Ascension Island (South Atlantic) give a mean⁸⁷Sr/⁸⁶Sr ratio of 0.70311 ± 17 for both volcanics and plutonic inclusions. The late-stage differentiated rocks (rhyolites and granitic inclusions) have much higher⁸⁷Sr/⁸⁶Sr ratios, up to 0.712. All these rocks display the same range of Nd isotopic compositions (ε_Ndvalues from6.9to11.1with a mean on12samples of8.4 ± 0.6) implying a cogenetic relation between the two sequences. The D/H systematics lead to the same conclusion.In the NdSr diagram, the data plot close to the mantle array and show a positive correlation. This suggests a mixing between a depleted MORB-type mantle, i.e. the upper mantle, and a hot-spot with less depleted geochemical characteristics, i.e. the OIB mantle source.The total range of δ¹⁸O values lies between 4.8‰ for plagioclase cumulates and 6.7‰ for the most evolved rocks (peralkaline granites and comendites). The basic rocks have values around 5.3‰, typical of mantle-derived material. These oxygen data indicate that the high⁸⁷Sr/⁸⁶Sr ratios in the most evolved rocks (both volcanic and plutonic terms) result from the combination of two different processes: incorporation of slight amounts (< 1%) of high-temperature altered oceanic crust by the magma in the late stages of the differentiation process and then in-situ Rb decay since the time of formation of these rocks. Both processes were very effective because of the high Rb and low Sr contents of these evolved rocks.Oxygen isotope systematics in the Ascension Island granites and rhyolites indicate that a fractional crystallization process alone does not produce δ¹⁸O values higher than 6.7‰, i.e. that the ultimate δ¹⁸O enrichment, relative to the initial basic magma, is not greater than 1.5‰.     

Isotopic evolution of lavas from Haleakala Crater, Hawaii

Pb and Sr isotopic ratios have been determined for tholeiitic shield-building, alkalic cap, and post-erosional stage lavas from Haleakala Crater. Pb isotopic compositions of the tholeiites overlap those of the alkalic cap lavas, although⁸⁷Sr/⁸⁶Sr ratios of these two suites are distinct. Alkalic cap and post-erosional lavas appear to be indistinguishable on the basis of Sr and Pb isotopic composition.Sr and Pb isotopic ratios of Haleakala post-shield-building lavas are positively correlated. Such a trend is previously undocumented for any suite of Hawaiian lavas and contrasts with the general negative correlation observed for data from Hawaiian tholeiites. These relations are consistent with a three-component petrogenetic mixing model. Specifically, it is proposed that magma batches at individual Hawaiian volcanoes formed by: (1) mixing of melts generated from mantle plumes containing two isotopically distinct mantle components (primitive vs. enriched), and (2) subsequent variable degrees of interaction between these plume melts and a third (MORB signature) mantle reservoir prior to their emplacement in a crustal magma chamber. These observations and inferences provide new constraints on physical models of Hawaiian magmatism. Based on observed temporal isotopic variations of Haleakala lavas, it is suggested that the ratio of enriched: primitive mantle components in the Hawaiian plume source decreases during the waning stages of alkalic volcanism. Over the same time interval, both decreasing melt production and protracted residence of ascending melts within the upper mantle contribute to a systematic increase in the ratio of depleted vs. plume component.     

Isotopic composition of calcrete deposits from Europe,Africa and India

The four models postulated for the formation of calcrete layers all involve the three-stage process: provision of a carbonate-rich solution, the movement of that solution and the precipitation of solid carbonate. The last step originates from either or both of two main alternatives: loss of CO₂ out of the solution increasing the pH and consequently the CO concentration, or evaporation of the water effecting an increase in the ionic concentration. This paper presents theoretical calculations for the isotopic composition of the carbonate resulting from these two alternative processes. The isotopic composition of calcrete deposits from France, Spain, Italy, Libya, Cyprus, Kenya, Morocco, South Africa and India are compared with the theoretical models. The calcrete samples from Spain, Cyprus and India are probably the result of evaporation processes.     

A comparison of TEC fluctuations and scintillations at Ascension Island

With increasing reliance on space-based platforms for global navigation and communication, concerns about the impact of ionospheric scintillation on these systems have become a high priority. Recently, the Air Force Research Laboratory (AFRL) performed amplitude scintillation measurements of L1 (1.575 MHz) signals from GPS satellites at Ascension Island (14.45° W, 7.95° S; magnetic latitude 16° S) during February–April, 1998, to compare amplitude scintillations with fluctuations of the total electron content (TEC). Ascension Island is located in the South Atlantic under the southern crest of the equatorial anomaly of F2 ionization where scintillations will be much enhanced during the upcoming solar maximum period. Ascension Island is included in the global network of the International GPS Service (IGS) and the GPS receivers in this network report the carrier to noise (C/N) ratio, the dual frequency carrier phase and pseudorange data at 30-s intervals. Such data with a sampling interval of 30 s were analyzed to determine TEC, the rate of change of TEC (ROT) and also ROTI, defined as the standard deviation of ROT. The spatial scale of ROTI, sampled at 30 s interval, will correspond to 6 km when the vector sum of the ionospheric projection of the satellite velocity and the irregularity drift orthogonal to the propagation path is of the order of 100 m/s. On the other hand, the scale-length of the amplitude scintillation index corresponds to the Fresnel dimension which is about 400 m for the GPS L1 frequency and an ionospheric height of 400 km. It is shown that, in view of the co-existence of large and small scale irregularities in equatorial irregularity structures, during the early evening hours, and small magnitude of irregularity drifts, ROTI measurements can be used to predict the presence of scintillation causing irregularities. The quantitative relationship between ROTI and S4, however, varies considerably due to variations of the ionospheric projection of the satellite velocity and the ionospheric irregularity drift. During the post-midnight period, due to the decay of small scale irregularities leading to a steepening of irregularity power spectrum, ROTI, on occasions, may not be associated with detectable levels of scintillation. In view of the power law type of irregularity power spectrum, ROTI will, in general, be larger than S4 and the ratio, ROTI/S4, in the present dataset is found to vary between 2 and 10. At high latitudes, where the ionospheric motion, driven by large electric fields of magnetospheric origin, is much enhanced during magnetically active periods, ROTI/S4 may be considerably larger than that in the equatorial region.     

Geochemistry of late Cenozoic lavas on Kunashir Island, Kurile Arc

Middle Miocene to Quaternary lavas on Kunashir Island in the southern zone of the Kurile Arc were examined for major, trace, and Sr–Nd–Pb isotope compositions. The lavas range from basalt through to rhyolite and the mafic lavas show typical oceanic island arc signatures without significant crustal or sub-continental lithosphere contamination. The lavas exhibit across-arc variation, with increasingly greater fluid-immobile incompatible element contents from the volcanic front to the rear-arc; this pattern, however, does not apply to some other incompatible elements such as B, Sb, and halogens. All Sr–Nd–Pb isotope compositions reflect a depleted source with Indian Ocean mantle domain characteristics. The Nd and Pb isotope ratios are radiogenic in the volcanic front, whereas Sr isotope ratios are less radiogenic. These Nd isotope ratios covary with incompatible element ratios such as Th/Nd and Nb/Zr, indicating involvement of a slab-derived sediment component by addition of melt or supercritical fluid capable of mobilizing these high field-strength elements and rare earth elements from the slab. Fluid mobile elements, such as Ba, are also elevated in all basalt suites, suggesting involvement of slab fluid derived from altered oceanic crust. The Kurile Arc lavas are thus affected both by slab sediment and altered basaltic crust components. This magma plumbing system has been continuously active from the Middle Miocene to the present.     

Quantitative evaluation of fractional crystallization in Bouvet Island lavas

Bouvet Island, situated near the southernmost end of the Mid-Atlantic Ridge, is characterized by lavas ranging in composition from hawaiite through mugearite and benmoreite to rhyolite. Major and trace elements vary systematically throughout the sequence, as do mineral compositions, and geochemical modelling indicates that the compositional variations observed in the differentiated lavas can be ascribed to extensive fractional crystallization of a parental hawaiite magma. Following this scheme the hawaiite parent magma experienced approximately 44% fractional crystallization of plagioclase + clinopyroxene + olivine + opaque oxides and minor apatite to give rise to the mugearite magma, which in turn experienced a further 69% fractional crystallization of the same phase assemblage (though with more evolved compositions) to give rise to the Bouvet Island rhyolite. Extensive fractional crystallization (64%), possibly separated in time, of the parental hawaiite gave rise to the benmoreite magma. In the latter scheme the fractionating phases are similar both in composition and proportion to those crystallizing in the passage from hawaiite to mugearite, suggesting that the Bouvet Island hawaiites correspond in composition to a six phase (plag + of + cpx + Fe-Ti oxide + ap+ lq) saturation surface and that the more differentiated lavas resulted from differing degrees of crystallization on this surface.     

Petrology of tholeiitic lavas from Tanna Island (New Hebrides): Importance of cumulative processes in Island arc magmatism

Tanna, one of the southernmost islands of the New Hebrides volcanic arc, is made of Late Pliocene to Recent island arc tholeiitic basalts and andesites, with SiO₂ contents ranging from 45 to 57%. These lavas are highly porphyritic (30–50% in volume): phenocrysts of plagioclase are the most abundant, together with olivine and clinopyroxene. The groundmass contain plagioclase, augite, olivine, magnetite and glass; pigeonite, tridymite, sanidine and, rarely, biotite may also occur. The olivines and clinopyroxenes show an iron enrichment from the cores of phenocrysts to their rims and the groundmass crystals, but their compositional variations are not correlated with the Mg/Fe ratio of bulk host rocks, the most Fe-rich compositions being found in Mg-rich lavas. Plagioclase compositions range from An₉₅ to An₆₀ in the basalts and An₆₀ to An₅₀ in the andesites, but, within each group, they are not correlated with SiO₂ or Na₂O contents of host lavas. Consequently, the bulk major element compositions of Tanna volcanic rocks cannot be considered as primarily controlled by crystal separation from successive liquids. The oxyde-SiO₂ variations diagrams, and the modal compositions and mineral chemistry show that crystal accumulation is the predominant mechanism accounting for bulk rock compositions. However, this does not exclude fractional crystallization: the variation of the calculated groundmass mineralogy strongly suggest the occurrence of crystal removal mainly clinopyroxene and magnetite.     

Magnetic stratigraphy and magnetic-petrologic properties of Precambrian Gardar lavas,South Greenland

We present major and trace element data for olivines and rapidly quenched groundmass material separated from eight samples of basaltic rock. From these data apparent olivine/liquid distribution coefficients (D) for the elements Mg, Mn, Ni, Co, Cr, Sc, Na, Sm, and Ca have been calculated. Petrographic and electron microprobe studies indicate that olivines in most of these samples are but slightly zoned and groundmass glasses are essentially uniform in composition; these data suggest that the co-existing olivine-groundmass pairs are in equilibrium. In addition, olivine crystallization temperatures were calculated using experimental calibrations of lnD vs.1/T for the elements Mg, Fe, Mn, Ni, and Co. A high degree of concordancy for the resulting temperatures, based on five different elements for most samples, also suggests that olivine-groundmass equilibrium was obtained. We conclude that the apparent olivine/liquidD's derived from this study are representative of equilibriumD's, but emphasize that variations in temperature, and possibly bulk composition, strongly affect suchD's in natural magmas.     

Genesis of lavas of the Taupo Volcanic Zone, North Island, New Zealand

The Taupo Volcanic Zone forms part of the Taupo-Hikurangi subduction system, and comprises five volcanic centres: Tongariro, Taupo, Maroa, Okataina and Rotorua. Tongariro Volcanic Centre is formed almost entirely of andesite while the other four centres contain predominantly rhyolitic volcanics and later fissure eruptions of high-Al basalt. Estimated total volume of each lava type are as follows: 2 km³ of high-Al basalt (< 0.1%); 260 km³ of andesite (< 2.5%); 5 km³ of dacite (< 0.1%); > 10,000 km³ of rhyolite and ignimbrite (> 97.4%).The location of the andesites and vent alignments suggest a source from a subduction zone underlying the area. However, the lavas differ chemically from island-arc andesites such as those of Tonga; in particular by having higher contents of the alkali elements, light REE and Sr and Pb isotopes. This suggests some crustal contamination, and it is considered that this may occur beneath the wide accretionary prism of the subduction system. Amphibolite of the subduction zone will break down between 80 and 100 km and a partial melt will rise. A multi-stage process of magma genesis is then likely to occur. High-Al basalts are thought to be derived from partial melting of a garnet-free peridotite near the top of the mantle wedge overlying the subduction zone, locations of the vents controlled largely by faults within the crust. Rhyolites and ignimbrites were probably derived from partial melting of Mesozoic greywacke and argillite under the Taupo Volcanic Zone. Initial partial melting may have been due to hydration of the base of the crust; the “water” having come from dehydration of the downgoing slab. The partial melts would rise to form granodiorite plutons and final release of the magma to form rhyolites and ignimbrites was allowed because of extension within the Taupo graben.Dacites of the Bay of Plenty probably resulted from mixing of andesitic magma with small amounts of rhyolitic magma, but those on the eastern side of the Rotorua-Taupo area were more likely formed by a higher degree of partial melting of the Mesozoic greywacke-argillite basement. This may be due to intrusion of andesite magma on this side of the Taupo volcanic zone.     

Elemental fractionation in lavas during post-eruptive degassing: Evidence from trachytic lavas,Rishiri Volcano,Japan

Trachytic lavas of Rishiri Volcano, northern Japan, show a peculiar geochemical variation across lava flow units. Samples collected systematically in a vertical cross section from a lava flow unit with a thickness of about 20 m are nearly homogeneous in major element compositions. However, some trace elements, including Li, B and Cs, are considerably depleted in samples collected from the main part of the flow unit, compared to those obtained from the surface of the lava flow (clinker layer). In particular, Cs content of the main flow unit is as low as ∼30% of the clinker layer. ¹¹B / ¹⁰B ratios of samples from the main flow unit are also slightly lower than those of the clinker samples, and the isotope compositions positively correlate with boron concentrations. These geochemical variations cannot be explained by magmatic processes in magma chambers, post-eruptive weathering, or alteration process. Rather, we infer these systematics resulted from escape of these elements from the lava flow unit during post-eruptive degassing. Vapor phases in which Li, B and Cs dissolved are suggested to have been transported through veins formed in the main flow unit as fractures due to slight shearing along the flow planes after lava emplacement. In the Tanetomi lava, only rocks of the clinker layer preserve original composition of magmas, although they are porous and brownish due to extensive oxidization. On the other hand, rocks of the main flow unit do not retain original magma compositions, although they are dense and grayish, and seem to be much fresher compared to the clinkers. A similar geochemical modification of lavas can occur in other volcanic systems, especially for lavas consisting of relatively thick flow units.     

The composition of basaltic lavas from Bayuda,Sudan and their place in the cainozoic volcanic history of north-east Africa

Six new analyses of young basaltic rocks from the Bayuda field show the predominant rock types to be strongly undersaturated basanites and nepheline trachybasalts. Both types are believed to represent magmas of deep-seated origin. Similar rocks are widely distributed in north-east Africa but mildly alkaline to tholeiitic basalts were erupted along the eastern margin of the continent in early and late Cainozoic times, whereas along the Tripoli-Tibesti zone to the west mildly alkaline basalts were probably confined to the early Tertiary. The Tripoli-Tibesti zone was one of uplift and strongly tensional tectonics in the late Mesozoic and early Cainozoic, and at this time may have been a line of potential lithospheric rifting, but a period of quiescence followed and resurgence of activity in the late Cainozoic produced weaker tensional structures and more strongly alkaline basic magmas. The region between these two main zones of activity was characterized throughout by intermittent alkaline volcanicity and weak tectonism. Neverthless, fracture zones which apparently controlled the volcanicity are beginning to be recognized in this area. It is argued that African volcanic activity is related to linear, rather than circumscribed, areas of mantle activity. Possible connections with epeirogenic movements within the Alpine orogenic belt appear to have been neglected in the debate on the causes of African igneous activity.     

Flow banding in basaltic pillow lavas from the Early Archean Hooggenoeg Formation, Barberton Greenstone Belt, South Africa

Well-preserved pillow lavas in the uppermost part of the Early Archean volcanic sequence of the Hooggenoeg Formation in the Barberton Greenstone Belt exhibit pronounced flow banding. The banding is defined by mm to several cm thick alternations of pale green and a dark green, conspicuously variolitic variety of aphyric metabasalt. Concentrations of relatively immobile TiO₂, Al₂O₃ and Cr in both varieties of lava are basaltic. Compositional differences between bands and variations in the lavas in general have been modified by alteration, but indicate mingling of two different basalts, one richer in TiO₂, Al₂O₃, MgO, FeO^t and probably Ni and Cr than the other, as the cause of the banding. The occurrence in certain pillows of blebs of dark metabasalt enclosed in pale green metabasalt, as well as cores of faintly banded or massive dark metabasalt, suggest that breakup into drops and slugs in the feeder channel to the lava flow initiated mingling. The inhomogeneous mixture was subsequently stretched and folded together during laminar shear flow through tubular pillows, while diffusion between bands led to partial homogenisation. The most common internal pattern defined by the flow banding in pillows is concentric. In some pillows the banding defines curious mushroom-like structures, commonly cored by dark, variolitic metabasalt, which we interpret as the result of secondary lateral flow due to counter-rotating, transverse (Dean) vortices induced by the axial flow of lava towards the flow front through bends, generally downward, in the tubular pillows. Other pillows exhibit weakly-banded or massive, dark, variolitic cores that are continuous with wedge-shaped apophyses and veins that intrude the flow banded carapace. These cores represent the flow of hotter and less viscous slugs of the dark lava type into cooled and stiffened pillows.     

The strontium isotopic composition of seawater,and seawater-oceanic crust interaction

The⁸⁷Sr/⁸⁶Sr ratio of seawater strontium (0.7091) is less than the⁸⁷Sr/⁸⁶Sr ratio of dissolved strontium delivered to the oceans by continental run-off (～0.716). Isotope exchange with strontium isotopically lighter oceanic crust during hydrothermal convection within spreading oceanic ridges can explain this observation. In quantitative terms, the current⁸⁷Sr/⁸⁶Sr ratio of seawater (0.7091) may be maintained by balancing the continental run-off flux of strontium (0.59 × 10¹² g/yr) against a hydrothermal recirculation flux of 3.6 × 10¹² g/yr, during which the⁸⁷Sr/⁸⁶Sr ratio of seawater drops by 0.0011. A concomitant mean increase in the⁸⁷Sr/⁸⁶Sr ratio of the upper 4.5 km of oceanic crust of 0.0010 (0.7029–0.7039) should be produced. This required⁸⁷Sr enrichment has been observed in hydrothermally metamorphosed ophiolitic rocks from the Troodos Massif, Cyprus.The post-Upper Cretaceous increase in the strontium isotopic composition of seawater(～0.7075–0.7091) covaries smoothly with inferred increase in land area. This suggests that during this period the main factor which has caused variability in the⁸⁷Sr/⁸⁶Sr ratio of seawater strontium could have been variation in the magnitude of the continental run-off flux caused by variation in land area. Variations in land area may themselves have been partly a consequence of variations in global mean sea-floor spreading rate.     

Biostratigraphy of Famennian in Hainan Island, South China

On the basis of the faunas including conodonts Palmatolepis gracilis sigmoidalis, P.gracilis gracilis, Polygnathus germanus, which are found for the first time, and corals Cystophrentis kalaohoensis it is confirmed that the Changjiang Formation refers to the Famennian, and may correlate with the Shaodong Formation and Menggongao Formation of central Hunan. This is the only Devonian deposits known in Hainan Island so far. It means that the outstanding problem of whether the Devonian rocks exist in that island is now settled. Meanwhile, the new evidences indicate that the lectostratotype section of Nanhao Formation of Baoting area refers to the Silurian,rather than the Lower Carboniferous as considered by most geologists formerly. Furthermore, the Member 3 of Nanhao Formation at Shilu-Jinbo district is proved to be the Upper Carboniferous,rather than Tournaisian or Middle-Upper Devonian as deemed to be before.