Showing papers on "Basalt published in 1998"

Peridotites from the Izu-Bonin-Mariana Forearc (ODP Leg 125): Evidence for Mantle Melting and Melt-Mantle Interaction in a Supra-Subduction Zone Setting

Abstract: Ocean Drilling Program Leg 125 recovered serpentinized harzburgites and dunites from a total of five sites on the crests and flanks of two serpentinite seamounts, Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu–Bonin forearc. These are some of the first extant forearc peridotites reported in the literature and they provide a window into oceanic, supra-subduction zone (SSZ) mantle processes. Harzburgites from both seamounts are very refractory with low modal clinopyroxene (<4%), chrome-rich spinels (cr-number = 0.40–0.80), very low incompatible element contents, and (with the exception of amphibole-bearing samples) U-shaped rare earth element (REE) profiles with positive Eu anomalies. Both sets of peridotites have olivine–spinel equilibration temperatures that are low compared with abyssal peridotites, possibly because of water-assisted diffusional equilibration in the SSZ environment. However, other features indicate that the harzburgites from the two seamounts have very different origins. Harzburgites from Conical Seamount are characterized by calculated oxygen fugacities between FMQ (fayalite–magnetite–quartz) – 1.1 (log units) and FMQ + 0.4 which overlap those of mid-ocean ridge basalt (MORB) peridotites. Dunites from Conical Seamount contain small amounts of clinopyroxene, orthopyroxene and amphibole and are light REE (LREE) enriched. Moreover, they are considerably more oxidized than the harzburgites to which they are spatially related, with calculated oxygen fugacities of FMQ – 0.2 to FMQ + 1.2. Using textural and geochemical evidence, we interpret these harzburgites as residual MORB mantle (from 15 to 20% fractional melting) which has subsequently been modified by interaction with boninitic melt within the mantle wedge, and these dunites as zones of focusing of this melt in which pyroxene has preferentially been dissolved from the harzburgite protolith. In contrast, harzburgites from Torishima Forearc Seamount give calculated oxygen fugacities between FMQ + 0.8 and FMQ + 1.6, similar to those calculated for other subduction-zone related peridotites and similar to those calculated for the dunites (FMQ + 1.2 to FMQ + 1.8) from the same seamount. In this case, we interpret both the harzburgites and dunites as linked to mantle melting (20–25% fractional melting) in a supra-subduction zone environment. The results thus indicate that the forearc is underlain by at least two types of mantle lithosphere, one being trapped or accreted oceanic lithosphere, the other being lithosphere formed by subduction-related melting. They also demonstrate that both types of mantle lithosphere may have undergone extensive interaction with subduction-derived magmas.

Experimental determination of partition coefficients for rare earth and high-field-strength elements between clinopyroxene, garnet, and basaltic melt at high pressures

Abstract: Clinopyroxene/melt and garnet/melt partition coefficients have been determined for Ti, Sr, Y, Zr, Nb, Hf, and rare earth elements from 19 doped experiments on 1921 Kilauea basalt The experiments were carried out from 20 to 30 GPa and 1310° to 1470 °C The purpose was to derive a set of partition coefficients for high-field-strength elements (HFSE) and rare earth elements (REE) in a systematic, linked set of experiments at P and T conditions relevant to basalt petrogenesis These data are used in melting models to understand the development of negative HFSE anomalies observed in many abyssal peridotite clinopyroxenes It is shown that melting can account for the observed trace element patterns in some residual peridotites, but that other processes may also be needed to account for most residual mantle compositions in mid-ocean ridge systems It is also shown that REE are more strongly fractionated by garnet at these P-T conditions than previously thought

The hydrous phase equilibria (to 3 kbar) of an andesite and basaltic andesite from western Mexico: constraints on water content and conditions of phenocryst growth

Abstract: We have conducted high pressure (to 3 kbar), water saturated melting experiments on an andesite (62 wt% SiO2) and a basaltic andesite (55 wt% SiO2) from western Mexico. A close comparison between the experimental phase assemblages and their compositions, and the phenocryst assemblages of the lavas, is found in water saturated liquids, suggesting that the CO2 content was minimal in the fluid phase. Thus the historic lavas from Volcan Colima (with phenocrysts of orthopyroxene, augite, plagioclase, and hornblende) were stored at a temperature between 950–975 °C, at a pressure between 700–1500 bars, and with a water content of 3.0–5.0 wt%. A hornblende andesite (spessartite) from Mascota, of nearly identical composition but with only amphibole phenocrysts, had a similar temperature but equilibrated at a minimum of 2000 bars pressure with a dissolved water content of at least 5.5 wt% in the liquid. Experiments on the basaltic andesite show that the most common natural phenocryst assemblages (olivine, ±augite, ±plagioclase) could have precipitated at temperatures from 1000–1150 °C, in liquids with a wide range of dissolved water content (∼2.0–6.0 wt%) and a corresponding pressure range. A lava of the same bulk composition with phenocrysts of hornblende, olivine, plagioclase, and augite is restricted to temperatures below 1000 °C and pressures below 2500 bars, corresponding to <5.5 wt% water in the residual liquid. Although there is some evidence for mixing in the andesites (sporadic olivine phenocrysts), the broad theme of the history of both lava types is that the phenocryst assemblages for both the andesitic magmas and basaltic andesitic magmas are generated from degassing and reequilibration on ascent of initially hydrous parents containing greater than 6 wt% water. Indeed andesitic magmas could be related to a basaltic andesite parent by hornblende-plagioclase fractionation under the same hydrous conditions.

Permeability within basaltic oceanic crust

Abstract: Water-rock interactions within the seafloor are responsible for significant energy and solute fluxes between basaltic oceanic crust and the overlying ocean. Permeability is the primary hydrologic property controlling the form, intensity, and duration of seafloor fluid circulation, but after several decades of characterizing shallow oceanic basement, we are still learning how permeability is created and distributed and how it changes as the crust ages. Core-scale measurements of basaltic oceanic crust yield permeabilities that are quite low (generally 10−22 to 10−17 m²), while in situ measurements in boreholes suggest an overlapping range of values extending several orders of magnitude higher (10−18 to 10−13 m²). Additional indirect estimates include calculations made from borehole temperature and flow meter logs (10−16 to 10−11 m²), numerical models of coupled heat and fluid flow at the ridge crest and within ridge flanks (10−16 to 10−9 m²), and several other methods. Qualitative indications of permeability within the basaltic oceanic crust come from an improved understanding of crustal stratigraphy and patterns of alteration and tectonic modification seen in ophiolites, seafloor samples and boreholes. Difficulties in reconciling the wide range of estimated permeabilities arise from differences in experimental scale and critical assumptions regarding the nature and distribution of fluid flow. Many observations and experimental and modeling results are consistent with permeability varying with depth into basement and with primary basement lithology. Permeability also seems to be highly heterogeneous and anisotropic throughout much of the basaltic crust, as within crystalline rocks in general. A series of focused experiments is required to resolve permeability in shallow oceanic basement and to directly couple upper crustal hydrogeology to magmatic, tectonic, and geochemical crustal evolution.

The Importance of Pahoehoe

Abstract: ▪ Abstract Pahoehoe lava flows are common in every basaltic province, and their submarine variants, pillow lavas and sheet flows, cover the bulk of the Earth. Pahoehoe flows are emplaced by inflation—the injection of molten lava underneath a solidified crust. Only in the past few years has an understanding of the inflation process and the ability to recognize ancient inflated lava flows been achieved. All large terrestrial basaltic flow fields studied to date, including flood basalts, were emplaced as thermally efficient, inflated, compound pahoehoe sheet flows. This leads us to propose that this is the standard way of emplacing large lavas (the SWELL hypothesis). The atmospheric impact of such flood basalt eruptions could have been protracted and severe, providing a plausible link between flood basalt eruptions and mass extinctions.

Magma Genesis in the New Britain Island Arc: Further Insights into Melting and Mass Transfer Processes

Abstract: Quaternary volcanic rocks from the New Britain island arc display INTRODUCTION a wide range in chemical compositions. The source of the lavas The New Britain region of Papua New Guinea represents shares isotopic characteristics with Indian Ocean type mid-ocean an outstanding opportunity to reach an understanding ridge basalt (MORB). In contrast, the high field strength elements of the processes of magma genesis in an oceanic island (HFSE) are extremely depleted in the volcanic front rocks compared arc. The Quaternary volcanoes found there define the with MORB. We propose that this results from a previous melteastern part of the Bismarck volcanic arc, and have extraction event—hypotheses invoking residual phases in either the formed in response to northward subduction of the small mantle wedge or subducting slab cannot account for the depletion Solomon plate beneath the Bismarck plate (Fig. 1). The relative to MORB. In addition, elements other than the HFSE are New Britain arc is outstanding for two main reasons: also affected. Chemical signatures in parts of the New Britain arc (1) Arc volcanism in the central sector of the island and Manus Basin may relate to a previous subduction episode has taken place over an exceptionally wide range of along the now inactive Vitiaz–West Melanesian trench. Ultradepths to the Wadati–Benioff zone—from ~100 km deep depleted volcanic front basalts invariably have strong ‘fluid’-related at the ‘volcanic front’ (closest to the submarine trench) trace element signatures, including high Sr/Nd and U/Th (and down to ~600 km in the northwest beneath the Witu U disequilibrium), together with positive Eu anomalies that can Islands. The reasons for the correspondingly large width be related to the mobility of Eu in the slab-derived flux. Negative of the volcanic arc are still unknown, but isotopic and Ce anomalies are attributed to a minor sedimentary component. elemental differences between the rocks are systematic Across-arc geochemical profiles record a decrease in the degree of as depths to the Wadati–Benioff zone increase. We believe partial melting and diminishing influence of a slab-derived fluid that these differences provide unparalleled insights into with depth, superimposed upon the depleted mantle composition the geochemical architecture of subduction-zone systems. beneath the volcanic front. Element partitioning into (and not (2) Rocks of the New Britain volcanic front are low in necessarily the source of) the fluid is considered to exert strong control potassium, range from basalt to rhyolite, and have (as on the chemistry of volcanic front magmas, a feature that may go illustrated below) exceptional depletions in high field some way to explaining the contradictory estimates of the slab flux strength elements (HFSE). Indeed, they may represent derived from isotope vs trace element data in many subduction suites. the most HFSE-depleted arc rocks known. These rocks

Chemical and Isotopic Composition of Lavas from the Northern Mariana Trough: Implications for Magmagenesis in Back-arc Basins

Abstract: We report the results of a geochemical and isotopic study of mostly basaltic glasses recovered from 25 dredge stations along the northernmost 500 km of the Mariana Trough extension axis. The distribution of samples links regions of seafloor spreading to the south with regions farther north where a progression of rifting styles accompanies the earliest stages of back-arc basin extension. Petrographic, chemical and isotopic compositions of igneous rocks reflect the changing styles of extension, with typical back-arc basin basalts in the south which become increasingly similar to arc lavas to the north. Felsic lavas also appear along the extensional axis in the north. Glassy, sparsely phyric basalts characterize regions of seafloor spreading. Felsic lavas and porphyritic basalts occur in the northern, rifting portion. Geochemical and isotopic compositions distinguish between mature arc portions (Ce/Pb 20; ^(206)Pb/^(204)Pb >18.5, ^(87)Sr/^(86)Sr >0.7032, ^eNd 10, Ba/La +7). Samples from along the extensional axis of the northern Mariana Trough show progressive changes in chemical and isotopic compositions, from back-arc basin basalts that formed by seafloor spreading northward through increasingly arc–like basalts, until lavas that are indistinguishable from arc lavas are encountered in the northernmost portion of the rift. Batch-melting models indicate that northernmost rift lavas reflect higher degrees of melting, with 13 ± 5% melting where seafloor spreading occurs, doubling to 28 ± 8% for the northernmost part of the rift axis. The greater degree of melting in the north reflects the greater amount of water added to the mantle source, reflecting the arc-like nature of the source region and melt generation style characteristic of the initial stages of back-arc basin formation. Our data indicate that F = 0.44W + 0.07, where F is the degree of mantle melting and W is the percent water in the mantle. ‘True’ back-arc basin basalts are generated by adiabatic decompression associated with mantle upwelling in mature extensional settings. Eruption of ‘true’ back-arc basin basalts accompanies seafloor spreading, which begins when the basin is 100–150 km wide. The arc is disrupted during early rift formation, because arc magmatism is captured by the extension axis, but the generation of arc melts by hydrous melting of the mantle wedge continues whether or not back-arc extension is occurring. Back-arc basin seafloor spreading requires development of an upwelling mantle flow regime, allowing melting by adiabatic decompression, similar to that responsible for mid-ocean ridge basalt (MORB). The arc begins to re-form once extension progresses nearly to the point of seafloor spreading.

The Roza Member, Columbia River Basalt Group: A gigantic pahoehoe lava flow field formed by endogenous processes?

Abstract: We present studies on the physical volcanology of the ∼15 Ma Roza Member of the Wanapum Formation in the Columbia River Basalt Group. The Roza Member represents a compound pahoehoe flood basalt lava flow field, with an area of ∼40,300 km 2 and a volume of 1300 km 3 . It consists of 4 major lava flows each composed of numerous, decimeter to kilometer long pahoehoe lobes. Roza lavas feature a wide range of pahoehoe surface structures, such as lava rise plateaus, tumuli, and surface breakouts, and we illustrate that the lava morphology is inconsistent with previous proposals of rapid emplacement for these lavas. An integral component of the Roza flow field is the sheet lobe with internal structures identical to those of inflated pahoehoe sheet lobes from Hawaii and Iceland, both at the same scale and at much larger scales. We identify a three-part division of the sheet lobes into basal crust, lava core, and lava crust, which are interpreted as the equivalent to the bottom crust, the liquid lava core, and the surface crust of an active inflating pahoehoe lobe. The upper lava crust grows continuously during lava emplacement and its growth rate can be approximated by conductive cooling. This relationship is used to calculate the emplacement time for individual Roza sheet lobes and to derive a first-order estimate on the duration of the Roza eruption. The results indicate that the emplacement of individual Roza lobes lasted for months to years and that the lava flow field was constructed over a period of at least 14 years. We propose that the Roza flows achieved great areal dimensions and thicknesses by inflation and endogenous growth. As the lava flowed from vent to flow front it traveled under an insulating crust which maintained cooling rates of <0.1°C/km and allowed for efficient transport of lava over distances up to 300 km.

Geochemical Evolution within the Tonga–Kermadec–Lau Arc–Back-arc Systems: the Role of Varying Mantle Wedge Composition in Space and Time

Abstract: New trace element and Sr, Nd, and Pb isotope data for lavas from the active Tonga-Kermadec arc in the southwest Pacific, the volcano of Niua fo'ou in the back-arc Lau Basin, and Pacific Ocean sediments from DSDP Sites 204 and 275, and ODP Site 596, are integrated with existing geochemical data for lavas from the Lau Basin, Samoa, the Louisville Ridge Seamount Chain (LR-SMC) and the extinct Lau Ridge arc, giving new insights into the petrogenesis of lavas in an active arc - back-arc system. Geochemical variations in Tonga-Kermadec arc lavas are the result of (1) differences in the amount and composition of the material being subducted along the arc, and (2) pre-existing heterogeneities in the upper mantle. Differences in the material being subducted beneath the arc have an important influence on the chemistry of the arc lavas. At the Kermadec Trench, ∼1 km thick layer of sediment is being subducted beneath the arc, compared with ∼200 m at the Tonga Trench. This results in the high Th/U and more radiogenic Pb isotope compositions of Kermadec lavas compared with Tonga lavas. The latter have Pb isotope compositions intermediate between those of Pacific sediments and Pacific mid-ocean ridge basalt (MORB), suggesting that much of the Pb in these lavas is derived from subducting Pacific Ocean crust. This is supported by the Pb isotope signatures of the subducting LR-SMC, which are also observed in lavas from the northern Tongan islands of Tafahi and Niuatoputapu. High field strength element (HFSE) and heavy rare earth element (HREE) concentrations are generally lower in Tongan lavas (particularly those from northern Tongan islands) than in Kermadec lavas. The Tonga Ridge basement, the proto-Tonga arc lavas (ODP Site 839) and the older Lau Ridge arc lavas are generally less depleted than the modern arc lavas. In the back-arc region, upper-mantle depletion as inferred from HFSE and HREE contents of the lavas broadly increases eastwards across the Lau Basin, whereas the subduction signature and volatile (CO and F) contents increase eastwards towards the modern arc. These observations suggest thai depletion is due to melt extraction during back-arc extension and vokanism, together with a long 'residence time' of mantle material within the mantle wedge. The upper mantle beneath the northernmost end of the Tonga arc and Lau Basin contains an ocean-island basalt (OIB) component derived from the Samoa plume to the north. This is reflected in high concentrations of Nb relative to other HFSE in lavas from Niua fo'ou, and Tafahi and Niuatoputapu islands at the northern end of the Tonga arc. Pb isotopes also suggest an LR-SMC contribution into Tafahi and Niuataputapu. Trace element and isotope modelling is used to investigate the combined effects of varying mantle source depletion and subduction on the geochemistry of the arc lavas. The results suggest that the arc lava geochemistry can be explained largely by the balance between a relatively constant subduction input of Pb, Th, U, Cs, Ba, Sr, Rb, K and Sc [corresponding to 0.001-0.005 weight fraction of the Stolper & Newman (1994, Earth and Planetary Science Letters, 121, 293-325] 'HO-rich component' composition), into the overlying, but variably depleted mantle wedge.

Nature and Evolution of Cenozoic Lithospheric Mantle beneath Shandong Peninsula, Sino-Korean Craton, Eastern China

Abstract: The Shanwang and Qixia basalts lie within the North China block and were erupted in Miocene to Pliocene time (18.1 to 4.3 Ma) and Pliocene time (6.4 to 5.9 Ma), respectively. The Shanwang area lies astride the Tancheng-Lujiang (Tanlu) fault zone, a major lithospheric fault, whereas the Qixia area lies east of the fault zone. The basaltic rocks (alkali olivine basalts, basanites, nephelinites) carry abundant deep-seated xenoliths including spinel lherzolite (dominant), dunite, and pyroxenite, and a megacryst suite including augite, anorthoclase, phlogopite, ilmenite, and garnet. Xenoliths with coarse-grained microstructures are common in the Qixia xenolith suite, but are absent in Shanwang. Reconstructed bulk compositions of the lherzolites range from relatively depleted ( 12% modal diopside). Equilibration temperatures of 850° to 1020°C indicate entrainment of these lherzolites from depths ≤45 km, within the lithosphere; the geotherm may have been higher beneath Shanwang. T...

Some physical requirements for the emplacement of long basaltic lava flows

Abstract: Long basaltic lava flows (over 100 km in length) require specific emplacement conditions to prevent the lava from freezing as it is transported to the flow front. The minimum dimensions of the lava transport systems (tubes, channels, or sheets) require that the flow have a volume greater than several cubic kilometers. Long lava flows are emplaced on slopes less than 10% (∼5°) and the lava being transported must cool at a rate less than 0.5°C/km. We show that there are two modes by which thermally efficient, long distance lava transport can be achieved: (1) “rapid” emplacement in which the lava flows so quickly that it does not cool excessively despite large heat losses and (2) “insulated” emplacement in which heat loss is minimized. We here estimate cooling in the rapid mode using a modified version of a previously published thermal model for aa flows and find that, for a range of inputs appropriate for subaerial terrestrial condition, effusion rates of at least 3100 to 11000 m3/s, channel flow velocities in excess of 4–12 m/s, and minimum channel depths of 3–17 m are required for basaltic flows >100 km in length. For emplacement in the insulated mode, we construct a very simple heat balance model for roofed sheet flows which shows that extremely long sheet-fed flows are possible with velocities as low as 0.2–1.4 m/s, flow thickness of 6–23 m, and minimum effusion rates of the order of 50–7100 m3/s. Also, earlier work has suggested that tube-fed flows more than 100 km long can be produced at effusion rates as low as several tens of m3/s and with tube diameters of a few tens of meters. We argue that flows emplaced in the rapid mode should be morphologically similar to channel-fed aa flows while those emplaced in the insulated mode should be similar to tube-fed or sheet-like inflated pahoehoe flows. This leads to several field criteria for distinguishing these two modes of emplacement in ancient lava sequences. Additional constraints on the emplacement of long lava flows are expected from the continued study of the formation and evolution of lava channels, tubes, and sheets.

Evidence for pressure-release melting beneath magmatic arcs from basalt at Galunggung, Indonesia

Abstract: The melting of peridotite in the mantle wedge above subduction zones is generally believed to involve hydrous fluids derived from the subducting slab1. But if mantle peridotite is upwelling within the wedge, melting due to pressure release could also contribute to magma production. Here we present measurements of the volatile content of primitive magmas from Galunggung volcano in the Indonesian arc which indicate that these magmas were derived from the pressure-release melting of hot mantle peridotite. The samples that we have analysed consist of mafic glass inclusions in high-magnesium basalts. The inclusions contain uniformly low H2O concentrations (0.21–0.38 wt%), yet relatively high levels of CO2 (up to 750 p.p.m.) indicating that the low H2O concentrations are primary and not due to degassing of the magma. Results from previous anhydrous melting experiments on a chemically similar Aleutian basalts2 indicate that the Galunggung high-magnesium basalts were last in equilibrium with peridotite at ∼1,320 °C and 1.2 GPa. These high temperatures at shallow sub-crustal levels (about 300–600 °C hotter than predicted by geodynamic models1,3), combined with the production of nearly H2O-free basaltic melts, provide strong evidence that pressure-release melting due to upwelling in the sub-arc mantle has taken place. Regional low-potassium4 and low-H2O (ref. 5) basalts found in the Cascade arc indicate that such upwelling-induced melting can be widespread.

High-Temperature Silicate Volcanism on Jupiter's Moon Io

Abstract: Infrared wavelength observations of Io by the Galileo spacecraft show that at least 12 different vents are erupting lavas that are probably hotter than the highest temperature basaltic eruptions on Earth today. In at least one case, the eruption near Pillan Patera, two independent instruments on Galileo show that the lava temperature must have exceeded 1700 kelvin and may have reached 2000 kelvin. The most likely explanation is that these lavas are ultramafic (magnesium-rich) silicates, and this idea is supported by the tentative identification of magnesium-rich orthopyroxene in lava flows associated with these high-temperature hot spots.

Pb isotopic variability in melt inclusions from oceanic island basalts, polynesia

Abstract: Previous studies have suggested that melting processes are responsible for the trace element variability observed in olivine-hosted basaltic melt inclusions. Melt inclusions from three individual lava samples (two from Mangaia, Cook Islands, and one from Tahaa, Society Islands) have heterogeneous Pb isotopic compositions, even though the erupted lavas are isotopically homogeneous. The range of Pb isotopic compositions from individual melt inclusions spans 50 percent of the worldwide range observed for ocean island basalts. The melt inclusion data can be explained by two-component mixing for each island. Our data imply that magmas with different isotopic compositions existed in the volcanic plumbing system before or during melt aggregation.

The Geochemistry of Volcanic Rocks from Pantelleria Island, Sicily Channel: Petrogenesis and Characteristics of the Mantle Source Region

Abstract: Major and trace element, Sr–Nd–Pb isotope and mineral chemical at least two distinct geochemical components: a mid-ocean ridge basalt (MORB) source, relatively depleted component, and a data are presented for mafic and felsic volcanic rocks from the island of Pantelleria. The mafic rocks, mostly basalts, range from hyHIMU-like enriched component. A further enriched component, normative transitional basalts, through alkali basalts, to basanites. similar to the Enriched Mantle 1 (EM 1) component, could also Clinopyroxene in the mafic rocks varies in composition from Al, Tihave been involved. According to geophysical data, the lithosphere poor diopside to Al, Ti-rich augite. These two populations can be is thinned beneath the island, and the asthenospheric mantle rises present simultaneously in the same sample and even in the same to a depth of 60 km. Rare earth element data require residual garnet crystal, suggesting polybaric fractionation in the pressure range 0–4 in the source and constrain the melting process to a depth of kbar, or mixing between basaltic magmas with different degrees of 70–80 km. The petrological and geochemical data suggest that the alkalinity. On the basis of their major and trace element and mafic magmas are generated within the asthenospheric mantle, from Sr–Nd–Pb isotope composition and age of eruption, two groups of a deep plume bringing the HIMU–EM 1 isotopic and trace element basalts are distinguished: a high TiO2–P2O5 group, erupted before signatures. Interaction of these OIB-like magmas with the shallower 50 ka BP, and a low TiO2–P2O5 group, erupted after 50 ka BP, asthenospheric mantle, providing a depleted MORB signature, separated by a caldera collapse. The felsic volcanic rocks have gives rise to magmas with the observed isotopic and geochemical compositions ranging from comenditic trachyte to comendite and characteristics. pantelleritic trachyte to pantellerite, with progressively increasing peralkalinity. The Sr–Nd isotope compositions of most of the felsic volcanic rocks are similar to those of the mafic volcanic rocks, except for some very Sr-poor pantellerites, which show post-depositional

Formation of lava tubes and extensive flow field during the 1991–1993 eruption of Mount Etna

Abstract: Detailed mapping during the 1991–1993 eruption of Mount Etna has shown that there is a relationship between tumuli, ephemeral vents, lava tubes, and their parent lava flows. During this eruption, many tubes formed in stationary, inflated ‘a’a lava flows. Ephemeral vents at the fronts of these stationary flows and above lava tubes fed secondary lava flows, many of which subsequently developed new tubes. The resulting complex network of tubes, ephemeral vents, and secondary flows was responsible for most of the widening, thickening, and lengthening of the 1991–1993 Etna lava flow field. The supply of relatively uncooled lava via tubes to distal parts of this flow field allowed lava to flow 3 km farther from the vent than the longest channel-fed lava flow. Our observations suggest that lava tubes play a more important role in the formation of extensive ‘a’a flow fields on Etna than has previously been recognized.

Origin of the Indian Ocean-type isotopic signature in basalts from Philippine Sea plate spreading centers: An assessment of local versus large-scale processes

Abstract: Basalts erupted from spreading centers on the Philippine Sea plate between 50 Ma and the present have the distinctive isotopic characteristics of Indian Ocean mid-ocean ridge basalt (MORB), such as high 208Pb/204Pb and low 143Nd/144Nd for a given 206Pb/204Pb compared with Pacific and Atlantic Ocean MORB. This feature may indicate that the upper mantle of the Philippine Sea plate originated as part of the existing Indian Ocean upper mantle domain, or, alternatively, that local processes duplicated these isotopic characteristics within the sub-Philippine Sea plate upper mantle. Synthesis of new and published isotopic data for Philippine Sea plate basin basalts and island arc volcanic rocks, radiometric ages, and tectonic reconstructions of the plate indicates that local processes, such as contamination of the upper mantle by subducted materials or by western Pacific mantle plumes, did not produce the Indian Ocean-type signature in Philippine Sea plate MORB. It is more likely that the plate originated over a rapidly growing Indian Ocean upper mantle domain that had spread into the area between Australia/New Guinea and southeast Asia before 50 Ma.

Etendeka Volcanism of the Goboboseb Mountains and Messum Igneous Complex, Namibia. Part I: Geochemical Evidence of Early Cretaceous Tristan Plume Melts and the Role of Crustal Contamination in the Paraná–Etendeka CFB

Abstract: The Goboboseb Mountains and Messum Complex represent a major Cretaceous (132 Ma) bimodal eruptive centre in the southern Etendeka continental flood basalt (CFB) province. The eruptives compris the Awahab Formation and are represented by a lower sequence of mafic lavas, followed by the Goboboseb quartz latite members, the Messum Mountain Basalts, and finally the Springbok quartz latite. The sequence is cut by numerous dolerite dykes, sills and plugs, rare rhyolite, and carbonatite. The mafic lavas comprise two distinct series, which although corresponding broadly to the Etendeka regional low Ti and Zr basalts (LTZ type), are distinguished by Ti/Zr ratios into the LTZ.H (higher Ti/Zr) and LTZ.L (lower Ti/Zr) series. The LTZ.H basalts have no previously described extrusive equivalent in the Etendeka (or Parana) CFB, and consist of magnesian, mildly alkaline to tholeiitic lavas, dominated by oliv + cpx phenocryst assemblages which fractionate (near the surface) to phono-tephrite. They are identified as predominantly mantle plume melts (Tristan-Walvis plume). The LTZ.L lavas are less magnesian, extending to icelandites, are tholeiitic, with cpx ± oliv + pl + Fe-Ti oxide phenocryst assemblages and groundmass pigeonite and subcalcic augite. Stratigraphically, the LTZ.H lavas are interbedded with LTZ.L types in the lower part of the sequence and also occur as dykes. Within the Messum Complex, a remnant early sequence of basalts (Messum Carter Basalts) are in part transitional between the LTZ.L and LTZ.H series. The LTZ.H, and at least some of the LTZ.L lavas are inferred to have been erupted from the Goboboseb-Messum Centre. Chemically, the LTZ.H melts are broadly intermediate between E-MORB and OIB magmas, with higher Ti/Zr Sm/ γb and Ti/γ ratios than the LTZ.L types, which suggest segregation depths between the garnet and spinel peridotite stability fields. The Pb-Nd-Sr isotopic compositions of the LTZ.H eruptives are similar to, but not identical with the modern Tristan plume composition, and the observed variability is attributable to limited lower-crust assimilation and/or Atlantic MORB source mixing. The LTZ.L lavas show evidence for crystal fractionation, have 'arc-like' trace element signatures, correlated e-SiO, e-Ti/γ and -Ti/Zr, e-1/Sr and 1/Nd-e variations, and relatively radiogenic Pb, evolved Sr(e, 58-174) and low Nd(e -6·1 to -9·5) isotopic compositions. Their geochemistry is inferred to be AFC (assimilation-fractional crystallization) controlled, and is modelled by three-component mixing involving mantle plume derived melt, mafic lower crust and silicic mid-upper crust. The voluminous quartz latites (Part II, Ewart et al., 1978) extend these geochemical trends.