Showing papers on "Basalt published in 2002"

Vapour undersaturation in primitive mid-ocean-ridge basalt and the volatile content of Earth's upper mantle

Abstract: The analysis of volatiles in magmatic systems can be used to constrain the volatile content of the Earth’s mantle and the influence that magmatic degassing has on the chemistry of the oceans and the atmosphere. But most volatile elements have very low solubilities in magmas at atmospheric pressure, and therefore virtually all erupted lavas are degassed and do not retain their primary volatile signatures. Here we report the undersaturated pre-eruptive volatile content for a suite of mid-ocean-ridge basalts from the Siqueiros intra-transform spreading centre. The undersaturation leads to correlations between volatiles and refractory trace elements that provide new constraints on volatile abundances and their behaviour in the upper mantle. Our data generate improved limits on the abundances of carbon dioxide, water, fluorine, sulphur and chlorine in the source of normal mid-oceanridge basalt. The incompatible behaviour of carbon dioxide, together with the CO2/Nb and CO2/Cl ratios, permit estimates of primitive carbon dioxide and chlorine to be made for degassed and chlorine-contaminated mid-ocean-ridge basalt magmas, and hence constrain degassing and contamination histories of mid-ocean ridges.

Whole-Mantle Convection and the Transition-Zone Water Filter

Abstract: Because of their distinct chemical signatures, ocean-island and mid-ocean-ridge basalts are traditionally inferred to arise from separate, isolated reservoirs in the Earth's mantle. Such mantle reservoir models, however, typically satisfy geochemical constraints, but not geophysical observations. Here we propose an alternative hypothesis that, rather than being divided into isolated reservoirs, the mantle is filtered at the 410-km-deep discontinuity. We propose that, as the ascending ambient mantle (forced up by the downward flux of subducting slabs) rises out of the high-water-solubility transition zone (between the 660 km and 410 km discontinuities) into the low-solubility upper mantle above 410 km, it undergoes dehydration-induced partial melting that filters out incompatible elements. The filtered, dry and depleted solid phase continues to rise to become the source material for mid-ocean-ridge basalts. The wet, enriched melt residue may be denser than the surrounding solid and accordingly trapped at the 410 km boundary until slab entrainment returns it to the deeper mantle. The filter could be suppressed for both mantle plumes (which therefore generate wetter and more enriched ocean-island basalts) as well as the hotter Archaean mantle (thereby allowing for early production of enriched continental crust). We propose that the transition-zone water-filter model can explain many geochemical observations while avoiding the major pitfalls of invoking isolated mantle reservoirs.

Noble gases and volatile recycling at subduction zones

Abstract: Volatiles are lost from the Earth’s mantle to the atmosphere, hydrosphere and crust through a combination of subaerial and submarine volcanic and magmatic activity. These volatiles can be primordial in origin, trapped in the mantle since planetary accretion, produced in situ, or they may be recycled—re-injected into the mantle via material originally at the surface through the subduction process. Quantifying the absolute and relative contributions of these various volatile sources bears fundamental information on a number of issues in the Earth Sciences ranging from the evolution of the atmosphere and hydrosphere to the nature and scale of chemical heterogeneity in the Earth’s mantle. Noble gases have a pivotal role to play in addressing the volatile mass balance between the Earth’s interior and exterior reservoirs. The primordial isotope 3He provides an unambiguous measure of the juvenile volatile flux from the mantle (Craig et al. 1975). As such, it provides a means to calibrate other volatiles of geological and geochemical interest. A prime example is the CO2 flux at mid-ocean ridges (MOR): by combining estimates of the 3He flux at MOR with measurements of the CO2/3He ratio in oceanic basalts, Marty and Jambon (1987) derived an estimate of the CO2 flux from the (upper) mantle. The approach of using ratios (involving noble gas isotopes) has also been extended to island arcs. Marty et al. (1989) found significantly higher CO2/3He ratios in arc-related geothermal fluids than observed at mid-ocean ridges, consistent with addition of slab-derived CO2 to the mantle wedge. Sano and Williams (1996) scaled the CO2 flux to 3He, showing that the output of CO2 at subduction zones was comparable in magnitude to that at spreading ridges. Therefore, for CO2 at least, subduction zones also …

Mesozoic lithosphere destruction beneath the North China Craton: evidence from major-, trace-element and Sr–Nd–Pb isotope studies of Fangcheng basalts

Abstract: Major- and trace-element as well as Sr–Nd–Pb isotopic data of the Mesozoic Fangcheng basalts provide an insight into the nature of their mantle source and the secular evolution of the lithospheric mantle beneath the North China Craton. Fangcheng basalts include alkali basalt and olivine tholeiite, both characterized by high Mg (Mg#=65–72), Si, and Ca and low K+Na, Ti, and P. They are extremely enriched in LREEs ((La/Yb)N=39.3–49.3) and LILEs (Ce, Rb, Ba, U, Th) and depleted in HFSEs (Nb, Ta, Zr, Hf, Ti), with slightly negative Pb anomaly. Correspondingly, these basalts are exceedingly high in e Sr (74.0~81.5) and low in e Nd (–13.1~–14.2) and 206Pb/204Pb (<17.8). Since crustal contamination during the magma ascent is insignificant, the Fangcheng basalts could reflect the nature of its mantle source. The isotopic data of these basalts cannot be explained by mixing of typical mantle components, but can be accounted for by interaction of an old lithospheric mantle with the lower/middle crust. Therefore, we consider that these basalts originated from the Mesozoic lithospheric mantle, which evolved from its Paleozoic counterpart through extensive interaction with a crust-derived melt. We propose that this melt was generated from the melting of the subducted lower crust of the Yangtze Craton. This peculiar Mesozoic lithospheric mantle somehow was in turn replaced later by the hot and thin Cenozoic lithospheric mantle.

Plume-Associated Ultramafic Magmas of Phanerozoic Age

Abstract: the importance of ultramafic magmatism is often diminA parameterization of experimental data in the 0·2–7·0 GPa pressure range constrains both forward models of potential primary ished in models of Phanerozoic hotspot volcanism because magma compositions that exit the melting regime in the mantle and basalt eruptives are more common, and the plume model inverse models for computing the effects of olivine fractionation for is subject to debate (Anderson, 2000). Even where ultraany olivine-phyric lava suite. This is used to infer the MgO contents mafic lavas occur, they often have a large olivine phenoof primary magmas from Gorgona, Hawaii, Baffin Island and cryst content and this complicates interpretations of the West Greenland. They typically contain 18–20% MgO for wide MgO content of the primary magma. Indeed, the invariations in assumed peridotite source compositions, but MgO can terpretation that picrites can only form by the acdrop to 14–17% for Fe-enriched sources, and increase to 24–26% cumulation of olivine in basaltic liquids (Bowen, 1928) for fractional melts from Gorgona. Primary magmas with 18–20% went unchallenged until petrographic evidence demMgO have potential temperatures of 1520–1570°C. For Gorgona onstrated that skeletal olivine can grow in situ in ultrabasic picrites with 24% MgO, the potential temperature and initial liquids (Drever, 1956; Drever & Johnston, 1957). melting pressure were about 1700°C and 8·0 GPa, respectively; Discussions on primary magma compositions are often melting was hot and deep, consistent with the plume model. There reduced to the following shortlist of frequently asked are important restrictions to magma mixing in mantle plumes. questions: How can the composition of a lava flow be Primary magmas that exit the melting regime are both well-mixed used to derive the composition of its primary magma aggregate fractional melts and isolated fractional melts. The latter extracted from the melting regime in the mantle? How can originate from a hot plume axis and be in equilibrium with can an evaluation be made of the effects of olivine olivines having mg-numbers of 93·0–93·6, but they have MgO fractionation in crust and mantle lithosphere, and the contents and thermal characteristics that are difficult to constrain. possible sampling of wall-rock olivines from the melting regime? How can we best describe potentially complex fractional melting products of the mantle with equilibrium experimental data? Can the physics of the melt collection process be understood by identifying geochemical differ

Apatite in Igneous Systems

Abstract: Apatite is a minor but ubiquitous mineral in most igneous rocks. Although the modal proportion of apatite in common rocks is generally low, it can reach high concentrations in enclaves, cumulates, and other rocks of low abundance (i.e., rocks that constitute a small volume of the crust and mantle; e.g., nelsonites). The presence of apatite in most rocks is due not only to its low solubility in naturally occurring melts and aqueous solutions, but also to the limited ability of common rock-forming minerals to accept the amount of phosphorus that occurs in most rocks into their structure. In this paper, we will discuss some aspects of the occurrence, texture, composition, physical chemistry and petrogenetic significance of apatite in felsic rocks (i.e., andesite to rhyolite, and their plutonic analogs), in mafic (i.e., basalts and related rocks, and plutonic analogs), and ultramafic rocks of the Earth’s crust and mantle. Fluorapatite is by far the most common member of apatite family found in igneous rocks. However, most natural fluorapatite contains some chlorine and hydroxyl as well, and these constituents can attain high concentrations in some cases. The other halogens, bromine and iodine, also occur in apatite, but their concentrations are much lower than chlorine and fluorine. Many cations commonly substitute for calcium and phosphorus in apatite, however, they rarely reach concentrations that warrant the definition of a separate mineral species. Apatite can be described by the general formula A5(XO4)3Z (following Sommerauer and Katz-Lehnert 1985). The A-site accommodates large cations (e.g., Ca2+, Sr2+, Pb2+, Ba2+, Mg2+, Mn2+, Fe2+, REE3+, Eu2+, Cd2+, Na+) (Pan and Fleet, Ch. 2 in this volume), and comprises two sites that exhibit VII-fold …

Kerguelen Hotspot Magma Output since 130 Ma

Abstract: The Kerguelen hotspot (Indian Ocean) has produced basalt for …130 Myr, among the longest known volcanic records from a single source. New and published 40Ar/39Ar age determinations from the Kerguelen Plateau, Broken Ridge, Rajmahal Traps, and Bunbury basalts, and of Indian and Antarctic dikes help to document the hotspot’s history. Using radiometric dates and crustal structure determined from geophysical data and drilling results, we calculate the magmatic output of the Kerguelen hotspot through time. Output rates have varied in ways not predicted by current geodynamic models; maximum eruption volumes postdate the initial surface manifestation of the hotspot as well as break-up between Antarctica and India by ∶15 Myr, and magma output rates were high, as well as geographically diverse, over an interval of 25 Myr, from …120 to …95 Ma. We propose two alternatives to the standard mantle plume paradigm, one involving multiple plume sources, and another consisting of a single, but dismembered plume source.

Geochemistry of oceanic carbonatites compared with continental carbonatites: mantle recycling of oceanic crustal carbonate

Abstract: Major and trace element and Sr–Nd–Pb–O–C isotopic compositions are presented for carbonatites from the Cape Verde (Brava, Fogo, Sao Tiago, Maio and Sao Vicente) and Canary (Fuerteventura) Islands. Carbonatites show pronounced enrichment in Ba, Th, REE, Sr and Pb in comparison to most silicate volcanic rocks and relative depletion in Ti, Zr, Hf, K and Rb. Calcio (calcitic)-carbonatites have primary (mantle-like) stable isotopic compositions and radiogenic isotopic compositions similar to HIMU-type ocean island basalts. Cape Verde carbonatites, however, have more radiogenic Pb isotope ratios (e.g. 206Pb/204Pb=19.3–20.4) than reported for silicate volcanic rocks from these islands (18.7–19.9; Gerlach et al. 1988; Kokfelt 1998). We interpret calcio-carbonatites to be derived from the melting of recycled carbonated oceanic crust (eclogite) with a recycling age of ~1.6 Ga. Because of the degree of recrystallization, replacement of calcite by secondary dolomite and elevated ∂13C and ∂18O, the major and trace element compositions of the magnesio (dolomitic)-carbonatites are likely to reflect secondary processes. Compared with Cape Verde calcio-carbonatites, the less radiogenic Nd and Pb isotopic ratios and the negative Δ7/4 of the magnesio-carbonatites (also observed in silicate volcanic rocks from the Canary and Cape Verde Islands) cannot be explained through secondary processes or through the assimilation of Cape Verde crust. These isotopic characteristics require the involvement of a mantle component that has thus far only been found in the Smoky Butte lamproites from Montana, which are believed to be derived from subcontinental lithospheric sources. Continental carbonatites show much greater variation in radiogenic isotopic composition than oceanic carbonatites, requiring a HIMU-like component similar to that observed in the oceanic carbonatites and enriched components. We interpret the enriched components to be Phanerozoic through Proterozoic marine carbonate (e.g. limestone) recycled through shallow, subcontinental–lithospheric–mantle and deep, lower-mantle sources.

Global mineral distributions on Mars

Abstract: [1] Determining the mineralogy of Mars is an essential part of revealing the conditions of the surface and subsurface. A deconvolution method was used to remove atmospheric components and determine surface mineralogy from Thermal Emission Spectrometer data at 1 pixel per degree (ppd). Minerals are grouped into categories on the basis of compositional and spectral similarity, and global concentration maps are produced. All binned pixels are fit well with RMS errors of ≤0.005 in emissivity. Higher RMS errors are attributed to short wavelength particle size effects on dust-covered surfaces. Significant concentrations (>0.10) of plagioclase, high-Ca pyroxene, sheet silicates/high-Si glass, and hematite are detected and display distributions consistent with previous studies. Elevated concentrations of plagioclase and high-Ca pyroxene are consistent with basaltic surfaces and are located in low-albedo highlands regions north of ∼45°S. Significant concentrations of plagioclase and sheet silicates/high-Si glass and low concentrations of high-Ca pyroxenes are consistent with andesitic surfaces and are concentrated in both southern and northern high-latitude, low-albedo regions. Andesitic surfaces in the southern hemisphere have a lower spectral contrast than northern surfaces. An isolated surface located in Solis Planum is spectrally distinct but compositionally similar to other surfaces interpreted to be andesitic in composition. Concentrations of olivine below the detection limit correctly identify its presence in two of three locations. Potassium feldspar, low-Ca pyroxene, basaltic glass, olivine, sulfate, carbonate, quartz, and amphibole are not detected with confidence at 1 ppd. The results presented here indicate a predominance of volcanic compositions within Martian dust-free surfaces.

Spectral evidence for weathered basalt as an alternative to andesite in the northern lowlands of Mars

Abstract: Mineral abundances derived from the analysis of remotely sensed thermal emission data from Mars have been interpreted to indicate that the surface is composed of basalt (Surface Type 1) and andesite (Surface Type 2)1. The global distribution of these rock types is divided roughly along the planetary dichotomy which separates ancient, heavily cratered crust in the southern hemisphere (basalt) from younger lowland plains in the north (andesite)1. But the existence of such a large volume of andesite is difficult to reconcile with our present understanding of the geological evolution of Mars. Here we reinterpret martian surface rock lithologies using mineral abundances from previous work1 and new mineralogies derived from a spectral end-member set representing minerals common in unaltered and low-temperature aqueously altered basalts. Our results continue to indicate the dominance of unaltered basalt in the southern highlands, but reveal that the northern lowlands can be interpreted as weathered basalt as an alternative to andesite. The coincidence between locations of such altered basalt and a suggested northern ocean basin implies that lowland plains material may be composed of basalts weathered under submarine conditions or weathered basaltic sediments transported into this depocentre.

40Ar/39Ar Dates from the West Siberian Basin: Siberian Flood Basalt Province Doubled

Abstract: Widespread basaltic volcanism occurred in the region of the West Siberian Basin in central Russia during Permo-Triassic times. New 40 Ar/ 39 Ar age determinations on plagioclase grains from deep boreholes in the basin reveal that the basalts were erupted 249.4 ± 0.5 million years ago. This is synchronous with the bulk of the Siberian Traps, erupted further east on the Siberian Platform. The age and geochemical data confirm that the West Siberian Basin basalts are part of the Siberian Traps and at least double the confirmed area of the volcanic province as a whole. The larger area of volcanism strengthens the link between the volcanism and the end-Permian mass extinction.

Silicic volcanism: an undervalued component of large igneous provinces and volcanic rifted margins

Abstract: Silicic volcanic rocks are associated with most, if not all, continental ×ood basalt provinces and volcanic rifted margins, where they can form substantial parts of the eruptive stratigraphy and have eruptive volumes >10 4 km 3 . Poor preservation of silicic volcanic rocks following kilometer-scale uplift and denudation of the volcanic rifted margins, however, can result in only deeper level structural features being exposed (i.e., dike swarms, major intrusions, and deeply subsided intracaldera µlls; e.g., North Atlantic igneous province). The role of silicic magmatism in the evolution of a large igneous province and rifted margin may therefore be largely overlooked. There are silicic-dominated igneous provinces with eruptive volumes comparable to those of maµc large igneous provinces ( >10 6 km 3 ), but that have low proportions of basalt expressed at the surface. Some silicic large igneous provinces are associated with intraplate magmatism and continental breakup (e.g., Jurassic Chon Aike province of South America, Early Cretaceous eastern Australian margin), whereas others are tectonically and geochemically associated with backarc environments (e.g., Sierra Madre Occidental). Silicic volcanic rocks formed in these two environments are similar in terms of total eruptive volumes, dominant l ithologies, and rhyolite geochemistry, but show fundamental differences in tectonic setting and basalt geochemistry. Large-volume ignimbrites are the dominant silicic volcanic rock type of continental flood basalt and silicic large igneous provinces. Individual silicic eruptive units can have thicknesses, areal extents, and volumes that are comparable to, or exceed, in

40Ar/39Ar Geochronology of the Rajmahal Basalts, India, and their Relationship to the Kerguelen Plateau

Abstract: During the mid-Cretaceous, extensive magmatism occurred in the Indian Ocean to form volcanic portions of the southern and central Kerguelen Plateau, Elan Bank and Broken Ridge. Basalt was erupted also along the rifted margin of eastern India (Rajmahal). We investigated the ages of these Indian basalts using Ar-40/Ar-39 incremental-heating experiments on whole rocks. Our results are consistent with the hypothesis that the lava pile of similar to230 m thickness in the Rajmahal Hills, Jharkhand, and alkalic basalts in the Bengal Basin were emplaced at similar to118 Ma. Dykes intruded to the SW of the Rajmahal Hills appear to be 2-3 Myr younger than these lavas. Magmatic activity in eastern India therefore was contemporaneous with the final stage of volcanism at Ocean Drilling Program Site 1136 on the Southern Kerguelen Plateau (119-118 Ma), but older than final magmatism at Sites 749 and 750 on the Southern Kerguelen Plateau (112-110 Ma), Site 1137 on Elan Bank (108 Ma) and Site 1138 on the Central Kerguelen Plateau (100 Ma). By combining these age data with plate reconstructions that take into account the motion of hotspots in a convecting mantle, we suggest that eruption of the Rajmahal basalts, formation of the Southern Kerguelen Plateau, and Elan Bank's separation from India are best explained by the presence of the Kerguelen hotspot close to the eastern Indian margin just after 120 Ma.

Characteristics of volcanic rifted margins

Abstract: Volcanic rifted margins evolve by a combination of extrusive flood volcanism, intrusive magmatism, extension, uplift, and erosion. The temporal and spatial relationships between these processes are influenced by the plate tectonic regime; the preexisting lithosphere (thickness, composition, geothermal gradient); the upper mantle (temperature and character); the magma production rate; and the prevailing climatic system. Of the Atlantic rifted margins, 75% are believed to be volcanic, the cumulative expression of thermotectonic processes over 200 m.y. Volcanic rifted margins also characterize Ethiopia-Yemen, India-Australia, and Africa-Madagascar. The transition from continental flood volcanism (or formation of a large igneous province) to ocean ridge processes (mid-ocean ridge basalt) is marked by a prerift to synrift transition with formation of a subaerial and/or submarine seaward-dipping reflector series and a significant thickness (to 15 km) of juvenile, high-velocity lower crust seaboard of the continental rifted margin. Herein we outline the similarities and differences between volcanic rifted margins worldwide and list some of their diagnostic features.

Are the regional variations in Central American arc lavas due to differing basaltic versus peridotitic slab sources of fluids

Abstract: Central American arc volcanism shows strong regional trends in lava chemistry that result from differing slab contributions to arc melting. However, the mechanism that transfers slab-derived trace elements into the mantle wedge remains largely unknown. By using a dynamic model for mantle flow and fluid release, we model the fate of three different slab-fluid sources: sediment, ocean crust, and serpentinized mantle. In the open subarc system, sediments lose almost all their highly fluid mobile elements by ∼50 km depth, so other fluid sources are necessary to explain the slab signal in arc-lava compositions. The well-documented transition from lavas with a strong geochemical slab signature (i.e., high Ba/La ratios) found in Nicaragua to lavas with a weaker slab signature (i.e., low Ba/La ratios) erupted in Costa Rica seems easiest to produce by a higher fraction of serpentine-hosted fluids released from the deeply faulted, highly serpentinized lithosphere subducting beneath Nicaragua than from the less deeply faulted, thicker, amphibolitic oceanic-crust and oceanic-plateau lithosphere subducting beneath Costa Rica.

The andesite aqueduct: perspectives on the evolution of intermediate magmatism in west-central (105–99°W) Mexico

Abstract: Intermediate calc-alkaline magma (52–65% SiO2) in western-central Mexico is the focus of this paper, and the typically porphyritic andesites (57–65% SiO2) form large central volcanoes, whereas basaltic andesites (52–57% SiO2) are less porphyritic, and they are found as cones and flows but are absent from central volcanoes. Several studies of experimental phase equilibria on these lavas relate water concentration to the phenocryst assemblages and to the degree of crystallinity, so that the abundance, composition and variety of phenocrysts can be used to constrain the amount of water dissolved in the magmas. Thus, the plagioclase-rich andesites of Volcan Colima, Mexico, become so as a result of decompressional crystallisation at ~950 °C (the pyroxene phenocryst temperature), and lose their dissolved water (2.5 to 4.5 wt% H2O) which is inversely proportional to the modal abundance of plagioclase. The feeding magma to V. Colima, North America's most productive central volcano, is represented by hornblende lamprophyre, a lava type without plagioclase phenocrysts which requires at least 6 wt% water to reproduce the phenocryst assemblage. Thus, degassing of the V. Colima magmas, and of those of the other central volcanoes in the western-central Mexican volcanic belt, contributes essentially all their dissolved water to the conduit or to the atmosphere. The source of this magmatic water is related to the source of the intermediate magmas. For some this must lie in the mantle, as the incorporation of hornblende-lherzolite nodules in a hydrous andesite with hornblende phenocrysts could only have occurred while ascending through the mantle. Consistent with a mantle source is the composition of the olivine phenocrysts in Mexican lavas with 10 to 5% MgO, which is in the mantle range of Fo88–92. Accordingly, basaltic andesites and andesites with >5% MgO are candidates for a mantle source. The equilibration of intermediate magmas with the mantle, as illustrated by the experiments of various workers, requires that the magmas be hydrous at pressure. An additional constraint is that the activity of silica in the mantle must be equal to that in the hydrous magma at equilibrium. Using published and new experiments to define RTlnγSiO2 in hydrous liquids, this quantity is shown to vary as a function of liquid composition (H2O, MgO, Na2O+K2O), and it approaches zero for quartz-saturated hydrous liquids. Using appropriate values of RTlnγSiO2 for three intermediate lavas, the amount of water required to equilibrate with an olivine-orthopyroxene mantle source is calculated, and within error indicates that only the most silica-rich magma is at water saturation in the mantle, in agreement with published experimental work. Hydrous intermediate magmas, ascending from their hornblende-lherzolite source regions (~1 to 1.5 GPa) along the hydrous adiabat, may not encounter any phase boundaries until 0.2–0.4 GPa because of the increase in the thermal stability of hornblende in water-undersaturated magmas. Therefore, the phenocryst assemblages of hornblende-free andesites equilibrate at low pressures. The virtual absence of basalt in west-central Mexico ( 50% crystallinity, evidently also an eruptible limit for V. Colima andesitic lavas. If the lower limit of water dissolved in Mexican intermediate magmas is accepted as that required for phenocryst equilibration (~6 wt% water), and the upper limit as saturation in the mantle source at 1 GPa (~16 wt%) then, with an estimate of the volcanic and plutonic magma delivery rate (km3/106 year) per km of volcanic arc, the flux of water returned from the mantle along the 35,000-km, global subduction-related arc system can be estimated. Measurements of the volcanic flux are woefully few, and estimates from Mexico, the Lesser Antilles and central America show a range from 4 to 20 km3/106 year×km which, if subtracted from the isotopically constrained continental growth rate, gives the plutonic flux rate. This suggests that, of the magma flux ascending to the continental crust, only about a fifth reaches the surface. If the dissolved magmatic water limits are coupled with the volcanic and plutonic emplacement rates, then the amount of water returned by magmatism to the crust is crudely in balance with that subducted.

Upwelling of deep mantle material through a plate window: Evidence from the geochemistry of Italian basaltic volcanics

Abstract: 87 Sr/ 86 Sr– 206 Pb/ 204 Pb–eNd–eHf space. The isotopic compositions of the two end-members of these mixing arrays are assessed by least-squares regression. The mantle-derived component ( 206 Pb/ 204 Pb = 19.8, 87 Sr/ 86 Sr = 0.7025, eNd = +8, eHf = +9) is a rather homogeneous mixture of the standard high-m (HIMU) and depleted mantle (DM) components. The crust-derived component ( 206 Pb/ 204 Pb = 18.5, 87 Sr/ 86 Sr > 0.715, eNd = � 12, eHf = � 11) accounts for the enrichment of K and other large-ion-lithophile elements in the Italian volcanics. As shown by the relationship in eHf–eNd space and the lower-thanchondritic Hf/Sm ratio, this crustal component is dominated by pelagic sediments rather than terrigenous material. The overall scarcity of calc-alkaline compositions in the Italian volcanics and the presence of a HIMU component, which is the hallmark of hot spot basalts, raise the question of how plume mantle source contributes to volcanism in a subduction environment. At about 13 Ma, the Apennine collision terminated the westward subduction of the Adria plate under the European margin and rotated the direction of convergence to the northwest. The cumulative differential of subduction between the fossil plate under Tuscany and the active plate under Sicily since the opening of the Tyrrhenian Sea amounts to at least 300 km and is large enough to rift the dipping plate and open a plate window beneath the southern part of the peninsula. This model is consistent with recent high-resolution seismic tomography. We propose that the counterflow of mixed upper and lower mantle passing the trailing edge of the rifted plate is the source of Italian mafic volcanism. Alternatively, material from a so-far unidentified plume may be channeled through the plate window. The crustal signature is probably acquired by interaction of the mantle advected through the window with the upper part of the subducted plate. INDEX TERMS: 1749 History of Geophysics: Volcanology, geochemistry, and petrology; 1025 Geochemistry: Composition of the mantle; 1040 Geochemistry: Isotopic composition/chemistry; KEYWORDS: Italian volcanism, HIMU, subduction, pelagic sediments, mixing, slab window

Explosive silicic volcanism of the Yellowstone hotspot: The ash fall tuff record

Abstract: Unaltered silicic ash fall tuffs are abundant in Neogene sedimentary basins of the western U.S. and constitute an important record of explosive silicic volcanism in this region. In particular, ash fall tuffs from silicic volcanic centers along the Yellowstone hotspot track are common in these basins and provide a detailed record of explosive volcanism along the hotspot track. The available hotspot ash fall tuff record commences at ca. 16 Ma, shortly after the initiation of hotspot silicic volcanism at ca. 16.5 Ma, and continues through the most recent explosive eruptions in the late Pleistocene. Post–16 Ma hotspot silicic volcanism has been dominated by eruption of metaluminous ash flow tuffs and rhyolites, and ash fall tuffs produced by these eruptions dominate the Yellowstone hotspot ash fall tuff record. Evaluation of a well-dated composite sequence of 142 of these tuffs reveals systematic variation in magma composition, magma temperature, eruption frequency, and, possibly, volumetric discharge as the hotspot migrated eastward from the western edge of the North America craton to its current location in the Yellowstone Plateau. On the basis of these variations, three primary stages of metaluminous rhyolite magmatism (M stages) are recognized: M1 (16.0–15.2 Ma), M2 (15.2–7.5 Ma), and M3 (7.5–0 Ma). Each of these stages is marked by distinctive magma compositions, eruption frequencies, and magma temperature ranges and trends, with an overall decline in average eruption frequency and magma temperature from stage to stage. The partitioning of explosive hotspot volcanism into stages likely reflects variation in the style and intensity of the interaction between the mantle anomaly powering the hotspot magmatic systems and a spatially and temporally heterogeneous lithosphere along the hotspot track. Although the ash fall tuff record does not provide definitive constraints on the nature of these variable interactions, correlations between changes in eruption frequency, ash fall tuff discharge, and magma temperature point to variable input of mantle basalt into hotspot crustal magmatic systems as a first order control on intensity of explosive volcanism. As future studies reveal more about the processes controlling the variations in hotspot silicic volcanism, a fuller understanding of both the nature of silicic magmatism and the nature of the Yellowstone hotspot should emerge.

Petrogenesis of the Back-arc East Scotia Ridge, South Atlantic Ocean

Abstract: The East Scotia Ridge is an active back-arc spreading centre located to the west of the South Sandwich island arc in the South Atlantic Ocean, consisting of nine main segments, E1 (north) to E9 (south). Major and trace element and Sr–Nd–Pb isotope compositions are presented, together with water contents, for lavas sampled along the active ridge axis. Magmatism along the East Scotia Ridge is chemically heterogeneous, but there is a common mid-ocean ridge basalt (MORB)-type source component for all the magmas. An almost unmodified MORB-source mantle appears to underlie the central part of the back-arc. Subduction components are found at the northern and southern ends of the ridge, and there is a marked sediment melt input of up to 2% in segment E4. Enriched (plume) mantle is present beneath segment E2 at the northern end of the ridge, suggesting that plume mantle is flowing westward around the edges of the subducting slab. The southern part of segment E8 is unique in that its magma source is similar to sub-arc depleted mantle.

Mineralogy of the mid-ocean-ridge basalt source from neodymium isotopic composition of abyssal peridotites

Abstract: Inferring the melting process at mid-ocean ridges, and the physical conditions under which melting takes place, usually relies on the assumption of compositional similarity between all mid-ocean-ridge basalt sources. Models of mantle melting therefore tend to be restricted to those that consider the presence of only one lithology in the mantle, peridotite. Evidence from xenoliths and peridotite massifs show that after peridotite, pyroxenite and eclogite are the most abundant rock types in the mantle. But at mid-ocean ridges, where most of the melting takes place, and in ophiolites, pyroxenite is rarely found. Here we present neodymium isotopic compositions of abyssal peridotites to investigate whether peridotite can indeed be the sole source for mid-ocean-ridge basalts. By comparing the isotopic compositions of basalts and peridotites at two segments of the southwest Indian ridge, we show that a component other than peridotite is required to explain the low end of the (143)Nd/(144)Nd variations of the basalts. This component is likely to have a lower melting temperature than peridotite, such as pyroxenite or eclogite, which could explain why it is not observed at mid-ocean ridges.

Involvement of continental crust in the formation of the Cretaceous Kerguelen Plateau: New perspectives from ODP Leg 120 sites

Abstract: The basaltic basement of the large igneous province formed by the Kerguelen Plateau and Broken Ridge in the southeastern Indian Ocean has been sampled by the three Ocean Drilling Program cruises (Legs 119, 120 and 183). Although the Cretaceous parts of this plateau formed in the embryonic Indian Ocean basin, presumably by melting associated with the Kerguelen plume, trace element abundances and isotopic ratios of Sr, Nd and Pb of Cretaceous basalt from several drill sites indicate that continental lithosphere was involved in their petrogenesis. On the basis of relative depletions in Nb, Ta and Th, and isotopic characteristics similar to those of EMI ocean island basalt, lavas from Leg 120 Site 747 in the Central Kerguelen Plateau contain a component derived from the lower continental crust. On the basis of relative abundances of Sr and Eu and EMI-like Pb isotopic ratios, the source of basalt from Leg 120 Site 750 in the northeastern part of the Southern Kerguelen Plateau also contained a component derived from lower continental crust; in this case, the crustal component formed as a plagioclase-rich, clinopyroxene-bearing cumulate. Basalts from Leg 120 Site 749 define two distinct isotopic (Sr, Nd and Pb) groups which differ from the isotopic fields for Site 747 and 750 basalts. Among Site 749 lavas, there is subtle evidence for a continental component, broadly similar (i.e. moderate 200Pb/201Pb ~ 18.0) to that expressed more obviously in basalt from Leg 119, Site 738 on the southern edge of the Southern Kerguelen Plateau and Leg 183 Site 1137 on Elan Bank. The continental components in the Kerguelen Plateau basalts may have resided in a heterogeneous mantle plume that was formed, in part, by deep recycling of crust. It is more likely, however, that slivers of Gondwana lithosphere reside within the lithosphere and athemosphere of the Indian Ocean mantle where they contaminate both plume-derived and mid-ocean ridge basaltic magmas.