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Showing papers on "Basalt published in 2000"


Journal ArticleDOI
TL;DR: The early stage of collision-related volcanism, which was most evident during the Early Miocene (<21 Ma), produced a considerable volume of lavas and pyroclastic deposits of basaltic andesite to rhyolite composition.

821 citations


Journal ArticleDOI
03 Mar 2000-Science
TL;DR: In this paper, the authors used TES data from the Mars Global Surveyor (MGS) to determine compositions and distributions of martian low-albedo regions, and two surface spectral signatures are identified from low albedo region.
Abstract: Thermal Emission Spectrometer (TES) data from the Mars Global Surveyor (MGS) are used to determine compositions and distributions of martian low-albedo regions. Two surface spectral signatures are identified from low-albedo regions. Comparisons with spectra of terrestrial rock samples and deconvolution results indicate that the two compositions are a basaltic composition dominated by plagioclase feldspar and clinopyroxene and an andesitic composition dominated by plagioclase feldspar and volcanic glass. The distribution of the two compositions is split roughly along the planetary dichotomy. The basaltic composition is confined to older surfaces, and the more silicic composition is concentrated in the younger northern plains.

657 citations


Journal ArticleDOI
TL;DR: In this article, the major, trace element, and Nd-Sr-Pb isotopic compositions of mantle xenolith-bearing Cenozoic basalts in southeastern China are measured to provide an insight into the nature of their mantle sources and processes.

449 citations



Journal ArticleDOI
TL;DR: In this paper, a detailed isotope and trace element study of volcanic rocks from Umnak Island, Central Aleutian Arc is presented, where the data require the addition of three distinct subduction components to the subarc mantle, which are characterized and quantified: (1) a hydrous fluid from subducted oceanic crust, with mid-ocean ridge basalt (MORB)-like isotopic compositions but high Pb/Nd, and (2) a sediment partial melt, with sediment-like isotope compositions and high Th/N
Abstract: [1] Plate processing at convergent margins plays a central role in the distribution of elements among major earth reservoirs. The mechanisms by which this distribution occurs, however, have remained poorly constrained. This paper provides new constraints through a detailed isotope and trace element study of volcanic rocks from Umnak Island, Central Aleutian Arc. The data require the addition of three distinct subduction components to the subarc mantle, which are characterized and quantified: (1) a hydrous fluid from subducted oceanic crust, with mid-ocean ridge basalt (MORB)-like isotopic compositions but high Pb/Nd; (2) a hydrous fluid from subducted sediment, with sediment-like isotopic compositions and an enrichment in fluid-mobile elements; (3) a sediment partial melt, with sediment-like isotopic compositions and high Th/Nd and Th/Nb compared to both regional and global sediments. The sediment melt is depleted in fluid-mobile elements, indicating loss of fluid prior to melting. The high Th/Nb of the sediment melt indicates presence of a Ti-rich residual phase such as rutile during partial melting. The observation that sediment fluid and sediment melt can be distinguished in different volcanic rocks suggests that they arrive separately at the sites of arc magma formation. This indicates release of multiple discrete fluid and melt phases from sediment to the overlying mantle wedge, which can be viewed as a natural consequence of progressive metamorphism of the subducting slab.

393 citations


Journal ArticleDOI
TL;DR: In this article, seismic reflection data along the Atlantic and Western Australia rifted margins reveals six characteristic volcanic seismic facies units named (1) Landward flows, (2) Lava Delta, (3) Inner Flows, (4) Inner Seaward Dipping Reflectors (Inner SDR), (5) Outer High, and (6) Outer SDR).
Abstract: Large-volume extrusive basaltic constructions have distinct morphologies and seismic properties depending on the eruption and emplacement environments. The presence and amount of water is of main importance, while local rift basin configuration, erosion, and resedimentation determine the overall geometry of the volcanic constructions. We have developed the concept of seismic volcanostratigraphy, a subset of seismic stratigraphy, to analyze volcanic deposits imaged on seismic reflection data. The method places special focus on identification and mapping of seismic facies units and the volcanological interpretation of these units. Interpretation of seismic reflection data along the Atlantic and Western Australia rifted margins reveals six characteristic volcanic seismic facies units named (1) Landward Flows, (2) Lava Delta, (3) Inner Flows, (4) Inner Seaward Dipping Reflectors (Inner SDR), (5) Outer High, and (6) Outer SDR. These units are interpreted in terms of a five-stage tectonomagmatic volcanic margin evolution model comprising (1) explosive volcanism in a wet sediment, broad basin setting, (2) subaerial effusive volcanism forming Gilbert-type lava deltas along paleoshorelines, (3) subaerial effusive volcanism infilling a fairly narrow rift basin, (4) shallow marine explosive volcanism as the injection axis is submerged below sea level, and finally (5) deep marine sheet flow or pillow-basalt volcanism. Further, erosion and resedimentation processes are particularly important during the shallow marine stages. Seismic volcanostratigraphy provides important constraints on rifted-margin development, in particular, on the prevolcanic basin configuration, relative timing of tectonomagmatic events, total amount of volcanic rocks, location of paleoshorelines, and margin subsidence history. These parameters give key boundary conditions for understanding the processes forming volcanic margins and other large-volume basaltic provinces.

380 citations


Journal ArticleDOI
TL;DR: In this article, the authors report on new crater size-frequency distribution data for 139 spectrally and morphologically defined basalt units which are exposed in six nearside impact basins (Australe, Tranquillitatis, Humboldtianum, Humorum, Serenitatis and Imbrium).
Abstract: The chronology of lunar volcanism is based on radiometric ages determined from Apollo and Luna landing site samples, regional stratigraphic relationships, and crater degradation and size-frequency distribution data for units largely defined prior to the end of the Apollo program. Here we report on new crater size-frequency distribution data for 139 spectrally and morphologically defined basalt units which are exposed in six nearside impact basins (Australe, Tranquillitatis, Humboldtianum, Humorum, Serenitatis, and Imbrium). Crater size-frequency distribution measurements are a statistically robust and accurate method to derive absolute model ages of unsampled regions of the Moon. Compared to crater degradation ages, crater size-frequency ages, performed on spectrally defined units, offer significant improvements in accuracy. Our investigation showed that (1) in the investigated basins, lunar volcanism was active for at least 1.5–2.0 b.y., starting at about 3.9–4.0 b.y. and ceasing at ∼2.0 b.y., (2) most basalts erupted during the late Imbrian Period at about 3.6–3.8 b.y., (3) significantly fewer basalts were emplaced during the Eratosthenian Period, (4) basalts of Copernican age were not found in any of the investigated basins, (5) lunar basin-filling volcanism probably started within ∼100 m.y. after the formation of the individual basins. We also assessed the relationship between impact basin age and the history of mare basalt emplacement in these basins. We found that (1) in all pre-Nectarian basins (Australe and Tranquillitatis) as well as in the Humboldtianum basin, which is of Nectarian age, the distribution of surface ages is clearly dominated by only a single peak in the number of erupted units at 3.6–3.8 b.y., (2) in the younger basins (Humorum, Serenitatis, and Imbrium) a second peak at 3.3–3.5 b.y. is observed, (3) basalt eruptions younger than 2.6 b.y. occur only intermittently, and (4) in the youngest basins, Serenitatis and Imbrium, we see an extended period of active basin-filling volcanism (1.5–1.6 b.y.) which is 500 m.y. longer than in the Australe and Humorum and even ∼1.0 b.y. longer than in Tranquillitatis and Humboldtianum.

324 citations


Journal ArticleDOI
TL;DR: In this article, the authors modeled the Moon's thermal evolution using a simple thermal conduction model and showed that as a result of the high abundance of heat-producing elements that are found in the Procellarum KREEP Terrane, partial melting of the underlying mantle is an inevitable outcome.
Abstract: Geophysical, remote-sensing, and sample data demonstrate that the Procellarum and Imbrium regions of the Moon make up a unique geochemical crustal province (here dubbed the Procellarum KREEP Terrane). Geochemical studies of Imbrium's ejecta and the crustal structure of the Imbrium and Serenitatis basins both suggest that a large portion of the lunar crust in this locale is composed of a material similar in composition to Apollo 15 KREEP basalt. KREEP basalt has about 300 times more uranium and thorium than chondrites, so this implies that a large portion of Moon's heat-producing elements is located within this single crustal province. The spatial distribution of mare volcanism closely parallels the confines of the Procellarum KREEP Terrane and this suggests a causal relationship between the two phenomena. We have modeled the Moon's thermal evolution using a simple thermal conduction model and show that as a result of the high abundance of heat-producing elements that are found in the Procellarum KREEP Terrane, partial melting of the underlying mantle is an inevitable outcome. Specifically, by placing a 10-km KREEP basalt layer at the base of the crust there, our model predicts that mare volcanism should span most of the Moon's history and that the depth of melting should increase with time to a maximum depth of about 600 km. We suggest that the 500-km seismic discontinuity that is observed in the Apollo seismic data may represent this maximum depth of melting. Our model also predicts that the KREEP basalt layer should remain partially molten for a few billion years. Thus the Imbrium impact event most likely excavated into a partially molten KREEP basalt magma chamber. We postulate that the KREEP basalt composition is a by-product of mixing urKREEP with shallow partial melts of the underlying mantle. Since Mg-suite rocks are likely derived from crystallizing KREEP basalt, the provenance of these plutonic rocks is likely to be unique to this region of the Moon.

306 citations


Journal ArticleDOI
TL;DR: In this article, the authors measured oxygen isotope ratios in 72 arc and back-arc lavas from five ocean-ocean subduction zone systems using laser-fluorination analyses of olivine and other phenocrysts and glass.
Abstract: Variations of oxygen isotope ratios in arc-related lavas can constrain the contributions of subducted crustal igneous rocks, sediments, and fluids to the sub-arc mantle. We have measured oxygen isotope ratios in 72 arc and back-arc lavas from five ocean–ocean subduction zone systems using laser-fluorination analyses of olivine and other phenocrysts and glass. Eighty percent of our samples have {delta}18O values for any given phase (olivine, plagioclase, glass, or biotite) within 0·2{per thousand} of the average value for that phase in upper-mantle peridotites and mid-ocean ridge basalt (MORB); the range for each phase is <=1·0{per thousand}. This result contrasts with previous studies of whole-rock samples (which are significantly more variable even after exclusion of samples believed to be altered or fractionated by magmatic differentiation) and demonstrates that most arc-related lavas contain <=1–2% of 18O-enriched crustal oxygen from any source (i.e. assimilation or subducted contributions). Elevations in {delta}18O that do occur in these basic, arc-derived magmas relative to values most common for mantle-derived lavas are associated both with ‘enriched’ radiogenic isotope signatures and, even more strongly, with chemical indices consistent with high integrated extents of melting of their peridotite sources. We interpret these relationships as evidence that melting in the sources of the high-{delta}18O lavas we have studied was fluxed by addition of high-{delta}18O aqueous fluid (or perhaps a hydrous melt) from the subducted slab, such that sources that contain relatively large components of slab-derived fluid or melt are both relatively 18O enriched and also experienced relatively large amounts of melting. We have developed a quantitative model linking the amount of melting to the extents of 18O, radiogenic isotope, and trace-element enrichment in a mantle source being fluxed by addition of aqueous fluid. Comparison of this model with observed variations in the geochemistry of lavas from the Vanuatu–Fiji–New Caledonia region (the suite of related samples showing the greatest range in {delta}18O observed in this study) constrains the amounts and chemical and isotopic compositions of slab-derived phases in the sources of these arc-related lavas. Assuming a {delta}18O value of 20{per thousand} for the slab-derived fluid, 0·5–1·0 wt % is added to the sources of most mantle-derived arc magmas; the maximum amount of slab-derived flux in the sources of arc magmas according to our results is 2·5 wt %.

287 citations


Journal ArticleDOI
TL;DR: In this article, a single section of abyssal peridotite from the Kane Transform area in the Atlantic Ocean was examined for 190Pt−186Os and 187Re−187Os systematics.

281 citations


Journal ArticleDOI
TL;DR: A linear spectral deconvolution model was applied to both surface-only Cimmeria spectra using a library of 60 minerals to determine the composition and abundance of the component minerals as discussed by the authors.
Abstract: The Mars Global Surveyor Thermal Emission Spectrometer (TES) instrument collected 4.8×106 spectra of Mars during the initial aerobraking and science-phasing periods of the mission (September 14, 1997, through April 29, 1998). Two previously developed atmosphere-removal models were applied to data from Cimmeria Terra (25°S, 213°W). The surface spectra derived for these two models agree well, indicating that the surface and atmosphere emission can be separated and that the exact atmosphere-removal model used has little effect on the derived surface composition. The Cimmeria spectra do not match terrestrial high-silica igneous rocks (granite and rhyolite), ultramafic igneous rocks, limestone, or quartz- and clay-rich sandstone and siltstone. A particulate (sand-sized) sample of terrestrial flood basalt does provide an excellent match in both spectral shape and band depth to the Cimmeria spectrum over the entire TES spectral range. No unusual particle size effects are required to account for the observed spectral shape and depth. The implied grain size is consistent with the thermal inertia and albedo of this region, which indicate a sand-sized surface with little dust. The identification of basalt is consistent with previous indications of pyroxene and basalt-like compositions from visible/near-infrared and thermal-infrared spectral measurements. A linear spectral deconvolution model was applied to both surface-only Cimmeria spectra using a library of 60 minerals to determine the composition and abundance of the component minerals. Plagioclase feldspar (45%; 53%) and clinopyroxene (26%; 19%) were positively identified above an estimated detection threshold of 10–15% for these minerals. The TES observations provide the first identification of feldspars on Mars. The best fit to the Mars data includes only clinopyroxene compositions; no orthopyroxene compositions are required to match the Cimmeria spectra. Olivine (12%; 12%) and sheet silicate (15%; 11%) were identified with lower confidence. Carbonates, quartz, and sulfates were not identified in Cimmeria at detection limits of ∼5, 5, and 10%, respectively. Their presence elsewhere, however, remains open. The Cimmeria spectra are not well matched by any one SNC meteorite spectrum, indicating that this region is not characterized by a single SNC lithology. The occurrence of unweathered feldspar and pyroxene in Cimmeria, together with the inferred presence of pyroxene and unweathered basalts in other dark regions and at the Viking and Pathfinder landing sites, provides evidence that extensive global chemical weathering of materials currently exposed on the Martian surface has not occurred.

Journal ArticleDOI
TL;DR: In this paper, the concentrations and fluxes of major, minor and trace metals were determined in 53 samples of groundwaters from around Mt Etna, in order to evaluate the conditions and extent of alkali basalt weathering by waters enriched in magma-derived CO 2 and the contribution of aqueous transport to the overall metal discharge.

Journal ArticleDOI
TL;DR: The trace element compositions of eclogites, blueschists and mafic granulites from high-pressure terranes have been analysed to investigate element losses and fractionation that occur during dehydration of oceanic basalt in subduction zones as mentioned in this paper.

Journal ArticleDOI
TL;DR: In this article, the major and trace element and radiogenic isotope ratios (Sr, Nd and Pb) are presented for a suite of Neogene to Recent basalts (MgO>4 wt%) from the axial regions of the Kenya Rift.

Journal ArticleDOI
TL;DR: In this article, the authors present a new Ar/Ar ages for Mts Bambouto and Oku (western Highlands) and of the Ngaoundere Plateau, in the continental sector of anhydrous lherzolite source (asthenospheric component) and melts the Cameroon Volcanic Line.
Abstract: from variable amounts of mixing between melts derived from an The volcanic activity of Mts Bambouto and Oku (Western Highlands) and of the Ngaoundere Plateau, in the continental sector of anhydrous lherzolite source (asthenospheric component) and melts the Cameroon Volcanic Line, Equatorial West Africa, ranges in from an amphibole-bearing peridotite source (lithospheric HSr age from Oligocene to Recent. It is characterized by basanitic, alkali component). New Ar/Ar ages for Mts Oku and Bambouto basaltic and transitional basaltic series. Mineral chemistry, major basalts, combined with previous Ar/Ar and K/Ar ages of and trace element bulk-rock compositions, and geochemical modelling basaltic and silicic volcanics, and with volcanic stratigraphy, suggest suggest that the magmatic series evolved mainly at low pressure a NE–SW younging of the peak magmatic activity in the Western (2–4 kbar) through fractional crystallization of clinopyroxene and Highlands. This SW younging trend, extending from the Oligocene olivine±magnetite, at moderately hydrated (H2O= 0·5–1 wt %) volcanism in northern Cameroon (e.g. Mt Oku) to the still active and QFM (quartz–fayalite–magnetite) to QFM + 1 fO2 conMt Cameroon, suggests that the African plate is moving above a ditions. Basalts from Ngaoundere (Miocene to Quaternary) and deep-seated mantle thermal anomaly. However, the age and location from the early activity (31–14 Ma) of the Western Highlands have of the Ngaoundere volcanism does not conform to the NE–SW incompatible trace element and Sr–Nd isotopic compositions similar younging trend, implying that the continental sector of the Cameroon to those of oceanic Cameroon Line basalts, pointing to a similar Volcanic Line cannot be easily interpreted as the surface expression asthenospheric mantle source. By contrast, the late (15–4 Ma) of a single hotspot system. Western Highlands basanites and alkali basalts have anomalously high concentrations of Sr, Ba and P, and low concentrations of Zr, which are exclusive features of continental Cameroon basalts. The genesis of these latter magmas is consistent with derivation from an incompatible element enriched, amphibole-bearing lithospheric

Journal ArticleDOI
01 Nov 2000-Lithos
TL;DR: In this article, major and trace element and Sr-Nd isotopie data for the intermediate to silicic suites, to provide constraints on their petrogenesis, were reported.

Journal ArticleDOI
01 Nov 2000-Lithos
TL;DR: In this article, the authors proposed an ion exchange type of enrichment, in which the HFSE, LILE and LREE, mobilized during slab melting, are preferentially enriched in the metasomatized mantle, resulting in a diversity of post-collision magma compositions.

Journal ArticleDOI
TL;DR: In this article, the authors show that the negative Ti anomaly commonly observed in clinopyroxene from mantle peridotites cannot be explained by the Ti in coexisting orthopyroxenes, but can be explain by small degrees of partial enrichment.
Abstract: depleted compositions occur in the upper part of the lithospheric Geological and geophysical evidence indicates that at least 100 km mantle, which now is >100 km thick. Garnet peridotites are of Archaean to Proterozoic lithospheric mantle has been removed essentially undepleted, and Y–Ga–Zr relationships of the garnets from beneath large areas of eastern and southeastern China during are typical of Phanerozoic mantle. The overall highly fertile nature late Mesozoic to Cenozoic time. Mantle-derived xenoliths in Tertiary of the existing lithosphere requires that the Archaean and Proterozoic basalts from several localities across this region have been studied mantle that existed beneath the region in Palaeozoic times has been by X-ray fluorescence, electron microprobe and laser ablation microlargely or completely removed, and replaced by younger, hotter and probe–inductively coupled plasma-mass spectrometry to characterize more fertile material. This probably occurred by upwelling of this thinner lithosphere. Trace element patterns of clinopyroxenes in asthenospheric material during late Mesozoic to Cenozoic time, the peridotites from southeastern China can be divided into four underplating to form new lithosphere. The occurrence of rare depleted groups: fertile garnet lherzolites, fertile spinel (± garnet) lherzolites, xenoliths may show that some older mantle material is residual and and depleted and enriched peridotites. The addition of Nb, Sr, light coexists with younger material beneath southeastern China. rare earth elements, but not of Ti and Zr, suggests a metasomatizing agent containing both H2O and CO2. This study also demonstrates that the negative Ti anomaly commonly observed in clinopyroxene from mantle peridotites cannot be balanced by the Ti in coexisting orthopyroxene, but can be explained by small degrees of partial

Journal ArticleDOI
J.G. McHone1
TL;DR: In this paper, the authors show that there is no physical evidence of a plume mechanism for Early Mesozoic Pangaean rifting and magmatism in the Atlantic Ocean.

Journal ArticleDOI
03 Feb 2000-Nature
TL;DR: It is shown that 18O/16O ratios of MORBs are correlated with aspects of their incompatible-element chemistry, consistent with control of the oxygen-isotope and incompatible- element geochemistry of MORB by a component of recycled crust that is variably distributed throughout their upper mantle sources.
Abstract: Mid-ocean-ridge basalts (MORBs) are the most abundant terrestrial magmas and are believed to form by partial melting of a globally extensive reservoir of ultramafic rocks in the upper mantle. MORBs vary in their abundances of incompatible elements (that is, those that partition into silicate liquids during partial melting) and in the isotopic ratios of several radiogenic isotope systems. These variations define a spectrum between 'depleted' and 'enriched' compositions, characterized by respectively low and high abundances of incompatible elements. Compositional variations in the sources of MORBs could reflect recycling of subducted crustal materials into the source reservoir, or any of a number of processes of intramantle differentiation. Variations in ^(18)O/^(16)O (principally sensitive to the interaction of rocks with the Earth's hydrosphere) offer a test of these alternatives. Here we show that ^(18)O/^(16)O ratios of MORBs are correlated with aspects of their incompatible-element chemistry. These correlations are consistent with control of the oxygen-isotope and incompatible-element geochemistry of MORBs by a component of recycled crust that is variably distributed throughout their upper mantle sources.

Journal ArticleDOI
TL;DR: Fryer et al. as mentioned in this paper measured trace element and rare earth element contents of clinopyroxenes (cpx) in peridotites from ophiolite complexes from the Hellenic Peninsula: Vourinos, Pindos, Othris (Greece), and Bulqiza (Albania).

Journal ArticleDOI
TL;DR: In this paper, the melting relations of homogeneous peridotite + oceanic basalt mixtures were investigated at 3.5 GPa on a fertile peridotiite (MPY90) and oceanic basin (GA1) compositional join.
Abstract: Flood basalt provinces may constitute some of the most catastrophic volcanic events in the Earth's history. A popular model to explain them involves adiabatic ascent of plumes of anomalously hot peridotite from a thermal boundary layer deep in the mantle, across the peridotite solidus. However, peridotitic plumes probably require unreasonably high potential temperatures to generate sufficient volumes of magma and high enough melting rates to produce flood volcanism. This lead to the suggestion that low melting eclogitic or pyroxenitic heterogeneities may be present in the source regions of the flood basalts. In order to constrain petrogenetic models for flood basalts generated in this way, an experimental investigation of the melting relations of homogeneous peridotite + oceanic basalt mixtures has been performed. Experiments were conducted at 3.5 GPa on a fertile peridotite (MPY90)–oceanic basalt (GA1) compositional join. The hybrid basalt + peridotite compositions crystallised garnet lherzolite at subsolidus temperatures plus quenched ne-normative picritic liquids at temperatures just above the solidus, over the compositional range MPY90 to GA150MPY9050. The solidus temperature decreased slightly from ∼1500 °C for MPY90 to ∼1450 °C for GA150MPY9050. Compositions similar to GA130MPY9070 have 100% melting compressed into a melting interval which is approximately 50–60% smaller than that for pure MPY90, due to a liquidus minimum. During adiabatic ascent of hybrid source material containing a few tens of percent basalt in peridotite, the lower solidus and compressed solidus–liquidus temperature interval may conspire to substantially enhance melt productivity. Mixtures of recycled oceanic crust and peridotite in mantle plumes may therefore provide a viable source for some flood volcanics. Evidence for this would include higher than normal Fe/Mg values in natural primary liquids, consistent with equilibration with more Fe-rich olivine than normal pyrolitic olivine (i.e.

Journal ArticleDOI
TL;DR: In this paper, the authors infer that emplacement of the feeder-dike to the largest historical eruption in Iceland (that of Laki in 1783) increased shear stress in the South Iceland Seismic Zone and almost certainly triggered the largest (M∼7.1 in 1784) historical earthquake in Iceland.
Abstract: Volcanic systems are swarms of tectonic fractures and basalt volcanoes formed as a result of plate-pull (as the plates are pulled apart) associated with the mid-ocean ridges and the magma dynamics of the Iceland Mantle Plume. Most systems are 40–150 km long, 5–20 km wide, and develop a central volcano. They supply magma to all eruptions in Iceland. Data obtained in the last few years have greatly improved our knowledge of their volcanotectonic environment; as a result, the geometry of the plume is better constrained, and the crust, previously considered thin (∼10 km), is now modeled as thick (∼20–40 km). Depending on the location of the volcanic systems, their activity either decreases or increases faulting in the two main seismic zones. From this, we can infer that emplacement of the feeder-dike to the largest historical eruption in Iceland (that of Laki in 1783) increased shear stress in the South Iceland Seismic Zone and almost certainly triggered the largest (M∼7.1 in 1784) historical earthquake in Ic...

Journal ArticleDOI
TL;DR: In this article, the dominant rocks are basalts with geochemical characteristics distinct from those of mid-ocean ridge basalts and the presence of wood fragments, charcoal, pollen, spores and seeds in the shallow water sediments overlying the igneous basement show that the growth rate of the plateau was sufficient to form subaerial landmasses.

Journal ArticleDOI
TL;DR: In this article, chemical compositions, ages, and Nd and Sr isotopic compositions of Miocene basaltic volcanic rocks from the southwestern Basin and Range Province were measured, and other data obtained from the literature show that temporal variations in the Nd isotopic and age data provide information about the amount and timing of changes of the overall lithosphere thickness during extension.

Journal ArticleDOI
TL;DR: In this article, a simple heat balance model for an evolving magma chamber is used to make predictions of the time scales for magma differentiation, which are compared with geological and isotopic constraints on the rates of crystallization and differentiation.
Abstract: A simple heat balance model for an evolving magma chamber is used to make predictions of the time scales for magma differentiation, which are compared with geological and isotopic constraints on the rates of crystallization and differentiation. In a 10 km3 magma chamber releasing thermal energy at a rate of 100 MW, basalt and rhyolite magmas should reach 50% crystallization after 2500 and <1500 years, respectively. The tendency for phenocrysts to remain suspended in a cooling magma increases with melt viscosity and hence the degree of magma differentiation. The time scales of crystallization and differentiation may be estimated on the basis of the U-series isotope compositions of separated crystals and bulk rocks, crystal size distributions, and trace element and Sr isotope profiles in phenocryst phases. The last of these indicate crystal residence ages in the melt of tens of years up to 100 years. Short residence and even differentiation times are also obtained from Ra–Th isotope studies of alkali feldspars, and rocks that experienced alkali feldspar fractionation. However, much older ages of 103–105 years for separated phases have been reported from recent volcanic centres in St Vincent in the Lesser Antilles, Vesuvius in Italy, the Kenya Rift Valley and Long Valley, California. These old crystal ages are all from relatively evolved igneous rocks, as predicted from their higher melt viscosities and simple models of cooling and crystal settling. However, the old ages are also typically obtained for complex minerals that are not in bulk equilibrium with their host rocks, and so, apart from offering a minimum estimate of the age of the particular magmatic system, their significance for models of differentiation of the host magmas is not clear. An alternative approach is therefore to determine the variations in U–Th–Ra isotope compositions of bulk rocks reflecting different degrees of magma differentiation, and such data indicate that differentiation in more mafic magmas takes much longer than in more evolved magmas. For example, 50% fractional crystallization of basanite to produce phonolite on Tenerife took 105 years, whereas a further 50% fractional crystallization to generate the more evolved phonolites occurred within a few hundred years of eruption. On Tenerife the more mafic magmas fractionated at greater depths, and the rates of fractional crystallization were higher in the more evolved magmas studied. This is readily explained by a cooling model in which a large volume of primitive magma deep in the crust has a longer cooling time than a smaller body of differentiated magma at shallower depths in the crust.

Journal ArticleDOI
TL;DR: In this paper, the authors present new trace element and isotopic data on a selection of least evolved lavas in Iceland, and the relationship between the Iceland plume and the mid-Atlantic Ridge can be reexamined.
Abstract: [1] We present new trace element and isotopic data on a selection of least evolved lavas in Iceland. Pb isotopic compositions are variable: 206Pb/204Pb ranges from 18.05 in picrites to 19.15 in alkali basalts. Large excesses of Sr and Pb and a deficiency of Hf and Zr relative to rare earth elements are characteristic of the picritic lavas (Ce/Pb ≈ 10, Sr/Nd ≈ 35, and Sm/Hf ≈ 1.75), whereas more normal ratios are observed in the alkali basalts (Ce/Pb ≈ 40, Sr/Nd ≈ 10, and Sm/Hf ≈ 1.4). These variations are correlated to systematic changes in Sr, Nd, and Pb isotopic compositions. Using these new data, the relationship between the Iceland plume and the Mid-Atlantic Ridge can be reexamined. The 87Sr/86Sr versus 206Pb/204Pb relationship demonstrates clearly that all isotopic variations in Iceland have their origin in the plume source. In addition, the composition of both Reykjanes and Kolbeinsey Ridges results from contamination of the depleted mantle by the Iceland plume. Trace element and isotopic heterogeneities within Iceland are related to rock type and not to location in the island. This is contrary to what would be expected if a uniform plume mixed with surrounding mid-ocean ridge basalt (MORB), in which case the geochemical values would be more MORB-like at the perimeter. Instead, it suggests that the Iceland plume itself was highly heterogeneous. We propose that the source of the Iceland plume was old recycled oceanic crust that was stored in the mantle for a long time (potentially since the end of the Archean). Melting of the basaltic portion of this crust led to the formation of the alkali basalts, and melting of the clinopyroxene-plagioclase gabbroic portion of the same oceanic crust yielded the picritic basalts. In both cases, melting of the harzburgitic portion of the recycled lithosphere provided the necessary Mg and Ni. The tholeiites represent mixtures from the two sources. Iceland is therefore, together with Hawaii, a clear example of melting of a complete section of recycled oceanic lithosphere.

Journal ArticleDOI
TL;DR: In this article, a similar process formed the Martian flood lava surfaces and attempt to place constraints on the eruption parameters using thermal modeling, concluding that the thermophysical properties of the lava should be similar to fluid basalt, and that the average eruption rates were probably of the order of 104 m3/s, with the flood-like surges having flow rates of 105-106 m3 /s.
Abstract: The recent flood lavas on Mars appear to have a characteristic “platy-ridged” surface morphology different from that inferred for most terrestrial continental flood basalt flows. The closest analog we have found is a portion of the 1783–1784 Laki lava flow in Iceland that has a surface that was broken up and transported on top of moving lava during major surges in the eruption rate. We suggest that a similar process formed the Martian flood lava surfaces and attempt to place constraints on the eruption parameters using thermal modeling. Our conclusions from this modeling are (1) in order to produce flows >1000 km long with flow thicknesses of a few tens of meters, the thermophysical properties of the lava should be similar to fluid basalt, and (2) the average eruption rates were probably of the order of 104 m3/s, with the flood-like surges having flow rates of the order of 105–106 m3/s. We also suggest that these high eruption rates should have formed huge volumes of pyroclastic deposits which may be preserved in the Medusae Fossae Formation, the radar “stealth” region, or even the polar layered terrains.

Journal ArticleDOI
30 Apr 2000
TL;DR: In this paper, the authors proposed a model for the Iceland plume in which a heterogeneous core, derived from the lower mantle, consists of enriched streaks or blobs dispersed in a more depleted matrix.
Abstract: New Sr–Nd–Pb–Hf data require the existence of at least four mantle components in the genesis of basalts from the the North Atlantic Igneous Province (NAIP): (1) one (or more likely a small range of) enriched component(s) within the Iceland plume, (2) a depleted component within the Iceland plume (distinct from the shallow N-MORB source), (3) a depleted sheath surrounding the plume and (4) shallow N-MORB source mantle. These components have been available since the major phase of igneous activity associated with plume head impact during Paleogene times. In Hf–Nd isotope space, samples from Iceland, DSDP Leg 49 (Sites 407, 408 and 409), ODP Legs 152 and 163 (southeast Greenland margin), the Reykjanes Ridge, Kolbeinsey Ridge and DSDP Leg 38 (Site 348) define fields that are oblique to the main ocean island basalt array and extend toward a component with higher 176Hf/177Hf than the N-MORB source available prior to arrival of the plume, as indicated by the compositions of Cretaceous basalts from Goban Spur (∼95 Ma). Aside from Goban Spur, only basalts from Hatton Bank on the oceanward side of the Rockall Plateau (DSDP Leg 81) lie consistently within the field of N-MORB, which indicates that the compositional influence of the plume did not reach this far south and east ∼55 Ma ago. Thus, Hf–Nd isotope systematics are consistent with previous studies which indicate that shallow MORB-source mantle does not represent the depleted component within the Iceland plume [Thirlwall, J. Geol. Soc. London 152 (1995) 991–996; Hards et al., J. Geol. Soc. London 152 (1995) 1003–1009; Fitton et al., Earth Planet. Sci. Lett. 153 (1997) 197–208]. They also indicate that the depleted component is a long-lived and intrinsic feature of the Iceland plume, generated during an ancient melting event in which a mineral (such as garnet) with a high Lu/Hf was a residual phase. Collectively, these data suggest a model for the Iceland plume in which a heterogeneous core, derived from the lower mantle, consists of ‘enriched’ streaks or blobs dispersed in a more depleted matrix. A distinguishing feature of both the enriched and depleted components is high Nb/Y for a given Zr/Y (i.e. positive ΔNb), but the enriched component has higher Sr and Pb isotope ratios, combined with lower eNd and eHf. This heterogeneous core is surrounded by a sheath of depleted material, similar to the depleted component of the Iceland plume in its eNd and eHf, but with lower 87Sr/86Sr, 208Pb/204Pb and negative ΔNb; this material was probably entrained from near the 670 km discontinuity when the plume stalled at the boundary between the upper and lower mantle. The plume sheath displaced more normal MORB asthenosphere (distinguished by its lower eHf for a given eNd or Zr/Nb ratio), which existed in the North Atlantic prior to plume impact. Preliminary data on MORBs from near the Azores plume suggest that much of the North Atlantic may be ‘polluted’ not only by enriched plume material but also by depleted material similar to the Iceland plume sheath. If this hypothesis is correct, it may provide a general explanation for some of the compositional diversity and variations in inferred depth of melting [Klein and Langmuir, J. Geophys. Res. 92 (1987) 8089–8115] along the MAR in the North Atlantic.

Journal ArticleDOI
01 Oct 2000-Lithos
TL;DR: The Paresis ring complex as mentioned in this paper consists of over 90% rhyolites and comendites, with minor amounts of silica-understood basalt, phonolite and lamprophyre.