Showing papers on "Basalt published in 2006"

The Genesis of Intermediate and Silicic Magmas in Deep Crustal Hot Zones

Abstract: A model for the generation of intermediate and silicic igneous rocks is presented, based on experimental data and numerical modelling. The model is directed at subduction-related magmatism, but has general applicability to magmas generated in other plate tectonic settings, including continental rift zones. In the model mantlederived hydrous basalts emplaced as a succession of sills into the lower crust generate a deep crustal hot zone. Numerical modelling of the hot zone shows that melts are generated from two distinct sources; partial crystallization of basalt sills to produce residual H2O-rich melts; and partial melting of pre-existing crustal rocks. Incubation times between the injection of the first sill and generation of residual melts from basalt crystallization are controlled by the initial geotherm, the magma input rate and the emplacement depth. After this incubation period, the melt fraction and composition of residual melts are controlled by the temperature of the crust into which the basalt is intruded. Heat and H2O transfer from the crystallizing basalt promote partial melting of the surrounding crust, which can include meta-sedimentary and meta-igneous basement rocks and earlier basalt intrusions. Mixing of residual and crustal partial melts leads to diversity in isotope and trace element chemistry. Hot zone melts are H2O-rich. Consequently, they have low viscosity and density, and can readily detach from their source and ascend rapidly. In the case of adiabatic ascent the magma attains a super-liquidus state, because of the relative slopes of the adiabat and the liquidus. This leads to resorption of any entrained crystals or country rock xenoliths. Crystallization begins only when the ascending magma intersects its H2O-saturated liquidus at shallow depths. Decompression and degassing are the driving forces behind crystallization, which takes place at shallow depth on timescales of decades or less. Degassing and crystallization at shallow depth lead to large increases in viscosity and stalling of the magma to form volcano-feeding magma chambers and shallow plutons. It is proposed that chemical diversity in arc magmas is largely acquired in the lower crust, whereas textural diversity is related to shallow-level crystallization.

Trace element composition of mantle end‐members: Implications for recycling of oceanic and upper and lower continental crust

Abstract: [1] Recycling of oceanic crust together with different types of marine sediments has become somewhat of a paradigm for explaining the chemical and isotopic composition of ocean island basalts. New high-precision trace element data on samples from St. Helena, Gough, and Tristan da Cunha, in addition to recent data from the literature, show that the trace element and isotope systematics in enriched mantle (EM) basalts are more complex than previously thought. EM basalts have some common characteristics (e.g., high Rb/La, Ba/La, Th/U, and Rb/Sr and low Nb/La and U/Pb) that distinguish them from HIMU basalts (high μ = 238U/204Pb). The isotopically distinct EM-1 and EM-2 basalts, however, cannot be clearly distinguished on the basis of incompatible trace element ratios. Ultimately, each suite of EM basalts carries its own specific trace element signature that must reflect different source compositions. In contrast, HIMU basalts show remarkably uniform trace element ratios, with a characteristic depletion in incompatible trace elements (Rb, Ba, Th, U, and Pb) and enrichment in Nb and Ta relative to EM basalts. Compositional similarities between HIMU and EM basalts (e.g., Nb/U, La/Sm, La/Th, Sr/Nd, Ba/K, and Rb/K) suggest that their sources share a common precursor, most likely recycled oceanic lithosphere. The compositional differences between HIMU and EM basalts, on the other hand, can only be explained if the EM sources contain an additional heterogeneous component. Parent-daughter ratios in subducted marine sediments have a unimodal distribution. Recycling of sediments alone can therefore not account for the isotopic bimodality of EM basalts. The upper and lower continental crust have similarly variable trace elements ratios but are systematically distinct in their Rb/Sr, U/Pb, Th/Pb, and Th/U ratios. Thus the upper and lower continental crust evolve along two distinct isotopic evolution paths but retain their complex trace element characteristics, similar to what is observed in EM basalts. We therefore propose that recycling of oceanic lithosphere together with variable proportions of lower and upper continental crust, which are introduced into the mantle together with the oceanic lithosphere via subduction erosion and/or subduction of marine sediments, respectively, provides a plausible explanation for the trace element and isotope systematics in ocean island basalts.

An overview of adakite petrogenesis

Abstract: The term adakite was originally proposed to define silica-rich, high Sr/Y and La/Yb volcanic and plutonic rocks derived from melting of the basaltic portion of oceanic crust subducted beneath volcanic arcs. It was also initially believed that adakite only occurs in convergent margins where young and thus still hot oceanic slabs are being subducted, but later studies have proposed that it also occurs in other arc settings where unusual tectonic conditions can lower the solidus of older slabs. Currently, adakite covers a range of arc rocks ranging from pristine slab melt, to adakite-peridotite hybrid melt, to melt derived from peridotite metasomatized by slab melt. Adakite studies have generated some confusions because (1) the definition of adakite combines com- positional criteria with a genetic interpretation (melting of subducted basalt), (2) the definition is fairly broad and relies on chemistry as its distinguishing characteristic, (3) the use of high pressure melting experiment results on wet basalts as unequivocal proofs of slab melting and (4) the existence of adakitic rocks with chemical characteristics similar to adakites but are clearly unrelated to slab melting. Other studies have shown that adakitic rocks and a number of the previously reported adakites are pro- duced through melting of the mafic lower crust or ponded basaltic magma, high-pressure crystal fractionation of basaltic magma and low-pressure crystal fractionation of basaltic magma plus magma mixing processes in both arc or non-arc tectonic environments. Despite the confusing interpretations on the petrogenesis of adakite and adakitic rocks, their in- vestigations have enriched our understanding of material recycling at subduction zones, crustal evolu- tionary processes and economic mineralization.

Long-term volumetric eruption rates and magma budgets

Abstract: [1] A global compilation of 170 time-averaged volumetric volcanic output rates (Qe) is evaluated in terms of composition and petrotectonic setting to advance the understanding of long-term rates of magma generation and eruption on Earth. Repose periods between successive eruptions at a given site and intrusive:extrusive ratios were compiled for selected volcanic centers where long-term (>104 years) data were available. More silicic compositions, rhyolites and andesites, have a more limited range of eruption rates than basalts. Even when high Qe values contributed by flood basalts (9 ± 2 × 10−1 km3/yr) are removed, there is a trend in decreasing average Qe with lava composition from basaltic eruptions (2.6 ± 1.0 × 10−2 km3/yr) to andesites (2.3 ± 0.8 × 10−3 km3/yr) and rhyolites (4.0 ± 1.4 × 10−3 km3/yr). This trend is also seen in the difference between oceanic and continental settings, as eruptions on oceanic crust tend to be predominately basaltic. All of the volcanoes occurring in oceanic settings fail to have statistically different mean Qe and have an overall average of 2.8 ± 0.4 × 10−2 km3/yr, excluding flood basalts. Likewise, all of the volcanoes on continental crust also fail to have statistically different mean Qe and have an overall average of 4.4 ± 0.8 × 10−3 km3/yr. Flood basalts also form a distinctive class with an average Qe nearly two orders of magnitude higher than any other class. However, we have found no systematic evidence linking increased intrusive:extrusive ratios with lower volcanic rates. A simple heat balance analysis suggests that the preponderance of volcanic systems must be open magmatic systems with respect to heat and matter transport in order to maintain eruptible magma at shallow depth throughout the observed lifetime of the volcano. The empirical upper limit of ∼10−2 km3/yr for magma eruption rate in systems with relatively high intrusive:extrusive ratios may be a consequence of the fundamental parameters governing rates of melt generation (e.g., subsolidus isentropic decompression, hydration due to slab dehydration and heat transfer between underplated magma and the overlying crust) in the Earth.

Potential for carbon dioxide sequestration in flood basalts

Abstract: [1] Flood basalts are a potentially important host medium for geologic sequestration of anthropogenic CO2. Most lava flows have flow tops that are porous and permeable and have enormous capacity for storage of CO2. Interbedded sediment layers and dense low-permeability basalt rock overlying sequential flows may act as effective seals allowing time for mineralization reactions to occur. Laboratory experiments confirm relatively rapid chemical reaction of CO2-saturated pore water with basalts to form stable carbonate minerals. Calculations suggest a sufficiently short time frame for onset of carbonate precipitation after CO2 injection that verification of in situ mineralization rates appears feasible in field pilot studies. If proven viable, major flood basalts in the United States and India would provide significant additional CO2 storage capacity and additional geologic sequestration options in certain regions where more conventional storage options are limited.

Geochemistry of Picritic and Associated Basalt Flows of the Western Emeishan Flood Basalt Province, China

Abstract: Picritic lava flows near Lijiang in the late Permian Emeishan flood basalt province are associated with augite-phyric basalt, aphyric basalt, and basaltic pyroclastic units. The dominant phenocryst in the picritic flows is Mg-rich olivine (up to 91.6% forsterite component) with high CaO contents (to 0.42 wt %) and glass inclusions, indicating that the olivine crystallized from a melt. Associated chromite has a high Cr-number (73–75). The estimated MgO content of the primitive picritic liquids is about 22 wt %, and initial melt temperature may have been as high as 1630– 1690 � C. The basaltic lavas appear to be related to the picritic ones principally by olivine and clinopyroxene fractionation. Agecorrected Nd–Sr–Pb isotope ratios of the picritic and basaltic lavas are indistinguishable and cover a relatively small range [e.g. eNd(t) ¼� 1. 3t oþ4.0]. The higher eNd(t) lavas are isotopically similar to those of several modern oceanic hotspots, and have oceanisland-like patterns of alteration-resistant incompatible elements. Heavy rare earth element characteristics indicate an important role for garnet during melting and that the lavas were formed by fairly small degrees of partial melting. Rough correlations of isotope ratios with ratios of alteration-resistant highly incompatible elements (e.g. Nb/La) suggest modest amounts of contamination involving continental material or a relatively low-eNd component in the source. Overall, our results are consistent with other evidence suggesting some type of plume-head origin for the Emeishan province.

Geochemical and mineralogical indicators for aqueous processes in the Columbia Hills of Gusev crater, Mars

Abstract: [1] Water played a major role in the formation and alteration of rocks and soils in the Columbia Hills. The extent of alteration ranges from moderate to extensive. Five distinct rock compositional classes were identified; the order for degree of alteration is Watchtower ≅ Clovis > Wishstone ≅ Peace > Backstay. The rover's wheels uncovered one unusual soil (Paso Robles) that is the most S-rich material encountered. Clovis class rocks have compositions similar to Gusev plains soil but with higher Mg, Cl, and Br and lower Ca and Zn; Watchtower and Wishstone classes have high Al, Ti, and P and low Cr and Ni; Peace has high Mg and S and low Al, Na, and K; Backstay basalts have high Na and K compared to plains Adirondack basalts; and Paso Robles soil has high S and P. Some rocks are corundum-normative, indicating that their primary compositions were changed by loss and/or gain of rock-forming elements. Clovis materials consist of magnetite, nanophase ferric-oxides (npOx), hematite, goethite, Ca-phosphates, Ca- and Mg-sulfates, pyroxene, and secondary aluminosilicates. Wishstone and Watchtower rocks consist of Fe-oxides/oxyhydroxides, ilmenite, Ca-phosphate, pyroxene, feldspar, Mg-sulfates, and secondary aluminosilicates. Peace consists of magnetite, npOx, Mg- and Ca-sulfates, pyroxene, olivine, feldspar, apatite, halides, and secondary aluminosilicates. Paso Robles consists of Fe3+-, Mg-, Ca-, and other sulfates, Ca-phosphates, hematite, halite, allophane, and amorphous silica. Columbia Hills outcrops and rocks may have formed by the aqueous alteration of basaltic rocks, volcaniclastic materials, and/or impact ejecta by solutions that were rich in acid-volatile elements.

Characterization and petrologic interpretation of olivine‐rich basalts at Gusev Crater, Mars

Abstract: Rocks on the floor of Gusev crater are basalts of uniform composition and mineralogy. Olivine, the only mineral to have been identified or inferred from data by all instruments on the Spirit rover, is especially abundant in these rocks. These picritic basalts are similar in many respects to certain Martian meteorites (olivine-phyric shergottites). The olivine megacrysts in both have intermediate compositions, with modal abundances ranging up to 20-30%. Associated minerals in both include low-calcium and high-calcium pyroxenes, plagioclase of intermediate composition, iron-titanium-chromium oxides, and phosphate. These rocks also share minor element trends, reflected in their nickel-magnesium and chromium-magnesium ratios. Gusev basalts and shergottites appear to have formed from primitive magmas produced by melting an undepleted mantle at depth and erupted without significant fractionation. However, apparent differences between Gusev rocks and shergottites in their ages, plagioclase abundances, and volatile contents preclude direct correlation. Orbital determinations of global olivine distribution and compositions by thermal emission spectroscopy suggest that olivine-rich rocks may be widespread. Because weathering under acidic conditions preferentially attacks olivine and disguises such rocks beneath alteration rinds, picritic basalts formed from primitive magmas may even be a common component of the Martian crust formed during ancient and recent times.

Immiscible Transition from Carbonate-rich to Silicate-rich Melts in the 3 GPa Melting Interval of Eclogite + CO2 and Genesis of Silica-undersaturated Ocean Island Lavas

Abstract: We explore the partial melting behavior of a carbonated silicadeficient eclogite (SLEC1; 5 wt % CO2) from experiments at 3 GPa and compare the compositions of partial melts with those of alkalic and highly alkalic oceanic island basalts (OIBs). The solidus is located at 1050–1075 C and the liquidus at 1415 C. The sub-solidus assemblage consists of clinopyroxene, garnet, ilmenite, and calcio-dolomitic solid solution and the near solidus melt is carbonatitic ( 20 wt %), may be plausible sources or contributing components to melilitites and melilititic nephelinites from oceanic provinces, as they have strong compositional similarities including their SiO2, FeO*, MgO, CaO, TiO2 and Na2O contents, and CaO/Al2O3 ratios. Carbonated silicate partial melts from eclogite may also contribute to less extreme alkalic OIB, as these lavas have a number of compositional attributes, such as high TiO2 and FeO* and low Al2O3, that have not been observed from partial melting of peridotite CO2. In upwelling mantle, formation of carbonatite and silicate melts from eclogite and peridotite source lithologies occurs over a wide range of depths, producing significant opportunities for metasomatic transfer and implantation of melts.

The development and refinement of continental arcs by primary basaltic magmatism, garnet pyroxenite accumulation, basaltic recharge and delamination: insights from the Sierra Nevada, California

Abstract: The lower crust of the Mesozoic Sierra Nevada batholith was made up of high MgO, garnet-poor and low MgO, garnet-rich pyroxenites. Both groups are genetically linked and are collectively complementary to the mafic to intermediate Sierran plutons. High MgO pyroxenites represent high pressure cumulates from a mantle-derived hydrous basalt or basaltic andesite, resulting in derivative magmas having unusually low MgO for a given SiO2 as represented by the numerous mafic enclaves found in many Sierran plutons. The low MgO pyroxenites are either (1) shallow pressure cumulates from these derivative magmas or (2) partial melting residues (restites) of these derivative magmas after they were emplaced and solidified at lower crustal levels. In both cases, the complementary melt to the low MgO pyroxenites is driven to higher SiO2 contents, generating diorites and granodiorites. However, this simple two-stage scenario for the origin of Sierran granitoids cannot explain the observation that the Mg# of Sierran intermediate magmas remains roughly constant at ∼0.45–0.50 with increasing SiO2. Basaltic recharge/mixing with the lower crust is suggested as one means of buffering Mg#s and re-melting the lower crust to generate granitic melts, the latter of which mix with more juvenile magmas to complete the Sierran differentiation series.

Contributions of Slab Fluid, Mantle Wedge and Crust to the Origin of Quaternary Lavas in the NE Japan Arc

Abstract: Quaternary lavas from the NE Japan arc show geochemical evidence of mixing between mantle-derived basalts and crustal melts at the magmatic front, whereas significant crustal signals are not detected in the rear-arc lavas. The along-arc chemical variations in lavas from the magmatic front are attributable almost entirely to geochemical variations in the crustal melts that were mixed with a common mantle-derived basalt. The mantle-derived basalts have slightly enriched Sr–Pb and depleted Nd isotopic compositions relative to the rear-arc lavas, but the variation is less pronounced if crustal contributions are eliminated. Therefore, the source mantle compositions and slab-derived fluxes are relatively uniform, both across and along the arc. Despite this, incompatible element concentrations are significantly higher in the rear-arc basalts. We examine an open-system, fluid-fluxed melting model, assuming that depleted mid-ocean ridge basalt (MORB)-source mantle melted by the addition of fluids derived from subducted oceanic crust (MORB) and sediment (SED) hybrids at mixing proportions of 7% and 3% SED in the frontal- and rear-arc sources, respectively. The results reproduce the chemical variations found across the NE Japan arc with the conditions: 0.2% fluid flux with degree of melting F ¼ 3% at 2 GPa in the garnet peridotite field for the rear arc, and 0.7% fluid flux with F ¼ 20% at 1 GPa in the spinel peridotite field beneath the magmatic front. The chemical process operating in the mantle wedge requires: (1) various SED–MORB hybrid slab fluid sources; (2) variable amounts of fluid; (3) a common depleted mantle source; (4) different melting parameters to explain across-arc chemical variations.

Magma Evolution and Open-System Processes at Shiveluch Volcano: Insights from Phenocryst Zoning

Abstract: Phenocryst zoning patterns are used to identify open-system magmatic processes in the products of the 2001 eruption of Shiveluch Volcano, Kamchatka. The lavas and pumices studied are hornblende–plagioclase andesites with average pre-eruptive temperatures of 840C and fO2 of 1.5–2.1 log units above nickel–nickel oxide (NNO). Plagioclase zoning includes oscillatory and patchy zonation and sieve textures. Hornblendes are commonly unzoned, but some show simple, multiple or patchy zoning. Apatite microphenocrysts display normal and reverse zoning of sulphur. The textural similarity of patchy hornblende and plagioclase, together with Ba–Sr systematics in patchy plagioclase, indicate that the cores of these crystals were derived from cumulate material. Plagioclase–liquid equilibria suggest that the patchy texture develops by resorption during H2O-undersaturated decompression. When H2O-saturated crystallization recommences at lower pressure, reduced pH2O results in lower XAn in plagioclase, causing more Al-rich hornblende to crystallize. Plagioclase cores with diffuse oscillatory zoning, and unzoned hornblende crystals, probably represent a population of crystals resident in the magma chamber for long periods of time. In contrast, oscillatory zoning in the rims of plagioclase phenocrysts may reflect eruption dynamics during decompression crystallization. Increasing Fe/Al in oscillatory zoned rims suggests oxidation as a result of degassing of H2O during decompression. A general lack of textural overlap between phenocryst types suggests that different phenocryst populations were spatially or temporally isolated during crystallization. We present evidence that the host andesite has mixed with both more felsic and more mafic magmas. Olivine and orthopyroxene xenocrysts with reaction or overgrowth rims and strong normal zoning indicate mixing with basalt. Sieve-textured plagioclase resulted from mixing of a more felsic magma with the host andesite. The mineralogy and mineral compositions of a mafic andesite enclave are identical to those of the host magma, which implies efficient thermal quenching, and thus small volumes of intruding magma. Mixing of this magma with the host andesite results in phenocryst zoning because of differences in dissolved volatile contents. We suggest that small magma pulses differentiated at depth and ascended intermittently into the growing magma chamber, producing incremental variations in whole-rock compositions.

Combined Trace Element and Pb-Nd–Sr-O Isotope Evidence for Recycled Oceanic Crust (Upper and Lower) in the Iceland Mantle Plume

Abstract: We present the results of a comprehensive major element, trace element and Sr–Nd–Pb–O isotopic study of post-glacial volcanic rocks from the Neovolcanic zones on Iceland. The rocks studied range in composition from picrites and tholeiites, which dominate in the main rift systems, to transitional and alkalic basalts confined to the off-rift and propagating rift systems. There are good correlations of rock types with geochemical enrichment parameters, such as La/Sm and La/Yb ratios, and with long-term radiogenic tracers, such as Sr–Nd–Pb isotope ratios, indicating a long-lived enrichment/ depletion history of the source region. Sr/Sr vs Nd/Nd defines a negative array. Pb isotopes define well-correlated positive arrays on both Pb/Pb vs Pb/Pb and Pb/Pb diagrams, indicating mixing of at least two major components: an enriched component represented by the alkali basalts and a depleted component represented by the picrites. In combined Sr–Nd–Pb isotopic space the individual rift systems define coherent mixing arrays with slightly different compositions. The enriched component has radiogenic Pb (Pb/Pb > 19 3) and very similar geochemistry to HIMU-type ocean island basalts (OIB). We ascribe this endmember to recycling of hydrothermally altered upper basaltic oceanic crust. The depleted component that is sampled by the picrites has unradiogenic Pb (Pb/Pb < 17 8), but geochemical signatures distinct from that of normal mid-ocean ridge basalt (N-MORB). Highly depleted tholeiites and picrites have positive anomalies in mantle-normalized trace element diagrams for Ba, Sr, and Eu (and in some cases also for K, Ti and P), negative anomalies for Hf and Zr, and low dOolivine values (4 6–5 0‰) below the normal mantle range. All of these features are internally correlated, and we, therefore, interpret them to reflect source characteristics and attribute them to recycled lower gabbroic oceanic crust. Regional compositional differences exist for the depleted component. In SW Iceland it has distinctly higher Nb/U ( 68) and more radiogenic Pb/Pb ratios (18 28–18 88) compared with the NE rift (Nb/U 47; Pb/Pb 1⁄4 18 07–18 47). These geochemical differences suggest that different packages of recycled oceanic lithosphere exist beneath each rift. A third and minor component with relatively high Sr/Sr and Pb/Pb is found in a single volcano in SE Iceland (Oraefajokull volcano), indicating the involvement of recycled sediments in the source locally. The three plume components form an integral part of ancient recycled oceanic lithosphere. The slope in the uranogenic Pb diagram indicates a recycling age of about 1 5 Ga with time-integrated Th/U ratios of 3 01. Surprisingly, there is little evidence for the involvement of North Atlantic N-MORB source mantle, as would be expected from the

The 26·5 ka Oruanui Eruption, Taupo Volcano, New Zealand: Development, Characteristics and Evacuation of a Large Rhyolitic Magma Body

Abstract: The caldera-forming 26•5 ka Oruanui eruption (Taupo, New Zealand) erupted ~530 km3 of magma, >99% rhyolitic, <1% mafic. The rhyolite varies from 71•8 to 76•7 wt % SiO2 and 76 to 112 ppm Rb but is dominantly 74–76 wt % SiO2. Average rhyolite compositions at each stratigraphic level do not change significantly through the eruption sequence. Oxide geothermometry, phase equilibria and volatile contents imply magma storage at 830–760°C, and 100–200 MPa. Most rhyolite compositional variations are explicable by ~28% crystal fractionation involving the phenocryst and accessory phases (plagioclase, orthopyroxene, hornblende, quartz, magnetite, ilmenite, apatite and zircon). However, scatter in some element concentrations and 87Sr/86Sr ratios, and the presence of non-equilibrium crystal compositions imply that mixing of liquids, phenocrysts and inherited crystals was also important in assembling the compositional spectrum of rhyolite. Mafic compositions comprise a tholeiitic group (52•3–63•3 wt % SiO2) formed by fractionation and crustal contamination of a contaminated tholeiitic basalt, and a calc-alkaline group (56•7–60•5 wt % SiO2) formed by mixing of a primitive olivine–plagioclase basalt with rhyolitic and tholeiitic mafic magmas. Both mafic groups are distinct from other Taupo Volcanic Zone eruptives of comparable SiO2 content. Development and destruction by eruption of the Oruanui magma body occurred within ~40 kyr and Oruanui compositions have not been replicated in vigorous younger activity. The Oruanui rhyolite did not form in a single stage of evolution from a more primitive forerunner but by rapid rejuvenation of a longer-lived polygenetic, multi-age ‘stockpile’ of silicic plutonic components in the Taupo magmatic system.

Alkaline volcanic rocks from the Columbia Hills, Gusev crater, Mars

Abstract: [1] Irvine, Backstay, and Wishstone are the type specimens for three classes of fine-grained or fragmental, relatively unaltered rocks with distinctive thermal emission spectra, found as float on the flanks of the Columbia Hills. Chemical analyses indicate that these rocks are mildly alkaline basalt, trachybasalt, and tephrite, respectively. Their mineralogy consists of Na- and K-rich feldspar(s), low- and high-Ca pyroxenes, ferroan olivine, Fe-Ti (and possibly Cr) oxides, phosphate, and possibly glass. The texture of Wishstone is consistent with a pyroclastic origin, whereas Irvine and Backstay are lavas or possibly dike rocks. Chemical compositions of these rocks plot on or near liquid lines of descent for most elements calculated for Adirondack class rocks (olivine-rich basalts from the Gusev plains) at various pressures from 0.1 to 1.0 GPa. We infer that Wishstone-, Backstay-, and Irvine-class magmas may have formed by fractionation of primitive, oxidized basaltic magma similar to Adirondack-class rocks. The compositions of all these rocks reveal that the Gusev magmatic province is alkaline, distinct from the subalkaline volcanic rocks thought to dominate most of the planet's surface. The fact that differentiated volcanic rocks were not encountered on the plains prior to ascending Husband Hill may suggest a local magma source for volcanism beneath Gusev crater.

Structure and evolution of hydrothermal vent complexes in the Karoo Basin, South Africa

Abstract: The Karoo large igneous province, formed at c . 183 Ma, is characterized by the presence of voluminous basaltic intrusive complexes within the Karoo Basin, extrusive lava sequences and hydrothermal vent complexes. These last are pipe-like structures, up to several hundred metres in diameter, piercing the horizontally stratified sediments of the basin. Detailed mapping of two sediment-dominated hydrothermal vent complexes shows that they are composed of sediment breccias and sandstone. The breccias cut and intrude tilted host rocks, and are composed of mudstone and sandstone fragments with rare dolerite boulders. Sandstone clasts in the breccias are locally cemented by zeolite, which represents the only hydrothermal mineral in the vent complexes. Our data document that the hydrothermal vent complexes were formed by one or a few phreatic events, leading to the collapse of the surrounding sedimentary strata. We propose a model in which hydrothermal vent complexes originate in contact metamorphic aureoles around sill intrusions. Heating and expansion of host rock pore fluids resulted in rapid pore pressure build-up and phreatic eruptions. The hydrothermal vent complexes represent conduits for gases and fluids produced in contact metamorphic aureoles, slightly predating the onset of the main phase of flood volcanism.

Experimental petrology of the basaltic shergottite Yamato-980459: Implications for the thermal structure of the Martian mantle

Abstract: The Martian meteorite Yamato (Y-) 980459 is an olivine-phyric shergottite. It has a very primitive character and may be a primary melt of the Martian mantle. We have conducted crystallization experiments on a synthetic Y-980459 composition at Martian upper mantle conditions in order to test the primary mantle melt hypothesis. Results of these experiments indicate that the cores of the olivine megacrysts in Y-980459 are in equilibrium with a melt of bulk rock composition, suggesting that these megacrysts are in fact phenocrysts that grew from a magma of the bulk rock composition. Multiple saturation of the melt with olivine and a low-calcium pyroxene occurs at approximately 12 ± 0.5 kbar and 1540 ±10 °C, suggesting that the meteorite represents a primary melt that separated from its mantle source at a depth of ~100 km. Several lines of evidence suggest that the Y-980459 source underwent extensive melting prior to and/or during the magmatic event that produced the Y-980459 parent magma. When factored into convective models of the Martian interior, the high temperature indicated for the upper Martian mantle and possibly high melt fraction for the Y-980459 magmatic event suggests a significantly higher temperature at the core-mantle boundary than previously estimated.

Nature and Origin of the Hematite-Bearing Plains of Terra Meridiani Based on Analyses of Orbital and Mars Exploration Rover Data Sets

Abstract: The ~5 km of traverses and observations completed by the Opportunity rover from Endurance crater to the Fruitbasket outcrop show that the Meridiani plains consist of sulfate-rich sedimentary rocks that are largely covered by poorly-sorted basaltic aeolian sands and a lag of granule-sized hematitic concretions. Orbital reflectance spectra obtained by Mars Express OMEGA over this region are dominated by pyroxene, plagioclase feldspar, crystalline hematite (i.e., concretions), and nano-phase iron oxide dust signatures, consistent with Pancam and Mini-TES observations. Mossbauer Spectrometer observations indicate more olivine than observed with the other instruments, consistent with preferential optical obscuration of olivine features in mixtures with pyroxene and dust. Orbital data covering bright plains located several kilometers to the south of the landing site expose a smaller areal abundance of hematite, more dust, and a larger areal extent of outcrop compared to plains proximal to the landing site. Low-albedo, low-thermal-inertia, windswept plains located several hundred kilometers to the south of the landing site are predicted from OMEGA data to have more hematite and fine-grained olivine grains exposed as compared to the landing site. Low calcium pyroxene dominates spectral signatures from the cratered highlands to the south of Opportunity. A regional-scale model is presented for the formation of the plains explored by Opportunity, based on a rising ground water table late in the Noachian Era that trapped and altered local materials and aeolian basaltic sands. Cessation of this aqueous process led to dominance of aeolian processes and formation of the current configuration of the plains.