Showing papers on "Basalt published in 2010"

Continental and Oceanic Crust Recycling-induced Melt^Peridotite Interactions in the Trans-North China Orogen: U^Pb Dating, Hf Isotopes and Trace Elements in Zircons from Mantle Xenoliths

Abstract: We present the first finding of continental crust-derived Precambrian zircons in garnet/spinel pyroxenite veins within mantle xenoliths carried by the Neogene Hannuoba basalt in the central zone of the North China Craton (NCC). Petrological and geochemical features indicate that these mantle-derived composite xenoliths were formed by silicic melt^lherzolite interaction. The Precambrian zircon ages can be classified into three age groups of 2·4^2·5 Ga, 1·6^2·2 Ga and 0·6^1·2 Ga, coinciding with major geological events in the NCC. These Precambrian zircons fall in the field of continental granitoid rocks in plots of U/Yb vs Hf and Y. Their igneous-type REE patterns and metamorphic zircon type CL images indicate that they were not crystallized during melt^peridotite interaction and subsequent high-pressure metamorphism.The 2·5 Ga zircons have positive eHf(t) values (2·9^10·6), whereas the younger Precambrian zircons are dominated by negative eHf(t) values, indicating an ancient continental crustal origin.These observations suggest that the Precambrian zircons were xenocrysts that survived melting of recycled continental crustal rocks and were then injected with silicate melt into the host peridotite. In addition to the Precambrian zircons, igneous zircons of 315 3 Ma (2 ), 80^170 Ma and 48^64 Ma were separated from the garnet/spinel pyroxenite veins; these provide evidence for lower continental crust and oceanic crust recycling-induced multi-episodic melt^peridotite interactions in the central zone of the NCC. The combination of the positive eHf(t) values (2·91^24·6) of the 315 Ma zircons with the rare occurrence of 302^324 Ma subduction-related diorite^granite plutons in the northern margin of the NCC implies that the 315 Ma igneous zircons might record melt^peridotite interactions in the lithospheric mantle induced by Palaeo-Asian oceanic crust subduction. Igneous zircons of age 80^170 Ma generally coexist with the Precambrian metamorphic zircons and have lower Ce/Yb and Th/U ratios, higher U/Yb ratios and greater negative Eu anomalies.The eHf(t) values of these zircons vary greatly from ^47·6 to 24·6.The 170^110 Ma zircons are generally characterized by negative eHf(t) values, whereas the 110^100 Ma zircons have positive eHf(t) values.These observations suggest that melt^peridotite interactions at 80^170 Ma were induced by partial melting of recycled continental crust. The 48^64 Ma igneous zircons are characterized by negligible Ce anomalies, unusually high REE, U and Th contents, and positive eHf(t) values.These features imply that the melt^peridotite interactions at 48^64 Ma could be associated with a depleted mantle-derived carbonate melt or fluid.

Fore-arc basalts and subduction initiation in the Izu-Bonin-Mariana system

Abstract: Recent diving with the JAMSTEC Shinkai 6500 manned submersible in the Mariana fore arc southeast of Guam has discovered that MORB-like tholeiitic basalts crop out over large areas These ''fore-arc basalts'' (FAB) underlie boninites and overlie diabasic and gabbroic rocks Potential origins include eruption at a spreading center before subduction began or eruption during near-trench spreading after subduction began FAB trace element patterns are similar to those of MORB and most Izu-Bonin-Mariana (IBM) back-arc lavas However, Ti/V and Yb/V ratios are lower in FAB reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid-ocean ridges and back-arc basins Some FAB also have higher concentrations of fluid-soluble elements than do spreading center lavas Thus, the most likely origin of FAB is that they were the first lavas to erupt when the Pacific Plate began sinking beneath the Philippine Plate at about 51 Ma The magmas were generated by mantle decompression during near-trench spreading with little or no mass transfer from the subducting plate Boninites were generated later when the residual, highly depleted mantle melted at shallow levels after fluxing by a water-rich fluid derived from the sinking Pacific Plate This magmatic stratigraphy of FAB overlain by transitional lavas and boninites is similar to that found in many ophiolites, suggesting that ophiolitic assemblages might commonly originate from near-trench volcanism caused by subduction initiation Indeed, the widely dispersed Jurassic and Cretaceous Tethyan ophiolites could represent two such significant subduction initiation events

The Magnetite Crisis in the Evolution of Arc-related Magmas and the Initial Concentration of Au, Ag and Cu

Abstract: The association of large Au–Cu–Ag ore deposits with convergent margins is commonly attributed to the higher content of chalcophile elements in the parental magmas generated at subduction zones compared with mid-ocean ridges. We present new geochemical data for arc-like, relatively oxidized mantle-derived basalt to rhyolite magmas from the Pual Ridge and vicinity, Eastern Manus Basin, which show that the initial abundances of base and precious metals in the parental basalts are similar to those of mid-ocean ridge basalt (MORB). The contents of Au, Cu, and Ag are built up in the evolving Pual Ridge liquids to levels considerably in excess of those in MORB because, unlike MORB, they are not saturated in a sulfide phase, which is a consequence of their being more oxidized than MORB. The behaviour of S during the evolution of the Pual Ridge magmas is obscured by late-stage SO2 loss during eruption, but we show that it may be inferred by using Se as a proxy, because this element follows S closely during magmatic evolution except it is not lost during low-pressure (near sea-floor) degassing. The onset of magnetite fractionation at ~60 wt % SiO2 and an Mg-number of ~40 is accompanied by an abrupt decrease in the contents of Au, Cu and Ag, previously attributed to separation of Cu–Au-rich fluid, which is also shown by Se, implying that magnetite fractionation triggers sulfide saturation. Petrological modelling reveals that the amount of magnetite fractionation involved is sufficient to convert most of the S originally dissolved in the magma as sulfate (SO42–) to sulfide (S2–), triggering saturation in a Cu-rich sulfide phase, tentatively identified as bornite (Cu5FeS4). This sulfide phase sequesters Au and Ag, elements with the same valence as Cu in sulfides, but not other potentially chalcophile elements such as Ni, Re, and Pt, which suggests that the phase is crystalline rather than an immiscible sulfide melt. The relatively high contents of Cu and Au characteristic of evolved convergent margin magmas requires no enrichment from subducted material. Instead, the association of major Cu–Au deposits with convergent-margin magmatism results specifically from the process of magmatic evolution under oxidizing conditions. This same property also leads to early magnetite fractionation, triggering the abrupt saturation in the Cu-rich sulfide. Hence the easily recognizable trend of magmatic evolution under oxidizing conditions (i.e. the sharp drop in chalcophile element concentrations) may be an exploration guide to economic Au–Ag–Cu provinces, or a crucial pre-enrichment step in the formation of such deposits. The decrease in P2O5 and Sr at the same stage in the fractionation sequence (~60 wt % SiO2) indicates that saturation in apatite is concomitant with magnetite–sulfide saturation in the Eastern Manus Basin

Simple mixing as the major control of the evolution of volcanic suites in the Ecuadorian Andes

Abstract: Examination of an extensive major and trace element database for about 700 whole rocks from the Ecuadorian Andes reveals series of local trends typified by three volcanoes: Iliniza and Pichincha from the Western Cordillera and Tungurahua from the Eastern Cordillera. These local trends are included in a more scattered global trend that reflects typical across-arc chemical variations. The scatter of the global trend is attributed to greater crustal contributions or decreasing melt fractions. Trace element modelling shows that the local trends are consistent with mixing, and not with any fractional crystallization or progressive melting dominated processes. These local trends are extendable to include samples from other Ecuadorian volcanoes, suggesting that mixing processes are dominant throughout the region. Mixing model using trace and major element analyses identifies two end-members: low-silica, basaltic and high-silica, dacitic magmas. It also shows that mixing occurred between magmas after their segregation, rather than earlier mixing between the solid sources prior to melting. As a consequence, there must exist efficient magma-mixing processes that can overcome the obstacles to mixing magmas with contrasting physical properties, and can produce series of hybrid liquids over regional-scale. Model calculations show that estimated silicic end-members are primary magmas and are not co-magmatic derivatives of the corresponding mafic end-members. Lavas of Ecuadorian volcanoes are likely originated from magmas of contrasting origins, such as basaltic magmas generated by fluxed melting of peridotites in the mantle wedge and dacitic, adakite-type magmas originating from the slab or the mafic lower crust.

Lunar apatite with terrestrial volatile abundances

Abstract: The Moon is thought to be depleted relative to the Earth in volatile elements such as H, Cl and the alkalis. Nevertheless, evidence for lunar explosive volcanism has been used to infer that some lunar magmas exsolved a CO-rich and CO_2-rich vapour phase before or during eruption. Although there is also evidence for other volatile species on glass spherules, until recently there had been no unambiguous reports of indigenous H in lunar rocks. Here we report quantitative ion microprobe measurements of late-stage apatite from lunar basalt 14053 that document concentrations of H, Cl and S that are indistinguishable from apatites in common terrestrial igneous rocks. These volatile contents could reflect post-magmatic metamorphic volatile addition or growth from a late-stage, interstitial, sulphide-saturated melt that contained ~1,600 parts per million H_2O and ~3,500 parts per million Cl. Both metamorphic and igneous models of apatite formation suggest a volatile inventory for at least some lunar materials that is similar to comparable terrestrial materials. One possible implication is that portions of the lunar mantle or crust are more volatile-rich than previously thought.

Large igneous provinces (LIPs) and carbonatites

Abstract: There is increasing evidence that many carbonatites are linked both spatially and temporally with large igneous provinces (LIPs), i.e. high volume, short duration, intraplate-type, magmatic events consisting mainly of flood basalts and their plumbing systems (of dykes, sills and layered intrusions). Examples of LIP-carbonatite associations include: i. the 66 Ma Deccan flood basalt province associated with the Amba Dongar, Sarnu-Dandali (Barmer), and Mundwara carbonatites and associated alkali rocks, ii. the 130 Ma Parana-Etendeka (e.g. Jacupiranga, Messum); iii. the 250 Ma Siberian LIP that includes a major alkaline province, Maimecha-Kotui with numerous carbonatites, iv. the ca. 370 Ma Kola Alkaline Province coeval with basaltic magmatism widespread in parts of the East European craton, and v. the 615–555 Ma CIMP (Central Iapetus Magmatic Province) of eastern Laurentia and western Baltica. In the Superior craton, Canada, a number of carbonatites are associated with the 1114–1085 Ma Keweenawan LIP and some are coeval with the pan-Superior 1880 Ma mafic-ultramafic magmatism. In addition, the Phalaborwa and Shiel carbonatites are associated with the 2055 Ma Bushveld event of the Kaapvaal craton. The frequency of this LIP-carbonatite association suggests that LIPs and carbonatites might be considered as different evolutionary ‘pathways’ in a single magmatic process/system. The isotopic mantle components FOZO, HIMU, EM1 but not DMM, along with primitive noble gas signatures in some carbonatites, suggest a sub-lithospheric mantle source for carbonatites, consistent with a plume/asthenospheric upwelling origin proposed for many LIPs.

Geochemistry and tectonics of Cenozoic volcanism in the Lesser Caucasus (Azerbaijan) and the peri-Arabian region: collision-induced mantle dynamics and its magmatic fingerprint

Abstract: The Lesser Caucasus occurs in the hinterland of the Arabia–Eurasia collision zone in the broad Alpine–Himalayan orogenic belt and includes Cenozoic plutonic and volcanic sequences that provide important clues for collision-driven continental magmatism and mantle dynamics. Two main magmatic episodes (Eocene and late Miocene–Quaternary) formed the volcanic landscape and the igneous assemblages in the Lesser Caucasus of Azerbaijan. (1) The Eocene sequence consists of trachybasalt and basaltic trachyandesite with subordinate tephrite-basanite, basaltic andesite, and trachyandesite, showing shoshonitic and mildly alkaline compositions. The Miocene–Quaternary magmatic episode is represented by (2a) an early phase of upper Miocene–lower Pliocene andesite, trachyandesite, trachydacite, dacite and rhyolite lavas, and by (2b) a late phase of upper Pliocene–Quaternary trachybasalt, basaltic trachyandesite, basaltic andesite, trachyandesite, trachyte, and rhyolite flows. The rocks of the early phase have high-K calc-...

Compound-specific carbon isotopes from Earth's largest flood basalt eruptions directly linked to the end-Triassic mass extinction.

Abstract: A leading hypothesis explaining Phanerozoic mass extinctions and associated carbon isotopic anomalies is the emission of greenhouse, other gases, and aerosols caused by eruptions of continental flood basalt provinces. However, the necessary serial relationship between these eruptions, isotopic excursions, and extinctions has never been tested in geological sections preserving all three records. The end-Triassic extinction (ETE) at 201.4 Ma is among the largest of these extinctions and is tied to a large negative carbon isotope excursion, reflecting perturbations of the carbon cycle including a transient increase in CO2. The cause of the ETE has been inferred to be the eruption of the giant Central Atlantic magmatic province (CAMP). Here, we show that carbon isotopes of leaf wax derived lipids (n-alkanes), wood, and total organic carbon from two orbitally paced lacustrine sections interbedded with the CAMP in eastern North America show similar excursions to those seen in the mostly marine St. Audrie’s Bay section in England. Based on these results, the ETE began synchronously in marine and terrestrial environments slightly before the oldest basalts in eastern North America but simultaneous with the eruption of the oldest flows in Morocco, a CO2 super greenhouse, and marine biocalcification crisis. Because the temporal relationship between CAMP eruptions, mass extinction, and the carbon isotopic excursions are shown in the same place, this is the strongest case for a volcanic cause of a mass extinction to date.

The Tarim picrite–basalt–rhyolite suite, a Permian flood basalt from northwest China with contrasting rhyolites produced by fractional crystallization and anatexis

Abstract: We report major and trace element composition, Sr–Nd isotopic and seismological data for a picrite–basalt–rhyolite suite from the northern Tarim uplift (NTU), northwest China. The samples were recovered from 13 boreholes at depths between 5,166 and 6,333 m. The picritic samples have high MgO (14.5–16.8 wt%, volatiles included) enriched in incompatible element and have high 87Sr/86Sr and low 143Nd/144Nd isotopic ratios (eNd (t) = −5.3; Sri = 0.707), resembling the Karoo high-Ti picrites. All the basaltic samples are enriched in TiO2 (2.1–3.2 wt%, volatiles free), have high FeOt abundances (11.27–15.75 wt%, volatiles free), are enriched in incompatible elements and have high Sr and low Nd isotopic ratios (Sri = 0.7049–0.7065; eNd (t) = −4.1 to −0.4). High Nb/La ratios (0.91–1.34) of basalts attest that they are mantle-derived magma with negligible crustal contamination. The rhyolite samples can be subdivided into two coeval groups with overlapping U–Pb zircon ages between 291 ± 4 and 272 ± 2 Ma. Group 1 rhyolites are enriched in Nb and Ta, have similar Nb/La, Nb/U, and Sr–Nd isotopic compositions to the associated basalts, implying that they are formed by fractional crystallization of the basalts. Group 2 rhyolites are depleted in Nb and Ta, have low Nb/La ratios, and have very high Sr and low Nd isotopic ratios, implying that crustal materials have been extensively, if not exclusively, involved in their source. The picrite–basalt–rhyolite suite from the NTU, together with Permian volcanic rocks from elsewhere Tarim basin, constitute a Large Igneous Province (LIP) that is characterized by large areal extent, rapid eruption, OIB-type chemical composition, and eruption of high temperature picritic magma. The Early Permian magmatism, which covered an area >300,000 km2, is therefore named the Tarim Flood Basalt.

Mantle Melting as a Function of Water Content beneath the Mariana Arc

Abstract: Subduction zone magmas are characterized by high concentrations of pre-eruptive H_2O, presumably as a result of an H_2Oflux originating from the dehydrating, subducting slab. The extent of mantle melting increases as a function of increasing water content beneath back-arc basins and is predicted to increase in a similar manner beneath arc volcanoes. Here, we present new data for olivine-hosted, basaltic melt inclusions from the Mariana arc that reveal pre-eruptive H_2O contents of ~1•5-6•0 wt %, which are up to three times higher than concentrations reported for the Mariana Trough back-arc basin. Major element systematics of arc and back-arc basin basalts indicate that the back-arc basin melting regime does not simply mix with wet, arc-derived melts to produce the observed range of back-arc magmatic H_2O concentrations. Simple melting models reveal that the trend of increasing extents of melting with increasing H_2O concentrations of the mantle source identified in the Mariana Trough generally extends beneath the Mariana volcanic front to higher mantle water contents and higher extents of melting. In detail, however, each Mariana volcano may define a distinct relationship between extent of melting and the H_2O content of the mantle source. We develop a revised parameterization of hydrous melting, incorporating terms for variable pressure and mantle fertility, to describe the distinct relationships shown by each arc volcano. This model is used in combination with thermobarometry constraints to show that hydrous melts equilibrate at greater depths (34-87 km) and temperatures (>1300°C) beneath the Mariana arc than beneath the back-arc basin (21-37 km), although both magma types can form from a mantle of similar potential temperature (~1350°C).The difference lies in where the melts form and equilibrate. Arc melts are dominated by those that equilibrate within the hot core of the mantle wedge, whereas back-arc melts are dominated by those that equilibrate within the shallow zone of decompression melting beneath the spreading center. Despite higher absolute melting temperatures (>1300°C), Mariana arc melts reflect lower melt productivity as a result of wet melting conditions and a more refractory mantle source.

The Chlorine Isotope Composition of the Moon and Implications for an Anhydrous Mantle

Abstract: Arguably, the most striking geochemical distinction between Earth and the Moon has been the virtual lack of water (hydrogen) in the latter. This conclusion was recently challenged on the basis of geochemical data from lunar materials that suggest that the Moon's water content might be far higher than previously believed. We measured the chlorine isotope composition of Apollo basalts and glasses and found that the range of isotopic values [from -1 to +24 per mil (per thousand) versus standard mean ocean chloride] is 25 times the range for Earth. The huge isotopic spread is explained by volatilization of metal halides during basalt eruption--a process that could only occur if the Moon had hydrogen concentrations lower than those of Earth by a factor of approximately 10(4) to 10(5), implying that the lunar interior is essentially anhydrous.

Composition of the marginal rocks and sills of the Rustenburg Layered Suite, Bushveld Complex, South Africa: Implications for the formation of the Platinum-group element deposits

Abstract: The Bushveld Complex contains large ore deposits of platinum-group elements (PGE), V, and Cr. Understanding how these deposits formed is in part dependent on estimates of the compositions of the magmas that filled the Bushveld chamber. Over the past 20 years, estimates for the major oxides and some trace elements in the magmas have been made using the marginal rocks of the intrusion. However, data for most of the trace elements have not been available. This paper presents the results for a full range of trace elements, including the platinum-group elements. The marginal rocks of the Lower and lower Critical zones (B-1 magmas) are tholeiitic Mg-rich basaltic andesites with Mg# 71. It had been suggested that they are boninites but their mantle-normalized incompatible lithophile trace element patterns (spidergrams) resemble those of the upper continental crust and the concentrations of the elements are much higher than those of boninites. The patterns resemble siliceous high magnesium basalts. An unusual feature is that the Pt/Pd ratios are >1.5. The Pt contents of the B-1 rocks (15–25 ppb) are slightly higher than those observed in most primary mantle melts, suggesting that the high Pt/Pd ratio is due to Pt enrichment rather than Pd depletion. The crystallization order and composition of the minerals formed in equilibrium with the B-1 magma matches that of the Lower and lower Critical zones and thus this magma appears to be representative of the parental magma of these zones. The marginal rocks to the upper Critical zone (B-2) are tholeiitic basalts in terms of major element composition, with Mg# 55. The spidergrams show some similarities with E-MORB; however, the B-2 rocks have strong positive Ba and Pb anomalies and negative P, Ti, Hf, and Zr anomalies, and thus they more closely resemble lower continental crust. The B-2 rocks have lower and more variable Pt + Pd contents than the B-1 magma, suggesting that some of the samples have experienced sulfide saturation, but in common with the B-1 magmas, the Pt/Pd ratios are high, in excess of 1.5. The crystallization order of the Upper Critical zone cannot be modelled by the B-2 magma alone. However, mixtures of B-2 magma and B-1 magma satisfy the crystallization order and mineral composition of the upper Critical zone. The marginal rocks of the Main zone (B-3) are also tholeiitic basalts in terms of major element composition but have a higher Mg# (62) than the B-2 rocks. Trace element patterns in part resemble those of B-2 magmas but are depleted in most incompatible elements with large positive Ba, Pb, and Eu anomalies and negative Nb, Ta, Hf, and Zr anomalies, suggesting the rocks contain a plagioclase component. The PGE contents of the B-3 rocks are lower than those of the B-1 magma and less variable than those of the B-2 magma, but in common with both the other magmas, they have high Pt/Pd. The crystallization order and composition of the minerals in equilibrium with the B-3 magma matches that of the Main zone. Two processes have been suggested to explain the compositions of the Bushveld magmas: mixing of primitive mantle melts with partial melts of continental crust and mixing of primitive mantle melts with melts derived from the subcontinental lithospheric mantle (SCLM). The trace element concentrations of the magmas can be modelled by crustal contamination. This interpretation is supported by oxygen isotopes, initial 87Sr/86Sr ratios and ɛNd of the cumulate rocks. However, the high Pt/Pd ratios of all of the magmas and the overall higher than normal Pt concentrations of the B-1 magma are difficult to explain by mixing of primary mantle melt with crustal components. The SCLM has high Pt/Pd ratios and mixing of primary mantle magma with SCLM-derived magma could account for the high Pt concentrations and high Pt/Pd ratios. This interpretation is supported by recent work on Os isotopes of the Kaapvaal SCLM. It should be kept in mind that the two processes are not necessarily exclusive. A magma with a SCLM component could have been emplaced into the crust and subsequently have been contaminated by partial melts of the crust.

Melting Relations of MORB–Sediment Mélanges in Underplated Mantle Wedge Plumes; Implications for the Origin of Cordilleran-type Batholiths

Abstract: This paper gives the results of a set of laboratory experiments designed to analyse the petrological implications of mantle wedge plumes—large buoyant structures predicted by thermomechanical numerical modelling of subduction zones. A particular design of layered capsule was used to simulate the complex multilayer formed by intense flow within the mantle wedge as predicted by numerical models. A basaltic [mid-ocean ridge basalt (MORB)-derived amphibolite] component was sandwiched between two adjacent layers of a sedimentary (Bt-rich metagreywacke) component. Conditions were fixed at temperatures of 1000–1200°C at pressures of 1·5–2·0 GPa. Our results suggest that significant volumes of hybrid, Cordilleran-type granodioritic magmas can be generated by sub-lithospheric partial melting of a mechanically mixed source. Partial melting of the end-members is not buffered, forming granitic (melting of metasediment) and trondhjemitic (melting of MORB) melts in high-variance assemblages Melt + Grt + Pl and Melt + Grt + Cpx, respectively. However, the composition of melts formed from partial melting of metasediment–MORB melanges is buffered for sediment-to-MORB ratios ranging from 3:1 to 1:3, producing liquids of granodiorite to tonalite composition along a cotectic with the lower-variance phase assemblage Melt + Grt + Cpx + Pl. Our model explains the geochemical and isotopic characteristics of Cordilleran batholiths. In particular, it accounts for the observed decoupling between major element and isotopic compositions. Large variations in isotopic ratios can be inherited from a compositionally heterogeneous source; however, major element compositions are more strongly dependent on the temperature of melting rather than on the composition of the source.

Crustal sulfur is required to form magmatic Ni–Cu sulfide deposits: evidence from chalcophile element signatures of Siberian and Deccan Trap basalts

Abstract: Process models for ore formation in magmatic Ni–Cu–platinum group element (PGE) sulfide systems require that S saturation is achieved in a mafic–ultramafic magma. Traditional models explain the achievement of S saturation or sulfide saturation either by the addition of crustal S, by the felsification of the magma by crustal contamination, or by mixing between primitive and evolved magmas. Which process matters most is important to industry-oriented exploration models where crustal S sources are believed to be encouraging features of a metallotect. Studies of the Siberian Trap flood basalts at Noril’sk have demonstrated that chalcophile element depletion is linked to assimilation of silica-rich crust, but it is less clear whether this contaminant contained an appreciable amount of S. At Noril’sk, the Ni–Cu–PGE sulfide deposits are associated with subvolcanic intrusions that were emplaced into Permian and Carboniferous sedimentary sequences rich in shales, marlstones, and evaporites. Similar to the Siberian Trap basalts, the Deccan Trap contains a volumetrically important suite of crustally contaminated tholeiitic basalts. We present new PGE data for samples from a stratigraphic sequence of basalts from the southern Deccan province. Two of the formations in this sequence (the Bushe and Poladpur Formations) have geochemical signatures indicative of a wide degree of crustal contamination of a magma type that gave rise to the stratigraphically higher Ambenali Formation (a product of transitional midocean ridge basalt magmatism). There are no known deposits or occurrences of Ni–Cu–PGE sulfides associated with subvolcanic intrusions in the Deccan province. Despite the fact that the Bushe Formation exhibits a stronger crustal contamination signature than the most contaminated Siberian Trap basalt formations, and the Poladpur lavas are also strongly crustally contaminated, the Bushe and Poladpur basalts are undepleted in Ni, Cu, or PGE. This indicates that the contaminated Deccan Trap lavas did not achieve S saturation. This, in turn, places constraints on the potential of the Deccan Trap in southern India to host significant magmatic sulfide deposits. Conversely, this observation also indicates that an S-rich crustal contaminant is required for the genesis of magmatic Ni–Cu–PGE sulfide deposits.

Highly silicic compositions on the Moon.

Abstract: Using data from the Diviner Lunar Radiometer Experiment, we show that four regions of the Moon previously described as "red spots" exhibit mid-infrared spectra best explained by quartz, silica-rich glass, or alkali feldspar. These lithologies are consistent with evolved rocks similar to lunar granites in the Apollo samples. The spectral character of these spots is distinct from surrounding mare and highlands material and from regions composed of pure plagioclase feldspar. The variety of landforms associated with the silicic spectral character suggests that both extrusive and intrusive silicic magmatism occurred on the Moon. Basaltic underplating is the preferred mechanism for silicic magma generation, leading to the formation of extrusive landforms. This mechanism or silicate liquid immiscibility could lead to the formation of intrusive bodies.

Secular Variation of Magmatic Sulfide Deposits and Their Source Magmas

Abstract: Magmatic sulfide deposits are divisible into two major groups, those that are valued primarily for their Ni and Cu and that are mostly sulfide rich (>10% sulfide), and those that are valued primarily for their PGE and tend to be sulfide poor (<5% sulfide). Most members of the Ni-Cu group formed as a result of an interaction of mantle-derived magma with the crust that gave rise to the early onset of sulfide immiscibility. Of the different classes of deposit in this group, the komatiite-related class ranges from 2.7 to 1.9 Ga in age, the Flood basalt-related class from 1.1 to 0.25 Ga, and the Mg basalt- and basalt-related group from the Archean to the present. There is only one example each of anorthosite complex- and impact-related deposits, so that one cannot generalize about their secular distribution, except to say that anorthosite complexes are Proterozoic. Ural-Alaskan intrusions are dominantly Phanerozoic (some Archean deposits have been included with this group), but as yet no examples have been found with economic sulfide bodies. Seventy-five percent of known PGE resources occur in three intrusions—the Bushveld, Great Dyke, and Stillwater, the rocks all of which have crystallized from two magma types, an unusual, high SiO2, MgO, and Cr and low Al2O3 type (U-type) that was emplaced at an early stage and a later, normal tholeiitic-type magma (T-type); the PGE are concentrated in layers close to the level at which the predominant crystallization switches from one magma type to the other. The U-type magma is interpreted as a PGE-rich, komatiitic magma (possibly the product of two-stage mantle melting) that has interacted to varying degrees with the crust, becoming SiO2 enriched in this way. These three intrusions are Neoarchean to Paleoproterozoic in age. All known examples of komatiites, with one exception, are Paleoproterozoic or older and their secular distribution is thought to be due to cooling of the Earth. Known deposits do not occur in the oldest (>3.0 Ga) komatiites but appear at around 2.7Ga in continental (Kambalda, Western Australia) or island-arc (Perseverance-Mount Keith, Western Australia) environments, possibly because it was these environments that offered the opportunity for interaction with felsic rocks. It is suggested that the development of these environments in the Archean was an additional control on the age distribution of these deposits. It is postulated that the restricted secular distribution of PGE-enhanced intrusions is also due to the need for a hot mantle to give rise to U-type magmas.

Global Silicate Mineralogy of the Moon from the Diviner Lunar Radiometer

Abstract: We obtained direct global measurements of the lunar surface using multispectral thermal emission mapping with the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment. Most lunar terrains have spectral signatures that are consistent with known lunar anorthosite and basalt compositions. However, the data have also revealed the presence of highly evolved, silica-rich lunar soils in kilometer-scale and larger exposures, expanded the compositional range of the anorthosites that dominate the lunar crust, and shown that pristine lunar mantle is not exposed at the lunar surface at the kilometer scale. Together, these observations provide compelling evidence that the Moon is a complex body that has experienced a diverse set of igneous processes.

Silica deposits in the Nili Patera caldera on the Syrtis Major volcanic complex on Mars

Abstract: The martian surface features abundant volcanoes and evidence for past liquid water. Extant or relict martian volcanic hydrothermal systems have therefore been sought in the pursuit of evidence for habitable environments. The Mars Exploration Rover, Spirit, detected deposits highly enriched in silica with accessory minerals, suggesting formation by hydrothermal leaching of basaltic rocks by low-pH solutions. However, extensive erosion has obscured the context of the formation environment of these deposits. Silica deposits have also been identified remotely, but also with limited contextual clues to their formation; aqueous alteration products of basalt and volcanic ash are the most likely sources. Here we report the detection from orbit of hydrated silica deposits on the flanks of a volcanic cone in the martian Syrtis Major caldera complex. Near-infrared observations show dozens of localized hydrated silica deposits. As a result of the morphology of these deposits and their location in and around the cone summit, we suggest that the deposits were produced by a volcanically driven hydrothermal system. The cone and associated lava flows post-date Early Hesperian volcano formation. We conclude that, if a relict hydrothermal system was associated with the silica deposits, it may preserve one of the most recent habitable microenvironments on Mars.