Showing papers on "Basalt published in 1999"

Petrology and geochemistry of Camiguin Island, southern Philippines: insights to the source of adakites and other lavas in a complex arc setting

Abstract: Camiguin is a small volcanic island located 12 km north of Mindanao Island in southern Philippines. The island consists of four volcanic centers which have erupted basaltic to rhyolitic calcalkaline lavas during the last ∼400 ka. Major element, trace element and Sr, Nd and Pb isotopic data indicate that the volcanic centers have produced a single lava series from a common mantle source. Modeling results indicate that Camiguin lavas were produced by periodic injection of a parental magma into shallow magma chambers allowing assimilation and fractional crystallization (AFC) processes to take place. The chemical and isotopic composition of Camiguin lavas bears strong resemblance to the majority of lavas from the central Mindanao volcanic field confirming that Camiguin is an extension of the tectonically complex Central Mindanao Arc (CMA). The most likely source of Camiguin and most CMA magmas is the mantle wedge metasomatized by fluids dehydrated from a subducted slab. Some Camiguin high-silica lavas are similar to high-silica lavas from Mindanao, which have been identified as “adakites” derived from direct melting of a subducted basaltic crust. More detailed comparison of Camiguin and Mindanao adakites with silicic slab-derived melts and magnesian andesites from the western Aleutians, southernmost Chile and Batan Island in northern Philippines indicates that the Mindanao adakites are not pure slab melts. Rather, the CMA adakites are similar to Camiguin high-silica lavas which are products of an AFC process and have negligible connection to melting of subducted basaltic crust.

Complex Shear Wave Velocity Structure Imaged Beneath Africa and Iceland.

Abstract: A model of three-dimensional shear wave velocity variations in the mantle reveals a tilted low velocity anomaly extending from the core-mantle boundary (CMB) region beneath the southeastern Atlantic Ocean into the upper mantle beneath eastern Africa. This anomaly suggests that Cenozoic flood basalt volcanism in the Afar region and active rifting beneath the East African Rift is linked to an extensive thermal anomaly at the CMB more than 45 degrees away. In contrast, a low velocity anomaly beneath Iceland is confined to the upper mantle.

Compositional stratification in the deep mantle

Abstract: A boundary between compositionally distinct regions at a depth of about 1600 kilometers may explain the seismological observations pertaining to Earth's lower mantle, produce the isotopic signatures of mid-ocean ridge basalts and oceanic island basalts, and reconcile the discrepancy between the observed heat flux and the heat production of the mid-ocean ridge basalt source region. Numerical models of thermochemical convection imply that a layer of material that is intrinsically about 4 percent more dense than the overlying mantle is dynamically stable. Because the deep layer is hot, its net density is only slightly greater than adiabatic and its surface develops substantial topography.

Major Lunar Crustal Terranes: Surface Expressions and Crust-Mantle Origins

Abstract: In light of global remotely sensed data, the igneous crust of the Moon can no longer be viewed as a simple, globally stratified cumulus structure, composed of a flotation upper crust of anorthosite underlain by progressively more mafic rocks and a residual-melt (KREEP) sandwich horizon near the base of the lower crust. Instead, global geochemical information derived from Clementine multispectral data and Lunar Prospector gamma-ray data reveals at least three distinct provinces whose geochemistry and petrologic history make them geologically unique: (1) the Procellarum KREEP Terrane (PKT), (2) the Feldspathic High-lands Terrane (FHT), and (3) the South Pole-Aitken Terrane (SPAT). The PKT is a mafic province, coincident with the largely resurfaced area in the Procellarum-Imbrium region whose petrogenesis relates to the early differentiation of the Moon. Here, some 40% of the Th in the Moon's crust is concentrated into a region that constitutes only about 10% of the crustal volume. This concentration of Th (average ∼5 ppm), and by implication the other heat producing elements, U and K, led to a fundamentally different thermal and igneous evolution within this region compared to other parts of the lunar crust. Lower-crustal materials within the PKT likely interacted with underlying mantle materials to produce hybrid magmatism, leading to the magnesian suite of lunar rocks and possibly KREEP basalt. Although rare in the Apollo sample collection, widespread mare volcanic rocks having substantial Th enrichment are indicated by the remote data and may reflect further interaction between enriched crustal residues and mantle sources. The FHT is characterized by a central anorthositic region that constitutes the remnant of an anorthositic craton resulting from early lunar differentiation. Basin impacts into this region do not excavate significantly more mafic material, suggesting a thickness of tens of kilometers of anorthositic crust. The feldspathic lunar meteorites may represent samples from the anorthositic central region of the FHT. Ejecta from deep-penetrating basin impacts outside of the central anorthositic region, however, indicate an increasingly mafic composition with depth. The SPAT, a mafic anomaly of great magnitude, may include material of the upper mantle as well as lower crust; thus it is designated a separate terrane. Whether the SPA basin impact simply uncovered lower crust such as we infer for the FHT remains to be determined.

The 15 June 1991 Eruption of Mount Pinatubo. I. Phase Equilibria and Pre-eruption P–T–fO2–fH2O Conditions of the Dacite Magma

Abstract: contents of the plagioclase. Compositional zoning of hornblende is Crystallization experiments were carried out on a representative consistent either with an early crystallization event at ~400 MPa sample of the crystal-rich dacite ejected during the 15 June 1991 and 840–900°C, or with mixing events before eruption. The eruption of Mt Pinatubo, to define the pre-eruption conditions of experiments show that addition of S leads to an increase in the this major volcanic event. Experiments were performed in the mg-number of hornblende in the redox range where pyrrhotite occurs temperature and pressure range of 750–900°C and 220–390 (Ζ NNO+ 1·4) The lack of Mg-rich overgrowth on hornblende MPa, respectively. Redox conditions were varied between those of shows that if sulfur was introduced into the magma in the course the NNO (nickel–nickel oxide) buffer and 2·7 log fO2 units above of its crystallization, such as by volatile infiltration from an (NNO + 2·7). Melt water contents ranged from 3 to 7 wt % underlying, triggering basalt magma, then it must have occurred (H2O saturation). Phase equilibria at 220 MPa reproduce the when the dacite was already oxidized. phase assemblage of the magma only at temperatures below 780°C and water-rich conditions: melt H2O content > 6 wt %, or XH2Ofluid > 0·80. Phase abundances and the compositions of hornblende, plagioclase, and melt indicate that the eruption tapped a magma body that was at a temperature of 760 ± 20°C and

Geochemical and Sr‐Nd isotopic characteristics of volcanic rocks from the Okinawa Trough and Ryukyu Arc: Implications for the evolution of a young, intracontinental back arc basin

Abstract: The Okinawa Trough is an incipient intracontinental back arc basin that has developed behind the Ryukyu arc-trench system. To explore its magmatic evolution and the nature of the mantle source, we present major and trace element and Sr-Nd isotopic data for mafic volcanic rocks from the Middle and Southern Okinawa Trough and the Central Ryukyu Arc. Two episodes of activity formed the latter: older (∼21–13 Ma) and younger (∼6–4 Ma). Although Quaternary basalts from the Middle Okinawa Trough (MOT) have major element compositions comparable to basalts from intra-oceanic back arc basins, they are characterized by relative enrichments of large ion lithophile elements and Pb and by depletions of high field strength elements analogous to those observed in Ryukyu Arc volcanics. Two components have been involved in the generation of MOT basalt, which we identify as an E-MORB type (or oceanic island basalt type) upwelling asthenospheric mantle and a “subduction component.” Quaternary basalts from the Southern Okinawa Trough (SOT) have uniform Nd but heterogeneous Sr isotopic ratios and incompatible trace element compositions. This may be ascribed to more complicated tectonic and magmatic processes in the SOT compared with the MOT, such as oblique subduction of the Philippine Sea Plate and interaction with postcollisional extension in the northern Taiwan orogenic belt. Integrating geological information available from nearby regions, we emphasize that the SOT is an “atypical” back arc basin because its development essentially occurred synchronous with or even prior to development of the arc-trench system.

Structure and petrology of Tauride ophiolites and mafic dike intrusions (Turkey): Implications for the Neotethyan ocean

Abstract: Cretaceous Neotethyan ophiolites occur in four east-west–trending subparallel zones within the Tauride tectonic belt in southern Turkey The ophiolites of the Inner, Intermediate, and Outer zones tectonically overlie the Mesozoic platform carbonates of the Tauride belt and are commonly underlain by a Cenomanian ophiolitic melange These ophiolites consist mainly of tectonized mantle rocks, mafic-ultramafic cumulates, and gabbros, and commonly lack sheeted dike complexes and extrusive rocks of a complete ophiolite sequence Metamorphic soles that are several hundred meters thick occur as thrust-faulted slices beneath these ophiolites and show well-developed metamorphic field gradients Ophiolitic units and the metamorphic soles are intruded by mafic dike swarms that are truncated at the contact with the underlying melange unit Dike rocks are made of subalkalic basalt to andesite typical of evolved island-arc tholeiites; they display large compositional variations, with SiO 2 content between 50 and 60 wt% and MgO between 8 and 4 wt%, and contain higher Ti augite phenocrysts and significantly less calcic plagioclase than their host cumulates The majority of the analyzed dike rocks show a slight depletion in light rare earth elements (REE) with low La/SmN ratios and are depleted in both high-field strength (HFS) and heavy REEs, while enriched in large-ion-lithophile elements (LILE) relative to normal mid-ocean ridge basalt (MORB) These characteristics suggest a mantle source that underwent previous melt extractions and subsequent metasomatism by LILE- and light REE-enriched fluids Geochemical modeling of trace elements shows that melting occurred at relatively low pressures under hydrous conditions and that it may have required the existence of an asthenospheric window, in which the dike magmas developed through tapping and mixing of melts generated within a rising melting column starting slightly within the garnet stability field, or in a transitional zone between the garnet and spinel stability fields at about 60 km depth This asthenospheric window was probably created during subduction of a Neotethyan ridge system; magmas ascending from the melt column within this window generated dikes that crosscut the metamorphic soles and were injected into the overlying mantle wedge and oceanic lithosphere The new 40 Ar/ 39 Ar hornblende dates of 92–90 Ma and 90–91 Ma from the metamorphic soles and dike swarms, respectively, show that evolution of these two geologic units was closely related in time and space and that they formed at the same intraoceanic subduction zone within the Inner Tauride seaway These data suggest that the Tauride ophiolites within the three zones to the north originated from the same root zone situated north of the Tauride carbonate platform, and that they constitute remnants of a single ophiolitic nappe sheet derived from the Inner Tauride seaway within the Neotethyan ocean

Hf isotope evidence for pelagic sediments in the source of hawaiian basalts

Abstract: Lead, oxygen, and osmium isotopic ratios measured on Hawaiian basalts can be matched with the isotopic ratios inferred for recycled ancient oceanic crust. High-precision hafnium isotopic data for lavas from several Hawaiian volcanoes identify old pelagic sediments in their source. These observations support the recycling hypothesis, whereby the mantle source of ocean island basalts includes ancient subducted oceanic crust. Hyperbolic lead-hafnium isotopic relations among Hawaiian basalts further indicate that upper mantle material is not involved in the production of hot spot basalts.

World-class Ni-Cu-PGE deposits: key factors in their genesis

Abstract: Magmatic Ni-Cu sulfide deposits form as the result of segregation and concentration of droplets of liquid sulfide from mafic or ultramafic magma, and the partitioning of chalcophile elements into these from the silicate melt. Sulfide saturation of a magma is not enough in itself to produce an ore deposit. The appropriate physical environment is required so that the sulfide liquid mixes with enough magma to become adequately enriched in chalcophile metals, and then is concentrated in a restricted locality so that the resulting concentration is of ore grade. The deposits of the Noril'sk region have developed within flat, elongate bodies (15 × 2 × 0.2 km) that intrude argillites, evaporites and coal measures, adjacent to a major, trans-crustal fault and immediately below the centre of a 3.5 km-thick volcanic basin. Studies of the overlying basalts have shown that lavas forming a 500 m-thick sequence within these have lost 75% of their Cu and Ni and more than 90% of their PGE. Overlying basalts show a gradual recovery in their chalcophile element concentrations to reach “normal” values 500 m above the top of the highly depleted zone. The ore-bearing Noril'sk-type intrusions correlate with those basalts above the depleted zone that contain “normal” levels of chalcophile elements. The high proportion of sulfide (2–10 wt.%) associated with the Noril'sk-type intrusions, the high PGE content of the ores, the extensive metamorphic aureole (100–400 m around the bodies), and the heavy sulfur isotopic composition of the ores (+8–+12 ∂34S) are explicable if the ore-bearing bodies are exit conduits from high level intrusions, along which magma has flowed en route to extrude at surface. The first magma to enter these intrusions reacted with much evaporitic sulfur, at a low “R” value and thus gave rise to sulfides with low metal tenors. Successive flow of magma through the system progressively enriched the sulfides in the conduits, losing progressively less of their chalcophile metals, and thus accounting for the upward increase in metals in successive lava flows above the highly depleted flows. The Voisey's Bay deposit lies partly within a 30–100 m-thick sheet of troctolite, interpreted as a feeder for the 1.334 Ga Voisey's Bay intrusion, and partly at the base of this intrusion, where the feeder adjoins it. Studies of olivine compositions indicate that an early pulse of magma through the feeder and into the intrusion was Ni depleted but that subsequent pulses were much less depleted. Trace element, Re-Os and S and O isotope data, and mineralogical studies indicate that the magma pulses interacted with country gneiss, probably principally in a deeper level intrusion, extracting SiO2, Na2O, K2O and possibly sulfur form the gneiss, which accounts for the magma becoming sulfide saturated. The Jinchuan deposit of north central China occurs within a 6 km-long dyke-like body of peridotite. The compositions of olivine within the dyke, the igneous rocks themselves, and the ore are all inconsistent with derivation of the body from ultramafic magma, as originally supposed, and indicate that the structure forms the keel of a much larger intrusion of magnesian basalt magma. Flow of magma into the intrusion has resulted in olivine and sulfide being retained where the keel was widening out into the intrusion. The West Australian komatiite-related deposits occur in thermal erosional troughs which have developed due to the channelisation of magma flow and the resulting thermal erosion of underlying sediments and basalt by the hot komatiite magma. The sediments are sulfide-rich, and may have contributed substantially to the sulfide of the ores. The mineralisation in the Duluth complex occurs in troctolitic intrusions along the western margin of the complex as a result of magma interacting with and extracting sulfur from the underlying graphite- and sulfide-bearing sediments. No magma flow channels have been identified so far, and the lack of magma flow subsequent to the development of sulfide immiscibility is regarded as the reason why these deposits are not of economic grade. When most major Ni-Cu sulfide deposits are compared, they prove to have a number of features in common; olivine-rich magma, proximity to a major crustal fault, sulfide-bearing country rocks, chalcophile element depletion in related intrusive or extrusive rocks, field and/or geochemical evidence of interaction between the magma and the country rocks, and the presence of or proximity to a magma conduit. The features are thought to explain the three key requirements (sulfide immiscibilty, adequate mixing between sulfides and magma, and localisation of the sulfides) discussed and have important implications with respect to exploration.

Cooling and crystallization of lava in open channels, and the transition of Pāhoehoe Lava to 'A'ā

Abstract: Samples collected from a lava channel active at Kilauea Volcano during May 1997 are used to con- strain rates of lava cooling and crystallization during early stages of flow. Lava erupted at near-liquidus tem- peratures (F1150 7C) cooled and crystallized rapidly in upper parts of the channel. Glass geothermometry indi- cates cooling by 12-14 7C over the first 2 km of trans- port. At flow velocities of 1-2 m/s, this translates to cooling rates of 22-50 7C/h. Cooling rates this high can be explained by radiative cooling of a well-stirred flow, consistent with observations of non-steady flow in proximal regions of the channel. Crystallization of plag- ioclase and pyroxene microlites occurred in response to cooling, with crystallization rates of 20-50% per hour. Crystallization proceeded primarily by nucleation of new crystals, and nucleation rates of F10 4 /cm 3 s are similar to those measured in the 1984 open channel flow from Mauna Loa Volcano. There is no evidence for the large nucleation delays commonly assumed for plagioclase crystallization in basaltic melts, possibly a reflection of enhanced nucleation due to stirring of the flow. The transition of the flow surface morphology from pahoehoe to 'a'aoccurred at a distance of 1.9 km from the vent. At this point, the flow was thermally stratified, with an interior temperature of F1137 7C and crystallinity of F15%, and a flow surface tempera- ture of F1100 7C and crystallinity of F45%. 'A'afor- mation initiated along channel margins, where crust was continuously disrupted, and involved tearing and clotting of the flow surface. Both observations suggest that the transition involved crossing of a rheological threshold. We suggest this threshold to be the develop- ment of a lava yield strength sufficient to prevent vis- cous flow of lava at the channel margin. We use this concept to propose that 'a'aformation in open chan- nels requires both sufficiently high strain rates for con- tinued disruption of surface crusts and sufficient groundmass crystallinity to generate a yield strength equivalent to the imposed stress. In Hawai'i, where lava is typically microlite poor on eruption, these combined requirements help to explain two common observations on 'a'aformation: (a) 'a'aflow fields are generated when effusion rates are high (thus promoting crustal disruption); and (b) under most eruption conditions, lava issues from the vent as pahoehoe and changes to 'a'aonly after flowing some distance, thus permitting sufficient crystallization.

Origin of enriched-type mid-ocean ridge basalt at ridges far from mantle plumes: The East Pacific Rise at 11°20′N

Abstract: The East Pacific Rise (EPR) at 11°20′N erupts an unusually high proportion of enriched mid-ocean ridge basalts (E-MORB) and thus is ideal for studying the origin of the enriched heterogeneities in the EPR mantle far from mantle plumes. These basalts exhibit large compositional variations (e.g., [La/Sm]N = 0.68–1.47, 87Sr/86Sr = 0.702508–0.702822, and 143Nd/144Nd = 0.513053–0.513215). The 87Sr/86Sr and 143Nd/144Nd correlate with each other, with ratios of incompatible elements (e.g., Ba/Zr, La/Sm, and Sm/Yb) and with the abundances and ratios of major elements (TiO2, Al2O3, FeO, CaO, Na2O, and CaO/Al2O3) after correction for fractionation effect. These correlations are interpreted to result from melting of a two-component mantle with the enriched component residing as physically distinct domains in the ambient depleted matrix. The observation of [Nb/Th]PM > 1 and [Ta/U]PM > 1, plus fractionated Nb/U, Ce/Pb, and Nb/La ratios, in lavas from the northern EPR region suggests that the enriched domains and depleted matrix both are constituents of recycled oceanic lithosphere. The recycled crustal/eclogitic lithologies are the major source of the enriched domains, whereas the recycled mantle/peridotitic residues are the most depleted matrix. On Pb-Sr isotope plot, the 11°20′N data form a trend orthogonal to the main trend defined by the existing EPR data, indicating that the enriched component has high 87Sr/86Sr and low 206Pb/204Pb and 143Nd/144Nd. This isotopic relationship, together with mantle tomographic studies, suggests that the source material of 11°20′N lavas may have come from the Hawaiian plume. This “distal plume-ridge interaction” between the EPR and Hawaii contrasts with the “proximal plume-ridge interactions” seen along the Mid-Atlantic Ridge. The so-called “garnet signature” in MORB is interpreted to result from partial melting of the eclogitic lithologies. The positive Na8-Si8/Fe8 and negative Ca8/Al8-Si8/Fe8 trends defined by EPR lavas result from mantle compositional (vs. temperature) variation.

Terrestrial Analogs and Thermal Models for Martian Flood Lavas

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.

Magmatic interactions as recorded in plagioclase phenocrysts of Chaos Crags, Lassen Volcanic Center, California

Abstract: The silicic lava domes of Chaos Crags in Lassen Volcanic National and mingling of this material into the silicic host. These processes Park contain a suite of variably quenched, hybrid basaltic andesite are commonplace in some orogenic magma systems and may be magmatic inclusions. The inclusions represent thorough mixing elucidated by isotopic microsampling and analysis of the plagioclases between rhyodacite and basalt recharge liquids accompanied by some crystallizing from them. mechanical disaggregation of the inclusions resulting in crystals mixing into the rhyodacite host preserved by quenching on dome emplacement. Sr/Sr ratios (~0·7037–0·7038) of the inclusions are distinctly lower than those of the host rhyodacite

Some Phase Equilibrium Constraints on the Origin of Proterozoic (Massif) Anorthosites and Related Rocks

Abstract: Experimental data in the range of 1 bar to 13 kbar enable us to INTRODUCTION map the liquidus equilibria relevant to Proterozoic (massif) anorThere is hardly a consensus on the petrogenesis of thosites and related mafic rocks. Massif anorthosites are widely Proterozoic (massif ) anorthosites, but many investigators believed to have formed by accumulation of plagioclase into highbelieve in a petrogenetic scheme that involves at least Al basaltic liquids. Mantle-derived basaltic liquids, fractionating two major stages: (1) extensive crystallization (± asat pressures sufficiently high (10–13 kbar) to crystallize the highly similation) of a mantle-derived magma ponded at or aluminous orthopyroxene megacrysts typically observed in anorthosite near the base of the crust that produces suspensions of massifs, reach plagioclase saturation at low normative silica contents. plagioclase in Fe-rich, high-Al gabbroic liquids; followed Peritectic-like equilibria (e.g. liq + opx → plag + cpx + sp) by (2) intrusion of these suspensions into the mid–upper and a thermal divide on the plagioclase+ pyroxene liquidus surface crust where they form complex magma chambers (e.g. ensure that mantle-derived liquids become nepheline normative with Duchesne, 1984; Emslie, 1985; Ashwal, 1993; Wiebe, further crystallization and crustal assimilation at depth. Such 1994). A key component of the scheme is the presence liquids cannot produce the full range of troctolitic–noritic to trocin most massifs of high-Al orthopyroxene megacrysts tolitic–gabbroic mineral assemblages observed in anorthosite massifs (HAOM, Emslie, 1975) that appear to preserve a record without extensive low-pressure granite assimilation. Conversely, the of the high-pressure stage (Maquil & Duchesne, 1984; array of plausible anorthosite parental liquids not only lies along Longhi et al., 1993). The structures of massifs vary from the trace of the plagioclase+ two-pyroxene cotectic from 10 to 13 those that are composites of multiple diapiric intrusions kbar, but also straddles the thermal divide on the plagioclase + (Harp Lake, Emslie, 1980; Rogaland, Duchesne, 1984) pyroxene liquidus surface. This condition requires mafic source to intrusive bodies with well-developed layering of regions, such as lower continental crust or foundered mafic plutons, leucocratic troctolites and leuconorites or leucogabbros for liquids parental to massif anorthosites and associated mafic overlain by masses of anorthosite (Michikamau, Emslie, intrusions. 1970; Laramie, Frost et al., 1993), so there may be

Quaternary volcanism near the Valley of Mexico: implications for subduction zone magmatism and the effects of crustal thickness variations on primitive magma compositions

Abstract: The Valley of Mexico and surrounding regions of Mexico and Morelos states in central Mexico contain more than 250 Quaternary eruptive vents in addition to the large, composite volcanoes of Popocatepetl, Iztaccihuatl, and Nevado de Toluca. The eruptive vents include cinder and lava cones, shield volcanoes, and isolated andesitic and dacitic lava flows, and are most numerous in the Sierra Chichinautzin that forms the southern terminus of the Valley of Mexico. The Chichinautzin volcanic field (CVF) is part of the E-W-trending Mexican Volcanic Belt (MVB), a subduction-related volcanic arc that extends across Mexico. The crustal thickness beneath the CVF (∼50 km) is the greatest of any region in the MVB and one of the greatest found in any arc worldwide. Lavas and scoriae erupted from vents in the CVF include alkaline basalts and calc-alkaline basaltic andesites, andesites, and dacites. Both alkaline and calc-alkaline groups contain primitive varieties that have whole rock Mg#, MgO, and Ni contents, and liquidus olivine compositions (≤Fo90) that are close to those expected of partial melts from mantle peridotite. Primitive varieties also show a wide range of incompatible trace element abundances (e.g. Ba 210–1080 ppm; Ce 25–100 ppm; Zr 130–280 ppm). Data for primitive calc-alkaline rocks from both the CVF and other regions of the MVB to the west are consistent with magma generation in an underlying mantle wedge that is depleted in Ti, Zr, and Nb and enriched in large ion lithophile (K, Ba, Rb) and light rare earth (La, Ce) elements. Extents of partial melting estimated from Ti and Zr data are lower for primitive calc-alkaline magmas in the CVF than for those from the regions of the MVB to the west where the crust is thinner. The distinctive major element compositions (low CaO and Al2O3, high SiO2) of the primitive calc-alkaline magmas in the CVF indicate a more refractory mantle source beneath this region of thick crust. In contrast, primitive alkaline magmas from the CVF and other regions of the MVB show compositional similarities to intraplate-type alkali basalts erupted behind the arc in the Mexican Basin and Range province. These similarities are consistent with the hypothesis that slab-induced convection in the mantle wedge beneath the MVB causes advection of asthenospheric mantle from behind the arc to the region of magma generation. Trace element systematics of primitive magmas in the MVB reveal substantial variability in both the extent of mantle wedge enrichment by subduction processes and in the composition of mantle heterogeneities that are related to previous extraction of alkaline to sub-alkaline basaltic melts.

Plume‐ridge interactions of the Discovery and Shona mantle plumes with the southern Mid‐Atlantic Ridge (40°‐55°S)

Abstract: We report on 66 Pb, Sr, and Nd isotope analyses of basalts dredged along the Mid-Atlantic Ridge (MAR) from 40° to 55°S. The results strongly indicate interaction and mixing between the off-ridge Discovery and ridge-centered Shona mantle plumes and the ambient asthenosphere beneath the MAR. In addition, the Bouvet mantle plume appears to be feeding the southernmost portion of the MAR as suggested earlier by le Roex et al [1987]. The Discovery and Shona plumes have enriched mantle and high-μ(μ = 238U/204Pb) affinities, respectively. Their proximity to one another suggests a genetic relationship, probably associated with subducted altered oceanic crust recycled through the mantle with some sediment (Discovery) or without sediment (Shona). The Discovery Ridge Anomaly exhibits Pb, Sr, and Nd isotopic discontinuities resulting from southward preferential plume flow across the Agulhas transform beginning ∼13 Ma. The presence of a component with unusually low 206Pb/204Pb accompanied by high 87Sr/86Sr and low 208Pb/204Pb and 143Nd/144Nd in the Discovery Ridge Anomaly and to a lesser extent in the Shona Ridge Anomaly indicates three-component mixing between the ambient asthenosphere, the Discovery and Shona plumes, and this low-μ (LOMU) component which possibly represents subcontinental lithospheric mantle material. We also note that in Pb, Sr, and Nd isotopic space, ocean island basalts from the Tristan, Gough, and Discovery family of plumes could be interpreted as resulting from binary mixing between a generic plume component similar to Bouvet or the “C” component [Hanan and Graham, 1994] and the LOMU component, which progressively increases southward. The LOMU component seems to be a characteristic feature of the South Atlantic and Indian Ocean mantles and is thought to reside passively in the shallow mantle because of delamination of subcontinental lithospheric mantle following the breakup of Gondwana.

Magmatic evolution of the Moon

Abstract: Although incomplete because of the imperfect and somewhat random sampling of rock types by the Apollo and Luna missions (1969-1976), the history of lunar magmatism has been reconstructed by numerous researchers over the past three decades. These reconstructions have illustrated the continuous nature of lunar magmatism (from 4.6 to approximately 2.0 Ga) and the large influence of early differentiation and catastrophic bombardment on lunar mantle dynamics, magmatism, and eruptive style. In this review, we group magmatism into multiple stages of activity based on sampled rock types and evaluate the models for each stage. Stage 1 is early lunar differentiation and associated magmatism. Partial melting of the Moon soon after accretion was responsible for producing an anorthositic crust and a differentiated lunar interior. The extent of lunar melting and mantle processing depends strongly on the mechanisms that induced melting. Estimates for the time over which melting and crystallization occurred range from tens to hundreds of millions of years. Stage 2 is the disruption of lunar magma ocean cumulates. Soon after the crystallization of most of the lunar magma ocean, the cumulate pile experienced gravitational overturn. This resulted in transport of late-forming cumulates into the deep lunar mantle and mixing of magma ocean cumulates on a variety of scales. Stage 3 is the post-magma ocean highland magmatism. Whereas the ferroan anorthositic crust was probably produced during the crystallization of a magma ocean, the slightly younger Mg suite and alkali suite plutonic rocks may have been generated by decompressional melting of early magma ocean cumulates during cumulate pile overturn. A KREEP and crustal signature was incorporated into these primitive basaltic magmas through assimilation near the base of the lunar crust or through melting of a hybridized mantle. The alkali suite could represent either the differentiation products of Mg suite parental magmas or a separate, but contemporaneous episode of basaltic magmatism. Stage 4 is pre-basin volcanism. Sample analysis and remote sensing data indicate that early lunar volcanism (KREEP basalts and high-alumina basalts) was contemporaneous with periods of highlands plutonism and catastrophic bombardment of the lunar surface. The relationship between early stages of lunar volcanism and the contemporaneous plutonism is not clear. The KREEP basalts may be volcanic equivalents to both the Mg suite and alkali suite. Stage 5 is the late remelting of magma ocean cumulates and eruption of mare basalts. Basin-associated eruption of mare basalts occurred during and following the late stages of catastrophic bombardment. This volcanic activity was possibly an extension of the thermal event that initiated pre-basin volcanism. Mare basalts exhibit a wide range of composition resulting from near-surface fractionation of chemically distinct primary basaltic magmas. Most likely, mare basalts were produced by small to moderate degrees of partial melting of hybrid cumulate sources in the deep lunar mantle. Alternatively, the mixed chemical signatures observed in many mare basalts may be interpreted as indicating assimilation of late-stage, evolved cumulates by melts produced deep in the cumulate pile. The wide range of compositions exhibited by the mare basalts compared with earlier episodes of basaltic magmatism may reflect the thermal regime in the lunar mantle that limited the extent of partial melting and melt-source homogenization.

Petrogenesis and Stratigraphy of the High-Ti/Y Urubici Magma Type in the Paraná Flood Basalt Province and Implications for the Nature of ‘Dupal’-Type Mantle in the South Atlantic Region

Abstract: within the asthenosphere following continental break-up, is preferred.Etendeka large igneous province has a restricted spatial extent, Thus the South Atlantic Dupal mantle anomaly cannot be considerednear the southeast Brazilian coast and in the northern Etendeka as a single entity: Urubici flood basalts and Walvis Ridge Site(Namibia). Urubici flows are interbedded with low-Ti/Y Gramado 525A basalts have a relatively shallow origin within originallyflows. Flow correlations indicate that local topographic relief was lithospheric mantle, whereas the Tristan plume is a deep mantleimportant in controlling emplacement of flows, and that lavas near upwelling.the coast have undergone up to 1 km of post-magmatic uplift relativeto inland areas. Urubici magmas have undergone extensive fractionalcrystallization (MgO <5·5 wt %). Stratigraphic variations high-light complexities of mixing and minor crustal assimilation indicativeof open-system magmatic plumbing. The least contaminated samples

Lu-Hf Isotope Systematics of Garnet Pyroxenites from Beni Bousera, Morocco: Implications for Basalt Origin.

Abstract: Six garnet pyroxenites from Beni Bousera, Morocco, yield a mean lutetium-hafnium age of 25 ± 1 million years ago and show a wide range in hafnium isotope compositions (ɛHf = –9 to +42 25 million years ago), which exceeds that of known basalts (0 to +25). Therefore, primary melts of garnet pyroxenites cannot be the source of basalts. The upper mantle may be an aggregate of pyroxenites that were left by the melting of oceanic crust at subduction zones and peridotites that were contaminated by the percolation of melts from these pyroxenites. As a consequence, the concept of geochemical heterogeneities as passive tracers is inadequate. Measured lutetium-hafnium partitioning of natural minerals requires a reassessment of some experimental work relevant to mantle melting in the presence of garnet.

Os Isotope Systematics in the Canary Islands and Madeira: Lithospheric Contamination and Mantle Plume Signatures

Abstract: Osmium concentrations and isotopic signatures were measured in 28 primarily Holocene basalts (22 of which have been analyzed for Sr–Nd–Pb isotope composition), two carbonatites and two mantle xenoliths from the Canary Islands, Selvagen Grande and Madeira in the eastern North Atlantic. 187Os/188Os ratios in the basalts range from 0.129 to 0.183. The Os isotope systematics indicate that the basalts fall into three petrogenetic groups: (1) a ‘radiogenic’ group with high 187Os/188Os from 0.152 to 0.183; (2) an ‘unradiogenic’ group with low 187Os/188Os from 0.129 to 0.138; (3) an ‘intermediate’ group with 187Os/188Os between 0.139 and 0.151. The Os isotope systematics of the radiogenic group samples are consistent with minor contamination of the basalts by marine sediment. All samples in the unradiogenic group contain mantle xenoliths, and the unradiogenic Os can be explained by bulk assimilation of ≤ 5% mantle peridotite in the form of disaggregated xenoliths. The radiogenic and unradiogenic groups are also characterized by higher 87Sr/86Sr and 208Pb/204Pb but lower 143Nd/144Nd than samples with similar 206Pb/204Pb from the intermediate group, which is interpreted to reflect interaction of plume magmas with the lithospheric mantle. The intermediate group samples are believed to represent the isotopic signature of the mantle plume. The Os isotopic composition of the Canary plume is among the most radiogenic found in ocean island basalts, comparable with the endmember HIMU islands Mangaia and Tubuaii, but at significantly lower 206Pb/204Pb. The radiogenic Os and moderate 206Pb/204Pb signature of the Canary plume is consistent with a plume which contains 25–35% of relatively young (∼1.2 Ga) recycled oceanic crust. Variable degree of mixing of the Canary Island plume source with shallow depleted asthenosphere containing a component of Paleozoic oceanic crust produces the limited range in Os isotopic signatures observed in the Madeira and Canary Island basalts despite a large range in 206Pb/204Pb isotopic composition.