Showing papers on "Basalt published in 2004"

Bulk-rock Major and Trace Element Compositions of Abyssal Peridotites: Implications for Mantle Melting, Melt Extraction and Post-melting Processes Beneath Mid-Ocean Ridges

Abstract: This paper presents the first comprehensive major and trace element data for � 130 abyssal peridotite samples from the Pacific and Indian ocean ridge–transform systems. The data reveal important features about the petrogenesis of these rocks, mantle melting and melt extraction processes beneath ocean ridges, and elemental behaviours. Although abyssal peridotites are serpentinized, and have also experienced seafloor weathering, magmatic signatures remain well preserved in the bulk-rock compositions. The better inverse correlation of MgO with progressively heavier rare earth elements (REE) reflects varying amounts of melt depletion. This melt depletion may result from recent sub-ridge mantle melting, but could also be inherited from previous melt extraction events from the fertile mantle source. Light REE (LREE) in bulk-rock samples are more enriched, not more depleted, than in the constituent clinopyroxenes (cpx) of the same sample suites. If the cpx LREE record sub-ridge mantle melting processes, then the bulk-rock LREE must reflect post-melting refertilization. The significant correlations of LREE (e.g. La, Ce, Pr, Nd) with immobile high field strength elements (HFSE, e.g. Nb and Zr) suggest that enrichments of both LREE and HFSE resulted from a common magmatic process. The refertilization takes place in the ‘cold’ thermal boundary layer (TBL) beneath ridges through which the ascending melts migrate and interact with the advanced residues. The refertilization apparently did not affect the cpx relics analyzed for trace elements. This observation suggests grain-boundary porous melt migration in the TBL. The ascending melts may not be thermally ‘reactive’, and thus may have affected only cpx rims, which, together with precipitated olivine, entrapped melt, and the rest of the rock, were subsequently serpentinized. Very large variations in bulk-rock Zr/Hf and Nb/Ta ratios are observed, which are unexpected. The correlation between the two ratios is consistent with observations on basalts that DZr/DHf < 1 and DNb/DTa < 1. Given

Recycled metasomatized lithosphere as the origin of the Enriched Mantle II (EM2) end‐member: Evidence from the Samoan Volcanic Chain

Abstract: An in-depth Sr-Nd-Pb-He-Os isotope and trace element study of the EMII-defining Samoan hot spot lavas leads to a new working hypothesis for the origin of this high 87Sr/86Sr mantle end-member. Systematics of the Samoan fingerprint include (1) increasing 206Pb/204Pb with time - from 18.6 at the older, western volcanoes to 19.4 at the present-day hot spot center, Vailulu'u Seamount, (2) en-echelon arrays in 206Pb/204Pb – 208Pb/204Pb space which correspond to the two topographic lineaments of the 375 km long volcanic chain – this is much like the Kea and Loa Trends in Hawai'i, (3) the highest 87Sr/86Sr (0.7089) of all oceanic basalts, (4) an asymptotic decrease in 3He/4He from 24 RA [Farley et al., 1992] to the MORB value of 8 RA with increasing 87Sr/86Sr, and (5) mixing among four components which are best described as the “enriched mantle”, the depleted FOZO mantle, the (even more depleted) MORB Mantle, and a mild HIMU (high 238U/204Pb) mantle component. A theoretical, “pure” EMII lava composition has been calculated and indicates an extremely smooth trace element pattern of this end-member mantle reservoir. The standard recycling model (of ocean crust/sediment) fails as an explanation for producing Samoan EM2, due to these smooth spidergrams for EM2 lavas, low 187Os/188Os ratios and high 3He/4He (>8 RA). Instead, the origin of EM2 has been modeled with the ancient formation of metasomatised oceanic lithosphere, followed by storage in the deep mantle and return to the surface in the Samoan plume.

Mineralogy at Meridiani Planum from the Mini-TES Experiment on the Opportunity Rover.

Abstract: The Miniature Thermal Emission Spectrometer (Mini-TES) on Opportunity investigated the mineral abundances and compositions of outcrops, rocks, and soils at Meridiani Planum Coarse crystalline hematite and olivine-rich basaltic sands were observed as predicted from orbital TES spectroscopy Outcrops of aqueous origin are composed of 15 to 35% by volume magnesium and calcium sulfates [a high-silica component modeled as a combination of glass, feldspar, and sheet silicates (approximately 20 to 30%)], and hematite; only minor jarosite is identified in Mini-TES spectra Mini-TES spectra show only a hematite signature in the millimeter-sized spherules Basaltic materials have more plagioclase than pyroxene, contain olivine, and are similar in inferred mineral composition to basalt mapped from orbit Bounce rock is dominated by clinopyroxene and is close in inferred mineral composition to the basaltic martian meteorites Bright wind streak material matches global dust Waterlain rocks covered by unaltered basaltic sands suggest a change from an aqueous environment to one dominated by physical weathering

Flood and shield basalts from Ethiopia: Magmas from the African superswell

Abstract: The Ethiopian plateau is made up of several distinct volcanic centres of different ages and magmatic affinities. In the NE, a thick sequence of 30 Ma flood basalts is overlain by the 30 Ma Simien shield volcano. The flood basalts and most of this shield volcano, except for a thin veneer of alkali basalt, are tholeiitic. In the centre of the province, a far thinner sequence of flood basalt is overlain by the 22 Ma Choke and Guguflu shield volcanoes. Like the underlying flood basalts, these shields are composed of alkaline lavas. A third type of magma, which also erupted at 30 Ma, is more magnesian, alkaline and strongly enriched in incompatible trace elements. Eruption of this magma was confined to the NE of the province, a region where the lava flows are steeply tilted as a result of deformation contemporaneous with their emplacement. Younger shields (e.g. Mt Guna, 10·7 Ma) are composed of Si-undersaturated lavas. The three main types of magma have very different major and trace element characteristics ranging from compositions low in incompatible elements in the tholeiites [e.g. 10 ppm La at 7 wt % MgO (=La7), La/Υb = 4.2], moderate in the alkali basalts (La7 = 24, La/Υb = 9.2), and very high in the magnesian alkaline magmas (La7 = 43, La/Υb = 17). Although their Nd and Sr isotope compositions are similar, Pb isotopic compositions vary considerably; 206Pb/204Pb varies in the range of ∼17·9-18·6 in the tholeiites and ∼19·0-19·6 in the 22 Ma shields. A conventional model of melting in a mantle plume, or series of plumes, cannot explain the synchronous eruption of incompatible-element-poor tholeiites and incompatible-element-rich alkali lavas, the large range of Pb isotope compositions and the broad transition from tholeiitic to alkali magmatism during a period of continental rifting. The lithospheric mantle played only a passive role in the volcanism and does not represent a major source of magma. The mantle source of the Ethiopian volcanism can be compared with the broad region of mantle upwelling in the South Pacific that gave rise to the volcanic islands of French Polynesia. Melting in large hotter-than-average parts of the Ethiopian superswell produced the flood basalts; melting in small compositionally distinct regions produced the magmas that fed the shield volcanoes. © Oxford University Press 2004; all rights reserved.

A hydrous melting and fractionation model for mid-ocean ridge basalts: Application to the Mid-Atlantic Ridge near the Azores

Abstract: The major element, trace element, and isotopic composition of mid-ocean ridge basalt glasses affected by the Azores hotspot are strongly correlated with H2O content of the glass. Distinguishing the relative importance of source chemistry and potential temperature in ridge-hotspot interaction therefore requires a comprehensive model that accounts for the effect of H2O in the source on melting behavior and for the effect of H2O in primitive liquids on the fractionation path. We develop such a model by coupling the latest version of the MELTS algorithm to a model for partitioning of water among silicate melts and nominally anhydrous minerals. We find that much of the variation in all major oxides except TiO2 and a significant fraction of the crustal thickness anomaly at the Azores platform are explained by the combined effects on melting and fractionation of up to ~700 ppm H2O in the source with only a small thermal anomaly, particularly if there is a small component of buoyantly driven active flow associated with the more H2O-rich melting regimes. An on-axis thermal anomaly of ~35°C in potential temperature explains the full crustal thickness increase of ~4 km approaching the Azores platform, whereas a ≥75°C thermal anomaly would be required in the absence of water or active flow. The polybaric hydrous melting and fractionation model allows us to solve for the TiO2, trace element and isotopic composition of the H2O-rich component in a way that self-consistently accounts for the changes in the melting and fractionation regimes resulting from enrichment, although the presence and concentration in the enriched component of elements more compatible than Dy cannot be resolved.

Early Life Recorded in Archean Pillow Lavas

Abstract: Pillow lava rims from the Mesoarchean Barberton Greenstone Belt in South Africa contain micrometer-scale mineralized tubes that provide evidence of submarine microbial activity during the early history of Earth. The tubes formed during microbial etching of glass along fractures, as seen in pillow lavas from recent oceanic crust. The margins of the tubes contain organic carbon, and many of the pillow rims exhibit isotopically light bulk-rock carbonate δ 13 C values, supporting their biogenic origin. Overlapping metamorphic and magmatic dates from the pillow lavas suggest that microbial life colonized these subaqueous volcanic rocks soon after their eruption almost 3.5 billion years ago.

Chemistry of Rocks and Soils in Gusev Crater from the Alpha Particle X-ray Spectrometer

Abstract: The alpha particle x-ray spectrometer on the Spirit rover determined major and minor elements of soils and rocks in Gusev crater in order to unravel the crustal evolution of planet Mars. The composition of soils is similar to those at previous landing sites, as a result of global mixing and distribution by dust storms. Rocks (fresh surfaces exposed by the rock abrasion tool) resemble volcanic rocks of primitive basaltic composition with low intrinsic potassium contents. High abundance of bromine (up to 170 parts per million) in rocks may indicate the alteration of surfaces formed during a past period of aqueous activity in Gusev crater.

Basaltic rocks analyzed by the Spirit rover in Gusev crater

Abstract: The Spirit landing site in Gusev Crater on Mars contains dark, fine-grained, vesicular rocks interpreted as lavas. Pancam and Mini-Thermal Emission Spectrometer (Mini-TES) spectra suggest that all of these rocks are similar but have variable coatings and dust mantles. Magnified images of brushed and abraded rock surfaces show alteration rinds and veins. Rock interiors contain

Acid-sulfate weathering of synthetic Martian basalt: The acid fog model revisited

Abstract: The acid fog model has received considerable attention as a model of soil formation on Mars. Previous evaluations of this model have focused on experimental weathering of terrestrial basalt samples. However, these samples differ significantly from what now is thought to be typical of Martian basalt. The acid fog model is tested here using synthetic basaltic analogs derived from Mars Pathfinder soil and rock compositions. Reaction of synthetic basalt with various acidic solutions and subsequent evaporation has led to the formation of several putative secondary mineral phases. Many of these phases were not produced in prior experimental studies aimed at aqueous interactions on Mars. Of these alteration phases, Mg, Fe, Ca, and Al sulfates were identified. In addition, secondary ferric oxide phases formed via rapid Fe oxidation under relatively high pH levels buffered by basalt dissolution. Amorphous silica is a ubiquitous product in these experiments and has formed by precipitation from solution and by the dissolution of minerals and glasses leaving behind leached surface layers composed of residual silica. The secondary products formed in these experiments demonstrate the importance of primary mineralogy when testing models of aqueous interactions on Mars. New constraints are placed on both the reactivity of primary basalt and the secondary mineralogy present at the Martian surface. Copyright 2004 by the American Geophysical Union.

Petrogenesis of Carboniferous rift-related volcanic rocks in the Tianshan, northwestern China

Abstract: The Tianshan Carboniferous rift-related volcanic rocks make up a large igneous province in northwestern China. On the basis of petrogeochemical data, the rift- related basic lavas can be classified into two major magma types. These are (1) a low-Ti/Y type situated in central and eastern Tianshan that exhibits low Ti/Y ( 2 (43–55 wt%) and relatively high Fe 2 O 3 T (Fe 2 O 3 as total Fe; 6.4–11.5 wt%); and (2) a high-Ti/Y type situated in western Tianshan that has high Ti/Y (>500), Ce/Y (>3), and SiO 2 (49–55 wt%) and relatively low Fe 2 O 3 T (5.8–7.8 wt%). Elemental data suggest that the chemical variations of the low-Ti/Y and high-Ti/Y lavas cannot be explained by crystallization from a common parental magma. The Tianshan Carboniferous basic lavas most likely originated from an asthenospheric oceanic-island-basalt–like mantle source ( 87 Sr/ 86 Sr ( t ) ≈ 0.703–0.705, ϵ Nd( t ) ≈ +4 to +7). Crustal contamination and continental lithospheric mantle have also contributed significantly to the formation of the basic lavas of the Tianshan Carboniferous rift. Our data show that spatial petrogeochemical variations exist in the volcanic rocks of the Tianshan large igneous province. The location of the thickest volcanic succession, which has dominantly tholeiitic lavas, in the eastern Tianshan may have been centered over the melting anomaly in the mantle. The eastern Tianshan basic magmas were generated by a higher degree of partial melting in the spinel-garnet transition zone of the mantle compared to the alkaline basaltic lavas that are the dominant magma type in the western Tianshan. The lower degree of partial melting in the garnet stability field of the mantle, as is characteristic of the western Tianshan basic lavas, may be the result of a relatively thicker lithosphere and lower geotherm.

Highly heterogeneous Late Mesozoic lithospheric mantle beneath the North China Craton: evidence from Sr–Nd–Pb isotopic systematics of mafic igneous rocks

Abstract: The lithospheric mantle beneath the North China Craton changed dramatically in its geophysical and geochemical characteristics from Palaeozoic to Cenozoic times. This study uses samples of Mesozoic basalts and mafic intrusions from the North China Craton to investigate the nature of this mantle in Mesozoic times. Sr–Nd–Pb isotopic data demonstrate that the Late Mesozoic lithospheric mantle was extremely heterogeneous. In the central craton or the Luzhong region, it is slightly Sr–Nd isotopically enriched, beneath the Taihangshan region it has an EM1 character (87Sr/86Sri=0.7050–0.7066; eNd(t)=−17–−10), and beneath the Luxi–Jiaodong region, it possesses EM2-like characteristics (87Sr/86Sri up to 0.7114). Compositional variation with time is also apparent in the Mesozoic lithospheric mantle. Our data suggest that the old lithospheric mantle was modified during Mesozoic times by a silicic melt, where beneath the Luxi–Jiaodong region it was severely modified, but in the Luzhong and Taihangshan regions the effects were much less marked. The silicic melt may have been the product of partial melting of crustal materials brought into the mantle by the subducted slab during the formation of circum-cratonic orogenic belts. This Mesozoic mantle did not survive for a long time, and was replaced by a Cenozoic mantle with depleted geochemical characteristics.

Geochemical Constraints for the Genesis of Post-collisional Magmatism and the Geodynamic Evolution of the Northern Taiwan Region

Abstract: Taiwan is an active mountain belt created by the oblique collision of the northern Luzon arc with Asia. Late Pliocene extensional collapse of the northern Taiwan mountain belt (NTMB) was accompanied by magmatism that formed the Northern Taiwan Volcanic Zone (NTVZ; 2 8---0 2Ma). The geochemical characteristics of the NTVZ magmas can thus provide constraints both for the mantle source composition and the geodynamic processes operating in the late orogenic stage of the region. The NTVZ volcanic rocks consist dominantly of calc-alkaline andesites and basalts, along with subordinate but heterogeneous lavas including low-K, shoshonitic and ultrapotassic magmas. From the NE to the SW in the NTVZ, the magmas show systematic compositional variations from low-K to calc-alkaline and then shoshonitic. This spatial geochemical variation, characterized by southwesterly increase in potassium and incompatible trace elements, appears to be subparallel to the southwestern part of the modern Ryukyu subduction system. Sr---Nd isotope ratios of the NTVZ volcanic rocks ( Sr/Sr 0 70376---0 70551; Nd/Nd 0 51259---0 51301) suggest that two mantle source components are involved in the magma generation, the asthenosphere and metasomatized subcontinental lithospheric mantle. These two components are represented by the 2 6MaMienhuayu high-Mg basaltic andesites and the 0 2Ma Tsaolingshan high-Mg potassic lavas, respectively. The latter are interpreted to be the products of small-degree melting of a phlogopitebearing, harzburgite lithospheric mantle source metasomatized recently by the nearby Ryukyu subduction zone processes. The Sr---Nd---Pb isotope systematics and specific trace-element ratios of the NTVZ volcanic rocks suggest that melts derived from subducted sediments and fluids released from slab dehydration reactions were both involved in metasomatizing this mantle source. The unique spatial geochemical variation of the NTVZ volcanic rocks can be successfully modelled using variable degrees of partial melting of the mantle source regions, coupled with mixing of different melt components from depleted asthenospheric and metasomatized lithospheric mantle components beneath individual volcanic fields. It is inferred that mixing of melts from specific mantle components and the degree of partial melting are spatially and temporally related to the tectonic evolution of the northern Taiwan region, and not simply due directly to subduction zone processes. The overall NTVZ geochemical characteristics can be explained by various degrees of melting within an ascending region of the asthenospheric mantle, triggered by extensional collapse of the NTMB, and interaction of these melts with overlying fluidand sediment-modified lithospheric mantle. JOURNAL OF PETROLOGY VOLUME 45 NUMBER 5 PAGES 975–1011 2004 DOI: 10.1093/petrology/egh001

Mantle dynamics and genesis of mafic magmatism in the intermontane Pacific Northwest

Abstract: [1] The Columbia Plateau, Oregon Plateau, Snake River Plain, and Northern Nevada Rift compose a single magmatic system containing all the essential characteristics ascribed to a mantle plume genesis. A mobile mantle is delineated by volcanic migrations, divisible into two types: (1) Rapid, radial migrations (∼10–100 cm/yr) are associated with impingement and spreading of the Yellowstone plume head along the Chief Joseph, Steens Mountain–Picture Gorge, and Northern Nevada Rift magmatic trends from ∼16.6 to 15.0 Ma. (2) Subsequent (post-15.0 Ma), slower migrations (1–5 cm/yr) are associated with shearing off of the plume head, generating the Snake River Plain hot spot track above the plume tail, and with westward asthenospheric drag of the plume head beneath the Oregon Plateau. The plume head provided a melt component to Imnaha and Grande Ronde Basalts. Depleted mantle lithosphere lying above the plume head provided a melt component to Steens Basalt and Picture Gorge Basalt and to younger eruptions of high-alumina olivine tholeiite. The plume head currently resides beneath a broad lithospheric swell, marked by young volcanism, high heat flow, and slow P wave travel times. The periphery of the plume head is delineated by the cratonic margin to the east, a gravity discontinuity and a set of wrinkle ridges to the north, and a prominent belt of young high-alumina olivine tholeiites and active volcanoes adjacent to the Cascade volcanic arc to the west.

Jurassic intraplate magmatism in southern Hunan–eastern Guangxi: 40Ar/39Ar dating, geochemistry, Sr–Nd isotopes and implications for the tectonic evolution of SE China

Abstract: The Mesozoic geology of SE China is characterized by intensive and widespread magmatism. However, the tectonic regime that accounted for the Mesozoic magmatism has been an issue with little consensus. A comprehensive study of Ar-40 -Ar-39 dating, geochemistry and Sr-Nd isotopes has been conducted on basalts from southern Hunan and syenite intrusions from eastern Guangxi. Three episodes of Jurassic magmatism, i.e. alkaline basalts of c. 175 Ma in age, syenitic intrusions of c. 160 Ma and high-Mg basalts of c. 150 Ma, are identified. The older, c. 175 Ma alkaline basalts are characterized by low Sr (I-Sr = 0.7035-0.7040) and high Nd (epsilon(Nd)(T) = 5 to 6) isotopic compositions and OIB-like trace-element patterns (e.g. Nb/La > 1). In contrast, the younger, c. 150 Ma high-Mg basalts have high Sr (I-Sr c. 0.7054) and low Nd (epsilon(Nd)(T) c. -2) isotopic compositions and incompatible trace-element patterns of arc affinity. The c. 160 Ma syenitic intrusions display a relatively large range of Sr and Nd isotopic compositions (I-Sr = 0.7032-0.7082, epsilon(Nd)(T) = 5.5 to -4.1), with the Qinghu syenites having the lowest I-Sr, highest epsilon(Nd)(T) and OIB-type incompatible trace-element patterns analogous to the c. 175 Ma alkaline basalts. Such a secular variation in rock types and geochemical and isotopic characteristics reveals changes in melt segregation depth and mantle sources, which are inferred to have resulted from the post-Indosinian orogenic lithosphere extension and thinning. The c. 175 Ma alkaline basalts are suggested to have formed by small degrees of decompression melting of the astheno-sphere or an enriched lithospheric mantle source accreted by asthenosphere-derived melts during the initial extension. The c. 160 Ma syenitic and c. 150 Ma high-Mg basaltic rocks mainly originated from the enriched lithospheric mantle that melted owing to a raised geotherm caused by lithosphere thinning. This interpretation is at odds with the active continental margin related to the subduction of palaeo-Pacific plate, but consistent with continental rifting and extension for the Mesozoic of SE China.

Geochronology of age-progressive volcanism of the Oregon High Lava Plains : implications for the plume interpretation of Yellowstone

Abstract: [1] The High Lava Plains province (HLP) is a late Cenozoic bimodal volcanic field at the northern margin of the Basin and Range province in southeastern Oregon that hosts a westward younging trend of silicic volcanism that crudely mirrors northeastward migration of silicic volcanism along the Yellowstone–Snake River Plain (YSRP) trend. We present 40Ar/39Ar ages for 19 rhyolite domes, 5 rhyolite ash flow tuffs, and 34 basaltic lavas from the HLP. The previously identified trend of westward migration of HLP rhyolites is confirmed. The rate of propagation is ∼33 km/m.y. from 10 to 5 Ma, slowing to ∼13 km/m.y. after 5 Ma. The duration of silicic volcanism at any locus is ∼2 m.y. Three older HLP dacite domes yielded ages of ∼15.5 Ma. Basalts are not age progressive. We identify several episodes of increased basaltic activity at 7.5–7.8, 5.3–5.9, and 2–3 Ma, with the younger episode likely continuing into the Recent. The HLP and YSRP trends emerged from the axis of middle Miocene basaltic volcanism of the Columbia River and Steens basalts. We propose a model in which (1) Miocene flood basalts and widespread silicic rocks are the result of emplacement of a plume head near the craton margin, enhanced by flow up a topographic gradient along the base of the lithosphere at the craton margin; (2) the HLP trend is the result of westward flow originating at the craton margin; and (3) the YSRP trend is the trace of the motion of the North American plate over the tail of the plume.