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177
Constraints from thorium/lanthanum on sediment recycling at subduction zones and the evolution of continents
- J. Petrol
"... Arc magmas and the continental crust share many chemical features, but a major question remains as to whether these features are created by subduction or are recycled from subducting sediment. This ques-tion is explored here using Th/La, which is low in oceanic basalts (<02), elevated in the cont ..."
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Arc magmas and the continental crust share many chemical features, but a major question remains as to whether these features are created by subduction or are recycled from subducting sediment. This ques-tion is explored here using Th/La, which is low in oceanic basalts (<02), elevated in the continents (>025) and varies in arc basalts and marine sediments (009–034). Volcanic arcs form linear mixing arrays between mantle and sediment in plots of Th/La vs Sm/La. The mantle end-member for different arcs varies between highly depleted and enriched compositions. The sedimentary end-member is typically the same as local trench sediment. Thus, arc magmas inherit their Th/La from subducting sediment and high Th/La is not newly created during subduction (or by intraplate, adakite or Archaean magmatism). Instead, there is a large fractiona-tion in Th/La within the continental crust, caused by the preferen-tial partitioning of La over Th in mafic and accessory minerals. These observations suggest a mechanism of ‘fractionation & foun-dering’, whereby continents differentiate into a granitic upper crust and restite-cumulate lower crust, which periodically founders into the mantle. The bulk continental crust can reach its current elevated Th/La if arc crust differentiates and loses 25–60 % of its mafic residues to foundering. KEY WORDS: arc magmatism; continental crust; delamination; thorium; sediment subduction
Recycled metasomatized lithosphere as the origin of the enriched mantle II endmember: evidence from the Samoan volcanic
"... [1] 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 ol ..."
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[1] 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
Similar V/Sc systematics in MORB and arc basalts: implications for the oxygen fugacities of their mantle source regions
- J. Petrol
"... V/Sc systematics in peridotites, mid-ocean ridge basalts and arc basalts are investigated to constrain the variation of fO2 in the asthenospheric mantle. V/Sc ratios are used here to ‘ see through’ those processes that can modify barometric fO2 determinations in mantle rocks and/or magmas: early fra ..."
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V/Sc systematics in peridotites, mid-ocean ridge basalts and arc basalts are investigated to constrain the variation of fO2 in the asthenospheric mantle. V/Sc ratios are used here to ‘ see through’ those processes that can modify barometric fO2 determinations in mantle rocks and/or magmas: early fractional crystallization, degassing, crustal assimilation and mantle metasomatism. Melting models are combined here with a literature database on peridotites, arc lavas and mid-ocean ridge basalts, along with new, more precise data on peridotites and selected arc lavas. V/Sc ratios in primitive arc lavas from the Cascades magmatic arc are correlated with fluid-mobile elements (e.g. Ba and K), indicating that fluids may subtly influence fO2 during melting. However, for the most part, the average V/Sc-inferred fO2s of arc basalts, MORB and peridotites are remarkably similar (125 to þ05 log units from the FMQ buffer) and disagree with the observation that the barometric fO2s of arc lavas are several orders of magnitude higher. These observations suggest that the upper part of the Earth’s mantle may be strongly buffered in terms of fO2. The higher barometric fO2s of arc lavas and some arc-related xenoliths may be due respectively to magmatic differentiation processes and to exposure to large, time-integrated fluid fluxes incurred during the long-term stability of the lithospheric mantle. KEY WORDS: vanadium; scandium; oxygen fugacity; mantle; arcs
A detailed geochemical study of the island arc crust: the Talkeetna arc section, south-central
, 2006
"... Chugach Mountains of south–central Alaska is 5–18 km wide and extends for over 150 km. This accreted island arc includes exposures of upper mantle to volcanic upper crust. The section comprises six lithological units, in order of decreasing depth: (1) residual upper mantle harzburgite (with lesser p ..."
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Chugach Mountains of south–central Alaska is 5–18 km wide and extends for over 150 km. This accreted island arc includes exposures of upper mantle to volcanic upper crust. The section comprises six lithological units, in order of decreasing depth: (1) residual upper mantle harzburgite (with lesser proportions of dunite); (2) pyroxenite; (3) basal gabbronorite; (4) lower crustal gabbronorite; (5) mid-crustal plutonic rocks; (6) volcanic rocks. The pyroxenites overlie residual mantle peridotite, with some interfingering of the two along the contact. The basal gabbronorite overlies pyroxenite, again with some interfingering of the two units along their contact. Lower crustal gabbronorite (10 km thick) includes abundant rocks with well-developed modal layering. The mid-crustal plutonic rocks include a heterogeneous assemblage of gabbroic rocks, dioritic to tonalitic rocks (30–40 % area), and concentrations of mafic dikes and chilled mafic inclusions. The volcanic rocks (7 km thick) range from
Geochemical consequences of melt transport in 2-D: The sensitivity of trace elements to mantle dynamics
, 1995
"... Dynamic models are presented to investigate the consequences of melting and melt transport for stable trace element geochemistry in open systems. These models show that including explicit melt transport in 2-D adds non-trivial behaviour because melts and residues can travel and mix along very differ ..."
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Cited by 14 (4 self)
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Dynamic models are presented to investigate the consequences of melting and melt transport for stable trace element geochemistry in open systems. These models show that including explicit melt transport in 2-D adds non-trivial behaviour because melts and residues can travel and mix along very different trajectories. Calculations are presented for both equilibrium and disequilibrium transport and passive and active mid-ocean ridge flows. These calculations demonstrate that trace elements are sensitive to mantle dynamics and can readily distinguish between different end-member flow fields. Passive plate driven flow with strong melt focusing produces enrichments of incompatible elements. Active small-scale solid convection within the partially molten region, however, can lead to extreme dilution of incompatible elements suggesting that this form of convection may not be significant beneath normal ridges. These calculations provide additional predictions about acrossaxis trends of geochemi...
Evolution of U–Pb and Sm–Nd systems in numerical models of mantle convection andplate tectonics. J.Geophys
, 2004
"... [1] The development of U-Th-Pb and Sm-Nd isotopic signatures in a convecting mantle is studied using a numerical convection model with melting-induced differentiation and tracking of major and trace elements. The models include secular cooling and the decay of heat-producing elements, a rudimentary ..."
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[1] The development of U-Th-Pb and Sm-Nd isotopic signatures in a convecting mantle is studied using a numerical convection model with melting-induced differentiation and tracking of major and trace elements. The models include secular cooling and the decay of heat-producing elements, a rudimentary ‘‘self-consistent’ ’ treatment of plate tectonics, and both olivine system and garnet-pyroxene system phase transitions. The system self-consistently evolves regions with a high m(=U/Pb) (HIMU)-like Pb signature and regions with low 143Nd/144Nd. However, the isotopic ‘‘age’ ’ determined from the slope in (207Pb/204Pb)–(206Pb/204Pb) space is much larger than observed. Several hypotheses are examined to explain this discrepancy. Sampling length scale has a minimal effect on age. The extent of crustal settling above the core-mantle boundary makes some difference but not enough. More frequent remelting is a possible explanation but requires the rate of crustal production to have been much higher in the past. Not introducing HIMU into the mantle prior to 2.0–2.5 Gyr before present, because of a change in the surface oxidization environment or subduction zone processes, can account for the difference, but its effect on other isotope systems needs to be evaluated. Improved
Osmium isotopes and mantle convection
- Phil. Trans. R. Soc. Lond. A
"... The decay of 187Re to 187Os (with a half-life of 42 billion years) provides a unique isotopic fingerprint for tracing the evolution of crustal materials and mantle residues in the convecting mantle. Ancient subcontinental mantle lithosphere has uniquely low Re/Os and 187Os/188Os ratios due to large- ..."
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The decay of 187Re to 187Os (with a half-life of 42 billion years) provides a unique isotopic fingerprint for tracing the evolution of crustal materials and mantle residues in the convecting mantle. Ancient subcontinental mantle lithosphere has uniquely low Re/Os and 187Os/188Os ratios due to large-degree melt extraction, recording ancient melt-depletion events as old as 3.2 billion years. Partial melts have Re/Os ratios that are orders of magnitude higher than their sources, and the subduction of oceanic or continental crust introduces into the mantle materials that rapidly accumulate radiogenic 187Os. Eclogites from the subcontinental lithosphere have extremely high 187Os/188Os ratios, and record ages as old as the oldest peridotites. The data show a near-perfect partitioning of Re/Os and 187Os/188Os ratios between peridotites (low) and eclogites (high). The convecting mantle retains a degree of Os-isotopic heterogeneity similar to the lithospheric mantle, although its amplitude is modulated by convective mixing. Abyssal peridotites from the ocean ridges have low Os isotope ratios, indicating that the upper mantle had undergone episodes of melt depletion prior to the most recent
Age and geochemistry of the Central American forearc basement (DSDP Leg 67 and 84): insights into Mesozoic arc volcanism and seamount accretion on the fringe of the Caribbean LIP
- J. Petrol
, 2008
"... The igneous forearc basement along the Pacific coast of northern ..."
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Cited by 8 (0 self)
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The igneous forearc basement along the Pacific coast of northern
North Atlantic Igneous Province: A review of models for its formation
- In: The Origins of Melting Anomalies: Plates, Plumes and Planetary Processes. Geological Society of America, Special Papers
"... The mantle plume concept is currently being challenged as an explanation for North Atlantic Igneous Province formation. Alternative models have been suggested, including delamination, meteorite impact, small-scale rift-related convection, and chemical mantle heterogeneities. We review available data ..."
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The mantle plume concept is currently being challenged as an explanation for North Atlantic Igneous Province formation. Alternative models have been suggested, including delamination, meteorite impact, small-scale rift-related convection, and chemical mantle heterogeneities. We review available datasets on uplift, strain localization, age and chemistry of igneous material, and tomography for the North Atlantic Igneous Province, and compare them with predictions from the mantle plume and alternative models. The mantle plume concept is quite successful in explaining formation of the NAIP, but unexplained aspects remain. Delamination and impact models are currently not supported. Rift-related small-scale convection models appear to be able to explain volcanic rifted margin volcanism well. However, the most important problem that non-plume models need to overcome is the continuing, long-lived melt anomaly extending via the Greenland-Faeroe Ridges to Iceland. Mantle heterogeneities, resulting from an ancient subducted slab, are included in plate tectonic models to explain the continuing melt production as an alternative to the mantle plume model, but there are still uncertainties related to this idea that need to be solved.
Hafnium isotope and trace element constraints on the nature of mantle heterogeneity beneath the central Southwest Indian Ridge (138E to 478E
, 2005
"... Hafnium isotope and incompatible trace element data are presented for a suite of mid-ocean ridge basalts (MORB) from 13 to 47E on the Southwest Indian Ridge (SWIR), one of the slowest spreading and most isotopically heterogeneous mid-ocean ridges. Variations in Nd–Hf isotope compositions and Lu/Hf r ..."
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Cited by 7 (0 self)
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Hafnium isotope and incompatible trace element data are presented for a suite of mid-ocean ridge basalts (MORB) from 13 to 47E on the Southwest Indian Ridge (SWIR), one of the slowest spreading and most isotopically heterogeneous mid-ocean ridges. Variations in Nd–Hf isotope compositions and Lu/Hf ratios clearly distinguish an Atlantic–Pacific-type MORB source, present west of 26E, characterized by relatively low eHf values for a given eNd relative to the regression line through all Nd–Hf isotope data for oceanic basalts (termed the ‘Nd–Hf mantle array line’; the deviation from this line is termed DeHf) and low Lu/Hf ratios, from an Indian Ocean-type MORB signature, present east of 32E, characterized by relatively high DeHf values and Lu/Hf ratios. Additionally, two localized, isotopically anomalous areas, at 13–15E and 39–41E, are characterized by distinctly low negative and high positive DeHf values, respectively. The low DeHf MORB from 13 to 15E appear
