Uranium–thorium disequilibria and partitioning on melting of garnet peridotite

Beattie, Paul

doi:10.1038/363063a0

Cited by 255 publications

(112 citation statements)

References 27 publications

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“…Garnet plays a key role for the fractionation between the U-series nuclides during melting because U is more compatible in garnet compared to Th, Pa and Ra (Beattie, 1993b;LaTourrette et al, 1993;Hauri et al, 1994;Salters and Longhi, 1999). In clinopyroxene (cpx), the partition coefficients are dependent on pressure and composition.…”

Section: U-series Nuclide Partitioningmentioning

confidence: 99%

The influence of source heterogeneity on the U–Th–Pa–Ra disequilibria in post-glacial tholeiites from Iceland

Koornneef

Stracke

Bourdon

2012

Geochimica et Cosmochimica Acta

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We investigate the relative influence of mantle upwelling velocity and source heterogeneity on the melting rates recorded by 230 230 Th excesses and distance from the inferred plume centre is consistent with a model of decreasing mantle upwelling velocity with increasing distance from the plume axis. However, the model is not substantiated by the ( 231 Pa/ 235 U) data as the correlation with distance from the plume centre is weak. On the scale of individual eruption centres, the observed U-series are influenced by variations in melt transport time, source porosity, and local variations in mantle upwelling velocity. Broad correlations between ( 230 Th/ 238 U) and ( 231 Pa/ 235 U) and highly incompatible trace element ratios for samples from the Western Volcanic Zone provide, however, evidence for a significant underlying effect of source heterogeneity on the U-series data. Low 230 Th and 231 Pa excesses in enriched samples from the Western Volcanic Zone with high U/Th, Nb/U and Nb/La indicate that partial melts from an enriched source component, characterised by high melt productivity but low bulk D U /D Th , influence the U-series systematics of the erupted melts. These results re-affirm the presence of comparatively larger abundances of enriched material in the mantle source beneath the South Western Rift of Iceland, which has been suggested based on relationships between highly incompatible element and Pb isotope ratios in Icelandic basalts. Overall, our results highlight the importance of lithological heterogeneity on the melting behaviour of the upper mantle and the composition of oceanic basalts.

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Section: U-series Nuclide Partitioningmentioning

confidence: 99%

The influence of source heterogeneity on the U–Th–Pa–Ra disequilibria in post-glacial tholeiites from Iceland

Koornneef

Stracke

Bourdon

2012

Geochimica et Cosmochimica Acta

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“…The lines of geochemical evidence are: (1) the Hf paradox (Salters and Hart, 1989); (2) 230Th excess (e.g., Beattie, 1993;LaTourrette et aI., 1993; among others); (3) Trace element patterns of some MORBs (e.g., Bender et aI.,…”

Section: Chapter One Introductionmentioning

confidence: 99%

Kinetic processes of mantle minerals

Koga¹

1999

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This dissertation discusses the experimental results designed to constrain the processes of MORE generation. The main focus of this study is to investigate the location and the related processes of the transformation boundary from spinel to garnet peridotite facies at subsolidus conditions, because the presence of garnet in melting residues has significant influence to the conclusion drawn from geochemical/geophysical observations. Using an approach that monitors the rate of reaction progresses, the experimental results confirmed the presence of a region that garnet and spinel coexist in peridotite compositions. The trace element distribution among the product phases (opx and cpx) subsequent to the garnet breakdown reaction is in disequilibrium, due to the differences of diffusivity between major and trace elements. The presence of disequilibrium distribution in nature may be used to infer time scales of geodynamic processes. Diffusion coefficients of Al in diopside are experimentally determined, and used for modeling the equilibration of major elements in pyroxene during MORE genesis. In summary, this dissertation contributes two major inferences: the location of the transformation boundaries of the gamet-spinel peridotite; the presence of disequilibrium trace elements distribution with equilibrium major elements distribution in mantle pyroxenes.3

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“…Firstly, it lends further support to the hypotheses based on U-Th disequilibrium studies and theoretical grounds, that small melt fractions are indeed present beneath spreading axes (McKenzie, 1985b;McKenzie and Bickle, 1988;Beattie, 1993) with the concomitant implications for trace element fractionation; indeed, the slab windowrelated basalts are probably the closest erupted analogues to MORB precursor melts yet reported. Secondly, the model implies that HREE-depleted alkaline basalts require neither lithospheric extension, nor high mantle temperatures for their formation.…”

Section: Discussionmentioning

confidence: 60%

“…We therefore have no direct evidence as to the composition of these small meltfraction precursors to MOR tholeiites. Indirect evidence for their existence, however, comes from recent studies of Th-U disequilibria and the partitioning of Th and U during melting of garnet lherzolite (Beattie, 1993). These studies imply that mid-ocean ridge basalts are produced by melting of the mantle initiated at depths where garnet is stable, corroborating the theoretical melting column models for MORB generation.…”

Section: Introductionmentioning

confidence: 57%

The generation of small melt-fractions in truncated melt columns: constraints from magmas erupted above slab windows and implications for MORB genesis

Hole

Saunders

1996

Mineral. mag.

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At a number of locations along the Pacific margin of the Americas and west Antarctica, small volumes of alkalic basalts were erupted following successive ridge crest-trench collisions. The basalts were generated as a result of upwelling of asthenosphere through windows in the subducted plate. There is no evidence for local high temperature mantle plumes or significant lithospheric extension associated with these basalts. For the best sampled area, the Antarctic Peninsula, mean values for fractionation-corrected iron content (FeO*) vary from 6.9 to 10.6 wt.%, and for Na20 (NAB.0) from 3.25 to 4.6 wt.%, implying generation of small meltfractions at variable mean pressures. The results of rare earth element inversion modelling yield a melt generation interval of 100 to 52 kin, with a maximum melt fraction of c. 7% generated from a MORB-like source at Tp 1300~ Trace element and isotope systematics are also consistent with the generation of the basalts from a MORB-like source.Mean pressures of melt generation increase with increasing distance from the original trench, but trace element and Nas.0 data suggest that there is no systematic variation in extent of melting with distance from the trench. The data are consistent with a model whereby a MORB melting column, initially intersecting the peridotite solidus at between 15 and 30 kbar, is truncated by a lithospheric cap which thickens from c. 15-20 km (~ 5-8 kbar) up to a maximum of c. 50 km (~ 15 kbar), such that the mean pressures of melting increase with increasing distance from the palaeo-trench. The MORB-like major element geochemistry of basalt samples closest to the ancestral trench are consistent with initial intersection of the peridotite solidus at low pressures (c. 15 kbar). For areas of thickest lithosphere, mean pressures of melt generation are higher, but extents of melting are lower than for a MORB melting column with a similar initial pressure of intersection of the peridotite solidus. All these basalts therefore potentially represent analogues for the small melt-fraction precursors to the generation of MOR tholeiites.Thermal constraints suggest that these low volume, small melt-fractions, were generated with CO2 on the solidus, because mean pressures of melt generation are greater than the pressure of intersection of the T v 1300~ mantle adiabat and the dry peridotite solidus. Potentially, all MORB may be generated initially with CO2 on the solidus, and if this is correct, it does not require thermal anomalies to generate large extents of melting at high mean pressures.

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Uranium–thorium disequilibria and partitioning on melting of garnet peridotite

Cited by 255 publications

References 27 publications

The influence of source heterogeneity on the U–Th–Pa–Ra disequilibria in post-glacial tholeiites from Iceland

The influence of source heterogeneity on the U–Th–Pa–Ra disequilibria in post-glacial tholeiites from Iceland

Kinetic processes of mantle minerals

The generation of small melt-fractions in truncated melt columns: constraints from magmas erupted above slab windows and implications for MORB genesis

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