Partitioning of U and Th during garnet pyroxenite partial melting: Constraints on the source of alkaline ocean island basalts

Elkins, L. J.; Gaetani, G. A.; Sims, K. W. W.

doi:10.1016/j.epsl.2007.10.034

Cited by 82 publications

(36 citation statements)

References 67 publications

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“…Experimentally determined mineral-melt partition coefficients for garnet and cpx in pyroxenite or eclogite (Klemme et al, 2002;Pertermann and Hirschmann, 2002;Pertermann et al, 2004;Elkins et al, 2008) overlap with partition coefficients determined for peridotite (Wood and Blundy, 1997;Salters et al, 2002;McDade et al, 2003a). Calculated bulk D U /D Th for pyroxenite range from being smaller to being larger than those in garnet-peridotite Stracke et al, 2006;Elkins et al, 2008;Prytulak and Elliott, 2009). The magnitude of the U-Th fractionation and the difference in bulk partitioning behaviour of pyroxenite compared to peridotite thus remains poorly constrained.…”

Section: U-series Nuclide Partitioningmentioning

confidence: 89%

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: 89%

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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“…wehr et al, 2001)), fractionation is mainly controlled by clinopyroxene with D Th > D U , creating small 238 U excesses in the melt (La Tourette and Burnett, 1992;La Tourette et al, 1993;Beattie, 1993a,b;Hauri et al, 1994;Lundstrom et al, 1994;Salters and Longhi, 1999;Wood et al, 1999;Landwehr et al, 2001;Salters et al, 2002;Elkins et al, 2008).…”

Section: à4mentioning

confidence: 99%

“…Elkins et al (2008) and Stracke et al (2006), there are relatively large deviations across experimental measurements of mineral/melt partitioning for U and Th that translate into a high degree of uncertainty in the resulting calculated parameters (e.g. Sims et al, 1999).…”

Section: à4mentioning

confidence: 99%

Understanding melt generation beneath the slow-spreading Kolbeinsey Ridge using 238U, 230Th, and 231Pa excesses

Elkins

Sims

Prytulak

et al. 2011

Geochimica et Cosmochimica Acta

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“…Because melting of garnet-lherzolite requires higher temperatures compared with spinel-lherzolite [43,50,51], it means that a deep mantle source region (garnet stability field) has a relatively lower melting degree. However, melting even a small amount of garnet, which has D U /D Th three times higher than clinopyroxene [16], would produce a high degree of excess 230 Th in basalt. Due to lack of accurate P-T experimental data, earlier modeling studies usually considered that the spinel-garnet transition zone lies in a 60-80 km wide range below the oceanic crust [48,[52][53][54].…”

Section: Mantle Melting Conditions and 230 Th-238 U Disequilibriummentioning

confidence: 99%

“…For example, Beattiea [10] and Bourdon et al [15] considered that the initial melt with high 230 Th excess is formed in the mantle containing garnet, and then uprises and mixes with melts in the spinel-lherzolite field ( 230 Th deficiency) to form primary basaltic magma with excess 230 Th. Recently, Elkins et al [16] and Pertermann et al [17] showed by partition experiments of natural garnet and clinopyroxene that only melting with the participation of garnet can produce excess 230 Th. However, some geologists still consider excess 230 Th as an indicator of mantle plume sources (garnet stability field) [18], and usually assume the presence of garnet in the mantle melting process [19,20].…”

mentioning

confidence: 98%

Genesis of 230Th excess in basalts from mid-ocean ridges and ocean islands: Constraints from the global U-series isotope database and major and rare earth element geochemistry

Zhang

Zeng

2010

Sci. China Earth Sci.

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Based on 230 Th-238 U disequilibrium and major element data from mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs), this study calculates mantle melting parameters, and thereby investigates the origin of 230 Th excess. ( 230 Th/ 238 U) in global MORBs shows a positive correlation with Fe 8 , P o , Na 8 , and F melt (Fe 8 and Na 8 are FeO and Na 2 O contents respectively after correction for crustal fractionation relative to MgO = 8 wt%, P o =pressure of initial melting and F melt =degree of melt), while 230 Th excess in OIBs has no obvious correlation with either initial mantle melting depth or the average degree of mantle melting. Furthermore, compared with the MORBs, higher ( 230 Th/ 238 U) in OIBs actually corresponds to a lower melting degree. This suggests that the 230 Th excess in MORBs is controlled by mantle melting conditions, while the 230 Th excess in OIBs is more likely related to the deep garnet control. The vast majority of calculated initial melting pressures of MORBs with excess 230 Th are between 1.0 and 2.5 GPa, which is consistent with the conclusion from experiments in recent years that D U >D Th for Al-clinopyroxene at pressures of >1.0 GPa. The initial melting pressure of OIBs is 2.2-3.5 GPa (around the spinel-garnet transition zone), with their low excess 226 Ra compared to MORBs also suggesting a deeper mantle source. Accordingly, excess 230 Th in MORBs and OIBs may be formed respectively in the spinel and garnet stability field. In addition, there is no obvious correlation of K 2 O/TiO 2 with ( 230 Th/ 238 U) and initial melting pressure (P o ) of MORBs, so it is proposed that the melting depth producing excess 230 Th does not tap the spinel-garnet transition zone. OIBs and MORBs in both ( 230 Th/ 238 U) vs. K 2 O/TiO 2 and ( 230 Th/ 238 U) vs. P o plots fall in two distinct areas, indicating that the mineral phases which dominate their excess 230 Th are different. Ce/Yb-Ce curves of fast and slow ridge MORBs are similar, while, in comparison, the Ce/Yb-Ce curve for OIBs shows more influence from garnet. The mechanisms generating excess 230 Th in MORBs and OIBs are significantly different, with formation of excess 230 Th in the garnet zone only being suitable for OIBs. excess 230 Th, clinopyroxene, garnet, spinel-lherzolite, mid-ocean ridge, ocean island Citation:Zhang G L, Zeng Z G. Genesis of 230 Th excess in basalts from mid-ocean ridges and ocean islands: Constraints from the global U-series isotope database and major and rare earth element geochemistry.More than 20 years of uranium-series disequilibrium studies have shown that newly formed MORBs frequently display excess 230 Th (( 230 Th/ 238 U)>1) [1, 2]. Excess 230 Th is potentially important in the studies of magma formation depth and upwelling rate [3][4][5][6][7]. However, in the international community the cause of excess 230 Th has never been satisfactorily explained [8][9][10][11]. The partition coefficients of Th and U between melt and mantle residue are affected by pressure, temperature, porosity, oxygen fu...

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Partitioning of U and Th during garnet pyroxenite partial melting: Constraints on the source of alkaline ocean island basalts

Cited by 82 publications

References 67 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

Understanding melt generation beneath the slow-spreading Kolbeinsey Ridge using 238U, 230Th, and 231Pa excesses

Genesis of 230Th excess in basalts from mid-ocean ridges and ocean islands: Constraints from the global U-series isotope database and major and rare earth element geochemistry

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