U-series disequilibria generated by partial melting of spinel lherzolite

Landwehr, Dirk; Blundy, Jon; Chamorro-Perez, Eva; Hill, E.; Wood, Bernard J.

doi:10.1016/s0012-821x(01)00328-4

Cited by 86 publications

(61 citation statements)

References 45 publications

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“…Partition coefficients determined experimentally generally agree that U is preferentially incorporated into garnet over Th [Beattie, 1993b;LaTourrette et al, 1993], and Th is preferred over U in clinopyroxene [Beattie, 1993a], except in the case of aluminous pyroxene, which at high pressures is documented to have D U solid/melt /D Th solid/melt > 1 [Wood et al, 1999]. Since most MORB measured have ( 230 Th/ 238 U) > 1, it is believed that U-Th fractionation occurs during mantle melting involving residual garnet [Landwehr et al, 2001] or possibly high-pressure (> 1.5 GPa) aluminous pyroxene [Beattie, 1993a;Landwehr et al, 2001;LaTourrette and Burnett, 1992;Wood et al, 1999].…”

Section: Effects Of Melting Processes On U-series Disequilibriamentioning

confidence: 73%

“…Re-melting of this fertilized peridotite could generate melts with trace element and isotopic compositions reflecting eclogite contributions, but with lower SiO 2 and higher MgO indicative of peridotite contributions Yaxley and Green, 1998], although this single lithology source is not consistent with other chemical, petrologic, and geologic constraints. Therefore, realizing that the above scenario is an end-member model, we propose that the U-Th disequilibrium measured in the oblique supersegment lavas was generated reflecting ( [Beattie, 1993a;Landwehr et al, 2001;LaTourrette and Burnett, 1992;Wood et al, 1999]. The involvement of cpx could occur in one of two ways, either partial melting of depleted spinel harzburgite at low pressures or partial equilibration of eclogite melt with depleted spinel harzburgite.…”

Section: Orthogonal Supersegment Versus Oblique Supersegmentmentioning

confidence: 90%

“…Even though the transport time is much slower during reactive porous flow, excess 226 Ra is produced and maintained to shallow levels, thereby dismissing the previous constraint of rapid melt transport. Yet, reactive porous flow cannot be the only mechanism generating U-series disequilibrium, as U-Th partitioning constrain excess 230 Th to deep melt extraction in the presence of garnet [Landwehr et al, 2001] and not shallow re-equilibration.…”

Section: Effects Of Melting Processes On U-series Disequilibriamentioning

confidence: 99%

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The influence of ridge geometry at the ultraslow-spreading Southwest Indian Ridge (9º-25ºE) : basalt composition sensitivity to variations in source and process

Standish¹

2006

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Between 9º-25º E on the ultraslow-spreading Southwest Indian Ridge lie two sharply contrasting supersegments. One 630 km long supersegment erupts N-MORB that is progressively enriched in incompatible element concentrations from east to west. The second 400 km long supersegment contains three separate volcanic centers erupting E-MORB and connected by long amagmatic accretionary segments, where mantle is emplaced directly to the seafloor with only scattered N-MORB and E-MORB erupted. Rather than a major break in mantle composition at the discontinuity between the supersegments, this sharp contrast in geometry, physiography, and chemistry reflects "source" versus "process" dominated generation of basalt.Robust along-axis correlation of ridge characteristics (i.e. morphology, upwelling rate, lithospheric thickness), basalt chemistry, and crustal thickness (estimated from gravity) provides a unique opportunity to compare the influence of spreading geometry and rate on MORB generation. What had not been well established until now is the importance of melting processes rather than source at spreading rates < 20 mm/yr. Along the orthogonally spreading supersegment (14 mm/yr) moderate degrees of partial melting effectively sample the bulk mantle source, while on the obliquely spreading supersegment (7-14 mm/yr) suppression of mantle melting to low degrees means that the bulk source is not uniformly sampled, and thus "process" rather than "source" dominates melt chemistry. AcknowledgementsThere are so many people to thank when reaching a milestone such as this, so let me start with those that have had the most recent impact on this accomplishment, my thesis committee; Henry Dick, Stan Hart, Ken Sims, Fred Frey, Anton le Roex, Mark Kurz, Jian Lin, and my thesis defense chairperson, Hans Schouten. With both Hans and Jian I have gained a new appreciation for and understanding of real-time seagoing geophysics, and I think all four of us (including Henry) have witnessed some pretty amazing human efforts and as a result grown together as people and scientists during the many days bouncing around the South Atlantic. Mark's analytical expertise, continuing support and encouraging words, and attention to detail, has kept my path over the last 6 years straight and narrow, and in the right direction. Anton has been a great source of information both regarding trace elements and more importantly the SW Indian Ridge. As one of the first geochemists to study the South Atlantic spreading ridges in detail, his early work through the 80's and 90's set the bar (and quite a high bar it was) for which all of my work will be gauged. It is a shame that our schedules did not coincide so that he could be here at WHOI for the defense, but I know much discussion will come from my work. Learning trace element geochemistry from Fred Frey was special. Fred's ability to get back to the basics and stress the importance to understanding "why" in the most fundamental terms is important in geochemistry. I very much appreciated his willingness to ...

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Section: Effects Of Melting Processes On U-series Disequilibriamentioning

confidence: 73%

Section: Orthogonal Supersegment Versus Oblique Supersegmentmentioning

confidence: 90%

Section: Effects Of Melting Processes On U-series Disequilibriamentioning

confidence: 99%

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The influence of ridge geometry at the ultraslow-spreading Southwest Indian Ridge (9º-25ºE) : basalt composition sensitivity to variations in source and process

Standish¹

2006

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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%

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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“…They also pointed out that in general, MORB magmas require a melting degree of >10%, and if clinopyroxene melting cannot produce excess 230 Th, then melt with excess 230 Th generated in garnet stability field will be overshadowed by the melt with a high melting degree ( 230 Th deficiency) during their upwelling and mixing process. Landwehr et al [24] showed by further experiments that, at pressures >1.0 GPa, the partition of U and Th between Al-clinopyroxene and silicate melt can form excess 230 Th, and also that in clinopyroxene D U /D Th increases linearly with increasing pressure, and the generated maximum excess 230 Th (1.38) is similar to the observed maximum excess 230 Th in MORBs. Although the recent experimental and theoretical studies have indicated that clinopyroxene can gradually become Al-rich with increasing pressure, this Al-clinopyroxene have not yet been found in the ultramafic rocks obtained from mid-ocean ridges [8,24].…”

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confidence: 52%

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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U-series disequilibria generated by partial melting of spinel lherzolite

Cited by 86 publications

References 45 publications

The influence of ridge geometry at the ultraslow-spreading Southwest Indian Ridge (9º-25ºE) : basalt composition sensitivity to variations in source and process

The influence of ridge geometry at the ultraslow-spreading Southwest Indian Ridge (9º-25ºE) : basalt composition sensitivity to variations in source and process

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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