Abstract
Based on 230Th-238U 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 230Th excess. (230Th/238U) in global MORBs shows a positive correlation with Fe8, P o, Na8, and Fmelt (Fe8 and Na8 are FeO and Na2O 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 230Th 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 (230Th/238U) in OIBs actually corresponds to a lower melting degree. This suggests that the 230Th excess in MORBs is controlled by mantle melting conditions, while the 230Th excess in OIBs is more likely related to the deep garnet control. The vast majority of calculated initial melting pressures of MORBs with excess 230Th 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 226Ra compared to MORBs also suggesting a deeper mantle source. Accordingly, excess 230Th in MORBs and OIBs may be formed respectively in the spinel and garnet stability field. In addition, there is no obvious correlation of K2O/TiO2 with (230Th/238U) and initial melting pressure (P o) of MORBs, so it is proposed that the melting depth producing excess 230Th does not tap the spinel-garnet transition zone. OIBs and MORBs in both (230Th/238U) vs. K2O/TiO2 and (230Th/238U) vs. P o plots fall in two distinct areas, indicating that the mineral phases which dominate their excess 230Th 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 230Th in MORBs and OIBs are significantly different, with formation of excess 230Th in the garnet zone only being suitable for OIBs.
Similar content being viewed by others
References
Tepley F J, Lundstromb C C, Sims K W W, et al. U-series disequilibria in MORB from the Garrett Transform and implications for mantle melting. Earth Planet Sci Lett, 2004, 223: 79–97
McKenzie D. Constraints on melt generation and transport from U-series activity ratios. Chem Geol, 2000, 162: 81–94
Sims K W W, Goldstein S J, Blichert-Toft J, et al. Chemical and iso topic constrains on the generation and transport of magma beneath the East Pacific Rise. Geochimi Cosmochi Acta, 2002, 66: 3481–3504
Sims K W W, Depaolo D J, Murrell D M, et al. Mechanisms of magma generation beneath Hawaii and Mid-Ocean Ridges: Uranium/ thorium and samarium/neodymium isotopic evidence. Science, 1995, 267: 508–511
Richardson C, McKenzie D. Radioactive disequilibria from models of melt generation by plumes and ridges. Earth Planet Sci Lett, 1994, 128: 425–437
Spiegelman M, Reynolds J R. Combined dynamic and geochemical evidence for convergent melt flow beneath the East Pacific Rise. Nature, 1999, 402: 282–285
Williams R W, Gill J B. Effects of partial melting on the uranium decay series. Geochi Cosmochi Acta, 1989, 53: 1607–1619
Turner S, Blundy J, Wood B, et al. Large 230Th-excesses in basalts produced by partial melting of spinel lherzolite. Chem Geol, 2000, 162: 127–136
Wood B J, Blundy J D, Robinson J A C. The role of clinopyroxene in generating U-series disequilibrium during mantle melting: Implications for uranium series disequilibria in basalts. Geochi Cosmochi Acta, 1999, 63: 1613–1620
Beattiea P. The generation of uranium series disequilibria by partial melting of spinel peridotite: Constraints from partitioning studies. Earth Planet Sci Lett, 1993, 117: 379–391
Koh K M, Tay E G, Lundstrom C C, et al. Investigating solid mantle upwelling rates beneath mid-ocean ridges using U-series disequilibria, 1: A global approach. Earth Planet Sci Lett, 1998, 157: 151–165
Blundy J D, Wood B J. Prediction of crystal-melt partition coefficients from elastic moduli. Nature, 1994, 372: 452–454
Spiegelman M, Elliott T. Consequences of melt transport for uranium series disequilibrium in young lavas. Earth Planet Sci Lett, 1993, 118: 1–20
Van Orman J A, Grove T L, Shimizu N. Uranium and thorium diffusion in diopside. Earth Planet Sci Lett, 1998, 160: 505–519
Bourdon B, Zindler A, Elliott T, et al. Constraints on mantle melting at mid-ocean ridges from global 238U-230Th disequilibrium data. Nature, 1996, 384: 231–235
Elkins L J, Gaetani G A, Sims K W W. Partitioning of U and Th during garnet pyroxenite partial melting: Constraints on the source of alkaline ocean island basalts. Earth Planet Sci Lett, 2008, 265: 270–286
Pertermann M, Hirschmann M M, Hametner K, et al. Experimental determination of trace element partitioning between garnet and silica-rich liquid during anhydrous partial melting of MORB-like eclogite. Geochem Geophy Geosys, 2004, doi: 10.1029/2003GC000638
Hawkesworth C, Scherstén A. Mantle plumes and geochemistry. Chem Geol, 2007, 241: 319–331
Prytulak J, Elliott T. Determining melt productivity of mantle sources from 238U-230Th and 235U-231Pa disequilibria: An example from Pico Island, Azores. Geochi Cosmochi Acta, 2009, 73: 2103–2122
Russo C J, Rubin K H, Graham D W. Mantle melting and magma supply to the Southeast Indian Ridge: The roles of lithology and melting conditions from U-series disequilibria. Earth Planet Sci Lett, 2009, 278: 55–66
Salters V J M, Hart S R. The hafnium paradox and the role of garnet in the source of mid-ocean ridge basalts. Nature, 1989, 342: 420–422
Salters V J M. The generation of mid-ocean ridge basalts from the Hf and Nd isotope perspective. Earth Planet Sci Lett, 1996, 141: 109–123
Blundy J D, Robinson J A C, Wood B J. Heavy REE are compatible in clinopyroxene on the spinel lherzolite solidus. Earth Planet Sci Lett, 1998, 160: 493–504
Landwehr D, Blundy J, Chamorro-Perez E M, et al. U-series disequilibria generated by partial melting of spinel lherzolite. Earth Planet Sci Lett, 2001, 188: 329–348
Iyer S D, Ray D. Structure, tectonic and petrology of mid-oceanic ridges and the Indian scenario. Current Sci, 2003, 85: 277–289
Herzberg C. Partial crystallization of mid-ocean ridge basalts in the crust and mantle. J Petrol, 2004, 45: 2389–2405
Shaw C S J, Dingwell D B. Experimental peridotite-melt reaction at one atmosphere: a textural and chemical study. Contrib Mineral Petrol, 2008, 155: 199–214
Le Roux P J, Le Roex A P, Schilling J G. Crystallization processes beneath the southern Mid-Atlantic Ridge (40°–55°S), evidence for high-pressure initiation of crystallization. Contrib Mineral Petrol, 2002, 142: 582–602
Zhang G L, Zeng Z G, Yin X B, et al. Deep fractionation of clinopyroxene in the East Pacific Rise 13°N: Evidence from high MgO MORB and melt inclusions. Acta Geol Sin, 2009, 83: 266–277
Kokfelt T F, Hoernle K, Lundstrom C, et al. Time-scales for magmatic differentiation at the Snaefellsjökull central volcano, western Iceland: Constraints from U-Th-Pa-Ra disequilibria in post-glacial lavas. Geochi Cosmochi Acta, 2009, 73: 1120–1144
Claude-Ivanaj C, Joron J L, Allegre C J. 238U-230Th-226Ra fractionation in historical lavas from the Azores: Long-lived source heterogeneity vs. metasomatism fingerprints. Chem Geol, 2001, 176: 295–310
Turner S, Hawkesworth C, Rogers N, et al. U-Th isotope disequilibria and ocean island basalt generation in the Azores. Chem Geol, 1997, 139: 145–164
Lundstrom C C, Hoernle K, Gill J. U-series disequilibria in volcanic rocks from the Canary islands: Plume versus lithospheric melting. Geochi Cosmochi Acta, 2003, 67: 4153–4177
Rogers N W, Thomas L E, Macdonald R, et al. 238U-230Th disequilibrium in recent basalts and dynamic melting beneath the Kenya rift. Chem Geol, 2006, 234: 148–168
Rubin K H, Macdougall J D. 226Ra excesses in mid-ocean-ridge basalts and mantle melting. Nature, 1988, 335: 158–161
Rubin K H, Van Der Zander I, Smith M C, et al. Minimum speed limit for ocean ridge magmatism from 210Pb-226Ra-230Th disequilibria. Nature, 2005, 437: 534–538
Tepley III F J, Lundstrom C C, Sims K W W, et al. U-series disequilibria in MORB from the Garrett transform and implications for mantle melting. Earth Planet Sci Lett, 2004, 223: 79–97
Cooper K M, Goldstein S J, Sims K W W, et al. Uranium-series chronology of Gorda ridge volcanism: New evidence from the 1996 eruption. Earth Planet Sci Lett, 2003, 206: 459–475
Bourdon B, Turner S P, Ribe N M. Partial melting and upwelling rates beneath the Azores from a U-series isotope perspective. Earth Planet Sci Lett, 2005, 239: 42–56
Peate D W, Hawkesworth C J, Van Calsteren P W, et al. 238U-230Th constraints on mantle upwelling and plume-ridge interaction along the Reykjanes ridge. Earth Planet Sci Lett, 2001, 187: 259–272
Pietruszka A J, Rubin K H, Garcia M O. 226Ra-230Th-238U disequilibria of historical Kilauea lavas (1790–1982) and the dynamics of mantle melting within the Hawaiian plume. Earth Planet Sci Lett, 2001, 186: 15–31
Bourdon B, Joron J L, Claude-Ivanaj C, et al. U-Th-Pa-Ra systematics for the Grande Comore volcanics: Melting processes in an upwelling plume. Earth Planet Sci Lett, 1998, 164: 119–133
Niu Y L, Batiza R. An empirical method for calculating melt compositions produced beneath mid-ocean ridges: Application for axis and off-axis (seamounts) melting. J Geophys Res, 1991, 96: 21753–21777
Hirose K, Kushiro I. Partial melting of dry peridotites at high pressures: Determination of compositions of melts segregated from periodotite using aggregates of diamond. Earth Planet Sci Lett, 1993, 114: 477–489
Klein E M, Langmuir C H. Global correlation of ocean ridge basalt chemistry with axial depth and crustal thickness. J Geophys Res, 1987, 92: 8089–8115
Hekiniana R, Francheteaub J, Armijoc R, et al. Petrology of the Easter microplate region in the South Pacific. J Volcanol Geother Res, 1996, 72: 259–289
Niu Y L, Waggoner D G, Sinton J M, et al. Mantle source heterogeneity and melting processes beneath seafloor spreading centres: The East Pacific Rise, 18°–19°S. J Geophys Res, 1996, 101: 27711–27733
Taylor B, Martinez F. Back-arc basin basalt systematics. Earth Planet Sci Lett, 2003, 210: 481–497
Shen Y, Forsyth D W. Geochemical constraints on initial and final depths of melting beneath mid-ocean ridges. J Geophys Res, 1995, 100: 2211–2237
Longhi J. Some phase equilibrium systematics of lherzolite melting: I. Geochem Geophys Geosys, 2002, 3, doi: 10.1029/2001GC000204
Tenner T J, Hirschmann M M, Withers A C, et al. Hydrogen partitioning between nominally anhydrous upper mantle minerals and melt between 3 and 5 GPa and applications to hydrous peridotite partial melting. Chem Geol, 2009, 262: 42–56
O’Hara M J, Richardson S W, Wilson G. Garnet-peridotite stability and occurrence in crust and mantle. Contrib Mineral Petrol, 1971, 32: 48–68
Jenkins D M, Newton R C. Experimental determination of the spinel peridotite to garnet peridotite inversion at 900°C and 1000°C in the system CaO-MgO-Al2O3-SiO2, and at 900°C with natural garnet and olivine. Contrib Mineral Petrol, 1979, 68: 407–419
Makenzie D, O’Nions R K. Partial melt distributions from inversion of rare earth element concentrations. J Petrol, 1991, 32: 1021–1091
Hirschmann M M, Stolper E M. A possible role for garnet pyroxenite in the origin of the “garnet signature” in MORB. Contrib Mineral Petrol, 1996, 124: 185–208
Klemme S, O’Neill H S. The near-solidus transition from garnet lherzolite to spinel lherzolite. Contrib Mineral Petrol, 2000, 138: 237–248
Robinson J A C, Wood B J. The depth of the spinel to garnet transition at the peridotite solidus. Earth Planet Sci Lett, 1998, 164: 277–284
Toomey D R, Wilcock W S, Solomon S C, et al. Mantle seismic structure beneath the MELT region of the East Pacific Rise from P and S wave tomography. Science, 1998, 280: 1224–1227
Saltzer R L, Humphreys E D. Upper mantle P wave velocity structure of the eastern Snake River Plain and its relationship to geodynamic models of the region. J Geophys Res, 1997, 102: 11829–11841
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Zhang, G., Zeng, Z. 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. Sci. China Earth Sci. 53, 1486–1494 (2010). https://doi.org/10.1007/s11430-010-4038-4
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s11430-010-4038-4