Abstract
Chemical data of 39 fresh basaltic glasses from the East Pacific Rise (EPR) between 6 and 30°S and Pb, Sr, and Nd isotopic compositions of 12 basalt glasses are presented. Major and trace element data indicate a wide compositional range, including primitive basalts (Mg#=0.67) and highly evolved FeTi-basalts (Mg#=0.34) [molMg/(Mg+Fe2+)]. The compositional range can be attributed to low-pressure fractional crystallization. Fractionation-corrected major element concentrations provide evidence for varying mantle melting conditions. Calculations of the melting conditions suggest melt generation in a rising upper mantle column between 20 and 10 kbar, at temperatures between 1430 and 1280°C, and total degrees of partial melting between 17 and 20% by weight. Leached and hand-picked basalt glasses display large variations in 87Sr/86Sr (0.70235–0.70270), 143Nd/144Nd (0.51312–0.51323), and 206Pb/204Pb (18.064–18.665), but are similar to other N-type MORB from the EPR. The isotopic ratios of basalts from 13 to 23°S show strong correlations and delineate two systematic trends. From 23 to 17°S, 87Sr/86Sr and Pb isotope ratios increase and 143Nd/144Nd decrease in agreement with previous results (Mahoney et al. 1989). A reverse trend is indicated by basalts from 17 to 13°S. However, K/Ti and (La/Sm)N continuously increase from 23 to 13°S. This opposite behavior indicates a recent decoupling of isotopic and minor element ratios in the mantle between 13 and 17°S. North of 13.5°S (Garrett Fracture Zone), isotopic data show no systematic variation with ridge location and display an overall weaker covariation. The results suggest that the isotopic variations and ridge segmentation appear to be unrelated and that major ridge offsets apparently coincide with changes in mantle melting conditions (P, T, F) (F, degrees of melting). There is no evidence for a systematic relationship between calculated melting conditions and second order ridge segmentation. Our isotopic data provide further evidence for regionally confined chemical variations in the mantle at 5 to 30°S. We interpret the isotopic trends as reflecting melting of distinct smallvolume and old enriched mantle components. In contrast, variations in trace elements are attributed to young mantle differentiation processes.
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References
Allègre JC, Sarda P, Staudacher T (1993) Speculations about the cosmic origin of He and Ne in the interior of the Earth. Earth Planet Sci Lett 117:229–233
Anderson DL (1993) Helium-3 from the mantle: primordial signal or cosmic dust? Science 261:170–176
Batiza R, Niu Y (1992) Petrology and magma chamber processes at the East Pacific Rise 9°30′ N. J Geophys Res 97:6779–6797
Batiza R, Rosendahl BR, Fisher RL (1977) Evolution of ocean crust. 3. Petrology and geochemistry of basalts from the East Pacific Rise and Siqueiros transform Fault. J Geophys Res 82:265–276
Bryan WB (1983) Systematics and model phenocryst assemblages in submarine basalts: petrologic implications. Contrib Mineral Petrol 83:62–74
Clague DA, Bunch TE (1976) Formation of ferrobasalts at East Pacific mid-ocean ridge spreading centers. J Geophys Res 81:4247–4256
Cohen RS, Evensen NM, Hamilton PJ, O'Nions RK (1980) U−Pb, Sm−Nd and Rb−Sr systematics of mid-ocean ridge basalt glasses. Nature 283:149–153
Detrick RS, Madsen JC, Buhl PE, Vera J, Mutter JM, Orcutt J, Brocker T (1987) Multichannel seismic imaging of an axial magma chamber along the East Pacific Rise between 4°N and 13°N. Nature 326:35–41
Detrick RS, Harding AJ, Kent GM, Orcutt JA, Mutter JC, Buhl P (1993) Seisic structure of the southern East Pacific Rise. Science 259:499–504
Dulce G, Schwartz HU (1992) GEOMETEP 5-bathymetrie und tektonik. Abschlußbericht GEOMETEP 5 (SO 62) BGR Hannover, pp 17–90
Dungan MA, Rhodes JM (1978) Residual glasses and melt inclusions in basalts from DSDP Legs 45 and 46: evidence for magma mixing. Contrib Mineral Petrol 67:417–433
Elthon D, Scarfe CM (1984) High-pressure phase equilibria of a high-magnesian basalt and the genesis of primary oceanic basalts. Am Mineral 69:1–15
Engeln JF, Stein S (1984) Tectonics of the Easter Plate. Earth Planet Sci Lett 68:259–270
Falloon TJ, Green DH (1987) Anhydrous partial melting of MORB pyrolite and other peridotite compositions at 10 kbar: implications for the origin of MORB glasses. Contrib Mineral Petrol 37:181–219
Francheteau J, Ballard RD (1983) The East Pacific Rise near 21°N, 13°N and 20°S: inferences for along-strike variability of axial processes of the mid-ocean ridge. Earth Planet Sci Lett 64:93–116
Fujii T, Scarfe CM (1985) Composition of liquids coexisting with spinel lherzolite at 10 kbar and the genesis of MORBs. Contrib Mineral Petrol 90:18–28
Galer SJG, O'Nions RK (1986) Magmagenesis and the mapping of chemical and isotopic variations in the mantle. Chem Geol 56:45–61
Habermehl K (1986) Zur Geochemie von Bor in Ozeanbodenbasalten. Diploma thesis, Univ Giessen, Germany
Hamelin B, Dupre B, Allegre CJ (1984) Lead-strontium isotopic variations along the East Pacific Rise and the Mid-Atlantic Ridge. Earth Planet Sci Lett 67:340–350
Hanan BB, Schilling J-G (1989) Easter Microplate evolution. J Geophys Res 94:7432–7448
Hekinian R, Walker D (1987) Diversity and spatial zonation of volcanics from the East Pacific Rise near 21°N. Contrib Mineral Petrol 96:265–280
Hekinian R, Thompson G, Bideau D (1989) Axial and off-axial heterogeneity of basaltic rocks from the East Pacific Rise 12°35′N–12°51′N. J Geophys Res 94:17,437–17,463
Henderson (1982) Inorganic geochemistry. Pergamon Press, Elmsford, NY
Ito E, White WM, Göpel C (1987) The O, Sr, Nd and Pb isotope geochemistry of MORB. Chem Geol 62:157–176
Kinzler RJ, Grove TL (1992) Primary magmas of mid-ocean ridge basalts. 2. Applications. J Geophys Res 97:6907–6926
Klein EM, Langmuir CH (1987) Global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness. J Geophys Res 92:8089–8115
Klein EM, Langmuir CH (1989) Local versus global variations in ocean ridge basalt composition: a reply. J Geophys Res 94:4241–4252
Klein EM, The Clipperton Transform Team, The CHEast Pacific Rise Team (1987) Geochemistry of basalts collected during Alvin dive within and adjacent to the Clipperton transform fault (10°N, East Pacific Rise). Eos Trans Am Geophys Union 68:1540
Kurz MD, Mahoney JJ, Sinton JM (1992) A helium isotope anomaly on the superfast spreading East Pacific Rise (abstract). Eos Trans Am Geophys Union 74:582
Langmuir CH, Bender JF (1984) The geochemistry of oceanic basalts in the vicinity of transform faults: observations and implications. Earth Planet Sci Lett 69:107–127
Langmuir CH, Bender JF, Batiza R (1986) Petrological and tectonic segmentation of the East Pacific Rise 5°30′–14°30′ N. Nature 322:422–429
Laschek D (1985) Geochemische Untersuchungen an Basalten vom Galapagos Spreading Center und vom East Pacific Rise. PhD thesis Univ Karlsruhe, Germany
Lin J, Phipps Morgan J (1992) The spreading rate dependence of three-dimensional mid-ocean ridge gravity structure. Geophys Res Lett 19:13–16
Lonsdale P (1977) Regional shape and tectonics of the equatorial East Pacific Rise. Marine Geophys Res 3:295–315
Lonsdale P (1985) Nontransformal offsets of the Pacific-Cocos plate boundary and their traces on the rise flanks. Geol Soc Am Bull 96:313–327
Macdonald KC, Haymon RM, Miller SP, Sempere JC (1988a) Deep tow and Sea Beam studies of dueling propagating ridges on the East Pacific Rise near 20°40′ S. J Geophys Res 93:2875–2898
Macdonald KC, Fox PJ, Perram LJ, Eisen MF, Haymon RM, Miller SP, Carbote SM, Cormier M-H, Shor AN (1988b) A new view of the mid-oxean ridge from the behavior of ridge axis discontinuities. Nature 335:217–225
Macdougall JD, Lugmair GW (1985) Extreme isotopic homogenity among basalts from the southern East Pacific Rise: mantle or mixing effect? Nature 313:209–211
Macdougall JD, Lugmair GW (1986) Sr and Nd isotopes in basalts from the East Pacific Rise: significance for mantle heterogeneity. Earth Planet Sci Lett 77:273–284
Mahoney JJ, Sinton JM, Spencer KC, Smaglik SM (1989) Isotopic characteristics of a superfast spreading ridge: East Pacific Rise 13–23° S (abstract). Eos Trans Am Geophys Union 70:1317
Mahoney JJ, Sinton JM, Kurz MD, Macdougall JD, Spencer KJ, Lugmair GW (1994) Isotope and trace element characterization of a superfast spreading ridge: East Pacific Rise, 13–23°S. Earth Planet Sci Lett (in press)
McKenzie DP (1984) The generation and compaction of partially molten rock. J Petrol 25:713–765
McKenzie DP, Bickle MJ (1988) The volume and compaction of melt generated by extension of the lithosphere. J Petrol 29:625–679
McKenzie DP, O'Nions RK (1991) Partial melt distribution from inversion of rare earth element concentrations. J Petrol 32:1021–1091
Michael PJ, Chase RL (1987) The influence of primary magma composition, H2O and pressure on mid-ocean ridge basalt differentiation. Contrib Mineral Petrol 96:245–263
Naar DF, Hey RN (1991) Tectonic evolution of the Easter Microplate. J Geophys Res 96:7961–7993
Newman S, Finkel RC, MacDougall JD (1983) 230Th−238U disequilibrium systematics in oceanic tholeiites from 21° N on the East Pacific Rise. Earth Planet Sci Lett 65:17–33
Nielsen RL (1988) A method for the simulation of combined major and trace element liquid line of descent. Geochim Cosmochim Acta 52:27–38
Nielsen RL (1990) Simulation of igenous differentiation processes. In: Nicholl J, Russell JK (eds) Modern methods of igneous petrology: understanding magmatic processes. (Reviews in mineralogy 24) Mineral Soc Am, Washington, DC, pp 65–105
Niu Y, Batiza R (1991) An empirical method for calculating melt compositions produced beneath mid-ocean ridges: application for axis and off-axis (seamounts) melting. J Geophys Res 96:21753–21777
Niu Y, Batiza R (1992) MORBCAL: A program for calculating the compositions of primary basaltic melts produced by decompression-induced melting below mid-ocean ridges. Comput Geosci 18:1277–1282
Niu Y, Batiza R (1993) Chemical variation trends at fast and slow spreading mid-ocean ridges. J Geophys Res 98:7887–7902
Oldenburg CM, Spera FJ, Yuen DA, Sewell G (1989) Dynamic mixing in magma bodies: theory, simulations and implications. J Geophys Res 94:9215–9236
Perfit MR, Fornari DJ, Malahoff A, Embley RW (1983) Geochemical studies of abyssal lavas recorded by DRSV Alvin from Easter Galapagos Rift, Inca Transform, and Ecuador Rift. 3. Trace element abundances and petrogenesis. J Geophys Res 88:10551–10572
Procelli DR, O'Nions RK, O'Reilly SY (1986) Helium and Sr isotopes in ultramafic xenolits. Chem Geol 54:237–250
Presnall DC, Dixon JR, O'Donnell TH, Dixon SA (1979) Generation of mid-ocean ridge tholeiites. J Petrol 20:3–35
Puchelt H, Emmermann R (1983) Petrogenetic implications of tholeiitic glasses from the East Pacific Rise and the Galapagos spreading center. Chem Geol 38:39–56
Rea DK (1981) Tectonics of the Nasca-Pacific divergent plate boundary Nasca Plate: crustal formation and Andean convergence. Geol Soc Am Mem 154:27–62
Reynolds JR, Langmuir CH, Bender JF, Kastens KA, Bryan WB (1992) Spatial and temporal variability in the geochemistry of basalts from the East Pacific Rise. Nature 359:493–499
Ribe NM (1988) On the dynamics of mid-ocean ridges. J Geophys Res 93:429–436
Richard P, Shimizu A, Allègre CJ (1976) 143Nd/144Nd a natural tracer: an application to oceanic basalts. Earth Planet Sci Lett 31:269–278
Rubin K, Mahoney JJ (1993) What's on the plume channel? Nature 362:109–110
Rusby RI (1992) GLORIA and other geophysical studies of the tectonic pattern and history of the Easter Microplate, southeast Pacific. In: Parsen LM, Murton BJ, Browning P (eds) Ophiolites and their modern oceanic analogues. Geol Soc Spec Publ 60:81–106
Ryan JG, Langmuir CH (1993) The systematics of boron abundances in young volcanic rocks. Geochim Cosmochim Acta 52:1489–1498
Salters VJM, Hart SR (1989) The hafnium paradox and the role of garnet in the source of mid-ocean ridge basalts. Nature 342:420–422
Schilling J-G Sigurdsson H, Davis AN, Hey RN (1985) Easter Microplate evolution. Nature 317:325–330
Sinton JM, Detrick RS (1992) Mid-ocean ridge magma chambers. J Geophys Res 97:197–216
Sinton JM, Smaglik SM, Mahoney JJ, Macdonald KC (1991) Magmatic processes at superfast spreading mid-ocean ridges: glass compositional variations along the East Pacific Rise 13–23°S. J Geophys Res 96:6133–6155
Sleep NH (1988) Tapping of melts by veins and dikes. J Geophys Res 93:10255–10272
Stolper E (1980) A phase diagram for mid-ocean ridge basalt: preliminary results and implications for petrogenesis. Contrib Mineral Petrol 74:13–27
Sun SS (1980) Lead isotopic study of young volcanic rocks from mid-ocean ridges, ocean islands and island arcs. Philos Trans R Soc London A 297:409–445
Thompson G, Bryan WB, Ballard RD, Hamuro K, Melson WG (1985) Axial processes along a segment of the East Pacific Rise 10–12°N Nature 318:429–433
Thompson G, Bryan WB, Humphris SE (1989) Axial volcanism on the East Pacific Rise 10–12° N. In: Saunders AD, Norry MD (eds) Magmatism in ocean basins. Geol Soc Spec Publ 42:181–200
Tighe SA (ed) (1988) East Pacific Rise data synthesis and final report. Joint Oceanographic Institution, Inc, Washington, DC
Toomey DR, Purdy GM, Solomon SC, Wilcock WSD (1990) The three-dimensional seismic velocity structure of the East Pacific Rise near latitude 9°30′ N. Nature 347:639–645
Vance D, Stone JOH, O'Nions RK (1989) He, Sr, and Nd isotopes in xenoliths from Hawaii and other occanic islands. Earth Planet Sci Lett 96:147–160
Volpe HM, Goldstein SJ (1990) Dating young MORB 226Ra−230Th isotopic disequilibria measured by mass spectrometer (abstract). Eos Trans Am Geophys Union 71:1702
Walker D, Shibata T, Delong SE (1979) Abyssal tholeiites from the Oceanographer fracture zone. II. Phase equilibria and mixing. Contrib Mineral Petrol 70:111–125
Weaver JS, Langmuir CH (1990) Calculation of phase equilibrium in mineral-melt systems. Comput Geosci 16:1–19
White WM (1993) 238U/204Pb in MORB and open system evolution of the depleted mantle. Earth Planet Sci Lett 115:211–226
White WM, Hofmann AW, Puchelt H (1987) Isotope geochemistry of Pacific mid-ocean ridge basalt. J Geophys Res 92:4881–4893
Zuleger E, Erzinger J (1988) Determination of REE and Y in silicate materials with ICP-AES. Fresenius Z Anal Chem 332:140–144
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Bach, W., Hegner, E., Erzinger, J. et al. Chemical and isotopic variations along the superfast spreading East Pacific Rise from 6 to 30°S. Contr. Mineral. and Petrol. 116, 365–380 (1994). https://doi.org/10.1007/BF00310905
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DOI: https://doi.org/10.1007/BF00310905