Transitional basalts and tholeites from the East Pacific Rise, 9/sup 0/N
Basalts from the base of a small seamount on approx.1.5-m.y.-old crust west of the East Pacific Rise (EPR) at 9/sup 0/N are intermediate in chemical and isotopic composition between light-rare-earth-element-depleted tholeiite (normal midocean ridge basalt (MORB)) and alkali basalt. Like oceanic alkali basalt, these rocks contain significantly more Ba, K, P, Sr, Ti, U, and Zr than normal MORB. Since the absolute abundances of these elements are still well below alkali basalt levels, the label transitional is adopted for these basalts. A series of fractionated MORB also occurs in this area, northwest of the Siqueiros Fracture Zone - Transform Fault. The normal tholeiites are either olivine-plagioclase or plagioclase-clinopyroxene phyric, while the transitional basalts are spinel-olivine phyric. Fractional crystallization quantitatively accounts for the chemical variability of the tholeiitic series but not for the transitional basalts. The tholeiitic series probably evolved in a crustal magma chamber approx.4 km below the crest of the East Pacific Rise. /sup 143/Nd//sup 144/Nd and other chemical data suggest that the large-ion-lithophile-enriched transitional basalts may represent a hybrid of normal MORB and Siqueiros area alkali basalt. Incompatible element plots of K, P, and U indicate possible derivation of the transitional basalts by magma mixing. Derivation of the transitional basalts from a 1:1 mixture is supported by all available chemical data. This magma mixing apparently occurred at < or approx. =30 km depth within a few tens of kilometers from the EPR axis. The Siqueiros area basalts reflect a profound chemical and isotopic heterogeneity in the upper mantle. The EPR shown no evidence of plumelike bulges and associated large-scale outpourings of nonnormal MORB resulting from these mantle heterogeneities. Severe constraints are placed on upper mantle models.
- Research Organization:
- Geological Research Division, A-015, Scripps Institution of Oceanography University of California, San Diego, La Jolla, California 92093
- OSTI ID:
- 6028599
- Journal Information:
- J. Geophys. Res.; (United States), Vol. 84:B4
- Country of Publication:
- United States
- Language:
- English
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