Complex Zoning in Plagioclase Feldspars from ODP Site 648

Abstract

ODP Legs 106 and 109 drilled zero-age crust at Site 648 on the axis of the Mid-Atlantic Ridge and recovered sparsely to moderately phyric but chemically homogeneous basalt. Plagioclase phenocrysts in Site 648 basalts exhibit a variety of zoning patterns and compositions including: (1) tabular (An 84) cores mantled by more sodic zones (An 80 and An 75), (2) tabular (An 80) cores mantled by narrow (An 84 and An 75) zones, (3) skeletal cores (An 80 _ 78) enclosing sodic (An 75) patches, and (4) rounded cores (An 75) surrounded by skeletal (An 80 _ 78) mantles. Backscattered electron images have been used to define zone morphologies in 28 crystals; major element compositions of the zones have been determined by electron microprobe; and Mg, K, Ti, Fe, and Sr concentrations have been determined by ion microprobe. Equilibrium partitioning of minor and trace elements between crystals and liquids occurred during stable growth of tabular cores and inner rims. Disequilibrium partitioning occurred during rapid growth of crystal rims, resulting in enrichment of incompatible trace and minor elements, and during growth of calcic zones with skeletal and acicular morphology, resulting in enrichment of Fe and Sr and depletion of Mg, K, and Ti. Crystal/liquid partition coefficients estimated from the composition of inner rims and glass are D M " = 0.03, D K = 0.15, D Ti = 0.028, D Fe = 0.05, and D Sr = 1.82. Using these partition coefficients, we have calculated the composition of primitive liquid in equilibrium with phenocryst cores. The estimated liquid composition is similar to primitive basalt compositions from the Kane Fracture Zone except for a much higher Ca/(Ca + Na) ratio which resembles values for MAR plume basalts. The diversity in plagioclase morphologies and compositions is explained by mixing of melts at different stages of evolution and stored in separated but interconnected reservoirs, as previously suggested by Kuo and Kirkpatrick (1982).

Melt Migration through High-Level Gabbroic Cumulates of the East Pacific Rise at Hess Deep: The Origin of Magma Lenses and the Deep Crustal Structure of Fast-Spreading Ridges

“…Reequilibration of olivine and clinopyroxene, but not Plagioclase, is also observed in abyssal gabbros from the Southwest Indian Ridge (Meyer et al, 1989). Transport and preservation of xenocrystic calcic Plagioclase in crustal gabbros was suggested by those authors and by Meyer and Shibata (1990).…”

“…Rock just above the contact with coarse gabbroic material at the bottom is oxide-rich (12% oxides in the mode; Table 1). with published partition coefficients (Meyer and Shibata, 1990). However, the plagioclases in this sample have considerably less MgO than the natural liquids which can have produced them, their Mg#s are too low for their range of An contents (anorthite to sodic labradorite), and their K 2 O and TiO 2 contents are too low for their Mg#s.…”

Experimental petrology of basement lavas from Ocean Drilling Program Leg 192: implications for differentiation processes in Ontong Java Plateau magmas

“…Meyer & Shibata 1990). To clarify the magmatic differentiation for the OJP, it is crucial to explain the origin of the high-An parts of type II plagioclase phenocrysts and we therefore examine the problem based on the experimentally determined compositional relationship between melt and plagioclase.…”

Petrological systematics of the Mid‐Atlantic Ridge south of Kane: Implications for ocean crust formation

“…Magma mixing. In the Kane region, the complex form and chemical zonation of many phenocrysts [e.g., Natland, 1978], as well as aspects of the major and trace element data, have been cited as evidence for magma mixing in a crustal magma chamber [e.g., Dungan et al, 1979;Meyer and Shibata, 1990]. Phenocrysts in the present sample set also have these complex characteristics and record a mixing history.…”