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Petrographic observations of dike and pillow chilled margins recovered during DSDP Leg 83 are presented. Chilled margins of pillows exhibit a maximum of four textural zones. From the margin inward these are (1) glass, (2) glass with isolated spherulites, (3) coalesced spherulites, and (4) plumose clinopyroxene coalesced with plagioclase in sheaf textures. Chilled margins of dikes exhibit a maximum of six zones that include from the margin inward (1) glass, (2) a cryptocrystalline texture ranging from equigranular (a) to porphyrotopic (b), (3) open spheruloids to radial-rim spheruloids in a cryptocrystalline matrix, (4) coalesced radial-rim spheruloids, (5) coalesced spherical to bowtie spherulites, and (6) plagioclase sheafs with interstitial plumose clinopyroxene. The sequence of zones occurring in a given dike margin is dependent on the nature of the contact with the country rock. Zones 1, 2a, 3, 4, and 5 are observed only in dikes in which the boundary with the country rock is irregular or convolute; in planar or subplanar dike/host contacts Zones 1, 2a, 3, 4, and 5 are absent and replaced by Zone 2b. Zone 6 occurs in both margin types. Although both dike and pillow margins demonstrate textures indicative of significant deviations from the equilibrium crystallization temperature and a progressive decrease in undercooling away from the margin, pillow margins formed under greater degrees of undercooling with higher cooling rates than dike contacts. In contrast, in dike chilled margins the undercooling and cooling rate are such that the nucleation density far surpasses that in pillow margins. The resulting textural features can be used to ascertain the mode of origin in incompletely recovered units.

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An Interpretation of Contrasting Nucleation and Growth Histories from the Petrographic Analysis of Pillow and Dike Chilled Margins, Hole 504B, Deep Sea Drilling Project, Leg 83