Abyssal tholeiites from the Oceanographer Fracture Zone - II. Phase equilibria and mixing

Unusual effects have been discovered in the major element phase relations of basalts from the Oceanographer Fracture Zone (OFZ) which suggest that magma mixing of primitive basalt with the differentiation residue of a previous batch of primitive magma has occurred. These effects include a reversal in mineral crystallization sequence which cannot happen during normal differentiation processes or be explained by any plausible change in physical conditions. This unusual effect is encountered as a result of curvature in composition space of the liquid-line-of-descent equilibria involving olivine, plagioclase, and high-calcium clinopyroxene. Mixing of magmas at different stages of their evolution produces mixtures that do not lie on the curved liquid-line-of-descent. Observation of such anomalous compositions in the OFZ suite supplements accumulating petrographic and trace-element geochemistry evidence that magma mixing is an important petrogenetic process. Mixing of fractionated residual liquids can produce mixtures which are either superheated or supercooled depending on the sense of 'thermal curvature' of the liquid-line-of-descent. Both senses are encountered in the tholeiitic system, and this effect may exert a qualitative control on the crystallization texture of the mixture. A comparison of approximately 2,000 abyssal tholeiite compositions to the experimental liquid-line-of-descent reveals that erupted differentiates which would be expected from advanced fractionation are scarce. Just this sort of phenomenon (the 'perched' steady state) was proposed by O'Hara (1977) as an earmark of the operation of a continuously fractionating magma chamber into which fresh magma is periodically remixed.