The effects of source composition and source evolution during progressive partial melting on the chemistry of mantle-derived mid-ocean ridge basalt (MORB) melts were tested using a comprehensive geochemical and Sr–Nd–Pb isotopic dataset for fresh, magnesian basaltic glasses from the Miocene Macquarie Island ophiolite, SW Pacific. These glasses: (1) exhibit clear parent–daughter relationships; (2) allow simple reconstruction of primary melt compositions; (3) show exceptional compositional diversity (e.g. K2O/TiO2 0·09–0·9; La/Yb 1·5–22; 206Pb/204Pb 18·70–19·52); (4) preserve changes in major element and isotope compositions, which are correlated with the degree of trace element enrichment (e.g. La/Sm). Conventional models for MORB genesis invoke melting of mantle that is heterogeneous on a small scale, followed by binary mixing of variably lithophile element-enriched melt batches. This type of model fails to explain the compositions of the Macquarie Island glasses, principally because incompatible element ratios (e.g. Nb/U, Sr/Nd) and Pb isotope ratios vary non-systematically with the degree of enrichment. We propose that individual melt batches are produced from instantaneous ‘parental’ mantle parageneses, which change continuously as melting and melt extraction proceeds. This concept of a ‘dynamic source’ combines the models of small-scale mantle heterogeneities and fractional melting. A dynamic source is an assemblage of locally equilibrated mantle solids and a related melt fraction. Common MORB magmas that integrate the characteristics of numerous melt batches therefore tend to conceal the chemical and isotopic identity of a dynamic source. This study shows that isotope ratios of poorly mixed MORB melts are a complex function of the dynamic source evolution, and that the range in isotope ratios within a single MORB suite does not necessarily require mixing of diverse components.
KEY WORDS: mid-ocean ridge basalt; Macquarie Island; radiogenic isotopes; mantle; geochemistry
VADIM S. KAMENETSKY1,* and ROLAND MAAS2,
1SCHOOL OF EARTH SCIENCES AND CENTRE FOR ORE DEPOSIT RESEARCH, UNIVERSITY OF TASMANIA, GPO BOX 252-79, HOBART, TASMANIA, 7001, AUSTRALIA
2DEPARTMENT OF EARTH SCIENCES, LA TROBE UNIVERSITY, BUNDOORA, VICTORIA 3083, AUSTRALIA