Magmagenesis and the mapping of chemical and isotopic variations in the mantle

Abstract

The variations of highly incompatible-element ratios, such as Th/U and Rb/K, observed in oceanic basalts have often been attributed to trace-element heterogeneities existing in their mantle sources. However, the constancy of source Th/U ratios deduced from ( 232 Th 230 Th ) activity ratios when compared with the range seen in erupted basalts suggests that magmagenesis is capable of mutally fractionating even the highly incompatible elements. This is also evident in the poor correlations that exist between the radiogenic isotopes of Pb, Sr and Nd and their respective parent/daughter ratios, which can only be relatively short-term features of the source. Of the processes operative during magmagenesis, magma migration and magma chamber differentiation may dominate the behaviour of major and more compatible elements, but are unlikely to be responsible for fractionating the incompatible elements to a significant degree, even for the case of open-system magma chambers. However, the melting of mantle peridotite containing ∼ 0.1% of accessory phases, whose presence is inferred from the volatile inventory (CO 2, H 2O, halogens) of ocean ridge basalts, may involve the release of a small discrete fraction of melt before more extensive melting takes place. This is considered to not only dominate the incompatible-element patterns of spreading ridge basalts, but also those of ocean island basalts and the so-called “metasomatic” features identified in ultramafic xenoliths, depending on whether these early formed melts mix with voluminous melts, as at spreading ridges, or react with the cooler mantle of the oceanic or continental lithosphere. The mapping of the source region through this and subsequent processes into the erupted basalts for even a homogeneous source is unlikely to be straightforward. Overall, the highly incompatible elements may be significantly enriched in the erupted liquids over that expected from a consideration of the melting contributing the bulk of the major elements, and in a heterogeneous manner. The relationship of chemical and isotopic heterogeneities in erupted basalts to those existing in their sources is considered from the standpoint of how advective mixing, extraction and residence time alter the signature evident for different elements that may be mapped through magmagenic processes. In particular, the short residence time of the incompatible elements imply that heterogeneities are ephemeral features of the convective portion of the mantle, whose signature is dominated by advective mixing and extraction, and whose radiogenic isotopes record “isolation” times outside this flow.