Isotopic evolution of the mantle: the role of magma mixing

Abstract

Trace element and isotopic data for magmas from the two major mantle reservoirs appear to be inconsistent. The incompatible elements and Sr and Nd isotopes show that abyssal tholeiities (MORB) are form a reservoir that has current and time-integrated depletions of the elements that are fractionated into a melt. MORB, however, have 206Pb/ 204Pb and 207Pb/ 204Pb ratios suggesting long-term enrichment in U/Pb (i.e. future single-stage ages). Alkali basalts and tholeiities from continents and oceanic islands are derived from LILE-and U/Pb-enriched reservoirs. Sr and Nd isotopic ratios, however, appear to indicate that some of these basalts are derived from unfractionated reservoirs and others from reservoirs with time-integrated depletions. The apparent stages of the mantle reservoirs are much younger than the ages of continental shields. These inconsistencies can be reconciled by assuming that oceanic and continental basalts are mixtures of magmas from depleted and enriched reservoirs. MORB are slightly contaminated, depleted magmas while ocean island and continental basalts are mixtures of MORB, or a depleted picritic parent magma, and an enriched end-member having trace element patterns similar to high-K magmas such as kimberlites or nephelinites. The mixing relations are such that mixtures can be enriched in U/Pb, Rb/Sr, Nd/Sm, 206Pb/ 204Pb relative to primitive mantle, yet appear to have time-integrated depletions in 143Nd/ 144Nd and 87Sr/ 86Sr. A small amount of contamination by material from an enriched reservoir can explain the Pb results for MORB. Depleted basalts are more sensitive to Pb than to Rb or Nd contamination. The 238U/ 204Pb of uncontaminated MORB may be about 7 compared to 7.9 for the primary growth curve and >10 for the enriched reservoirs. Similarly, continental and ocean island basalts may represent mixtures of enriched and depleted magmas. If so, the ages of mantle reservoirs have been underestimated. We propose that both of the major mantle magmas source are in the upper mantle and that both are global in extent. The shallower one, <200 km depth, is inhomogenous, having been enriched (metasomatised) at various time by fluids from the depleting layer. It may also be the sink of subducted sediments and hydrothermally altered oceanic crust. The deeper source, 220–670 km depth, is more homogenous, more garnet-rich and provides depleted magmas which however, become contaminated as they rise through the shallow encriched layer. Partial melting and diapiric ascent originated in the thermal boundary layer between the two chemically distinct layers. Magma mixing, prior to eruption, is an inevitable consequence of a chemically layered mantle with depleted magmas rising through an enriched uppermost mantle. This allows quantification of the heretofore nebolous concept of mantle metasomatism.