A short comment on geochemistry & plumes
Michele Lustrino
Dipartimento di Scienze della Terra, Università degli Studi di Roma La Sapienza, P.le A. Moro, 5, 00185 Rome, Italy & Istituto di Geologia Ambientale e Geoingegneria (IGAG) – CNR, P.le A. Moro, 5, 00185 Rome, Italy, michele.lustrino@uniroma1.it
The ability of geochemistry and petrology to identify magma sources and locations is typically overestimated in the same way as the ability of seismic tomography to detect plumes is overestimated. Geochemistry and petrology have little ability to determine melt source depths. Models that assign geochemical signatures to specific layers in the mantle, including the transition zone, the lower mantle, and the core-mantle boundary, are based on speculative models not verified or verifiable. For the geochemical signatures usually interpreted as “evidence” for a deep-mantle-plume origin, less extreme alternatives exist but rarely taken into consideration.
Inferring the chemical and mineralogical composition, and the thermal state, of igneous rock sources is a very difficult task. Some gross features can be inferred, but going into detail requires assuming a large number of unconstrainable parameters that undermine the basis of the results.
In order to reconcile the evidence with mantle plumes, more than 60 different definitions of plumes have been proposed in the literature, some of them in complete disagreement with the original plume model of Morgan. Too often the absolute low volume of magma, as well as the calculated low degrees of melting to produce it, the depleted Sr-Nd isotopic compositions, the low 3He/4He and the variable Pb isotopic ratios of supposed mantle-plume-related melts cannot be explained with the classical plume theory. Several attempts have been made to explain away these paradoxes (e.g., by invoking cold plumes, incompatible-element-depleted plumes, degassed plumes etc. etc.) but these generate even more paradoxes including cool mantle plumes made of depleted mantle (harzburgitic matrix) containing various amounts of once-shallow recycled lithologies including Mn nodules, sedimentary carbonates, shallow lithospheric mantle, basalts metamorphosed into plagioclase-free lithologies and so on.
What is essential to point out is that the presence of recycled shallow-Earth materials is a feature of all magmas claimed to come from mantle plumes. Mantle peridotite alone cannot reproduce the chemical composition of these magmas. This is agreed by essentially all geochemists. The main debate then revolves around where these lithologies are considered to reside (in the deep mantle, the transition zone or the shallow upper mantle) and the difference in potential temperature between the lower and the upper mantle (i.e., on whether there is an adiabatic or sub-adiabatic whole mantle except for the surface- and core-mantle-boundary super-adiabatic layers).
As regards trace elements, the simple process of decompression and cooling is responsible for a series of chromatographic reactions with ambient mantle (e.g., variation of the olivine-pyroxenes ratio) that can strongly modify the original trace-element budget of the melt and the mantle that is infiltrated by it. The interpretation of trace-element contents of basalts is, as a result, highly ambiguous.
