:: Quote of the week ::
or false, others must judge; for the firmest conviction
of the truth of a doctrine by its author, seems, alas,
not to be the slightest guarantee of truth."
Charles Darwin, letter to Lyell,
25th June, 1858
Website visitor statistics.
Click on image for enlargement.
Dear WM, This recent paper published by Mallik & Dasgupta concludes "mantle potential temperatures of 1330-1350°C appear sufficient to produce high-MgO, primitive basanite-nephelinite if carbonated eclogite melt and peridotite interaction is taken into account.".
This is more evidence that the geochemistry of mafic Mg-rich alkaline rocks can be explained with "normal" temperature, simply assuming a non-pyrolitic mantle source.–Michele Lustrino
Dear WM, Regarding the rapid eruption rates in flood basalts, I offer the following comment. Megacryst zircons from kimberlites record a range of ages from approximately the date of actual kimberlite eruption to about 5-10 Ma earlier than eruption (and sometimes more) (Data in Moore et al, 2008, EPSL 268:151-164, Fig.1). I am aware of unpublished data which shows the same relationship. Megacryst suites from individual kimberlites apparently crystallize isobarically over a wide temperature range, although the depth of formation differs from one kimberlite to another.
While there is considerable debate regarding the origin of the megacryst suite, field, petrographic and experimental evidence suggests a close genetic link between megacrysts and the host kimberlite. However, irrespective of the origin of the megacrysts, this suite reflects an isobaric magmatic event in the mantle over a time period of the order of 10 Ma immediately prior to kimberlite eruption. One possible interpretation of the range in megacryst zircon ages is that they reflect a period of kimberlite melt accumulation in the mantle, with eruption only occurring once a critical volume of melt has accumulated.
Episodes of kimberlite eruption in southern Africa also seems to follow continental epeirogenic episodes by about the same period (5-10 Ma) that is recorded by the zircon megacrysts. Further, these epeirogenic events appear to be roughly synchronous with reorganization of mid-oceanic spreading regimes around southern Africa. Post-Gondwana kimberlite and other alkaline activity in southern Africa thus seems more readily explained by tectonic melting triggers than plumes. In essence, the zircon dates suggest that the "anomaly" of disparate melting rates and eruption rates is not an anomaly at all, at least for kimberlites. But maybe also a great deal of other magmatic activity.–Andy Moore
Dear WM, I would like to bring your attention to this analysis of the thermodynamics of Hell. I wonder if this information is relevant to plumes.–Andy Moore
Dear WM, It has long been taught in geophysics and planetary physics courses, but not in mantle geochemistry courses, that the Earth started hot and was extensively differentiated during accretion. This knowledge goes back to the extremely influential papers of Francis Birch, including his classic 1952 paper, his 1965 Presidential Address and his energetics-of-core-formation papers. These papers form the foundations of modern geophysics but they are the antithesis of the Urey school of geochemistry (Urey, 1952) which produced many advocates of cold accretion, primordial Earth, crustal growth, continuous differentiation and undegassed mantle. Thus, 1952 was a pivotal year for both mantle geophysics and mantle geochemistry. The two sciences diverged from that point on.
Schilling (Nature 242, 1973, page 565) states "Contrary to earlier views (Birch 1965), the model implies that plumes are transporting to the Earth's surface more primordial mantle material(s) than present in the low-velocity layer lying beneath those mid-ocean ridge segments remote from plumes." This was the nucleus of the geochemical version of the plume hypothesis. Tatsumoto (1978) and O'Hara (1973) almost immediately showed the flaws in Schilling's argument, and Tozer (1973 ) demonstrated the fluid dynamic implausibility of the geochemical model. Birch’s papers had already demonstrated that classical physics ruled out the assumptions in what became the canonical model of geochemistry.
The Birch ideas were extended to mantle geochemistry by Tatsumoto, Armstrong and Kay who developed top-down models of geochemistry. The mass balance associated with hot accretion and early differentiation was developed in many early papers and summarized in Chapter 8 of Theory of the Earth and Chapter 13 of New Theory of the Earth. The mass balance shows clearly that the whole mantle had to be processed to form the crust plus the kimberlites and carbonatites.–Don Anderson