Superkimberlites: A geodynamic diamond window to the Earth's core

Carbon is the fourth most abundant element in the solar system. In the Earth carbon is in atmospheric CO2, limestone, other organic products, graphite and trace diamond; interstellar diamond, however, is ubiquitous. Diamond is well known for some unique physical and chemical properties, but it is perhaps less well known that the mineral is geologically ancient (3.3 Ga), that its origins are deep in the mantle (> 180 km), and that diamonds are among the deepest solid objects to reach the surface of the Earth; rare diamonds are from the transition zone (400–670 km), and other diamonds possibly nucleated in the lower mantle (> 670 km). Transport to the surface is in volatile (COHNS)-charged highly explosive kimberlite and lamproite volcanoes. These volcanoes are sited exclusively in the oldest (> 1.7 Ga), tectonically most stable, and thickest (∼ 200 km) regions of crust and upper mantle lithosphere. The energetics required for volcanism are so exceptional and the sources so deep that possible connections between and among the core, geomagnetism, plumes and diamonds are explored. Some correlations are established and others are implied. The results are sufficiently enticing to propose that kimberlites and geographically and temporally associated carbonatites are continental recorders of plumes dating back to at least 2.8 Ga, and that some diamonds may have recorded core events dating back to 3.3 Ga, or possibly earlier. Peaks in kimberlite magmatic activity correlate, on average, with normal and reverse superchron and subchron behavior of the geomagnetic field. The time lag between magnetohydrodynamic activity in the core and kimberlite eruptive cycles at the Earth's surface is of the order of 25–50 Ma, consistent with the travel times modeled for the passage of plumes from D′' layer to the subcontinental lithosphere. Although the existence of plumes and the nature of D′' are debated, the correlations established for the past 500 Ma between and among superchrons, subchrons, kimberlites and entrained diamonds weigh heavily in favor of the following scenario: solid core growth, the consequent release of Si, O, C, H, S, K and possibly N and B to D′', disruption of D′' at some critically unstable threshold thickness (200–300 km), enhanced core convection and the stabilization of a constant non-reversing magnetic dipole field, rising plumes and subsequent volcanism. If protokimberlitic magma and entrainment begin at the core-mantle boundary, a number of geochemical and mineralogical anomalies in diamonds are at present best satisfied if D′' is invoked. These include but are not limited to intensely reduced (i.e., oxygen deficient) SiC, metallic Fe, an abundance of sulfides, silicate perovskite and wustite-periclase mineral inclusions in diamonds. The most abundant source of diamonds is unequivocally from cratonic root zones with C possibly implanted by ancient plumes; eclogitic suite diamonds are equivocal, and diamonds transported from the transition zone and the lower mantle are best explained by entrainment in highly reduced plumes. Carbon in the overwhelming majority of diamonds appears to be primordial. By analogy with a chondritic Earth and chondrites, carbon was acquired during accretion in gaseous complexes, in the form of nanometer-size amorphous C, and as hydrocarbon particles; but carbon was possibly also added as crystalline nanodiamonds that served as seeds for subsequent diamond growth.