Liiga lihtne? Luiz A.
Peslier, Anne H. This has mystified Earth scientists as the dynamic and relatively hot asthenosphere should have eroded away these keels over time e. Three key factors have been invoked to explain cratonic root survival: 1 Low density makes the cratonic mantle buoyant e.
Here we address the mechanism of the longevity of continental mantle lithosphere by focusing on the water parameter. Although nominally anhydrousolivine, pyroxene and garnet can accommodate trace amounts of water in the form of H bonded to structural O in mineral defects e. Olivine softens by orders of magnitude if water ppm H2O is added to its structure e.
Our recent work has placed constraints on the distribution of water measured in peridotite minerals in the cratonic root beneath the Kaapvaal in southern Africa Peslier et al. At P greater than 5 GPa, the water contents of pyroxene remain relatively constant while those of olivine systematically decrease from 50 to less than 10 ppm H2O at 6. We hypothesized that at P greater than 6.
As olivine likely controls the rheology of the mantle, we calculated that the dry olivines could be responsible for a contrast in viscosity between cratonic lithosphere and surrounding asthenosphere large enough to explain the resistance of cratonic root to asthenospheric delamination.