Earth's Mantle below the Oceans [Preview]

Samples collected from the ocean floor reveal how the mantle's convective forces shape Earth's surface, create its crust and perhaps even affect its rotation

A hot spot would seem to be the cause of so much melting. In fact, assuming that temperature alone causes the melting in the Azores hot-spot region, we calculated that the hot-spot mantle would need to be more than 100 degrees C hotter than the mantle from elsewhere below the ridge.

Is there a way of testing the validity of this temperature estimate and its underlying assumptions? A number of geothermometers have been proposed. They are based on the observation that certain mineral pairs that coexist in equilibrium in the mantle undergo temperature-dependent chemical reactions. For instance, the orthopyroxene and clinopyroxene in a mantle peridotite react with each other until they reach an equilibrium composition that depends on temperature. Laboratory experiments have calibrated that relation. Thus, determining the composition of the coexisting mineral pair can indicate the temperature at which the members of the pair reached equilibrium.

I applied two geothermometers, one devised by Donald H. Lindsley of Stony Brook University and the other by Peter R. A. Wells of the University of Oxford, to the Mid-Atlantic Ridge peridotites. The results were surprising. They did not show higher temperatures in the hot-spot region; if anything, the region gives temperatures that are slightly lower.

A mantle spiked with water
WHY DID WE NOT FIND higher mantle temperatures for a region that displays high melting? One possibility is that the upper mantle there has a composition that causes it to melt more easily. Water could be the main factor. Experiments by Peter J. Wyllie of the California Institute of Technology and Ikuo Kushiro of the University of Tokyo and the Carnegie Institution of Washington, among others, demonstrated that trace amounts of water and other volatile elements in peridotite drastically decrease its melting temperature. Therefore, if such a "wet" mantle upwelled under a stretch of mid-ocean ridge, it would start melting more deeply in Earth than normal, "dry" mantle would. By the time the peridotite reached the surface, it would have undergone a degree of melting significantly greater than that of dry mantle under similar temperatures [see box on opposite page].

Is there any evidence that the upper mantle below the Azores hot-spot area is wetter than the mantle elsewhere below the Mid-Atlantic Ridge? Indeed there is. Jean-Guy E. Schilling and his co-workers at the University of Rhode Island reported that basalts from the segment of the hot spot situated between 35 and 45 degrees north latitude contain three to four times more water than do normal mid-ocean ridge basalts, as well as higher concentrations of several chemical elements (mostly light rare-earth elements). The anomalously high concentration of those elements means that the parent mantle in the hot-spot area harbors an enriched supply of these elements.

It seems, therefore, that the mantle below the Azores hot spot differs from the normal sub-Mid-Atlantic Ridge mantle not so much by being hotter as by having incorporated at some stage water and other fluids that changed its chemical composition and melting behavior. This chemical transformation of mantle peridotite by fluids is called metasomatism. It would explain why wet mantle near the surface would have experienced more melting than normal mantle would. It may also explain why the equilibrium temperatures estimated from peridotites at the Azores hot spot do not appear higher than average. Melting reactions consume heat, so that partial melting of upwelling mantle may actually have cooled the surrounding mantle. The higher the degree of melting, the greater the heat loss.

So the Azores hot spot may not be linked to a thermal plume originating from the deep mantle or the core-mantle boundary. Instead it may be a melting anomaly of relatively superficial origin in the mantle. These hot spots may not be truly hot and perhaps are best classified as "wet spots" because of the key role that fluids may play in their formation.

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