Geophysics

Hawaiian Carats

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Science  13 Jun 2003:
Vol. 300, Issue 5626, pp. 1623
DOI: 10.1126/science.300.5626.1623a

Diamonds, which require high pressures and generally moderate temperatures in order to form, are found primarily in volcanic rocks (kimberlites) that erupt through old, thick continental crust. The geothermal gradient means that these locations are much cooler, and the volcanic rocks derive from several hundred kilometers or more beneath the continents. A few diamonds have been found to be derived from crustal carbon in subduction zones (which are cool, too). Only one diamond has been found in an oceanic setting: an ancient oceanic plateau in the southeast Pacific.

Wirth and Rocholl report the recovery of microdiamonds from a recent volcanic eruption in the Hawaiian Islands (on Oahu). Their occurrence is enigmatic because the mantle beneath Hawaii is hot and Hawaiian lavas are thought to come from outside the regions where diamond can exist stably. These microdiamonds may have formed in a reducing fluid deep in the mantle beneath Hawaii, followed by subsequent transport of this mantle upward to the region where Hawaiian lavas originate.—BH

Earth Planet. Sci. Lett. 10.1016/S0012-821X(03)00204-8 (2003)

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