Geochemistry

Included Clues

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Science  18 Sep 2009:
Vol. 325, Issue 5947, pp. 1475-1477
DOI: 10.1126/science.325_1475d

When natural diamonds form deep within Earth's interior, they often acquire small inclusions of other minerals that can change physical properties such as hardness and color. Although impurities tend to decrease a gem's aesthetic and economic value, mineral inclusions can provide valuable clues to how and where their more glamorous hosts were formed. Using high-resolution transmission electron microscopy and spectroscopy, Wirth et al. identified assemblages of mineral inclusions, including several carbonate and high-pressure silicate phases, in ∼1-carat diamonds from central Brazil. Because some inclusions are only stable across a narrow range of temperatures and pressures, the diamonds can be traced as far down as the upper mantle, and possibly much deeper. Volatile-rich inclusions, such as PbCl2 nanoparticles and KCl, suggest that salty hydrothermal fluids played an important role during formation. The isotopic signature of the host indicates that the carbon in these diamonds may have been derived from sedimentary organic material that was first deposited in the ocean—hence the salty inclusions—and then subsequently pushed several hundred kilometers below the surface at an ancient subduction zone.

Earth Planet. Sci. Lett. 10.1016/j.epsl.2009.06.043 (2009).

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