Ferric Iron Content of Mineral Inclusions in Diamonds from Sao Luiz: A View into the Lower Mantle

摘要

Moessbauer spectroscopy on inclusions in diamonds from Sao Luiz, Brazil, show that (Mg,Fe)O inclusions contain little Fe~(3+) (Fe~(3+)/ΣFe ≤ 7%), whereas inclusions with pyroxene composition (believed to have been originally in the perovskite structure) and tetragonal almandine-pyrope phase (TAPP) inclusions contain high relative amounts of Fe~(3+) (Fe~(3+)/ΣFe = 20 to 75%). These observations are consistent with high-pressure experiments on synthetic samples and support the idea that the inclusions originated in the lower mantle. Fe-Mg partitioning data for co-existing (Mg,Fe,Al)(Si,Al)O_3 - (Mg,Fe)O pairs also indicate that the phases were in equilibrium at the time of diamond formation in the lower mantle.