Ferric iron content of mineral inclusions in diamonds from Sao Luiz: A view into the lower mantle

Mossbauer spectroscopy on inclusions in diamonds from Sao Luiz, Brazil, show that Mg,Fe)O inclusions contain little Fe³⁺ (Fe³⁺/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 Fe³⁺ (Fe³/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)O₃- (Mg,Fe)O pairs also indicate that the phases were in equilibrium at the time of diamond formation in the lower mantle.