Minor Crystalline Solids and Elements: Physicochemical Behavior and Geologic Significance
<ul><li>Ti 4+ , P 5+ associate with (silica) network-modifying cations, e.g., Mg, Fe </li></ul><ul><li>Strong effect on melt composition and structure, mineral-melt equilibria, liquid line of descent </li></ul>
P 5+ , geochemical significance Less of a phosphate mineral may be needed to account for the bulk P 2 O 5 of Solid Earth & Planetary systems (e.g., mantle) as silicate minerals may host P 5+ In the absence or prior to saturation with a phosphate mineral, mafic silicate melts may become enriched in P 2 O 5 , especially in the geochemically important low melt fraction regime Interactions between P 5+ -hosting silicates and accessory phosphates may affect the stability of the latter P 5+ -bearing silicates as a source of phosphorus for microorganisms in nutrient-limited, sub-surface environments.
Reports of extreme enrichment in Olivine and Pyroxenes: 2-4 wt.% P 2 O 5 in olivine and pyroxene in Pallasites [Buseck & Clark 1984] and in reduced terrestrial basalts [Goodrich 1984] 17 wt.% P 2 O 5 in zoned olivine crystals from ancient slags [Kresten et al. 1998] P oorly characterized silico-phosphates in Angrites [Mittlefehldt 2002] Isostructural solid solutions of silicates and phosphates are not common; Si 4+ , P 5+ and Al 3+ have similar radii but also different EN The higher the degree of SiO 4 -polymerization, the lower the P 2 O 5 content, e.g., Gt > Ol >> Px ( Koritnig 1965; Henderson 1968; Anderson & Greenland 1969; Thompson 1975; Bishop et al. 1978; Brunet & Chazot 2001) EN=1.8 Phosphorus in silicate minerals Al 3+ 0.39 Å EN=1.5 P 5+ 0.31 Å EN=2.1 Si 4+ 0.26 Å
5-9 GPa supersolidus experiments Sample in graphite capsule “ hard” Al 2 O 3 sleeve crushable Al 2 O 3 sample WC cubes Opx 9 GPa, 1775 °C Ol Gt
All JSC data for P 2 O 5 wt% in orthopyroxene [0.011(6) wt.%], olivine [0.07(3) wt%], and garnet [0.24(5) wt.%] in equilibrium with liquid [0.7-1.5 wt%]
… .Partitioning coefficients…. <ul><li>depend on ionic size and charge of the element </li></ul><ul><li>may depend on the composition and structure of the involved crystalline and liquid phases phase </li></ul><ul><li>may depend on intensive parameters </li></ul><< 1 1 incompatible element: compatible element : phosphorus behaves incompatibly in pyroxenes (chain silicate) and becomes more compatible in olivine and garnet (orthosilicates) All JSC data D xl/liq = C i xl /C i liq
<ul><li>Common silicates (Opx, Ol, Gt) can host phosphorus (P) </li></ul><ul><li>Possible positive correlation between P and Na in Gt, but no apparent correlation between P and other elements in Ol and Opx </li></ul><ul><li>P 2 O 5 (wt%) in Gt > Ol > Cpx > Opx > Plag: P incorporation more structure than site specific(?) </li></ul><ul><li>Ol/Liq, Opx/Liq DP 2 O 5 probably constant </li></ul><ul><li>Gt/Liq, Cpx/Liq, Plag/Liq DP 2 O 5 may vary with P and/or T, and liquid and garnet composition </li></ul>