PHASE-RELATIONS IN THE MGO-P2O5-H2O SYSTEM AND THE STABILITY OF PHOSPHOELLENBERGERITE - PETROLOGICAL IMPLICATIONS

The polymorphic relations for Mg-3(PO4)(2) and Mg2PO4OH have been dete rmined by reversed experiments in the temperature-pressure (T-P) range 500-1100 degrees C, 2-30 kbar. The phase transition between the low-p ressure phase farringtonite and Mg-3(PO4)(2)-II, the ME analogue of sa rcopside, is very pressure dependent and was tightly bracketed between 625 degrees C, 7 kbar and 850 degrees C, 9 kbar. The high-temperature , high-pressure polymorph, Mg-3(PO4)(2)-III, is stable above 1050 degr ees C at 10 kbar and above 900 degrees C at 30 kbar. The low-pressure stability of farringtonite is in keeping with its occurrence in meteor ites. The presence of iron stabilizes the sarcopside-type phase toward s lower P. From the five Mg2PO4OH polymorphs only althausite, holtedah lite, beta-Mg2PO4OH (the hydroxyl analogue of wagnerite) and epsilon-M g2PO4OH were encountered. Relatively speaking, holtedahlite is the low -temperature phase (< 600 degrees C), epsilon-Mg2PO4OH the high-temper ature, low-pressure phase and beta-Mg2PO4OH the high-temperature, high -pressure phase, with an intervening stability field for althausite wh ich extends from about 3 kbar at 500 degrees C to about 12 kbar at 800 degrees C. Althausite and holtedahlite are to be expected in F-free n atural systems under most geological conditions; however, wagnerite is the most common Mg-phosphate mineral, implying that fluorine has a ma jor effect in stabilizing the wagnerite structure. Coexisting althausi te and holtedahlite from Modum, S. Norway, show that minor fluorine is strongly partitioned into althausite (KDF/OH approximate to 4) and th at holtedahlite may incorporate up to 4 wt% SiO2. Synthetic phosphoell en-bergerite has a composition close to (Mg(0.91)square(0.1))(2)Mg-12- P8O38H8.4. It is a high-pressure phase, which breaks down to Mg2PO4OH + Mg-3(PO4)(2) + H2O below 8.5 kbar at 650 degrees C, 22.5 kbar at 900 degrees C and 30 kbar at 975 degrees C. The stability field of the ph osphate end-member of the ellenbergerite series extends therefore to m uch lower P and higher T than that of the silicate end-members (stable above 27 kbar and below ca. 725 degrees C). Thus the Si/P ratio of in termediate members of the series has a great barometric potential, esp ecially in the Si-buffering assemblage with clinochlore + talc + kyani te + rutile + H2O. Application to zoned ellenbergerite crystals includ ed in the Dora-Maira pyrope megablasts, western Alps, reveals that gro wth zoning is preserved at T as high as 700-725 degrees C. However, th e record of attainment of the highest T and/or of decreasing P through P-rich rims (1 to 2 Si pfu) is only possible in the presence of an ad ditional phosphate phase (OH-bearing or even OH-dominant wagnerite in these rocks), otherwise the trace amounts of P in the system remain se questered in the core of Si-rich crystals (5 to 8 Si pfu) and can no l onger react.