Ferric-ferrous ratios, H 2 O contents and D/H ratios of phlogopite and biotite from lavas of different tectonic regimes

Complete chemical analyses, including ferric and ferrous iron, H2O contents and dD values for 16 phlogopite and biotite and 2 hornblende separates are presented. Samples were obtained from volcanic rocks from four localities: (1) phlogopite phenocrysts from minette lavas from the western Mexico continental arc, (2) biotite and hornblende phenocrysts from andesite lavas from Mono Basin, California, (3) phlogopite and biotite from clinopyroxenite nodules entrained in potas- sic lavas from the East African Rift, Uganda, and (4) phlogopite phenocrysts from a wyomingite lava in the Leucite Hills, Wyoming. The Fe 2 O 3 contents in the mi- cas range from 0.8 to 10.5 wt%, corresponding to 0.09 to 1.15 Fe 31 per formula unit (pfu). Water contents vary from 1.6 to 3.0 wt%, corresponding to 1.58 to 3.04 OH pfu, significantly less than would be expected for a site fully occupied by hydroxyl. Cation- and anion-based normalization procedures provide accurate mineral for- mulae with respect to most cations and anions, but are unable to generate accurate estimates of Fe 31 yFe T ,a nd overestimate OH at the expense of O on the hydroxyl site. These inaccuracies are present despite acceptable adjust- ed totals and stoichiometric calculated site occupancies. The phlogopite and biotite phenocrysts in arc-related lavas from western Mexico and eastern California have the highest Fe 31 yFe T ratios (56-87%), reflecting high magmatic oxygen fugacities (DNNO 51 2t o 15), in contrast to those from Uganda (25-40%) and the Leucite Hills (23%). There is no correlation between the OH content and the Fe 31 yFe T ratio in the micas. Values of K MgyFe 21 D (+ 2s errors) were calculated for three phlogo- pite-olivine pairs (0.12 + 0.12, 0.26 + 0.14, 0.09 + 0.12), two biotite-hornblende pairs (0.73 + 0.08 and 1.22 + 0.10) and a single phlogopite-augite pair (1.15 + 0.12). Values of K FyOH D for two biotite and hornblende pairs could not be determined without significant error because of the extremely low F contents (, 0.2 wt%) of the four phases. The dD values obtained in this study encompass a large range (2137 to 243‰). The phlogo- pite and biotite separates from Uganda have dD values of 270 to 249‰, which overlap those believed to represent "primary" mantle. There is a larger range in dD values (2137 to 243‰) for phlogopite phenocrysts from west- ern Mexico minette lavas, although their range in d 18 O values (5.2-6.2‰) is consistent with "normal" mantle. It is unlikely, therefore, that the variable dD values reflect heterogeneity in the mantle source region of the minette magmas. Nor can the extremely low dD values reflect degassing of H2 or H2O since almost 100% loss of dis- solved water in the magma is required, an unrealistic scenario given the stability of the hydrous phenocrysts. The very low dD values of the Mascota minette phlogo- pites require that the hydrogen be introduced from an external source (e.g., meteoric water). Whatever the pro- cess responsible for the observed hydrogen isotope com- position, it had no effect on the d 18 O value, fO2 , aH2O or bulk composition of the host magmas.