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

Abstract

Complete chemical analyses, including ferric and ferrous iron, H 2 O 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 potassic 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 micas range from 0.8 to 10.5 wt%, corresponding to 0.09 to 1.15 Fe 3ϩ 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 formulae with respect to most cations and anions, but are unable to generate accurate estimates of Fe 3ϩ ͞Fe T , and overestimate OH at the expense of O on the hydroxyl site. These inaccuracies are present despite acceptable adjusted 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 3ϩ ͞Fe T ratios (56-87%), reflecting high magmatic oxygen fugacities (DNNO ϭ ϩ2 to ϩ5), in contrast to those from Uganda (25-40%) and the Leucite Hills (23%). There is no correlation between the OH content and the Fe 3ϩ ͞Fe T ratio in the micas. Values of K Mg͞Fe 2ϩ D (+ 2s errors) were calculated for three phlogopite-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 F͞OH D for two biotite and hornblende pairs could not be determined without significant error because of the extremely low F contents (Ͻ 0.2 wt%) of the four phases. The dD values obtained in this study encompass a large range (Ϫ137 to Ϫ43‰). The phlogopite and biotite separates from Uganda have dD values of Ϫ70 to Ϫ49‰, which overlap those believed to represent "primary" mantle. There is a larger range in dD values (Ϫ137 to Ϫ43‰) for phlogopite phenocrysts from western 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 H 2 or H 2 O since almost 100% loss of dissolved 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 phlogopites require that the hydrogen be introduced from an external source (e.g., meteoric water). Whatever the process responsible for the observed hydrogen isotope composition, it had no effect on the d 18 O value, f O 2 , a H...

Mössbauer study of Fe3+/Fe2+ ratio in amphiboles to search correlation with hydrogen isotope fractionation

“…3). Such a relationship may be expected for minerals that have experienced an interaction with, for example, a reducing fluid rich in molecular hydrogen, or that may have experienced oxidation [6][7][8]. Our data clearly also does not support interaction with a very reducing fluid (e.g.…”

“…Fe 3+ content of amphiboles may be very informative for interpretations of their hydrogen isotope compositions [7,8]. For 17 representative samples, Mössbauer spec- Table 1 troscopy analyses were performed in order to determine the Fe 3+ /Fe 2+ ratios of the amphiboles ( Fig.…”

Mechanisms and Crystal Chemistry of Oxidation in Annite: Resolving the Hydrogen-Loss and Vacancy Reactions

“…Redhammer et al (1993) also concluded that the dominant oxidation mechanism in hydrothermally synthesized annite is the vacancy mechanism, based on an interpretation of infrared (IR) spectra that has been contested by Rancourt et al (1994a). Virgo and Popp (2000) performed a detailed study of the oxidation and H-loss mechanisms in mantle-derived phlogopites (including 40 synthetic re-equilibrations) and have re-examined the conclusions of Rebbert et al (1995), of Redhammer et al (1993), and of researchers who analyzed ferric/ferrous and H contents in natural micas (Feldstein et al, 1996;Feeley and Sharp, 1996). In opposition to the suggestions of Rebbert et al (1995), they have shown that the Fe 3+ in their natural phogopites occurs exclusively via the oxybiotite reaction (reaction 1) and that this interpretation is consistent with all considered published data on natural samples, where quantitative lettic/ferrous and H analyses were available.…”

Change in D/H ratio, water content and color during dehydration of hornblende.

“…This pro cess, Eq. (1), has also been proposed to be op erative in other hydrous minerals such as micas (e.g., Wones, 1963;Vedder and Wilkins, 1969), though it was reported that there was no clear correlation between Fe3+ and OH (per formula unit) of micas in several lava flows from different tectonic settings, analyzed by Feldstein et al (1996). It should be noted that actual dehydration mechanism of hornblende may involve other pro cesses rather than being solely associated with the dehydrogenation-oxidation reaction, and is still the subject of investigation (e.g., Addison et al, 1962a, b;Hodgson et al, 1965;Clowe et al, 1988).…”