M. Darby Dyar

Metasomatic oxidation of upper mantle periodotite

Examination of Fe3+ in metasomatized spinel peridotite xenoliths reveals new information about metasomatic redox processes. Composite xenoliths from Dish Hill, California possess remnants of magmatic dikes which were the sources of the silicate fluids responsible for metasomatism of the peridotite part of the same xenoliths. Mössbauer spectra of mineral separates taken at several distances from the dike remnants provide data on Fe3+ contents of minerals in the metasomatized peridotite. Clinopyroxenes contain 33% of total iron (FeT) as Fe3+ (Fe3+/FeT=0.33); orthopyroxenes contain 0.06–0.09 Fe3+/FeT; spinels contain 0.30–0.40 Fe3+/FeT; olivines contain 0.01–0.06 Fe3+/FeT; and metasomatic amphibole in the peridotite contains 0.85–0.90 Fe3+/FeT. In each mineral, Fe3+ and Fe2+ cations per formula unit (p.f.u.) decrease with distance from the dike, but the Fe3+/FeT ratios of each mineral do not vary. Clinopyroxene, spinel, and olivine Fe3+/FeT ratios are significantly higher than in unmetasomatized spinel peridotites. Metasomatic changes in Fe3+/FeT ratios in each mineral are controlled by the oxygen fugacity of the system, but the mechanism by which each phase accommodates this ratio is affected by crystal chemistry, kinetics, rock mode, fluid composition, fluid/rock ratio, and fluid-mineral partition coefficients. Ratio increases in pyroxene and spinel occur by exchange reactions involving diffusion of Fe3+ into existing mineral grains rather than by oxidation of existing Fe2+ in peridotite mineral grains. The very high Fe3+/FeT ratio in the metasomatic amphibole may be a function of the high Fe3+/FeT of the metasomatic fluid, crystal chemical limitations on the amount of Fe3+ that could be accommodated by the pyroxene, spinel, and olivine of the peridotite, and the ability of the amphibole structure to accommodate large amounts of 3 + valence cations. In the samples studied, metasomatic amphibole accounts for half of the bulk-rock Fe2O3. This suggests that patent metasomatism may produce a greater change in the redox state of mantle peridotite than cryptic metasomatism. Comparison of the metasomatized samples with unmetasomatized peridotites reveals that both Fe2+ and Fe3+ cations p.f.u. were increased during metasomatism and 50% or more of iron added was Fe3+. With increasing distance from the dike, the ratio of added Fe3+ to added Fe2+ increases. The high Fe3+/FeT of amphibole and phlogopite in the dikes and in the peridotite, and the high ratios of added Fe3+/added Fe2+ in pyroxenes and spinel suggest that the Fe3+/FeT ratio of the metasomatic silicate fluid was high. As the fluid perolated through and reacted with the peridotite, Fe3+ and C−O−H volatile species were concentrated in the fluid, increasing the fluid Fe3+/FeT.

Neglected Fe3+/Fe2+ ratios—A study of Fe3+ content of megacrysts from alkali basalts

Petrologists are generally faced with the dilemma of estimating Fe 2+ /Fe 3+ ratios for minerals on the basis of microprobe analyses that only identify total iron, commonly given as FeO. Common solutions to this problem include the assumption that all iron is ferrous, or simply calculating Fe 3+ on the basis of stoichiometry from the microprobe analyses. In order to test these assumptions, ferric iron content has been directly measured on Al-augite, kaersutite, and spinel high-pressure megacrysts from alkali basalts by using the technique of Mossbauer spectroscopy. Measured Fe 3+ content does not agree with Fe 3+ calculated from microprobe analyses. Calculated Fe 3+ content varies widely between three augite megacrysts that have nearly identical microprobe analyses and consistent Mossbauer Fe 3+ values. Ferric iron calculations are found to be highly sensitive to very slight variations in Si, 1+, 3+, and 4+ cations, and should be considered generally unreliable. Mantle petrologists generally assume low or neglible ferric iron content in mantle minerals. This study found that 28%-40% of iron was ferric in these eight megacrysts. Similarity of Al-augite and kaersutite megacryst compositions to minerals in mantle xenoliths suggests that minerals in mantle xenoliths may have higher Fe 3+ content than generally believed.

Nonstoichiometric hydrogen contents in common rock-forming hydroxyl silicates

Abstract H+ and Fe3+ have been analyzed in conjunction with electron microprobe studies of over 500 petrologically well-constrained rock-forming Fe-bearing hydroxyl silicates. H+ and Fe3+ vary widely in the minerals studied. In most cases, biotite, muscovite, and chlorite are deficient in H+ relative to the generally assumed stoichiometries. Results indicate that oxysubstitution (or deprotonation) involving only H+ and Fe3+ exchange is rare, although the related substitution of H+mineral (Fe, Mg)2+mineral → (Fe3+, A13+, 1 2 Ti 4+ , etc.)mincral + 1 2 [H 2 ] gas is extremely common. In biotites from metapelites in western Maine, this relationship can be expressed quantitatively as H + = −1.120 ± 0.302 × (6 − 4 Si 4+ + 6 Fe 3+ + 6A13+ + 6Ti4+ × 2)× + 4.9006 ± 0.2547 6A13+ + 6Ti4+ × 2)× + 4.9006 ± 0.2547 for a 24-oxygen formula unit. Similar equations will be developed for other hydroxyl phases. This mechanism accounts for the great majority of R3+ and R4+ substitution (apart from the Tshermak substitution), reducing or eliminating the need to invoke vacancies for charge balance and should also be considered in formulation of site substitution models, geothermobarometry and any other applications dependent on full characterization of mineral compositions.

Fe3+/H+ and D/H in kaersutites—Misleading indicators of mantle source fugacities

Hornblende megacrysts from basaltic flows in several locations have been studied by means of the electron microprobe, Mossbauer spectroscopy, uranium extraction techniques, and mass spectrometry to define the interrelations between the concentrations of Fe 3+ and H + , and the hydrogen isotope partitioning (D/H) in mantle amphiboles. Results indicate a nearly perfect inverse relation between Fe 3+ content and H + content; no relation is observed between D/H partitioning and wt% H 2 O. A review of diffusion data for amphiboles suggests that the observed increase in Fe 3+ content with decreasing H + content may represent partial dehydration of the amphiboles during ascent. Thus, Fe 3+ and H + contents measured on amphiboles at Earth9s surface may not represent mantle conditions. Instead, high Fe 3+ contents may be indicative of high H + contents in the mantle source region rather than high mantle oxygen fugacities.

Crystal chemistry of iron in two styles of metasomatism in the upper mantle

Redox characteristics and crystal chemical changes caused by Fe-Ti metasomatism are evident in an anhydrous composite nodule from Kilboume Hole, New Mexico. Data collected using electron microprobe and Mossbauer spectroscopy show that olivines have no evidence of Fe-oxidation from metasomatism, although changes in Fe2+↔ Mg2+ exchange and Fe2+ ordering are observed in the peridotite with distance from the dike contact. Orthopyroxenes exhibit constant Fe3+/σFe ratios at all distances from the dike, although the proportion of Fe2+ in sites with trivalent neighbors and total Fe decrease with increasing distance from the contact. Iron valence and occupancy remain fairly constant for clinopyroxene, in which the dominant metasomatic change is a Tschermak substitution of Al for Si and Mg. Spinel displays constant values of Fe3+/σFe and total iron, but grains near the dike contact have high Mg and Al contents. These results contrast with analogous studies of composite xenoliths which show evidence of hydrous metasomatism. The data suggest that hydrous modal metasomatism may be related to conditions of higher ƒo2 than may be produced by melts that do not form new minerals.

IN SITU MEASUREMENT OF FERRIC IRON IN LUNAR GLASS BEADS USING FE-XAS

Through use of a new X-ray Absorption Spectroscopy (XAS) calibration for Fe 3 + analysis in silicate glasses, the first direct measurements of ferric iron in natural lunar picritic glasses are presented. Lunar glass beads from the Apollo sample collection contain up to 60.0% Fe 3 + . No correlation with melt chemical properties, such as Mg# or weight % TiO 2 , or physical properties, such as bead diameter, was observed. Fe 3 + / Fe is negatively correlated with NBO/T. These elevated Fe 3 + / Fe values reflect eruption and posteruption oxidation due to magmatic degassing of H or OH. Glass beads observed to be zoned to lower Fe 3 + / Fe rims may represent a subsequent reduction in the lunar vacuum prior to cooling through the glass transition temperature.