The Influence of Redox State On Mica Crystallization In Leucogranitic and Pegmatitic Liquids

Abstract

International audienceExperiments have been performed both on a peraluminous leucogranitic (DK89) and a F-, Li-, P-rich pegmatitic (B2) melt to constrain the stability of micas in evolved crustal silicic magmas and refine mica-melt partition coefficients for F, Li, and Be. The experiments were conducted in parallel in two fO2 ranges, “oxidizing” (NNO +1 to +3) and “reducing” (NNO –1.6 to –1.4). One two-stage reducing-oxidizing experiment was conducted in a vessel fitted with a H2-permeable Shaw-type membrane. The approach toward equilibrium was tested by imposing long experimental durations and combining mica crystallization experiments with mica dissolution experiments using mica seeds. Experimental micas and melts were analyzed for major elements by electron microprobe and for light elements by nuclear microprobe. At 3 kbar, 620 °C, and under oxidizing conditions, B2 crystallized only muscovite, the biotite seeds reacting to form a new mica intermediate between phengite and Li-rich phengite. Under reducing conditions, biotite (siderophyllite composition) appeared as the stable mica. The two-stage experiment yielded a composite mica assemblage with siderophyllite cores mantled by muscovite rims. At 3.5–3.8 kbar, 720 °C, and under oxidizing conditions, DK89 crystallized only aluminous biotite and muscovite seeds reacted to form biotite-bearing assemblages; muscovite appeared together with biotite at 700 °C. Under reducing conditions, Al-rich biotite is also the stable mica at 720 °C. Partition coefficients show that F and Li are preferentially incorporated in biotite rather than in muscovite, the opposite as for Be. Biotite fractionation buffers the F and increases the Li and Be contents of the residual melt. Muscovite increases the Li content of the melt and has little influence on F and Be concentrations. Our experiments reproduce mica assemblages and compositions typical of Variscan pegmatites and leucogranites, yet very Li-rich micas (e.g., lepidolites) were not obtained. The results stress the differential influence of fO2 on mica stability in moderately and highly fractionated crustal melts. Mica crystallization in leucogranites does not appear to be strongly dependent on fO2. In contrast, a very strong influence of fO2 on stable mica assemblages is demonstrated for the pegmatitic melt. The reducing experiments emphasize the existence of a stability field for biotite in melts poor in Fe, Mg, and Ti. If fO2 is reducing, biotite must crystallize in moderately to highly evolved peraluminous crustal melts. In contrast, the crystallization of muscovite as the sole mica in evolved crustal melts constitutes an indicator of oxidizing fO2. Such an oxidizing evolution that deviates from classical buffered T-logfO2 trajectories is the consequence of a mechanism of magma “self-oxidation” that is proposed to result from dissociation of H2O in the melt

Prolonged Slip on the South Tibetan Detachment Constrains Tectonic Models for Synorogenic Extension in the Central Himalaya

“…Chemical analyses of individual white mica flakes (Table S10 in Supporting Information S1) confirm that they are true muscovite with minor celadonite substitution. Some crystals do show minor variations in some elements (e.g., Mg), but the range of compositions in all analyzed muscovites is comparable to the range of muscovite compositions found in undeformed leucogranite samples from the Himalaya (Figure 6g; Castelli & Lombardo, 1988;Pichavant et al, 2016;Liu et al, 2019;Liu et al, 2020;Xie et al, 2020). Given the petrographic observations, the major element chemical data, and the fact that MsAr plateaus are roughly the same for all samples, we interpret the MsAr dates as cooling ages that help constrain the cooling history of the samples.…”

Mica-liquid trace elements partitioning and the granite-pegmatite connection: The St-Sylvestre complex (Western French Massif Central)

“…For example, the Kd biotite/liquid for F and Li both decrease when lowering temperature from 720-750°C to 620°C. The Kd muscovite/liquid for Li drops by a factor > 5 between 650 and 620°C, but the lowest values correspond to charges more oxidized than the highest (Pichavant et al, 2016;Icenhower and London, 1995). For Rb, Ba and Cs, the data at 650°C show significant dispersions (figure 2).…”

The Role of Sulphur on the Melting of Ca-Poor Sediment and on Trace Element Transfer in Subduction Zones: An Experimental Investigation

“…The main effects of an increase of run duration, at fixed S content, are therefore summarized as follows (Table 2): for the sulphur-undoped runs, longer duration promotes the crystallization of phengite whereas biotite is not stable anymore. The recent experimental study of Pichavant et al (2016) shows that at low fO 2 ( NNO-1.6 to -1.4) biotite is the stable mica in peraluminous melts, whereas at high fO 2 (NNO+1 to +3) it is replaced by muscovite.…”