Boron isotope record of peak metamorphic ultrahigh-pressure and retrograde fluid–rock interaction in white mica (Lago di Cignana, Western Alps)

Abstract

This study presents boron (B) concentration and isotope data for white mica from (ultra)high-pressure (UHP), subductionrelated metamorphic rocks from Lago di Cignana (Western Alps, Italy). These rocks are of specific geological interest, because they comprise the most deeply subducted rocks of oceanic origin worldwide. Boron geochemistry can track fluidrock interaction during their metamorphic evolution and provide important insights into mass transfer processes in subduction zones. The highest B contents (up to 345 μg/g B) occur in peak metamorphic phengite from a garnet-phengite quartzite. The B isotopic composition is variable (δ 11 B = − 10.3 to − 3.6%) and correlates positively with B concentrations. Based on similar textures and major element mica composition, neither textural differences, prograde growth zoning, diffusion nor a retrograde overprint can explain this correlation. Modelling shows that B devolatilization during metamorphism can explain the general trend, but fails to account for the wide compositional and isotopic variability in a single, well-equilibrated sample. We, therefore, argue that this trend represents fluid-rock interaction during peak metamorphic conditions. This interpretation is supported by fluid-rock interaction modelling of boron leaching and boron addition that can successfully reproduce the observed spread in δ 11 B and [B]. Taking into account the local availability of serpentinites as potential source rocks of the fluids, the temperatures reached during peak metamorphism that allow for serpentine dehydration, and the high positive δ 11 B values (δ 11 B = 20 ± 5) modelled for the fluids, an influx of serpentinite-derived fluid appears likely. Paragonite in lawsonite pseudomorphs in an eclogite and phengite from a retrogressed metabasite have B contents between 12 and 68 μg/g and δ 11 B values that cluster around 0% (δ 11 B = − 5.0 to + 3.5). White mica in both samples is related to distinct stages of retrograde metamorphism during exhumation of the rocks. The variable B geochemistry can be successfully modelled as fluid-rock interaction with low-to-moderate (< 3) fluid/rock ratios, where mica equilibrates with a fluid into which B preferentially partitions, causing leaching of B from the rock. The metamorphic rocks from Lago di Cignana show variable retention of B in white mica during subduction-related metamorphism and exhumation. The variability in the B geochemical signature in white mica is significant and enhances our understanding of metamorphic processes and their role in element transfer in subduction zones.

Boron isotope fractionation in subducted serpentinites: A modelling attempt

“…3A). These results are compatible with the positive δ 11 B in natural antigorite-bearing serpentinites and metamorphic olivine from high-P veins (e.g., Scambelluri and Tonarini, 2012;De Hoog et al, 2014) and well match with the scenario requiring a serpentinite-derived component in the arc magma region to explain arcs with positive δ 11 B (e.g., Tonarini et al, 2011) and with the δ 11 B signature of serpentinite-derived fluids percolating within the slab at high-P (e.g., Halama et al, 2020). However, considering the δ 11 B atg of -6.2‰ obtained during serpentine phase transition at neutral pH conditions (Fig.…”

“…This positive δ 11 B signature is shared also by shallow forearc serpentinites (Benton et al, 2001), thus reflecting the 11 B-rich nature of the fluids extracted from the subducting slab by early devolatilization reactions. For this reason, δ 11 B is commonly used as a geochemical tracer to unravel chemical exchange between slab lithologies and mantle as well as to estimate the serpentinite contribution to the genesis of arc magmatism characterized by high δ 11 B signatures (e.g., De Hoog and Savov, 2018;Halama et al, 2020;Prigent et al, 2018;Tonarini et al, 2011).…”

Extensive fluid–rock interaction and pressure solution in a UHP fluid pathway recorded by garnetite, Lago di Cignana, Western Alps

“…The extensive evidence for garnet dissolution presented here is not coupled with concomitant reprecipitation, which implies that the whole process took place under open system conditions, with overall loss of material. The fluids responsible for mineral dissolution were externally derived and likely made available by dehydration of nearby serpentinite (Halama et al., 2020). Because garnet was concentrated into garnetite by dissolution of more soluble interstitial minerals along a fluid pathway, the garnetite can be considered a stylolite.…”

“…This evidence suggests the dissolution of a polymineralic component of the system during garnet growth, beyond quartz and carbonates (Frezzotti et al., 2011; Hawkins et al., 2009). Boron concentrations and isotopic ratios in white mica within several lithologies in the LCU area indicate that a fluid derived from partially dehydrated serpentinites was the driver behind fluid–rock interaction in quartzite, but not in eclogite (Halama et al., 2020).…”

“…Angiboust et al., 2017; Hadjzobir et al., 2017; Spry et al., 2000; Vrijmoed et al., 2006), no origin mechanism has been established for the LCU garnetites. Infiltration of external fluids and extensive fluid–rock interaction within this unit have been revealed by detailed fluid‐inclusion and geochemical studies, suggesting influx of locally solute‐rich serpentinite‐derived fluids (Frezzotti et al., 2011; Halama et al., 2020; Hawkins et al., 2009). Therefore, the LCU potentially includes remnants of a large‐scale fluid pathway, which has not yet been identified and described in detail.…”