Estimation of a maximum Lu diffusion rate in a natural eclogite garnet

Abstract

Lutetium zoning in garnet within eclogites from the Zermatt-saas Fee zone, Western Alps, reveal sharp, exponentially decreasing central peaks. they can be used to constrain maximum Lu volume diffusion in garnets. A prograde garnet growth temperature interval of ~450-600 °c has been estimated based on pseudosection calculations and garnet-clinopyroxene thermometry. the maximum pre-exponential diffusion coefficient which fits the measured central peak is in the order of D 0 = 5.7*10 -6 m 2 /s, taking an estimated activation energy of 270 kJ/mol based on diffusion experiments for other rare earth elements in garnet. this corresponds to a maximum diffusion rate of D (~600 °c) = 4.0 Introduction the closure temperature, t c , has been defined by Dodson (1973) to be "the temperature at the time corresponding to its apparent age". It is dependant on the diffusion rate, the cooling rate, grain size, and grain shape. Following Dodson (1973) for spherical minerals one obtains:(1) (Q = activation energy; r = universal gas constant, A = numerical geometry factor, D 0 = diffusion coefficient at infinitely high temperatures, r = radius, dt/dt = cooling rate). In terms of geochronology, the mineral specific t c determines, among other factors, whether its age represents the timing of mineral crystallization or whether it corresponds to an age where a particular isotope geochronometer was closed.Garnet geochronology is particularly useful for systems such as sm-Nd and Lu-Hf because this mineral has high 147 sm/ 144 Nd and 176 Lu/ 177 Hf ratios (e.g. Mezger et al. 1992;Duchêne et al. 1997). the stability relations of garnet are generally well understood, hence constraints can be placed on the age(s) of specific P-t conditions at which the mineral grew (e.g. Lapen et al. 2003;Whitehouse & Platt 2003) provided garnet has not been heated beyond its t c . Most t c studies so far concern sm-Nd garnet geochronology because diffusion experiments exist for sm and/or Nd in garnet (Harrison & Wood 1980;coghlan 1990;Ganguly et al. 1998; Van Orman et al. 2002;tirone et al. 2005). However, published estimates for sm-Nd t c ' span a large range in temperature from around 500 °c (e.g. Mezger et al. 1992) to around 800 °c (e.g. Jagoutz 1988). Not much is known about the t c of the Lu-Hf system, and, to our knowledge, no Lu and Hf diffusion data in garnet have been determined. In general, Lu-Hf ages have been found to give older ages when compared to sm-Nd ages. scherer et al. (2000) Geosci. 101 (2008) their high grade rocks this is likely due to a higher t c for the Lu-Hf, compared to the sm-Nd system. Lapen et al. (2003) interpreted a similar age difference in lower temperature rocks to reflect prograde zoning, based on the fact that Lu, in contrast to sm, will be strongly partitioned into garnet and hence, will be concentrated in the early grown core region. the knowledge whether prograde Lu-Hf zoning is preserved in a sample or not can be used to distinguish the two interpretations.1661-8726/08/030637-14 DOI 10.1007/s00015-008-...

Evidence for protracted prograde metamorphism followed by rapid exhumation of the Zermatt‐Saas Fee ophiolite

“…Matrix paragonite and glaucophane likely belong to the prograde assemblage for reasons discussed in Skora et al . (). Epidote, Fe‐rich dolomite, rutile, apatite and quartz often occur in various but small amounts.…”

Current challenges for explaining (ultra)high‐pressure tectonism in the Pennine domain of the Central and Western Alps

“…High‐ P metamorphism peaked at 37.1 ± 0.9 Ma and 38.8 ± 4.3 Ma in the central nappe and at 34.1 ± 2.8 Ma in the southern nappe (Herwartz et al ., ; Sandmann et al ., ). The latter age was interpreted as a minimum age because the rock may have been re‐equilibrated under temperatures higher than the closure temperatures in the Lu–Hf garnet system (≥630 °C, Skora et al ., ). Later, before c .…”

Combined geochemistry and geochronology constrains coupled subduction of oceanic and continental crust in the Huwan shear zone, central China

“…3) of the garnets indicates that the overall garnet growth occurred during eclogite-facies metamorphism. Published estimates of the closing temperature of the garnet Lu-Hf system are >630 °C (Skora et al 2008), >680 °C; (Scherer et al 2000), >800 °C (Schmidt et al 2011), and approximately 1000 °C (Shu et al 2013) and hence well above the peak temperatures of the Xuehe eclogite (<580 °C). This, together with the preserved prograde chemical zoning in the garnet, discredits the interpretation of these ages as a re-equilibration or cooling age.…”

“…Garnet is well suited for combined chronological and thermobarometric research because of its ability to preserve original chemical and isotopic compositions in relatively low-and medium-temperature metamorphic rocks (e.g., Spear and Selverstone 1983;Ague and Carlson 2013;Massonne 2013;Baxter and Caddick 2013). Garnet is ideal for Lu-Hf geochronology because this phase strongly partitions Lu over Hf (Otamendi et al 2002) and also because of its (presumed) high closure temperature (Dodson 1973; no less than 900 °C, Skora et al 2008;Smit et al 2013;Shu et al 2013). The high affinity of garnet for Lu may produce strong core-to-rim zonation in Lu, resulting in Lu-Hf ages that are biased toward the early (prograde) garnet growth (Lapen et al 2003).…”

“…To specify the geological meanings of these ages, bulk rock/inclusions bearing inherited Hf, complexities in garnet growth and post-garnet crystallization resetting must be clarified carefully (Cheng et al 2012). REE zonations in garnet, coupled to crystal size distributions, provide a powerful means for understanding prograde metamorphic paths when combined with Lu-Hf geochronology (Skora et al 2008).…”