Thermochemistry of monazite-(La) and dissakisite-(La): implications for monazite and allanite stability in metapelites

Abstract

Thermochemical properties have been either measured or estimated for synthetic monazite, LaPO 4 , and dissakisite, CaLaMgAl 2 (SiO 4 ) 3 OH, the Mg-equivalent of allanite. A dissakisite formation enthalpy of -6,976.5 ± 10.0 kJ mol -1 was derived from high-temperature drop-solution measurements in lead borate at 975 K. A third-law entropy value of 104.9 ± 1.6 J mol -1 K -1 was retrieved from low-temperature heat capacity (C p ) measured on synthetic LaPO 4 with an adiabatic calorimeter in the 30-300 K range. The C p values of lanthanum phases were measured in the 143-723 K range by differential scanning calorimetry. In this study, La(OH) 3 appeared as suitable for drop solution in lead borate and represents an attractive alternative to La 2 O 3 . Pseudo-sections were calculated with the THERIAK-DOMINO software using the thermochemical data retrieved here for a simplified metapelitic composition (La = P REE + Y) and considering monazite and Fe-free epidotes along the dissakisite-clinozoïsite join, as the only REE-bearing minerals. Calculation shows a stability window for dissakisite-clinozoïsite epidotes (T between 250 and 550°C and P between 1 and 16 kbar), included in a wide monazite field. The P-T extension of this stability window depends on the bulk-rock Ca-content. Assuming that synthetic LaPO 4 and dissakisite-(La) are good analogues of natural monazite and allanite, these results are consistent with the REE-mineralogy sequence observed in metapelites, where (1) monazite is found to be stable below 250°C, (2) around 250-450°C, depending on the pressure, allanite forms at the expense of monazite and (3) towards amphibolite conditions, monazite reappears at the expense of allanite.

Developing an inverted Barrovian sequence; insights from monazite petrochronology

“…Allanite and xenotime inclusions in the HREE-enriched garnet cores indicate these phases were stable during prograde metamorphism. Their absence in the current matrix assemblage suggests they became unstable during garnet growth at higher grades (Fitzsimons et al, 2005;Janots et al, 2007;Smith and Barreiro, 1990). We therefore interpret the monazite cores, which are depleted in HREE, to have formed from the breakdown of allanite and xenotime at 20.8 ± 0.2 Ma (MSWD = 1.2), on the prograde path.…”

“…Emerging developments in linking 'age to stage', and particularly the use of combined petrography and traceelement data from major and accessory phases (petrochronology), allow high-spatial precision geochronological data to be more firmly linked to the P -T conditions of accessory phase crystallisation reactions (Foster et al, 2004;Gasser et al, 2012;Janots et al, 2008;Rubatto et al, 2013). This is achieved by detailed observation of textural relationships, geochemical analysis of coexisting accessory phases and systematic documentation of trace-element signatures in major phases that 'fingerprint' monazite-forming reactions (Foster et al, 2002(Foster et al, , 2000(Foster et al, , 2004Gasser et al, 2012;Hermann and Rubatto, 2003;Hoisch et al, 2008;Janots et al, 2006Janots et al, , 2007Janots et al, , 2008Janots et al, , 2009Kingsbury et al, 1993;Pyle and Spear, 2003;Rubatto et al, 2006;Smith and Barreiro, 1990;Spear, 2010;Wing et al, 2003). Such advances in metamorphic and geochronological analysis offer the potential to unravel the complex tectonic and thermal evolution of thrust zones associated with inverted metamorphism.…”

Early Carboniferous P-T path from the Upper Gneiss Unit of Haut-Allier (French Massif Central) - reconstructed by geothermobarometry and EMP-Th-U-Pb monazite dating

“…As a consequence, the interpretation of the Devonian high-Y monazites to have crystallised prior to the garnet is also confirmed. Furthermore, it is interesting to compare the P-T path from Agnat to the stability limits and the modelled modal isopleths of monazite which are mainly controlled by the bulk Ca contents (Janots et al 2007;Spear 2010;Spear and Pyle 2010). Even for low bulk Ca, the P-T path enters the monazite stability field at high pressures and markedly increasing temperatures (Fig.…”

“…Stability fields of monazite and allanite at different bulk-rock CaO contents (wt. %) according to Janots et al (2007) and Spear (2010). The xenotime (Xtm), aluminosilicate and muscovite-out univariant lines are after Spear (1993;.…”

Neoproterozoic metamorphic evolution of the Isbjørnhamna Group rocks from south-western Svalbard

“…Partial replacement of biotite by chlorite may also be related to this phase of metamorphic evolution. Additionally, the partial or even complete replacement of monazite by apatite-allanite-epidote coronas in some samples is a common reaction indicative of late-metamorphic cooling (e.g., Ferry 2000; Majka & Budzyń 2006, Janots et al 2007, Broska & Majka 2008. The character of the retrogressive changes and the mineral assemblage synchronously formed suggest that retrogression took place shortly after the peak of M1 metamorphism.…”