Stability of monazite and disturbance of the Th-U-Pb system under experimental conditions of 250–350 °C and 200–400 MPa

Abstract

Ab stract: This ex per i men tal study pro vides im por tant data fill ing the gap in our knowl edge on monazite sta bil ity un der con di tions of fluid-me di ated low-tem per a ture meta mor phic al ter ation and post-mag matic hy dro ther mal al ter ations. The sta bil ity of monazite and main te nance of orig i nal Th-U-to tal Pb ages were tested ex per i men tally un der P-T con di tions of 250-350 °C and 200-400 MPa over 20-40 days. The start ing ma te ri als in cluded the Bur net monazite + K-feld spar ± al bite ± lab ra dor ite + mus co vite + bi o tite + SiO2 + CaF2 and 2M Ca(OH)2 or Na2Si2O5 + H2O fluid. In the runs with 2M Ca(OH)2, monazite was un al tered. REE-en riched ap a tite formed at 350 °C and 400 MPa. The pres ence of the Na2Si2O5 + H2O fluid pro moted the strong al ter ation of monazite, the for ma tion of sec ond ary REE-en riched ap a tite to fluorcalciobritholite, and the for ma tion of REE-rich steacyite. Monazite al ter ation in cluded the newly de vel oped po ros ity, patchy zon ing, and par tial re place ment by REE-rich steacyite. The un al tered do mains of monazite main tained the com po si tion of the Bur net monazite and its age of (or close to) ca. 1072 Ma, while the al tered do mains showed ran dom dates in the in ter vals of 375-771 Ma (250 °C, 200 MPa run), 82-253 Ma (350 °C, 200 MPa), and 95-635 Ma (350 °C, 400 MPa). The compositional al ter ation and dis tur bance of the Th-U-Pb sys tem re sulted from fluid-me di ated cou pled dis so lu tion-reprecipitation. In na ture, such age dis tur bance in monazite can be at trib uted to post-mag matic al ter ation in gra nitic rocks or to metasomatic al ter ation dur ing meta mor phism. Rec og ni tion of po ten tially al tered do mains (dark patches in highcon trast BSE-im ag ing, de vel oped po ros ity or in clu sions of sec ond ary min er als) is cru cial to the ap pli ca tion of Th-U-Pb geo chron ol ogy.

Monazite stability and the maintenance of Th-U-total Pb ages during post-magmatic processes in granitoids and host metasedimentary rocks: A case study from the Sudetes (SW Poland)

“…This compositional de pend ence was later con firmed ex per i men tally across a wider P-T range of 200-1000 MPa and 450-750°C by the al ter ation of monazite to al la nite and ap a tite or britholite . Re cent ex per i ments con ducted at 250-350°C and 200-400 MPa did not re sult in al ter ation of monazite, which sug gests that monazite re mains sta ble in high-Ca bulk com po si tion un der low-tem per a ture con di tions (Budzyñ et al, 2015b).…”

“…The fluid-me di ated com plete re moval of Pb in al tered monazite do mains pro vides a pow er ful tool for dat ing metasomatic and hy dro ther mal processes, as was ex per i men tally dem on strated at 450°C and 450 MPa . Re cent ex per i ments at 250-350°C and 200-400 MPa showed that the al kali-bear ing en vi ron ment pro motes dis so lu tion-reprecipitation al ter ation of monazite with only par tial Pb re moval (Budzyñ et al, 2015b). The lat ter case has far im pli ca tions to geo chron ol ogy re gard ing post-mag matic pro cesses that may af fect monazite and re sult in geo log i cally mean ing less, dis turbed Th-U-Pb dates.…”

“…Compositional al ter ations that may re sult in dis tur bance of the Th-U-Pb sys tem were pre vi ously doc u mented in na ture (Poitrasson et al, 1996(Poitrasson et al, , 2000Townsend et al, 2000;Petrík and Koneèný, 2009;TartÀse et al, 2011) and ex per i ments (Teufel and Hein rich, 1997;Seydoux-Guillaume et al, 2002;Harlov and Hether ing ton, 2010;Hether ing ton et al, 2010;Budzyñ et al, 2011Budzyñ et al, , 2015bHarlov et al, 2011;Wil liams et al, 2011). Ex per i ments at 900°C and 1000 MPa with monazite in the pres ence of an al kali-bear ing fluid Na 2 Si 2 O 5 + H 2 O re sulted in de ple tion of Pb or Th en richment .…”

“…Such compositional al terations, which af fect monazite dur ing post-mag matic, hy dro thermal and metasomatic pro cesses, have been rec og nized in var ious gra nitic rocks (e.g., Poitrasson et al, 1996Poitrasson et al, , 2000Townsend et al, 2000;Petrík and Koneèný, 2009;TartÀse et al, 2011). Ex per i men tal rep li ca tions of nat u ral pro cesses in dicate that the re-equil i bra tion of monazite via cou pled dis so lution-reprecipitation may sig nif i cantly dis turb the Th-U-Pb system un der low-to high-grade con di tions (Teufel and Hein rich, 1997;Seydoux-Guillaume et al, 2002;Harlov and Hether ington, 2010;Hether ing ton et al, 2010;Budzyñ et al, 2011Budzyñ et al, , 2015bHarlov et al, 2011), which can re sult in the com plete reset ting of the Th-U-Pb sys tem even at 450°C . Our cur rent knowl edge pro vides a better un der stand ing of the al ter ation mech a nisms and im por tance of geo chron ol ogical data to avoid mean ing less dates in pet ro log i cal re con structions and to link the true ages with par tic u lar geo log i cal processes.…”

The stability of xenotime in high Ca and Ca-Na systems, under experimental conditions of 250-350°C and 200-400 MPa: the implications for fluid-mediated low-temperature processes in granitic rocks

“…Four gently flat tened cap sules, two for xeno time ex per i ments with 2M Ca(OH) 2 and Na 2 Si 2 O 5 + H 2 O flu ids, and two for "twin" ex per i ments on monazite (Budzyñ et al, 2013(Budzyñ et al, , 2015, were placed in each au to clave. Three sets of ex per i ments were run un der P-T con di tions and with a du ra tion of: (1) Du ra tions of the ex per i ments were cho sen based on pre vi ous ex per i ments on the sta bil i ties of monazite and xeno time (Hether ing ton et al, 2010;Harlov et al, 2011).…”