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
Bartosz BUDZYÑ1, 2, * and Gabriela A. KOZUB-BUDZYÑ3
1 Pol ish Acad emy of Sci ences, In sti tute of Geo log i cal Sci ences, Re search Cen tre in Kraków, Senacka 1, 31-002 Kraków, Po land
2 Jagiellonian Uni ver sity, In sti tute of Geo log i cal Sci ences, Oleandry 2a, 30-063 Kraków, Po land
3 AGH Uni ver sity of Sci ence and Tech nol ogy, Fac ulty of Ge ol ogy, Geo phys ics and En vi ron men tal Pro tec tion, al. A. Mickie - wicza 30, 30-059 Kraków, Po land
Budzyñ, B., Kozub-Budzyñ, G. A., 2015. The sta bil ity of xeno time in high Ca and Ca-Na sys tems, un der ex per i men tal con di - tions of 250–350°C and 200–400 MPa: the im pli ca tions for fluid-me di ated low-tem per a ture pro cesses in gra nitic rocks. Geo - log i cal Quar terly, 59 (2): 316–324, doi: 10.7306/gq.1223
The sta bil ity of xeno time was tested by ex per i ments in the pres ence of a sil i cate min eral as sem blage and two dif fer ent flu ids, 2M Ca(OH)2 or Na2Si2O5 + H2O, un der P-T con di tions of 200–400 MPa and 250–350°C. The xeno time was sta ble in runs with 2M Ca(OH)2, rep li cat ing the low-tem per a ture metasomatic al ter ations of gra nitic rocks, ex cept in ex per i ment at 350°C and 400 MPa, where some (Y,REE)-rich fluorapatite formed. Ex per i ments with Na2Si2O5 + H2O re sulted in sig nif i cant xeno time al ter ation and par tial re place ment by an un known (Y,HREE)-rich sil i cate, and in the for ma tion of mi nor amounts of (Y,REE)-rich fluorapatite. The lat ter in di cate pref er en tial par ti tion ing of Y and REE into sil i cates over phos phates dur ing low-tem per a ture, metasomatic pro cesses in a high Na-Ca sys tem, sim i lar to peralkaline gra nitic rocks.
Key words: xeno time, fluorapatite, yt trium sil i cate, rare earth el e ments, ex per i men tal pe trol ogy.
INTRODUCTION
Xeno time, (Y,HREE)PO4, is an ac ces sory min eral of gra - nitic rocks, pegmatites, low- to high-grade meta mor phic rocks, and migmatites (Förster, 1998). Xeno time is also pres ent as de tri tal min eral in sed i men tary rocks or as an authigenic phase, com monly formed on zir con in siliciclastic rocks (Fletcher et al., 2000; Ras mus sen et al., 2004; Ras mus sen, 2005). Be cause of its U and Th con tents, xeno time has ap pli ca tions in iso to pic U-Pb dat ing us ing mass spec trom e try (Ras mus sen et al., 2004;
Ras mus sen, 2005) and in chem i cal U-Th-to tal Pb dat ing us ing elec tron microprobe (Cocherie and Legendre, 2007; Hether ing - ton et al., 2008; Suzuki and Kato, 2008). The ap pli ca tion of microanalytical meth ods, such as sen si tive high-res o lu tion ion microprobe (SHRIMP) or elec tron microprobe, to date xeno - time pro vides age data in tex tural con text to con strain the age of par tic u lar ig ne ous, meta mor phic or diagenetic pro cesses. Al - though xeno time is a rel a tively sta ble min eral, fluid-me di ated al - ter ation may lead to xeno time break down and re place ment by
Y-rich ap a tite and Y-rich epidote rec og nized in meta mor phosed gra nitic rocks (Broska et al., 2005) or by fluorapatite and hingganite-(Y), doc u mented in Skoddefjellet peg ma tite from Svalbard (Majka et al., 2011). The P-T con di tions of xeno time al ter ations are not tightly con strained, and ex per i men tal data are nec es sary to fully un der stand the sta bil ity of xeno time as a func tion of pres sure-tem per a ture con di tions, ac com pa ny ing min eral as sem blage, and fluid com po si tion.
Pre vi ous ex per i men tal works on xeno time fo cused on rel a - tively sim ple sys tems. The sta bil ity of xeno time in the pres ence of com mon meta mor phic and ig ne ous flu ids (H2O, NaCl and KCl brines, CaF2 + H2O, 1M and 2M HCl, 1M and 2M H2SO4, 1M NaOH, and Na2Si2O5 + H2O) was tested at 500 MPa and 600°C, and 1000 MPa and 900°C, doc u ment ing the dis so lu tion and etch ing of xeno time crys tal faces, the for ma tion of po rous tex tures or the growth of small xeno time grains in some ex per i - ments – but no in ter nal compositional al ter ations (Hether ing ton et al., 2010). The ex per i ments at the same con di tions of 500 MPa and 600°C, and 1000 MPa and 900°C, with start ing com po si tions of xeno time + SiO2 + Al2O3 + ThSiO4 + Na2Si2O5
+ H2O, re sulted in compositional al ter ation of xeno time and en - rich ment in ThSiO4 along rims (Harlov and Wirth, 2012), par - tially rep li cat ing the Th en rich ment of xeno time in gra nitic pegmatites from the Hidra an or tho site, Nor way (Hether ing ton and Harlov, 2008). Xeno time has also been tested for its sol u - bil ity in H2O and H2O-NaCl flu ids at 1 GPa and 800°C, show ing higher sol u bil ity of YPO4 than CePO4 in pure H2O to XNaCl = 0.27
* Corresponding author, e-mail: ndbudzyn@cyf-kr.edu.pl Received: December 3, 2014; accepted: January 8, 2015; first published online: February 25, 2015
and lower sol u bil ity with in creas ing NaCl con cen tra tion (Tropper et al., 2011). The ex per i ments rep li cat ing sys tems of nat u ral rocks, in volv ing xeno time + al bite + K-feld spar + bi o tite + mus co vite + SiO2 + CaF2 with a va ri ety of flu ids, 2M KOH, 2M NaOH, 2M Ca(OH) 2 or Na2Si2O5 + H2O, ran un der con di tions of 450°C and 590 MPa, de creas ing to 540 MPa over 16 days, re - sulted in xeno time al ter ation in all runs, with the for ma tion of Y-rich britholite and fluorapatite (Budzyñ and Harlov, 2011).
There is a lack of pub lished ex per i men tal works re gard ing the sta bil ity of xeno time in the pres ence of sil i cates and flu ids rep li - cat ing con di tions of low-tem per a ture meta mor phism or hy dro - ther mal, post-mag matic al ter ations in gra nitic rocks.
This study ex per i men tally ex plores the sta bil ity of xeno time un der con di tions of 200–400 MPa and 250–350°C, in the pres - ence of sil i cate min er als as sem blages and fluid. Ex per i ments with 2M Ca(OH)2 al ka line fluid rep li cate con di tions of the low-tem per a ture meta mor phism of gra nitic rocks. The sec ond set of ex per i ments, with Na2Si2O5 + H2O al kali fluid, shows Y and REE par ti tion ing be tween phos phates and sil i cates, dur ing the low-tem per a ture meta mor phic over print of gran ites or dur - ing fluid-me di ated post-mag matic pro cesses in peralkaline gra - nitic rocks.
EXPERIMENTAL AND ANALYTICAL METHODS
EXPERIMENTAL METHODS
The xeno time used for the ex per i ments is a por tion of the crys tal from peg ma tite from the North-West Fron tier Prov ince (NWFP), Pa ki stan, also used in pre vi ous ex per i men tal works by Hether ing ton et al. (2010), Budzyñ and Harlov (2011), Harlov and Wirth (2012) and Budzyñ et al. (2014). The other nat u ral min er als used in clude gem-qual ity al bite (Ab100; Rožòava, Slovakia), lab ra dor ite (An60Ab27Kfs3; Chi hua hua, Mex ico), sanidine (Eifel re gion, Ger many), mus co vite (peg ma - tite, Siedlimowice, SW Po land), bi o tite (gneiss, Sikkim Himalaya, In dia), and gar net (Gore Moun tain, NY, USA). The cho sen min eral com po si tions and used weight pro por tions (Ta - ble 1) roughly rep li cate that of gra nitic rock. Rel a tively high amounts of xeno time (ca. 5 mg of xeno time in ca. 34.6 to 40.0 mg to tal charge of cap sule) were added to guar an tee ob - ser va tion of xeno time in spec i mens with ex per i men tal prod ucts, be cause the ma te rial from each cap sule was planned to be split be tween 3-4 por tions. CaF2 (Suprapure, Merck) was used in ex per i ments in ex cess as a source of Ca and F to in crease re - ac tion rates, and to form fluorapatite. Syn thetic SiO2 was used in stead of nat u ral quartz to in crease re ac tion rates. Gar net was added to test the fluid-me di ated par ti tion ing of Y be tween xeno - time and gar net.
Crushed and sieved min er als to a frac tion of 50–250 µm were washed in eth a nol in an ul tra sonic bath, and for eign or al - tered min eral grains were picked by hand un der a bin oc u lar mi - cro scope. The flu ids used in cluded 2M Ca(OH)2 and Na2Si2O5
+ H2O, as they were the most ag gres sive flu ids in pre vi ous ex - per i ments on monazite (Budzyñ et al., 2011) and xeno time (Budzyñ and Harlov, 2011). The min er als and re agents were weighed and mixed to gether while dry. The mixed sol ids were loaded into 3 mm wide, 15 mm long Au cap sules, into which fluid was added us ing a sy ringe, which were then pinched shut.
Then the cap sules were arc-welded shut us ing a Lampert PUK U3 ar gon plasma torch. Prior to runs, the cap sules were checked for leaks by weigh ing, dry ing in a 105°C oven over - night, and then weigh ing again.
The ex per i ments were con ducted at the Deut sche GeoForschungsZentrum (Potsdam, Ger many) us ing a stan - dard cold-seal, 6 mm bore, René metal au to claves with H2O as the pres sure me dium. Four gently flat tened cap sules, two for xeno time ex per i ments with 2M Ca(OH)2 and Na2Si2O5 + H2O flu ids, and two for “twin” ex per i ments on monazite (Budzyñ et al., 2013, 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) 200 MPa, 250°C, 40 days; (2) 200 MPa, 350°C, 40 days; and (3) 400 MPa, 350°C, 20 days (Ta ble 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; Budzyñ and Harlov, 2011; Budzyñ et al., 2011; Harlov et al., 2011). Pres sures and tem per a tures were sta ble dur ing the runs. At the end of the ex per i ments, the au to claves were cooled us ing com pressed air, reach ing tem - per a tures of <100°C within <1 min. Af ter the runs, the cap - sules were weighed, opened, and dried in a 105°C oven. The ex per i men tal prod ucts were mounted in ep oxy and pol ished for elec tron microprobe anal y ses. The sec ond por tion was sprin kled on the SEM mount with ad he sive car bon tape for back-scat tered elec tron (BSE) im ag ing.
ANALYTICAL METHODS
The pri mary ob ser va tions and analyses of the start ing min - er als and ex per i men tal prod ucts were per formed us ing a Hitachi S-4700 field emis sion scan ning elec tron mi cro scope (SEM) equipped with an en ergy-dispersive spec trom e ter (EDS) at the In sti tute of Geo log i cal Sci ences, Jagiellonian Uni ver sity (Kraków, Po land).
The chem i cal com po si tions of xeno time, (Y,REE)-rich fluorapatite, and un named (Y,HREE)-rich sil i cate were de ter - mined us ing a Cameca SX 100 elec tron microprobe equipped with four-wave length-dispersive spec trom e ters (WDS) at the De part ment of Spe cial Lab o ra to ries, Lab o ra tory of Elec tron Microanalysis, Geo log i cal In sti tute of Dionýz Štúr (Bratislava, Slo vak Re pub lic). The xeno time was an a lyzed un der con di tions of 15 kV ac cel er at ing volt age, 180 nA beam cur rent, and 3 µm beam size, fo cused on the grain mount and coated with ca. 25 nm car bon film. The nat u ral and syn thetic stan dards, and the cor re spond ing spec tral lines used for stan dard iza tion were as fol lows: ap a tite (P Ka), PbCO 3 (Pb Mb), ThO2 (Th Ma), UO2
(U Mb), YPO4 (Y La), LaPO4 (La La), CePO4 (Ce La), PrPO4(Pr Lb), NdPO4 (Nd La), SmPO4 (Sm La), EuPO4
(Eu Lb), GdPO4 (Gd La), TbPO4 (Tb La), DyPO4 (Dy Lb), HoPO4 (Ho Lb), ErPO4 (Er Lb), TmPO4 (Tm La), YbPO4
(Yb La), LuPO4 (Lu Lb), faya lite (Fe Ka), bar ite (S Ka), wollastonite (Ca Ka, Si Ka), SrTiO3 (Sr La), Al2O3 (Al Ka), and GaAs (As La). The count ing times (peak/back ground, in sec.) were as fol lows: P 10/5, Pb 300/150, Th 60/30, U 30/20, Y 10/5, La 10/5, Ce 10/5, Pr 30/15, Nd 10/5, Sm 10/5, Eu 30/15, Gd 20/10, Tb 20/10, Dy 30/15, Ho 60/30, Er 60/30, Tm 30/15, Yb 30/15, Lu 100/50, Fe 20/20, S 10/10, Ca 20/10, Sr 20/10, Al 10/10, Si 10/10, and As 20/20. (Y,REE)-rich fluorapatite and un - named (Y,HREE)-rich sil i cate were analysed us ing two con di - tions of: (1) 15 kV, 20 nA for F (30/15 sec), Si (10/5), Na (10/5), Al (10/5), Mg (10/5) P (10/5), Ca (10/5), K (10/5), Cl (10/5), Fe (10/5), Mn (10/5), Ti (10/5); fol lowed by (2) 15 kV, 80 nA for Y (30/15), Sr (60/30), Pb (30/15), Ce (40/20), La (40/20), Nd (30/15), Pr (50/25), Sm (30/15), Eu (60/30), Gd (40/20), Tb (20/10), Dy (60/30), Th (30/15), U (40/20), and 1–5 µm beam size, de pend ing on the size of the grain an a lysed.
Chem i cal anal y ses of sil i cates and compositional WDS X-ray maps of xeno time from ex per i ments were per formed us - The stability of xenotime in high Ca and Ca-Na systems, under experimental conditions of 250–350°C and 200–400 MPa: ... 317
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ing a JEOL SuperProbe JXA-8230 elec tron microprobe equipped with five wave length-dispersive spec trom e ters in the Lab o ra tory of Crit i cal El e ments AGH-KGHM, at the Fac ulty of Ge ol ogy, Geo phys ics and En vi ron men tal Pro tec tion, AGH Uni - ver sity of Sci ence and Tech nol ogy (Kraków, Po land). The analyses were per formed us ing 15 kV ac cel er at ing volt age, 20 nA beam cur rent, and 5 µm beam size for feld spars, 3 µm for micas, and a fo cused beam for gar net. The count ing times on peak/back ground (in sec.) were 20/10 for Si, and 10/5 for the other el e ments in feld spars and micas; 10/5 for all el e ments ex - cept 30/15 for Y in gar net. Compositional WDS X-ray maps were col lected us ing con di tions of 15 kV, 100 nA, 100 ms dwell time, 0.33 mm step size and 0.3 mm beam size.
RESULTS
EXPERIMENTS WITH XENOTIME AND 2M CA(OH)2 FLUID
Xeno time and the other start ing min er als in the ex per i men - tal prod ucts from runs with 2M Ca(OH)2 are not al tered re gard - ing tex tures and com po si tion (Fig. 1A, C, Ta ble 2 and Ap pen di -
ces 1–3*). Wollastonite is the only phase formed in the low est P-T run at 200 MPa and 250°C (Fig. 1B). Del i cate, nee dle-like crys tals of Y-rich fluorapatite (iden ti fi ca tion based on the EDS anal y sis) are pres ent on the xeno time sur face in prod ucts of the 400 MPa and 350°C run (Fig. 1D).
EXPERIMENTS WITH XENOTIME AND NA2SI2O5 + H2O FLUID
All ex per i ments with Na2Si2O5 + H2O re sulted in xeno time al ter ation and in the for ma tion of new phases (Fig. 2). The xeno time shows par tial dis so lu tion and some etch ing on the sur face (Fig. 2B, F, K, L). The xeno time in ex per i men tal prod - ucts shows no compositional vari a tions com pared to the start - ing NWFP xeno time (Ta ble 2). Del i cate crys tals of (Y,REE)-rich fluorapatite are pres ent on the xeno time sur face (Fig. 2D–G) or form in di vid ual, pris tine grains (Fig. 2O) in all ex per i men tal P-T ranges. (Y,REE)-rich fluorapatite shows high vari a tions of REE con tent (Ap pen dix 4), i.e. 21.97–22.16 wt.% (Y+REE)2O3 in X12N-05 (200 MPa, 350°C) vs. 6.42–12.85 wt.% (Y+REE)2O3
in X12N-15 (400 MPa, 350°C). These cor re spond to Ca and P vari a tions of 32.86-37.81 wt.% CaO and 27.71-28.16 wt.%
P2O5 (X12N-05), and 41.55–50.61 wt.% CaO and The stability of xenotime in high Ca and Ca-Na systems, under experimental conditions of 250–350°C and 200–400 MPa: ... 319
* Supplementary data associated with this article can be found, in the online version, at doi: 10.7306/gq.1223 Fig. 1. Experimental products from runs with 2M Ca(OH)2 fluid (BSE images)
A – xenotime, K-feld spar, bi o tite, and mus co vite show ing no signs of al ter ations in run at 200 MPa and 250°C; B – wollastonite formed in a run at 200 MPa and 250°C; C – unaltered sil i cates in prod ucts from runs at 400 MPa and 350°C; D – delicate, nee dle-like crys tals of Y-bear ing fluorapatite, formed on the xeno time sur face in an ex per i ment run at 400 MPa and 350°C; mineral names ab bre vi a tions: Bt – bi o tite, Grt – gar net, Kfs – K-feld spar, Ms – mus co vite, Qz – quartz, Xtm – xeno time, YAp – Y-bear ing fluorapatite
36.01–37.90 wt.% P2O5 (X12N-15). The (Y,REE)-rich fluorapatite is char ac ter ized by a high flu o rine con tent of 3.41–7.48 wt.% in X12N-05 and 3.26–6.12 wt.% in X12N-15.
The small size of (Y,REE)-rich fluorapatite from the X12N-04 (200 MPa, 250°C) run pre vented elec tron microprobe anal y ses.
Be side (Y,REE)-rich fluorapatite, an un named (Y,HREE)-rich sil i cate formed in all runs with Na2Si2O5 + H2O.
The un named (Y,HREE)-rich sil i cate forms crys tals with sizes of a few micrometres across and sev eral to 50 µm long in 200 MPa ex per i ments (250–350°C; X12N-04, X12N-05;
Fig. 2H, I), tetragonal grains reach ing 200 µm-across in the 400 MPa and 350°C run (X12N-15; Fig. 2N), or par tially re places xeno time (Figs. 2C, L, M, 3). The chem i cal com po si tion also var ies be tween ex per i ments. The larg est dif fer ences are no - ticed in Si con tent (Ap pen dix 4), i.e. 67.05–73.68 wt.% SiO2
(200 MPa, 250°C), 48.62–49.37 wt.% SiO2 (200 MPa, 350°C) and 58.86–66.67 wt.% SiO2 (400 MPa, 350°C), and Y con cen - tra tions 11.17–12.98 wt.% Y2O3 (200 MPa, 250°C), 20.20–22.00 wt.% Y2O3 (200 MPa, 350°C), and 11.08–12.09 wt.% Y2O3 (400 MPa, 350°C). The el e vated Ca con tent of 1.70–2.42 wt.% CaO was no ticed at 200 MPa and 350°C, vs. lower 0.08-0.37 wt.% CaO at 200 MPa, 250°C, and 0.21–0.54 wt.% CaO at 400 MPa, 350°C. The un named (Y,HREE)-rich sil i cate has rel a tively low con cen tra tions of Th and U, i.e. <0.03–0.11 wt.% ThO2, 0.04–1.49 wt.% UO2
(200 MPa, 250°C), <0.03 wt.% ThO2 and <0.04 wt.% UO2
(200 MPa, 350°C) and <0.03 wt.% ThO2, 0.39–2.11 wt.% UO2
(400 MPa, 350°C). There were no flu o rine or chlo rine sub sti - tutes in the formed un named (Y,HREE)-rich sil i cate.
In all runs, K-feld spar was par tially re placed by al bite due to the albitisation pro cess (Fig. 2J, M and Ap pen dix 1). Del i cate, nee dle-like crys tals of Na, Fe, Mg, and Al-rich am phi bole (iden - ti fi ca tion based on EDS anal y ses) formed in all runs (Fig. 2B, O). The small size of am phi bole pre vented elec tron microprobe an a lyzes. The start ing al bite, gar net, and micas were not al - tered (Ap pen di ces 1–3).
DISCUSSION
The ex per i ments show sig nif i cant dif fer ences in xeno time sta bil ity, de pend ing on fluid com po si tion. The ex per i ments with 2M Ca(OH)2 were prom is ing re gard ing the al ter ation of xeno - time and the for ma tion of sec ond ary fluorapatite. Pre vi ous ex - per i ments, un der con di tions of 450°C and 590 MPa de creas ing to 540 MPa over 16 days, and with start ing ma te ri als of xeno - time + al bite + K-feld spar + bi o tite + mus co vite + SiO2 + CaF2 + 2M Ca(OH)2, re sulted in par tial dis so lu tion of the xeno time grain sur faces, and in the for ma tion of Y-rich britholite and fluorapatite (Budzyñ and Harlov, 2011). Fur ther more, re cent ex per i ments con strain ing the sta bil ity of xeno time, us ing the same start ing ma te ri als as used in this work, re ported xeno time al ter ation and the for ma tion of (Y,HREE)-rich fluorapatite or (Y,HREE)-rich britholite in the pres ence of 2M Ca(OH)2 fluid in the wide P-T range of 200-1000 MPa and 450–750°C, as well as the for ma tion of (Y,HREE)-rich epidote at 800–1000 MPa and 650°C (Budzyñ et al., 2014). Here, the ex per i ments show that xeno time re mains sta ble in the pres ence of al ka line fluid with high-Ca ac tiv ity un der ex per i men tal P-T con di tions of 200 MPa and 250-350°C, rep re sent ing the low est P-T con di tions of the meta mor phism of gra nitic rock. How ever, the pres ence of (Y,HREE)-rich fluorapatite in di cates that lim ited al ter ation of xeno time may oc cur un der con di tions of 400 MPa and 350°C.
In na ture, pro gres sive meta mor phism may re sult in the grad ual pres ence of xeno time. In lower greenschist to up per am phi bo lite fa cies metasedimentary rocks from the Paleoproterozoic Mount Bar ren Group (south west ern Aus tra - lia), de tri tal xeno time is sta ble be low 450°C, dis ap pears dur ing el pmaST ]C°[P [aPM]noi ta ruD )syad(nP2O5OiS2OhT2OU2Y2O3rP2O3dN2O3mS2O3uE2O3dG2O3bT2O3yD2O3oH2O3rE2O3mT2O3bY2O3uL2O3OS3la toT emi tonex gn itratS8365.5372.090.011.034.0461.041.066.084.006.340.100.843.127.417.044.315.010.0<72.101 02.062.070.080.0 37.020.080.040.050.024.021.065.040.090.030.003.050.0 40-C21X052002045126.5312.090.001.026.0461.031.086.025.056.330.157.782.125.476.072.325.020.0 58.001 82.070.060.070.0 48.010.080.030.050.084.011.024.060.071.040.052.040.0 20.0 50-C21X053002047 84.5322.080.001.019.9361.031.056.045.016.310.117.792.116.496.084.335.010.0<91.001 23.090.060.070.0 90.120.090.030.050.036.041.084.050.012.050.005.060.0 51-C21X053004023126.4342.011.031.081.9361.071.066.015.005.399.056.713.146.417.026.335.010.0<17.89 52.080.050.060.0 56.010.050.050.060.066.021.094.030.041.050.024.050.0 40-N21X052002049 73.5342.001.031.021.0451.051.046.005.074.300.197.743.117.417.014.315.020.0 43.001 63.021.070.001.0 45.020.040.020.060.015.041.097.060.090.030.072.020.0 20.0 50-N21X053002048 44.5362.021.041.045.0451.071.056.094.022.359.025.703.186.427.006.375.010.0<15.001 82.070.030.040.0 36.020.030.030.040.023.080.025.040.091.050.093.040.0 51-N21X053004029 16.4302.080.090.049.8351.071.056.005.026.360.161.853.156.417.063.394.010.0<08.89 32.060.040.050.0 82.020.020.030.040.092.080.093.040.070.020.052.040.0 nor tcele fo timilnoi tce ted wo leb stne no pmoc fo seulav ;noi t ai ved dra dnats – cilati ,%.tw ni nevig era se ulav llAeborporcimsA ;el batehtni detne serp ton era 2O5lA ,60.0 < 2O3aL ,01.0 < 2O3,60.0 < eC2O3 ,80.0 < OeF ,10.0 < OaC ,10.0 < OrS ,20.0 < ObP30.0 <
2el baT nor tcele eht fo stlu sereg ar evAeborporcimfo sesylana eht emi tonex
The stability of xenotime in high Ca and Ca-Na systems, under experimental conditions of 250–350°C and 200–400 MPa: ... 321
Fig. 2. Experimental products from runs with Na2Si2O5 + H2O fluid (BSE images)
A – unaltered minerals from an experiment at 200 MPa and 250°C; B – xenotime grains showing partial dissolution on the surface and delicate crystals of amphibole from runs at 200 MPa and 250°C; C – the unnamed (Y,HREE)-rich silicate replacing xenotime (200 MPa, 250°C); D–G – small crystals of (Y,REE)-rich fluorapatite formed on the xenotime surface in runs at 200 MPa and 250°C (D), and 200 MPa and 350°C (E–G); H, I – grains of the unnamed (Y,HREE)-rich silicate formed during an experiment at 200 MPa and 350°C; J – grain of K-feldspar with pseudomorphic replacement by albite along the rim due to an albitisation process; K – etched surface of the xenotime (400 MPa, 350°C); L – grains of the unnamed (Y,HREE)-rich silicate filling cracks of the xenotime (400 MPa, 350°C); M – xenotime partially replaced by the unnamed (Y,HREE)-rich silicate with the (Y,REE)-rich fluorapatite formed, and K-feldspar partially replaced by albite due to albitisation; N – large grains of the unnamed (Y,HREE)-rich silicate from 400 MPa and 350°C run; O – the (Y,REE)-rich fluorapatite and needle-like crystals of amphibole formed at 400 MPa and 350°C conditions; mineral names abbreviations: Ab – albite, Amph – amphibole, YSi – (Y,HREE)-rich silicate; other explanations as in Figure 1
the pro gres sive meta mor phism of pelites un der mid-greenschist to am phi bo lite fa cies, and is pres ent again un - der con di tions of 650°C and ca. 8 kbar (Ras mus sen et al., 2011). The growth of meta mor phic xeno time was also doc u - mented on de tri tal xeno time un der tem per a ture con di tions of 450°C in quartz-mus co vite-chlorite schist (Ras mus sen et al., 2011). The overgrowths were in ter preted as re sult ing from compositional al ter ations or from the re place ment of de tri tal xeno time by meta mor phic xeno time, due to a fluid-me di ated cou pled dis so lu tion-reprecipitation pro cess (Ras mus sen et al., 2011). The grad ual pres ence of xeno time was also re ported in Al pine metapelites. The tem per a ture con di tions of xeno time break down and the for ma tion of sec ond ary HREE-rich epidote and ap a tite were con strained to 450–528°C, with the dis ap - pear ance of xeno time and co-ex ist ing monazite re placed by al - la nite or epidote in the bi o tite zone (Janots et al., 2008). The re - versed re ac tions, i.e. al la nite break down and the for ma tion of monazite and xeno time, oc curred at tem per a tures from 560 to 610°C, de pend ing on the bulk CaO con tent and Ca/Na ra tios (Janots et al., 2008). Ac cord ing to these works, xeno time is sta - ble un der tem per a ture con di tions be low 450°C. The xeno time al ter ations re lated to fluid-me di ated over print were also re -
ported in S-type gran ites af fected by greenschist- to am phi bo - lite fa cies meta mor phism (Broska et al., 2005). The for ma tion of sec ond ary Y-rich ap a tite and Y-rich epidote rims around xeno - time were doc u mented in meta mor phosed S-type gran ites, how ever, in low-Ca gran ites, the al ter ation of xeno time re sulted in the for ma tion of Y-en riched epidote, but not ap a tite (Broska et al., 2005). Our ex per i men tal setup had the bulk CaO con tent sig nif i cantly in creased in re la tion to that of the nat u ral rocks due to lab ra dor ite, and high amounts of CaF2 and 2M Ca(OH)2 were used in ex cess to in crease re ac tion rates. De spite the high avail abil ity of Ca, no epidote formed and lim ited for ma tion of (Y,REE)-rich fluorapatite oc curred only at 400 MPa and 350°C.
The ex per i men tal re sults are con sis tent with ob ser va tions in na ture, show ing that xeno time re mains sta ble in a high-Ca sys - tem un der con di tions of 200–400 MPa and 250-350°C, with lim - ited al ter ation at 400 MPa and 350°C.
There are lim ited ex per i men tal data on the sta bil ity of xeno - time in the pres ence of the al kali fluid used in this work. Pre vi - ous ex per i ments, uti liz ing a sim ple sys tem of xeno time and Na2Si2O5 + H2O fluid, per formed un der high-grade con di tions of 1000 MPa and 900°C, doc u mented the dis so lu tion of xeno time grain edges (Hether ing ton et al., 2010). An other study used a Fig. 3. BSE image and compositional WDS X-ray maps of the xenotime (Xtm) partially replaced by the unnamed (Y,HREE)-rich
silicate (YSi) under conditions of 400 MPa and 350°C
Note that false enrichment of Y in grain boundaries of the xenotime and enrichment of Na and Ca in grain boundaries of the albite and CaF2, respectively, are related to relief of grains in a polished grain mount; explanations as in Figure 2
mix of xeno time + ThO2 + SiO2 + Al2O3 and Na2Si2O5 + H2O fluid, re sult ing in ThSiO4 en rich ment along rims of xeno time grains un der con di tions of 500 MPa and 600°C, and 1000 MPa and 900°C (Harlov and Wirth, 2012). Be cause there was no source of other el e ments pres ent in the nat u ral rock sys tem, the al ter ation mech a nisms and for ma tion of sec ond ary phases were lim ited. In our study, the xeno time re acted in the pres ence of the al kali fluid and sil i cate min er als as sem blage, roughly rep - li cat ing gra nitic rock com po si tion. Fluid-aided al ter ations re - sulted in dis so lu tion of the xeno time sur face (Fig. 2B, F, K), with out af fect ing the com po si tion of in ter nal do mains, sim i larly to the pre vi ous ex per i ments from Hether ing ton et al. (2010).
How ever, dis solved xeno time sup plied P, Y, and HREE to form (Y,REE)-rich fluorapatite, while lab ra dor ite and CaF2 were a source of the re main ing Ca and F. It has to be noted that (Y,REE)-rich fluorapatite is not the main phase ac cu mu lat ing com po nent re leased from the xeno time. The al kali fluid with high F ac tiv ity pro moted the mo bil ity of Y and REE, in cor po - rated in the un named (Y,HREE)-rich sil i cate that crys tal lised ei - ther as in di vid ual grains to grain ag gre gates (Fig. 2H, I, N) or formed subhedral to anhedral grains re plac ing the xeno time (Figs. 2C, L, M and 3). In the lat ter case, the un named (Y,HREE)-rich sil i cate, pres ent along with the xeno time rims or in side xeno time grains, prob a bly formed due to fluid pen e tra tion along cracks in the xeno time grains (Figs. 2L, M and 3). The al - ter ation mech a nism is in ter preted as a pseudomorphic re place - ment, due to a fluid-me di ated cou pled dis so lu tion-repre - cipitation pro cess (cf. Putnis, 2002, 2009; Harlov et al., 2011).
There is also a dis tinct pat tern of Y and REE sub sti tu tion in the (Y,HREE)-rich sil i cate phase, show ing sim i lar con cen tra tions of Y and REE in runs at 200 MPa and 250°C to 400 MPa and 350°C, and sig nif i cant en rich ment in Y and REE with an iso - baric (200 MPa) tem per a ture in crease (250 to 350°C) from 11.08–12.98 to 20.20–21.27 wt.% Y2O3, and from 5.28–7.24 to 12.85–13.57 wt.% REE2O3. The com po si tion of the un named (Y,HREE)-rich sil i cates is ap par ently vari able and strongly de - pends on the P-T con di tions.
An an a logue of the (Y,HREE)-rich sil i cate phase in nat u ral ex am ples is dif fi cult to find. Gerenite-(Y), (Ca,Na)2(Y,REE)3Si6O18·2H2O, a rare min eral rec og nized in mag matic aplite-pegmatites from the Strange Lake peralkaline gra nitic pluton in north east ern Can ada (Jambor et al., 1998), con tains most of the cat ions as in the ex per i men tal prod ucts, how ever, its com po si tion is very dif fer ent. An un named (Y,HREE)-bear ing hy drated sil i cate as so ci ated with turkestanite, (Th,REE)(Ca,Na)2K1-xx[Si8O20]·nH2O, was de - scribed in peralkaline gran ites of the Morro Redondo Com plex in Brazil (Vilalva and Vlach, 2010). Both phases are re lated to pre cip i ta tion dur ing the post-mag matic stage, in the pres ence of HFSE-rich flu ids at tem per a ture con di tions of ca. 450°C (Vilalva and Vlach, 2010). The for ma tion of a phase with a sim i - lar com po si tion to turkestanite was ex per i men tally achieved dur ing the al ter ation of monazite in the pres ence of a sil i cate as - sem blage and Na2Si2O5 + H2O at 450-600 MPa and 450–500°C (Budzyñ et al., 2011). The lower P-T ex per i ments with monazite, sil i cates, and Na2Si2O5 + H2O fluid doc u mented the for ma tion of a phase with a com po si tion sim i lar to steacyite, Th(Na,Ca)2K1-xx[Si8O20], re plac ing monazite at 200–400 MPa and 250-350°C (Budzyñ et al., 2013, 2015). The for ma tion of fluorapatite in cor po rat ing Y and REEs re leased from ex per i - men tally-al tered xeno time is lim ited, most likely due to the high avail abil ity of Si in the bulk sys tem, pro mot ing the for ma tion of the un named (Y,HREE)-rich sil i cate in this study, or a Th- and LREE-bear ing sil i cate phase in monazite ex per i ments (Budzyñ et al., 2013, 2015).
Xeno time, sim i larly to monazite, serves as a Th-U-Pb geochronometer, char ac ter ized by a high clo sure tem per a ture.
The vol ume dif fu sion of Pb in xeno time is ex tremely slow and ex per i men tally con strained as slower than in zir con or monazite (Cherniak, 2006). The Pb loss in 40 µm grain at 800°C is es ti - mated to ca. 5% of to tal Pb over 100 Ma (Cherniak, 2006). Sim i - larly, the clo sure tem per a ture lim it ing the vol ume dif fu sion of Pb in monazite is re stricted to 800–900°C (Cherniak et al., 2004;
Gardes et al., 2006). How ever, the fluid-aided al ter ations may cause the dif fu sion of REE, Th, U, and Pb in monazite, re sult ing in Th-U-Pb age dis tur bance (Teufel and Hein rich, 1997;
Seydoux-Guillaume et al., 2002; Harlov et al., 2011; Budzyñ et al., 2013, 2015) or a re set ting of the Th-U-Pb ages (Wil liams et al., 2011). The mech a nism of al ter ations lead ing to compositional mod i fi ca tion in monazite is re lated to a fluid-aided cou pled dis so lu tion-reprecipitation pro cess. Al - though con cen tra tions of U, Th, and Pb in the NWFP xeno time were low, there were no no ta ble changes af ter the ex per i ments (Ta ble 2). There are no mod i fi ca tions of Y and REE con cen tra - tions that may in di cate al ter ation of the xeno time via a fluid-aided cou pled dis so lu tion-reprecipitation pro cess, as doc - u mented in the ex per i men tal stud ies on monazite, in clud ing re - cent ex per i ments us ing the same P-T con di tions and du ra tion (Budzyñ et al., 2013, 2015). Tak ing these into ac count, as well as the ex cess of CaF2 and fluid used to in crease re ac tion rates, it is un likely that a lon ger du ra tion of ex per i ments could sig nif i - cantly af fect the sta bil ity of the xeno time. Sum ma riz ing, the ex - per i ments in di cate that al though al kali fluid me di ates dis so lu tion and par tial re place ment of the xeno time by sec ond ary phases, the struc ture of xeno time pre vents fluid in fil tra tion from lead ing to compositional al ter ation of its in ter nal do mains.
CONCLUSIONS
The ex per i ments re gard ing the sta bil ity of xeno time, to - gether with cor re spond ing ex per i ments on monazite from Budzyñ et al. (2013, 2015), pro vide im por tant data on the par ti - tion ing of Y, REE, Th, and U be tween phos phates and sil i cates dur ing low-tem per a ture metasomatic pro cesses.
1. Xeno time is sta ble in the pres ence of al ka line fluid with high Ca ac tiv ity, show ing lim ited al ter ation re lated to the for ma - tion of Y-rich fluorapatite un der P-T con di tions of 400 MPa and 350°C.
2. The ex per i ments in di cate that the for ma tion of (Y,HREE)-rich sil i cates and (Y,REE)-rich fluorapatite can be re - lated to low-tem per a ture al ter ation of xeno time, me di ated by al - kali fluid with high Na ac tiv ity. A large amount of the un named (Y,HREE)-rich sil i cates formed in the ex per i ments in di cates a pref er en tial par ti tion ing of Y and HREE into sil i cates, rather than phos phates, dur ing low-tem per a ture metasomatic pro - cesses.
3. The xeno time in ter nal do mains are not af fected by compositional al ter ation in high Ca and Na-Ca sys tems, sug - gest ing that xeno time can serve as a sta ble geochronometer in metasomatically al tered rocks of gra nitic com po si tion.
Ac knowl edge ments. The anal y ses of the start ing min er als were fi nan cially sup ported by the Na tional Sci ence Cen tre re - search grant num ber 2011/01/D/ST10/04588. This work was fi - nan cially sup ported by the In sti tute of Geo log i cal Sci ences, Pol - ish Acad emy of Sci ences as the re search pro ject “EXP”. B.
Budzyñ thanks W. Hein rich and D.E. Harlov for the ac cess to the ex per i men tal lab o ra to ries of the Deut sche GeoForschungsZentrum, Potsdam, Ger many. P. Koneèný is thanked for his as sis tance with elec tron microprobe analyses.
The re views from I. Broska and R. Kryza, and ed i to rial han dling by T. Peryt are greatly ac knowl edged. M. Jastrzêbski is thanked for his com ments on an early ver sion of this ar ti cle.
The stability of xenotime in high Ca and Ca-Na systems, under experimental conditions of 250–350°C and 200–400 MPa: ... 323
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