The chemical composition of REE-Y-Th-U-rich accessory minerals in peraluminous granites of the Erzgebirge-Fichtelgebirge region, Germany; Part II, Xenotime

Abstract

Xenotime, from a geochemically heterogeneous series of mildly to strongly peraluminous granites of the Erzgebirge, Germany, displays extended compositional variability with respect to abundances of HREE, Y, U, and Th. With few exceptions, the maximum and minimum concentrations for the lanthanides and actinides exceed those noted for this mineral from other geologic environments. Xenotime chemistry is dominated by the theoretical end-members HREE-PO 4 and YPO 4 (Ͼ90 mol% in total). Typical xenotime grains from the Erzgebirge granites and from other granites worldwide contain 70-80 mol% YPO 4 , and 16-25 mol% HREE-PO 4. This study documents the occurrence of xenotime, abnormally rich in HREE in place of Y, with up to 45 mol% HREE-PO 4. Substitutions of thorite-coffinite, (Th,U,Pb)SiO 4 , brabantite, (Ca,Th,U)(PO 4) 2 , and monazite, (La-Sm)PO 4 , are of minor importance, reaching maximum levels of 5-6 mol% each with a total contribution typically K10 mol%. Typically, the incorporation of actinides in xenotime from these and other granites, as well as from other rocks, is dominated by thorite-coffinite substitutions. However, in some xenotime grains, the U and Th concentrations are largely accounted for by the substitution mechanism 2 (REE,Y) 3ϩ ↔ (Th,U) 4ϩ ϩ Ca 2ϩ. The total lanthanide and actinide contents in xenotime and host granite are not strongly correlated. However, formation of xenotime unusually rich in HREE occurs only in the A-type Limica granites that are strongly enriched in these elements. The shapes of HREE patterns of xenotime and host rock are similar which, combined with mass-balance calculations, indicate the importance of this mineral in affecting the HREE as well as Y evolution of late-stage granitic melts. Late xenotime may show strong Y/Ho fractionation, thus recording the decoupling of Y and the HREE, which can occur in the latest stages of crystallization of evolved granites and which may be associated with deuteric alteration. Note: A/CNK ϭ molar Al 2 O 3 /(Na 2 O ϩ K 2 O ϩ CaO). * Lower values are due to post-magmatic U mobility.

REE and Y Mineralogy of the Krudum Granite Body (Saxothuringian Zone)

“…In the analyzed xenotime from high-F, high P 2 O 5 granites (iii, iv) both substitutions mechanisms exist. Both mechanisms were also found in the S-type, high-F, Li-mica granites from the German part of Krušné Hory/Erzgebirge area [32]. However, according to Pérez-Soba et al [7], unlike zircon, xenotime from highly fractionated peraluminous granites from the Belvís de Monroy pluton in the Iberian Variscan belt showed predominance of one substitution, the cheralite substitution, over the thorite-coffinite substitution.…”

“…Like for monazite, two main mechanisms exist for the replacement of Y by REE, U and Th in xenotime: charge balancing coupled substitutions involving Si and Ca (thorite-coffinite-type and cheralite-type substitutions respectively) [2,5,31,32]. In the analyzed xenotime from high-F, high P 2 O 5 granites (iii, iv) both substitutions mechanisms exist.…”

Corrigendum to: Phosphate minerals in granitic pegmatites from the Mount Wills District, north-eastern Victoria

“…The monazite-(Ce) crystals from the two pegmatites have unusually high Th and U contents and low U/Th ratios (Table 3) suggesting crystallisation from a highly fractionated magma with a pegmatitic origin (Gramaccioli & Segalstad 1978;Demartin et al 1991;Foerster 1998), rather than being restite or carried over from a parent granite. The high U contents (5-10 wt % U) exceed those both in the standards used and for which the CHIME method was designed, leading to concern over possible systematic errors.…”

REE-bearing minerals in Drava river sediments, Slovenia, and their potential origin

“…Therefore it presents a powerful tool for monazite-based radiometric dating (HaRRison et al, 1995;ZHu & o'nions, 1999;scHandl & GoRton, 2004;GRand'HoMMe et al, 2016). Xenotime, in contrast to monazite, beside Y contains also a significant amount of HREE, among which most commonly occurring are Dy, Yb, Er, and Gd while Tb, Ho, Tm, and Lu are occurring less often (FöRsteR, 1998b;ŠVecoVá et al, 2016;Voncken 2016). Depending on its geological genetic environment, xenotime is considered to be a primary source of HREE and actinides and it is a common accessory mineral in many non-basic igneous rocks, most granitic rocks and granitic pegmatites where it accounts for significant fraction of Y and HREE of bulk rock composition (bea, 1996;FöRsteR, 1998b), while it is also present in migmatites and high-grade metamorphic rocks (FöRsteR, 1998b).…”

“…Xenotime, in contrast to monazite, beside Y contains also a significant amount of HREE, among which most commonly occurring are Dy, Yb, Er, and Gd while Tb, Ho, Tm, and Lu are occurring less often (FöRsteR, 1998b;ŠVecoVá et al, 2016;Voncken 2016). Depending on its geological genetic environment, xenotime is considered to be a primary source of HREE and actinides and it is a common accessory mineral in many non-basic igneous rocks, most granitic rocks and granitic pegmatites where it accounts for significant fraction of Y and HREE of bulk rock composition (bea, 1996;FöRsteR, 1998b), while it is also present in migmatites and high-grade metamorphic rocks (FöRsteR, 1998b). Minerals such as monazite and xenotime are considered to be of economic importance especially when enriched in actinides (i.e., U and Th) for their use in nuclear industry, however they may pose an environmental hazard due to increased natural radiation (alaM et al, 1999;Vassas et al, 2006;Rao & MisRa, 2009).…”