The compositional variability of eudialyte-group minerals

Abstract

Eudialyte-group minerals (EGM) represent the most important index minerals of persodic agpaitic systems. Results are presented here of a combined EPMA, Mössbauer spectroscopy and LA-ICP-MS study and EGM which crystallized in various fractionation stages from different parental melts and mineral assemblages in silica over- and undersaturated systems are compared. Compositional variability is closely related to texture, allowing for reconstruction of locally acting magmatic to hydrothermal processes. Early-magmatic EGM are invariably dominated by Fe whereas hydrothermal EGM can be virtually Fe-free and form pure Mn end-members. Hence the Mn/Fe ratio is the most suitable fractionation indicator, although crystal chemistry effects and co-crystallizing phases play a secondary role in the incorporation of Fe and Mn into EGM. Mössbauer spectroscopy of EGM from three selected occurrences indicates the Fe3+/ΣFe ratio to be governed by the hydration state of EGM rather than by the oxygen fugacity of the coexisting melt. Negative Eu anomalies are restricted to EGM that crystallized from alkali basaltic parental melts while EGM from nephelinitic parental melts invariably lack negative Eu anomalies. Even after extensive differentiation intervals, EGM reflect properties of their respective parental melts and the fractionation of plagioclase and other minerals such as Fe-Ti oxides, amphibole and sulphides.

Eudialyte-group minerals from the Monte de Trigo alkaline suite, Brazil: composition and petrological implications

“…These differences are also found in the bulk composition of the EGM-bearing nepheline microsyenite dyke (Table 1), which suggests that EGMs crystallized in an environment where Eu, Sr, and Ba were already depleted. This depletion could be assigned to previous extensive fractionation of plagioclase and/or alkali feldspar (e.g., Blundy & Wood 1991, Bédard 2006, Brotzu et al 2007, Henderson & Pierozynski 2012, Carvalho & Janasi 2012) from the parental melt, similar to the interpretation by Schilling et al (2011b) regarding the same features observed in the EGMs from Ilímaussaq and Mont Saint-Hilaire.…”

“…Most of the trace element contents of the EGMs from Monte de Trigo vary within the ranges of EGMs from other localities (Wu et al 2010, Schilling et al 2011b, especially the HFSEs and REE. Major differences of the analyzed EGMs include low-Sr and Ba contents and a negative Eu/ Eu* ratio (Fig.…”

“…The EGMs from Monte de Trigo are distinguished by relatively high contents of Ca, moderate enrichment of Nb and REE, and a somewhat low content of Na when compared with the compositions from other localities worldwide (e.g., Deer et al 1986, Johnsen & Gault 1997, Johnsen & Grice 1999, Wu et al 2010, Schilling et al 2011b). The EGMs from Monte de Trigo do have Na, Nb, Ca, and REE contents similar to Coulson & Chambers (1996), Gualda & Vlach (1996), Coulson (1997), Johnsen & Gault (1997), Olivio & Williams-Jones (1999), Johnsen & Grice (1999), Mitchell & Liferovich (2006), Marks et al (2008), Schilling et al (2009Schilling et al ( , 2011b, and Wu et al (2010).…”

“…Significant amounts of other elements, such as Fe, Mn, Nb, rare earth elements (REE), Sr, and Cl are reported in EGM compositions (Johnsen & Gault 1997, Johnsen & Grice 1999, Schilling et al 2011b). According to the International Mineralogical Association (Johnsen et al 2003), the general formula of the EGMs is Na 12 [N(4) 4 X 2 , in which the common elements assigned to each site are as it follows: ■ Si at M(4); ■ Nb, Si,Ti,and W at M(3); ■ Fe, Mn, Na, Zr, K, and H 3 O at M(2); ■ Ca, Mn, and Fe at M(1); ■ Na, REE,Sr,K,Ca,Mn,and H 3 O at N(4); ■ Cl, OH, F, and CO 3 at X.…”

“…Since then, EGMs have been recognized in nearly all occurrences of agpaitic rocks, i.e., peralkaline nepheline syenites (and phonolites) that contain complex silicates of Zr, Ti, REE, F, and other volatiles (Sørensen 1997, Le Maitre 2002. EGMs have been described in agpaitic rocks from Khibiny and Lovozero, Kola Peninsula (Khomyakov 1995, Johnsen & Gault 1997, Schilling et al 2011b); Mont Saint-Hilaire, Canada (Johnsen & Gault 1997, Johnsen & Grice 1999, Schilling et al 2011b); Langesundsfjord, Norway (Andersen et al 2010, Schilling et al 2011b); Tamazeght, Morocco (Schilling et al 2009, Schilling et al 2011b); Poços de Caldas, Brazil (Johnsen & Gault 1997, Ulbrich & Ulbrich 2000, Ulbrich et al 2005, Schilling et al 2011b); Cerro Boggiani, Paraguay (Gomes et al 1996, Carbonin et al 2005; Pilanesberg, South Africa (Mitchell & Liferovich, 2006, Schilling et al 2011a); Sushina Hill, India (Mitchell & Chakrabarty, 2012); Agua de Pau volcano, Azores, Portugal (Ridolfi et al 2003); and Kilombe volcano, Kenya Rift Valley (Ridolfi et al 2006). The typical minerals associated with EGMs include alkali feldspar, nepheline, sodalite, aegirine, arfvedsonite, and several complex Zr-, Nb-, Ti-, and REE-bearing silicates, such as astrophyllite, lamprophyllite, rosenbuschite, rinkite, and wöhlerite (e.g., Sørensen 1997, Andersen et al 2010, Marks et al 2011, Schilling et al 2011b).…”