The tetrad effect and geochemistry of apatite from the Altay Koktokay No. 3 pegmatite, Xinjiang, China: implications for pegmatite petrogenesis

Abstract

In order to better constrain the evolution and petrogenesis of pegmatite, geochemical analysis was conducted on a suite of apatite crystals from the Altay Koktokay No. 3 pegmatite, Xinjiang, China and from the granitic and amphibolitic wall rocks. Apatite samples derived from pegmatite zones show convex tetrad effects in their REE patterns, extremely negative Eu anomalies and nonchondritic Y/Ho ratios. In contrast, chondritic Y/Ho ratios and convex tetrad effects are observed in the muscovite granite suggesting that different processes caused non-chondritic Y/Ho ratios and lanthanide tetrad effects. Based on the occurrence of convex tetrad effects in the host rocks and their associated minerals, we propose that the tetrad effects are likely produced from immiscible fluoride and silicate melts. This is in contrast to previous explanations of the tetrad effect; i.e. surface weathering, fractional crystallization and/or fluid-rock interaction. Additionally, we put forward that extreme negative Eu and non-chondritic Y/Ho in apatite are likely caused by the large amount of hydrothermal fluid exsolved from the pegmatite melts. Evolution of melt composition was found to be the primary cause of inter and intra-crystal major and trace element variations in apatite. Mn entering into apatite via substitution of Ca is supported by the positive correlation between CaO and MnO. Different evolution trends in apatite composition imply different crystallization environments between wall rocks and pegmatite zones. Based on the geochemistry of apatite samples, it is likely that there is a genetic relationship between the source of muscovite granite and the source of the pegmatite.

Sc- and REE-rich tourmaline replaced by Sc-rich REE-bearing epidote-group mineral from the mixed (NYF+LCT) Kracovice pegmatite (Moldanubian Zone, Czech Republic)

“…M-type tetrads are common in the minerals crystallized from residual silicate melt. In contrast, the exsolved F-rich hydrosaline liquids (according to Veksler et al 2005;Peretyazhko and Savina 2010;Wu et al 2011;Cao et al 2013) or F-rich aqueous fluid (according to Irber 1999;Dolejš and Štemprok 2001) could extract REE and acquire a W-type tetrad effect complementary to the pattern in the silicate melt.…”

“…5c) as well as the patterns of other REE-bearing minerals in the Kracovice pegmatite (unpublished data of authors) which crystallized either from viscous melt (garnet I, zircon), or from flux-rich liquid (TurP2, garnet II), or from melt-derived hydrothermal fluids (TurS), exhibit M-type tetrad effect. Tetrad effect is typical for highly evolved volatile-rich (e.g., F, H 2 O) melts at the final stages of crystallization when F-rich aqueous fluid phase is exsolved from the melt (Irber 1999;Dolejš and Štemprok 2001) or when an F-bearing hydrosaline magmatic liquids are separated from granitic magma above its solidus (Veksler et al 2005;Peretyazhko and Savina 2010;Wu et al 2011;Cao et al 2013). A possible factor contributing to the tetrad effect is a fluorine complexation (Irber 1999).…”

Lanthanide tetrad effect in limestone: a tool to environment analysis of the Ruteh Formation, NW Iran.

“…Convex shapes (M -like) in the normalized REE patterns are indicative of igneous and related systems such as hydrothermal activities, alteration and evolution processes (Irber, 1999;Monecke et al, 2007;Cao et al, 2013;Lv et al, 2018;Yang et al, 2018). In contrast, concave shapes (W-like) are mostly observed in low-temperature deposits such as marine sediments, limestones, cherts, phosphatic shales, and underground waters (Masuda et al, 1987;Akagi et al, 2004;Rossi et al, 2011;Feng et al, 2014;Abedini et al, 2017).…”

“…Despite their similar geochemical and CHARAC behavior, these elements show many irregular curves in their normalized distribution patterns have been reported from various depositional environments. This is related to non-CHARAC behaviors of REE such as lanthanides tetrad effect (Censi et al, 2007;Yasnygina and Rasskazov, 2008;Peretyazhko and Savina, 2010;Feng et al, 2011;Nardi et al, 2012;Cao et al, 2013;Lee et al, 2013;Abedini et al, 2017;Rezaei Azizi et al, 2017;Abedini et al, 2018). This phenomenon for the first time, labeled as 'tetrad effect', 'zigzag', 'kinked', and/or 'double-double' (Lee et al, 1994;Kawabe, 1995), reflects a unique characteristic of normalized REE distribution patterns which was reported for the first time in the aqueous extraction of rare earth elements studies in natural materials (Fidelis and Siekierski, 1966;Peppard et al, 1969).…”

REE Tetrad Effect as a Powerful Indicator of Formation Conditions of Karst Bauxites: A Case Study of the Shahindezh Deposit, NW Iran

“…as a new geochemical/mathematical-based tool to interpret irregular curves in normalized REE distribution patterns in a wide range of deposits (Kawabe, 1995;Lee et al, 1994;Irber, 1999;Jahn et al, 2001;Broska et al, 2006;Feng et al, 2011;Cunha et al, 2012;Nardi et al, 2012;Cao et al, 2013;Lee et al, 2013;Duc-Tin and Keppler, 2015;Abedini et al, 2017;Rezaei Azizi et al, 2017;Abedini et al, 2018a). Lee et al (1994) went on to suggest that the zigzag pattern of normalized REE patterns are indicative of an imperfect tetrad effect phenomenon in geochemical systems.…”