Lomonosov Moscow State University Calculation of changes in

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Lomonosov Moscow State University Calculation of changes in the Ta/Nb ratio in differentiates of

Lomonosov Moscow State University Calculation of changes in the Ta/Nb ratio in differentiates of granite melt based on experimental data Ya. O. Alferyeva, E. N. Gramenitskiy Corresponding author. E-mail adress: Yana. Alf@ya. ru (Ya. O. Alferyeva) 1

Based on experimental data (T = 650 -800 °C, P = 1 -2 kbar)

Based on experimental data (T = 650 -800 °C, P = 1 -2 kbar) on the solubility of columbite and tantalite in silicate melt and on the distribution of Ta and Nb among coexisting silicate melt, biotite, aqueous fluid, and aluminum fluoride melt, we calculated a possible change in the Ta/Nb indicator ratio in residual highly evolved granite melts. The Clarke values of these metals in acid rocks of the Earth’s crust were taken as their initial contents in the granite melt: Nb=2, 0∙ 10 -3, Ta=3, 5∙ 10 -4 wt. %, Ta/Nb =0, 18 2

We studied the following model systems: 1. Silicate melt and phases with Ta and

We studied the following model systems: 1. Silicate melt and phases with Ta and Nb impurities: 1. 1. melt and biotite, 1. 2. melt and aqueous fluid (only closed fluid-magmatic system), 1. 3. silicate melt and aluminum fluoride melt. 2. Silicate melt and essentially Ta–Nb ore phases within the tantalite–columbite series. 3

Formulas obtained in the [Alferyeva et al. , 2020] can be used to estimate

Formulas obtained in the [Alferyeva et al. , 2020] can be used to estimate the contents of various trace elements under interaction of various phases. The numerical values obtained below are examples of calculation by these formulas and are not universal. In this work we consider only normal and subalkalic acid rocks, plumasitic or having the mole ratio Al 2 O 3/(Na 2 O + K 2 O) = A/NK ≈ 1. Topaz- and/or cryolite-containing high-F granites are their most differentiated varieties. 4

1. 1. Silicate melt and biotite Consider a closed model system under conditions of

1. 1. Silicate melt and biotite Consider a closed model system under conditions of ideal fractional crystallization. Rock-forming minerals crystallize from aluminosilicate melt with initial Clarke contents of Ta and Nb. One of minerals (biotite) contains Nb and Ta impurities. The coefficients of Ta and Nb partition between the acid melt and biotite are Kp. Ta = 1. 9– 3. 6 and Kp. Nb = 8. 7– 9. 1 (Nash and Crecraft, 1985; Acosta. Vigil et al. , 2010). 5

b b b bb b Dependence of the metal content CMe. L in the

b b b bb b Dependence of the metal content CMe. L in the residual melt L on the amount of this melt ML (ML=1 system consists from melt, ML=0 system fully crystallised). The amount of biotite in sequential differentiates of rocks is constant (0, 5, 20 wt%). In the ultimate case, when biotite is absent (red line) Ta/Nb ratio remains constant. This is hypothetical case when Ta and Nb are not dispersed in crystalline or any other phases and accumulate in maximum possible contents in the residual melt. Lines based on experimental data of: а) Nash&Crecraft, 1985, b) Acosta-Vigil at all, 2010 6

Even 5 wt. % biotite contributes to a significant increase in Ta/Nb and deceleration

Even 5 wt. % biotite contributes to a significant increase in Ta/Nb and deceleration of the accumulation of Ta and Nb in the residual melt. A constant 20 wt. % content of Bt ensures rapid increase in Ta/Nb, but the dispersion of metals in the crystalline material is so high that the accumulation of Nb in the residual melt becomes impossible. 7

In most of natural intrusive granite complexes, the content of mafic minerals significantly decreases

In most of natural intrusive granite complexes, the content of mafic minerals significantly decreases from early to late intrusive phases. Below we qualitatively assess whether a decrease in the amount of biotite in successive portions of crystalline material can affect the above-obtained results. We accept that the amount of biotite in the elementary crystallization volume depends linearly on the mass of the residual melt ML and decreases from 5 or 20 wt. % in the first portions of crystalline substance to zero in the last ones. 8

Changes in Ta/Nb , Ta and Nb in the residual melt depending on the

Changes in Ta/Nb , Ta and Nb in the residual melt depending on the content of biotite. The initial content of biotite in the melt (wt. %): 0; 5; 20. The content of biotite linearly decreases to 0 wt. % in the final portions of melt. The curves were constructed based on experimental data: a, (Nash and Crecraft, 1985); b, (Acosta-Vigil et al. , 2010). With given parameters providing a gradual reduction in the “color index” of sequential differentiates, biotite can cause an increase in Ta/Nb at the initial stages of crystallization and prevents the accumulation of these metals in the residual melt not as seriously as in the first case. b b b 9

1. 2. Silicate melt and aqueous fluid (closed fluid-magmatic system) Consider a system in

1. 2. Silicate melt and aqueous fluid (closed fluid-magmatic system) Consider a system in which the Ta/Nb ratio can change only under interaction of silicate melt L and aqueous fluid fl. The primary melt has Clarke contents of Ta and Nb and is saturated with water. During crystallization of rock-forming minerals that do not contain Ta and Nb, the amount of the melt in the system decreases, and Ta, and Nb in residual melt increase. The values of partition coefficients Kp. Me=Cfl. Me/CLMe at equilibrium of silicate melt and aqueous fluid for F-containing high- and mediumplumasite granite systems were earlier obtained at 650– 750 °C and 1 kbar (Chevychelov et al. , 2005; Borodulin et al. , 2009): Кр. Ta = 0, 001 -0, 006; Кр. Nb = 0, 003 - 0, 01 10

Changes in Ta/Nb, Ta and Nb in the residual silicate melt under interaction of

Changes in Ta/Nb, Ta and Nb in the residual silicate melt under interaction of silicate melt L and aqueous fluid fl: 1) Curve for a fluid-free system. Coincides with curve of system with ideal fractionation of silicate melt and aqueous fluid (the each new elementary portion of the fluid is removed from the magmatic camera and does not interact with the system substance any longer). The water content in the initial melt is 15 wt. % 2) Equilibrium system (entire fluid produced during crystallization remains in the system and comes to equilibrium with each new composition of the melt). The solubility of fluid is 4 wt. %. 3) Equilibrium system. The solubility of fluid is 10 wt. %. 11

The interaction of silicate melt L and aqueous fluid fl within a closed system

The interaction of silicate melt L and aqueous fluid fl within a closed system has the least effect on the change in Ta/Nb among all the considered processes. As in case 1. 1, the formation of fluid somewhat retards the accumulation of Ta and Nb in the residual silicate melt. 12

1. 3. Silicate melt and aluminum fluoride melt Consider a system in which Ta

1. 3. Silicate melt and aluminum fluoride melt Consider a system in which Ta and Nb are distributed between silicate (L) and aluminofluoride (LF) melts. The Ta/Nb ratio can change only under interaction of silicate melt and aluminum fluoride melt. The primary melt has Clarke contents of Ta and Nb and F=0, 2 wt. %. The Kp. Me values were earlier determined at 800°C and 1 kbar (Gramenitskii and Shchekina, 1993; Gramenitskii et al. , 2005; Alferyeva et al. , 2011): 1) mole ratio in the melt Al 2 O 3/(Na 2 O + K 2 O)=A/NK=0, 95 Kp. Ta=0, 12, Kp. Nb=0, 73; 2) mole ratio in the melt Al 2 O 3/(Na 2 O + K 2 O)=A/NK=1, 1 Kp. Ta=0, 15, Kp. Nb=0, 64. 13

b Changes in Ta/Nb, Ta and Nb in the residual silicate melt under interaction

b Changes in Ta/Nb, Ta and Nb in the residual silicate melt under interaction of silicate melt and aluminum fluoride melt : 1) system without aluminum fluoride melt formation, 2) system with ideal fractionation of two melts, 3) equilibrium systems: a. A/NK = 0. 95; b. A/NK = 1. 1. b b 14

Aluminofluoride melt affects the distribution of metals stronger than an aqueous fluid. As in

Aluminofluoride melt affects the distribution of metals stronger than an aqueous fluid. As in systems 1. 1 and 1. 2, the formation of aluminofluoride melt affects negatively the accumulation of Ta and Nb in the residual silicate melt and leads to partial dispersion of these metals. 15

2. Silicate melt and essentially Ta–Nb ore phases within the tantalite–columbite series We consider

2. Silicate melt and essentially Ta–Nb ore phases within the tantalite–columbite series We consider crystallization of Ta–Nb ore minerals by the example of the isomorphous columbite (Col)– tantalite (Tan) series. Crystallization differentiation leads to an increase in the contents of Ta and Nb in the residual melt up to the solubility product (SP) of columbite–tantalite (Col –Tan), which is represented by a Fe-free end-member in this model and has general formula Mn. Ta 2 x. Nb 2(1– x)O 6 (0 ≤ x ≤ 1). The content of Mn. O in the silicate melt is taken to be constant and equal to 0. 2 wt. %. 16

SP = K N / A Averaged solubility products (SP) of minerals of the

SP = K N / A Averaged solubility products (SP) of minerals of the isomorphous columbite– tantalite series (based on data of Linnen and Keppler, 1997; Chevychelov et al. , 2010; Fiege et al. , 2011; Aseri et al. , 2015) depending on the molar Ta/(Ta+Nb) ratio in columbite. Dashed lines connect the SP values of the extreme endmembers, obtained at the same A/NK values of the melt: 0. 6, 0. 8, or 0. 9– 1. 1. Solid line is approximation of the SP values at A/NK = 1. 3– 1. 7 (triangular icons). 17

The coefficients of Ta and Nb distribution between the melt (L) and minerals of

The coefficients of Ta and Nb distribution between the melt (L) and minerals of the columbite –tantalite series (Col) were obtained from experimental data (Chevychelov et al. , 2010): 1) For a medium-plumasite (A/NK = 1. 3) system, KD = 1. 7, 2) For a high-plumasite (A/NK = 1. 7) system, KD = 6. 9. 18

The solubility product of columbite is achieved when the content of crystalline substance in

The solubility product of columbite is achieved when the content of crystalline substance in the system is about 99 wt. % 1) KD=6, 9, A/NK = 1, 7 2) KD=1, 7, A/NK = 1, 3. The difference in the distribution coefficients leads to different values of Ta/Nb in the residual melt of different compositions 19

Crystallization of minerals of the isomorphic columbite–tantalite series also increases Ta/Nb in the residual

Crystallization of minerals of the isomorphic columbite–tantalite series also increases Ta/Nb in the residual silicate melt. The achievement of the columbite–tantalite solubility product does not lead to dispersion of Ta and Nb. 20

Сonclusions • All the processes considered can lead to an increase in the Ta/Nb

Сonclusions • All the processes considered can lead to an increase in the Ta/Nb ratio in the residual silicate melt. • As a result of the interaction of the silicate melt with biotite, aqueous fluid, and salt melt, an increase in the Ta/Nb ratio is achieved due to the dispersion of these metals in the corresponding phases, which prevents their accumulation in the residual melt. • Crystallization of columbite is the most efficient (of the considered) Ta/Nb growth process, which does not lead to significant scattering of these metals. 21

REFERENCES • • • Acosta-Vigil, A. , Buick, I. , Hermann, J. , Cesare,

REFERENCES • • • Acosta-Vigil, A. , Buick, I. , Hermann, J. , Cesare, B. , Rubatto, D. , London, D. , Morgan, G. B. , 2010. Mechanisms of crustal anatexis: A geochemical study of partially melted metapelitic enclaves and host dacite, SE Spain. J. Petrol. 51, 785– 821. Alferyeva Ya. O. , E. N. Gramenitskiy, T. I. Shchekina Changes in the Ta/Nb Ratio in Successively Formed Differentiates of Granite Melt (Calculations Based on Experimental data)// Russian Geology and Geophysics. 2020. Vol. 61, № 1, P. 26– 35. DOI: 10. 15372/RGG 2019101 Alferyeva, Ya. O. , Gramenitskii, E. N. , Shchekina, T. I. 2011. Experimental study of phase relations in a lithium-bearing fluorinerich haplogranite and nepheline syenite system. Int. Geochem. 49 (7), 676– 690. Aseri, A. , Linnen, R. , Xu Dong Che, Thibault, Y. , Holtz, F. , 2015. Effects of fluorine on the solubilities of Nb, Ta, Zr and Hf minerals in highly fluxed water-saturated haplogranitic melts. Ore Geol. Rev. , No. 64, 736– 746. Borodulin, G. P. , Chevychelov, V. Yu. , Zaraysky, G. P. , 2009. Experimental study of partitioning of tantalum, niobium, manganese, and fluorine between aqueous fluoride fluid and granitic and alkaline melts. Dokl. Earth Sci. 427 (2), 868– 873 Chevychelov, V. Yu. , Zaraisky, G. P. , Borisovskii, S. E. , Borkov, D. A. , 2005. Effect of melt composition and temperature on the partitioning of Ta, Nb, Mn, and F between granitic (alkaline) melt and fluorinebearing aqueous fluid: fractionation of Ta and Nb and conditions of ore formation in rare-metal granites. Petrology 13 (4), 305– 321. Chevychelov, V. Yu. , Borodulin, G. P. , Zaraiskiy, G. P. , 2010. Solubility of columbite, (Mn, Fe)(Nb, Ta)2 O 6, in granitoid and alkaline melts at 650– 850 ºC and 30– 400 MPa: an experimental investigation. Geochem. Int. 48 (5), 456– 464. Fiege, A. , Kirchner, C. , Holtz, F. , Linnen, R. L. , Dziony, W. , 2011. Influence of fluorine on the solubility of manganotantalite (Mn. Ta 2 O 6) and manganocolumbite (Mn. Nb 2 O 6) in granitic melts—An experimental study. Lithos, No. 122, 165– 174. Gramenitskii, E. N. , Shchekina, T. I. , 1993. Phase relations in the liquidus part of the fluorine-containing granite system. Geokhimiya, No. 6, 821– 840. Gramenitskii, E. N. , Shchekina, T. I. , Devyatova, V. N. , 2005. Phase Relations in Fluorine-Containing Granite and Nepheline– Syenite Systems and Distribution of Elements between the Phases [in Russian]. GEOS, Moscow. Linnen, R. L. , Keppler, H. , 1997. Columbite solubility in granitic melts: consequences for the enrichment and fractionation of Nb and Ta in the Earth’s crust. Contrib. Mineral. Petrol. 128, 213– 227. Nash, W. , Crecraft, H. , 1985. Partition coefficients for trace elements in silicic magmas. Geochim. Cosmochim. Acta 49, 2309 – 2322. 22

Thank you for the attention 23

Thank you for the attention 23