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Ferrous Freudenbergite in Ilmenite Megacrysts: a Unique Paragenesis from the Dalnaya Kimberlite, Yakutia

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Ferrous Freudenbergite in Ilmenite Megacrysts: a Unique Paragenesis from the Dalnaya Kimberlite, Yakutia American Mineralogist, Volume 82, pages 991±1000, 1997 Ferrous freudenbergite in ilmenite megacrysts: A unique paragenesis from the Dalnaya kimberlite, Yakutia ALLAN D. PATCHEN,1 LAWRENCE A. TAYLOR, 1 AND NIKOLAI POKHILENKO2 1Planetary Geosciences Institute, Department of Geological Sciences, The University of Tennessee, Knoxville, Tennessee 37996, U.S.A. 2Institute of Mineralogy and Petrography, Russian Academy of Science, Siberian Branch, Novosibirsk, Russia ABSTRACT A suite of picroilmenite megacrysts from the Dalnaya kimberlite, Siberia, was found to fall into one of two groups, the most abundant having 11±12 wt% MgO and 650±1500 ppm Nb, and the others having lower MgO (8.8±10.2 wt%) and higher Nb (1700±2700 ppm). Ferrous freudenbergite (Na2FeTi7O16) crystals were found included in many of the megacrysts from the ®rst group. The freudenbergite-bearing ilmenite megacrysts are also pervaded by micrometer-size spots that have elevated Al2O3 (.2 wt%), SiO2 (.0.4 wt%), and Na2O(.0.15 wt%) contents. The low Cr2O3 vs. Nb content of the second group may re¯ect clinopyroxene crystallization. This may be a factor in¯uencing the lack of freuden- bergite in these megacrysts. All ferrous freudenbergite samples studied previously are manifested as metasomatic reaction mantles replacing rutile. The freudenbergite from the Dalnaya kimberlite described in this paper occurs as small (max. 150 mm 3 40 mm), euhedral, prismatic inclusions in picroilmenite (11±12 wt% MgO) megacrysts, with no associated rutile. Minor-element (Cr, Al, and Mg) substitutions for Fe are more extensive than in previously studied freuden- bergite, with up to 1.4 wt% Cr2O3, 1.9 wt% Al2O3, and 3.1 wt% MgO. Nb is relatively low, typically less than 0.3 wt% Nb2O5 with a maximum of 1.1 wt%. Reaction of some of the freudenbergite with an alkalic ¯uid has resulted in thin, discontinuous rims and embayments of perovskite and an unidenti®ed hydrous calcium titanate, around most crys- tals. Rapid ascent from depth and shielding by ilmenite may have been contributing factors to the preservation of freudenbergite in these samples. The signi®cance of the euhedral nature of freudenbergite and the lack of any genetic relationship with rutile suggest that it crystallized by a process other than simple meta- somatic replacement of rutile. Indeed, the freudenbergite probably crystallized directly from a Na 1 Ti-rich ¯uid in®ltrating the ilmenite megacrysts. The several occurrences (Liberia, Bultfontein, and Dalnaya) of ferrous freudenbergite suggest that it may be more common in kimberlites than previously recognized. INTRODUCTION an alkali-rich ¯uid, metasomatically precipitating freu- The type locality for the rare mineral freudenbergite is denbergite ®rst (Haggerty 1983). A sodium titanate, sim- the apatite-rich Katzenbuckel (Odenwald, Germany) al- ilar to freudenbergite in composition, was also observed kali-syenite complex, where it occurs as a xenomorphic mantling rutile in a kimberlite dike in the west Ukikit accessory mineral associated with hematite (Frenzel ®eld, Yakutia (Oleinikov 1995). In the present study, the 1961; Frenzel et al. 1971). Freudenbergite, sensu stricto observed mineral occurs as small euhedral crystals em- Na Fe31 Ti O , has only been observed in this complex. bedded in picroilmenite megacrysts and, importantly, 2 2 6 16 with no associated rutile or hematite. Ferrous analogs of freudenbergite (Na2FeTi7O16) have been reported from xenoliths entrained in kimberlites. Frenzel (1961) originally reported freudenbergite as These were in lower crustal granulite xenoliths from Li- hexagonal Na2Fe2Ti7O18, but McKie (1963) and McKie beria (Haggerty 1983) and upper mantle zircon-bearing and Long (1970) corrected this to monoclinic xenoliths with lindsleyite-mathiasite from Bultfontein, Na2Fe2Ti6O16 on the the basis of observations of Wadsley South Africa (Haggerty and Gurney 1984; Haggerty (1964) that the X-ray data are similar to synthetic NaxTiO2 1991). In the Liberian samples, ferrous freudenbergite oc- ``bronze.'' Bayer and Hoffman (1965) demonstrated ex- curs as the inner-most phase of reaction mantles on rutile, tensive solid solution of the synthetic NaxTiO2 ``bronze,'' followed concentrically by ilmenite, perovskite, and resulting in a general formula ofABTixy 82y O16 , where A sphene. This assemblage suggests that rutile reacted with 5 Na, Rb, and K; B 5 Fe, Mg, Al, Cr, Mn, Zn, Ni, Co, 0003±004X/97/0910±0991$05.00 991 992 PATCHEN ET AL.: FREUDENBERGITE IN ILMENITE TABLE 1. Average electron microprobe analyses of Dalnaya ilmenite megacrysts Sample 315/84 251/84 367/79 370/84 114/84 393/84 377/84 331/84 243/84 4/84 304/84 n* 34 36 35 34 50 36 37 36 42 36 35 Nb2O5 0.09(2)² 0.10(2) 0.12(2) 0.16(2) 0.17(1) 0.22(2) 0.10(2) 0.16(3) 0.28(2) 0.25(3) 0.39(4) TiO2 52.2(2) 52.2(4) 53.1(4) 52.2(4) 52.0(2) 51.7(5) 52.3(4) 52.6(2) 50.5(2) 47.7(3) 48.5(3) ZrO2 0.03(1) ,0.03 0.03(1) 0.03(1) 0.04(1) 0.04(1) 0.03(1) 0.03(1) 0.05(1) 0.06(1) 0.08(1) Al2O3 0.67(9) 0.69(10) 0.59(6) 0.53(9) 0.58(5) 0.55(7) 0.66(9) 0.58(11) 0.53(4) 0.57(3) 0.49(7) Cr2O3 1.44(2) 1.53(4) 0.83(2) 0.61(1) 0.56(1) 0.52(1) 1.37(6) 0.65(1) 1.03(2) 1.03(4) 0.72(2) Fe2O3³ 7.63 7.46 7.07 7.86 7.94 8.49 7.81 7.32 9.47 13.9 12.5 FeO 25.7(2) 25.1(3) 25.7(9) 26.4(3) 27.2(2) 26.6(10) 24.5(4) 26.4(2) 27.2(1) 27.2(3) 27.9(2) MgO 11.7(1) 12.1(2) 12.2(6) 11.4(1) 10.9(2) 11.1(7) 12.5(3) 11.6(2) 10.2(1) 8.73(15) 8.83(12) MnO 0.23(1) 0.22(1) 0.25(3) 0.24(1) 0.27(1) 0.26(1) 0.23(1) 0.27(2) 0.28(1) 0.24(1) 0.30(2) NiO 0.15(1) 0.17(1) 0.14(1) 0.11(1) 0.11(1) 0.10(1) 0.16(1) 0.12(1) 0.11(1) 0.10(1) 0.07(1) Total 99.89 99.55 100.04 99.56 99.67 99.51 99.61 99.71 99.58 99.70 99.74 Cations on basis of three O atoms Nb 0.001 0.001 0.001 0.002 0.002 0.002 0.001 0.002 0.003 0.003 0.004 Ti 0.910 0.910 0.921 0.916 0.914 0.910 0.909 0.920 0.895 0.854 0.868 Zr 0.000 Ð 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.001 0.001 Al 0.018 0.019 0.016 0.015 0.016 0.015 0.018 0.016 0.015 0.016 0.014 Cr 0.026 0.028 0.015 0.011 0.011 0.010 0.025 0.012 0.019 0.019 0.014 Fe31 0.133 0.130 0.123 0.138 0.140 0.149 0.136 0.128 0.168 0.250 0.224 Fe21 0.499 0.486 0.496 0.515 0.531 0.520 0.473 0.514 0.535 0.542 0.556 Mg 0.406 0.418 0.420 0.396 0.380 0.387 0.431 0.402 0.358 0.310 0.313 Mn 0.005 0.005 0.005 0.005 0.005 0.005 0.004 0.005 0.006 0.005 0.006 Ni 0.003 0.003 0.003 0.002 0.002 0.002 0.003 0.002 0.002 0.002 0.001 Total 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 Ilm 51.4 50.2 50.7 52.6 54.1 53.0 48.7 52.4 54.8 55.5 56.7 Gk 41.8 43.1 43.0 40.4 38.7 39.4 44.3 41.0 36.6 31.7 31.9 Hem 6.9 6.7 6.3 7.0 7.1 7.6 7.0 6.5 8.6 12.8 11.4 * n 5 Number of analyses in average. ² Number in parentheses is 1 s standard deviation in terms of the least unit cited. ³Fe2O3 calculated assuming ilmenite stoichiometry. Cu, and Nb. Haggerty (1983) further simpli®ed the for- ished for electron microprobe analysis. Freudenbergite mula to A2B8O16, where A represents a large cation and was identi®ed in six of these samples. One sample (315/ B represents a smaller cation. 84) was also mounted as an intact megacryst. Chemical Experiments by Flower (1974) in the system Na2O- analyses were performed with a CAMECA SX50 electron Fe2O3-Al2O3-TiO2-SiO2 at 1000 bars pressure found that microprobe in the Department of Geological Sciences at freudenbergite is stable over a range of temperatures the University of Tennessee. Digital backscattered elec- (700±900 8C) and bulk compositions, at the Mn2O3- tron (BSE) images and X-ray maps were acquired using Mn3O4 fO2 buffer. Although these conditions are consistent an Oxford Instruments eXL-II Energy Dispersive Spec- with the Katzenbuckel association, the freudenbergite ob- trometer (EDS) interfaced to the CAMECA SX50. Op- served in kimberlite xenoliths (Haggerty 1983; this study) erating conditions for chemical analyses were 15 keV ac- probably formed atfO2 closer to FMQ or WM.
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