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Home , IIE iron meteorite, Merrillite

44th Lunar and Planetary Science Conference (2013) 1342.pdf

FLUORINE-BEARING FROM A SILICATE INCLUSION OF THE ELGA (IIE) IRON . M. A. Nazarov1, S. N. Teplyakova1, F. Brandstaetter2, Th. Ntaflos3; 1Vernadsky Institute of Geo- chemistry and Analytical Chemistry, Kosygin St. 19, Moscow 119991, Russia ([email protected]), 2Natural Histo- ry Museum, A-1010 Vienna, Austria, 3Institute for Lithospheric Research, University of Vienna, A-1090 Vienna, Austria.

Introduction: Merrillite has been reported from limits. The apatite formula is (Ca, lunar rocks [e.g., 1,2], SNC [3,4,5], some Fe,Mn,Mg,Na,K)10.03(P,Si,Al)6.02(O23.95F0.05)24(F,Cl)2.0 ordinary [e.g., 6], Angra dos Reis [7], and In transmitted and reflected light the F-bearing merril- the Springwater [8]. In general, the phase was lite is not distinguishable from apatite. The phases have considered as H-free with a crystal struc- practically the same refractive indexes and birefrin- ture of β-whitlockite [2,6,7,9]. Lunar merrillites are gence. However, in BSE images the merrillite is slight- high in REEs and poor in Na as compared to meteoritic ly darker than apatite. The Raman spectrum of F- ones which are very close to Ca18Mg2Na2P14O56. Mer- bearing merrillite (Fig. 2) shows a well resolved dou- rillites of SNC meteorites are appear to be F-bearing blet at 959 and 973 ∆cm-1. This doublet is the charac- [10,11] and could correspond to bobdownsite teristic feature of Raman spectra of terrestrial whitlock- (Ca18Mg2P14O54F2). Here we report on a new F-bearing ite [2], meteoritic Na-whitlockite, REE-poor synthetic phosphate found in a silicate inclusion of the Elga (IIE) merrillite [2], and merrillite in ALH84001 [13]. The . The phosphate associates with apatite doublet reflects two types of phosphate groups [2]. and is the main carrier for F in the inclusion. Hydrogen-bearing whitlockite and bobdownsite have -1 Results: The silicate inclusion (#2315-2a) sepa- an additional peak at 923 ∆cm . This peak that is at- 2- rated from the metal matrix has a size of 3x1.4 mm and tributed to the presence of (HPO4) groups [2] is not consists of (wt%) 68.2 glass and 29.7 pyroxenes (Fig. observed in the recorded spectrum (Fig. 2). Preliminary 1). Both clino- and orthopyroxenes are present but or- TEM studies indicate that the Elga phosphate has a thopyroxene (En75.1Wo2.6) is a minor phase. The clino- merrillite . pyroxene is augite (En51.2Wo37.8). Pyroxene grains are Discussion: Obviously, the F-bearing merrillite of present as euhedral prismatic crystals up to 50 x 200 the Elga inclusion differs from bobdownsite and is a μm in size. The glass composition (wt%) is: SiO2 70.0, new mineral of the whitlockite group. In the inclusion MgO 0.13, Na2O 4.90, Al2O3 15.8, K2O 6.82, CaO the phase is the main carrier for F whereas Cl occurs 0.30, TiO2 0.65, FeO 0.47, P2O5 0.23, Cr2O3 and MnO mainly in apatite. Based on the modal content of the <0.02. Accessories are phosphates, ilmenite and phases the bulk F and Cl concentrations of the inclu- . The phosphates are the F-bearing merrillite and sion are estimated to be 0.08 and 0.003 wt% respec- F,Cl-apatite. Their modes (wt%) are 1.64 and 0.54, tively. Such a high F content has never been reported in respectively. The bulk composition of the inclusion as silicate inclusions of IIE irons as well as in other mete- inferred from mineral chemistry and mineral modes is orites and lunar rocks. Concerning the F content, the (wt%): SiO2 63.7, TiO2 0.69, Al2O3 11.0, Cr2O3 0.38, inclusion is comparable with terrestrial crustal rocks FeO 2.33, MnO 0.11, MgO 5.32, CaO 6.57, Na2O (Fig. 3). However, the Cl concentration is not so en- 3.60, K2O 4.66, P2O5 1.12. The merrillite occurs as hanced (Fig. 3) and it is much lower than that of chon- needle-like euhedral crystals embedded into the glass drites and similar to that of SNCs, and lunar matrix (Fig. 1). They are up to 150 μm in length but rocks. usually < 20 μm in width. Rare apatite inclusions usu- The high F content is compatible with the rhyolitic ally have rounded outlines. The mean (8 analyses) mer- composition of the Elga inclusion. However, the inclu- rillite composition (wt%) is SiO2 0.33, Al2O3 0.11, FeO sion is relatively poor in refractory incompatible ele- 1.39, MnO 0.07, MgO 3.04, CaO 45.8, Na2O 2.72, ments, e.g., La, when compared to differentiated rocks K2O 0.14, P2O5 45.0, F 3.77. Chlorine, La and Ce are (Fig. 4). The F-La relationship suggests that the most below detection limits. The corresponding formula is probable source of the Elga inclusion could be E chon- Ca17.84(Mg,Fe,Mn)2.09(Na,K)1.98(P,Si,Al)14.01(O53.67F0.33) drites which are highest in F and poorest in La among 54F4. The atomic proportions are very similar to those chondritic meteorites. The inclusion can be derived of merrillite in which 2O is replaced by 4F, i.e., from the E source by partial melting at cer- Ca18Mg2Na2P14O54F4. The mean apatite composition tain oxidizing conditions. The Cl depletion of the in- (wt%) is: SiO2 0.64, Al2O3 0.17, FeO 0.54, MnO 0.08, clusion could be due to vaporization of the element MgO 0.05, CaO 54.2, Na2O 0.08, K2O 0.10, P2O5 40.9, from the melt because Cl is much more volatile as F 3.51, Cl 0.55. Lanthanum and Ce are below detection compared to F. Most differentiated extraterrestrial 44th Lunar and Planetary Science Conference (2013) 1342.pdf

rocks show a similar depletion in Cl relative to chon- drites (Fig. 3).

Fig 2. Raman spectrum of F-bearing merrillite.

1000 The Earth primitive mantle

Eucrites

SNC 100 Chondrites

Lunar rocks m

p The Earth crust

p 10

,

a Fig. 1. F-bearing merrillite (Mer) is represented by L well shaped crystals whereas apatite occurs as round-

ed inclusions. The phases associate with pyroxene 1 (gray) and are embedded in a glass matrix (dark grey). Elga EL EH

1000 EH 0.1

CI 10 100 1000 CO EL The Earth crust F, ppm CV CM Fig. 4. La vs F. In spite of the F enrichment the Elga LL 100 H inclusion is very poor in La (determined by Laser Ab- L Elga lation ICP-MS). Data are from [14-17]. Lunar rocks

m References: [1] Hughes J.M. et al. (2006) Am. Miner., p

p 10

, 91, 1547-1552. [2] Jolliff B.L. et al. (2006) Am. Mineral., l

C 91, 1583-1595. [3] Greenwood J.P. et al. (2003) GCA, 67, 2289-2298. [4] Wang A. et al. (2004) Am. Mineral., 89, 665- 680. [5] Lundberg L.L. et al. (1988) GCA, 52, 2147-2163. 1 The Earth primitive mantle [6] Prewitt C.T. & Rothbard D.R. (1975) LPS VI, 646-648. Eucrites [7] Dowty E. (1977) EPSL, 35, 347-351. [8] Davis A.M. & SNC Olsen E.J. (1991) Nature, 353, 637-640. [9] Gopal R. & Chondrites Calvo C. (1972) Nature Phys. Sci., 237, 30-32. 0.1 [10] Mikouchi T. et al. (2001) MAPS, 36, 531-548. 10 100 1000 F, ppm [11] Gnos E. et al. (2002) MAPS, 37, 835-854. [12] Chen Fig.3. Cl vs F. The Elga inclusion is very rich in F M. et al. (1995) LPS XXVI, 237-238. [13] Cooney T.F. et al. relative to chondrites and differentiated space rocks. (1999) Am. Mineral., 84, 1569-1576. [14] Treatise on Geo- Data are from [14-17]. chemistry (2003) v.1-3. [15] Wasson J. & Kallemeyn G.W. (1988) Phil.Trans.R.Soc.London, A325, 535-544. [16] Kitts Acknowledgments: This study was supported by K. and Lodders K. (1998) MAPS,v.33, A197-A213. [17] Austrian Academy of Sciences and the Program #22 of Lodders K. (1998) MAPS,v.33, A183-A190. the Russian Academy of Sciences.