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Lunar and Planetary Science XXVIII

1020.PDF

PARENT MAGMAS OF SNC HARZBURGITES: PHASE EQUILIBRIA MODELING

Alexei A. Ariskin, Vernadsky Institute of Geochemistry and Analytical Chemistry, Kosygin St. 19, Moscow 117975, Russia (ariskin@glas.apc.org)
The clan of SNC meteorites includes two igneous harzburgites - ALHA77005 and LEW88516, both having similar textures and mineralogy [1,2]. A distinctive feature of these rocks is the presence of large (up to few mm) low-Ca pyroxene oikocrysts (hereafter Opx) enclosing olivine (Ol) and chromite (Chr). Mineral assemblage in non-poikilitic areas is represented by maskelynite, Ol, Opx, high-Ca pyroxenes (Aug), Chr, ilmenite, and minor phosphates (±sulfides). Ol in ALH is almost uniform in composition (Fo70-73) [2-4], whereas olivines in LEW are more iron-rich ranging from Fo64 to Fo70 [1,4]. Pyroxene compositions display similar relations. Based on the textural and mineralogical data, the SNC harzburgites have been interpreted to be igneous cumulates solidified from primary magma mixtures of with cumulus crystals (Ol+Chr±Opx) intercumulus liquid [2,4]. Despite some doubts that the intercumulus minerals were crystallized as a closed system [5], few attempts have been done to estimate the parent magma compositions (PMC) for ALH using its bulk rock and mineral compositions [2,5], as well for LEW using the components from magmatic inclusions [1]. These attempts, however, did not result in an unambiguous solution, mainly because of the lack of constraints following from phase equilibria. Knowledge of the phase equilibria for the natural cumulates could allow one to estimate more correctly the range of trapped liquid compositions, and finally to make it narrower the range of parent magmas to be searched.

Calculating phase equilibria for SNC harzburgites. To accomplish the goal, we used the METEOMOD program designed for the calculations of melting-crystallization relationships in iron-rich meteoritic igneous systems [6]. The basic block of METEOMOD is a set of which empirically calibrated equations, describe equilibria between silicate melt and minerals such as Ol, Opx, Aug, and Pl in terms of temperature, fO2, and liquid compositions. The program calculates crystallization sequences step by step, as the total amount of crystals is increased. The accuracy of the calculations for temperature is of ±10-15°C, with the contents of major end-members in the minerals calculated within ±1-2 mol% [6].

T, C
1600 1500 1400 1300 1200 1100

LEW88516
Melt,% Fo 100 88 76 64 51 37 23
87.7 86.1 83.6

1600 1500 1400

Melt,% Fo 100 88 76 64 51
89.3 88.0 86.0 83.5

ALHA77005

80.5

80.0

En
76.4 76. 75. 73. 72. 71. 6 6 6 4 4

En
51.2 50.8 50.5

1300
37
75.9

En
76.3

72.5 69.6

1200
23
72.4

En
51.7

An
74.4

74.8 72.8 71.4

Ol

Opx

Au g

1100

Ol

Opx

Aug

Pl

Using METEOMOD we calculated the course of equilibrium crystallization for 2 melts corresponding to the bulk compositions of ALH [2] and LEW [4]. These calculations were carried out at 1 atm, with the crystal increment of 1%, at WM buffer, and in the range of melting 100
Fig.1. Modeled crystallization sequences


Lunar and Planetary Science XXVIII

1020.PDF

PARENT MAGMAS OF SNC HARZBURGITES: Ariskin, A.A
These compositions can be used to These constraints may include proportions estimate the PMC for ALH and LEW, if average or/and compositions of initial cumulus minerals. The first method, based on the modal compositions of their cumulus minerals were mineralogy analysis, results in too large similar to those which should be in equilibrium differences between the estimated Ol proportion with the trapped melt. The main problem of for ALH (45.8 wt% [2,5] and 60.2% [4]). The accurate defining the PMC along the calculated problem of mafic mineral reequilibration during a liquid lines of descent is the search for additional post-cumulus process was also widely discussed constraints following from primary cumulus in the literature. An alternative approach includes mineral-melt equilibria. the use of chromite core Table. Calculated mineral-melt compositions and phase compositions. proportions in terms of the temperature Constraints from spinel LEW88516 (bulk) ALHA77005 (bulk) Component, compositions. o o o o o wt% 1135.6 C 1226.2 C 1140.3 C 1229.6 C 1336.5 C The chromite cores SiO2 50.45 51.03 50.16 51.53 47.66 in SNC harzburgites were TiO2 1.07 0.67 1.94 1.30 0.99 found to contain small Al2O3 12.68 7.35 12.88 9.33 6.98 amounts of Al2O3, mostly in FeO 14.17 17.99 13.10 16.32 21.10 the range of 6-9 wt.% [3,4,8]. MnO 0.36 0.47 0.34 0.43 0.53 Experimental data obtained MgO 5.93 9.33 6.32 9.19 13.51 CaO 10.58 10.60 10.97 9.14 7.16 in eucrite/SNC igneous Na2O 2.28 1.23 2.15 1.43 1.07 systems indicate that such K2O 0.14 0.08 0.19 0.12 0.09 spinels could be crystallized P2O5 2.33 1.25 1.96 1.22 0.91 only from the melts 100.0 100.0 100.0 100.0 100.0 Total containing no more than 8 Mg/(Mg+Fe) 0.427 0.480 0.462 0.501 0.533 wt% Al2O3 [7,8]. In fact, Ca/(Ca+Al) 0.431 0.567 0.436 0.471 0.483 there is a strong dependence Calculated proportions of melts and minerals, wt% between the observed spinel Melt 21.0 39.1 21.2 34.1 45.5 and experimental glass Ol 54.4 53.9 64.6 65.6 54.5 compositions (Fig. 2), which Opx 15.4 7.0 12.1 0.3 could be used to predict Aug 9.2 0.9 alumina content in the Pl 1.1 trapped melt (PMC?), if an Calculated mineral compositions, mol% average composition of 69.1 73.0 72.2 74.8 78.5 Fo (Ol) chromite cores is known. For 69.3-6.6 74.8-3.5 71.4-6.7 76.3-3.1 En-Wo (Opx) example, if the Chr cores in 51.7-34.3 En-Wo (Aug) 50.4-33.5 ALH contain in average of 74 An (Pl) 16 about 7.5% Al2O3, one can Longhi&Pan,1989 (T=1240 oC) estimate the alumina content Bartels&Grove,1991 in its trapped melt to be of 7 wt.%. Such a melt o corresponds to the temperature of 1340 C (see 12 2 Table). More detail studies of Chr chemistry are x 1140 18 .0 necessary to estimate the PMC for LEW. 0 1180 x-

Al2O3 in melt, wt%

8

34 .9 +0 72 .1 =0 y

4

Chromite cores in ALHA77005

AAAAAAA AAAAAAA AAAAAAA

Al2O3 in Chr, wt%
16 20 24

0 0 4 8 12

Fig. 2. Experimental Chromite and Glass compositions

References. [1] Harvey et al. (1993) GCA 57, 4769-4783. [2] Lundberg et al. (1990) GCA 54, 2535-2547. [3] Ikeda Y. (1994) Proc. NIPR Symp. Antarct. Meteorites 7, 9-29. [4] Treiman et al. (1994) Meteoritics 29, 581-592. [5] McSween et al. (1988) LPSC 19, 766-767. [6] Ariskin et al. (1997) Meteoritics and Planet. Sci. 32 (in press). [7] Longhi J., Pan V. (1989) PLPSC 19, 451-464. [8] Bartels&Grove (1991) PLPSC 21, 351-365.