rEOXHMHH, 2003, № 4, c. 355-365
PETROGENESIS OF PARENT MAGMAS OF SNC METEORITES © 2003 r. I. D. Ryabchikov* and H. Wanke**
*Institute for Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences,
Staromonetny pereulok 35, Moscow 119017, Russia
**Max-Planck-Institut fur Chemie, Abteilung Kosmochemie, Becher-Weg 27, D-55128 Mainz, Germany
Received October 31, 2002
Primary magmas for the investigated SNC meteorites have been estimated on the basis of the bulk compositions of "basaltic shergottites" and melt inclusion data for other SNCs. All estimated compositions are similar in terms of major elements. These compositions are high in Mg + Fe and Si mole fractions, and they are similar in this respect to terrestrial boninites.
These primary magma compositions could only have been produced by small fraction partial melting of oliv-ine-rich source, which could be related to Martian primitive mantle either by previous episodes of the extraction of basaltic melts, which formed very thick crust, or by formation of olivine-rich cumulates during the consolidation of primordial magma ocean, which originally had composition of Martial primitive mantle (similar to terrestrial pyrolite, but with significantly higher Fe/Mg ratio).
The estimated contents of heat producing elements in this olivine-rich source are very low, and they are likely to be not sufficient to provide energy for magma genesis, if the whole of Martian mantle consists now of such very depleted harzburgite.
The hypothesis is favored, which assumes primitive magma generation in the olivine-rich cumulate pile, originally formed during the consolidation of the primordial global magma ocean. The solidified layer of residual melt, which underlay olivine-rich cumulate pile, may be enriched in incompatible elements, including K, U and Th, whose radioactive decay might have provided heat for the initiation of plumes in which primary magmas of SNC meteorites originated at higher levels. This scenario is consistent with trace element and isotope data, which provide evidence that Martian mantle retains geochemical heterogeneities from the earliest stages of its history resulting from an early isolation of, and subsequent inefficient mixing between, mantle reservoirs in Mars.
INTRODUCTION
The SNC meteorites (called after Shergotty, Nakhla and Chassigny) differ from other differentiated meteorites in their isotopic and trace element characteristics. Their young radiometric ages first observed for Sher-gotty [1] were the first lines of evidence in support of their origin on a planetary-sized body, probably Mars [2, 3]. Later these young ages for the crystallization of SNC meteorites were confirmed by a number of methods [4]. The most convincing evidence for the origin of these rocks on Mars is provided by isotopic composition of trapped gases in shock-melted glass in shergot-tites, which was demonstrated to be indistinguishable from Martian atmosphere as measured by mass spectrometer on board of the Viking lander [5-7].
The recognition of shergottites as the fragments of Martian magmatic rocks permitted to estimate the bulk composition of the silicate part of Mars on the basis of the geochemical analyses of SNC meteorites using the general principles of element behavior in terrestrial magmatic systems and the regularities governing the composition of chondritic meteorites [8-12]. These estimates are also broadly consistent with the data provided by Earth-based and orbital spectra, and in situ spectral and chemical observations from the Viking and
Pathfinder landers [13, 14]. The estimated composition of Martian primitive mantle provided foundations for the interpretation of other geochemical and geophysical data emerging for Mars during the last few years.
It has been noted that the composition of the immediate mantle source(s) of the parental magmas for SNC meteorites must differ significantly from undifferentiated primitive lherzolites such as Wanke-Dreibus model composition [15-17], and it permits to assess the processes and scale of the global differentiation of Mars. The calculated compositions of the primary melts of SNC meteorites allow to estimate P-T conditions of their generation providing insight into the thermody-namic parameters of Martian interiors.
Previously such estimations were done for Shergotty and Zagami [17] - basaltic shergottites for which petrological and geochemical data were available since long ago. In this paper we include into consideration all the published data on the composition of intercumulus melts and melt inclusions in SNC meteorites. This has been done in order to test the possibility that the composition of mantle sources and parameters of magma genesis process may be different for various groups of SNC meteorites.
Table 1
1 2 3 4 5 6
SiO2 49.359 51.726 49.385 48.407 47.589 50.058
TiO2 0.757 0.525 0.678 0.665 1.103 0.694
AÍ2O3 6.247 5.974 5.183 5.408 6.085 6.347
FeO 18.657 17.281 18.464 19.638 19.861 18.806
MgO 15.739 14.573 15.577 16.565 16.755 15.864
CaO 7.722 7.488 8.960 7.649 5.925 7.220
Na2O 0.941 0.701 1.080 0.987 1.598 0.912
K2O 0.058 0.053 0.109 0.122 0.537 0.099
MnO 0.518 0.563 0.544 0.547
1—parent melt estimated as the matrix between xenocrysts of olivine, orthopyroxene and chromite in the lithology A of meteorite EETA 79001 [19] recalculated to equilibrium with olivine Fo82 (slightly more magnesian than in the original publication).
2—primary magma, calculated from the bulk composition of Dar al Gani 476 [24], assuming that it is in equilibrium with homogeneous grains of olivine Fo82.
3—primary magma, calculated from Zagami bulk composition [25], by fractional addition of equilibrium olivine, assuming its composition in mantle source as Fo82.
4—similar calculations for the bulk composition of Shergotty [8].
5—similar calculations for the melt, trapped as microinclusions in cumulus olivine in Chassigny [20].
6—similar calculations for the bulk composition of Dhofar 019 [23].
In all cases ^(Mg/Fe) for olivine—melt equilibria was assumed to be 0.33.
EVALUATION OF PRIMARY MAGMA COMPOSITIONS
All SNC meteorites consist of varying proportions of phenocrysts (or cumulus minerals) and products of crystallization of intergranular melt. Detailed petro-graphic investigations permitted to estimate compositions of intercumulus melts for a number of SNC meteorites [18, 19] partly based on the analyses of melt inclusions [20, 21]. For the melts, which are in equilibrium with olivine (or nearly in equilibrium), compositions of primary magmas may be approximately estimated by the fractional addition of equilibrium olivine until its Mg-number would be high enough to be compatible with the model composition of Martian mantle.
One of two lithologies of EETA 79001 contains zoned megacrysts of olivine and orthopyroxene (presumably disaggregated xenoliths of harzburgite) with Mg-numbers in the cores up to 84 [19]. The groundmass of lithology A, which has been suggested to reflect parental melt composition [19], corresponds to equilibrium with olivine Fo81 (assuming, that ^(Fe/Mg) = 0.33). This is higher than Mg-number of the estimated primitive Martian mantle [11], and therefore, this composition may be accepted as one of the Martian primary magmas (Table 1).
Dar al Gani 476 is a rock composed of olivine mega-crysts (about 24 vol. %) set in a fine-grained ground-mass of pyroxene and maskelynitized plagioclase. The most magnesian cores of olivine megacrysts are Fo79, and assuming, that originally homogeneous grains of this composition were immersed into equilibrium melt, we may estimate composition of this hypothetical melt by subtracting olivine from bulk composition, which
yields about 11 % MgO, 16 % FeO and 53 % SiO2. In this case, however, the calculated amount of olivine phenocrysts would be about 36 % which is much larger by comparison with the observed value. Assuming that originally olivine phenocrysts were more magnesian but later changed their composition due to the exchange with the residual melt or crystalline phases in intercumulus matrix and intracrystalline diffusion of divalent cations, the calculations would yield lower proportion of olivine phenocrysts. At olivine composition with Fo82 (similar to that which should be in equilibrium with the melt of matrix in EETA 79001, see above) this value would be 27 wt %, which is sufficiently close to the measured proportion of this phase. We, therefore, have chosen olivine of this composition to estimate parent magma for Dar al Gani 476 meteorite, which would contain 51.7 % SiO2 and 14.6 % MgO (see Table 1).
Shergotti and Zagami do not contain olivine among early minerals, but experiments with Zagami composition showed, that olivine (Fo75.8) appears at the liquidus together with pigeonite [22]. It implies, that the parent magma of this meteorite was close to olivine saturation. The same probably applies to Shergotty bulk composition and its estimated intercumulus melt, because these compositions are fairly similar to Zagami. Chassigny is a dunitic cumulate, containing Fo68 olivine, and, therefore, its parent melt assessed from melt inclusion studies [20] was undoubtedly saturated with respect to oli-vine. Shergottite Dhofar 019 also represents olivine saturated magma, because it contains zoned olivine grains with the cores approaching Fo60 [23]. We, therefore, may estimate primary melt compositions for these four meteorites using numerical procedure, which models
fractional addition of equilibrium olivine to the initial melt until olivine Mg-number becomes identical with that of assumed mantle source (reverse to fractional crystallization of olivine). For this we have assumed, that olivine in peridotitic source contains 82% of for-sterite (same as for Dar al Gani 476). We have also recalculated composition of the matrix in lithology A of EETA 79001 (see above), assuming equilibrium with olivine Fo82. The res
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