Lecture held by Nóra Fáy on 11 July, 1997

on the XXIIth Symposium on Antarctic

Meteorites, NIPR, Tokyo

 

 

HIGH TITANIUM BASALTS IN THE SOLAR SYSTEM

 

Bérczi Sz.1, Brezsnyánszky K.2, Detre Cs.2, Ditrói-Puskás Z.1, Fáy Nóra3, Holba Agnes3, Józsa S.1, Kubovics I.1, Lukács B.3, Szakmány Gy.1, Tóth I.4

 

1 Eötvös Univ. Petrology Dept., H-1088 Budapest Múzeum krt. 4/a

2 Geological Institute of Hungary H-1143 Budapest Stefánia út 14.

3 CRIP RMKI H-1525 Bp. 114. Pf. 49., Budapest, Hungary

4 Konkoly Observatory, H-1525 Bp. 114. Pf. 67., Budapest, Hungary

 

ABSTRACT

            Reviewing the Ti/Si relations in the Solar System we concentrate on anomalous Ti enrichments. It seems that the enrichment was connected mainly with differentiations in residual liquids of upper mantle origin, especially on Earth, Luna and the unidentified angrite parent body. Here we compare data for chondrites, achondrites, lunar and terrestrial basalts on the basis of the NIPR Antarctic Meteorite Catalog, Apollo samples and Carpathian Basin geologic investigations.

 

THE PROBLEM

            The surprisingly high Ti content of lunar surfaces is an old problem in selenology. In some "blue" lunar basaltic layers the weight share of TiO2 is above 10 % while the standard Solar System abundance is smaller by 2 orders of magnitude. Even in Archean terrestrial basalts it is not above 1 % (Condie, 1981). So either Luna has been composed of a very special assemblage, or lunar volcanism was very special.

            It is rather difficult to choose until real in situ work does not become again possible. Still, the NIPR Catalog of Antarctic Meteorites (Yanai, Kojima & Haramura, 1995), henceforth the Catalog, is an excellent database to investigate this problem (appended, where necessary, with some terrestrial and lunar data), because it is the largest available cross section of the Solar System. By comparing the Solar System Ti data to lunar ones and by trying to find lunar analogons one may get some insight into the lunar Ti "anomaly". Henceforth no specific reference is given if the source is the Catalog.

 

COMPOSITIONAL DATA

            Chondritic data show that the primordial Ti/Si ratio is cca. 0.003. The highest chondritic ratio measured is cca. 0.0065, except for C chondrites where this value is the average. We have no explanation for this C chondrite speciality; maybe this is a condensational dependence on the primordial temperatures in the nebula.

            Now let us see the effects of thermal/gravitational transformation/differentiation. In one wide group of achondrites the average does not go above 0.003; this group contains mesosiderites, lodranites, aubrites and ureilites. The opposite group is that of "volcanic" achondrites. Let us see first the diogenite → howardite → eucrite sequence. There a gradual (Mg → Al,Ca) substitution is seen, which may be interpreted as nonequilibrium thermodynamic process when ascending mantle material crosses the thickening (Al,Ca)-rich crust of the parent body (Lukács & Bérczi, 1997) With substantial variations, the averages are as follows:

Rock

<Ti/Si>

Diogenites

0.005

Howardites

0.011

Eucrites

0.019

It seems as if the Ti enrichment were parallel with the (Mg → Al,Ca) substitution. Such a phenomenon gets a simple explanation if the viscosity and/or melting point of the lava, caeteris paribus, decreases with Ti content.

            But if this is true, similar trend should be seen on planetary basalts. Indeed, the Martian Ti/Si level, if calculable from the only 2 measured samples of the Catalog, is 0.017, near to eucrites and old and new terrestrial values.

            Still there are a few Solar System basalts well above the eucritic Ti level. Lunar "red" (Apollo 12, 14 & 15) basalts are at Ti/Si ~ 0.1, and "blue" ones (Apollo 11 & 17) at ~0.4. The lower tail of the reds is represented in the Catalog by Yamato-793169, Yamato-793274 & Asuka-881757, but blues have not been identified so far on Antarctica. (Lunar anorthosites are at the double of chondritic level.) But lunar basalts do have analogons for Ti level in the Solar System.

 

DISCUSSION

            One such is the angrites. They are rather rare and the Catalog has only 2 measured samples, the archetypic Angra dos Reis and Asuka-881371. Hence the angrite level is Ti/Si = 0.05±0.02, comparable to the lunar red one. The other analogons are some C/T Carpathian Basin basalts. (That time is the prehistory of the Basin whose actual formation ended in the Miocene.) Mecsek (Harangi, 1994) and Gyergyóditró (Lengyel, 1957; Pál Molnár, 1995) basalts are at the red lunar level and Szarvaskö (Szentpétery, 1953; Lengyel, 1957) ones at the lunar blue one (for the comparison see Bérczi & Lukács, 1996). The Carpathian Basin is special, but not quite singular; much older "red" kimberlites are known from Greenland (Nixon, 1987).

            The lunar bulk composition may or may not be special, but at some spots the formation processes of the Carpathian Basin produced lunar Ti level from terrestrial bulk one. So probably rather the lunar volcanic mechanism produced the high enrichment in Ti. Unfortunately just now the details of the lunar mechanism cannot be investigated, while in the Carpathian Basin the process was so complicated that one cannot tell apart the important and irrelevant details for high Ti content.

            Still, we can give here the comparative Fig. 1. It is a completed form of a similar Figure in Bérczi & Lukács (1996), and for lunar data see the references therein. The samples are lunar red and blue basalts, the analogous Mecsek and Szarvaskő ones, and the measured shergottites (Zagami & ALH-77005), angrites (Angra dos Reis & Asuka-881371) and basaltic lunars (Yamato-793169, Yamato-793274 & Asuka-881757) of the Catalog.

            The picture is partly interpretable. Both red samples (lunar and Mecsek) show anticorrelation between Ti and Mg, conform with the idea that somehow the more and more thicker and aluminoferous crust makes the lava enriched in Ti. Of course, it is not advisable to calculate correlation coefficients from 2 or 3 points, but at least the locations of the angrite, basaltic lunar and shergottitic points support an anticorrelation. But, in the contrary, for both blue data (lunar and Szarvaskő) the correlations are positive.

            At this moment there are no lunar expeditions. Still, a comparative investigation on lunar basalts, basaltic meteorites (lunar or not) and Carpathian Basin basalts may help. We close this presentation with a statement and a question.

            1) Lunar "blue" basalts are still absent among measured lunar meteorites.

            2) Where is in the Solar System the angrite parent body having mimicked a substantial planet with thick and more or less aluminiferous crust?

 

ACKNOWLEDGEMENTS

            Discussions with Dr. S. Harangi are acknowledged. Partly supported by: the Hungarian-Japanese S&T Cooperation "Impact and Extraterrestrial Spherules" 31/96; OTKA T/014958, MÜI-TP-15/95 & OMFB-96-97-47-1265-MÜI-TP-055/96.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

REFERENCES

Bérczi Sz. &. Lukács B., 1996: KFKI-1996-08

Condie K. C., 1981: Archean Greenstone belts. Elsevier, Amsterdam

Harangi Sz., 1994: Lithos 33, 303

Lengyel E., 1957: Ann. Inst. Geol. Publ. Hung. XLVI/2

Lukács B. & Bérczi Sz., 1997: in this Volume

Nixon P. H. (ed.), 1987: Mantle Xenoliths. J. Wiley & Sons, N.Y.

Pál Molnár E., 1995: Petrologic and Geologic Investigations on

            the Gyergyóditró Syenite Complex. University Press, Szeged

Szentpétery Zs., 1953: Ann. Inst. Geol. Publ. Hung. XLI/1

Yanai K., Kojima H. & Haramura H., 1995: Catalog of the Antarctic

            Meteorites. NIPR, Tokyo