[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Ibitira

	I have noticed the sudden intrest in Ibitira, since it has become 
availible to collectors. I have spent a couple of days looking at past 
papers on Ibitira that span 20 yrs. I will sum up this information, and 
try to give exactly what is known about it right now. I am sure that it 
will change in the next few months.
	Ibitira is a vesiculated basaltic meteorite(eucrite). The 
uniqueness is due to the little holes found in the matrix, and it's 
unbrecciated texture.It records what the very first volcanic episodes 
must have been like in the early Solar System.
	Lets look at the vesicles, and possible origin for a moment. The 
Vesicles were formed as the basalt flow went from a high pressure 
interior to a low pressure enviroment(surface) (Mckinnon 1993). Ibitira 
never reached the surface of the parent body it came from. This known 
from the fact that at the absolute surface, the vesicles would have been 
removed completely due to a rapid cooling of the lava. Instead if it were 
below a certain limit, vesicles would be preserved due to slower 
cooling(Wilkening and Anders 1975). The depth of the lava flow was 
between 2.5 meters, and 20 meters, with Ibitira at the bottom 
100-150cm(Wilkening and Anders 1975).
	Ibitira shows a strong shocking event. This indicated by the 
presence of cracks in the pyroxenes of the meteorite(Steele and Smith 1976)
However, it was not brecciated. It is difficult to constrain anything on 
the shock, and metamorphic history of the meteorite. It just leaves more 
questions than answers, it seems from the literature.
	Now, where did it come from? A 4Vesta type asteroid seems to be 
the best fit. This is based on reflectance spectra data, and oxygen isotopes.
Much data was gathered, and it all seems to coincide with the hypothesis 
of a 4Vesta asteroid type origin.
	The reflectance data for minerals in eucrites, and 4vesta type 
asteroids are almost a perfect match. McKinnon (1993) discusses this in 
more detail. In any case, Ibitira matches just as well as any other 
eucrite, to a 4Vesta type asteroid.
	Oxygen isotopes in Ibitira, match well with other eucrites(almost 
exactly). I am looking at a chart from Clayton and Mayeda (1983),that 
shows the result of oxygen istope study of eucrties,SNC's, and 
terrestrial basalts. When the compostion of all these rocks(meteorites) 
are plotted on a graph, they each make up a distinct little cluster. 
Eucrites (Ibitira), have the lowest oxygen isotope ratio's, as compared 
with other solar system basalts.
	I think based on previous literature, Ibitira is a Eucrite derived 
from a 4Vesta type asteroid. I know now that it is under revision, but I 
doubt the relationships thus far noticed will change appreciably. I am 
curious as to who is doing the reclassification;can anyone tell me?
	Well I just wanted to take some time to throw that in.Maybe it 
cleared something up, or perhaps created more confusion. If you have a 
question let me know, as always, I will answer it.

Frank Stroik
University of Wyoming

Clayton,Robert R. Mayeda, Toshiko K.Oxygen isotopes in eucrites, 
                 shergottites,nakhlites, and chassignites. Earth and
                 Planetary Science Letters 62,pp1-6, 1983

McKinnon Willam B. Vestal voyagers unvieled Nature 363,pp.211-212 1993

Steele, I.M and Smith J.V. Mineralogy of the Ibitira eucrite and comparisons
                with other eucrite and lunar samples. Earth and Planetary 
                Science Letters 33 pp 67-78 1976

Wilkening, Laurel L. and Anders, Edward. Some studies of an unusual eucrite:
                         Ibitira. Geochimica et Cosmochimica Acta.39 pp.
                         1205-1210. 1975

Follow-Ups: References: