[meteorite-list] Black inclusions in NWA R-Chondrites?

From: bernd.pauli_at_paulinet.de <bernd.pauli_at_meteoritecentral.com>
Date: Mon Jul 24 14:26:25 2006
Message-ID: <DIIE.0000003900000933_at_paulinet.de>

Hi Jeff and List,

"I'm hoping someone may know of an abstract/personal/web info on the black
 (xenolithic?) inclusions found in some of the NWA R-Chondrites? Any info
 would be appreciated either on or off-list." Here is an example:
 
http://www.meteorites.com.au/features/nwa2921.html
 
Beautiful specimen with a very conspicuous, large, dark clast and another
equally large but light greyish-brown clast set in a medium-gray matrix
(this light-dark structure is typical of regolith breccias).

As for the black inclusion, I don't think it is xenolithic. It represents
unequilibrated material of low(er) petrologic type. Hence all those tiny
chondrules set in a dark matrix that resembles that of carbonaceous chon-
drites.

The light- to medium-colored matrix areas show only few chondrules and/or
chondrule relics and thus correspond to petrologic types 5 or 6. The dark,
chondrule-rich clasts are unequilibrated and correspond to type 3.x (about
3.8 some sources say).

But: the dark clasts are NOT carbonaceous but are due to the dispersion of
tiny grains of sulfides and Cr-spinels, which causes silicate darkening.

My beautiful NWA 3098 (R5) and my latest addition, NWA 2943 (R3-6), which
is still in San Diego with Cap'n Blood, also show these dark, unequilibrated
clasts.

But, whereas my NWA 3098 from Stefan clearly shows chondrule-poor, light
clasts and a few small but chondrule-rich dark clasts (see JPEGs in my
private mail to you), the NWA 2943 from Michael Blood has an overall
higher abundance of chondrules - the chondrules are more evenly distributed.
throughout the busy matrix.

Best wishes,

Bernd

Here are some useful references:

BLAND P. et al. (1992a) A unique type 4 chondrite from
the Sahara - Acfer 217 (abs. Meteoritics 27, 1992, 204-205).

BISCHOFF A. et al. (1994a) Acfer 217 - a new member of
the Rumuruti chondrite group (Meteoritics 29, 264-274).

DIXON E.T. et al. (2003) 39Ar-40Ar chronology
of R chondrites (MAPS 38-3, 2003, pp. 341-355).

JACKEL A. et al. (1996) DaG 013 - A new Saharan Rumuruti-
chondrite (R3-6) with highly unequilibrated (Type 3) fragments
(abs. Lun.Plan. Sci. 27, 595-596).

BISCHOFF A. (2000) Mineralogical characterization of
primitive, type-3 lithologies in Rumuruti chondrites
(MAPS 35-4, 200, pp. 699-706).

BISCHOFF A. et al. (2001) Mineralogy, Chemistry, and noble
gases of the unpaired Rumuruti chondrites NWA 753 and NWA 755
(MAPS 36-9, 2001, A021).

LINGEMANN C.M. et al. (2000) Rumuruti chondrites: Origin and
evolution of primitive components (MAPS 35-5, 2000, A098).
Received on Mon 24 Jul 2006 02:25:19 PM PDT