  The insoluble organic matter (IOM) of an unequilibrated enstatite chondrite
Sahara (SAH) 97096 has been investigated using a battery of analytical
techniques. As the enstatite chondrites are thought to have formed in a reduced
environment at higher temperatures than carbonaceous chondrites, they
constitute an interesting comparative material to test the heterogeneities of
the IOM in the solar system and to constrain the processes that could affect
IOM during solar system evolution. The SAH 97096 IOM is found in situ: as
submicrometer grains in the network of fine-grained matrix occurring mostly
around chondrules and as inclusions in metallic nodules, where the carbonaceous
matter appears to be more graphitized. IOM in these two settings has very
similar $\delta^{15}N$ and $\delta^{13}C$; this supports the idea that
graphitized inclusions in metal could be formed by metal catalytic
graphitization of matrix IOM. A detailed comparison between the IOM extracted
from a fresh part and a terrestrially weathered part of SAH 97096 shows the
similarity between both IOM samples in spite of the high degree of mineral
alteration in the latter. The isolated IOM exhibits a heterogeneous
polyaromatic macromolecular structure, sometimes highly graphitized, without
any detectable free radicals and deuterium-heterogeneity and having mean H- and
N-isotopic compositions in the range of values observed for carbonaceous
chondrites. It contains some submicrometer-sized areas highly enriched in
$^{15}N$ ($\delta^{15}N$ up to 1600 permil). These observations reinforce the
idea that the IOM found in carbonaceous chondrites is a common component
widespread in the solar system. Most of the features of SAH 97096 IOM could be
explained by the thermal modification of this main component.
