Ip. Wright et al., THE CONTENT AND STABLE ISOTOPIC COMPOSITION OF CARBON IN INDIVIDUAL MICROMETEORITES FROM GREENLAND AND ANTARCTICA, Meteoritics & planetary science, 32(1), 1997, pp. 79-89
The C contents and delta(13)C values of eleven individual micrometeori
tes have been determined using a combination of stepped combustion and
static mass spectrometry. A new low-blank procedure, involving pretre
atment of the samples with a solvent to remove surficial contaminants,
has enabled samples of 6-84 mu g to be analysed successfully. The ele
ven samples (seven separated from Greenland cryoconite and four from A
ntarctic ice) were each split prior to C determination and a fragment
taken for study using analytical electron microscopy. In this way, the
chemical compositions were obtained thereby allowing comparison with
other investigations. As with previous studies of micrometeorites coll
ected at the Earth's surface, the major difficulty with interpreting t
he results involves distinguishing indigenous components from terrestr
ial contaminants. Overall C contents were typically <0.2 wt%, although
one of the Greenland samples contained 1.5 wt% C, considered to arise
mainly from algal contamination. For the other samples, around 0.05-0
.15 wt% Of the total C in each micrometeorite was considered to be org
anic in nature with at least some of this (if not all) being terrestri
al in origin; the remainder was probably indigenous, being analogous t
o the macromolecular organic material found in primitive carbonaceous
chondrites. The generally low content of this indigenous organic mater
ial, compared to conventional meteorites, is presumably a reflection o
f C loss from the micrometeorites either during atmospheric heating, o
r subsequent weathering. For that C combusting between 500 and 600 deg
rees C, ten of the samples appeared to show a simple two-component sys
tem (i.e., a mixture of blank and an isotopically fight component; del
ta(13)C less than or equal to -32 parts per thousand). It is possible
that the light component is C-delta, a fine-grained form of presolar d
iamond which is known to be prevalent in primitive chondritic meteorit
es. If so, then it is present in the micrometeorites at concentrations
of similar to 30-600 ppm (typically 200 ppm), which is a similar leve
l to that in meteorites. An analysis of algae separated from Greenland
cryoconite shows tentative evidence for the presence of extraterrestr
ial silicon carbide; however, further work will be needed to substanti
ate this.