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DECREPITATION SAMPLE SPECIFICATIONS
The following items should be carefully considered
in collecting samples as part of a decrepitation survey. Each major
consideration
is discussed in detail below.
The decrepitation method can be used on opaque,
translucent
and transparent minerals with equal ease and provides information on
the
abundances, formation temperatures and the fluid compositions of the
fluid
inclusions. Although it is sometimes possible to infer absolute
formation
temperatures, this involves making several assumptions about the fluid
composition and so it is generally preferable to interpret the results
empirically in a manner similar to interpreting a soil geochemical
survey.
It may often be of advantage to examine some of the samples in thin
section
in order to gain a better appreciation of the nature of the fluid
inclusions
involved. If concomitant microscope work is planned, great care should
be taken to ensure that the sample sectioned is exactly the
same
as the one decrepitated. The decrepitation sample in such cases should
therefore be the offcuts from the thin section preparation to ensure
that
both samples are from the same growth zone. (Growth zoning in quartz is
more common than many people care to admit!)
CHOICE OF MINERAL PHASE TO BE USED
Generally, quartz is the preferred sampling medium as
thin
section work can be done to confirm and complement the decrepitation
results
if necessary. Quartz is also resistant to weathering and can retain
it's
fluid inclusions despite lateritization. It also has low ductility,
which
is theoretically favourable for generating counts in the
decrepitometer.
The quartz may be either as ordinary veins or as silicification of host
rocks. Silicification haloes give low level responses and it is
suggested
that more samples than usual be collected when using this medium as
some
20% of the samples typically give responses too weak to interpret. Many
other mineral phases can be used and the following have been tested
successfully
to date:- haematite, magnetite, pyrite, fluorite, barite, calcite,
dolomite,
olivine, feldspars (MUST be fresh), galena and sphalerite.
Unsuitable
minerals include chlorite, muscovite, sericite (and presumably all
layer
silicates), kaolin (and presumably all clays). Note that these hydrous
minerals do NOT produce decrepitation counts due to dehydroxylation. In
theorey, ductile minerals like galena, fluorite and the carbonates
should
not decrepitate, although in practice they decrepitate very well.
(Theorey
has yet to catch up with the facts on this!) However, it would be best
to avoid these minerals if possible until we develop a better
understanding
of them. Many other minerals could also be used although several trial
samples of other minerals should be done first to check their
efficiency.
MONOMINERALIC SAMPLES
Different minerals with different mechanical properties
require
varying excess pressures to cause decrepitation. Hence, to ensure
consistency,
it is preferable to avoid mixing minerals either within a sample or
across
a single survey. If it is impossible to collect monomineralic samples,
mixtures can sometimes be used (such as quartz-feldspar in granites) or
mineral separations can be done. Please discuss such problems before
collecting
such mineralogically diverse samples. Inclusion of clays and layer
silicates
within the sample is not a serious problem (except inasmuch as it
dilutes
the active mineral), but inadvertant inclusion of carbonates and some
sulphides
can cause serious problems. (See below) (Acid washing of the samples
can
be used to remove carbonate contamination.)
ZONING OF VEIN QUARTZ
Many quartz veins have pronounced growth zoning
developed
inwards from the vein walls. Often this is readily visible, but
sometimes
it is not. In general, veins which posess vugs and cavities with well
formed
quartz crystals are growth zoned. Uniform, massive milky veins tend not
to be zoned. If they don't look completely homogenous from wall to
wall,
they probably aren't! Failure to recognize the possibility of growth
zoning
could render the results uninterpretable, so veins in unfamiliar areas
should be sampled at several positions across their width to check for
growth zoning. Variation along the length of a quartz vein does occur
within
limits. Depending on the objectives of the survey it may be necessary
to
ascertain the amount of such variation by collecting samples at various
points along strike in the same vein. Variation down dip in quartz
veins
is theoretically likely, but rarely observed. Unless the thermal
gradient
up the vein was exceptionally high, a vertical section of some 500
metres
would be necessary to observe this effect. Only rarely would it be
necessary
(or possible) to collect samples to check this effect in a survey.
Variation
between different vein sets, even when they spatially overlap, is
usually
far more pronounced than variation due to any of the above effects.
Test
samples should be planned and collected to assess whether any of the
above
variations interfere with the variations you plan to interpret.
POST EMPLACEMENT CHANGES
Metamorphism and recrystallization during folding
subsequent
to emplacement can alter or erase the fluid inclusions from the
minerals
of interest. The effects are quite variable and in a great many cases
useful
decrepitation data can still be obtained despite these processes.
Metamorphism
to greenschist facies rarely affects quartz veins, but at higher
grades,
new inclusions might be introduced and swamp the original inclusion
signature.
Recrystallization typically reduces the numbers of inclusions present
but,
even when severe, sufficient inclusions usually remain to allow
interpretation.
These effects do need to be considered carefully, but they are not
usually
as serious as they theoretically might be. But be careful if you are in
high grade metamorphics.
SAMPLE QUANTITY AND FORM
The amount of sample actually analysed is 0.5 to
2.0grams.
The form of the sample analysed is as grains, pre-sieved to -420+200
microns
(-40+80 mesh). In finer grain sizes, many of the inclusions have
already
been mechanically broken open during sample comminution, so pulverized
samples are unsuitable. Either lump samples or samples passed through a
jaw crusher should be submitted for analysis. A minimum of about 10
grams
is preferred to allow for losses into fine grain fractions during
sample
preparation. (Smaller samples can be used by special arrangement.) Lump
samples some 2 to 4 cm. across are convenient and are cheap to freight.
Note that because there are typically billions of fluid inclusions per
gram (in vein quartz), there is no need to send large samples for
reason
of obtaining a representative 0.5 gram split. The samples should be as
monomineralic as possible (see above) unless mineral separations have
been
arranged.
CARBONATE AND SULPHIDE CONTAMINATION
Carbonate minerals all decrepitate violently (well
below
their pyrolytic decomposition temperatures) and almost always give
exceedingly
high count rates. (They can saturate the decrepitometer counter
circuits.)
It is quite possible for contaminant amounts (say 5%) of carbonate in a
quartz sample to completely swamp the quartz result. If this happens,
the
decrepigram cannot be interpreted. (The presence of carbonate is
immediately
obvious). Consequently, unless you are deliberately using carbonates
for
your samples, you should scrupulously avoid them. If necessary, acid
washing
can be arranged to remove carbonate contamination. Sulphides can give
rise
to strange peaks on the decrepigrams. Generally these do not swamp out
the result from the dominant phase, but they can make the
interpretation
more difficult. Some sulphides oxidise within the heating range of the
instrument and this generates many decrepigram counts. Such oxidation
is
exothermic and thus self sustaining and the decrepigram peaks resulting
are of little interpretative use. Some sulphides melt and some have
been
observed to form molten native sulphur during the analysis. These are
definitely
undesireable as they mess up the sample tubes. Unless you are
deliberately
using sulphides it is best to try and exclude them from the sample,
although
their effect is nowhere near as bad as carbonates and trace amounts can
be tolerated.