The processes involved in the formation and
modification of continental crust -- igneous processes, erosion
and formation of sediments, tectonic-metamorphic processes -- tend
to have unique chemical signatures. Such signatures may involve
the patterns of abundance of the elements, isotope systematics or
the fractionation of light stable isotopes such as those of hydrogen,
carbon, oxygen or sulphur.
Fragments of crust accreted as parts of the assemblage of a continent
carry with them records of their unique geochemical history. Thus,
geochemistry adds to information from paleomagnetism, geochronology
and structural studies to form a more complete description of the
processes of crustal generation and evolution.
It is now recognized that in all major parts of the plate tectonic
cycle, fluids play a major role. At ocean ridges where new, hot
lithosphere is formed, sea water interacts with igneous rocks and
modifies both rock and ocean water. When ocean floor is subducted,
fluids are mobilized, lubricate the subduction thrusts and eventually
catalyse the formation of gassy, subduction-related volcanic products.
When continental blocks collide (as in the Himalayan or Alpine mountain-building
events), the great continental thrust structures are lubricated
by the extrusion of fluids from the compressed and heated rocks
pushed to greater depth.
Every time a geologic fluid moves, whether it is an igneous melt,
metamorphic H2O - CO2, or fluids from a sedimentary basin, chemical
and isotopic changes occur along the fluid pathways. The type of
fluid can often be identified from its stable isotope systematics
and its composition derived from the study of small inclusions of
fluid preserved in the rock or chemical changes along the flow path.
Thus chemical change can be used to identify tectonic processes
both in terms of scale and style.
Almost all of our mineral and hydrocarbon resources are related
to the movement of such fluids. Sulphides of iron-zinc-copper-silver
are associated with sea water heated in the oceanic crust and cooled
as it rises at ridges. Gold-silver-tungsten-copper-molybdenum deposits
appear to be associated with fluid processes above subduction zones.
Most of the great gold deposits of Canada appear to be associated
with deep fluids from igneous and metamorphic processes. Every time
large volumes of fluid are moved in tectonic processes, there is
potential for the rearrangement of chemical components, at times
into valuable ore deposits.
Such a plot allows us to determine the source of a magma from which
a rock has solidified, and much about how this has happened. Yes,
another detective story, Sherlock Holmes would have loved it. Is
it difficult to do? Very! But the principles behind this scientific
detective work are straightforward.
Let's start with a bottle of pop. Open it when it's very cold and
the bubbles will escape, sometimes forming a froth. Open the bottle
or can when it's warm and the same will happen, only more so, because
the bubbles will wish to get out much faster yet. What we have here
is a splendid demonstration of how pressure (under which the pop
is sealed in its container) and temperature control the balance
in which the carbon dioxide or CO2 is held in solution in the pop.
Remove the pressure on the solution, that is remove the cap, and
the bubbles no longer can be held in solution, they are "volatile"
and escape. Increase the temperature and more volatiles will tend
to escape. Easy? Surely, it is. Now, we must add the "solids"
which also will fall out within a certain temperature-cum-pressure
regime, something one has in shakes and sugar or salt solutions.
Likewise, all minerals have their specific temperatures-cum-pressure
ranges at which they will melt into or precipitate from a magma.
The magma from which a rock-type(s) has solidified can have formed
from an infinite selection of rocks and circumstances. It may have
come from the mantle deep below the crust, or it may have formed
when rock in the crust was melted. Then again, only certain parts
of a rock may have melted, or all of it, or much, or little. On
its way upward, parts of the magma may have solidified, and what
was left moved on, then being different in its chemical character
because of the missing constituents.
The possibilities are endless, depending on prevailing pressures
and temperatures. One can only dissolve so much sugar in a cup of
tea, and when it cools, some of the sugar will precipitate (or fall
out).
And in that fact lies the handle to the scientific analysis of
a solidified rock. The chemical constituents in the rock can indicate
to the geochemist under what pressure and temperature conditions,
and from which likely original mix of magma, the rock has formed.
Pretty neat, eh?
The triangular AFM diagram that we see allows the researcher to
plot different "suites" of igneous rocks, which describe
the environment (such as temperature and pressure) and the source
of the rock's formation. The "A" stands for alkalies,
the "F" for iron (or ferrous and ferric) oxides, and the
"M" for magnesium oxide. Exactly where each rock sample
will be plotted on this diagram depends on the relative weight percentage
of these three oxide groups found in the sample.
Samples from the Garibaldi volcanic belt near Vancouver will plot
dominantly in the calc-alkaline field. The granitic rocks from the
Coast Plutonic Complex west of the volcanic belt also plot in the
same field. In contrast, the freshly formed rocks from the magmas
oozing out of the Juan de Fuca rift offshore Vancouver Island plot
in the tholeiitic area of the diagram. If such rocks were subsequently
thrust onto the continent (as happened in western Newfoundland),
they could be distinguished from the volcanic rocks.
The role of geochemistry in LITHOPROBE is two-fold. Determination
of the deep structure and processes of the crust requires, first,
an understanding of the pressure-temperature (P-T) conditions applicable
at depth and, second, of the fluids that are present. P-T conditions
at depth can be estimated from the distribution of the elements
and isotopes found in the rocks now exposed at the surface that
were formerly buried, and from intrusive rocks that have risen from
depth carrying elements or inclusions with them. The fluids at depth
can be estimated both from the chemical traces that they have left
behind and from the fluids that are currently escaping at the Earth's
surface.
In a more general way, one can say that one way of reading the
story of rocks is through the distinct chemical signature which
igneous, metamorphic, erosional and sedimentary processes impart
on the rock formations.
For instance, fragments of oceanic and continental crust which
have joined our continent carry the records of their unique geochemical
histories. Geochemistry and its tools add one more discipline for
correlating information from several disciplines, such as seismic,
structural, tectonic, and other studies.
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