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A triangular (or ternary) diagram used to plot the chemical analysis of igneous rocks.

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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