to enstatite by discontinuous reaction with the remaining melt at the peritectic accompanied by additional isothermal crystallization of enstatite until the melt is used up. Some forsterite always remains because there is insufficient silica component to convert all of it into the intermediate compound enstatite. This indicates that the system was undersaturated with respect to silica. The peritectic reaction that converts the olivine mineral forsterite to the pyroxene mineral enstatite, as note previously, is called a discontinuous reaction. This phase diagram provides an excellent example of how early formed crystals can react with remaining melt to produce an entirely different mineral. These reactions are characteristic of the minerals in the discontinuous reaction series of Bowen's reaction series (Chapter 8).
Systems with exactly 30% silica component have the composition of the intermediate compound enstatite. They are precisely saturated with respect to silica. Line B (Figure 3.11) indicates the cooling behavior of such a system. Initial crystallization at the liquidus produces forsterite crystals. Continued cooling, accompanied by addition, separation, and growth of forsterite crystals, causes the remaining melt to evolve down the liquidus toward the peritectic with forsterite and melt percentages and melt compositions given by the lever rule. When this system reaches the peritectic, it contains 14% forsterite crystals and 86% melt with 35% silica component. Discontinuous reaction between the remaining melt and the forsterite crystals converts all the forsterite crystals to enstatite, while simultaneous crystallization of enstatite causes the melt to be used up. The final rock is 100% enstatite and is neither undersaturated (lacks forsterite) nor oversaturated (lacks quartz), but is saturated with respect to silica.
Systems with silica component contents of 30–35% silica are oversaturated with respect to silica and thus exhibit another set of behaviors. Line C (33% silica component) is representative of these behaviors (Figure 3.11). The system cools to the liquidus at 1650 °C where forsterite crystals begin to separate. Continued separation of forsterite causes melt composition to evolve down the liquidus toward the peritectic. As the melt composition reaches the peritectic, the lever rule shows that the system contains ~6% (2/35) forsterite crystals and ~94% (33/35) melt with ~35% silica component. At the peritectic, all the forsterite is converted to enstatite with the remaining melt and additional enstatite crystallizes, but additional melt remains. The lever rule shows that as the system leaves the peritectic and enters the enstatite plus liquid stability field it contains ~40% (2/5) enstatite and ~60% (3/5) melt. Further cooling leads to additional crystallization of enstatite (30% silica component), which causes the remaining melt to evolve down the liquidus toward the eutectic (at 46% silica component). As the system reaches the eutectic, it contains ~81% enstatite (13/16) and ~19% (3/16) melt with 46% silica component. At the eutectic, enstatite and quartz crystallize simultaneously until the melt is used up. The final rock contains 96% (67/70) enstatite and 4% (3/70) quartz and records a melt that was oversaturated with respect to silica.
Compositions in this system between 30 and 35% silica component clearly show that minerals, such as forsterite, that are undersaturated with respect to silica can crystallize from magmas that are oversaturated with respect to silica. If equilibrium conditions between these crystals and the melt are maintained, they will eventually be converted to the intermediate compound and therefore will not be preserved. However, if disequilibrium conditions exist, of the kinds that commonly occur during fractional crystallization, early formed crystals may well be preserved in the final rock. In addition, because silica in the remaining melt was not used to convert forsterite to enstatite, the melt will be more enriched in silica than would otherwise be the case. As discussed in Chapter 8, such concepts are very important in understanding the evolution of magma composition.
The system forsterite–silica (Figure 3.11) clearly illustrates the concept of silica saturation. Compositions of >30% silica (SiO2) end member component by weight are oversaturated with respect to silica, so that there is sufficient silica to convert all the forsterite into enstatite, with additional silica remaining. Equilibrium crystallization in such silica‐rich systems produces the intermediate compound enstatite with excess silica to form quartz. As discussed in connection with the nepheline–silica diagram (see Figure 3.10), quartz forms by equilibrium crystallization of melts that are oversaturated with respect to silica. On the other hand, compositions of <30% silica component by weight are undersaturated with respect to silica, so that there is insufficient silica to convert all the forsterite into enstatite. Equilibrium crystallization in such silica‐poor systems produces forsterite plus as much of the intermediate compound enstatite as can be formed at the peritectic. Forsterite forms by equilibrium crystallization of melts that are undersaturated with respect to silica. As detailed in Chapter 7, both forsterite‐rich olivine and feldspathoids suggest crystallization from systems undersaturated with respect to silica. Systems with exactly 30% silica component are exactly saturated with respect to silica because they possess precisely the amount of silica component required to convert forsterite into enstatite without excess silica remaining.
One can also investigate melting behaviors in this system. For compositions of >35% silica, the system behaves as a simple eutectic, producing first melts with an invariant composition of 46% silica. Melts possess this composition until either quartz (for systems 35–46% silica component) or enstatite (for systems >46% silica component) is completely melted. Subsequent melting of the remaining mineral causes the liquid to change composition up the liquidus. These behaviors once again demonstrate the ways in which melt compositions depend both on the percentage of partial melting and on the composition of the original rock. For compositions of <35% silica, melting involves peritectic reactions. In these systems, whenever the system reaches the peritectic, some or all of the enstatite remaining in the solid fraction melts to produce both forsterite and melt at the peritectic (35% silica component). This behavior is essentially the reverse of what happens when systems cool through the peritectic, and melt plus olivine yields enstatite. Such behavior, in which the melting of one crystalline material produces both a new crystalline material and a melt of different composition, is called incongruent melting. It also illustrates how silica oversaturated melts might be obtained from the partial melting of silica undersaturated, forsterite‐rich rocks such as ultramafic peridotites in the mantle.
3.3 ISOTOPES
This section provides a brief introduction to the uses of some radioactive isotopes and stable isotopes important in the understanding of Earth materials and processes. Isotope studies provide powerful insights concerning the age, behavior and history of Earth materials. In geology, a thorough understanding of both stable and radioactive isotopes is essential for determining the ages and origin of minerals and rocks. Isotope ratios, determined by mass spectroscopy, are also instrumental in understanding a variety of other phenomena discussed in this book, including the determination of:
1 Source rocks from which magmas are derived.
2 Origin of water on Earth's surface.
3 Timing of mountain building events involving igneous intrusions and metamorphism.
4 Timing of unroofing of such rocks and the dispersal of their erosional products by sedimentary agents.
5 Source rocks for petroleum and natural gas.
6 Changes in ocean water temperatures, biological productivity and circulation.
7 History of ice age glacial expansions and contractions.
8 Climate
