what is binary magma
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Answer:
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Explanation:
The binary eutectic phase diagram explains the chemical behavior of two immiscible (unmixable) crystals from a completely miscible (mixable) melt, such as olivine and pyroxene, or pyroxene and Ca plagioclase. Here we are going to generalize to two minerals, A and B, or P and Q. We want to observe the behavior of this system under two conditions, one of complete equilibrium during crystallization when all chemical reactions can run to completion, and the second of disequilibrium when fractionation occurs and igneous rocks evolve.
The conventions for the phase diagram include the following (illustration below):
Two immiscible components: such as CaAl2Si2O8 (calcic plagioclase) and FeMgSiO4 (olivine) plotted along the horizontal axis, OR olivine (isolated tetrahedra) and pyroxene (single chain tetrahedra). They are immiscible because they have different crystal structures.
One variable, temperature, plotted along the vertical axis. Pressure is held constant at 1 atmosphere.
Three phases, crystal A, crystal B, and melt.
.Complete miscibility of the melt (magma)
The assumptions are:
The system remains in equilibrium throughout its history so that all reactions can take place and everything can come to stability.
Everything in the original melt remains in communication throughout the crystallization process.
Organization of the Binary Eutectic Phase Diagram
NOTE the following about reading the diagram:
1. The liquidus line separates the all melt phase from the melt+crystal phase.
2. The solidus line separates the melt+crystal phase from the all crystal phase.
NOTE that the solidus and liquidus lines are experimental, they have been determined by melting and cooling many melts at different percent compositions.
3. The eutectic is the point at which all three phases can exist simultaneously, A, B, and melt. The eutectic here is 50% B, but can be any percent depending on the minerals involved.
An Example of Crystallization From An Equilibrium Melt With 50/50 Eutectic
1. For pure A (far left of diagram) the melting (crystalizing) temperature is TA about 1380o.
2. For pure B (far right of diagram) the melting (crystalizing) temperature is TB about 1485o.
3. If we add some B to a melt of A (say 20% B; red arrow) the temperature of melting (crystallization) is lowered to about 1360o. The more B we add the lower the melting temperature becomes; that is, it moves down the liquidus line toward the eutectic.
4. Any mixture of A and B lowers the melting (crystallizing) temperature. For example, a mix of 60% B (blue arrow) crystallizes at about 1320o where pure B crystallizes (melts) at about 1480o.
An Example of Equilibrium Crystallization From a Melt With a 50/50 Eutectic
The First Crystal
(numbers on phase diagram correspond with numbers below)
1. Assume a melt comp- osition of 70% B and 30% A.
2. Cool melt to liquidus line along red arrow. Only B crystals form at about 1380o (B is immiscible with A). Only B crystallizes because we are on the Melt+B liquidus line; no A can crystallize until the eutectic is reached.
3. Removing (crystallizing) B changes the melt composition making in richer in A. Therefore the melt composition begins to migrate to the left, but down the liquidus line toward the eutectic point. The system must stay on the liquidus line since going above it would raise the temperature high enough to melt everything