1.6c Questions (continued)

1.06 The P-T phase diagram in Figure 1.12 shows the crystallization sequence for primitive basalt. In this kind of diagram, phase relationships of a single starting magma composition (in this case, that of primitive basalt) are investigated as variables of both pressure and temperature.

1. Starting from the “all liquid” field of this diagram (a temperature greater than the liquidus temperature), draw a cooling/crystallization path on the diagram for a magma that crystallizes at low pressure (less than 1 GPa), and one at higher pressure. Describe the predicted sequence of crystallization for each cooling/crystallization path.
2. Describe how the crystallization history of a primitive basalt differs in these two different cooling pathways?
3. Using the mineral and textural observations from your thin-sections (and your interpreted sequence of crystallization), at what range of pressures do you think the thin-section samples crystallized? Justify your answer.

1.07 Figure 1.13 is the four-component (Anorthite-Forsterite-Clinopyroxene-Quartz) phase diagram projected from the plane of plagioclase saturation (after Walker and others, 1979. Plagioclase is assumed to be saturated and crystallizing in all fields. (Note that this is a different kind of phase diagram than that of Figure 1.12. In this diagram composition and temperature are the variables and the pressure is held constant.) Point A and B represent starting compositions for liquid lines of decent (LLD) discussed below. Points P and E are invariant reaction (P) and eutectic (E) points, respectively.

1. Draw onto Figure 1.13 the liquid line of decent that you would expect from magma that crystallizes, starting from either point A or point B. (Draw one LLD from each point.) Describe the sequence of crystallization for each LLD? (Remember, plagioclase is assumed to be crystallizing in all cases on this diagram.)
2. Use the phase relations you observed in the thin-sections to approximate where each of the samples would plot on your proposed liquid line of decent, and plot them on Figure 1.13. Note that although you may not have found it, the mineral pigeonite (sub-calcic clinopyroxene) occurs in sample DR13-3.
3. From this information, place the samples in order from most primitive to most evolved.
4. Consider the chemistry and petrography of sample T178-G4; it is a ferroandesite. One possible petrogenetic origin for this sample could be a mix between a basaltic melt and a silicious melt. Draw a mixing line onto Figure 1.13 that connects samples T179-G10 and T735-G12. Predict the mineralogy of a sample produced by a mix of these two compositions. How does your prediction compare to the observed mineralogy of sample T178-G4? Hypothesize at least two petrographic observations that might be indicative of magma mixing and found in a sample formed by mixing of these two end-member magmas.

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