M
- PDF / 10,388,754 Bytes
- 42 Pages / 595.276 x 841.89 pts (A4) Page_size
- 19 Downloads / 194 Views
Temperature (C)
MAGMATIC PROCESSES Introduction Magmatic processes comprise any process that affects the melting or crystallization of a magma. This includes partial melting of rocks of different composition under different conditions of temperature and pressure (total and fluid such as H 2 0) and the processes that modify the composition of the melt after melting. The processes that can potentially modify the melt include evolution of a fluid phase (H 2 0 or C0 2 ), fractional crystallization, assimilation of country rocks, immiscibility of liquids, and flow differentiation. These processes range from equilibrium to various degrees of disequilibrium and may be modeled mathematically as described below.
0
200
Mechanisms of melting A single mineral like ice melts at a constant temperature at constant pressure with the composition of the melt (water) being the same as the solid (ice). In contrast a rock consisting of two or more minerals melts over a range of temperature at a constant pressure, and the composition of the melt is different from that of the solid. The composition of the melt will continuously evolve as different ratios of minerals are melted. Phase diagrams can summarize the temperature and composition of phases at a given pressure or the solidus and liquidus at different temperature and pressure (Figure Ml). The solidus is the temperature at a given composition and pressure of a system in which only solid exists below that temperature, but both solid and liquid exist above that temperature. The liquidus is the temperature at a given composition and pressure of the system in which solid and liquid exists below that temperature,
1000
1500
2000
Solidus
400
600 Partial melting
500
Rising Diapir (solid)
Figure Ml Partial melting of peridotite at oceanic rises is favored by decompression of relatively dry, solid peridotite ( < 0.4 wt% H 2 0) close to its melting point (after Wyllie, 1981 ). A solid diapir may rise through the lower mantle and could intersect the solidus at about 250 km. Further rise could cause more melting with gradual decrease in temperature.
but only liquid exists above that temperature. Thus, an assemblage of minerals in a rock at a given pressure will only begin to melt if the temperature of the rock is brought above the solidus. Normally the upper mantle and crust of the Earth are solid, but some circumstances such as decompression, heating, or addition of H 2 0 may bring rocks above the solidus and favor partial melting of upper mantle or lower crustal rocks. For example, much of the upper mantle at oceanic rises or hot spots on the ocean floor may consist of relatively dry peridotite
374
MAGMATIC PROCESSES
with a small percent H 2 0 that is close to the melting point of the peridotite. Gradual rise of the solid peridotite to shallower levels may result in the intersection of the peridotite solidus and begin partial melting (Figure Ml). Further rise and decompression could result in 20-30% melting of the peridotite, producing the abundant tholeiitic basalts observed
Data Loading...
