TEE BEHAVIOR OF INCLUSIONS IN IGNEOUS MAGMAS 527 



adjustments took place very rapidly compared with concentration 

 adjustments) a mass of liquid about (AbaAuj) containing in it about 

 20 per cent of crystals of the kind in equilibrium with it (about 

 AbiAng) and containing also the inclusions converted to liquid, the 

 whole at a temperature of about 1470°. There is no objection, 

 therefore, to the conversion of an inclusion into liquid if the inclusion 

 is suf&ciently contrasted with the magma in composition in the 

 proper direction; nor is such melting of an inclusion to be regarded 

 as evidence of superheat in the magma. 



This condition, in which liquid inclusions are contained in the 

 magma, is of course a temporary one and it is rather the end result 

 that has particular significance in petrogenesis. Final adjustment 

 of concentration takes place by formation of a single homogeneous- 

 liquid and adjustment of the composition of the crystals to that in 

 equilibrium with this liquid. This necessitates a further drop in 

 temperature to about 1460° where the whole mass consists of the 

 liquid Ab45Anss containing somewhat less than 20 per cent crystals 

 of the composition about AbiAn4. Thus the net result of the addi- 

 tion of inclusions of composition more sodic than the liquid is that 

 the inclusions become a part of the liquid and at the same time calcic 

 crystals are formed in amount slightly less than the amount of inclu- 

 sions added, the heat needed in order to make the inclusions part of 

 the liquid being supplied by the formation of crystals. In the sense 

 that they become a part of the liquid the inclusions are dissolved, 

 but the process is not simply the formation of a liquid whose com- 

 position is the sum of that of the magma and the inclusions. 



Let us now observe how these effects are carried over into more 

 complex systems. Figure 6 is the equiUbrium diagram of the system 

 diopside: anorthite: albite for which we have discussed the heat 

 quantities on an earlier page and found that the solution heat of any 

 solid phase can be regarded as substantially equal to its latent heat 

 of melting. A liquid of composition A is, at 1250°, just saturated 

 with plagioclase of composition AbiAn4 approximately. If foreign 

 inclusions consisting of the plagioclase AbiAn^ were added to this 

 liquid we would have an effect strictly analogous to that described 

 for simple plagioclase mixtures. The liquid would tend to make the 

 inclusions over into AbiAn4 which takes place with evolution of heat. 



