Careful studies of the mineralogy of andesitic lavas from the Montserrat Soufriere Hills volcano have provided evidence that magma within the underlying magma chamber has been subject to strong convective mixing.
Geologists
from the University of Bristol (Couch et al., 2001) have found that many of the
mineral crystals within the porphyritic Soufriere Hills andesite are not in
chemical equilibrium with the surrounding glassy matrix; and that many crystals
have mineralogically distinctive outer rims, suggesting that the outer (later)
part formed under different conditions than the inner (earlier) part. The plagioclase feldspars, for example, commonly have rims
which are more calcic than their cores – which is contrary the characteristics
of typical plagioclases which have formed from a progressively cooling melt.
| As predicted by the Bowen Reaction Series, a plagioclase crystal which forms early in the cooling sequence (when the magma is hot) will tend to be anorthitic (Ca-rich). As the magma cools, plagioclase of progressively more albitic composition (Na-rich) will form around the original crystal. The result is that most plagioclase crystals in the rock will have a relatively anorthitic core and a relatively albitic rim. |
One
possible explanation for the features observed at Soufriere Hills is that magma
of two different compositions have mixed together, and that the
crystals which formed early from a relatively felsic magma have been
exposed to a more mafic magma later in their history.
Couch et al. propose an alternative hypothesis, in which the body of magma with an essentially consistent composition (andesitic in this case) has been heated from below by an influx of hot basaltic magma. They suggest that the resulting strong thermal gradient between the two parts of the magma has caused the original magma to start convecting. The consequence of this is that minerals formed at relatively low temperatures (< 875° C) in the upper and middle parts of the chamber are then exposed to higher temperatures (900 to 1000° C), and that they form rims which are mineralogically consistent with those higher temperatures.
Based on estimates of the temperature differences between the two magmas (~100° C), the authors suggest that convection is taking place at quite significant rates – between 5 and 20 m/day.
Reference
Couch, S., Sparks, R. and Carroll, M., Mineral disequilibrium in lavas explained by self-mixing in open magma chambers, Nature, V. 411, p. 1037-1039 (June 2001).
Steven Earle, 2001. Return to Earth Science News