عنوان

The petrology and geochemistry of Merapi Volcano, central Java, Indonesia

TLpq303874255

انگلیسی

The petrology and geochemistry of Merapi Volcano, central Java, Indonesia

[Thesis]

M.-A. del Marmol

The Johns Hopkins University

1990

402

Ph.D.

The Johns Hopkins University

1990

Merapi (125 km) is a large, active strato volcano on the segmented Indonesian volcanic front. It is composed of numerous pyroclastic flows, lava flows, and lahars, and characterized by classic flank domes. Its lavas are calc-alkaline hi-Al basalts and andesites ranging in SiO2 from 49 to 56 (% wt.), having the usual characteristics of low MgO, P2O2, TiO2, and high Al2O3, FeO and CaO. However, these lavas are all unusual in their almost uniform K2O of about 2.0 wt.%. Aside from the constant potash content and significant differences in Sr, Rb and initial Sr-87/86 ratios, Merapi lavas are exceedingly similar to central Aleutian lavas (e.g. Atka). The lavas themselves are highly crystalline (25-55 vol %) and porphyritic. They contain plagioclase, clinopyroxene, magnetite, olivine (basalts), orthopyroxene (andesites), and amphibole (domes and high crystalline lavas). The plagioclase population is dominated by twinned and zoned crystals of An70-60, which are accompanied by a small population of unzoned high An (80-90) crystals. These high An crystals evidently originate from gabbroic xenoliths. The common olivine is Fo69-72, but occasional more forsteritic xenocrysts and xenoliths (up to Fo90) are also present. "Mantle" olivine as well as crustal sediments and gabbros have contaminated the magma. Lindsley's two pyroxene thermometer gives temperatures of 1050-900 for the highly crystalline lavas, and the fugacity of oxygen is 2-4 units above FMQ. Comparison of crystallinity, temperature, and fO2 with Baker's experiments on HAB of Atka indicates crystallization of the basaltic magmas at 2-5 kb and 2% H2O, whereas the andesites crystallized at <2 kb with 2% H2O. Merapi magmas are undersaturated with respect to water. When one also considers the fact that olivine is never a liquidus phase, it indicates that these lavas can not be the results of a mantle partial melt. The cyclic activity of Merapi is controlled by the high crystallinity and viscosity of the magma, as well as the small amount of water. A slight change in one of these parameters may trigger a devastating eruption, an example of which occurred in 928 A.D. (Abstract shortened with permission of author.)

Earth sciences

Geology

Geophysics

M.-A. del Marmol

p

[Thesis]

276903

a

Y