High-magnesian Andesite Research Articles

46. Magma mixing process during ascent and eruption of high-magnesian andesite and related magmas from Setouchi zone(Abstracts of Papers Presented at the Fall Meeting of the Society)

46. Magma mixing process during ascent and eruption of high-magnesian andesite and related magmas from Setouchi zone(Abstracts of Papers Presented at the Fall Meeting of the Society)

Melting experiments on a high-magnesian andesite

Melting experiments were conducted on a high-magnesian bronzite olivine andesite (Teraga-Ike andesite) which is considered to be a primary andesite. The high-magnesian andesite magma is in equilibrium with both olivine and orthopyroxene at about 15.5 kbar and 1080°C under H 2O-saturated conditions and at lower pressure and higher temperature under H 2O-undersaturated conditions. This suggests that high-magnesian andesites could be generated by the partial melting of upper mantle peridotite containing a small amount of H 2O.

Existence of andesitic primary magma: An example from southwest Japan

A high-magnesian andesite (SiO 2 58.50%, MgO 9.47%) occurs at Teraga-Ike in southwest Japan. It belongs to the Setouchi volcanic rocks of middle Miocene age and carries olivine and bronzite as phenocrysts (4.2 and 1.4 modal percent, respectively). This andesite is characterized by Mg-values as high as 75, suggesting that it may be a primary andesite. Olivine phenocrysts (Fo 87–91) are in equilibrium with the groundmass (= liquid) on the basis of Fe-Mg exchange partitioning between olivine and liquid, and they have high NiO contents (up to 0.45%). Chromite inclusions in olivine and rarely bronzite have high Cr 2O 3 contents (max. 54.87%). These features strongly suggest that the Teraga-Ike andesite keeps the chemical composition of the primary magma generated in the upper mantle, and therefore verify the existence of primary andesite magmas.

Origin of high magnesian andesites : genetic relation to coesisting basalts(Abstracts of Papers Presented at the Spring Meeting of the Society, 1981)

Origin of high magnesian andesites : genetic relation to coesisting basalts(Abstracts of Papers Presented at the Spring Meeting of the Society, 1981)