Petrochemical variations among mildly peralkaline (comendite) obsidians from the oceans and continents
✍ Scribed by D. K. Bailey; R. Macdonald
- Book ID
- 104751054
- Publisher
- Springer
- Year
- 1970
- Tongue
- English
- Weight
- 650 KB
- Volume
- 28
- Category
- Article
- ISSN
- 0010-7999
No coin nor oath required. For personal study only.
✦ Synopsis
Eleven new analyses and modes of comendite obsidians are presented, and compared with all available data on similar rocks. Most specimens are aphyrie or contain only sparse phenocrysts, most commonly alkali feldspar. The oxides Si02, M~O3, Na~O and K~O total over ninety percent by weight in all analyses. Iron, as FeO, is the only other constituent rising above one percent by weight. When the analyses are projected into the system Na~O-K20-AI~Oa-SiO 2, oceanic and continental samples group differently. Oceanic specimens have a compositional spread ranging from trachytie to the quartz-feldspar cotectic zone, consistent with derivation through a traehyte magma stem. Continental eomendites show a strong correlation with the experimentally determined quartz-feldspar minima along a path of increasing peralkalinity. These differences presumably reflect the contrasting environments of magma generation, and suggest an origin by partial melting within the continental crust for the continental eomendite obsidians.