Diversity and origin of abyssal tholeiite from the Mid-Atlantic Ridge near 24° and 30° North latitude
✍ Scribed by Akiho Miyashiro; Fumiko Shido; Maurice Ewing
- Publisher
- Springer
- Year
- 1969
- Tongue
- English
- Weight
- 887 KB
- Volume
- 23
- Category
- Article
- ISSN
- 0010-7999
No coin nor oath required. For personal study only.
✦ Synopsis
On cursory examination of hand specimens and thin sections, the abyssal tholeiite in a dredge haul may appear to be uniform in composition. Chemical analyses of a considerable number of fragments, however, have always revealed the existence of regular compositional variation in them. The MgO content decreases with increasing SIC2. In abyssal tholeiites with relatively low A120 a contents, the SiCk, total iron, Na20 and P~O~ contents tend to increase and the MgO content tends to decrease with increasing iron/magnesia ratio, probably owing to crystallization differentiation.
In a certain dredge haul, high-alumina abyssal tholeiites (with AlzO a contents near or over 17%) occur in association with low-alumina abyssal tholeiites. The magma of high-alumina abyssal tholeiites would be generated from that of low-alumina abyssal tholeiites by differentiation at a depth around 30 kin.
In pillow lavas of abyssM tholeiite free from weathering and metamorphism, the chilled rim of the pillow usually has virtually the same chemical composition as the more crystalline core except for a decrease of KzO content toward the rim. On the other hand, the weathered rim of pillow lavas shows marked compositional change. The Fe2Oz/FeO ratio of unweathered abyssal tholeiite is in the range of 0.1 to 0.3. This ratio and the I-I~O-and H~O + contents increase with advancing weathering.