| Autor: |
R.J. Bultitude, David H. Green |
| Rok vydání: |
1967 |
| Předmět: |
|
| Zdroj: |
Earth and Planetary Science Letters. 3:325-337 |
| ISSN: |
0012-821X |
| DOI: |
10.1016/0012-821x(67)90055-6 |
| Popis: |
The compositions of basaltic rocks containing high pressure xenoliths are used to select olivine nephelinite and picritic nephelinite compositions for study of high pressure melting and crystal fractionation relationships. In the olivine nephelinite composition (26% normative olivine) under anhydrous conditions, olivine is the liquidus phase up to 18 kb and is joined by clinopyroxene at lower temperatures. At 27 kb, clinopyroxene is the liquidus phase and both garnet and clinopyroxene occur in runs near the liquidus at 36 kb. A simlar sequence is found in the picritic nephelinite (36% normative olivine) but olivine persists as the liquidus phase up to 27 kb and garnet is the liquidus phase at 31.5 kb and 36 kb. Orthopyroxene was not observed in the melting interval of either composition under dry conditions. Experiments were carried out in which water added to the sample resulted in lowering of the liquidus temperature by 150–250°C at pressures of 13.5–30 kb. Under these conditions olivine is the liquidus phase up to 18 kb. At pressures of 18–22.5 kb in the olivine nephelinite and 22.5–27 kb in the picritic nephelinite aluminous orthopyroxene is the major near-liquidus phase (at temperatures of 1150°C–1250°C). Minor olivine and garnet accompany the orthopyroxene. Electron microprobe analyses of pyroxenes, olivine and garnet enable calculation of high pressure fractionation trends for both the dry melting and wet melting conditions. While fractionation under dry conditions at high pressure may account for some of the variation among natural undersaturated magmas, the fractionation trend does not conform to the natural chemical variation in the series olivine basanite → olivine nephelinite → olivine melilite nephelinite. However, the fractionation trend at 18–27 kb under ‘wet’ conditions (PH2O < Pload) is dominated by orthopyroxene and separation of orthopyroxene accompanied initially by minor olivine, and at lower temperatures by garnet, produces derivative liquids closely matching the olivine nephelinite → olivine melilite nephelinite series. The results provide evidence for development of the highly undersaturated olivine and melilite nephelinite lavas by either extreme fractionation of picritic magmas or by low degrees of partial melting of the mantle at depths of 60–100 km. An essential factor in their genesis is the presence of small amounts of water so that the magmas are produced at temperatures of 150–250°C below that required for dry magma production. |
| Databáze: |
OpenAIRE |
| Externí odkaz: |
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