Jadeite, albite and nepheline as inclusions in spinel of chromitite from Hess Deep, equatorial Pacific: their genesis and implications for serpentinite diapir formation |
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| Authors: | Kyoko Matsukage Shoji Arai |
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| Institution: | (1) Department of Earth Science, Kanazawa University, Kakuma-machi, Kanazawa 920-11, Japan Fax: 81-762-64-5746 E-mail: kyokoala@kenroku.ipc.kanazawa-u.ac.jp, JP;(2) E-mail: ultrasa@kenroku.ipc.kanazawa-u.ac.jp, |
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| Abstract: | Polymineralic inclusions which consist of a few grains of diopside, enstatite, jadeite, nepheline, albite, pargasite, phlogopites
and olivine were found in chromian spinel in a chromitite pod and in troctolite from Hess Deep, equatorial Pacific. The inclusion
mineral suite in chromitite is characterized by Na-Al silicates, such as jadeite, nepheline and albite. Jadeite and nepheline
commonly coexist with enstatite, and tend to occur as interstitial grains between subhedral enstatite (or other minerals)
and host spinel. Albite, diopside and enstatite occur as equant inclusions. The mafic minerals in the inclusions have similar
chemistry to those found in the troctolite and dunite. The modes of occurrence and mineral chemistry of the inclusions are
controlled by magmatic precipitation, and subsequent reequilibration due to decrease of temperature in the uppermost mantle.
The mafic minerals in spinel inclusions were crystallized from a melt enriched in Cr and some incompatible components formed
by melt-mantle interaction process mixed to various extent with subsequently supplied more primary melt. Albite and nepheline
could also be formed at the magmatic stage. Jadeite was formed by a subsolidus reaction of albite and nepheline at low temperatures
(250–300 °C) at slightly less than 3 kbar. This requires a remarkable temperature decrease, at least locally, of the uppermost
mantle and crust. The Hess Deep rocks were formed in the uppermost mantle beneath a spreading-ridge axis at more than 1000 °C,
and were transposed outwards from the axis by corner flow. At the off-ridge conditions, the rocks were cooled and serpentinized
by circulation of sea water at the mantle depth to form jadeite in chromitite. The serpentinized portion could have risen
as a kind of serpentinite diapir through the thin crust up to the ocean floor.
Received: 24 January 1997 / Accepted: 6 November 1997 |
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