Kiyoaki Niida


Peridotites drilled at Site 920 of Ocean Drilling Program Leg 153 in the Mid-Atlantic Ridge near the Kane Transform (MARK) area were examined to understand the mode of magma channeling and modification of the upper mantle peridotites. This paper presents mineralogical data from a slow-spreading ocean ridge for peridotites that have been modified by magma channeling and for those that are free from the modification.

The chemical compositions of primary minerals in spinel harzburgite and lherzolite are highly uniform. Olivine core compositions concentrate around Fo = 91.0 and NiO = 0.37 wt%. Spinel cores also have a small compositional range of Mg# = 70-75 and Cr# = 27-31. Orthopyroxenes with Mg# = 90.5-91.6 have Al2O3 contents of 3.17 to 4.77 wt%. TiO2 and Na2O contents in primary clinopyroxenes are extremely low, less than 0.15 and 0.11 wt%, respectively. These results suggest that the MARK peridotites represent moderately depleted upper mantle peridotite. The chemical compositions of primary minerals are modified where the peridotites are crosscut by gabbro and pyroxenite dikes and veins. Such peridotites are enriched in FeO* and TiO2, and depleted in MgO, NiO, and Al2O3.

Spatial variations in chemical composition of minerals were examined across channels in Samples 153-920D-10R-2 (Piece 4, 35-41 cm) and 153-920D-13R-2 (Piece 7, 136-143 cm). Primary minerals in spinel harzburgite are enriched in FeO* and TiO2 within narrow replacement bands next to websterite veins. No evidence for melt extractions was detected in the harzburgite wall, indicating that the replacement occurred as a one-way modification process by melt addition from the channeling magma. The Fo-NiO systematics for the modified olivines support a model of open-system reaction between the wall harzburgite and the infiltration melt with a highly evolved composition. All of the results emphasize that the final magma channeling occurred in “cold” peridotites at a shallow level of the upper mantle.

Date of initial receipt: 7 August 1995
Date of acceptance: 14 December 1995

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