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Podiform chromitite formation in a low-Cr/high-Al system: An example from the Southwest Indian Ridge (SWIR) |
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| Authors: | Betchaida D Payot Shoji Arai Henry J B Dick Natsue Abe Yuji Ichiyama |
| Institution: | 1. Department of Earth Sciences, Kanazawa University, Kakuma, Kanazawa, Ishikawa, 920-1192, Japan 2. National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City, 1101, Philippines 3. Department of Marine Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, 02543, USA 4. Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan 5. Japan Agency for Marine-Earth Science and Technology, Kochi Core Center, 200 Monobe-Otsu Nankoku, Kochi, 783-8502, Japan |
| Abstract: | Recent reassessment of abyssal peridotites obtained during the dredging of the oblique supersegment and the easternmost subsection of the Southwest Indian Ridge by the R/V Knorr Cruise 162 and the R/V Yokosuka YK98-07 revealed the occurrence of dunites containing podiform chromitites and dunites with variable chromite concentration closely associated with lherzolite and harzburgite. The size of the chromitite pods varies from a few mm to 2 cm in width. Chromites in the podifom chromitites have very low Cr# (=0.22–0.23) and low TiO2 (<0.17 wt%). They are almost free of silicate inclusions except for a few euhedral sulfide grains which occur far from cracks and lamellae and are considered primary in origin. The lherzolite which possibly represents the wallrock hosting the dunites with podiform chromitites also show low spinel Cr#(=0.16) and low Cr# in the clinopyroxenes (=0.09–0.10) and orthopyroxenes (=0.07–0.09). The small size of the SWIR podiform chromitites is strongly controlled by the low Cr/Al available in the wallrock and the invading melt. The presence of sulfide inclusions and the absence of PGEs further attest to the low Cr/Al (i.e. low refractoriness) in the system involved in the genesis of the SWIR podiform chromitites. Lastly, the discovery of podiform chromitites in the SWIR implies that the formation of podiform chromitite at mid-oceanic ridges, regardless of its spreading rate, is highly possible. |
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