V21A-3004
The origin of olivine-rich troctolites/plagioclase-dunites

Tuesday, 15 December 2015
Poster Hall (Moscone South)
Gordana Garapic1, Ulrich Faul2, Seth C Kruckenberg3, Joshua Wiejaczka1 and John Newton1, (1)SUNY College at New Paltz, Department of Geology, New Paltz, NY, United States, (2)Massachusetts Institute of Technology, Cambridge, MA, United States, (3)Boston College, Chestnut Hill, MA, United States
Abstract:
Olivine-rich troctolites or plagioclase dunites are a type of rock commonly found in oceanic crust and peridotite massifs that does not fit into a IUGS classification. Part of the reason is that their origin is poorly constrained, in particular whether these rocks are cumulates or residual mantle rocks. To avoid implications for origin or process Blackman et al. 2006 defined as olivine-rich troctolites rocks that contain > 70% olivine,with plagioclase, cpx and spinel.

We examined this type of rock in Krivaja peridotite massif in Bosnia-Herzegovina where it occurs as massive outcrops with an area of several tens of square kilometres. The plagioclase dunites are underlain by peridotites that contain plagioclase patches indicative of melt migration. These peridotites are progressively depleted of pyroxene and cross-cut by gabbro veins.

The plagioclase dunites have Mg# predominantly from 89 - 90 and Ni contents from 2500 - 3500 ppm, similar to the peridotites. EBSD mapping of whole thin sections shows orientation distribution functions (odf) that are unlike any of the fabric types observed in naturally or experimentally deformed rocks with a strong maximum in (001) near the foliation plane (although this plane is poorly defined) and weak girdles of the other two axes perpendicular to it. For comparison, we also mapped known cumulates from the Rum and Stillwater layered intrusions with somewhat lower Mg# of 84 - 86. The odf of these samples show a strong maximum of (010) perpendicular to the foliation plane and weak girdles of the other two axes in the plane. This fabric type has been explained by crystal settling and compaction in a magma chamber. Together these observations show that the plagioclase dunites from Krivaja, as well as rocks with similar characteristics e.g. from the Mid-Atlantic Ridge (Drouin et al. 2010) and Italy (Renna and Tribuzio, 2011) are not cumulates but are of mantle origin. An important characteristic of the plagioclase dunites is that the plagioclase, as well as more rare poikilitic cpx occurs interstitial to olivine. These interstitial phases mimic the geometry of melt observed in experiments with olivine and a basaltic melt. These observations confirm that the melt distribution at mantle grain sizes is similar to the experimental melt distribution at grain sizes of a few tens of microns.

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