OLIVINE-RICH TROCTOLITES: GEOCHEMICAL AND MICROSTRUCTURAL EVIDENCE OF THEIR MANTLE ORIGIN

Olivine-rich troctolites (plagioclase dunites) conist of > 70% of olivine, with interstitial plagioclase, clinopyroxene and spinel. They are commonly found in oceanic lithosphere and peridotite massifs. Since their origin (mantle vs. cumulate) is poorly constrained their name within the rock classification schemes is still a subject of debate.

Olivine-rich troctolites from Krivaja peridotite massif in Bosnia-Herzegovina occur as massive outcrops with an area of several tens of square kilometres. They 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 olivine-rich troctolites 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 similar microstructures but 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 olivine-rich troctolites from Krivaja, as well as rocks with similar characteristics e.g. from the Mid-Atlantic Ridge (Drouin et al. 2010) and Italy (Renna and Tribuzo, 2011) are not cumulates but are of mantle origin and should therefore be called plagioclase dunites. An important characteristic of these plagioclase dunites is that the geometry of their interstitial phases (plagioclase and Cpx) mimics the geometry of melt observed in experiments with olivine and a basaltic melt. This shows that the melt distribution at mantle grain sizes is similar to the experimental melt distribution at grain sizes of a few tens of microns.