NI-RICH SULFIDE INCLUSIONS IN EARLY LAMPROITE MINERALS

Abstract

Magmatic sulfide inclusions were found in early lamproite minerals from three localities: in olivine-1 of olivine hyalolamproite from Smoky Butte (Montana, USA), in olivine-1 of olivine orendite from Leucite Hills (Wyoming, USA), in orthopyroxene and phlogopite of fortunite from SE Spain. Sulfide blebs usually coexist with fluid and silicate melt inclusions in the host mineral. Occasionally they are found in silicate melt inclusions. Sulfide inclusions contain the following assemblages: pentlandite + chalcopyrite, monosulfide solid solution (Mss) + chalcopyrite, violarite — in the Smoky Butte lamproites; Mss + pentlandite and Mss + chalcopyrite in the Leucite Hills orendites; pentlandite + heazlewoodite, pentlandite + pyrrhotite and pentlandite + godlevskite — in the SE Spain fortunites. Pentlandite broadly varies in Ni/(Ni+Fe) ratio from 0.44 to 0.71. The Smoky Butte Mss has the highest concentrations of Ni (up to 58.2 wt.%) and is close to ideal (Ni,Fe)S (Me/S — 0.98–1.00), while the Leucite Hills Mss contains up to 27.5 wt.% Ni and has a composition intermediate between hexagonal (Fe,Ni) 9 S 10 (Me/S — 0.9) and monoclinic (Fe,Ni) 7 S 8 (Me/S — 0.875) compositions. Chalcopyrite is similar to ideal composition of the CuFeS 2 -type. Pyrrhotite compositions vary from FeS to (Fe,Ni) 9 S 10 . Ni-rich minerals were found in enstatite of fortunites. They are identified chemically as heazlewoodite solid solution (Ni — 51.32.6 wt.%). Probable composition for initial sulfide melt (ISM) calculated for globules vary for different lamproite localities. In the Smoky Butte lamproites it is undersaturated in sulfur and close in composition to pentlandite (Me/S — 1.10) with approximately equal Ni/(Ni+Fe) ratio (0.51–0.53) and contains up to 33.3 wt.% Ni and up to 5.3 wt.% Cu. Initial sulfide melt in fortunites is also depleted in sulfur (Me/S — 1.11–1.21), but it shows broad variations in Ni content, from 27 to 51 wt.%. Initial sulfide melt in the Leucite Hills orendites is saturated in sulfur (Me/S — 0.91) and contains up to 23.4 wt.% Ni and up to 5.1 wt.% Cu. The presence of sulfide globules indicates that a small volume of sulfide melt was separated from the silicate liquid at the early stage of evolution of a primitive lamproite magma under T >> 1000 °C, P >> 1 kbar and f O 2 << NNO. High Ni concentrations in the initial mantle substratum from which a primitive lamproite magma is derived is likely to be responsible for the Ni-rich composition of the initial sulfide melt.