Sample Description

Sample Description

Sample description

In the Ural Mountains 15 Uralian-Alaskan-type mafic-ultramafic complexes form a 900 km long linear belt in the western section of the Tagil-Magnitogorsk zonealong the 60-th meridian. The belt is known as the Uralian Platinum belt and isbordered by two N-S trending fault zones, the Main Uralian Fault in the west and the Serov-Mauk Fault in the east (Online ResourceMOESM1a). Details on the geology of these complexes are summarized by Efimof (1977) and more recently Savelieva et al. (2002).

The samples for the present study were collected from the complexes of Nizhny Tagil, Sevtley Bor, and Kytlym (Online ResourceMOESM1). The Nizhny Tagil complex predominantly consists of dunite, which is overlain by clinopyroxenites with, in places, a few meters of wehrlite at the contact. Mafic rocks occur only in some small lens-shaped bodies in the northern part and at the south-eastern rim of the complex (Online ResourceMOESM1d; Chashukhin et al., 2002; Savelieva et al., 2002).

A dunite core that is surrounded by a rim of clinopyroxenite forms the Svetley Bor complex (Online ResourceMOESM1c). The clinopyroxenite locally contains hornblendites where clinopyroxene is replaced by hornblende. Mafic rocks are not exposed (e.g. Garuti et al., 1997).

In the western part of the Kytlym complex a number of dunite bodies (Kosva Block, and Tilay-Konjak block) are surrounded by clinopyroxenite (Online ResourceMOESM1b; Garuti et al., 1997; Savelieva et al., 1999, 2002; Chashukhin et al., 2002). Several successions of dunite – wehrlite – clinopyroxenite in the western part of the Tilay-Konjak block are surrounded by mafic rocks (Online ResourceMOESM1b). The mafic and ultramafic rocks in the SW part of the Kytlym complex (Kosva block), Nizhny Tagil and Svetley Bor have similar compositions and are thought to be formed from alkaline silica undersaturated parental melts (Krause et al., 2007; Krause, 2008). The eastern part of the Kytlym complex is interpreted to be derived from a tholeiitic parental melt. The Tilay-Konjak block represents a transition zone where melts from both sources were mixed producing tholeiitic to subalkaline mafic rocks (Krause, 2008).

A detailed lithological and mineralogical description of the studied ultramafic rocks is given by Krause et al. (2007). Dunite and wehrlite contain, medium to fine-grained, subhedral to euhedral grains of chromian spinel up to 2 mm in the matrix or as inclusion up to 200 µm in silicates(Online Resource MOESM3a). The degree of serpentinization varies between 5 and 20 in some cases up to 50 percent. The matrix of clinopyroxenite cumulates and hornblendite dykes and veinscontain subhedral to anhedral spinel grainsup to 3 mm (Online Resource MOESM3b) and euhedral spinel inclusions up to 250 µm in silicate phases.

The clinopyroxene-rich mafic rocks in the Uralian-Alaskan-type complexes in the Ural Mountains were first described as Tilaites by Dupark and Tikhonowitch (1920). These are coarse grained rocks which often show a porphyric texture and contain mainly clinopyroxene, less feldspar and a significant amount of olivine. In accordance with the IUGS nomenclature (Gillespie and Styles, 1999) we will call them feldspar-bearing clinopyroxenite. Based on their different mineralogical and chemical composition three groups of mafic rocks have been distinguished (Krause, 2008). Nepheline (Ne-) clinopyroxenites represent the most fractionated products of the melt that initially formed the ultramafic cumulates. Beside coarse grained clinopyroxene phenocrysts Nepheline (Ne-) -clinopyroxenites contain olivine, clinopyroxene, phlogopite, hornblende, intermediate plagioclaseAn28-48, K-feldspar, nepheline and anhedral to subhedral spinel up to 1.5 mm in the matrix (Online ResourceMOESM3c). Euhedral spinel up to 150 µm is enclosed in olivine and clinopyroxene. Ne-clinopyroxenites occur in Nizhny Tagil and to the southwest of the Kytlym complex.

Bytownite (By-) clinopyroxenite shares many mineralogical and textural features with Ne-clinopyroxenite, but it is devoid of nepheline and K-feldspar, has plagioclase with a higher An content (predominantly bytownite; An = 68-87) and contains rare orthopyroxene. Spinel forms anhedral to subhedral grains in the matrix and euhedral inclusions in the silicate phases. By-clinopyroxenites occur together with Ne-clinopyroxenites in the Nizhnii Tagil and in the Kosva Block in the western part of Kytlym. In the Tilay-Konjak Block in the eastern part of the Kytlym complex Ne-clinopyroxenites are absent and due to the different composition of silicate minerals these By-clinopyroxenites are recognized as a separate group (Krause, 2008).

Analytical Methods

Thirty-eight polished thin sections were used for the microprobe measurements. Spinel was analysed with the Jeol JXA 8200 microprobe of the Max-Planck-Institute for Chemistry and the Jeol JXA 8900RL microprobe at the Institute of Geosciences of the University of Mainz. We used natural minerals and oxides (Si, Ti, Al, Fe, Mg, Mn, Cr and Zn) and pure element standards (V, Co) as standards. The oxide minerals were measured with five WDS spectrometers at 20 kVacceleration voltage, a probe current of 12 or 20 nA and either a focussed beam or a beam diameter of 2 µm. Counting times varied between 20 and 80 s on the peak and 10 and 40 s on the background. The spinel stoichiometry was calculated using the method of Barns et al. (2004) assuming that all Ti is in the ulvöspinel component and that all V is present as Fe7V2O10. Ferrous and ferric iron were then calculated assuming stoichiometry.

X-ray element distribution maps of exsolved and homogeneous spinels were obtained with the Jeol JXA 8900RL microprobe at the Institute of Geosciences of the University of Mainz at 15 or 20 kV and 12-20 nA with a focussed electron beam. Chromium, Mn, Al, Mg and Ti were measured with WDS spectrometers at counting times between 100 and 200 ms, Fe, Ca and Si with an EDS spectrometer, and in addition the BSE signal was recorded. The step size varied between 0.2 and 2 µm.

We calculated the total area and area fractions of the different spinel phases in 176 exsolved spinel grains (Online ResourceMOESM6). X-ray element maps and reflected light images are converted to gray scale maps and the matrix is removed using a vector graphic software (Corel Draw®). The area fractions of spinel are then calculated using the ImageJ® software (Abramoff et al., 2004). Small grains (<5 µm) have not been used for the estimation of the initial composition because in this case the calculated area fractions are strongly affected by thin section sectioning.