The composition and origin of sulfides in peridotitic xenoliths from Ruddon’s Point (Fife, Scotland)

Permian mafic volcanic rocks occurring in southern terrains of Scotland (United Kingdom) are rich in peridotitic xenoliths providing insight into the composition of the Subcontinental Lithospheric Mantle (SCLM) beneath this area. Peridotites from the Ruddon’s Point (Fife) xenolith suite form four textural groups: (1) protogranular and (2) porphyroclastic lherzolites, (3) equigranular wehrlites and (4) lherzolites transitional between protogranular and equigranular peridotites. The SCLM beneath southern Scotland was affected by reaction with an alkaline melt resulting in clinopyroxene crystallization (wehrlitization) and decrease of Fo in olivine from primary (protogranular and porphyroclastic) lherzolites (Fo_88.5-90.0) through transitional to equigranular (Fo_80.0-85.0) peridotites (Matusiak-Małek et al., 2022).

The sulfides occurring in the peridotites form oval, elongated or irregular grains enclosed in pyroxenes and olivine, or interstitial between these phases. The abundance of sulfides  increases from the transitional lherzolites (mean = 0.009 vol.‰), through equigranular and porphyroclastic peridotites (0.026 and 0.029 vol.‰, respectively) to protogranular lherzolites (0.050 vol.‰). Sulfide minerals present in all textural groups are pentlandite (Pn) and chalcopyrite (Ccp). There is generally an absence of pyrrhotite (Po), but protogranular and “transitional” lherzolites contain minor amounts. Porphyroclastic lherzolites occasionally contain millerite (Mlr) and covellite (Cv). The sulfides from the equigranular and protogranular peridotites are more enriched in Cu-, and depleted in Ni-phases (Po₀Pn₇₁Ccp₂₉ and Po₄Pn₆₈Ccp₂₇, respectively) in comparison to sulfides from the porphyroclastic and transitional peridotites (Po₀Pn₈₀Ccp₂₀ and Po₆Pn₈₃Ccp₁₂, respectively). The Cu/(Cu+Fe) is homogenous in sulfides of all the textural types, whereas Ni/(Ni+Fe) in pentlandite is homogenous only in transitional and equigranular peridotites (0.64–0.65 and 0.55–0.59, respectively) in contrast to porphyroclastic and protogranular ones (0.54–0.68 and 0.52–0.64, respectively). The only significant difference in trace element composition of sulfides appears in the concentrations of Co and Zn which  are  4894 ppm and 2214 ppm, respectively, in the protogranular peridotites, compared to 30090 ppm and 1391 ppm, respectively, in the transitional peridotites.

The more primitive protogranular and porphyroclastic lherzolites  are characterized by the highest sulfide abundances in comparison to the sulfides from melt-metasomatized equigranular wehrlites, with no significant differences  in sulfide mineral and chemical (major and trace elements) composition between groups. Thus, activity of the alkaline silicate melts responsible for wehrlitization of the primary lherzolites seems not to influence the sulfide enrichment in the SCLM beneath S Scotland. The presence of Cv and Mlr in lherzolites suggests alteration by hydrothermal, post-volcanic activity, affecting the xenoliths after the exhumation to the surface by basaltic lavas.

Matusiak-Małek, M., Kukuła, A., Matczuk, P., Puziewicz, J., Upton, B.J.G., Ntaflos, T., Aulbach, S., Grégoire, M., Hughes H.S.R. (2022). Evolution of upper mantle and lower crust beneath Southern Uplands and Midland Valley Terranes (S Scotland) as recorded by peridotitic and pyroxenitic xenoliths in alkaline mafic lavas. 4th EMAW TOULOUSE 2021 Book of Abstracts.