Aenigmatite, sodic pyroxene, arfvedsonite and associated minerals in syenites from Morotu, Sakhalin

Abstract

Aenigmatite, sodic pyroxene and arfvedsonite occur as interstitial minerals in metaluminous to weakly peralkaline syenite patches in alkali dolerite, Morotu, Sakhalin. Aenigmatite is zoned from Ca, Al, Fe³⁺-rich cores to Ti, Na, Mn, Si-rich rims reflecting the main substitutions Fe²⁺Ti⁴⁺⇌Fe³⁺, NaSi⇌CaAl and Mn²⁺⇌Fe²⁺. Aenigmatite replaces aegirine and ilmenite supporting the existence of a ‘no-oxide’ field in $f_{{\text{O}}_{\text{2}} }$ — T space. In one case aenigmatite has apparently formed by reaction between ilmenite and arfvedsonite. Titanian aegirine (up to 3.0 wt% TiO₂) and Fe-chlorite may replace aenigmatite. Sodic pyroxene occurs as zoned crystals with cores of aegirine-augite rimmed by aegirine and in turn by pale green aegirine containing 93 mol% NaFe³⁺Si₂O₆. Additional substitution of the type NaAl⇌CaFe²⁺ is indicated by significant amounts (up to 6 mol%) of NaAlSi₂O₆. Arfvedsonite is zoned with rims enriched in Na, Fe and depleted in Ca which parallels the variation of these elements in the sodic pyroxenes.

The high peralkalinity of the residual liquid from which the mafic phases formed resulted from the early crystallization of microperthite (which makes up the bulk of the syenites) leading to an increase in the Na₂O/(Na₂O+K₂O) and (Na₂O+K₂O)/Al₂O₃ ratios of the remaining interstitial liquid which is also enriched in Ti, Fe, and Mn. Bulk composition of the melt, $f_{{\text{O}}_{\text{2}} }$, temperature and volatile content were all important variables in determining the composition and stability of the peralkaline silicates. $f_{{\text{O}}_{\text{2}} }$ in the residual liquid appears to have been buffered by arfvedsonite-aegirine and later by the arfvedsonite-aenigmatite and aenigmatite-aegirine equilibria under $f_{{\text{O}}_{\text{2}} }$ conditions of a ‘no-oxide’ field. An increase in $f_{{\text{O}}_{\text{2}} }$, above that of the alkali buffer reactions, is inferred by an increase of Ti and Mn in aenigmatite rims. The latest postmagmatic vapour crystallization stage of the syenites is marked by extremely low $f_{{\text{O}}_{\text{2}} }$ which may have been facilitated by exsolution of a gas phase. Low $f_{{\text{O}}_{\text{2}} }$ is supported by the replacement of aenigmatite by titanian aegirine, and the formation of rare Ti-rich garnet with a very low (ΣTi⁴⁺+Fe³⁺)/(ΣTi+ΣFe) ratio of 0.51, associated with leucoxene alteration of ilmenite.