Control factors in Na pyroxene compositions in lawsonite blueschist subfacies rocks: coexisting phases and kinetics

Abstract

Na pyroxene (Napx) of the aegirine-jadeite series occurs as a major metamorphic mineral in metacherts and metabasalts of the lawsonite blueschist (LBS) subfacies in the Kurosegawa belt, Kyushu, Japan. Its jadeite content [X_Jd = ^VIAl/(^VIAl + Fe³⁺ + Ca)] is highly variable at the thin section scale and is crucially influenced by associated minerals, such as albite and sodium amphibole (Namp). Whereas the coexistence with albite and quartz imposes an upper limit on X_Jd in Napx through the equilibrium reaction Ab = Jd + Qz, this paper reveals that uniformly low X_Jd was observed when albite was absent in the metacherts and the coexistence with Namp also effectively reduced X_Jd. In the former case available Na and Al content is limited in metacherts and in the latter case the Al/Fe³⁺ distribution between Napx and Namp controlled the X_Jd in metacherts and metabaslts. Notably, the distribution coefficient [K_D = (Al/Fe³⁺)^Namp/(Al/Fe³⁺)^Napx systematically varied depending on the mode of growth: K_D ∼ 4 in metabasalts, where Napx developed replacing relict igneous pyroxenes (i.e., topotaxial growth), whereas K_D ∼ 1 in metacherts and metamorphic veins developed in metabasalts, where no relict phase was involved (i.e., nucleation and growth). Previous studies confirmed this trend, suggesting a convergence of K_D to 1 in higher grade rocks, such as epidote-blueschist and eclogite facies. This implies a kinetic control on the distribution process, particularly at lower temperatures (< 300 °C). In summary, low-grade Napx associated with igneous pyroxene may represent a metastable state; thus, those in metacherts can serve as a more reliable proxy for the equilibrium conditions of the clinopyroxene system in these grades.