Halogen (F, Cl and Br) systematics in alkaline to peralkaline magmatic rocks and their constituting minerals

Abstract

Comprehensive investigations were conducted on halogen (F, Cl and Br) distributions in alkaline to peralkaline rocks and their constituting halogen-bearing minerals (e.g., apatite, biotite, amphibole, titanite, etc.). Olivine melilitites and olivine nephelinites from Upper Rhine graben contain small amounts of Cl and Br with respect to associated evolved rocks such as tephrites and phonolites. Significant differences are not found in their F contents. The low Cl and Br contents of olivine melilitites and olivine nephelinites are similar to estimates for the primitive mantle, whereas the F contents of these samples are much higher. This probably points to a relatively F-rich mantle beneath Central Europe. Similar to oceanic basaltic samples, the investigated alkaline rocks generally present fairly uniform Cl/Br ratios (370 ± 120), suggesting that magmatic processes have limited effects on the fractionation of Cl and Br. Apatites from silicate rocks of the Kaiserstuhl Alkaline Complex contain higher Cl, Br, S, Fe, Mn, Th, U and Si contents but lower Sr and Nb contents than apatites from associated carbonatites. Apatites from some transitional samples (a carbonate-bearing melilititic dyke rock and a diatreme breccias which contains both carbonatitic and silicate fragments) show systematic zonations and record the compositional variations between apatites from silicate rocks and apatites from carbonatites. This provides new constraints for the genetic relations between alkaline silicate rocks and associated carbonatites in Kaiserstuhl, and also suggests that Cl and Br contents of apatite are potentially ideal indicators for magma evolution. Apatite, biotite, amphibole and titanite from a series of alkaline rocks of Tamazeght complex show variable halogen concentrations. In general, the Cl contents of the minerals from gabbros and monzonites are higher than those from pyroxenites and syenitic rocks. Calculations of the partitioning coefficients for F and Cl between coexisting biotite and amphibole show relatively constant values of 1.1 and 0.3, respectively. However, the KD values of F and Cl for other coexisting mineral pairs are highly variable depending on the crystal chemistry of mineral, halogen contents and prevailing temperature of melt. The generally observed and preferred partitioning sequence for F is apatite ≫ biotite, amphibole, titanite, whereas for Cl it is apatite > amphibole > biotite.