An experimental study of the role of partial melts of sediments versus mantle melts in the sources of potassic magmatism. (15th June 2019)

• Record Type: Journal Article
• Authors: Förster, Michael W.; Prelević, Dejan; Buhre, Stephan; Mertz-Kraus, Regina; Foley, Stephen F.
• Is Part Of: Journal of Asian earth sciences. Volume 177(2019)
• Page Start: 76
• Page End: 88
• Publication Date: 2019-06-15
• Subjects: Lamproite -- High-pressure experiments -- Metasomatism -- Post-collisional magmatism -- Mediterranean magmatism -- High-K lavas; Earth sciences -- Asia -- Periodicals; Sciences de la terre -- Asie -- Périodiques; Earth sciences; Asia; Periodicals; 555.05
• DOI: 10.1016/j.jseaes.2019.03.014
• Languages: English
• ISSNs: 1367-9120

Abstract

Graphical abstract: Highlights: We simulate mantle metasomatism by reacting sediment or basanite with dunite. Sediment/dunite reaction is a precursor for K-rich magmatism with orogenic signature. The reaction of melt with refractory mantle dynamically increases K/Na ratio. K-enrichment of melts is independent from the experiments' starting composition. Abstract: Potassium-rich lavas with K/Na of >2 are common in orogenic and anorogenic intraplate magmatic provinces. However, in the primitive mantle, the concentration of Na exceeds that of K by 10 times. The source of K-rich lavas thus needs to be either K-enriched or Na-depleted to account for high K/Na ratios. The geochemical and isotopic compositions of high 87 Sr/ 86 Sr post-collisional lavas show that their mantle source contains a recycled crustal component. These highly K-enriched lavas with crustal like trace element patterns are termed "orogenic lamproites" and are compositionally distinct from K-rich "anorogenic lamproites" that show lower 87 Sr/ 86 Sr and a trace element pattern that resembles that of primary mantle melts. For both groups the processes of K-enrichment within their source are uncertain and are thought to be linked to melts of sedimentary rocks for "orogenic lamproites" and low-degree melts of ultramafic mantle rocks for "anorogenic lamproites". In both cases, metasomatism of the mantle lithosphere is the precursor to K-rich magmatism. In this study we experimentally determine the effects of mantle metasomatism by sediment- and hydrous mantle melts. The experiments simulate the interaction of refractory lithospheric mantle and metasomatizing melt in a 2-layer reaction experiment. The sediment/dunite reaction experiments lead to formation of a strongly K-enriched phlogopite-pyroxenite layer sandwiched between the two starting materials. The low temperature of the sediment/dunite reaction runs at <1000 °C simulates a fore-arc subduction environment, in which the melts of sediment are consumed during interaction with dunite as the temperature is below the solidus of the produced phlogopite-pyroxenites. The hydrous mantle melt/dunite reaction run is simulated by reacting a hydrated basanite with dunite. Since the temperature of the reaction is higher than the solidus of the resulting phlogopite-pyroxenites (1200 °C), the hydrous melt is not consumed but flows further, increasing in K2 O and K/Na as it reacts with the refractory peridotite. In both cases, melts are enriched in K and K/Na increases by crystallizing a low K and low K/Na eclogitic residue. Compositions of glass and phlogopite from both types of reactions are comparable to glasses and phlogopites found within post-collisional lavas. Since the enrichment of K within the reaction zone is strongly controlled by the formation of low K/Na and low-K residues, metasomatic enrichment of the mantle lithosphere in K does not need a highly K-enriched metasomatic agent.