Probing the atomic structure of basaltic melts generated by partial melting of upper mantle peridotite (KLB-1): Insights from high-resolution solid-state NMR study

Abstract:

Probing the structural disorder in multi-component silicate glasses and melts with varying composition is essential to reveal the change of macroscopic properties in natural silicate melts. While a number of NMR studies for the structure of multi-component silicate glasses and melts including basaltic and andesitic glasses have been reported (e.g., Park and Lee, Geochim. Cosmochim. Acta, 2012, 80, 125; Park and Lee, Geochim. Cosmochim. Acta, 2014, 26, 42), many challenges still remain. The composition of multi-component basaltic melts vary with temperature, pressure, and melt fraction (Kushiro, Annu. Rev. Earth Planet. Sci., 2001, 71, 107). Especially, the eutectic point (the composition of first melt) of nepheline-forsterite-quartz (the simplest model of basaltic melts) moves with pressure from silica-saturated to highly undersaturated and alkaline melts. The composition of basaltic melts generated by partial melting of upper mantle peridotite (KLB-1, the xenolith from Kilbourne Hole) also vary with pressure. In this study we report experimental results for the effects of composition on the atomic structure of Na₂O-MgO-Al₂O₃-SiO₂ (NMAS) glasses in nepheline (NaAlSiO₄)-forsterite (Mg₂SiO₄)-quartz (SiO₂) eutectic composition and basaltic glasses generated by partial melting of upper mantle peridotite (KLB-1) using high-resolution multi-nuclear solid-state NMR. The Al-27 3QMAS (triple quantum magic angle spinning) NMR spectra of NMAS glasses in nepheline-forsterite-quartz eutectic composition show only ^[4]Al. The Al-27 3QMAS NMR spectra of KLB-1 basaltic glasses show mostly ^[4]Al and a non-negligible fraction of ^[5]Al. The fraction of ^[5]Al, the degree of configurational disorder, increases from 0 at X_MgO [MgO/(MgO+Al₂O₃)]=0.55 to ~3% at X_MgO=0.79 in KLB-1 basaltic glasses while only ^[4]Al are observed in nepheline-forsterite-quartz eutectic composition. The current experimental results provide that the fraction of ^[5]Al abruptly increases by the effect of composition as well as pressure in natural silicate melts. The changes of the fraction of highly coordinated Al in multi-component silicate glasses and melts with composition can provide insight into the changes of macroscopic properties (e.g., entropy, viscosity, and diffusivity) with varying composition of melt.