On the preservation of mantle information in ultramafic nodules: glass inclusions within minerals versus interstitial glasses

Abstract

This study questions the assumption that silicate melts preserved as glass inclusions in minerals and as interstitial films or pockets in mantle xenoliths have identical chemical compositions to one another and are both suitable for inferring deep mantle melt compositions. Theoretical models of the elastic behavior of melt inclusions indicate that only limited decompression of the inclusion takes place during ascent of the host xenolith, whereas the pressure of an interstitial melt follows the external pressure. Consequences of such behavior are considered using simple model systems, such as the alkali-bearing system forsterite–nepheline–SiO 2. With decreasing pressure, phase boundaries shift to silica normative compositions. Consequently, reequilibration of small-degree melts of peridotite, which are characterized by olivine–nepheline normative compositions at moderate pressure, yield quartz normative compositions at a lower pressure. Also, melt inclusions are simpler systems, i.e. the trapped melts are in contact with a single mineral phase, in contrast with interstitial glasses. We show here that experimental heating of the inclusions restores the original melt composition of melt inclusions prior to cooling, which is not possible for interstitial glasses. Data obtained for rehomogenized glass inclusions and interstitial glasses associated in the same xenoliths from intraplate ocean islands and subduction zones confirm the model predictions. In intraplate nodules, the highly silicic, alkali-rich melts preserved as glass inclusions inside minerals are olivine–nepheline normative and represent high-pressure (1 GPa) near-solidus melts in equilibrium with a peridotitic assemblage. In contrast, the silica-normative composition of the interstitial glasses records their last pressure of equilibration, i.e., shallow-level conditions. In subduction zone settings, exsolution of the oversaturated H 2O-rich volatile phase and decompression cause conflicting effects on the composition of the melts trapped in xenoliths. The composition of glass inclusions is characterized by higher levels of volatile elements, mainly H 2O, and by a higher quartz norm than interstitial glasses. This is consistent with the hypothesis that the glass inclusions represent quenched melts from H 2O-saturated peridotite or amphibolite/eclogite systems, whereas the composition of the interstitial glasses indicates reequilibration at low pressure, probably induced by the H 2O-rich volatile loss