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

This study questions the assumption that silicate melts preserved as glass inclusions in minerals and as interstitial films or pockets in mantle xenol iths have identical chemical compositions to one another and are both suita ble for inferring deep mantle melt compositions. Theoretical models of the elastic behavior of melt inclusions indicate that only Limited decompressio n of the inclusion takes place during ascent of the host xenolith, whereas the pressure of an interstitial melt follows the external pressure. Consequ ences of such behavior are considered using simple model systems, such as t he alkali-bearing system forsterite-nepheIine-SiO2. With decreasing pressur e, phase boundaries shift to silica normative compositions. Consequently, r eequilibration of small-degree melts of peridotite, which are characterized by olivine-nepheline normative compositions at moderate pressure, yield qu artz normative compositions at a lower pressure. Also, melt inclusions are simpler systems, i.e. the trapped melts are in contact with a single minera l phase, in contrast with interstitial glasses. We show here that experimen tal heating of the inclusions restores the original melt composition of mel t inclusions prior to cooling, which is not possible for interstitial glass es. Data obtained for rehomogenized glass inclusions and interstitial glass es associated in the same xenoliths from intraplate ocean islands and subdu ction zones confirm the model predictions. In intraplate nodules, the highl y silicic, alkali-rich melts preserved as glass inclusions inside minerals are olivine-nepheline normative and represent high-pressure (1 GPa) near-so lidus 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 z one settings, exsolution of the oversaturated H2O-rich volatile phase and d ecompression cause conflicting effects on the composition of the melts trap ped in xenoliths. The composition of glass inclusions is characterized by h igher levels of volatile elements, mainly H2O, and by a higher quartz norm than interstitial glasses. This is consistent with the hypothesis that the glass inclusions represent quenched melts from H2O-saturated peridotite or amphibolite/eclogite systems, whereas the composition of the interstitial g lasses indicates reequilibration at low pressure, probably induced by the H 2O-rich volatile loss (C) 1999 Elsevier Science B.V. All rights reserved.