Supercooled Diopside Melt: Confirmation of Temperature-Dependent Expansivity Using Container-Based Dilatometry

Joachim Gottsmann (joachim.gottsmann @uni-bayreuth.de) & Donald B. Dingwell

Bayerisches Geoinstitut, Uni Bayreuth, D-95502 Bayreuth, Germany

One of the most fundamental aspects of the nature of silicate melts is their PVT equation of state. Yet the detailed nature of the temperature-dependence of volume was poorly approximated until quite recently. This changed with the application of dilatometric methods to melt densities by Knoche et al. (1992) and their observation of temperature-dependent expansivities. That method has remained, nevertheless, somewhat controversial up to the present, based as it was on an assumption of the equivalence of volumetric and enthalpic relaxation. Container-based dilatometry provides the opportunity to test that method and its assumptions. The expansivity of supercooled diopside liquid has been determined using techniques of container-based dilatometry. Two thermal strategies have been employed, one in which the sample is brought to equilibrium by long duration dwells at low temperatures (817°C) and one in which scanning dilatometry of the sample has been performed at somewhat higher temperatures (890-913°C). The results of both experiments yield a supercooled liquid expansivity for diopside liquid in the temperature range of 817-913°C of 84.4(2.8)x10⁴ cc/mol-deg. The expansivity is 65% higher than that obtained for diopside liquid obtained at superliquidus temperatures using buoyancy methods. Combined fitting of the new low temperature volume-temperature data from this study and the buoyancy-based superliquidus data yields a molar volume relationship for which the standard error of the fit is well within the experimental errors. This result reconciles the disparate values of expansivity measured at low temperatures in the supercooled state and at superliquidus temperatures and confirms the temperature-dependence of the expansivity of diopside liquid. Knoche et al.'s (1992) discovery that the expansivities of silicate melts are temperature-dependent, is thus confirmed.

Raman Spectroscopic Study of Fe(III)-Si Complexing in Acidic Solutions from 25 to 110°C

Robert Gout (gout@cict.fr)¹, Jacques Schott (schott@lucid.ups-tlse.fr)¹, Gleb S. Pokrovski (gleb@cnrs-orleans.fr)² & Julia Bidault¹

¹ Laboratoire de Géochimie, CNRS-OMP-Université Paul Sabatier, 39 Allées Jules Guesde, 31000 Toulouse, France

² Centre de Recherches sur la Synthèse et la Chimie des Minéraux (CNRS-CRSCM), 1A rue de la Férollerie, 45071 Orléans Cedex 2, France

Raman spectroscopic measurements were performed on aqueous acid silica-bearing solutions, Fe(III)-bearing solutions and Fe-silica bearing solutions at temperatures from 20 to 110°C. The spectrum of iron-free silica-bearing solutions (m_Si ¾ 0.02 mol/kg) is characterized by a single, completely polarized band at 785 cm^-1, which is attributed to the tetrahedral Si(OH)₄ molecule. The intensity of this band is proportional to Si concentrations up to 0.02 m of Si and is not affected by the increase of temperature. The spectrum of silica-free Fe(III)-bearing solutions at 20°C and pH = 0.8 (m_Fe = 0.1 mol/kg) exhibits a band at 500 cm^-1, which can be attributed to the vibration of the hydration sphere of the ferric ion (Fe(H₂O)₆³⁺). When the solution pH and/or temperature increase, one observes a new band at 380 cm^-1. This band can be attributed to Fe-O-Fe vibrations in the polymeric Fe oxy-hydroxide species produced via Fe³⁺ hydrolysis. The intensity of the 500 cm^-1 band allows direct calculation of the amount of Fe³⁺ ion in solution. In Fe(III)-silica solutions (m_Si = 0.02 mol/kg, m_Fe = 0.1 mol/kg, pH = 0.8), one observes a decrease in the intensity of the silicic acid band at 785 cm^-1 at temperature higher than 90°C. This decrease of the free Si(OH)₄ concentration is attributed to the formation of a Fe-silica complex according to the reaction:

Fe³⁺ + Si(OH)₄ = FeH₃SiO₄²⁺ + H⁺

From the intensities of the Raman bands at 785 cm^-1 and 500 cm^-1, the equilibrium constant of this reaction has been derived as a function of temperature.

Primary Magmas and Mantle Temperatures ­ An Overview

David Headley Green (director.rses@anu.edu.au)

Research School of Earth Sciences, The Australian National University, Australia

The sub-solidus mineralogy and the melting behaviour of lherzolitic upper mantle are experimentally well-determined, including the effects of small quantities of carbon and hydrogen (peridotite-C-H-O). Magmas of possible mantle origin can thus be evaluated with respect to possible pressure, temperature and (Source-Magma+Residue) relationships. Picritic N-MORB and E-MORB magmas are shown to have liquidus temperatures at 1 bar of 1325±20°C and are compatible with lherzolitic residue at 1.4 to 1.9 GPa, 1450-1480°C. Mineralogical and chemical data on Hawaiian picrites from six tholeiitic centres similarly have liquidus temperatures of 1320±25°C and are partial melts from lherzolitic (Loihi) to harzburgitic residues at 1.5 GPa approximately. The modern mantle has a potential temperature of 1430-1450°C with no deep seated thermal anomaly associated with, for example, the Hawaiian 'plume'. The buoyancy plume inferred beneath the Hawaiian Swell is primarily compositional, deriving from a major component of refractory harzburgite (Mg^#_Ol = 91-92, Cr^#_Sp = 70-80). The Hawaiian source region probably has a pre-history of 'old subducted slab' and provides a 4-component mixing signature on Hawaiian magmas. [(a) refractory harzburgite; (b) residual pyroxenite and garnet pyroxenite within locally re-fertilized peridotite, from partial melting of subducted oceanic crust; (c) variable addition of incipient melt (olivine nephelinite) from garnet harzburgite/lherzolite at P>2.5 GPa, T~1250-1550°C; and (d) CH₄+H₂O fluid phase from deeper mantle at fO₂ = IW+1.] It is inferred that neutrally buoyant, old, subducted slabs in the subasthenospheric mantle provide the locus for melting and initiation of diapirism, initially with melt retention but then with melt segregation and extraction at 1.5 GPa, leaving harzburgitic residues. A dominantly lherzolitic mantle with potential temperature of 1450°C and small contents of carbon and hydrogen, provides rheological and petrogenetic responses consistent with the plate tectonics paradigm and with mid-ocean ridge, intraplate, back-arc and island arc volcanism.