Density of phonolitic magmas and time scales of crystal fractionation in magma chambers

- First in situ density measurements of phonolitic melts at crustal and upper mantle conditions using synchrotron X-ray absorption.
- EoS accurately predicts the density of phonolitic melts at geologically relevant conditions.
- Crystal settling velocities constrain the storage time before eruption of phonolitic magma to about 100 yr.

Understanding magmatic processes and their evolution in the Earthʼs interior requires a better knowledge of silicate melts and their physical properties. Among them, density is one of the most important to constrain the residence or ascent time of magma in the upper mantle and crust. However, the volumetric properties of volatile-bearing and highly polymerized silicate melts are still poorly constrained due to scarce experimental data. In this study, the density of dry and hydrous (4.35 wt% H2O) phonolitic melt was measured in situ using the X-ray absorption method in a Paris-Edinburgh press at 0.73-3.1 GPa and 1484-1855 K. Calculated melt densities range from 2.49±0.02 to 2.66±0.03 g/cm3 and from 2.31±0.02 to 2.52±0.02 g/cm3 for the dry and hydrous compositions at depths of 30 to 100 km. The results are used to calibrate the equation of state (EoS) of phonolitic liquids for crustal and upper mantle conditions and to derive the partial molar volume of water: a least-squares fit of the P-T-ρ data to a third-order Birch-Murnaghan EoS yields: V0=28.10−0.74+0.10cm3/mol, KT=14.6−3.0+3.2 GPa, K′=8.9−2.6+3.3 and α=47−30+28×10−6/K for the dry melts and V0=20.6−0.8+7.5 cm3/mol, KT=4.6−3.0+2.3 GPa, K′=5.5−0.5+6.1 and α=135−115+114×10−6/K for the hydrous component at 1673 K. Combined with literature data, our EoS for water indicates that V¯H2O does not depend strongly on the silicate liquid composition at the investigated conditions.Crystal settling velocities in phonolitic liquids were calculated from the EoS to constrain the time scales of crystal fractionation in magma chambers. The high compressibility of phonolitic melts results in a small density contrast between nepheline and sanidine crystals and melt (Δρneph-phon=0.13-0.27 g/cm3 and Δρsan-phon=0.05-0.21 g/cm3) with corresponding settling times of about 380 and 510 yr/km, respectively, for crystal sizes of 2 cm. Because of their higher density, similar-sized magnetite and augite only require about 31 and 100 yr/km, respectively. The observed phenocryst populations in phonolitic magmas indicate that the residence time was sufficiently long to fractionate augite and magnetite, but not sanidine and nepheline. Thus, our density data, combined with the phenocryst populations of the erupted lavas point to a residence time in the crust between a few tens to hundreds of years, consistent with estimates from U-Th-Ra isotope compositions.