Rock density

  • (1) Mineralogisch-petrologiches Institut, Universität Basel, Switzerland
  • (2) Laboratoire de Géologie, ENS Paris-UMR 8538, France
  • (3) Collège de France - Chaire de Géodynamique, Europole de l'Arbois , BP 80 - 13545 Aix en Provence, France
  • Citation of this article:

    Bousquet, R., B. Goffé, X. Le Pichon, C. de Capitani, C. Chopin, and P. Henry (2005), Comment on ‘‘Subduction factory: 1. Theoretical mineralogy, densities, seismic wave speeds, and H2O contents’’ by Bradley R. Hacker, Geoffrey A. Abers, and Simon M. Peacock, J. Geophys. Res., 110, B02206, doi:10.1029/2004JB003450



    Introduction

    Hacker et al. (2003a) presented an interesting work on metamorphic influence on the physical properties (density, H2O content, seismic wave velocities) of mafic and ultra- mafic rocks and in a companion paper, the consequences on the localization of earthquakes in subducting slabs (Hacker et al., 2003b). While this work is very interesting and well presented, this paper appears to ignore similar works for densities calculations (Bousquet et al., 1997; Le Pichon et al., 1997; Henry et al., 1997; Gerya et al., 2002), for H2O content in rocks (Bousquet et al., 1997; Kerrick & Connolly, 2001) as well for the seismic waves velocities calculations (Sobolev and Babeyko, 1989, 1994). This absence of discussion might be without major consequence if all papers agreed. But this is not the case. The results of densities calculations for high-grade conditions are in some TP conditions in contradiction with previous (Hacker, 1996; Bousquet et al., 1997) and newest (Goffé et al., 2003; Vasilyev et al., 2004) works as well with measured densities of natural rocks (Austrheim, 1987) or experimental works (Green and Ringwood, 1967; Ito and Kennedy, 1971) with similar compositions. Density should vary with physical parameters according to the thermodynamic laws.

    Density calculation

    For example we notice for MORB-type rocks a strong increase of density with temperature increasing around 800°C for pressures between 10-15 kbar, and a decrease of density with pressure increasing around 27 kbar for temperatures between 500-700°C (Figure 1). These results seem to follow from mineral compositions of metamorphic facies chosen by Hacker et al. (2003a), in which some assumptions are questionable: - In high-temperature conditions, the increase of density depends mainly on the amount of garnet in rock (Bousquet et al., 1997). In the granulite facies, slopes of reaction of garnet breakdown into plagioclase are all positive in a P-T diagram (Ringwood, 1975). Knowing that garnets are denser than plagioclases, density of rocks should become less dense with an increase of temperature and not denser as claimed by Hacker et al. (2003a). - Authors claim “there is no stability field of chloritoid (in their Figure 1) consistent with its absence in naturally metamorphosed rocks of MORB composition”. However among the many mountain belts where mafic rocks were metamorphosed under blueschist and eclogite conditions, chloritoid has been described in several of them. For example, chloritoid is well known in mafic rocks of the European Alps from east (Miller, 1986) to Liguria (Messiga et al., 1995) through the Central and the Western Alps (Bearth, 1973, see Droop et al., 1990) as well as in rocks of Greece (Schliestedt, 1986). Their assumption also contradicts experimental work (e.g. Schmidt and Poli, 1998) and previous modeling of these authors (e.g. Peacock and Wang, 1999). - It is also unusual to assume that the Na-amphiboles (Glaucophane and Fe- glaucophane) are stable in amphibolite facies from 600 to 800°C at 0.8-1.2 GPa (Table 3 of Hacker et al., 2003a). Such stability field of Na-amphibole is in contradicction with their own assumption presented in table 4 in which they assume that in ”amphibolite, granulite, and related facies, all Na is in albite and pargasite”. Since the extensive work of W. G. Ernst in the Franciscan mélange, it is well known that Glaucophane-bearing rocks derive from low-temperature gradient (e.g. Ernst, 1973), and this assumption is used in many modeling works (e.g. Peacock, 1993; Bousquet et al., 1997; Kerrick and Connolly, 2001). Experimental works have shown that stability field of Na-amphibole is strongly temperature-dependent (Maresch, 1977). The stability field is below 600°C for pressures up to 1.5 GPa (Evans, 1990) and goes up to 800°C for pressures higher than 2.5 GPa (Tropper et al., 2000).

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    Figure 1

    Figure 1: Densities of metamorphosed MORB. a) From Hacker et al. (2003a). b) Calculated using Gibbs free energy minimization method (De Capitani and Brown, 1987) and the database of Holland and Powell (1998) for an unaltered MORB composition described in Hacker et al. (2003a). Calculations were done assuming no melting. We note major differences between two kinds of diagram particularly in high- temperature field up to 3 GPa. Results of this study are in better accordance with previous works that computed densities from mineral assemblages of natural rocks (Bousquet et al., 1997) as well on measured densities of natural work (Austrheim, 1987).

    Discussion-Conclusions

    Uncertainties in the densities of rocks presented by Hacker et al. (2003a) have also a great impact on the prediction of seismic wave velocities (e.g. for comparison Sobolev and Babeyko, 1994). Similar to the density, water content of a rock is strongly dependent on the mineral composition. Inconsistencies in the mineral assemblages also explain the discrepancies of water-content phase diagrams with previous studies (Hacker, 1996; Bousquet et al., 1997, Kerrick and Connolly, 2001). In conclusion, we regret that these innovative papers showing the dependency between seismic wave velocities of rocks and metamorphic reactions, do not mention previous published works with which they have significant disagreements. Also, we do not think their paper would have been hurt by noting that Sobolev and Babeyko (1994) had already modeled the evolution of seismic wave velocities in function of metamorphic reactions for different bulk composition of rocks and Bousquet et al. (1997) had already modeled the water-contents of rocks and their consequences on the localization of crustal earthquakes in convergent zones.

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