A note on the sensitivity of mantle convection models to composition-dependent phase relations

Numesical simulations of mantle convection are inhibit mantle circulation across such phase transitions and highly sensitive to the dependence of phase relations upon that strong time-dependent instabilitiesmay be induced near pressure, temperature, and composition. Phase transitions in phase boundaries. the (relatively silica-poor) olivine components of the manIn such studies, the phase transitions are approximated tle tend, with increasing iron content, to inhibit convective by univariant phase boundaries with fixed Clapeyron slopes mass transfer between the upper and lower mantle. Those in simple wmpositions (e.g., the a -+ 0 transition in pure a w i n g in the (relatively silica-rich) non-olivine wmpoMgzSi04). n u s , the 660 km seismic disconlinuity is reprenents fail to inhibit (and indeed promote) such mass transfer. sented by theunivariant y + pv+mw transitioninMgzSiO4, Behavior in the real mantle must entail an interplay between whose fixed negative Clapeyron slope inhibits transport besucb competing effects, with the finite width of multivariant tween the upper and lower mantle. Similarly, the 410 km phase fields actiug tomitigateimpediments to flow. Attempts seismic discontinuity is represented by the &variant a P to dirsccj corJate computed flow or temperature fields transitionin MgzSi04. whose fixed positiveClapeyron slope with models derived from physical observables should be does not inhibit transport. approached with caution, given the sensitivity ofthe former The magnitudes of sucb Clapeyron slopes, however, are to compositional heterogeneity and uncertainties in phase not necessarily constant; they may be h c t i o m of presrelations. sure and temperature [cf. Bina and Helffrich, 19941. Moreover, the actual identity of the televanl phase iransilion, and Introduction thus the magnitude and sign 01 the wrresponding Clapeyron slope, may change with pressure and temperature. This The m a c e of mineralogjd phase transitions in the Was explored recently by Liu [19941, who nqted that the E & ~ ~ [ ~ i ~ ~ ~ ~ ~ d , 19701 has long been inferred to Y + P + 7JZW transition in endmember MgzSi04 olivine impose significant constraints upon the convective dynamis replaced by the P + pv + m w transition at high temics of the interior. Early work [Richter, 1973; Schubert et peratures (Figure la). While the former Possesses a negaal., 1975; Christensen and Yuen, 1984, 19851 demonstrated tive C1apeyron slope, so that descent of cool downwellings that the sign and mapoitude of the Clapeyron brasurewhose geothpass below the triple point is inhibited, the temperature) slopes of phase transitions are imponant in delatter Possesses a zero or w&y positive Clapeyron slope termining the extent to which the and lower (due to its greater volume decrease and more negative enmay mix or remain stably stratified. More recent studies mPY change), so that ascent of warm upwellings whose [Nakakuki et al., 1994; Solheim and Peltier, 1993, 1994: geotherms pass above the trip1e point is not inhibited. ne Tackley et al., 1993, 1994; Honda et al., 1993; Weinstein, ~*gas~-etr~ whichresults. in which the phase bound1992, 1993; Peltier and Solheim, 1992; Zhao et al., 1992; IVY resists the descent of subducting but ascent Liu et al., 1991; Machete1 and Weber, 19911 have suggested rising plumes, can be s e a in Fi!Pe 2a. Higher Rayleigh that, because of its negative Clapeyron slope, the disproserve to amplify portionation of y-spinel into magnesiowiistite and silicate Here we expand upon the work of Liu [1994], noting that perovskite (the y pv + m w transition) at 660 km depth phase relations depend not only upon pressure and temperahas the potential to inhibit mantle and to induce ture but also upon composition. The identity of the relevant time-dependent instabilities so that even a stably stratified phase transitions. and thus the nature of any corresponding mantle may occasionally mix through catastrophic overturn Clapeyron slopes, are functions of chemical composition. events. We may briefly summarize such work by noting Because the mantle is a complex multiwmponeat chemical that the local body forces induced by t h d distortion of System, and because it should be characterized by varying phase boundaries with negative Clapeyron slopes tend to degrees of wm~ositional heterogeneity [e.g., Metcalfe et al., 19951, we explore the sensitivity of mantle wnvection simCopyright 1995 by the American Geophysical Union. ulations to such wmposition-dependent variations. Compositional heterogeneity may also induce chemical buoyancy effects [Liu and Chase. 19911, such as the reversal whereby Paper number 95GLO2546 oceanic crustal compositions become intrinsically less dense 0094-8534195195GL-02546$03 .OO thanpyrolite mantle compositions over the670-71Okmdepth