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Phase Transition Clapeyron Slopes and Transition Zone Seismic Discontinuity Topography Craig R

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Phase Transition Clapeyron Slopes and Transition Zone Seismic Discontinuity Topography Craig R JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. B8, PAGES 15,853-15,860, AUGUST 10, 1994 Phase transition Clapeyron slopes and transition zone seismic discontinuity topography Craig R. Bina Department of Geological Sciences, Northwestern University, Evanston, Illinois George Helffrichl Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. Abstract. The depths, widths, and magnitudes of the 410-km and 660-km seismic discontinuities are largely consistent with an isochemical phase change origin, as is the observation that the topography on these discontinuities is negatively correlated and significantly smaller than predicted for chemical changes. While most thermodynanlic studies of the relevant phase changes predict greater topography on the 410 than the 660, recent seismic studies suggest greater topography on the 660. The seismic results are consistent with some recent thermochemical studies which suggest that the Clapeyron slopes of the perovskite-forming reactions exceed in magnitude those of the spinel-forming reactions; however, we have reexamined the relevant Clapeyron slopes in light of other, more recent, experimental studies as well as the requirements of internal thermodynamic consistency. We conclude that the bulk of the evidence indicates a greater Clapeyron slope magnitude for the 410 than for the 660. Thus the recent seismic results are unexpected. One explanation might be that lateral temperature variations near 660 km depth exceed those near 410, consistent with a model of the 660 as a thermal boundary layer. An alternate interpretation, which requires neither a thermal boundary nor metastable olivine, is that the 410 does possess greater topography but is simply less visible seismically than the 660. This latter idea, and recent short-period observations of P'410P' seismic phases in conjunction with an elevated 660, is consistent with thermodynamic modeling of subduction zones illustrating the extreme broadening of the olivine n+P transition in cold slab interiors and, conversely, its sharpening in regions of high temperature. Introduction tually requires that they primarily be manifestations of phase changes. Recent seismological studies of man- The depths, widths, and magnitudes of the major tle structure [Revenaugh and Jordan, 1989,19911reveal seismic discontinuities in Earth's mantle, at 410 km and that lateral variations in the depths to these major seis- 660 km depth, are consistent with their being primar- mic discontinuities are negatively correlated: the "410" ily due to isochemical phase transformations [Bernal, is elevated when the "660" is depressed, and vice versa. 1936; Ringwood, 1969; Bina and Wood, 1987; Wood This result is consistent with the thermochemical prop and Helfich, 1990; Bana, 19911. Indeed, the obser- erties of mantle mineral phases. \.atton that thc "ropography" or geographic variation Upper mantle phase transitions involving the trans ill (Irr>~11of occurrence of these discotlti~~uitirs Ishearer. formation of olivine to p or y-spinel possess positive 1991; Shearer and Masters, 1992; Richards and Wicks, Clapeyron (dP/dT)slopes [Suito, 1977; Kawada, 1977; 19901 is an order of magnitude less than that predicted Fxkizawa, 1982; Ashida et al., 1987; Akimoto, 1987; for chemical contrasts in a convecting mantle [Chris- Katsura and Ito, 19891 so that a rise in temperature re- tesaen and Yuen, 1984; Ifincaid and Olson, 19871 vir- sults in an increase in the pressure (depth) of the phase change. On the other hand, both the predicted [Navrot- sky, 19801 and measured [Ito and Yamada, 1982; Ito and Takahashi, 19891 Clapeyron slopes of lower man- 'Now at Department of Geology, Univwsity of Bristol, Bristol, tle perovskite-forming phase changes are negative, so England. that a rise in tem~eratnreresults in a decrease in the pressure (depth) 0: the phase change. Thus attribution Copyright 1994 by the American Geophysical Union. of the the 410-km discontiuuitv to the a-sninel-forming., Paper number 94JB00462. reaction and the 660 to a pe;ovskite-for&ng reaction 014SOZ27/94/94JB-00462805.00 implies anticorrelation of the 410 and the 660: where 15,854 BINA AND HELFFRICR: CLAPEYRON SLOPES one is deflected downward, the other should be deflected Shearer [I9911 and Vidale and Benz [I9921 are unex- upward. Furthermore, with a recent exception [lto et pected. 'They are telling us something interesting, either al., 19901, studies of the magnitudes of these Clapeyron about Earth's mantle or about our seismological meth- slopes indicate a greater magnitude for the @-spinel- ods of investigating it. We illustrate that the broaden- forming reaction than for the perovskite-forming reac- ing of the olivine a+@ transition at lower temperatures tions. This, in turn, implies greater deflection of the should cause upward deflection of the 410 to be less 410-km discontinuity than of the 660 for a given tem- visible seismically, leading to underestimation of 410 perature perturbation. topography. In addition, the small Clapeyron slope for Recent studies of seismic waves interacting with these the 660-km phase transformation found herein should discontinuities, however, demonstrate just the opposite present a weaker barrier to subducting slabs than is effect. Shearer [I9911 found greater variation in the commonly assumed, rendering the geodynamical argu- depths to the 660 compared to the 410 recorded in ment for segregated upper and lower mantle reservoirs reflections and conversions of long-period body waves less compelling. from these discontinuities. The depths were estimated CNJIII the arrival lin~esor P +S and S--tP conversions Seismologically Observed Clapeyron and of topside and bottomside reflections. Different methods of analyzing this data set consistently show Slopes greater standard errors for 660 depth estimates than for 410 estimates. Moreover, near the northwest Pa- What is meant by the Clapeyron slope of a seismo- cific subduction zones where the observations are most logically observed mantle phase transition? In complex abundant, 410 and 660 depths are positively correlated, systems like the mantle where multicomponent phase not negatively correlated as the Clapeyron slopes would transformations such as thosc involving Lhc olivine poly- predict. morphs and perovskite occur, a Clapeyron slope can- Vidale and Benz [I9921 also found greater 660 vari- not be tbermodymamically defined. A useIu1 analogy ability but by different means. They stacked short- with phase transformations in single-component sys- period waveforms from deep earthquakes to image the tems may rescue thc concept, howcvcr. In such systems, arrivals between P and pP, arrivals which correspond if a phase boundary is crossed, all of the newly unstable phase is transformed to the newly stable phase, yielding to bottomside reflections and conversions of upgoing P a first-order discontinuity in seismic properties. In mul- and near-source SI-P conversions mediated by man- ticomponent systems of fixed bulk composition, regions tle discontinuities. Discontinuity depths are estimated where two or more stable phases coexist commonly ap- from the arrival times of these phases. Assuming nom- pear when a phase boundary is crossed, yielding seismic inal depths of 410 and 660 km for these discontinuities propcrtics which arc somc weightcd avcragc of those in "typical" mantle distant from the subduction zones of the individual phases [Bina and Wood, 19871. Sup- in which the events occurred, the 660 is displaced 25 - pose that this occurs as one follows a mantle adiabat km downward whereas the 410 is displaced 10 km up- to higher pressures. Imaged seismically at an appro- ward. Though the signs of the displacements are consis- priate frequency from a distance above the boundary, tent with a phase change origin for both discontinuities, a radially finite multiphase field resembles a first-order their relative magnitudes are not. discontinuity at some depth, even though it is really Thus the observed variability in the depth to the 660- a gradient zone. Referenced to this adiabat, the dis- km discontinuity exceeds the observed topography on placement of this apparent discontinuity from its un- the 410. The authors of these studies cite the consis- perturbed depth by some temperature variation may tency of their observations with recent thermochemical be qnantified and a "seismic" Clapeyron slope defined. studies [Akaogi et al., 1989; Ito et al., 19901, suggest- Though dependenl upon frequency, such a slope other- ing that the Clapeyron slopes of the perovskite-forming, wise depends only 11pon t,hermodynamic properties. reactions may exceed in magnitude those of the spinel- forming reactions and thus predicting greater vertical deflection of the 660 for a given temperature increment. Methods Here we reexamine the magnitudes of the relevant Clapeyron slopes, reassessing the available thermochem- The primary constraints upon the Clapeyron slopes ical data in light of the most recent phase equilibrium of mantle phase transitions arise from phase equilib- studies and the requirements of internal thermodynamic rium experiments which attempt to map the reaction consistency. We find that the magnitude of the Clapey- boundaries at high pressures (P)and temperatures (T) ron slope of the 410-km phase change does indeed ap- [Katsura and Ito, 1989; Ito and Takahashi, 19891. How- pear to exceed that of the 660-km phase change for the ever, since a finite overstep of a reaction boundary in Mg-bearing end-members of the Mg-Fe
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