A Model for the Layered Upper Mantle

Home , Silicate perovskite

PHYSICS OFTHE EARTH : , "| AND PLAN ETARY INTERIORS ELSEVIER Physics of the Earth and Planetary Interiors 100 (1997) 197 -212

A model for the layered upper mantle Tibor Gasparik Center for Nigh Pressure Research, Department of Earth and Space Sciences, State Universi~ of New York at Stony Brook, Stony Brook, NY 1 1794, USA Received 15 August 1995; accepted 21 May 1996

Abstract

Numerical modeling of mantle convection by Liu (1994, Science, 264: 1904-1907) favors a two-layer convection, if the results are reinterpreted for the correct phase relations in (Mg,Fe)zSiO 4. The resulting chemical isolation of the upper and lower mantle suggests a highly differentiated and layered upper mantle to account for the discrepancy between the observed compositions of mantle xenoliths and the cosmic abundances of elements. It is shown that a layered upper mantle with a hidden reservoir can have a structure consistent with the observed seismic velocity profiles and an average bulk composition corresponding to the cosmic abundances. The evolution of the upper mantle and the origin of komatiites are discussed in the context of the proposed model. © 1997 Elsevier Science B.V.

I. Introduction join (Gasparik, 1989). Until then, the transformation from (Mg,Fe)2SiO 4 olivine to beta phase was the Major progress in our understanding of the Earth's only explanation based on a phase transition. How- interior has been made in the last decade, primarily ever, the concept of a refractory shell residing in the owing to advances in seismology, experimental upper mantle (Gasparik, 1990a) was crucial in find- petrology, geochemistry and mineral physics. The ing the way to incorporate the new information into evidence shows that the currently preferred models a consistent model for the Earth's interior. Other of a largely homogeneous Earth's mantle, such as important discoveries taking place at the same time pyrolite (Ringwood, 1962) or piclogite (Bass and have been essential for further developments and Anderson, 1984), do not provide a satisfactory expla- improvements of the model. This paper is an attempt nation of the new observations. to summarize the new evidence and propose a model The present model for a layered upper mantle has for the mineralogy, chemistry, structure, and evolu- been developed over the last 8 years in response to tion of the upper mantle consistent with these obser- the new information obtained in phase equilibrium vations. studies with a split-sphere anvil apparatus (USSA- 2000) at the Stony Brook High Pressure Laboratory. The key information was the discovery of an alterna- 2. Evidence for a layered mantle tive explanation for the 400 km discontinuity, corresponding to the transformation from pyroxene to Liu (1994) carried out numerical modeling of the garnet in compositions close to the enstatite-jadeite effect of the transition from (Mg,Fe) 2 SiO 4 spinel to

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I0 ] I % I ,ooo /, 20 40 60 I0 15 20 25 30 Fe 2 SiO4 (rnol %) Pressure (GPo) Fig. 2. Pressure-composition phase diagram for Mg2SiO4 - Fig. 1. Phase relations for Mg2SiO4 (continuous lines) and for F%SiO4 join at 1600°C. After Katsura and Ito (1989). lherzolite (dashed lines). Phases: O1, olivine; Bt, beta phase; Sp, spinel; Pv, perovskite; Mw, magnesiowiistite; L, liquid. After Liu (1994). The correct phase relations are shown in Fig. 3. The phase transition corresponding to the breakdown (Mg,Fe)SiO 3 perovskite and magnesiowiistite on of spinel to perovskite and magnesiowiistite has a mantle convection by considering two different cases. negative Clapeyron slope at all temperatures below In the first case, the Clapeyron slope of the transition the solidus, as considered by Liu (1994) in the first was negative at all relevant temperatures. Both case. Thus, the flow of material across the 670 km downwelling and upwelling flows in the mantle were discontinuity would be impeded in both directions, impeded by the negative slope, which resulted in a resulting in a two-layer convection (Knopoff, 1964; two-layer convection. In the second case, the slope Anderson, 1979; Weinstein, 1992). of the transition changed from negative at lower A major argument in favor of a layered mantle is temperatures to positive at higher temperatures. Con- the discrepancy between the olivine-rich composition sequently, whereas the downwelling flow was still of the primitive mantle derived from the composi- impeded, hot plumes ascended across the positive tions of mantle xenoliths (Ringwood, 1962; Jagoutz boundary with ease. The resulting layering was much weaker and disappeared in a short time. 2500] i I I I I i I I i I ; ~,fe I ' ; I r I Liu (1994) favored the second case on the basis of his analysis of the phase relations reproduced in Fig. ~jp~e~in~ 1, where the phase boundaries for lherzolite were superimposed on the phase diagram for pure MgESiO 4. However, even in chemically complex : ! o, // /\ Pv+Mw compositions, such as lherzolite, the three relevant /,,/ /,, polymorphs, olivine, beta phase and spinel, are still / \ relatively simple binary solid solutions of Mg2SiO 4 ~- I I I and Fe2SiO 4. Phase relations in this binary system / t~ 1 1~, / oo~,~ru ~ were experimentally determined several times, and ,?/ ,',/ - are well established (Fig. 2). There is no doubt that I I I i I I I ioooF ,i ,I /,I,,, /I l I I~ I l I I I the addition of iron to the system Mg2SiO 4 expands IO 15 20 25 30 Pressure (GPa) the stability of spinel at the expense of beta phase to Fig. 3. Temperature-pressurephase relations for Mg2SiO4 (con- lower pressures. The phase relations used by Liu tinuous lines) and (Mgo.9Fe0.J)2SiO4 (dashed lines). Based on (1994) show the opposite, and thus are clearly incor- Katsura and Ito (1989), Ohtani et al. (1989), Gasparik (1990b) and rect. Presnall and Walter (1993). T. Gasparik / Physics of the Earth and Planetary. Interiors 100 (1997) 197-212 199 et al., 1979; Palme and Nickel, 1985) and the com- quires much higher magma ocean temperatures than positions of meteorites and the Sun, which have the necessary for the melting of the upper mantle to melt Mg/Si ratio close to unity (e.g. Anders and Grevesse, more than a minor portion of the lower mantle. If the 1989). Meteorites are considered to be representative melting curve of Sweeney and Heinz (1995) is cor- of the material from which the Earth formed. Some rect, even a mantle-deep magma ocean would de- meteorites did not undergo differentiation, others velop a perovskite septum at depths around 1000 km, correspond to the material from differentiated, and effectively dividing the mantle into two regions, subsequently disrupted, planetary bodies; these rep- which could then crystallize and evolve separately. resent material from various layers in these bodies, These considerations suggest that an early magma and by analogy in the Earth. MacDonald and Knopoff ocean responsible for the differentiation of the ob- (1958) were first to point out that a mantle corre- servable mantle was limited to depths not greatly sponding in composition to chondritic meteorites exceeding 670 kin. would have to be composed mostly of pyroxene and A two-layered mantle is the minimum require- garnet. ment when considering the geochemical evidence. Mantle xenoliths typically originate at the depths For example, Hart (1988) used various isotopic data less than 150-200km. Because this observable part to make a case for at least four different reservoirs in of the mantle is rich in olivine, a chondritic mantle the mantle. Arguments based on the noble gas sys- has to be layered; a large-scale differentiation pro- tematics also require multiple reservoirs established ducing an olivine-rich upper layer requires the pres- very early in the Earth's history (Allbgre, 1995; ence of a complementary silica-rich layer at greater O'Nions and Tolstikhin, 1996). The evidence for a depths. Liu (1979) suggested the possibility that the high degree of chemical isolation between the upper excess silica is concentrated in the lower mantle and and lower mantle throughout most of the Earth's the 670km discontinuity is a chemical boundary history thus appears robust. between an olivine-rich upper mantle and a more silica-rich lower mantle. This could result from frac- 3. Evidence for a hidden reservoir tionation of (Mg,Fe)SiO 3 perovskite in a completely molten Earth (Kumazawa, 1981; Ohtani, 1985; Agee The 400km discontinuity has traditionally been and Walker, 1988). However, Kato et al. (1988a,b)), explained by the phase transformation from Drake et al. (1993) and Gasparik and Drake (1995) (Mg,Fe) 2SiO 4 olivine to beta phase. Gasparik (1989) showed conclusively that the perovskite fractionation reported that the transformation from pyroxene to is not consistent with the near-chondritic Ca/AI and garnet occurring in compositions close to the en- Sc/Sm ratios observed in mantle xenoliths; a signifi- statite-jadeite join at 130-135 kbar is univariant and cant fractionation of MgSiO 3 perovskite from sili- thus could also produce a sharp discontinuity at the cate melt during a magma ocean phase early in the 400km depth (Fig. 4). Continued advances in seis- Earth's history would cause these ratios to be dis- mic techniques have since shown that the 400km tinctly nonchondritic. discontinuity is less than 2 km wide (Benz and Vi- Most recently, arguments based on the partition- dale, 1993). In contrast, the transformation from ing of siderophile elements between silicate melt and olivine to beta phase is divariant (Fig. 2) and would sulfur-bearing molten iron suggest that the extraction occur over a depth interval of 10-20kin (e.g. Kat- of iron from the observable mantle during core for- sura and Ito, 1989). Thus, the 400km discontinuity mation occurred at 28-30GPa, thus at depths not cannot be caused by the transformation from olivine exceeding greatly 670km (Agee and Li, 1996; to beta phase, and the univariant transformation from Righter et al., 1996). The experimentally determined pyroxene to garnet is now the most likely explana- melting curve of (Mg,Fe)SiO 3 perovskite shows a tion. major increase in the melting temperatures with pres- If the mantle at 400 km depth has a composition sure at the low-pressure stability limit of perovskite close to the enstatite-jadeite join, it must be chemi- (Zerr and Boehler, 1993; Sweeney and Heinz, 1995). cally stratified, because such a composition is very The resulting deep cusp in the mantle solidus re- different from the compositions of mantle xenoliths. 200 T. Gasparik / Physics of the Earth and Planetary Interiors 100 (1997) 197-212

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J f T. Gasparik / Physics of the Earth and Planetary Interiors 100 (1997) 197-212 201

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.~ L / '1-7t/ / / \ ~----/~-- I\ 2Pv I J ,=ooF / oo+=0 I\G? Id /"""::o;:V,..//./' / £t / S I"I! [O00FII , ,;~" [ l ~l I itS,~fl l, t ~S,.I.:t.~GQ:CQ~PvI~i ] 0 5 I0 15 20 25 Pressure (GPo) Fig. 5. Temperature-pressure phase relations for the proposed model of the Earth's upper mantle. CEn, Clinocnstatite; I1, ilmenite; Pv, perovskite; St, stishovite; other symbols as in Fig. 1 and Fig. 4. Based on data from Gasparik (1992, 1996a) and Herzberg and Gasparik (1991). Mantle geotherm is the model continental geotherm of Pollack and Chapman (1977) for the surface heat flow of 40 mW m-2, with the adiabatic segment extrapolating to 1280°C at 1 bar (McKenzie and Bickle, 1988).

The relatively high sodium content suggests that the al., 1983; Shirey and Hanson, 1986; Galer and Gold- mantle at 400 krn depth formed by crystallization of stein, 1991). A complementary enriched reservoir residual melts, presumably during the solidification formed by differentiation of the mantle prior to of a magma ocean. The corresponding layer has to 3.8 Ga is required to balance this depletion. Because be refractory to remain preserved in the upper mantle even the komatiite source regions were depleted, the throughout the Earth's history in spite of convection. enriched reservoir had to be located at depths greater Gasparik (1990a, 1992, 1993) proposed that this than 200-300km, which is the depth of origin of the refractory shell is located between depths of 220 and oldest Barberton komatiites (Herzberg, 1992). It is 520 km, was completely dehydrated during its forma- proposed here that the enriched reservoir responsible tion, and has remained unaffected by mantle pro- for the depletion of the Archean mantle is in the cesses owing to its extremely high melting tempera- shell. tures exceeding 2000°C (Fig. 5). Another contribut- ing, possibly even decisive, factor responsible for the refractory nature of the shell could be the high 4. Mantle convection mechanical strength of majorite garnet predicted by Karato et al. (1995). The presence of layering in the upper mantle is Additional evidence for a hidden reservoir is the inconsistent with the whole mantle convection which observed depletion of the Archean mantle in incom- would produce a homogeneous mantle. Such a man- patible elements, as is evident from the high Sm/Nd tle must be rich in olivine because that is the compo- ratios of even the oldest Archean rocks (McCulloch sition of the upper 200 km of the mantle accessible and Compston, 1981, Basu et al., 1981; Hamilton et to observation through xenoliths. The idea of a ho-

Fig. 4. Phase relations in the enstatite-diopside-jadeite system at the mantle temperatures (T(°C)= 1280 + P(GPa)/0.055), shown by continuous lines, and at the solidus temperatures (T(°C) = 1880 + P(GPa)/0.055), shown by dashed lines. Shaded area is the stability field of garnet, with the isobars indicating the expansion of the garnet stability with pressure. Bold lines indicate the compositions of garnet coexisting with two pyroxenes; arrows point to a pressure minimum. Phases: Opx, orthopyroxene; Cpx, clinopyroxene; Ga,CM, garnet; CaPv, CaSiO3 perovskite. Calculated from the data of Gasparik (1989, 1992, 1996a,b). 202 T. Gasparik / Physics ¢)[ the Earth and Planetary Interiors 100 (1997) 197-212

mogeneous mantle received widespread support with ing requires a major revision of our views on mantle the discovery of plate tectonics, which revealed a convection. highly dynamic nature of the Earth's surface consis- Gasparik (1993) proposed that a significant trans- tent with a vigorously convecting interior. port by convection may only occur in the presence of A two-layered mantle is expected to have inde- an interstitial melt. This study concluded that the pendent convection regimes in the upper and lower expected mantle temperatures are higher than the mantle, separated by the mostly impermeable 670 km temperatures of the volatile solidus; hence, in the discontinuity. A major argument against this view is presence of volatiles, the mantle should be partially the evidence from tomography suggesting that some molten. Van den Berg et al. (1993) pointed out that slabs penetrate into the lower mantle. As will be the experimentally determined high melting tempera- discussed later, this could either be a very recent tures of (Mg,Fe)SiO 3 perovskite (Zerr and Boehler, development in the evolution of mantle dynamics, or 1993) would prohibit convection in the lower mantle may not occur in some cases. The evidence is subject unless the melting temperatures are decreased by to interpretation of the tomographic images, in which volatiles. Any estimate of the rheological properties the lower velocities are traditionally associated with of the Earth, for example from a postglacial uplift, higher temperatures (e.g. Anderson et al., 1992). would reflect the properties of a partially molten However, the presence of an interstitial volatile- Earth, and cannot be used in support of the solid-state bearing melt would have the same effect. In extreme convection. This view allows a convection regime in cases, the observed contrast in seismic velocities is which any volatile-free, completely solid region po- so great that the corresponding temperature contrast tentially present in the mantle is mostly excluded would be unrealistic, and the presence of melt is the from convection. In this case, the theological proper- only plausible explanation (Zielhuis and Nolet, 1994). ties of the deep mantle would be analogous to the Some of the seismically slowest regions on the global properties of the Earth's crust, where the sharp con- scale are situated above the subducting slabs, and trast between the rheology of the solid and the most likely reflect partial melting caused by the partially molten rocks is evident. volatiles released from the slabs (Revenaugh and The purpose of this line of reasoning is to suggest Sipkin, 1994; Nolet et al., 1994). As a slab warms up an explanation for the inferred relative immobility of to sufficiently high temperatures during subduction, the transition zone (e.g. Ringwood, 1994) and for the partial melting of the slab is likely to occur in the preservation of deep roots under some continents transition zone (400-670 km), owing to the presence (MacDonald, 1963; Jordan, 1975). It is proposed that of volatiles (Gasparik, 1993). As the degree of par- the region between depths of 220 and 520 km, corre- tial melting of a slab increases with increasing tem- sponding to the refractory shell, is excluded from perature, the effect of the melt on the seismic veloci- convection because it was completely dehydrated ties could more than compensate for the low temper- during its formation from the magma ocean. Thus, ature of the slab. At the same time, the underlying whereas convection can take place in the whole mantle is cooled by the slab, and could potentially lower mantle owing to the ubiquitous presence of attain even higher velocities than the adjacent slab. volatiles, convection in the upper mantle is limited to Such velocity inversion would obscure the continua- the volatile-bearing regions, which include the tion of the slab in the transition zone and can result oceanic lithosphere, asthenosphere, transition zone in a false image of the slab descending into the lower between 520 and 670km depth, and the narrow mantle. conduits associated with the spreading centers, sub- Another argument against a two-layered mantle is duction zones and hotspots. the large size of the oceanic plates, which is more The presence of volatiles is essential for the recy- consistent with whole mantle convection; convection cling of the subducted oceanic lithosphere. Tomo- cells limited to the upper mantle would have to be graphic images of subduction zones indicate that extremely flat. But even the two-layered convection slabs maintain their rigidity and integrity in the model does not allow for layering within the upper upper mantle, encounter resistance at the 670kin mantle. Hence, the potential presence of such layer- discontinuity, and could stall there and be overrun by T. Gasparik / Physics of the Earth and Planetary Interiors 100 (1997) 197-212 203 the upper mantle. Ultimately, the slabs seem to 1962), are not likely to give the correct estimate for soften; they deform, thicken and bend, and are in- the composition of a layered mantle, especially in the serted in the transition zone between 520 and 670 km presence of a hidden reservoir. A more reliable depth (Van der Hilst et al., 1991; Fukao et al., 1992). estimate should be based on the cosmic abundances This behavior is consistent with an incipient melting of elements. expected in the presence of volatiles during the If the transition zone is a graveyard for the sub- process of thermal equilibration of the slab with the ducted oceanic lithosphere, it can be assumed that surrounding mantle, and is similar to the behavior the upper mantle to the depth of 150km and the implied by Ringwood and Irifune (1988). transition zone between 520 and 670 km depth, which A slab anchored at the 670 km discontinuity could together comprise 45% of the upper mantle, have the lose support owing to the incipient melting caused pyrolite composition. The remaining 55% is consid- by volatiles. This could result in a sudden movement ered to correspond to the remains of the refractory of the slab with respect to the surrounding mantle, shell. The composition of the shell was calculated by potentially triggering deep-focus earthquakes subtracting the pyrolite portion of the upper mantle (Gasparik, 1993). Such a mechanism is consistent from the cosmic abundances of Anders and Grevesse with the Bolivian earthquake of 1994, which was (1989). apparently caused by a sudden movement along a The result is given in Table 1. An average shell preexisting hydrated fault and was triggered by the should be composed of 37% of modal olivine and lubricating effect of the released volatiles (Silver et 63% of pyroxene. Because the sodium and aluminum al., 1995). contents of the shell are the same, the pyroxene In the presence of volatiles, the whole slabs resid- composition can be fully expressed in terms of three ing in the transition zone could ultimately turn into components, enstatite, diopside and jadeite, if the crystal mush on further heating. This is supported by relatively small amount of iron is included with several recent viscosity models, which predict a magnesium. Phase relations in this ternary system low-viscosity layer in the transition zone (Ricard et were determined (Gasparik, 1989, 1992, 1996a,b), al., 1989; Forte et al., 1991; King and Masters, 1992). The slabs are then completely recycled by upwelling under the spreading centers. At the speeds Table 1 of upwelling in meters per year, the conduits feeding Estimates of the chemical compositionof the Earth's upper mantle the spreading centers need to be only a kilometer (atoms per 1000 Si), corresponding modal contents of olivine and wide to allow for the complete recycling of the pyroxene, and the pyroxene components(mole per cent) oceanic lithosphere. Such narrow conduits could not Element Anders and Pyrolite Shell be detected by seismic methods. Grevesse (1989) The distribution of the conduits in the shell con- Si (Si+Ti) 1000 1000 1000 trois the position of plate boundaries and hotspots on AI (AI + Cr) 98 107 91 Mg (Mg +Mn) 1082 1255 940 the surface of the Earth. A minor mismatch between Fe 126 146 110 the plate boundaries and the system of conduits in Ca 61 73 51 the shell can be compensated by flow in the astheno- Na (Na + K) 60 21 91 sphere, along which the two systems of plates are Olivine 48 60 37 decoupled. A major mismatch results in the seces- Pyroxene 52 40 63 sion of spreading, displacement of hotspots, and Enstatite (Mg2Si206) 49.5 31.7 60.4 breakup of continents. Diopside (CaMgSi206) 19.4 27.7 14.2 Jadeite (NaA1Si206) 19.0 8.0 25.4 Pyrope (Mgl 5AISi1506) 12.1 32.6 0 5. Chemical composition and phase relations The Fe/(Fe + Mg) ratio in the cosmic abundances of Anders and Grevesse (1989) is the same as in pyrolite from Table 8-1 of The current mantle models based on the composi- Anderson (1989); Fe is included with Mg in the pyroxene compo- tions of mantle xenoliths, such as pyrolite (Ringwood, nents. 204 "£ Gasparik / Physics qf the Earth and Planeta O, Interiors 100 (1997) I97-212 and an updated phase diagram is shown in Fig. 4. mation of beta phase to spinel in a limited depth The diagram shows the phase relations at the temper- range around 560kin, the breakdown of spinel to atures corresponding to the adiabatic portion of the perovskite and magnesiowtistite responsible for the mantle geotherm, and thus relevant to the present 670 km discontinuity, and the high-pressure stability mantle, and at the solidus temperatures relevant to limit of garnet at 750km. A chemical boundary at the formation of the shell by solidification of a 520kin depth between the shell and the subducted magma ocean. slabs is reflected primarily in the garnet composition The calculated composition of the shell is identi- by an increase in the pyrope content of garnet from cal to the composition of the first garnet forming in zero in the shell to about 30 mol % in pyrolite. This the ternary system from two pyroxenes at the solidus causes a major expansion in the stability of the temperatures and the pressure corresponding to two-phase assemblage of garnet and spinel to lower 400 km depth. This would be an unlikely coincidence temperatures. unless the average composition of the shell is controlled by the crystallization of garnet. At the present mantle temperatures and pressures 6. Mineral composition of the upper mantle less than 13 GPa, the shell composition produces an assemblage of two pyroxenes and olivine. At the The corresponding mineral composition of the pressure corresponding to 400km depth, the jadeitic upper mantle can be calculated using the bulk com- pyroxene transforms to garnet. This transformation is positions given in Table 1 and the phase relations univariant, and thus can produce a sharp discontinu- from Fig. 4 and Fig. 5. The result is shown in Fig. 6 ity consistent with the seismic observations of the and Fig. 7. Fig. 6 summarizes the upper-mantle 400 km discontinuity. An important requirement for mineralogy under the assumption that the two chemi- a sharp discontinuity in this case is the absence of cally distinct units, the oceanic lithosphere and the the pyrope-grossular-almandine component in gar- shell, are homogeneous. A transition region is in- net, which would smear the transformation to lower cluded between the depths of 100 and 200km to pressures (Gasparik, 1989). The calculated shell account for the likely mixing of the two units in the composition satisfies this requirement. At higher asthenosphere. In this region, the olivine content pressures, the enstatite-rich pyroxene dissolves in the decreases from 60 to 37%, the pyropic garnet is garnet. This process is completed at 490km depth, eliminated, and the clinopyroxene increases, chang- where the pyroxene portion of the shell transforms ing in composition from diopsidic to jadeitic. The fully to garnet. subducted oceanic lithosphere consists of only two A simplified temperature-pressure phase diagram phases, garnet and an (Mg,Fe) 2SiO 4 polymorph (beta for the proposed upper mantle is shown in Fig. 5. This diagram accounts for the effect of all major Mineral Composition (real%] elements on the phase relations, and is based on an 0 20 40 60 80 I00 ¢= updated diagram for the system CaO-MgO-AI203- Cpx Olivine SiO 2 (Gasparik, 1990b, 1996a; Herzberg and Gas- I00 ...... parik, 1991), which was corrected for the effects of sodium (Gasparik, 1992) and iron (Ohtani et al., J°deiti:e / 1991). With increasing pressure along the mantle 300- Ciinopyrox ne CEn Orivine geotherm, the phase boundaries shown correspond to 400 ~ CEn I , Garnet the transformation of orthopyroxene to the high-pres- Beta Phase ~ 500 sure polymorph of enstatite-rich clinopyroxene, the Bt transformation of jadeitic clinopyroxene to garnet 600 Garne~ Spinel responsible for the 400 km discontinuity, the trans- 700 Lower Mantle formation of olivine to beta phase occurring at around i I I [ i I I i I 420km depth, the high-pressure stability limit of Fig. 6. Mineral composition of an upper mantle with chemically enstatite-rich clinopyroxene at 490km, the transfor- homogeneous layers of pyrotite and shell. T. Gasparik/ Physics of the Earth and Planetary Interiors 100 (1997) 197-212 205

Mineral Composition (mol%) A major problem with this 'homogeneous' model is that, in addition to a sharp discontinuity at 400 km depth, there would be another, more gradual discontinuity at slightly greater depths, caused by the transformation from olivine to beta phase. Moreover, this model cannot explain the presence of the high-velocity gradients in the slab portion of the transition zone (e.g. Bass and Anderson, 1984; Rigden et al., 1994). These discrepancies can be eliminated by adopting a layered model. In the layered model (Fig. 7), the layering reflects the crystallization of liquidus phases in a magma ocean. The accumulation of olivine at shallow depths by olivine flotation is expected to produce an olivine septum at 150-300km depth (Stolper et al., 1981; Fig. 7. Mineral composition of a layered upper mantle. Shaded Agee and Walker, 1988, 1993; Ohtani et al., 1993, layers correspond to oceanic lithosphere. 1995). The crystallization of garnet at greater depths forms a garnet layer in the transition zone (Ander- son, 1982). The shell composition crystallizes pri- phase or spinel). At 400 km depth, jadeitic clinopy- marily as pyroxene between 300 and 400 km depth. roxene transforms to garnet, and at 415-435km, The 400kin discontinuity is the boundary between olivine transforms to beta phase. the pyroxene and the garnet layer, and a chemical

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~, I- aaaen. Err

20 4O 60 80 I00 Modification (%) Fig. 8. Variations in the modal contents of olivine, pyroxene, and pyroxene components resulting from two different modifications of the proposed model (see text). 206 T. Gasparik / Physics of the Earth and Planetary Interiors I00 (19971 197-212

boundary between the shell and the subducted slabs 7. Evolution of the upper mantle is at 520km depth. Inherent compositional variations in the slabs residing between 520 and 670km depth The simplest explanation for the inferred layering are expected to cause a gradual change in the min- of the upper mantle is the well-documented change eral composition with depth from garnet to spinel, in the liquidus phase with depth from olivine to responsible for the observed high-velocity gradients garnet and its effect, taking place during the solidifi- in this region (Gasparik, 1993). The 670 km disconti- cation of a magma ocean, on the mineralogy, chem- nuity is both a phase and a compositional boundary, istry and structure of the upper mantle. However, and the composition of the lower mantle is cosmic. simple fractionation of these liquidus phases is in- Finally, two possible modifications of this model consistent with some observations. are considered (Fig. 8). The first modification, indi- It is now accepted that olivine becomes buoyant cated in Fig. 8 by continuous lines, assumes that the in ultramafic melts at a depth around 300km (Rigden Earth does not have the high sodium content implied et al., 1984; Agee and Walker, 1988; Miller et al., by the cosmic abundances of Anders and Grevesse 1991). It is not clear that this will happen in the more (1989), perhaps because sodium, being a volatile silica- and sodium-rich residual melts formed by element, was lost during the giant impact which partial crystallization of a chondritic mantle. The produced the Moon and was probably responsible for effect of garnet fractionation on the near-chondritic the formation of the magma ocean on Earth (Hart- Ca/A1 ratio of the observable upper mantle allows mann and Davis, 1975; Wetherill, 1976; Cameron 10-30% of garnet fractionation (Herzberg and Gas- and Ward, 1976; Cameron and Benz, 1991). The parik, 1991). However, this amount of garnet frac- observed low sodium content of the Moon supports tionation would deplete the observable mantle in this assumption. The chosen boundary case for the heavy rare earth elements, in contrary to what is composition of the upper mantle has the sodium observed. In addition, arguments were made that content of pyrolite. This would result in a major vigorous convection in a magma ocean could prevent decrease in the jadeite content, thus eliminating the fractionation (Tonks and Melosh, 1990; Solomatov possibility that the 400 km discontinuity is caused by and Stevenson, 1993). the transformation from pyroxene to garnet. How- The only kind of fractionation that does not seem ever, this would also result in a major decrease in the to cause problems is the fractionation of olivine. olivine content from 37 to 20%, which is insufficient Accumulation of olivine at shallow depths is neces- to explain the 400km discontinuity by the transfor- sary to account for the elevated Mg/Si ratio of the mation from olivine to beta phase. In this case, the observable mantle. Such fractionation could leave discontinuity would be much smaller, more gradual, the residual melt sufficiently rich in sodium and and present at greater depths than observed. silicon to produce a garnet-rich layer in the transition The second modification, shown in Fig. 8 by zone even without garnet fractionation. dashed lines, assumes that the fractionation in a The proposed evolution of the upper mantle is magma ocean led to a higher average iron content in shown in Fig. 9, starting with the crystallization of a the shell than in pyrolite, because the residual melt magma ocean resulting from the giant Moon-forming was likely to be enriched in iron. This would in- impact. Owing to the oblique nature of the impact, crease the olivine content of the shell. The chosen only about 30% of the mantle melts. The magma boundary case for the composition of the upper engulfs the remaining unmelted portion of the Earth, mantle has the olivine content of pyrolite. The result- producing a magma ocean of a uniform depth, ap- ing pyroxene content is lower, thus its jadeite content proximately corresponding to the upper mantle is higher, which will be reflected in a higher propor- (Stevenson, 1987; Tonks and Melosh, 1993). Crys- tion of the jadeitic clinopyroxene with respect to the tallization starts when the temperature drops below enstatite-rich clinopyroxene. Because it is the trans- the liquidus, thus precipitating liquidus phases (Fig. formation of the jadeitic clinopyroxene which causes 9(a)). Vigorous convection may prevent the fraction- the 400km discontinuity, this modification of the ation of olivine or garnet. Thin crust composed model is consistent with the observations. mainly of olivine forms at the surface, sinks and T. Gasparik / Physics of the Earth and Planetary Interiors I00 (1997) 197-212 207

A Olivine + Liquid 200

Liquid 400

Gornet ~- Liquid 60O

800 0 B

200

400 ~'.'(i:..:,:":::':':.:i::'i::.'y.:~:':,':~.:,:.?.,:.'.~".'."..'..;: ~...... :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

600 , i

== 800 mE , c-m 400 ~~'i~'r'i~i~i, i'~ ~-':'.-:'-'.i, ":."..'-." -'::':~: ~ :.::'~:

o'~..-y.'.?o'..'*'.o ._ ,o ! oo : ,, ,~t,y'-

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9nn 1:~2:-".;'-':"-'~i%'?:":?'::'~':;"~-"-ff~'-~':-'?:~"."~:;Z~~ st h enosp her • "220krn d|lcontinuit¥

800 ~1 ~£~~ Fig. 9. Evolution of the Earth's upper mantle (see text).

dissolves before reaching the bottom of the magma preferentially at the centers of convecting cells (see ocean. With further cooling and corresponding in- also Ohtani, 1985; Agee and Walker, 1988; Tonks crease in viscosity of the magma ocean, the crust and Melosh, 1990; Abe, 1993; Morse, 1993). becomes thicker and sinks and dissolves more slowly. The residual olivine-depleted melt crystallizes in If the rate of formation of the crust is faster than the transition zone, producing a garnet-rich layer dissolution, the olivine-rich crust will accumulate in (Fig. 9(b)). Because the d P/dT slope of the solidus the upper 200 km of the magma ocean before sinking at the corresponding pressures is close to adiabatic to deeper levels and dissolving. Partial segregation of (Fig. 5), the whole garnet layer may solidify at the the residual melt may occur as the crust accumulates same time, thus minimizing the possibility of garnet 208 T. Gasparik / Physics of the Earth and PlanetaD' Interiors 100 (1997) 197-212

fractionation. The crust starts accumulating at the top and lower mantle has been minimal, in agreement of the highly viscous and mostly solid garnet layer. with the limited mixing suggested by the noble gas This cools the top of the garnet layer and produces a systematics (O'Nions and Tolstikhin, 1996). barrier which prevents further mixing between the two regions. The garnet layer completely solidifies, forming a refractory shell. Continental crust segre- 8. The origin of komatiites gates subsequently from the depleted olivine-rich asthenosphere. Komatiites have received increased attention in Fig. 9(c) shows the upper mantle in the Archean. recent years, since it was understood that komatiitic The garnet shell breaks into segments, perhaps ow- melts originated at much higher pressures and tem- ing to the contraction on further cooling and to the peratures than the more common basaltic melts (Bi- slowdown in the Earth's rotation (Dicke, 1969). ckle et al., 1977). Numerous experimental studies Subduction is predominantly shallow (McCulloch, have shown that the komatiitic melts have the com- 1993; Rapp and Shimizu, 1996). The oceanic crust is positions of the partial melts forming on the anhy- recycled by partial melting in hot slabs. However, drous solidus of common mantle compositions (e.g. colder slabs dehydrate before melting, because the Herzberg et al., 1990; Wei et al., 1990). Thus, the interval between the dehydration and the melting compositions of komatiites are controlled by phase temperatures at the corresponding pressures is sev- relations rather than the bulk composition of the eral hundred degrees Celsius, and the H20 fluid source region (Arndt, 1986). This makes it possible escapes. As the presence of H20 in the slabs is to estimate the depth of origin of the komatiitic melts deemed essential for recycling, such dehydrated slabs and the temperature of the source region from exper- cannot be recycled and accumulate in the astheno- imental studies of melting relations. sphere. Thus, colder regions of the asthenosphere, Herzberg (1992) discussed the experimental evi- particularly under continents, are filled with nonrecy- dence indicating that all komatiitic melts formed at clable dehydrated slabs, forming the deep roots of pressures lower than 10GPa, and that the age and some continents. On rare occasions, the oldest and depth are correlated; older komatiites originated at thickest slabs penetrate through the shell and are greater depths (see also Takahashi (1990)). For ex- inserted between the shell and the lower mantle. This ample, the 3.5 Ga Barberton komatiites originated at raises the overlying segment of the shell, and the 240-300km depth, whereas the Mesozoic Gorgona garnet from the top portion of the segment trans- komatiites originated at 120km depth. Herzberg forms to pyroxene. (1992) estimated that the Barberton komatiites re- The post-Archean upper mantle is shown in Fig. quired a source region which was 500°C hotter than 9(d). Most of the oceanic plates are sufficiently thick the present mantle. It is proposed here that the to be able to subduct to the top of the lower mantle, komatiitic volcanism occurred as the direct conse- and this type of subduction has become dominant. quence of the thermal structure of the upper mantle The slabs are now cold enough to carry hydrous formed by the solidification of the magma ocean. phases to the transition zone. Because the interval The refractory shell produced during the solidifi- between the dehydration and the melting tempera- cation of the magma ocean could cool only by tures in the transition zone is narrow or absent, the conduction, and thus cooled much slower than the slabs are again fully recycled by partial melting convecting asthenosphere located closer to the Earth's (Gasparik, 1993). The garnet shell has been dis- surface. Knopoff (1964) calculated that in a noncon- placed to shallower levels world-wide, as is evident vective Earth, the surface would receive heat only from the global occurrence of the 400 km discontinu- from the uppermost 400-600kin in the lifetime of ity. The evidence from tomography suggests that, at the Earth. Hence, the cooling of the 300kin thick present, some of the oldest slabs seem to be able to shell could have occurred throughout a major span of penetrate into the lower mantle. It is proposed that the Earth's history, comparable with the age span of this style of subduction has developed only recently komatiites. At the time of solidification, the shell and its contribution to the mixing between the upper was sufficiently hot to generate the most magne- T. Gasparik / Physics of the Earth and Planetary Interiors 100 (1997) 197-212 209

sium-rich komatiitic melts required, for example, for through the overlying segments of the shell. Exten- the Barberton-type komatiites, and its slow cooling sive melting, generating komatiitic melts, occurred at throughout the Earth's history could have produced the intersection of the adiabat corresponding to the the whole range of temperatures necessary for the P-T trajectories of these rising overheated plumes origin of the younger komatiites. The absence of and the anhydrous mantle solidus. komatiitic volcanism after the Mesozoic indicates Fig. 10 shows schematically the thermal structure that the shell has reached thermal equilibrium with of the upper mantle containing a shell segment dis- the surrounding mantle. This is supported by the placed to shallower depths by the underlying slab. observed correlation between the age of komatiites Modified KLB-1 solidus (Gasparik, 1992) is an ap- and the depth of origin or the temperature of the proximation of the mantle solidus. Takahashi (1990) source region (Herzberg, 1992). estimated the maximum potential mantle temperature The oldest komatiitic volcanism was presumably (adiabat at zero pressure) of the corresponding started by the first slabs subducting below the shell Archean mantle at 1750°C from the origin of the and displacing it to shallower depths. The volatile- oldest komatiites. The resulting origin at 300km beating plumes formed by partial melting of these depth and at a temperature in excess of 1900°C is slabs underwent substantial heating by passing similar to the conditions reported by Herzberg (1992).

2500

2000

g i Bt+ St +Ga+

E* St I000 Opx + Cpx +Go÷OIj s B + St + Ga +H20 / A+ Cpx + Go

5000 5 I0 15 20 25 Pressure (GPo) Fig. I0. Temperature-pressure phase diagram for a volatile-bearing mantle. Dash-dot isotherms indicate shell temperatures in excess of the normal mantle temperatures corresponding to the mantle geotherm based on the potential mantle temperature of 1280°C. Shell geotherm is based on the potential mantle temperature for the Archean of 1750°(] (Takahashi, 1990). A, Phase A; C, phase C (could also include clinohumite or ehondrodite); D, phase D; E, phase E; sB, superhydrous phase B; T, talc; other symbols as in Fig. 1, Fig. 4 and Fig. 5. Phase relations are based on Presnall and Gasparik (1990), Gasparik (1992, 1993) and Zhang and Herzberg (1994). 210 T. Gasparik / Physics of the Earth and Planetary Interiors 100 (1997) 197-212

This can be compared with the potential temperature Agee, C.B. and Li, J., 1996. Pressure effect on partitioning of of the basalt-generating mantle at 1280°C (McKenzie nickel, cobalt, and sulfur during mantle-core lormation. J. and Bickle, 1988), which has remained fairly con- Conf. Abstr., 1(1): 510. Agee, C.B. and Walker, D., 1988, Mass balance and phase density stant during the Earth's history. constraints on early differentiation of chondritic mantle. Earth Planet. Sci. Lett., 90: 144-156. Agee, C.B. and Walker, D., 1993. Olivine flotation in mantle 9. Conclusion melt. Earth Planet. Sci. Lett., 114: 315-324. All~gre, C.J., 1995. Geochemical structure and evolution of the The model for the Earth's upper mantle shown in mantle. EOS Trans. Am. Geophys. Union, Spring Meet. Suppl., Fig. 7 is similar to that proposed by Gasparik (1993). 76(17): $294. The main contribution of the present study is the Anders, E. and Grevesse, N., 1989. Abundances of the elements: meteoritic and solar. Geochim. Cosmochim. Acta, 53: 197- verification that such a layered upper mantle can 214. have an average composition corresponding to the Anderson, D.L., 1979. Chemical stratification of the mantle. J. cosmic abundances of elements. At the same time, Geophys. Res., 84: 6297-6298. its structure is consistent with the experimentally Anderson, D.L., 1982. Isotopic evolution of the mantle: a model. determined phase relations, and appears to have suf- Earth Planet. Sci. Lett., 57: 13-24. Anderson, D.L., 1989. Theory of the Earth. Blackwell Scientific, ficient flexibility to match the observed seismic ve- Oxford. locity profiles. The proposed structure can be pro- Anderson, D.L., Tanimoto, T. and Zhang, Y., 1992. Plate tecton- duced by accumulation of olivine-rich crust in the ics and hotspots: the third dimension. Science, 256: 1645- upper 200km of the magma ocean, and does not 1651. require garnet fractionation, which seems inconsis- Arndt, N.T., 1986. Komatiites: a dirty window to the Archean mantle. Terra Cognita, 6: 59-66. tent with the geochemical evidence. Bass, J.D. and Anderson, D.L., 1984. Composition of the upper The implications of a hidden reservoir in the mantle: geophysical tests of two petrological models. Get- upper mantle for our understanding of mantle chem- phys. Res. Lett., 11: 229-232. istry are far reaching. The potential existence of such Basu, A.R., Ray, S.L., Saha, A.K. and Sarkar, S.N., 1981. Eastern a reservoir could provide an explanation for the Indian 3800-million-year-old crust and early mantle differentiation. Science, 212: 1502-1506. discrepancy between the chemistry of the observable Benz, H.M. and Vidale, J.E., 1993. Sharpness of upper-mantle portion of the upper mantle and the cosmic abun- discontinuities determined from high-frequency reflections. dances of elements, including depletions in many Nature, 365: 147-150. trace elements. If the sodium content of the upper Bickle, M.J., Ford, C.E. and Nisbet, E.G., 1977. The petrogenesis mantle corresponds to its cosmic abundance, it is of peridotitic komatiites: evidence from high-pressure melting experiments. Earth Planet. Sci. Lett., 37: 97-106. likely that neither potassium is depleted, and its high Cameron, A.G.W. and Benz, W., 1991. The origin of the moon concentration in the shell could be a major source of and the single impact hypothesis. Icarus, 92: 204-216. radiogenic heating. The radiogenic heating could Cameron, A.G.W. and Ward, W.R., 1976. The origin of the moon. also play a more important role in the lower mantle Lunar Sci., 7: 120-122. than is generally assumed. Dicke, R.H., 1969. Average acceleration of the Earth's rotation and the viscosity of the deep mantle. J. Geophys. Res., 74: 5895-5902. Drake, M.J., McFarlane, E.A., Gasparik, T. and Rubie, D.C., Acknowledgements 1993. Mg-perovskite/silicate melt and majorite garnet/silicate melt partition coefficients in the system CaO-MgO-SiO 2 This study was funded by National Science Foun- at high temperatures and pressures. J. Geophys. Res., 98: dation Grant EAR 93-03865. 5427-5431. Forte, A.M., Peltier, W.R. and Dziewonski, A.M., 1991. Infer- ences of mantle viscosity from tectonic plate velocities. Get- References phys. Res, Lett., 18: 1747-1750. Fukao, Y., Obayashi, M., Inoue, H. and Nenbai, M., 1992. Abe, Y., 1993. Thermal evolution and chemical differentiation of Subducting slabs stagnant in the mantle transition zone. J. the terrestrial magma ocean. Geophys. Monogr. (IUGG 14), Geophys. Res., 97: 4809-4822, 74: 41-54. Galer, S.J.G. and Goldstein. S.L., 1991. Early mantle differentia- T. Gasparik / Physics of the Earth and Planetary Interiors 100 (1997) 197-212 211

tion and its thermal consequences. Geochim. Cosmochim. Katsura, T. and Ito, E., 1989. The system Mg2SiO4-Fe2SiO4 at Acta, 55: 227-239. high pressures and temperatures: precise determination of Gasparik, T., 1989, Transformation of enstatite-diopside-jadeite stabilities of olivine, modified spinel, and spinel. J. Geophys. pyroxenes to garnet. Contrib. Mineral. Petrol., 102: 389-405. Res., 94: 15663-15670. Gasparik, T., 1990a. Immiscibility in the majorite garnets. EOS King, S.D. and Masters, G., 1992. An inversion for radial viscos- Trans. Am. Geophys. Union, 71: 1587. ity structure using seismic tomography. Geophys. Res. Lea., Gasparik, T., 1990b. Phase relations in the transition zone. J. 19: 1551-1554. Geophys, Res., 95: 15751-15769. Knopoff, L., 1964. The convection current hypothesis. Rev. Geo- Gasparik, T., 1992. Enstatite-jadeite join and its role in the phys., 2: 89-122. Earth's mantle. Contrib. Mineral. Petrol., 111: 283-298. Kumazawa, M., 1981. Origin of materials in the Earth's interior Gasparik, T., 1993. The role of volatiles in the transition zone. J. and their layered distribution. Jpn. J. Petrol. Mineral. Econ. Geophys. Res., 98: 4287-4299. Geol., Spec. Issue, 3: 239-247. Gasparik, T., 1996a. Melting experiments on the enstatite-di- Liu, L, 1979. On the 650-km seismic discontinuity. Earth Planet. opside join at 70-224kbar, including the melting of diopside. Sci. Lett., 42: 202-208. Contrib. Mineral. Petrol., 124: 139-153. Liu, M., 1994. Asymmetric phase effects and mantle convection Gasparik, T., 1996b. Diopside-jadeite join at 16-22 GPa. Phys. patterns. Science, 264: 1904-1907. Chem. Minerals, 23: 476-486. MacDonald, G.J.F., 1963. The deep structure of continents. Rev. Gasparik, T. and Drake, M.J., 1995. Partitioning of elements Geophys., 1: 587-665. among two silicate perovskites, superphase B, and volatile- MacDonald, G.J.F. and Knopoff, L., 1958. On the chemical bearing melt at 23GPa and 1500-1600°C. Earth Planet. Sci. composition of the outer core. Geophys. J. R. Astron. Soc., 1: Lett., 134: 307-318. 284-297. Hamilton, P.J., O'Nions, R.K., Bridgwater, D. and Nutman, A., McCulloch, M.T., 1993. The role of subducted slabs in an evolv- 1983. Sm-Nd studies of Archaean metasediments and ing earth. Earth Planet. Sci. Lett., 115: 89-100. metavolcanics from West Greenland and their implications for McCulloch, M.T. and Compston, W., 1981. Sm-Nd age of Kam- the Earth's early history. Earth Planet. Sci. Lett., 62: 263-272. balda and Kanowna greenstones and heterogeneity in the Hart, S.R., 1988. Heterogeneous mantle domains: signatures, gen- Archean mantle. Nature, 294: 322-327. esis and mixing chronologies. Earth Planet. Sci. Lett., 90: McKenzie, D. and Bickle, M.J., 1988. The volume and composi- 273-296. tion of melt generated by extension of the lithosphere. J. Hartmann, W.K. and Davis, D.R., 1975. Satellite-sized planetesi- Petrol., 29: 625-679. mals and lunar origin. Icarus, 24: 504-515. Miller, G.H., Stolper, E.M. and Ahrens, T.J., 1991. The equation Herzberg, C., 1992. Depth and degree of melting of komatiites. J. of state of molten komatiite: 1. Shock wave compression to Geophys. Res., 97: 4521-4540. 36GPa, 2. Application to komatiite petrogenesis and the Herzberg, C. and Gasparik, T., 1991. Garnet and pyroxenes in the Hadean mantle. J. Geophys. Res., 96:11831-11864. mantle: a test of the majorite fractionation hypothesis. J. Morse, S.A., 1993. Behavior of a perched crystal layer in a Geophys. Res., 96: 16263-16274. magma ocean. J. Geophys. Res., 98: 5347-5353. Herzberg, C., Gasparik, T. and Sawamoto, H., 1990. Origin of Nolet, G., Grand, S.P. and Kennett, B.LN.. 1994. Seismic hetero- mantle peridotite: constraints from melting experiments to geneity in the upper mantle. J. Geophys. Res., 99: 23753- 16.5 GPa. J. Geophys. Res., 95: 15779-15803. 23766. Jagoutz, E., Palme, H., Baddenhausen, H., Blum, K., Cendales, O'Nions, R.K. and Tolstikhin, I.N., 1996. Limits on the mass flux M., Dreibus, G., Spettel, B., Lorenz, V. and Wanke, H., 1979. between lower and upper mantle and stability of layering. The abundances of major, minor, and trace elements in the Earth Planet. Sci. Lett., 139: 213-222. earth's mantle as derived from primitive ultramafic nodules. Ohtani, E., 1985. The primordial terrestrial magma ocean and its Proc. 10th Lunar Planet. Sci. Conf., pp. 2031-2050. implications for stratification of the mantle. Earth Planet. Sci. Jordan, T.H., 1975. The continental tectosphere. Rev. Geophys. Lett., 38: 70-80. Space Phys., 13(3): 1-12. Ohtani, E., Kawabe, I., Moriyama, J. and Nagata, Y., 1989. Karato, S., Wang, Z., Liu, B. and Fujino, K., 1995. Plastic Partitioning of elements between majorite garnet and melt and deformation of garnets: systematics and implications for the implications for petrogenesis of komatiite. Contrib. Mineral. rheology of the mantle transition zone. Earth Planet. Sci. Lett., Petrol., 103: 263-269. 130: 13-30. Ohtani, E., Kagawa, N. and Fujino, K., 1991. Stability of majorite Kato, T., Ringwood, A.E. and Irifune, T., 1988a. Experimental (Mg,Fe)SiO 3 at high pressures and 1800°C. Earth Planet. Sci. determination of element partitioning between silicate per- Lett., 102: 158-166. ovskites, garnets and liquids: constraints on early differentia- Ohtani, E., Suzuki, A. and Kato, T., 1993. Flotation of olivine in tion of the mantle. Earth Planet. Sci. Lett., 89: 123-145. the peridotite melt at high pressure. Proc. Jpn. Acad., 69(B): Kato, T., Ringwood, A.E. and Irifune, T., 1988b. Constraints on 23-28. element partition coefficients between MgSiO 3 perovskite and Ohtani, E., Nagata, Y., Suzuki, A. and Kato, T., 1995. Melting liquid determined by direct measurements. Earth Planet. Sci. relations of peridotite and the density crossover in planetary Lett., 90: 65-68. mantles. Chem. Geol., 120: 207-221. 212 T. Gasparik / Physics of the Earth and Planetao ~Interiors 100 (1997) 197-212

Palme, H. and Nickel, K.G., 1985. Ca/AI ratio and the composi- Solomatov, V.S. and Stevenson, D.J., 1993. Nonfractional crystal- tion of the Earth's mantle. Geochim. Cosmochim. Acta, 49: lization of a terrestrial magma ocean. J. Geophys. Res., 98: 2123-2132. 5391-5406. Pollack, H.N. and Chapman, D.S., 1977. On the regional variation Stevenson, D.J., 1987. Origin of the moon--the collision hypoth- of heat flow, geotherms and lithospheric thickness. Tectono- esis. Annu. Rev. Earth Planet. Sci., 15: 271-315. physics, 38: 279-296. Stolper, E.M., Walker, D., Hager, B.H. and Hays, J.F., 1981. Melt Presnall, D.C. and Gasparik, T., 1990. Melting of enstatite segregation from partially molten source region: the impor- (MgSiO 3) from 10 to 16.5GPa and the forsterite (Mg2SiOa)- tance of melt density and source region size. J. Geophys. Res., majorite (MgSiO 3) eutectic at 16.5 GPa: implications for the 86: 6161-6271. origin of the mantle. J. Geophys. Res., 95: 15771-15777. Sweeney, J.S. and Heinz, D.L., 1995. Irreversible melting of Presnall, D.C. and Walter, M.J., 1993. Melting of forsterite, magnesium-iron-silicate perovskite at lower mantle pres- Mg2SiO4, from 9.7 to 16.5GPa. J. Geophys. Res., 98: 19777- sures. EOS Trans. Am. Geophys. Union, Fall Meet. Suppl., 19783. 76(46): F553. Rapp, R.P. and Shimizu, N., 1996. Arc magmatism in hot subduc- Takahashi, E., 1990. Speculations on the Archean mantle: missing tion zones: interactions between slab-derived melts and the link between komatiite and depleted garnet peridotite. J. Geo- mantle wedge, and the petrogenesis of adakites and high-mag- phys. Res., 95: 15941-15954. nesian andesites (HMA). J. Conf. Abstr., 1(1): 497. Tonks, W.B. and Melosh, H.J., 1990. The physics of crystal Revenaugh, J. and Sipkin, S.A., 1994. Seismic evidence for settling and suspension in a turbulent magma ocean. In: N.E. silicate melt atop the 410-km mantle discontinuity. Nature, Newsom and J.H. Jones (Editors), Origin of the Earth. Oxford 369: 474-476. University Press, New York, pp. 151-174. Ricard, Y., Vigny, C. and Froidevaux, C., 1989. Mantle hetero- Tonks, W.B. and Melosh, H.J., 1993. Magma ocean formation geneities, geoid and plate motion: a Monte Carlo inversion. J. due to giant impacts. J. Geophys. Res., 98: 5319-5333. Geophys. Res., 94: 13739-13754. Van den Berg, A.P., van Keken, P.E. and Yuen, D.A., 1993. High Rigden, S.M., Ahrens, T.J. and Stolper, E.M., 1984. Densities of melting temperature of perovskite: dynamic implications for liquid silicates at high pressures. Science, 226: 1071-1074. creep in the lower mantle. EOS Trans. Am. Geophys. Union, Rigden, S.M., Gwanmesia, G.D. and Liebermann, R.C., 1994. Fall Meet. Suppl., 74(43): 557. Elastic wave velocities of a pyrope-majorite garnet to 3 GPa. Van der Hilst, R., Engdahl, R., Spakman, W. and Nolet, G., 1991. Phys. Earth Planet. Inter., 86: 35-44. Tomographic imaging of subducted lithosphere below north- Righter, K., Drake, M.J. and Yaxley, G., 1996. Predicting west Pacific island arcs. Nature, 353: 37-43. siderophile element metal-silicate partition coefficients. J. Wei, K., Tr~nnes, R.G. and Scarfe, C.M., 1990. Phase relations of Conf. Abstr., 1(1): 510. aluminum-undepleted and aluminum-depleted komatiites at Ringwood, A.E., 1962. A model for the upper mantle. J. Geophys. pressures of 4-12GPa. J. Geophys. Res., 95: 15817-15827. Res., 67" 857-867. Weinstein, S.A., 1992. Induced compositional layering in a con- Ringwood, A.E., 1994. Role of the transition zone and 660km vecting fluid layer by an endothermic phase transition. Earth discontinuity in mantle dynamics. Phys. Earth Planet. Inter., Planet. Sci. Lett., 113: 23-39. 86: 5-24. Wetherill, G.W., 1976. The role of large bodies in the formation Ringwood, A.E. and Irifune, T., 1988. Nature of the 650-km of the solar system. Proc. 7th Lunar Planet. Sci. Conf., pp. seismic discontinuity: implications for mantle dynamics and 3245-3257. differentiation. Nature, 331: 131-136. Zerr, A. and Boehler, R., 1993. Melting of (Mg,Fe)SiO3-per- Shirey, S. and Hanson, G.N., 1986. Mantle heterogeneity and ovskite to 625kilobars: indications of a high melting tempera- crustal recycling in Archean granite-greenstone belts: evi- ture in the lower mantle. Nature. 262: 553-555. dence from Nd isotopes and trace elements in the Rainy Lakes Zhang, J. and Herzberg, C., 1994. Melting experiments on anhy- area, Superior Province, Ontario, Canada. Geoehim. Cos- drous peridotite KLB-1 from 5.0 to 22.5GPa. J. Geophys. mochim. Acta, 50: 2631-2651. Res., 99: 17729-17742. Silver, P.G., Beck, S.L., Wallace, T.C., Meade, C., Myers, S.C., Zielhuis, A. and Nolet, G., 1994. Deep seismic expression of an James, D.E. and Kuehnel, R., 1995. Rupture characteristics of ancient plate boundary in Europe. Science, 265: 79-81. the deep Bolivian earthquake of 9 June 1994 and the mechanism of deep-focus earthquakes. Science, 268: 69-73.