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Seismic Evidence for Water Deep in Earth's Upper Mantle

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Seismic Evidence for Water Deep in Earth's Upper Mantle R EPORTS million years ago (Ma), subduction mainly took place in the eastern Mediterranean with Seismic Evidence for Water Deep mostly south- and eastward-dipping subduc- tion. Remnants of the subducted slab can be in Earth’s Upper Mantle found in the lower mantle between 1300 and 1900 km depth under locations such as Egypt Mark van der Meijde,* Federica Marone, Domenico Giardini, and Libya (18). Subduction was ubiquitous Suzan van der Lee after 80 Ma. At present, active subduction zones are found in southern Italy and Greece. Water in the deep upper mantle can influence the properties of seismic dis- We investigated seismic discontinuities in the continuities in the mantle transition zone. Observations of converted seismic deep upper mantle of the Mediterranean re- waves provide evidence of a 20- to 35-kilometer-thick discontinuity near a gion by searching seismograms for seismic S depth of 410 kilometers, most likely explained by as much as 700 parts per waves that converted from P waves at the million of water by weight. discontinuities. Our search was facilitated by a recent temporary deployment of mobile Two major seismic velocity discontinuities, lies in the mantle transition zone would broadband seismic stations [MIDSEA (19)] at nominal depths of 410 and 660 km, border have no seismically observable effect on along the plate boundary between Eurasia the transition zone of Earth’s mantle. The the thickness of the 660-km discontinuity and Africa. We analyzed over 500 seismo- 410-km discontinuity is the result of the tran- (7), whereas the effect on the 520-km dis- grams recorded in the Mediterranean region sition from olivine to wadsleyite. The 660- continuity is unknown. at 18 temporary and 6 permanent broadband km discontinuity is caused by the transition Both temperature and water content of the stations (Fig. 1). We identified converted from ringwoodite to perovskite and magne- upper mantle vary spatially. A variety of waves by analyzing receiver functions (20). siowu¨stite. Both phase transition intervals are evidence (11–14) has indicated that amounts We found P-to-S converted waves from reported to be sharp (about 4 to 10 km thick) of 1500 parts per million (ppm) of water can the 410- and 660-km discontinuity for prac- (1–3). However, there are conditions under be present locally in the uppermost mantle. tically all stations. The conversions from the which the transition interval of the 410-km This water is carried there by subducting 410-km discontinuity are small in amplitude discontinuity can be thickened, such as by an oceanic plates, which release most of the for receiver functions that are low-passed increase in water content in olivine or by a subducted water at depths shallower than 150 with corner frequencies higher than 0.5 Hz. decrease in temperature. Theoretical studies km into the mantle wedge overlying the plate The amplitudes increase with decreasing cor- (4, 5) have shown how water influences the (15). Reports of water being able to reside ner frequency. This effect is not observed for properties of the olivine-wadsleyite phase below this depth are based on high-pressure conversions from the 660-km discontinuity, transition. Because water is preferentially in- experiments (16) and seismic tomography which show comparable amplitudes through- corporated into wadsleyite rather than oli- (17). It is, however, unknown whether and out the whole frequency range. vine, it stabilizes wadsleyite over a wider how much water is transported down to 400 Because of the temporary deployment of range of pressures and temperatures, thereby km and below. To investigate whether water the network and differences in site character- thickening the transition interval. This has resides near the mantle transition zone, we istics, not all seismic stations yielded equally been confirmed by experiments (6) showing studied the properties of seismic discontinui- robust results. Receiver functions for nine that under hydrous conditions, the zone of ties beneath the Mediterranean region (Fig. stations, however, had a sufficiently high coexistence of olivine and wadsleyite can 1). This region has been heavily affected by signal-to-noise ratio to show unambiguous thicken to as much as 40 km. A decrease in the subduction of oceanic lithosphere for the evidence for this frequency dependence of temperature would also thicken the transition past 190 million years. Between 190 and 110 the P-to-S conversion from the 410-km dis- interval (7), but the effect of temperature on the transition-interval thickness is much smaller than the effect of water. A large temperature reduction of 800 K would thick- en the discontinuity by only 10 km (5). In- creased water content in the mantle transition zone would have the opposite effect on the thickness of the phase transition interval at 660 km. This phase transition interval would become thinner because ringwoodite can hold much more water than perovskite, thereby extending its stability phase (8). Moreover, recent studies indicate that the phase transi- tion interval from wadsleyite to ringwoodite, nominally at a depth of 520 km, can be sharper under hydrous conditions (9)(Ͻ15 km thick) than under anhydrous conditions (10)(ϳ60 km thick). Temperature anoma- Fig. 1. Map of the Mediterranean region with seismic station and approximate plate boundary locations. Stations for which results are discussed in this paper are represented by black triangles Institute of Geophysics, Eidgeno¨ssische Technische and labeled with the station name (table S1); other analyzed stations are represented by black Hochschule–Ho¨nggerberg (HPP)/Swiss Federal Insti- squares. Curves with a sawtooth pattern indicate the present location of the convergent boundary, tute of Technology CH–8093 Zu¨rich, Switzerland. with sawteeth pointing in the direction of subduction or underthrusting. Strike-slip is represented *To whom correspondence should be addressed. E- by arrows. The gray scale indicates topography ranging from higher than 2.5 km in white to deeper mail: [email protected] than –2.5 km in dark gray. 1556 6 JUNE 2003 VOL 300 SCIENCE www.sciencemag.org R EPORTS continuity (Fig. 2). For the other stations, the servations (1–3), we found that the phase- 410-km discontinuity. Trace elements in oli- receiver functions were more ambiguous and transition interval at 660 km is relatively thin vine are unlikely to be the cause of the broad- did not allow a significant interpretation of (Ͻ5 km) beneath seven of the nine stations ening, because their presence would thicken the frequency dependence. For these nine (Fig. 2, fig. S1, and table S2). the phase transition interval by no more than stations, the amplitude of the converted Because of limited azimuthal coverage, 2km(26). waves from the 410-km discontinuity de- owing to the location of the Mediterranean We interpret the broadening of the 410-km pends on frequency with 75% confidence, on with respect to seismogenic zones, we could discontinuity deduced here as being caused by average, whereas this confidence is only 21% not investigate the azimuthal influence on the water in the mantle minerals at this depth. for the 660-km discontinuity (table S2). This frequency dependence of the converted These minerals can contain a substantial indicates that the observed frequency depen- waves. However, the conversion points at the amount of water: as much as 1000 ppm by dence is almost four times more probable for 410-km discontinuity beneath most stations weight of water in olivine and about 20,000 conversions from the 410-km discontinuity fall within a radius of 100 km. Synthetic tests ppm by weight of water in wadsleyite (27). The than for those from the 660-km discontinuity. (24) (fig. S3) showed that topography on the estimated water content in olivine for a transi- We observed converted energy from the discontinuity as large as 20 to 40 km on a tion interval of 20 to 25 km is around 500 ppm 410-km discontinuity that is significant (20) 100-km length scale cannot be responsible by weight (Fig. 3). To broaden the transition to above the 95% confidence level for corner for the observed frequency dependence. To- 30 to 40 km, olivine would need to contain 700 frequencies around 0.2 and 0.25 Hz for most pography on the 410-km discontinuity would to 1000 ppm by weight of water. This amount stations. For station PAB, the amplitude of mainly decrease the amplitudes of P-to-S of water is an underestimate because the con- the wave converted at the 410-km disconti- converted waves but could not be responsible verted waves are the most sensitive to the lower nuity becomes comparable to that of the wave for the observed frequency dependence. We part of the phase transition interval, where the converted at the 660-km discontinuity for also excluded scattering of converted energy fraction of wadsleyite increases more rapidly frequencies as low as 0.15 Hz. This frequen- from discontinuities at shallower depths as a with increasing depth (Fig. 3) and the corre- cy dependence of the amplitudes and the possible cause for the observed frequency sponding gradient in seismic velocity is the significance allows us to estimate the thick- dependence, because the standard deviation steepest. ness of the phase transition interval (20). of the converted wave from the 410-km dis- For some stations (Fig. 2), we also ob- These findings indicate that the phase continuity is comparable to the standard de- served, above the 95% confidence level, transition around 410 km depth is not sharp viation of the converted wave from the 660- waves that converted at the 520-km disconti- and occurs over depth intervals of about 20 to km discontinuity. Seismic tomography (25) nuity. Water would make the corresponding 25 km for these stations (fig.
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