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Wet vs. Dry Petrologic Anomalies in the Mantle Transition Zone Wang-Ping Chen 1, Tai-Lin Tseng 1, Michael Brudzinski 2, & Harry W. Green 3 1 University.

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Presentation on theme: "Wet vs. Dry Petrologic Anomalies in the Mantle Transition Zone Wang-Ping Chen 1, Tai-Lin Tseng 1, Michael Brudzinski 2, & Harry W. Green 3 1 University."— Presentation transcript:

1 Wet vs. Dry Petrologic Anomalies in the Mantle Transition Zone Wang-Ping Chen 1, Tai-Lin Tseng 1, Michael Brudzinski 2, & Harry W. Green 3 1 University of Illinois, Urbana-Champaign (CIDER participants, supported by CSEDI ) 2 Miami University, Oxford, OH (supported by CSEDI) 3 University of California, Riverside (supported by CSEDI) [Green, ’01]

2 Outline Overall theme –Intricate interplays between temperature and petrology Two key examples with mutually incompatible causes Subduction (Tonga & Izu-Bonin; effects of kinetics) –Anomaly in seismic anisotropy, variations in concordant anomalies of V P & V S, & changing patterns in seismic strain Four stages of slab evolution Involving metastable olivine (dry) –Counter-intuitive Extremely fast subduction of cold slab results in impounding of buoyant slab in transition zone TZ a thermo-petrologic obstacle for deep slab-penetration Continental collision (Tibet; effects of volatiles) –Discordant anomalies of V P & V S Hydroxyl in nominally anhydrous minerals (“wet”) –Convective removal of thickened mantle lithosphere A novel pathway to bring water into the TZ

3 Four Stages of Slab Evolution 3: Detached Slab - Outboard earthquakes - No pattern in seismic strain - Lacks high V - Strong radial anisotropy Indicates petrofabric (V SH > V SV ) - Large remnant of buoyant slab 4: Aseismic Remnant - High V - Gradually decreasing outward from outboard earthquakes - Null anisotropy - Thermal “aureole” - Negatively buoyant [Chen & Brudzinski, Science, ’01; GRL, ’03; Brudzinski & Chen, JGR, ‘03]

4 Reconstruction to -15 Ma [Chen and Tseng, JGR, ‘07] 30 – 15 Ma (d) –IM advanced beneath Lhasa terrane –IMF impinged upon S. edge of Qiangtang terrane Convective removal of thickened root –Onset of most recent volcanism in N. Tibet 15 Ma – present (c) –Indian mantle lithosphere advanced N. beyond BNS Elevation of Qiangtang terrane was only 3 km

5 Discordant  V P and  V S  V P ~+1.2%,  V S ~ Null Need petrologic anomaly to –Counteract effect of low temp (which raises V S ) –Leave V P unaffected Hydroxyl in Nominally Anhydrous Minerals (NAM) [Tseng & Chen, EPSL, ‘08]

6 Other Evidences and Consequences Prior to thickening, Qiangtang was part of a mantle wedge –Modern mantle wedges are wet Hydrous mica (phlogopite) is common in potassic volcanics of Cenozoic age in N. Tibet. Hydroxyls facilitate thickening of continental lithosphere –Contraction = advection of cold materials to depth –Low temp. inhibits plastic deformation –Hydroxyls enhance plastic deformation [Newman et al., ‘99]

7 For Reprints –http://www.Illinois.edu/goto/pubshttp://www.Illinois.edu/goto/pubs and look in that directory…

8 Metastable OlivineHydrous/ Norminally Anhydrous Phases Partial Melt Stability in Transition Zone Favorable Induced by dehydration Trigger for Earthquakes Transformational faulting Embrittlement/ Need new hypothesis Embrittlement 3% reduction in V P 60% of olivine- polymorph in  - phase ~2.2 wt % water/ Little reduction ~2% melt (~0.04 wt% water) Corresponding reduction in V SH ConsistentConsistent/Too much Too much (~6%) SH-SV splittingPlausible, fossil fabric Minor effect by themselves Oriented magma pockets (SPO) No pattern in Focal Mechanisms Localized self- stress (anticrack model) Inconsistent, need other localized source of stress Inconsistent, need regional stress for SPO Buoyancy of slab2%, but only in TZ1%, buoyant in the upper mantle/~0.2% ~No effect (depending on compressibility)

9 Some Fundamental Issues Mass imbalance –Fast subduction of cold slab but only modest anomalies in the lower mantle Tonga subduction zone –Rate of convergence > 200 mm/yr (from GPS) –Slab older than 100 Ma –2/3 of world’s deep seismicity –Best example of lower mantle anomaly Caribbean anomaly (remnant of Farallon plate?) Thermal-Petrologic “window” for deep slab penetration? [Grand et al., 1997]

10 Transition Zone beneath the Back-Arc I: Outboard EQs, strong anisotropy, but a lack of high V P & V S -- Need petrologic anomaly to counteract effects of low temp. which raises V -- Metastable olivine is the most likely candidate -- Buoyant, but only in the TZ II: No EQs, null anisotropy, but high V -- Gradually decreasing in amplitude -- Pure thermal anomaly III: No EQs, null anisotropy, no anomaly in V [Chen & Brudzinski, GRL, ’03; Brudzinski & Chen, JGR, ’00; ‘03]

11 Global Study of Seismic Strain Two populations Down-dip compression or extension, if –Slab dip > 20  –Down-dip comp. > 1/3 (sin[20  ]) of total slab-pull No clear pattern of strain, if –Slab dip < 20  –Vanishing down-dip comp. < 1/3 of total slab-pull ObservationsRegional Stress (Slab-Pull)Localized Stress Numerous seismicity in sub- horizontal slabs Small to absent, thus not necessary Only alternative, thus both necessary and sufficient Great pressure at depthSlab pull too small to be sufficient, ~0.5% negative buoyancy Only alternative, thus necessary and sufficient Down-dip compression from depths of 100 to 700 km, but gap in seismicity near 300 km InsufficientNot present? Localized, self-stress is both necessary and sufficient for generating deep earthquakes [Brudzinski & Chen, JGR, ‘05]


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