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Comparative compressibility of hydrous wadsleyite and ringwoodite: Effect of H 2 O and implications for detecting water in the transition zone
Author(s) -
Chang YunYuan,
Jacobsen Steven D.,
Bina Craig R.,
Thomas SylviaMonique,
Smyth Joseph R.,
Frost Daniel J.,
Boffa Ballaran Tiziana,
McCammon Catherine A.,
Hauri Erik H.,
Inoue Toru,
Yurimoto Hisayoshi,
Meng Yue,
Dera Przemyslaw
Publication year - 2015
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1002/2015jb012123
Subject(s) - ringwoodite , transition zone , mantle (geology) , stishovite , mineralogy , compressibility , analytical chemistry (journal) , geology , diamond anvil cell , chemistry , thermodynamics , high pressure , geochemistry , physics , environmental chemistry
Review of recent mineral physics literature shows consistent trends for the influence of Fe and H 2 O on the bulk modulus ( K 0 ) of wadsleyite and ringwoodite, the major phases of Earth's mantle transition zone (410–660 km). However, there is little consensus on the first pressure derivative, K 0 ′ = (d K/ d P ) P =0 , which ranges from about 4 to >5 across experimental studies and compositions. Here we demonstrate the importance of K 0 ′ in evaluating the bulk sound velocity of the transition zone in terms of water content and provide new constraints on the effect of H 2 O on K 0 ′ for wadsleyite and ringwoodite by conducting a comparative compressibility study. In the experiment, multiple crystals of hydrous Fo 90 wadsleyite containing 2.0 and 0.25 wt % H 2 O were loaded into the same diamond anvil cell, along with hydrous ringwoodite containing 1.4 wt % H 2 O. By measuring their pressure‐volume evolution simultaneously up to 32 GPa, we constrain the difference in K 0 ′ independent of the pressure scale, finding that H 2 O has no effect on K 0 ′, whereas the effect of H 2 O on K 0 is significant. The fitted K 0 ′ values of hydrous wadsleyite (0.25 and 2.0 wt % H 2 O) and hydrous ringwoodite (1.4 wt % H 2 O) examined in this study were found to be identical within uncertainty, with K 0 ′ ~3.7(2). New secondary‐ion mass spectrometry measurements of the H 2 O content of these and previously investigated wadsleyite samples shows the bulk modulus of wadsleyite is reduced by 7.0(5) GPa/wt % H 2 O, independent of Fe content for upper mantle compositions. Because K 0 ′ is unaffected by H 2 O, the reduction of bulk sound velocity in very hydrous regions of transition zone is expected to be on the order of 1.6%, which is potentially detectible in high‐resolution, regional seismology studies.