Earth and Space Science Open Archive This work has been accepted for publication in Journal of Geophysical Research - Solid Earth. Version of RecordESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary. Learn more about preprints. preprintOpen AccessYou are viewing the latest version by default [v1]Geochemical Profiles Across the Listvenite-Metamorphic Transition in the Basal Megathrust of the Semail Ophiolite: Results from Drilling at Oman DP Hole BT1BAuthorsMargueriteGodardiDElliot JCarteriDThierryDecrausaziDRomainLafayiDEmmaBennettFatmaKourimJuan Carlosde ObesoiDKatsuyoshiMichibayashiiDMichelleHarrisJude A.CoggonDamonTeagleiDPeter BKelemeniDSee all authors Marguerite GodardiDCorresponding Author• Submitting AuthorUniversite de MontpellieriDhttps://orcid.org/0000-0003-3097-5135view email addressThe email was not providedcopy email addressElliot J CarteriDTrinity College DubliniDhttps://orcid.org/0000-0002-0180-0419view email addressThe email was not providedcopy email addressThierry DecrausaziDUniversity of MontpellieriDhttps://orcid.org/0000-0002-4285-4974view email addressThe email was not providedcopy email addressRomain LafayiDUniversity of MontpellieriDhttps://orcid.org/0000-0001-6295-5871view email addressThe email was not providedcopy email addressEmma BennettSchool of Earth and Ocean Sciencesview email addressThe email was not providedcopy email addressFatma KourimAcademia Sinicaview email addressThe email was not providedcopy email addressJuan Carlos de ObesoiDUniversity of CalgaryiDhttps://orcid.org/0000-0002-9962-8177view email addressThe email was not providedcopy email addressKatsuyoshi MichibayashiiDNagoya UniversityiDhttps://orcid.org/0000-0003-1845-7298view email addressThe email was not providedcopy email addressMichelle HarrisUniversity of Plymouthview email addressThe email was not providedcopy email addressJude A. CoggonNational Oceanography Centreview email addressThe email was not providedcopy email addressDamon TeagleiDNational Oceanography Centre SouthamptoniDhttps://orcid.org/0000-0002-4416-8409view email addressThe email was not providedcopy email addressPeter B KelemeniDColumbia UniversityiDhttps://orcid.org/0000-0003-4757-0855view email addressThe email was not providedcopy email address
Abstract A large portion of the middle to lower crust beneath the continents and oceanic island arcs consists of amphibolites dominated by hornblende and plagioclase. We have measured P and S wave velocities ( V p and V s ) and anisotropy of 17 amphibole‐rich rock samples containing 34–80 vol % amphibole at hydrostatic pressures (P) up to 650 MPa. Combined petrophysical and geochemical analyses provide a new calibration for mean density, average major element contents, mean V p ‐ P and V s ‐ P coefficients, intrinsic V p and V s anisotropy, Poisson's ratios, the logarithmic ratio R s/p , and elastic moduli of amphibole‐rich rocks. The V p values decrease with increasing SiO 2 and Na 2 O + K 2 O contents but increase with increasing MgO and CaO contents. The maximum (≤0.38–0.40 km/s) and minimum S wave birefringence values occur generally in the propagation direction parallel to Y and normal to foliation, respectively. Amphibole plays a critical role in the formation of seismic anisotropy, whereas the presence of plagioclase, quartz, pyroxene, and garnet diminishes the anisotropy induced by amphibole crystallographic preferred orientations (CPOs). The CPO variations cause different anisotropy patterns illustrated in the Flinn diagram of V p (X)/ V p (Y)‐ V p (Y)/ V p (Z) plots. The results make it possible to distinguish, in terms of seismic properties, the amphibolites from other categories of lithology such as granite‐granodiorite, diorite, gabbro‐diabase, felsic gneiss, mafic gneiss, eclogite, and peridotite within the Earth's crust. Hence, amphibole, aligned by dislocation creep, anisotropic growth, or rigid‐body rotation, is the most important contributor to the seismic anisotropy of the deep crust beneath the continents and oceanic island arcs, which contains rather little phyllosilicates such as mica or chlorite.
Oxidation states within the planetary interior are intrinsically linked with the broad scale tectonism; however, it is difficult to estimate the actual oxidation conditions. Orthopyroxene–magnetite symplectite formed by olivine oxidation may provide a significant clue into such oxidation events. Here we report detailed mineralogical and petrological synthesis of such orthopyroxene–magnetite symplectites from olivine gabbros of Oman Ophiolite (Hole GT2A, ICDP Oman Drilling Project). In order to understand how oxidation affects different olivine compositions, we employed a phase equilibria approach and computed several temperature–composition diagrams at a fixed pressure (1 kbar). Our experiments predict the coexistence of olivine with Fo75–76 and Fo71 with the orthopyroxene (En79 and En76), respectively, which is remarkably similar to the mineral chemistry obtained from the Oman lower crustal gabbros. From the magnetite content, we also infer that the symplectite formation may have taken place over a range of temperatures (600–1000 °C) via subsolidus olivine oxidation and/or melt (oxidizing)–olivine interaction. The latter is more probable, considering the partial occurrence of orthopyroxene and clinopyroxene rim adjacent to the symplectites.
