Phase transition of wadsleyite-ringwoodite in the Mg2SiO4-Fe2SiO4 system

Intraplate, or hot-spot, volcanism is typically interpreted as the result of plate motion over a spatially localized region of long-lived deep-mantle upwelling (Morgan, 1971). Arguably the most famous example of this process is the Hawaiian-Emperor Seamount Chain (e.g., Steinberger et al., 2004). Whether Bermuda belongs in this category of an intraplate volcano originating from a deep-mantle plume source remains uncertain. The island of Bermuda, situated atop a roughly 10 6 km 2 bathymetric swell in the Western Atlantic Ocean (Figure 1), lacks an age-progressive seamount chain and present-day active volcanism. Radiometric dating of borehole samples from the island and adjacent swell area revealed that the volcanic pedestal was formed during the Middle Eocene (ca. 48-45 Ma), and reached subaerial extent in the Late Eocene, approximately 40-36 Ma (Vogt & Jung, 2007). The pedestal has remained volcanically dormant since, and is now overlain by a fossiliferous carbonate platform. In spite of these facts, which suggest an isolated volcanic episode leading to Bermuda's formation, the region is included in some plume-hot-spot catalogs (King & Adam, 2014; Sleep, 1990), though it fails to meet others' definitions of a mantle plume (Courtillot et al., 2003) due to its lack of a hotspot track and weak evidence for a significant upper mantle low velocity anomaly from seismic tomography. Additionally, several studies suggest a link between the igneous activity which formed Bermuda and other volcanic events, such as the formation of the Mississippi Embayment (Cox & Van Arsdale, 2002; L. Liu et al., 2017) and the reactivation of the New Madrid rift system (Chu et al., 2013), though it may not be linked to all the magmatism along its purported track (Mazza et al., 2014). Recent work in whole mantle seismic tomographic imaging has facilitated the observation and interpretation of several of these deep-mantle plume structures (e.g., French & Romanowicz, 2015), particularly in association with the Pacific and African LLSVPs (Lekić et al., 2012). The presence of a strong, high temperature mantle upwelling should be visible in tomographic images of mantle velocity. To explore this hypothesis in the context of Bermuda, we made cross sections through two seismic tomography models (Figure 2), the joint P and S velocity model LLNL_G3D_JPS (Simmons et al., 2015) and the radially anisotropic S velocity model SEMUCB_WM1 (French & Romanowicz, 2014). Both models display a low velocity anomaly beneath Bermuda relative to the one-dimensional (1-D) average velocity of the profile interrogated. This anomaly extends as a continuous feature to roughly 1,500 km depth, and then is deflected eastward

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“…This would be consistent with mineralogical understanding of the 520 as a broader discontinuity than the 410 or 660 (e.g. Tsujino et al., 2019).…”

Section: Resultssupporting

confidence: 87%

Mantle Transition Zone Receiver Functions for Bermuda: Automation, Quality Control, and Interpretation

2021

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“…Similarly, previous experimental works also found that the pressure interval is not greater than 0.2 GPa in the Mg2SiO4 system (Inoue et al 2010a). By comparison, the pressure interval of two-phase coexistence for the (Mg0.9Fe0.1)2SiO4 system is about 1.0 GPa at 1873 K (Akaogi et al 1989;Katsura and Ito 1989;Tsujino et al 2019).…”

Section: Water Effect On Wadsleyite-ringwoodite Phase Transitionsupporting

confidence: 62%

“…The 410-km and 660-km discontinuities, which separate the upper mantle from the lower mantle, caused by the olivine-wadsleyite and post-spinel phase transitions (Bina and Helffrich 1994;Helffrich and Wood 2001;Higo et al 2001;Hirose 2002;Fei et al 2004;Katsura et al 2004), respectively. In the mid mantle transition zone (MTZ), wadsleyite is expected to undergo a phase transition to ringwoodite (Akaogi et al 1989;Katsura and Ito 1989;Inoue et al 2006;Yu et al 2008;Tsujino et al 2019), and some seismic studies have observed a seismic discontinuity around 520 km depth in some regions such as Northeastern China and central Asia (Shearer 1990;Gossler and Kind 1996;Deuss and Woodhouse 2001;Tian et al 2016). However, unlike the 410-km and 660-km discontinuities, the 520-km discontinuity is not ubiquitously observed and absent in other regions such as the northeastern Pacific Ocean (Gossler and Kind 1996;Deuss and Woodhouse 2001).…”

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

“…Many experimental studies have investigated the phase boundary between wadsleyite and ringwoodite and found that for the (Mg0.9Fe0.1)2SiO4 system under dry condition, this phase transition occurs at ~520 km along the mantle adiabat, with a Clapeyron slope of ~+4.2 MPa/K (Akaogi et al 1989;Katsura and Ito 1989;Inoue et al 2006;Tsujino et al 2019). The effective width of the two-phase coexistence is 20-22 km, which is much thicker than that of the olivine-wadsleyite binary loop (Inoue et al 2006;Tsujino et al 2019). The velocity and density contrasts (ΔVP, ΔVS, and Δρ) between ringwoodite and wadsleyite are ~3.6%, 4.3%, and 1.9% (Núñez Valdez et al 2012), respectively, smaller than those across the olivine-wadsleyite transition (Núñez- Valdez et al 2013;Wang et al 2019).…”

Section: Introductionmentioning

confidence: 99%

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