Sarafian Emily

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Sarafian
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Emily
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  • Article
    Structure of the lithosphere beneath the Barotse Basin, western Zambia, from magnetotelluric data.
    (American Geophysical Union, 2019-01-30) Evans, Rob L. ; Elsenbeck, James R. ; Zhu, Jian ; Abdelsalam, Mohamed ; Sarafian, Emily ; Mutamina, Daniel ; Chilongola, F ; Atekwana, Estella ; Jones, Alan
    A magnetotelluric survey in the Barotse Basin of western Zambia shows clear evidence for thinned lithosphere beneath an orogenic belt. The uppermost asthenosphere, at a depth of 60–70 km, is highly conductive, suggestive of the presence of a small amount of partial melt, despite the fact that there is no surface expression of volcanism in the region. Although the data support the presence of thicker cratonic lithosphere to the southeast of the basin, the lithospheric thickness is not well resolved and models show variations ranging from ~80 to 150 km in this region. Similarly variable is the conductivity of the mantle beneath the basin and immediately beneath the cratonic lithosphere to the southeast, although the conductivity is required to be elevated compared to normal lithospheric mantle. In a general sense, two classes of model are compatible with the magnetotelluric data: one with a moderately conductive mantle and one with more elevated conductivities. This latter class would be consistent with the impingement of a stringer of plume‐fed melt beneath the cratonic lithosphere, with the melt migrating upslope to thermally erode lithosphere beneath the orogenic belt that is overlain by the Barotse Basin. Such processes are potentially important for intraplate volcanism and also for development or propagation of rifting as lithosphere is thinned and weakened by melt. Both models show clear evidence for thinning of the lithosphere beneath the orogenic belt, consistent with elevated heat flow data in the region.
  • Article
    The electrical structure of the central Pacific upper mantle constrained by the NoMelt experiment
    (John Wiley & Sons, 2015-04-18) Sarafian, Emily K. ; Evans, Rob L. ; Collins, John A. ; Elsenbeck, James R. ; Gaetani, Glenn A. ; Gaherty, James B. ; Hirth, Greg ; Lizarralde, Daniel
    The NoMelt experiment imaged the mantle beneath 70 Ma Pacific seafloor with the aim of understanding the transition from the lithosphere to the underlying convecting asthenosphere. Seafloor magnetotelluric data from four stations were analyzed using 2-D regularized inverse modeling. The preferred electrical model for the region contains an 80 km thick resistive (>103 Ωm) lithosphere with a less resistive (∼50 Ωm) underlying asthenosphere. The preferred model is isotropic and lacks a highly conductive (≤10 Ωm) layer under the resistive lithosphere that would be indicative of partial melt. We first examine temperature profiles that are consistent with the observed conductivity profile. Our profile is consistent with a mantle adiabat ranging from 0.3 to 0.5°C/km. A choice of the higher adiabatic gradient means that the observed conductivity can be explained solely by temperature. In contrast, a 0.3°C/km adiabat requires an additional mechanism to explain the observed conductivity profile. Of the plausible mechanisms, H2O, in the form of hydrogen dissolved in olivine, is the most likely explanation for this additional conductivity. Our profile is consistent with a mostly dry lithosphere to 80 km depth, with bulk H2O contents increasing to between 25 and 400 ppm by weight in the asthenosphere with specific values dependent on the choice of laboratory data set of hydrous olivine conductivity and the value of mantle oxygen fugacity. The estimated H2O contents support the theory that the rheological lithosphere is a result of dehydration during melting at a mid-ocean ridge with the asthenosphere remaining partially hydrated and weakened as a result.
  • Thesis
    Geophysical and petrological constraints on ocean plate dynamics
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2017-06) Sarafian, Emily K.
    This thesis investigates the formation and subsequent motion of oceanic lithospheric plates through geophysical and petrological methods. Ocean crust and lithosphere forms at mid-ocean ridges as the underlying asthenosphere rises, melts, and flows away from the ridge axis. In Chapters 2 and 3, I present the results from partial melting experiments of mantle peridotite that were conducted in order to examine the mantle melting point, or solidus, beneath a mid-ocean ridge. Chapter 2 determines the peridotite solidus at a single pressure of 1.5 GPa and concludes that the oceanic mantle potential temperature must be ~60ºC hotter than current estimates. Chapter 3 goes further to provide a more accurate parameterization of the anhydrous mantle solidus from experiments over a range of pressures. This chapter concludes that the range of potential temperatures of the mantle beneath mid-ocean ridges and plumes is smaller than currently estimated. Once formed, the oceanic plate moves atop the underlying asthenosphere away from the ridge axis. Chapter 4 uses seafloor magnetotelluric data to investigate the mechanism responsible for plate motion at the lithosphere-asthenosphere boundary. The resulting two dimensional conductivity model shows a simple layered structure. By applying petrological constraints, I conclude that the upper asthenosphere does not contain substantial melt, which suggests that either a thermal or hydration mechanism supports plate motion. Oceanic plate motion has dramatically changed the surface of the Earth over time, and evidence for ancient plate motion is obvious from detailed studies of the longer lived continental lithosphere. In Chapter 5, I investigate past plate motion by inverting magnetotelluric data collected over eastern Zambia. The conductivity model probes the Zambian lithosphere and reveals an ancient subduction zone previously suspected from surface studies. This chapter elucidates the complex lithospheric structure of eastern Zambia and the geometry of the tectonic elements in the region, which collided as a result of past oceanic plate motion. Combined, the chapters of this thesis provide critical constraints on ocean plate dynamics.