Geophysical and petrological constraints on ocean plate dynamics

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2017This 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.Funding for this research was provided by the National Science Foundation Division of Earth Sciences (EAR) grant number 1010432, Division of Ocean Sciences (OCE) grant numbers 1459649 and 0928663, WHOI Deep Ocean Exploration Institute, and the WHOI Academic Programs Office

Structure of the lithosphere beneath the Barotse Basin, western Zambia, from magnetotelluric data.

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Tectonics, 38(2), (2019):666-686. doi:10.1029/2018TC005246.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.Funding for MT acquisition and analysis was provided by the National Science Foundation grant EAR‐1010432 through the Continental Dynamics Program. The data used in this study are available for download at the IRIS Data Management Center through the DOI links cited in Jones et al. (2003–2008; https://doi.org/10.17611/DP/EMTF/SAMTEX) and Evans et al. (2012; https://doi.org/10.17611/DP/EMTF/PRIDE/ZAM). We would like to thank the field crew from the Geological Survey Department, Zambia, for their assistance in collecting data. Matthew Chamberlain, David Margolius, and Colin Skinner, formerly of Northeastern University, are also thanked for their field assistance. Data are available from the corresponding author pending their submission to the IRIS DMC repository at which point they will be publically available. This is Oklahoma State University, Boone Pickens School of Geology contribution number 2019‐99.2019-07-3

The electrical structure of the central Pacific upper mantle constrained by the NoMelt experiment

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 16 (2015): 1115–1132, doi:10.1002/2014GC005709.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.Funding for the NoMELT experiment was provided by the National Science Foundation through the following grant numbers: OCE-0927172, OCE-0928270, OCE-1459649, and OCE-0928663.2015-10-1

Geophysical and petrological constraints on ocean plate dynamics

Thesis: Ph. D., Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2017.Cataloged from PDF version of thesis.Includes bibliographical references.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 -60 °C 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.by Emily Kathryn Sarafian.Ph. D

Imaging Precambrian Lithospheric Structure in Zambia using Electromagnetic Methods

The Precambrian geology of eastern Zambia and Malawi is highly complex due to multiple episodes of rifting and collision, particularly during the formation of Greater Gondwana as a product of the Neoproterozoic Pan-African Orogeny. The lithospheric structure and extent of known Precambrian tectonic entities of the region are poorly known as there have been to date few detailed geophysical studies to probe them. Herein, we present results from electromagnetic lithospheric imaging across Zambia into southern Malawi using the magnetotelluric method complemented by high-resolution aeromagnetic data of the upper crust in order to explore the extent and geometry of Precambrian structures in the region. We focus particularly on determining the extent of subcontinental lithospheric mantle (SCLM) beneath the Archean-Paleoproterozoic cratonic Bangweulu Block and the Mesoproterozoic-Neoproterozoic Irumide and Southern Irumide Orogenic Belts. We also focus on imaging the boundaries between these tectonic entities, particularly the boundary between the Irumide and Southern Irumide Belts. The thickest and most resistive lithosphere is found beneath the Bangweulu Block, as anticipated for stable cratonic lithosphere. Whereas the lithospheric thickness estimates beneath the Irumide Belt match those determined for other orogenic belts, the Southern Irumide Belt lithosphere is substantially thicker similar to that of the Bangweulu Block to the north. We interpret the thicker lithosphere beneath the Southern Irumide Belt as due to preservation of a cratonic nucleus (the pre-Mesoproterozoic Niassa Craton). A conductive mantle discontinuity is observed between the Irumide and Southern Irumide Belts directly beneath the Mwembeshi Shear Zone. We interpret this discontinuity as modified SCLM relating to a major suture zone. The lithospheric geometries determined from our study reveal tectonic features inferred from surficial studies and provide important details for the tectonothermal history of the region

LA-ICP-MS and EPMA Data;Size of APB discussion from Early accretion of water and volatile elements to the inner solar system: evidence from angrites

Inner solar system bodies are depleted in volatile elements relative to chondrite meteorites, yet the source(s) and mechanism(s) of volatile element depletion and/or enrichment are poorly constrained. The timing, mechanisms and quantities of volatile elements present in the early inner solar system have vast implications for diverse processes from planetary differentiation to the emergence of life. We report major, trace and volatile element contents of a glass bead derived from the D'Orbigny angrite, the hydrogen isotopic composition of this glass bead and that of coexisting olivine and silicophosphates, and the ²⁰⁷Pb–²⁰⁶Pb age of the silicophosphates, 4568 ± 20 Ma. We use volatile saturation models to demonstrate that the angrite parent body must have been a major body in the early inner solar system. We further show via mixing calculations that all inner solar system bodies accreted volatile elements with carbonaceous chondrite H and N isotope signatures extremely early in solar system history. Only a small portion (if any) of comets and gaseous nebular H-species contributed to the volatile content of the inner solar system bodies

Alcohol use among emergency medicine department patients in Tanzania: A comparative analysis of injury versus non-injury patients.

BackgroundAlcohol is a leading behavioral risk factor for death and disability worldwide. Tanzania has few trained personnel and resources for treating unhealthy alcohol use. In Emergency Medicine Departments (EMDs), alcohol is a well-known risk factor for injury patients. At Kilimanjaro Christian Medical Center (KCMC) in Moshi, Tanzania, 30% of EMD injury patients (IP) test positive for alcohol upon arrival to the ED. While the IP population is prime for EMD-based interventions, there is limited data on if non-injury patients (NIP) have similar alcohol use behavior and potentially benefit from screening and intervention as well.MethodsThis was a secondary analysis of a systematic random sampling of adult (≥18 years old), KiSwahili speaking, KCMC EMD patients surveyed between October 2021 and May 2022. When medically stable and clinically sober, participants provided informed consent. Information on demographics (sex, age, years of education, type of employment, income, marital status, tribe, and religion), injury status, self-reported alcohol use, and Alcohol Use Disorder (AUD) Identification Test (AUDIT) scores were collected. Descriptive statistics were analyzed in RStudio using frequencies and proportions.ResultsOf the 376 patients enrolled, 59 (15.7%) presented with an injury. The IP and NIP groups did not differ in any demographics except sex, an expected difference as females were intentionally oversampled in the original study design. The mean [SD] AUDIT score (IP: 5.8 [6.6]; NIP: 3.9 [6.1]), drinks per week, and proportion of AUDIT ≥8 was higher for IP (IP:37%; NIP: 21%). However, alcohol preferences, drinking quantity, weekly expenditure on alcohol, perceptions of unhealthy alcohol use, attempts and reasons to quit, and treatment seeking were comparable between IPs and NIPs.ConclusionOur data suggests 37% of injury and 20% of non-injury patients screen positive for harmful or hazardous drinking in our setting. An EMD-based alcohol treatment and referral process could be beneficial to reduce this growing behavioral risk factor in non-injury as well as injury populations