On the serpentinization degree (S) of IODP expedition 357 Atlantis massif rocks; insights from rock magnetic properties and microscopic magnetic mineralogy study of six sites

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http://abstractsearch.agu.org/meetings/2016/FM/OS31D-2053.html
Author(s): Herrero-Bervera, Emilio; Whattam, Scott Andrew; Frederichs, Thomas
Author Affiliation(s): Primary:
University of Hawaii at Manoa, Honolulu, HI, United States
Other:
Korea University, South Korea
University of Bremen, Germany
Volume Title: AGU 2016 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2016; American Geophysical Union 2016 fall meeting, San Francisco, CA, Dec. 12-16, 2016. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: We have studied the magnetic properties of 37 serpentinized samples recovered via drilling during IODP Expedition 357, Atlantis Massif. We have recovered various lithologies including ultamafic rocks (primarily extensively serpentinized), subordidate gabbros, dolerites (small-scale melt injections) and schists. We have conducted remanence and induced magnetic experiments on the samples to determine for instance the degree of serpentinization (S). Stepwise alternating field and thermal demagnetization experiments from 2.5 to 70 mT and from 28 to 700°C, respectively, yielded univectorial diagrams showing the removal of secondary components (e.g., VRM, IRM, CRM) by isolating a characteristic component (ChRM) at various fields and temperatures. The normalized intensity of demagnetization (J/Jo) shows that the decrease of the magnetization of the specimens where about 50% of the original magnetization and is lost at about 5 mT and 100°C (i.e., Median Destructive Field). The stereograms show magnetic stability of the specimens by determining the directional behavior after 4 demagnetization steps (from 7.5-10 mT fields and low temperatures). Induced magnetization such as SIRM's, hysteresis saturation loops, back-fields and FORC experiments were performed. Diagnostic values of Mrs/Ms and Brc/Bc determine the domain structure of a magnetic sample. The magnetic grain sizes were determined using the protocol of Dunlop [2000]. Most of the samples were distributed over the Single (SD), Pseudo-Single Domain (PSD) and a few over the Multi Domain (MD) ranges with a certain degree of clustering on the PSD range. Curie points were obtained by measuring their low-field susceptibility vs. temperature from 28°C up to 700°C in an Argon atmosphere showing a minimum of 1-4 magnetic mineral phases with temperatures ranging from ∼100°C up to 640°C. These phases are predominantly Ti-poor, Ti-rich magnetite, maghemite and magnetite as corroborated by microscopic analysis as well as the Verwey transition (Tv≈110-120K). Samples studied show appreciable variation in bulk susceptibility (77.8 x 10-3 to 0.31 x 10-3 SI units). The samples are characterized by low, intermediate and high degree of serpentinization based on the results of their magnetic properties (e,g, Kappa, density, magnetic stability and Mrs/Ms vs Bcr/Bc).
Year of Publication: 2016
Research Program: IODP2 International Ocean Discovery Program
Key Words: 07 Marine Geology and Oceanography; Atlantic Ocean; Atlantis Massif; Expedition 357; International Ocean Discovery Program; Magnetic properties; Metasomatism; Mid-Atlantic Ridge; North Atlantic; Serpentinization
Coordinates: N300728 N301000 W0420350 W0421100
Record ID: 2017068226
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