Browsing by Author "Bowles, Julie"
Now showing 1 - 15 of 15
- Results Per Page
- Sort Options
Item Curie temperatures of titanomagnetite in ignimbrites: Effects of emplacement temperatures, cooling rates, exsolution, and cation ordering(American Geophysical Union, 2014) Jackson, Mike; Bowles, JuliePumices, ashes, and tuffs from Mt. St. Helens and from Novarupta contain two principal forms of titanomagnetite: homogeneous grains with Curie temperatures in the range 350–500 C and oxyexsolved grains with similar bulk composition, containing ilmenite lamellae and having Curie temperatures above 500 C. Thermomagnetic analyses and isothermal annealing experiments in combination with stratigraphic settings and thermal models show that emplacement temperatures and cooling history may have affected the relative proportions of homogeneous and exsolved grains and have clearly had a strong influence on the Curie temperature of the homogeneous phase. The exsolved grains are most common where emplacement temperatures exceeded 600 C, and in laboratory experiments, heating to over 600 C in air causes the homogeneous titanomagnetites to oxyexsolve rapidly. Where emplacement temperatures were lower, Curie temperatures of the homogeneous grains are systematically related to overburden thickness and cooling timescales, and thermomagnetic curves are generally irreversible, with lower Curie temperatures measured during cooling, but little or no change is observed in room temperature susceptibility. We interpret this irreversible behavior as reflecting variations in the degree of cation ordering in the titanomagnetites, although we cannot conclusively rule out an alternative interpretation involving fine-scale subsolvus unmixing. Shortrange ordering within the octahedral sites may play a key role in the observed phenomena. Changes in the Curie temperature have important implications for the acquisition, stabilization, and retention of natural remanence and may in some cases enable quantification of the emplacement temperatures or cooling rates of volcanic units containing homogeneous titanomagnetitesItem IRM Quarterly, Volume 18, Number 2 (Summer 2008). Cover article: 2008 International Conference on Rock Magnetism(University of Minnesota. Institute for Rock Magnetism, 2008) Bowles, JulieItem IRM Quarterly, Volume 18, Number 3 (Fall 2008). Cover article: IRM gets new shielded room and u-channel magnetometer(University of Minnesota. Institute for Rock Magnetism, 2008) Bowles, JulieItem IRM Quarterly, Volume 18, Number 4 (Winter 2008-2009). Cover article: New IRM Visitor Programs(University of Minnesota. Institute for Rock Magnetism, 2009) Bowles, JulieItem IRM Quarterly, Volume 19, Number 1 (Spring 2009). Cover article: SQUID Attack!(University of Minnesota. Institute for Rock Magnetism, 2009) Bowles, JulieItem IRM Quarterly, Volume 19, Number 2 (Summer 2009). Cover article: Seven Secrets to Sample Prep Set Stage for Success! (or: What to Do Before You Come to the IRM)(University of Minnesota. Institute for Rock Magnetism, 2009) Bowles, Julie; Jackson, Mike; Solheid, PeatItem IRM Quarterly, Volume 19, Number 3 (Fall 2009). Cover article: Interpretation of Low-Temperature Data Part 1: Superparamagnetism and Paramagnetism(University of Minnesota. Institute for Rock Magnetism, 2009) Bowles, Julie; Jackson, Mike; Chen, Amy; Solheid, PeatItem IRM Quarterly, Volume 19, Number 4 (Winter 2009-2010). Cover article: New Low-Temp Probe for 3-Axis Magnetic Measurements(University of Minnesota. Institute for Rock Magnetism, 2010) Bowles, Julie; Solheid, Peat; Jackson, Mike; Feinberg, Joshua; Moskowitz, BruceItem IRM Quarterly, Volume 20, Number 1 (Spring 2010). Cover article: Interpretation of Low-Temperature Data Part II: The Hematite Morin Transition(University of Minnesota. Institute for Rock Magnetism, 2010) Bowles, Julie; Jackson, Mike; Banerjee, SubirItem IRM Quarterly, Volume 20, Number 2 (Spring 2010). Cover article: The Eighth Santa Fe Conference on Rock Magnetism(University of Minnesota. Institute for Rock Magnetism, 2010) Bowles, JulieItem IRM Quarterly, Volume 20, Number 4 (Winter 2011). Cover article: Interpretation of Low-Temperature Data Part III: The Magnetite Verwey Transition (Part A)(University of Minnesota. Institute for Rock Magnetism, 2011) Jackson, Mike; Moskowitz, Bruce; Bowles, JulieItem IRM Quarterly, Volume 21, Number 2 (Summer 2011). Cover article: Inaugural Summer School for Rock Magnetism Held at IRM(University of Minnesota. Institute for Rock Magnetism, 2011) Bowles, JulieItem IRM Quarterly, Volume 21, Number 4 (Winter 2011). Cover article: Interpretation of Low-Temperature Data Part V: The Magnetite Verwey Transition (Part B): Field-Cooling Effects on Stoichiometric Magnetite Below TV(University of Minnesota. Institute for Rock Magnetism, 2011) Jackson, Mike; Bowles, Julie; Banerjee, SubirItem IRM Quarterly, Volume 22, Number 1 (Spring 2012). Cover article: "Blowin' in the Wind" and "Beneath Still Waters": A Report on the 9th Santa Fe Conference on Rock Magnetism(University of Minnesota. Institute for Rock Magnetism, 2012) Bowles, Julie; Jackson, Mike; Moskowitz, BruceItem IRM Quarterly, Volume 22, Number 2 (Summer 2012). Cover article: Interpretation of Low-Temperature Data Part VI: The MagnetiteVerwey Transition (Part C): Low-Temperature Demagnetization of Stoichiometric Magnetite(University of Minnesota. Institute for Rock Magnetism, 2012) Bowles, Julie; Jackson, Mike; Banerjee, Subir