Experimental calibration of the aluminum-in-hornblende geobarometer with application to Long Valley caldera (California) volcanic rocks

Research Article| September 01, 1989 Experimental calibration of the aluminum-in-hornblende geobarometer with application to Long Valley caldera (California) volcanic rocks Marie C. Johnson; Marie C. Johnson 1Department of Geological Sciences, Brown University, Providence, Rhode Island 02912 Search for other works by this author on: GSW Google Scholar Malcolm J. Rutherford Malcolm J. Rutherford 1Department of Geological Sciences, Brown University, Providence, Rhode Island 02912 Search for other works by this author on: GSW Google Scholar Geology (1989) 17 (9): 837–841. https://doi.org/10.1130/0091-7613(1989)0172.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Marie C. Johnson, Malcolm J. Rutherford; Experimental calibration of the aluminum-in-hornblende geobarometer with application to Long Valley caldera (California) volcanic rocks. Geology 1989;; 17 (9): 837–841. doi: https://doi.org/10.1130/0091-7613(1989)0172.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract A new geobarometer based on the Al content of igneous hornblendes in equilibrium with melt, fluid, biotite, quartz, sanidine, plagioclase, sphene, and magnetite or ilmenite has been calibrated experimentally. The calibration was performed by equilibrating the required phase assemblage over the pressure range 2-8 kbar at 740-780 °C, and then analyzing euhedral hornblendes in equilibrium with glass (melt). Experiments were performed on natural samples of both volcanic and plutonic rocks. Earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks. The experimental calibration differs from the empirical calibrations, especially above 5 kbar, and shows that the Al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought. The geobarometer's uncertainty is dramatically reduced. The derived equation is P (±0.5 kbar) = 3 -3.46 (±0.24) + 4.23 (±0.13) (AlT). The geobarometer is applied to post-Bishop Tuff volcanic rocks from Long Valley caldera, California, and reveals that most rhyodacites in this complex erupted from depths of about 6 km. These eruptions occurred over 500 000 yr, suggesting that the rhyodacitic magma reservoir beneath Long Valley had reached a steady P (depth)-T state. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.