Research Article| April 01, 2002 Phenocrysts versus xenocrysts in the youngest Toba Tuff: Implications for the petrogenesis of 2800 km3 of magma James E. Gardner; James E. Gardner 1Geophysical Institute and Department of Geology and Geophysics, University of Alaska, Fairbanks, Alaska 99775-7320, USA Search for other works by this author on: GSW Google Scholar Paul W. Layer; Paul W. Layer 1Geophysical Institute and Department of Geology and Geophysics, University of Alaska, Fairbanks, Alaska 99775-7320, USA Search for other works by this author on: GSW Google Scholar Malcolm J. Rutherford Malcolm J. Rutherford 2Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA Search for other works by this author on: GSW Google Scholar Author and Article Information James E. Gardner 1Geophysical Institute and Department of Geology and Geophysics, University of Alaska, Fairbanks, Alaska 99775-7320, USA Paul W. Layer 1Geophysical Institute and Department of Geology and Geophysics, University of Alaska, Fairbanks, Alaska 99775-7320, USA Malcolm J. Rutherford 2Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA Publisher: Geological Society of America Received: 05 Jul 2001 Revision Received: 10 Dec 2001 Accepted: 21 Dec 2001 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2002) 30 (4): 347–350. https://doi.org/10.1130/0091-7613(2002)030<0347:PVXITY>2.0.CO;2 Article history Received: 05 Jul 2001 Revision Received: 10 Dec 2001 Accepted: 21 Dec 2001 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation James E. Gardner, Paul W. Layer, Malcolm J. Rutherford; Phenocrysts versus xenocrysts in the youngest Toba Tuff: Implications for the petrogenesis of 2800 km3 of magma. Geology 2002;; 30 (4): 347–350. doi: https://doi.org/10.1130/0091-7613(2002)030<0347:PVXITY>2.0.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 The petrogenesis of the 2800 km3 of magma erupted as the youngest Toba Tuff has been investigated using experimental petrology and 40Ar/39Ar dating of biotite, sanidine, hornblende, and plagioclase from the tuff. We find that hornblende does not crystallize experimentally from the magma at temperatures and pressures indicated by the natural mineral assemblage. Hornblende is also not in isotopic equilibrium with biotite and sanidine, both of which grew experimentally. Hornblende thus appears xenocrystic, despite being a major phase in the tuff. Some plagioclase is also xenocrystic, on the basis of Ar isotopes, but others are probably phenocrystic, because plagioclase grew experimentally. Crystal clots of hornblende + plagioclase observed in the tuff suggest that the xenocrysts came from a common source, which was at least 1.5 Ma (the oldest hornblende 40Ar/39Ar age). Our results suggest that the Toba Tuff magma resided at nearly water-saturated pressures of 100–150 MPa and that xenocrysts were entrained as recently as 10 yr before the eruption. The ubiquitous presence of hornblende in the tuff indicates that entrainment occurred throughout the 2800 km3 of magma. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.