Textures and Sr, Ba, Mg, Fe, K and Ti compositional profiles in volcanic plagioclase clues to the dynamics of calc-alkaline magma chambers

Abstract

Other| August 01, 1995 Textures and Sr, Ba, Mg, Fe, K and Ti compositional profiles in volcanic plagioclase: Clues to the dynamics of calc-alkaline magma chambers Bradley S. Singer; Bradley S. Singer Universite de Geneve, Department de Mineralogie, Geneva, Switzerland Search for other works by this author on: GSW Google Scholar Michael A. Dungan; Michael A. Dungan Search for other works by this author on: GSW Google Scholar Graham D. Layne Graham D. Layne Search for other works by this author on: GSW Google Scholar American Mineralogist (1995) 80 (7-8): 776–798. https://doi.org/10.2138/am-1995-7-815 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Bradley S. Singer, Michael A. Dungan, Graham D. Layne; Textures and Sr, Ba, Mg, Fe, K and Ti compositional profiles in volcanic plagioclase: Clues to the dynamics of calc-alkaline magma chambers. American Mineralogist 1995;; 80 (7-8): 776–798. doi: https://doi.org/10.2138/am-1995-7-815 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 SocietyAmerican Mineralogist Search Advanced Search Abstract Concentration profiles for Ca, Na, Al, K, Fe, Ti, Mg, Sr, and Ba obtained by electron microprobe and secondary ion mass spectrometry from plagioclase crystals, together with textural observations from interference contrast microscopy, are consistent with contrasting magma dynamics in two subvolcanic reservoirs from which silicic lavas erupted at the Tatara-San Pedro volcanic complex, Chilean Andes. The 1 km3 late Pleistocene (68 ka) Tatara dacite is chemically homogeneous, phenocryst-poor, and contains crystals of opa-citized hornblende, orthopyroxene, and titanomagnetite, plus 2 mm euhedral plagioclase phenocrysts with simple zoning patterns. Except at their rims, analyzed phenocrysts show nearly monotonie core-to-rim changes from An52 to An31, including slight decreases in MgO and FeO′ and slight increases in Ba, Sr, and K concentrations. Abrupt, but small, chemical shifts are associated with many subtle dissolution surfaces. In contrast, the 0.1 km3 of chemically and texturally heterogeneous Holocene San Pedro dacite contains crystals of clinopyroxene, orthopyroxene, hornblende, biotite, titanomagnetite, ilmenite, and abundant 2 mm euhedral plagioclase phenocrysts, plus plagioclase and olivine xenocrysts derived from fragmented quenched basaltic inclusions. Analyzed phenocrysts span the range from An72 to An32. Abrupt increases of 15 mol% An, large increases in MgO, FeO′, and Sr, and decreases in Ba and K2O concentrations are associated with a few major dissolution surfaces. Plagioclase xenocrysts of An74–86 have higher Sr, MgO, and FeO′ and lower Ba and K concentrations than the phenocrysts, reflecting crystallization from a basaltic melt. In both dacites, phenocryst rims decrease in Sr and increase in K and Ti relative to equilibrium values as a consequence of rapid crystal growth at high undercooling during magma ascent and eruption. Modeling of variations in apparent DSrplag/melt implies growth rates for the ≤100 μm phenocryst rims of 10−9 cm/s, suggesting that magma ascent may have taken several months. Provided these kinetic effects are identified, the remainder of the concentration profiles for Sr, Ba, Mg, and K in plagioclase phenocrysts can be inverted using partition coefficient expressions to monitor the temporal evolution of melts in the preemptive magma chambers.Monotonie melt trends and periodic thermal dissolution of Tatara dacite phenocrysts imply a magma chamber closed to inputs of new magma and heat. Repeated cycles of crystal growth and weak dissolution occurred during phenocryst retention in thermally driven convection cells characterized by steady-state laminar flow. In contrast, dissolution events associated with large, abrupt shifts in melt Sr, Mg, and K/Ba toward basaltic values indicate that in the Holocene magma chamber heating of silicic melt adjacent to basaltic inclusions during magma mingling caused plagioclase dissolution. Renewed plagioclase growth from the resulting small volumes of hybrid melt recorded highly localized chemical mixing by diffusion and accompanying cycles of fractionation. 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.