HR: 1330h
AN: V12D-0616    [PDF]
TI: Tracking Pyroxenes Through a Magma System Using Trace Elements: an Example From Tenerife, Canary Islands
AU: * Olin, P
EM: polin@mail.wsu.edu
AF: Department of Geology, Washington State University, Pullman, WA 99164 United States
AU: Wolff, J
EM: jawolff@mail.wsu.edu
AF: Department of Geology, Washington State University, Pullman, WA 99164 United States
AU: Knaack, C
EM: knacck@wsu.edu
AF: Department of Geology, Washington State University, Pullman, WA 99164 United States
AB: Although much attention is currently focused on applications of radiogenic isotope microanalysis to petrogenetic problems, many volcanic suites do not exhibit a large enough range in, for example, Sr isotope ratios for the technique to be useful. Advances in LA-ICPMS instrumentation allow rapid and routine trace element microanalyisis of minerals and glasses at a spatial resolution approaching that of the electron microprobe, providing a microanalytical complement to isotope studies in igneous petrogenesis. We apply this approach to clinopyroxenes from the basanitic through phonolitic mixed-magma rocks of the bimodal Diego Hernandez Formation (DHF), Tenerife, Canary Islands. Two distinct phonolite types, distinguished on the basis of REE and HFSE abundances, have alternated in volumetric significance during the ~180 ky eruptive history of the DHF. Also, there is abundant petrographic evidence for repeated mingling of phonolitic and basanitic magmas prior to explosive eruptions, while whole-rock compositions of several associated mafic lavas are consistent with more complete hybridization of basanitic and phonolitic liquids. Clinopyroxenes from the suite fall into three groups: 1) titanaugites with unremarkable REE patterns; 2) salites with elevated REE contents and negative Eu anomalies; 3) salites with negative Eu anomalies and strong MREE depletion. Type 1 titanaugites crystallized from basanitic liquids, Type 2 salites from tephriphonolitic and phonolitic liquids, and Type 3 from highly-evolved phonolitic melts saturated with titanite. All three types are found, as discrete grains and as zones within complex grains, in the products of the largest explosive eruptions in the DHF, clearly demonstrating repeated crystal transfer between basanite, phonolite, and highly evolved phonolite, prior to the mingling events that triggered the individual eruptions. The ability to fingerprint crystals derived from different phonolite types is an important step in unravelling complex shallow-level magmatic processes on Tenerife. The large gap in REE abundances between Type 2 and 3 pyroxenes is consistent with earlier suggestions that the highly-evolved phonolites may contain a component of strongly differentiated syenite recycled from within the island edifice.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
DE: 8400 VOLCANOLOGY
DE: 8404 Ash deposits
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting