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Title:Mineralogy and geochemistry associated with hydrothermal alteration of a rhyolitic hyaloclastite from Ponza Island, Italy
Author(s):Ylagan, Robert Francis
Doctoral Committee Chair(s):Altaner, Stephen P.
Department / Program:Geology
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:A rhyolitic hyaloclastite from Ponza island, Italy, has been hydrothermally altered producing four distinct alteration zones based on XRD and field textures: (1) non-pervasive argillic zone; (2) propylitic zone; (3) silicic zone; and (4) sericitic zone. The unaltered hyaloclastite is a volcanic breccia with clasts of vesiculated obsidian in a matrix of predominantly pumice lapilli. Incomplete alteration of the hyaloclastite resulted in the non-pervasive argillic zone, characterized by smectite and disordered opal-CT. The other three zones exhibit more complete alteration of the hyaloclastite. The propylitic zone is characterized by mixed-layer illite/smectite (I/S) with 10 to 85% I, mordenite, opal-C, and authigenic K-feldspar (akspar). The silicic zone is characterized by I/S with $\ge$90% I, pure illite, quartz, akspar, and occasional albite. In the sericitic zone the hyaloclastite altered primarily to illitic I/S with $\ge$66% I, pure illite, quartz, and minor akspar and pyrite. K/Ar ages of I/S indicate hydrothermal alteration occurred at 3.32 $\pm$ 0.13Ma.
Oxygen isotope compositions of I/S systematically decrease from zones 1 through 4. In the argillic zone, smectite has a $\delta\sp{18}$O value of 21.7-22.0$\perthous$; I/S from the propylitic zone ranges from 16.3-14.5$\perthous$; highly illitic I/S from the silicic zone has a composition of 14.0-12.5$\perthous$; and illitic US from the sericitic zone has a composition of 11.9-8.6$\perthous$. $\delta\sp{18}$O values for quartz from the silicic and sericitic zones range from 15.9 to 12.6$\perthous$. Using isotope fractionation equations and data from quartz fluid inclusions, alteration temperatures ranged from 50-65$\sp\circ$C for the argillic zone, 85-125$\sp\circ$C for the propylitic zone, 110-210$\sp\circ$C for the silicic zone, and 145-225$\sp\circ$C for the sericitic zone. The melting point temperature from the fluid inclusions is approximately $-$1.6$\sp\circ$C, which indicates that hydrothermal fluids had a salinity similar to seawater. In addition, calculated isotopic water compositions of 1.4-3.0$\perthous$ are similar to local seawater.
Mass transfer calculations (assuming A1 immobility) based on XRF analysis of bulk rock samples reveal that in the argillic, propylitic, and silicic zones alteration proceeded in a relatively open chemical system, and sericitic zone alteration involved extensive loss of most chemical species, especially Si. Systematic gains in Mg are observed in all alteration zones due to I/S clay mineral formation, and systematic losses of Na, Ca, and K commonly occur for all zones except for the silicic zone which shows a gain in K due to akspar formation. With the exception of Ca, calculations of mass transfer associated with hydrothermal alteration on Ponza agree with chemical fluxes observed in laboratory experiments involving hydrothermal reactions of rhyolite and seawater.
Based on changes in morphology, polytype, and chemistry, smectite illitization on Ponza involved a dissolution and crystallization mechanism with multiple stages of nucleation and crystal growth. Particle shapes undergo abrupt changes from anhedral plates to laths and then to euhedral plates and hexagons. I/S polytype progressively changes from cis-vacant, turbostratic smectite to interstratified cis- and trans-vacant, 1M$\sb{\rm d}$ I/S to trans-vacant, 1M illite, and then to 2M$\sb1$ illite. I/S chemistry progressively changes during illitization, evolving toward a phengitic end-member composition with $\sim$0.89 fixed interlayer cations per half unit cell. End-member smectite contains $\sim$0.38 exchangeable interlayer cations.
Issue Date:1996
Rights Information:Copyright 1996 Ylagan, Robert Francis
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9712494
OCLC Identifier:(UMI)AAI9712494

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