Alteration of antarctic hyaloclastites

P.J. ELLERMAN

Geology DepartmentUniversity of Colorado

Boulder, Colorado 80309

W.E. LEMASURIER

Geology DepartmentUniversity of Colorado

Denver, Colorado 80302

W. C. MCINTOSH

New Mexico Bureau of MinesNew Mexico Institute of Mining and Technology

Socorro, New Mexico 87801

Palagonitized hyaloclastites were collected from several ant-arctic localities during the 1982 - 1983, 1983 - 1984, and 1984 -1985 field seasons by geologists from the University of Coloradoand the New Mexico Institute of Mining and Technology. Theserocks have been the subject of an ongoing study in an effort tocharacterize the diagenetic or hydrothermal alteration minerals.

Hyaloclastites are fragmental volcanic rocks consisting pri-marily of glass. They form when lava interacts with waterseawater, lake water, or glacial meltwaterduring an eruption. Thedeposits commonly alter to palagonite, a yellow mineraloid(figure 1); zeolites and clay minerals, becoming lithified in theprocess. Palagonitization is thought to occur early in the di-agenetic history of the rock (Jakobsson 1972).

Figure 1. Scanning electron microscope photomicrograph of ves-icular sideromelane fragment from Castle Rock. Vesicle walls havebeen replaced by palagonite. 1 centimeter = 6.5 micrometers.

Because hyaloclastites represent subaqueous eruption, pres-ent-day exposures of these deposits have been used to postulatehigher ice levels at the time of their eruption in Antarctica(LeMasurier and Rex 1983). However, primary characteristics of

both subglacial and submarine deposits are similar. Determina-tions of the characteristics of aqueous eruptive environmentrely upon intercalated tills or marine sediments, the presence offossils, glacial polish, and geologic setting. Where these fea-tures are absent or inconclusive, other evidence is required todistinguish hyaloclastites resulting from former marine trans-gressions from those representing glacial advances and to de-velop conclusions with respect to rates of uplift. Secondaryauthigenic minerals commonly reflect pore-water chemistry. Bycomparing neoformed mineral assemblages and selected traceelements in hyaloclastites of subglacial and submarine origin,we hope to develop additional criteria to distinguish betweenthe two. Because glass dissolution and palagonitization appearto exert the primary controls on pore-water chemistry, however,similarities between the alteration of hyaloclastites from bothtypes of eruptive environment were observed.

Basaltic pillow lavas and associated hyaloclastites are amongthe most common volcanic rocks on Earth, comprising much ofthe sea floor. Alteration of these volcanic rocks is thought tomake an important contribution to the chemical budget of theoceans and may affect subduction zone magma genesis(Staudigel and Hart 1983). Exposed basaltic hyaloclastites inAntarctica not only provide an opportunity to characterize bet-ter this alteration, they permit the study of its distribution,variation, and relationship to dikes and other rock units in thefield.

In this study, samples were collected from Turks Head, Tryg-gve Point, and Castle Rock on Ross Island; from Minna Bluff;Marie Byrd Land; and from localities along the eastern coast ofnorthern Victoria Land. The original composition of the rocksvaries from alkali olivine basalt to trachyte. Ages range from 23million years to less than 1 million years old, although dates arenot available for some samples. Magma chemistry, eruptionenergy, and water depth or ice thickness are variables whichcontribute to the physical characteristics of the deposits andtherefore, presumably, to the character of the subsequent al-teration. Hyaloclastite samples of this study encompass a largerange in clast size, sorting, vesicularity, original porosity, andpercentage of crystal or lithic fragments. Some of the samplesare pillow breccias; some shallow water deposits, transitional tosubaerial, contain accretionary lapilli.

Common authigenic minerals in palagonitized antarctichyaloclastites are phillipsites, chabazite, analcime, carbonate,smectite, illite, and chlorite. Other zeolites are much less com-mon and occur in minor amounts in some samples. Silica is rareand appears to be primarily associated with pervasive hydro-thermal alteration. Gypsum and halite are occasionally present.Phillipsite (figure 2) forms early and is found in nearly everysample. Similar authigenic mineral assemblages were observedin alkaline hyaloclastites of both subglacial and marine originand have been reported from hydrovolcanic rocks outside Ant-arctica (Hay and lijima 1968).

Electron microprobe analyses of glass fragments were per-formed to determine the chemical variation from the relativelyfresh glass in the center of the shard to the palagonitized rim.Preliminary results indicate silicon, aluminum, sodium, andmagnesium are lost by the glass and gained by the palagonitewhich is conspicuously hydrated relative to the glass. Theseresults are comparable to those of Fumes (1977). Variation inother major elements and selected trace elements is underinvestigation.

A lack of alteration may also provide evidence in interpretingthe diagenetic history of the rock. At Castle Rock, for example,the base of the outcrop is relatively unpalagonitized and con-

1985 REVIEW 33

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tains only small amounts of phillipsite, smectite, and calcite.The distinct upper contact of this unit cuts across bedding. Thetuffs above have been extensively altered to palagonite,zeolites, smectite, and calcite. This relationship suggests thatduring alteration of the upper part of Castle Rock, the lower partwas protected by ice, and authigenic mineral formation wasrestricted by frozen interstitial water. With lowering of the icelevel, the unaltered base was exposed to percolating meteoricwater. The glass, unprotected by more stable secondary miner-als and palagonite, has undergone extensive dissolution. Fluc-tuations in ice level, therefore, may affect the intensity or styleof postdepositional alteration.

Figure 2. Scanning electron microscope photomicrograph of phillip-site coating a glass fragment from Turks Head. 1 centimeter = 1micrometer.

The distribution and paragenesis of the alteration minerals inthe upper, palagonitized section at Castle Rock suggest theirformation was largely controlled by increasing pH resulting

from progressive dissolution of basaltic glass. Hydrothermalalteration associated with dike emplacement has also producedpalagonitization at some localities. It is probable that the altera-tion history of many of these deposits is complicated by theoperation of both mechanisms.

The presence of early formed phillipsite and the persistenceof relatively fresh glass in many of the antarctic hyaloclastitesamples unrelated to age, suggest that following initial pal-agonitization and authigenic mineral formation, diagenesis pro-ceeds slowly due to reduction in porosity and available exposedglass (Fumes 1974) and to the relative lack of free water in theantarctic environment.

Thanks to the other geologists in the field: Anne Wright,Philip Kyle, and Harry Keys (1982 1983), David Johnson (1983- 1984), and Nelia Dunbar (1984 - 1985). This research wassupported by National Science Foundation grant DPP 80-20836to W.E. LeMasurier.

References

Fumes, H. 1974. Volume relations between palagonite and authigenicminerals in hyaloclastites, and its beating on the rate of palagonitiza-tion. Bulletin Volcanologique, 38, 173 - 186.

Fumes, H. 1977. Element mobility during palagonitization of a sub-glacial hyaloclastite in Iceland. Chemical Geology, 22, 249 - 264.

Hay, R.L., and A. lijima. 1968. Nature and origin of palagonite tuffs ofthe Honolulu Group on Oahu, Hawaii, Geological Society of AmericaMemoir, 116, 338 - 376.

Jakobsson, S.P. 1972. on the consolidation and palagonitization of thetephra of the Surtsey volcanic island Surtsey Progress Report, 61. 1 8.

LeMasurier, WE., and D.C. Rex. 1983. Volcanic record of Cenozoicglacial history in Marie Byrd Land and western Ellsworth Laid:Revised chronology and evaluation of tectonic factors. In C. Crad-dock (Ed.), Antarctic geoscience. Madison: University of WisconsinPress.

Staudigel, H., and S.R. Hart. 1983. Alteration of basaltic glass: Mecha-nisms and significance for the oceanic crust-seawater budget. Geo-chimica et Cosmochimica Acta, 47, 337 - 350.

Geologic studies in the English Coast,eastern Ellsworth Land, Antarctica

P.D. ROWLEY and K.S. KELLOGG

U.S. Geological SurveyDenver, Colorado 80225

W.R. VENNUM

Department of GeologySonoma State University

Rohnert Park, California 94928

The first visit to the previously unexplored English Coast ofthe Bellingshausen Sea was made during the 1984 1985 australsummer by a U.S. Geological Survey (UsGs) field party. Despiteexceptionally poor weather, the field party completed reconnaissance geologic mapping of rocks exposed in nunataks scattered over about 16,000 square kilometers (figure 1). The fielparty also studied rocks in the Behrendt Mountains and othernearby parts of eastern Ellsworth Land that had not beermapped before. We found that most rocks in the English Coas

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and adjacent parts of eastern Ellsworth Land resemble those ofthe southern Antarctic Peninsula (Black, Lassiter, and 0rvillCoasts) and other parts of eastern Ellsworth Land (Laudon1972, 1982; Williams et al. 1972; Rowley and Williams 1982;Rowley et al. 1983). Mesozoic igneous rocks in all these areasdefine an Andean magmatic arc that continues northward upthe spine of the Antarctic Peninsula (figure 1). This arc de-

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