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On the Time Scales of Magma Genesis, Melt Evolution, Crystal

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On the Time Scales of Magma Genesis, Melt Evolution, Crystal JOURNAL OF PETROLOGY VOLUME 54 NUMBER 2 PAGES 235 ^ 271 2013 doi:10.1093/petrology/egs068 On the Time Scales of Magma Genesis, Melt Evolution, Crystal Growth Rates and Magma Degassing in the Erebus Volcano Magmatic System Using the 238U, 235Uand 232Th Decay Series Downloaded from KENNETH W.W. SIMS1,2*, SYLVAIN PICHAT3,MARKK.REAGAN4, PHILIP R. KYLE5, HENRIETTA DULAIOVA2,6, NELIA W. DUNBAR7, JULIE PRYTULAK8,9,GEORGINASAWYER10,GRAHAMD.LAYNE2,11, 3 12 JANNE BLICHERT-TOFT ,PIERREJ.GAUTHIER , http://petrology.oxfordjournals.org/ MATTHEW A. CHARETTE2 AND TIMOTHY R. ELLIOTT8 1WYOMING HIGH-PRECISION ISOTOPE LABORATORY, DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF WYOMING, LARAMIE, WY 82070, USA 2WOODS HOLE OCEANOGRAPHIC INSTITUTION, WOODS HOLE, MA 02543, USA 3LABORATOIRE DE GEOLOGIE DE LYON, CNRS UMR 5276, ECOLE NORMALE SUPERIEURE DE LYON, 46, ALLEE D’ITALIE 69364 LYON CEDEX 7, FRANCE 4DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF IOWA, IOWA CITY, IA 52242, USA at MBLWHOI Library on July 18, 2013 5DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, NEW MEXICO INSTITUTE OF MINING AND TECHNOLOGY, SOCORRO, NM 87801, USA 6DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF HAWAI’I AT MANOA, HONOLULU, HI 96822, USA 7NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES, NEW MEXICO INSTITUTE OF MINING AND TECHNOLOGY, SOCORRO, NM 87801, USA 8BRISTOL ISOTOPE GROUP, SCHOOL OF EARTH SCIENCES, UNIVERSITY OF BRISTOL, BRISTOL, BS8 1RJ, UK 9DEPARTMENT OF EARTH SCIENCE AND ENGINEERING, IMPERIAL COLLEGE, LONDON, SW7 2AZ, UK 10DEPARTMENT OF GEOGRAPHY, UNIVERSITY OF CAMBRIDGE, DOWNING PLACE, CAMBRIDGE CB2 3EN, UK 11DEPARTMENT OF EARTH SCIENCES, MEMORIAL UNIVERSITY OF NEWFOUNDLAND, ST. JOHN’S, NL A1B 3X5, CANADA 12LABORATOIRE MAGMAS ET VOLCANS, CNRS, CLERMONT-FERRAND 63000, FRANCE RECEIVEDMARCH25,2012;ACCEPTEDSEPTEMBER4,2012 ADVANCE ACCESS PUBLICATION OCTOBER 13, 2012 Weinvestigate the time scales of magma genesis, melt evolution, crystal 232Th^228Ra^228Th and 235U^231Pa^227Ac. These are the first growth rates and magma degassing in the Erebus volcano magmatic measurements of 231Pa^ 227Ac in volcanic samples and represent the system using measurements of 238U^230Th^226Ra^210Pb^210Po, first set of data in a volcanic system to examine the entire suite of *Corresponding author. Present address:Wyoming High-Precision Iso- ß The Author 2012. Published by Oxford University Press. All tope Laboratory, Department of Geology and Geophysics, University rights reserved. For Permissions, please e-mail: journals.permissions@ of Wyoming, Laramie,WY 82070, USA. E-mail: [email protected] oup.com JOURNAL OF PETROLOGY VOLUME 54 NUMBER 2 FEBRUARY 2013 relevant 238U, 235U and 232Th decay series nuclides. Our sample suite The Erebus lava lake is the top of a convecting magma consists of 22 phonolite volcanic bombs, erupted between 1972 and conduit, which is continuously and quiescently degassing. 2005, and five anorthoclase megacrysts separated from bombs erupted Infrequent Strombolian eruptions often eject phonolite in 1984, 1989, 1993, 2004 and 2005. The 238U^230Th, bombs onto the crater rim where they can be sampled. 230Th^226Ra and 235U^231Pa systems are uniform over the 34 years These bombs have provided an almost annual record of examined. The anorthoclase megacrysts and phonolite glasses show the magma composition from the early 1970s to the pre- complementary 226Ra/ 230Th disequilibria with (226Ra/230Th) sent. The highly evolved phonolite magma contains 40 in the anorthoclase and 0·75 in the phonolite glass. In all sam- 30% anorthoclase megacrysts reaching up to 10cm in ples, (210Pb/ 226Ra) is in radioactive equilibrium for both phases. In length, which makes it a rarity among alkaline systems. A two phonolite glass samples (227Ac/ 231Pa) is unity. For the phonolite wealth of observational, geophysical, petrological and geo- glasses (228Ra/232Th) is in equilibrium, whereas in the anorthoclase chemical data provides a comprehensive and detailed per- megacrysts it is significantly greater than unity. Instantaneous crystal spective of the genesis and evolution of the Erebus fractionation, with magma residence times greater than 100 years and volcanic system and the dynamics of the current lava lake less than 10 kyr, can account for the measured (Giggenbach et al., 1973; Kyle & Cole, 1974; Kyle et al., 238 230 226 210 235 231 227 U^ Th^ Ra^ Pb and U^ Pa^ Ac. However, the sig- 1982, 1992; Kyle, 1990a, 1990b; Reagan et al., 1992; Dunbar Downloaded from 228 232 nificant Ra/ Th disequilibria in the anorthoclase megacrysts et al., 1994; Zreda-Gostynska et al., 1997; Esser et al., 2004; preclude this simple interpretation. To account for this apparent dis- Harpel et al., 2004; Sims & Hart, 2006; Oppenheimer & crepancy we therefore developed an open-system, continuous crystalliza- Kyle, 2008, and papers therein; Oppenheimer et al. , 2011). tion model that incorporates both nuclide ingrowth and decay during The magmatic evolution of the Erebus lavas has crystallization. This open-system model successfully reproduces all of been studied extensively and is convincingly interpreted http://petrology.oxfordjournals.org/ 238 232 the measured U and Th disequilibria and suggests that the shal- in terms of simple magma differentiation processes low magma reservoir at Erebus is growing. The implication of this (Kyle et al., 1992). In this contribution, we investigate modeling is that when the time scale of crystallization is comparable the time scales of magma genesis, melt evolution, 226 with the half-life of the daughter nuclide of interest (e.g. Ra) the crystal growth rates and magma degassing in the simple isochron techniques typically used in most U-series studies can Erebus magmatic system using measurements of 210 226 provide erroneous ages. The observation that ( Pb/ Ra) and 238U^230Th^226Ra^210 Pb^210 Po, 232Th^228Ra^228Th and 227 231 ( Ac/ Pa) are in radioactive equilibrium suggests that the resi- 235U^231Pa^227Ac. These different nuclides have starkly dence time of the magmas is4100 years.When considering the effect contrasting chemistries resulting in significant elemental 222 210 226 of Rn degassing on Pb/ Ra, the data indicate that the majority fractionations during a variety of magmatic processes, of magma degassing is deep and long before eruption, consistent with and their different half-lives allow the investigation of at MBLWHOI Library on July 18, 2013 melt inclusion data. Additionally, for the 2005 lava bomb, whose erup- time scales ranging from less than 1 year to 105 years. This 210 tion date (16 December 2005) is known explicitly, Po was not com- work builds on the dataset presented by Reagan et al. pletely degassed from the magma at the time of eruption. Incomplete 210 (1992), which investigated a limited range of U-series nu- degassing of Po is atypical for subaerially erupted lavas and sug- clides and was based on samples from only two eruptions. gests that the Erebus shallow magma degasses about 1% of its Po per 238 232 We note that the present study presents the first measure- day. The combined U and Th data further indicate that the ments of 231Pa^227Ac in volcanic samples and hence is the pyroclasts ejected by Strombolian eruptions at Erebus have compos- first time all the relevant nuclides from the 238U, 235U, itions that are close to what would be expected for a near-steady-state and 232Th decay series have been measured in the same system, reflecting inmixing of degassed magmas, crystal fractionation, lavas of a volcanic system. and aging. BACKGROUND ON EREBUS KEY WORDS: Erebus volcano; U-series isotopes; HIMU; Antarctica; magma chamber residence time; open-system crystallization; magma VOLCANO degassing Erebus (Fig. 1) is an active composite volcano and the lar- gest of four volcanic centers forming Ross Island: Mt. Erebus (3794 m elevation, 2170 km3), Mt. Terror (3262 m, 170 0 k m 3), Mt. Bird (1800 m, 470 km3), and Hut Point INTRODUCTION Peninsula (100 km3). About 4520 km3 of volcanic material The persistent lava lake at Erebus volcano, Antarctica, has been erupted on Ross Island over the last 4Myr provides an unparalleled opportunity to study the time (Esser et al., 2004). Ross Island is emplaced on thin scales and rates of crystallization and magma degassing in (17^25 km) rifted continental crust at the southern bound- an active alkaline magmatic system. Determining these ary of the Terror Rift, which is a major graben located time scales is fundamental to our physical understanding within the Victoria Land Basin on the western margin of eruption dynamics and critical for hazard assessment. of the West Antarctic rift system (Cooper et al., 1987; 236 SIMS et al. EREBUS MAGMATIC SYSTEM EVOLUTION 167°00' E 168°00' E 169°00' E 77°10' S Map of Ross Sea Ross Island, Mt. Bird 1800 m Antarctica Contour interval 77°20' S 1200 meters Mt. Terror Mt. Terra 3262 m 0 10 20 Mount Nova Kilometers N Erebus 2130 m 77°30' S 3794 m Ross Ice Shelf Downloaded from 77°40' S McMurdo Sound Lower Hut Scott (11 ± 4) McMurdo Base Station 77°50' S http://petrology.oxfordjournals.org/ Northeast Tramways (13 ± 2) (9 ± 2) Northwest Nausea 3 2 Knob 0 0 (0 ± 4) (10 ± 5) at MBLWHOI Library on July 18, 2013 3 0 5 0 0 7 3300 0 n Cra 3 ai te M r 3400 0 70 3 00 36 Lava Lake rate r C e d i x x S 3647m 3794 m 180° Western Crater Ross Island 65°S 36 00 75°S 75°S 3400 85°S 90°E Antarctica 90°W Lower Ice Tower Ridge Southeast Southwest South (12 ± 3) (9 ± 7) Upper Ice (17 ± 8) (6 ± 2) 0 500 1000 Tower Ridge 0° km (4 ± 3) Caldera Rim Legend Contour interval 100 meters Extent of Extent of Sept. - Dec. pre-1984 bombs 0250 500 1000 1984 bombs Meters N Fig. 1. Map of Mt. Erebus showing its location and the location and 40Ar/39Ar ages of its lavas.
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