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Journal of Volcanology and Geothermal Research, 7 (1980) 11-37 11 © Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands

UKINREK , ALASKA, I. APRIL 1977 ERUPTION SEQUENCE, PETROLOGY AND TECTONIC SETTING

JUERGEN KIENLE', PHILIP R. KYLE 2 , STEPHEN SELF', ROMAN J. MOTYKA' and VOLKER LORENZ4 ' Geophysical Institute, University of Alaska, Fairbanks, AK 99701 (U.S.A.) 'Institute of Polar Studies, Ohio State University, Columbus, OH 43210 (U.S.A.) Department of Earth Sciences, Dartmouth College, Hanover, NH 03755 (U.S.A.) 4 Institut fur Geowissenschaften, Johannes Gutenberg-Universitilt, 6500 Mainz (Federal Republic of Germany) (Revised version accepted September 3, 1979)

ABSTRACT

Kienle, J., Kyle, P.R., Self, S., Motyka, R.J. and Lorenz, V., 1980. Ukinrek Maars, Alaska, I. April 1977 eruption sequence, petrology and tectonic setting. J. Volcanol. Geo- therm. Res., 7: 11-37.

During ten days of phreatomagmatic activity in early April 1977, two maars formed 13 km behind the Aleutian arc near Peulik on the Alaska Peninsula. They have been named "Ukinrek Maars", meaning "two holes in the ground" in Yupik Eskimo. The western formed at the northwestern end of a low ridge within the first three days and is up to 170 m in diameter and 35 m in depth. The eastern maar formed during the next seven days 600 m east of West Maar at a lower elevation in a shallow saddle on the same ridge and is more circular, up to 300 m in diameter and 70 m in depth. The maars formed in terrain that was heavily glaciated in Pleistocene times. The contained in the underlying till and silicic volcanics from nearby Peulik volcano controlled the domi- nantly phreatomagmatic course of the eruption. During the eruptions, steam and ash clouds reached maximum heights of about 6 km and a thin blanket of fine ash was deposited north and east of the vents up to a distance of at least 160 km. started to pool on the floor of East Maar after four days of intense phreatomagmatic activity. The new melt is a weakly undersaturated alkali olivine (Ne = 1.2%) showing some transitional character toward high-alumina . The chemistry, an anomaly in the tholeitic basalt--dominated Aleutian arc, suggests that the new melt is primitive, generated at a depth of 80 km or greater by a low degree of partial melting of garnet peri- dotite mantle with little subsequent fractionization during transport. The Pacific plate subduction zone lies at a depth of 150 km beneath the maars. Their position appears to be tectonically controlled by a major regional fault, the Bruin Bay fault, and its intersection with cross-arc structural features. We favor a model for the emplacement of the Ukinrek Maars that does not link the Ukinrek conduit to the plumbing system of nearby Peulik volcano. The Ukinrek eruptions probably represent a genetically distinct magma pulse originating at asthenospheric depths beneath the conti- nental lithosphere.

Onlvenity enntri .nutinns Series

rairbantz.3, idc:ka 9970 13

Jima Island in 1957 (Corwin and Foster, 1959) and in the Nilahue River Val- ley, in 1955 (Muller and Veyl, 1957; Zuniga, 1956; flies, 1959). Pos- sibly a maar was also produced by a very large eruption of Ksudach volcano in Kamchatka in 1907 (Vlodavetz and Piip, 1959), by the interaction of magma and water from a pre-existing . Maars form when the crust is perforated by phreatic or phreatomagmatic explosions that result when rising magma contacts groundwater at shallow depth. Phreatic is used here to denote explosions involving steam alone or steam and country rock; phreatomagmatic denotes explosions involving steam and magma (with or without country rock). By far the most common occur- rence of maars is in association with basic where groundwater inter- cepts magma that would, without the intervention of water, egress to the sur- face to produce strombolian explosions, scoria cones and flows. However, maars can be formed by other magma types, including carbonatitic, interme- diate or even acidic melts (Lorenz, 1973). Possibly most maars are the surface expression of diatremes (Lorenz, 1975; Lorenz et al., 1970; McCallum et al., 1976). Between March 30 and April 9, 1977, two maars formed near the crest of a short east-west-trending ridge in generally low-lying glacial terrain, 2.0 km south of the shore of Lake Becharof on the Alaska Peninsula (Kienle et al., 1978). The rim of the maars are located about 70 m above the lake level. The eruption site lies behind the Aleutian , 13 km northwest of Mt. Peulik, a 1525-m-high which erupted last in 1852. Fig. 1 shows the location of the maars and their geologic setting.

Quaternary Volcanics Volcanoes FTI Mesozoic Shallow Marine ' a Shale 1..1 Jurassic Batholith —Fault

UKINREK MAARS Pilot4 Point Regional Ash Fallout (7• 77 n , r _ _, 0

Fig. 1. Map of the upper Alaska Peninsula, showing the location of the Ukinrek Maars (star), Quaternary and Recent volcanoes and simplified geology after Beikman (1978). The minimum area of fine ash fall is also indicated. 15

C

Fig. 2. (a) Easterly view across the two Ukinrek Maars on April 3, 1977, 10 AST, West Maar in foreground with hot black blanket and steam rising from the lake, East Maar in in the background (photograph by Larry Conyers, CITGO). (b) "Mushroom" cloud produced by phreatic/phreatomagmatic explosion on April 5, 1977, 15:30-16:00 AST. Note the "cap" of white steam followed by a column of dark ash and steam (photograph looking south by Jim Faro, ADFG). (c) Vigorous phreatomagmatic activity at the East Maar on the evening of April 6, 1977, 17:00 AST (photograph looking south by R. Russel, ADFG). 17 66 •8

71.4 B

D 61 9

NI 67.8 BM2 70.2 73 0 67.2 BM1 71626112 • 72• 78.177A3 ,75 7 •69.2 • 71.8 0 79.16 IF 79 1 • 75.0 BM1A 78.3 72.2 75 96" 0 50 100 meters WEST MAAR

EAST MAAR

Fig. 4. Map of the Ukinrek Maars (survey by R. Motyka), also showing the thickness of the rim deposit. Accuracy of absolute elevations, given in meters above sea level, is unknown; elevation differences between surveyed points are 0.5 m or better. Date of survey: August 30, 1977. 21 the Ukinrek Maars are small (see also Table 1) and once again the lack of ero- sional modification is evident. We were surprised to find very high temperatures within the ejecta of two distinct scoriaceous* fall lobes on the high southeast and southwest rims of West Maar (Fig. 6a, b). The highest temperature measured was 805°C near its rim at 1.1 m depth, temperatures on a profile between the two maars decreas- ed from 210°C at 15 cm depth at the rim of the West Maar to 20°C at the same depth at the rim of the East Maar (D.J. Lalla, personal communication). Similar high temperatures at a distance of 250 m west of the West Maar are in- dicated by the fact that willow trunks were completely charcoaled at a depth greater than 20 cm below the surface. The high temperatures on the rim probably indicate very rapid accumulation of spatter and scoriaceous lapilli and bombs during the latter stages of West Maar activity, suggesting that magma also reached the surface at this maar. We have no visual observations to confirm magma in the crater of West Maar when it was active during the first three days of the eruption. However, irregular dike-like intrusions were observed in the till exposed in the southern crater wall during a 1979 visit by one of the authors (Lorenz). In April 1977 West Maar contained a lukewarm shallow (4.6 m deep) lake of slightly acidic (pH — 6) water (Fig. 6a). Subsequently, the lake drained (Fig. 6c) sometime between our second visit, May 20-29, and our third visit in late August. The water of the remaining hot spring (next to the person in Fig. 6c) was sampled by G. McCoy and the U.S. Geological Survey on August 24: It had a pH of 6.3 and a temperature of 81°C. The gas in the thermal wa- ters contained 98% CO 2 with a 13C composition of —6.36%o, a range that is common for CO 2 from the mantle (Barnes et al., 1978). By July 1979 a shal- low lake had again formed within the northern half and at the location of the former spring two lines of gas bubbles were observed with headings of N47°E and N151°E.

Ejecta and maar volumes

The two maars are surrounded by deposits of fine, grey-tan-colored ash fall, grey medium-coarse vesicle-poor lapilli and lithic fall, black juvenile scoriace- ous fall including many cauliflower bombs (Lorenz, 1974a), spherical bombs and ribbon bombs, and lithic (country rock) ejecta blankets (Figs. 2a, 6a, f). No accretionary lapilli have been observed. However, vesiculated tuffs (Lorenz, 1974b) were deposited by base surge against obstacles. Several fall units compose any one section and reflect the changing style of activity (Fig. 6d). The crater walls of West Maar (Fig. 6b, c) expose till; the walls of East Maar (Fig. 6d—f) are not readily accessible but appear to consist of till sheets and interbedded silicic pyroclastic (?) Peulik volcanics, as viewed from the crater rim.

*Within the scoriaceous deposits of both West and East Maar a great number of cauliflower bombs indicate a phreatomagmatic component. 23

Unfortunately, the limited thickness data for proximal deposits and the widely scattered, extremely thin and intermittent nature of the distal fall deposits makes a volume estimate very approximate. A total bulk volume of 26 X 106 m3 is probably an underestimate for the deposits within a 5-km ra- dius of the maars. The volume of non-juvenile ejecta should be approximately that of the craters. East Maar is estimated to have a crater volume of 4 X 106 m3 and that of West Maar is 0.25 X 10 6 m3 . The dome in East Maar above the crater floor has a volume of 0.9 X 10 6 m3 . The total volume of the ejecta represents perhaps 10 X 10 6 m3 of dense rock which is substanti- ally greater than the combined crater volume, 4.3 X 10 6 m3 . The excess vol- ume, 5.7 X 10 6 m3 of dense rock, is mostly accounted for by that of juvenile airfall material. Detailed descriptions of the ejecta, ballistics and the mechanism of maar formation are given in the companion paper, Ukinrek II (Self et al., this issue).

PETROLOGY AND GEOCHEMISTRY

Petrology

The juvenile Ukinrek ejecta is typically dark grey to black porphyritic basalt ranging in vesicularity from scoria to dense lapilli, bombs and blocks. White felsic xenoliths occur in several samples, mostly in the cauliflower and cored bombs and probably represent digested volcanics or sediments from the deep conduit wall. The mode of a representative sample is given in Table 3. Olivine (Fo80_ 87) is the dominant phenocryst phase; it is seriate and ranges in size from 0.2 to 2.0 mm. The olivine is generally euhedral but larger grains often show some resorption. Inclusions of glass, spinel, and magnetite are common. Euhedral plagioclase laths (An 75 ) are the most common microphenocryst. Clino- pyroxene and opaque oxide microphenocrysts are rare and usually average 0.25 mm. The clinopyroxene is euhedral to subhedral, usually greenish and may show weak pleochroism. The groundmass textures are variable, depend- ing on the type of material examined. Samples from the center of bombs have microcrystalline intergranular textures, whereas on the rim of bombs pilotax- itic textures were dominant: flow banding was observed in the groundmass of the denser bombs. Highly scoriaceous samples have hyalo-ophitic textures. All mineral phases seen as microphenocrysts occurred in the groundmass. Representative electron microprobe analyses of mineral phases in sample JRC10b are given in Table 4. Olivine is abundant and is generally uniform

1977). (c) Interior view of West Maar, looking south, taken in August 1977 after the lake had drained out. Strand lines stand 4.6 m above bottom of cracked mud. Person stands next to a small hot spring. (d) Ejecta blanket on south wall of East Maar, 26 m thick, arrow points to willow bush in the pre-eruption soil horizon (May 1977). (e) View of steaming of East Maar in May 1977, looking southeast, with a lake beginning to form. The lava dome still showed cherry-red glowing cracks at this time. (f) Vertical view of East Maar in August 1977, the lake level had risen considerably as compared to May. TABLE 4 25 Representative electron microprobe analyses (wt.%) of mineral phases in Ukinrek ejecta

Olivine Clinopyroxene Spinel Plagioclase 1 2 3A 3B 4 5

Sio, 39.6 39.1 45.7 - 48.5 Al203 - 8.22 11.2 32.5 TiO2 - - 1.87 9.08 - FeO* 13.6 18.6 8.22 58.5 0.94 MnO 0.03 0.23 0.13 0.59 - MgO 46.2 41.9 12.9 5.47 0.12 CaO 0.23 0.33 22.4 16.1 Na2 O -- - 0.26 - 2.38 K2 O - - - - 0.14 Cr2O3 - - 0.28 11.2 - Sum 99.66 100.16 99.98 96.04 100.68 Fe 2O3 - 5.54 28.8 FeO - 3.24 33.1 Total - 100.54 99.44

Formula proportions

0 4 4 6 6 32 8 Si 0.993 0.998 1.718 1.696 2.217 Al - - 0.364 0.360 3.665 1.750 Ti - - 0.053 0.052 1.889 - Fe" - - 0.155 5.998 - Fe" 0.284 0.398 0.258 0.100 7.661 0.036 Mn 0.001 0.005 0.004 0.004 0.138 -- Mg 1.723 1.592 0.724 0.715 2.257 0.008 Ca 0.006 0.009 0.903 0.891 - 0.788 Na - 0.019 0.019 - 0.211 K - - - - 0.008 Cr - - 0.008 0.008 2.448 - Total 3.007 3.002 4.052 4.000 24.056 5.018 Mg 85.8 79.8 Ca 47.8 - Usp** 16.7 Ca 78.2 Fe 14.2 20.2 Mg 38.3 - Na 21.0 Fe 13.9 - K 0.8

Wo 43.8 En 49.0 Fs 7.2 * FeO = total Fe as FeO. **Usp = calculated using method of Carmichael (1967). 1 = phenocryst core; 2 = groundmass phase; 3A = microphenocryst, as analysed; 3B = charge balance determination of Fe 2 O3 and FeO, Wo, En, Fs from recalculated end-members; 4 = inclusion in olivine phenocryst; 5 = microphenocryst.

Analyses made on an automated four-spectrometer ARL-EMX-SM electron probe at State University of New York, Stony Brook. Data was reduced using the Bence and Albee (1968) procedure. 27 Geochemistry

Bulk analyses and CIPW norms of representative samples are given in Table 3, along with comparative data on other basalts. Partial major element analyses of additional samples of Ukinrek ejecta are similar to the analyses in Table 3, and suggest that there was no significant variation in the chemistry of the erupted material. The ejecta are classified as alkali olivine basalt (AOB) on the basis of their weak undersaturation (Ne = 1.2%) and because they lie above the dividing line, proposed by Macdonald and Katsura (1964), separating alkaline from subal- kaline rocks on an alkalies versus silica plot (Fig. 8). The alkalic nature of the clinopyroxene is also supportive of their classification as AOB. Ukinrek basalt is moderately aluminous and shows some transitional character toward high- alumina basalts. It is more alkaline than most tholeiitic and high-alumina basalts from elsewhere along the Aleutian arc (Fig. 8), but is similar to horn- blende basalt from Bogoslof Island, which is just behind the arc. The Ukinrek basalt is similar in many respects to averages of world-wide AOB from differ- ent crustal settings (Fig. 8), however it is distinctly lower in Ti0 2 (Manson, 1968; Schwarzer and Rogers, 1974). Bass et al. (1973) have suggested that basalts from different tectonic settings can be characterised on a P 2 0 5 versus TiO 2 plot; Ukinrek basalt plots in the ocean ridge basalt field. In the Lesser Antilles volcanic arc AOB is common in Grenada; it shows a close similarity to the Ukinrek basalt, including low TiO 2 contents (Table 3).

10 — Ukinrek Moors Ukinrek Moors - groundmass p Bogoslof Island 8 IAB x Other Alaskan basalts 1:1 Average alkali olivine basalts 0 6 0.1 Fi.\-\°‘°4- A BC/ 9,1 0 CV 4

x X

o 1 1 1 1 1 1 I I I 42 44 46 48 50 52 54 56 Si02 Fig. 8. Total alkalies versus silica (in weight percent) for alkali olivine basalt (AOB) from Ukinrek Maars. The dashed line connects a whole rock analysis with the groundmass glass as determined by electron microprobe (Table 4). IAB is average high-alumina basalt (Jake and White, 1971); the hornblende basalt, Bogoslof Island is from Byers (1961); others refer to selected basalt analyses from the Aleutian Islands (Byers, 1961) and (Hoare et al., 1968); average AOB from A = oceanic, B = island arc and C = continental settings from Schwarzer and Rogers (1974). The boundary between alkaline and subalkaline rock series is that shown by Macdonald and Katsura (1964) to separate alkali and tholeiitic fields in Hawaiian basaltic rocks. •

29

100 a w 50 a Ukinrek Moors 0

Model 2 5% Partial Melt I 3e Chondrite Montle __J 0_ ...... 10 (r) Hornblende Basalt, Bogostof Island .....

5 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu

200

100

10

5 I I I 1111111 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu Fig. 9. Chondrite-normalized REE abundances of alkali basalt from the Aleutian volcanic arc and comparative data from other locations. (a) Ukinrek Maars, spark source mass spectrometer analyses (dot), instrumental neutron activation analysis (circles). REE of model calculation is for 2.5% modal melting of a mantle with 1.3 times chondrite REE abundance and a residual composition of 55% olivine, 25% orthopyroxene, 18% clino- pyroxene and 2% garnet. Distribution coefficients used were clinopyroxene/liquid and clinopyroxene/olivine (Kay and Gast, 1973); garnet/liquid (Shimizu, 1975) and clino- pyroxene/orthopyroxene (Sun and Hanson, 1975). (b) REE abundances of alkali basalts from the Aleutian and Lesser Antilles volcanic arcs and oceanic islands. References: 1 = Arculus et al. (1977); 2 = Sun and Hanson (1975); 3 = Shimizu and Arculus (1975). ratio it was necessary to include garnet as a residual phase. A model involving 2.5% modal melting of a mantle with 1.3 times chondrite REE abundances shows excellent agreement with that observed in Ukinrek basalt. Experimental studies based on model mantle compositions (Green, 1970, 1971, 1973; Frey et al., 1978) suggest that AOB can form by partial melting of a pyrolite source with — 0.1-0.2% H 2O or the presence of CO2-rich volatiles (Mysen and Boettcher, 1975; Mysen et al., 1975) at pressures of 25-30 kbars. Garnet is a residual phase under such conditions. In conclusion it is believed that the AOB erupted at Ukinrek Maars was • - -

Fig. 10. Maar-associated microearthquake swarm recorded on April 18, 1977, 00:24- 23:02 UT, within a few 100 m of East Maar. The tick marks are 1 minute apart. 33

DISTANCE (KM) ALEUTIAN TRENCH -100 100 200 300 400 0 t + ++414-,44-1-4v4 :+0 + 4- 4 44- 4- 4 + 4,4 414. 4- +4 4. 4 + + + AMERICAN + t 4°444.4 4. 4- PLATE )) + +.„4„ + + 4- 4++ + 44- 4-

100 - I

+ +11' + + ++ 4c?.. -t -4- 200 -

Fig. 11. Best located regional hypocenters (1976-1978) in the Katmai-Kodiak region pro- jected onto a vertical plane striking N125°E, showing Benioff zone associated with sub- ducting Pacific plate, 150 km deep beneath Ukinrek. Vertical grouping of shallow events at Ukinrek is believed to be associated with feeder dikes; the deeper part of the conduit system, inferred from geochemical evidence, is aseismic (from Pulpan and Kienle, 1979). complex models (Kay, 1977; Ringwood, 1974, 1977), where magma derived by partial melting of the underthrusting oceanic plate may be mixed with the overlying peridotite mantle, which then rises and segregates to form the finally erupted magma, the alkaline nature of the Ukinrek ejecta suggests that it was derived by a low degree of single-stage partial melting from a garnet peridotite mantle at the base of the continental lithosphere (Green, 1970, 1973). Based on our preliminary seismic and geochemical data, we presently favor a tectonic model for the emplacement of the Ukinrek Maars that does not link the Ukinrek conduit directly to the plumbing system of Peulik volcano. We feel that the Ukinrek eruptions represent a genetically distinct magma pulse originating at asthenospheric depths beneath the continental lithosphere. The position of the conduit 13 km behind the volcanic axis of the Aleutian arc could have been controlled by an old regional lithospheric fracture, the Bruin Bay fault, and its intersection with cross-arc structural features.

ACKNOWLEDGEMENTS

We gratefully acknowledge the help of the following individuals during the field study: from the University of Alaska D.J. Lalla (temperature measure- ments), J.R. Carden (petrology), J. Siwik (seismology) and V. Ferrell (mor- phology), and from Darmouth College J. Bratton (SO 2 measurements). B. Globerman and E.H. Brown, Western State College, J.R. Sans and A.T. Anderson, University of Chicago, provided microprobe and geochem- ical data. We thank C.A. Wood for data used in compiling Table 1. The University of Alaska team was supported by State of Alaska, Depart- 35

Green, D.H., 1971. Compositions of basaltic as indicators of conditions of origin: applications to oceanic volcanism. Philos. Trans. R. Soc. London, Ser. A, 268: 707-725. Green, D.H., 1973. Experimental melting studies on a model upper mantle composition at high pressure under water-saturated and water-undersaturated conditions. Earth Planet. Sci. Lett., 19: 37-53. Haggerty, S.E., 1976. Opaque mineral oxides in terrestrial igneous rocks. In: Oxide Minerals. Min. Soc. Am. Short Course Notes, Vol. 3, Chapter 8. Hoffmeister, J.E., Ladd, H.S. and Ailing, H.L., 1929. Falcon Island. Am. J. Sci., 18: 461. lilies, H., 1959. Die Entstehungsgeschichte eines Maares in Sildchile. Geol. Rundsch., 48: 232-247. Jaggar, T.A. and Finch, R.M., 1924. The of Kilauea, Hawaii, 1924. Am. J. Sci., Ser. 5, 8: 353-374. Jake'g, P. and White, A.J., 1971. Composition of island arcs and continental growth. Earth Planet. Sci. Lett., 12: 224-230. Kay, R.W., 1977. Geochemical constraints on the origin of Aleutian magmas. In: M. Talwani and W.C. Pitman, III (Editors), Island Arcs, Deep Sea Trenches and Back- Arc Basins. Am. Geophys. Union, Maurice Ewing. Ser., 1: 229-242. Kay, R.W., and Gast, P.W., 1973. The rare earth content and origin of alkali-rich basalts. J. Geol., 81: 653-682. Keller, A.S. and Reiser, H.N., 1959. Geology of the Mt. Katmai Area, Alaska. U.S. Geol. Surv. Bull., 1058-G: 261-298. Kienle, J., Motyka, R.J., Lalla, D.J., Estes, S.A. and Huot, J.-P., 1978.•Formation of two maars behind the Aleutian volcanic arc, Alaska Peninsula, April 1977. Preliminary re- sults: field reconnaissance, geochemistry and seismicity. Univ. Alaska, Geophys. Inst. Rep., UAG R-257: 26 pp. Kyle, P.R. and Rankin, P.C., 1976. Rare earth element geochemistry of late Cenozoic alkaline of the McMurdo Volcanic Group, Antarctica. Geochim. Cosmochim. Acta, 40: 1497-1507. Leeman, W.P. and Scheidegger, K.F., 1977. Olivine/liquid distribution coefficients and a test for -liquid equilibrium. Earth Planet. Sci. Lett., 33: 247-257. Lorenz, V., 1973. On the formation of maars. Bull. Volcanol., 37: 183-204. Lorenz, V., 1974a. Studies of the tephra deposits. Surtsey Progr. Rep., VII: 72-79. Lorenz, V., 1974b. Vesiculated tuffs and associated features. Sedimentology, 21: 273-291. Lorenz, V., 1975. Formation of phreatomagmatic maar-diatreme volcanoes and its rele vance to diatremes. Phys. Chem. Earth, 9: 17-27. Lorenz, V., McBirney, A.R. and Williams, H., 1970. An investigation of volcanic depres- sions, III. Maars, rings, tuff cones, and diatremes. Progr. Rep., NASA NGR 38-003-012. Macdonald, G.A. and Katsura, T., 1964. Chemical composition of Hawaiian lavas. J. Petrol., 5: 82--133. McCallum, M.E., Woolsey, T.S. and Schumm, S.A., 1976. A fluidization mechanism for subsidence of bedded tuffs in diatremes and related volcanic vents, Bull. Volcanol., 39: 512-527. Manson, V., 1968. Geochemistry of basaltic rocks: major elements. In: H.H. Hess and A. Poldervaart (Editors), Basalts, 1. Interscience, New York, N.Y., pp. 215-269. Markhinin, E.K. and 12 others, 1974. The eruption of the Mt. Tyatya volcano in the Kurile Islands in July of 1973. So y . Geol. Geophys., 15: 14-24. McGetchin, T.R. and Ullrich, G.W., 1973. Xenoliths in maars and diatremes with inferences for the Moon, Mars and Venus. J. Geophys. Res., 78: 1833 -1853. Muller, G. and Veyl, G., 1957. The birth of Nilahue, a new maar-type volcano at Rininahue, Chile. 20th Int. Geol. Congr. Rep., Sect. 1, pp. 375-396. Mysen, B.O. and Boettcher, A.L., 1975. Melting in a hydrous mantle, II. Geochemistry of and liquids formed by anatexis of mantle peridotite at high pressures and high temperatures as a function of controlled activities of water, hydrogen and carbon dioxide. J. Petrol., 16: 549-592. 37

Wood, C.A., 1974. Reconnaissance geophysics of Pinacate Centers, Sonora, Mexico. Bull. Volcanol., 38: 149--172. Wood, C.A., 1980. Maars. In: Encyclopedia of Volcanology (in press). Zhegalov, Y.V., 1964. Komandorsky Islands. In: G.H. Vlasov (Editor), Geology of the U.S.S.R., Vol. 31. Kamchatka, Kuril and Komandorsky Lslands, 1: Geological Description (English translation). U.S. Department of Commerce, Springfield, Va., Natl. Tech. Info Center, Doc. N68-15812. Zuniga, C.C., 1956. ErupciOn del VoLein Nilahue. Inst. Geol. Univ. Chile, Publ., 7: 73-96.

POSTSCRIPT

In a recent article Barnes and McCoy (1979) argue that the Ukinrek Maars may have been created by the explosive discharge of mantle-derived CO, rather than by phreatic explosions. Our geochemical data discussed in this paper indeed confirms a mantle (asthenospheric) origin of the Ukinrek basalt. However, we have also presented evidence in this paper and in the companion article Ukinrek II (Self et al., this issue) that shows that the eruption columns were wet (e.g., cauliflower bombs, base surge deposits, photographs of plumes) and hence that water was the more important gas phase driving the predominant- ly phreatomagmatic eruptions. ADDITIONAL REFERENCE

Barnes, I. and McCoy, G.A., 1979. Possible role of mantle-derived CO, in causing two "phreatic" explosions in Alaska. Geology, 7: 434-435.