Laacher See Tephra: a Widespread Isochronous Late Quaternary Tephra Layer in Central and Northern Europe

Laacher See Tephra: A widespread isochronous late Quaternary tephra layer in central and northern Europe

PAUL v.d. BOGAARD* i , , ,...... D ,,, . HANS-ULRICH SCHMINCKE f ¡"stUuíJur Mmeralogie, Ruhr-Um'sitat Bochum, Postfach 10 21 48, D-4630 Bochum 1, West Germany

ABSTRACT Tephra (LST), erupted some 11,000 yr B.P. from Laacher See Volcano (East Eifel Volcanic Field, West Germany), has been unusually well pre- A late Quaternary tephra layer, widespread in central and north- served in peat bogs and sediments of small lakes throughout much of ern Europe, resulted from explosive Plinian and phreatomagmatic central and northern Europe. Although recognized as a marker of the eruptions of the Laacher See Volcano 11,000 yr B.P. The tephra is European late Quaternary "Allerod-Interstadial" since Frechen (1952) distinguished bom other late Quaternary andesitic-rhyolitic airfall tuff and Firbas (1953), its areal distribution was poorly understood. Reliable layers in northern Europe and from basaltic or trachytic tuff deposits identification and correlation criteria so far have not been developed, and in southern Euruope by its phonolitic composition and abundance of sanidine, plagioclase, clinopyroxene, amphibole, and sphene. The 9° 18° proximal tephia sequence at Laacher See is divided into three main deposits: the predominantly Plinian deposits of Lower and Middle Laacher See Tephra (LLST and MLST) and phreatomagmatic deposits of the Upper Laacher See Tephra (ULST). The MLST member is further subdivided into beds A, B, and CI, C2, and C3. The chemical composition olr the magma is highly differentiated phonolite in the LLST to MLST B sections but mafic phonolite in the MLST CI to ULST section!;. All deposits are considered to be isochronous, the frequency maximum of 16 radiocarbon datings indicating an eruption about 11,000 ±50 yr B.P. Distal ash was deposited in three main fans directed to the northeast (LST traced up to 1,100 km distance), south (LST traced up to 600 km), and southwest (LST traced up to 100 km). Tephrostrati- graphic correlation of the distal ash deposits is based on (a) the major- element composition of glass shards, (b) lithology, and (c) heavy- mineral analyses. The northeastern fan consists of deposits from LLST, MLST B, and MLST CI eruptive phases, the southern fan comprises MLST A, MLST C2, and ULST deposits, and the southwestern fan consists exclusively of ash from the ULST eruptive phase. Northeastern transport of ash during eruptive phases, with high Pli- nian eruption columns, but southern and southwestern transport of ash along phases of relatively low eruption columns, are interpreted in terms of prevailing southwesterly paleowinds at high altitudes (tropopause level?) but northerly winds dominating in the lower atmosphere. The Laacher See eruption columns were emplaced into an atmosphere vertically zoned with respect to paleowind directions, which also explains the near-vent shifting of LLST, MLST B, and MLST CI isopach axes fro«» east-southeast to northeast within the first 20 km of transport

INTRODUCTION

Widespread tephra layers, formed from instantaneous explosive volcanic eruption!!, are ideal isochronous marker beds. The Laacher See Figure 1. Areal distribution of Laacher See Tephra. Dots are selected sites of ash layers. Numbers of sites (italics) and samples refer to Table B (GSA Data Repository). Broken lines indicate distal fan •Present address: Geology Department, University of Toronto, Scarborough margins. Boundary of the Scandinavien ice shield at 11,000 yr B.P. Campus, 1265 Military Trail, Scarborough, Ontario MIC 1A4, Canada. (after Berglund, 1979).

Additions! data for this article (two tables) may be secured free of charge by requesting Supplementary Data 85-32 from the GSA Documents Secretary.

Geological Society of America Bulletin, v. 96, p. 1554-1571,17 figs., 1 table, December 1985.

1554

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loc. 15

Dune sections Reworked LST

Lapilli layers

Ash layers

Breccia beds Figure 2. Strati- graphic units of proximal Laacher See Tephra. Correlation of two near-vent profiles with index map of lo- Laacher See calities (black triangles Tuft 5 (LST 51 symbolize morphologically prominent alkali Laacher See Tuff i, (LST L) basalt scoria cones southeast of Laacher Laacher See See volcano). LLST to Tuff 3 (LST 31 ULST = members of the LST formation; Laacher See MLST A to MLST C3 Tuff 2 (LST 2) = subunits; I to XXI = beds. Stratigraphic Laacher See subdivision according Tuff I (LST I) to Frechen (1953) or is shown on the very Frauenkirch Trass right.

Niedermendiger Tuft (NMT)

Frauenkirch Tuff (FT)

Me erboden Tuff (MT) Obermendiger Tuft (OUT)

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the synchronous character of all Laacher See Tephra deposits has been questioned by some workers (Windheuser and Bunnacker, 1979; Juvigne, 1984). The data presented in our paper are concerned with (a) the present state of knowledge of the areal distribution of Laacher See Tephra, (b) discussion of criteria to identify LST and distinguish it from other late Quaternary tephra layers, (c) mineralogical and chemical differences within and between ash layers of the main depositional areas, and (d) a model of its formation. We will begin by briefly discussing Laacher See Volcano and its eruptive history.

ERUPTIVE HISTORY OF LAACHER SEE VOLCANO

Laacher See Volcano is located in the East Eifel Volcanic Field (West Germany), some 40 km south-southeast of Bonn (Fig. 1). Its explosive eruption 11,000 yr B.P. (the first and only) produced at least 5 km3 phonolite magma (dense rock equivalent) and resulted in a widely dispersed tephra blanket more than 50 m thick close to the eruptive center. All Laacher See Tephra was erupted from overlapping vents that were located inside the present Laacher See basin (Bogaard and Schmincke, 1984). A complsx "Laacher See-type" eruptive history is deduced from near-vent tephra sections, including deposits from alternating Plinian as well as phreatomagmatic eruptive processes (Bogaard, 1983). The sequence is subdivided into deposits from three main eruptive phases: the Lower (LLST), Middle (MLST), and Upper (ULST) Laacher See Tephra. These are members of the Laacher See Tephra Formation (Fig. 2). At the base of Lower Laacher See Tephra, there is a poorly sorted, massive ash bed, rich in plant remains, tree molds, and accretionary lapilli—a deposit derived from an initial phreatomagmatic explosion. This so-called LLST basal tuff extends only up to 20-km distance from the vent. The overlying main volume of LLST consists of well-sorted, white layers of highly inflated pumice that was deposited from sustained Plinian erup- Figure 3. Distal Laacher See Tephra layer in Allerdd-dated fossil- tion columns. Isopach maps indicate transport directions that changed rich lake marl deposits from Triittlikon (eastern Switzerland; site no. from east-southeast close to the vent toward east-northeast at distances 75), some 350 km south of Laacher See volcano. The ash layer greater than 10 km. Simultaneous, phreatomagmatic explosions produced (arrow) here occurs as dark olive-gray parting. a minor base surge and fallout fan (LLST*) directed to the south. Middle Laj.cher See Tephra is subdivided into deposits from three ash-flow deposits at the base to laminated ash beds and massive and eruptive phases (MLST A, B, and C), which differ in (1) eruptive mecha- laminated silt beds at the top of the sequence (Schmincke and others, nisms, (2) prominent bedforms, and (3) pyroclast petrography. MLST A 1973; Bogaard, 1978; Fisher and others, 1983). The isopach pattern indi- consists of phreatomagmatic surge deposits and minor Plinian fallout lay- cates a major fallout fan directed to the south-southeast, as well as a minor ers. Its isopach map shows changing transport directions from south (near- ill-defined lobe directed to the south-southwest. vent) to southeast. The MLST B eruptive phase was characterized by LLST, LLST* and MLST A deposits were erupted from vents lo- magmatic eruptions from alternating convecting and collapsing Plinian cated in the southern part of the Laacher See basin. All tephra deposited eruption columns. Unwelded ash flow deposits (local term: "Trass") dom- after the MLST A eruptive phase was erupted from vents in the northern inate at the base, and well-sorted white pumice lapilli beds at the top of crater area (MLST B to ULST). The entire eruptive sequence (LLST to this sequence. The ash flows descended through about six passes in the rim ULST) is believed to have been deposited in less than 10 days (Elogaard, of the Laacher See basin, and accumulated in adjacent paleovalleys, after 1983). traveling as much as 10 km (Bogaard and Schmincke, 1984; Freundt and Schmincke, 198:5). Isopach maps constructed for MLST B fallout layers CRITERIA TO IDENTIFY LAACHER SEE TEPHRA indicate transport directions changing from east-southeast to east-northeast within the first 15 km from the vent. During MLST C eruptions, light Criteria for tephrostratigraphic correlations were recently summa- greenish-gray to gray, increasingly dense pumice was deposited in Plinian rized by Westgate and Gorton (1981). These are based on a tephra layer's fallout layers and intercalated minor ash flows. Transport directions of the stratigraphic, paleontologic, palynologic, paleomagnetic, and radiometric fallout clouds, inferred from isopach maps, are highly variable, ranging relationships and the properties of glass shards, crystals, and litliics that from east-southeast to northeast (section MLST CI), southeast (section make up the tephra. MLST C2) to east (section MLST C3). During the late phreatomagmatic eruptive phase which formed the Stratigraphy, Palynology, and Radiometric Age Upper Laacher S>ee Tephra, dense, gray to black, phenocryst-rich, phonolite clasts are the typical essential components. Bedforms range from mod- The Laacher See Tephra forms a more or less continuous pyroclastic erately sorted la.pilli-breccias, intensively duned sections, and massive cover up to 20 to 30 km from Laacher See volcano, usually underlain by

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2. The glass shards are phonolitic in chemical composition. 10 550 10 750 10 950 II 150 1 350 3. Sanidine, plagioclase (± hauyne) are abundant essential light- 1 1 1 1 1 1 1 1 1 1 mineral phases, with sanidine » plagioclase. Quartz is a common acci- 7 dental light mineral. 4. Clinopyroxene, amphibole, and sphene are the major heavy minerals, apart from titanomagnetite and accessory apatite, phlogopite, and olivine. 5 Other late Quaternary explosive volcanism in Europe has been recorded from Italy, southern France, the German West Eifel Volcanic dat a U - ri 1 Field, and Iceland. Fallout ash layers erupted by these volcanoes, however,

o f 1 1 : 1 3 - - appear to differ from Laacher See Tephra in their age, their chemical

No . 1 : 1 1 : 1 and/or mineralogical composition, and in their distribution patterns. 1 1 J : Italian airfall tephra of the appropriate age has a trachytic composi- : 1 1 1 tion and has been deposited predominantly in the eastern Mediterranean .. (Keller and Ninkovich, 1972; Keller and others, 1978; Keller, 1981). : 1 ; 1 1 : 1 1 1 1 1 • 1 • 1 Explosive volcanism in the French Massif Central, about 10,000 to 10 600 10 800 II 000 II 200 11 400 BP 12,000 B.P., is restricted to basaltic Strombolian eruptions (Brousse and

IA others, 1969; Camus, 1975), and these are unlikely to have had wide- C - Age spread tephra layers. Trachytic tephra from the "Chaine des Puys" is radiocarbon-dated as 8,400 to 9,000 yr B.P. (Geyh and others, 1970; Figure 4. Absolute frequency histogram showing 16 radiocarbon Condomines and others, 1982), and is described to occur stratigraphically ages of Laacher See Tephra. Broken line: lower scale and classes. above LST in late Quaternary sedimentary cores from the Lake Constance Dotted line: upper scale and classes. Data from Firbas (1953), Frecheu area and western Switzerland (Geyh and others, 1970; Martini, 1971). (1959), Straka (1957, 1975), Pissart and Juvignt (1980), Roesch Mineralogically, these ashes are characterized by dominance of amphibole (1982), and M. A. Geyh (1974, personal commun.). and plagioclase, and the lack of sanidine and sphene (Martini, 1971). To our knowledge, nothing is published on the nature or chemical composi- Quaternary loess or soil and overlain by reworked Laacher See Tephra tion of vitric particles in these trachytic tuffs. and/or soil. At greater distances, some 120 late glacial sedimentary sec- In the West Eifel Volcanic Field, explosive volcanism culminated at tions of the younger Alleröd Interstadial in central and northern Europe 30,000 yr B.P., long before the LST eruption, and until the present time, are presently known. These contain Laacher See Tephra either as only the "Ulmener Maar" is considered to be a younger eruptive center, millimetre- to centimetre-thick discrete ash layer (Fig. 3) or dispersed 10,920 yr B.P., (Büchel and Lorenz, 1982). Confusion with LST is ex- volcaniclastic material. Most of the sections represent deposits from silted- cluded, however, because the erupted magma has a melilite-nephelinite up ancient lakes or peat bogs; the substrata are composed of organic or composition. In Iceland, basaltic-andesitic and rhyolitic magmas were detritic gyttja, lake marl, and peat. For detailed discussions of the Laacher erupted explosively many times during the past few thousand years See Tephra's palynological environment, see Ammann and Tobolski (Sigurdsson and Loebner, 1981; Thorarinsson, 1981), but as yet, only one (1983), Firbas (1953), Lang (1954), Müller (1965), Schloss (1979), Usin- vitric tuff similar in age to the LST was detected in late glacial deposits of ger (1978), and Wegmüller and Welten (1973). Norway. This so-called "Vedde Ash" was erupted 10,600 yr B.P. by Katla Radiometric ages (14C) are available for charcoal inclusions from Volcano, but it differs from Laacher See Tephra in its glass chemistry near-vent ash-flow deposits and also for organic sediments (gyttja, peat) (andesitic and rhyolitic) and its extreme paucity of crystals (Mangerud and covering or underlying distal ash layers (Table A in the GSA Data Reposi- others, 1984). tory).1 The mean of all data is 10,920 yr B.P.. Analyses on charcoal (x = In summary, even if the exact radiocarbon or pollen age of a late 10,915 yr B.P.) differ only slightly from those on organic sediments (x = Quaternary tuff layer found in Europe is in doubt, the Laacher See Tephra 10,925 yr B.P.). Moreover, there is no significant difference between mean can be definitively identified by its phonolitic glass shards and its character- values calculated for the MLST member (x = 10,950 yr B.P.), and the istic mineral assemblage, and thus it can be distinguished from time- ULST member (x = 10,890 yr B.P.), as the absolute error of individual equivalent trachytic or basaltic tuffs in the southern European depositional measurements ranges from ±85 to ±500 yr. Judging from the frequency area or andesitic and rhyolitic tuffs in northern Europe. Nevertheless, more distribution of the data (Fig. 4), however, a radiometric age between microprobe analyses of vitric shards are desirable in order to characterize 10,950 yr B.P. and 11,050 yr B.P. for the entire Laacher See Tephra seems tephra layers of similar age in Europe. to be most likely. Areal Distribution of Distal Laacher See Tephra Compositional Criteria Immediately after eruption and deposition, Laacher See Tephra Several compositional criteria allow the Laacher See Tephra to be more than 1 mm thick covered an area of -700,000 km2. Ash was easily distinguished from other late Quaternary tephra layers in Europe: deposited predominantly in two discrete fans: one extending to the north- 1. Primary LST deposits always contain fresh or slightly altered, east (NE-fan), and another extending to the south (S-fan). The NE-fan of colorless and vesicle-rich, and/or pale brownish and vesicle-poor glass Laacher See Tephra is recognized up to 1,100 km from the source in cores shards. from the Baltic Sea, northeast of Gotland, Sweden (Fig. 1, site no. 40). The actual width of the fan, however, is difficult to define, because so far LST 'For free copies of Tables A and B, request Supplementary Data 85-32 from ash has not been detected in sites from northern Poland, Russia, or the the GSA Documents Secretary. southeastern Swedish mainland. The maximum distance to which Laacher

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Eruptive Modal composition of olivine- No. of phases essential lapilli phlogopite— thin sections titano magnetite-] sphene clinopyroxene-| amphibole-i | pheno- u crysts 3 U 6 MLST C3 I

MLST C2 I

MLST 01

MLST B

MLST A

LLST

ground -mass 15

sanidine- plagioclase-l I hauyne -J cancrinite nepheline vol - % 60 80 100 50 60 70 80 90 100 vol -% i _l I I i i L_

Figure 5. Modal composition of essential phonolite lapilli in the near-vent reference section of Laacher See Tephra. The diagram shows (a) the variation in bulk phenocryst content, and (b) the composition of the phenocryst assemblage, both cumulative. Simplified stratigraphie section on the left.

See ash can possibly be traced in the Baltic area is defined by the southern 1968) (Fig. 1, site no. 106). The heavy-mineral composition of colluvial boundary of the Scandinavian ice shield 11,000 yr B.P. (Fig. 1). deposits in Luxembourg, Belgium, and the southern Netherlands as far The width of the southern depositional fan is also difficult to define. west as the Paris basin, containing abundant clinopyroxene, amphibole, Laacher See Tephra extends southward over the Alps and is traced as and sphene (Lucius, 1961; Gullentops, 1952; Jungerius and Reizebos, much as -600 km distance from the source (Torbiere di Trana, Torino, 1976; Juvigne, 1977, 1980), however, suggests that LST extended much Italy) (Fig. 1, site no. 98). Northern Italy, southeastern France, Sardinia, farther westward. Corsica, the Balearic Islands, and the surrounding Mediterranean Sea are promising areas to look for Laacher See Tephra in the future. Criteria To Correlate Laacher See Tephra A third minor fan (SW-fan) is here postulated to extend from Laacher See Volcino to the west-southwest, based, however, on only three Ahrens (1929), who first recognized the bilobate distribution af LST primary sites of Laacher See Tephra. In the SW-fan, LST ash is traced as south and northeast of Laacher See Volcano, thought that the two direc- much as 100 km from the vent (Vance, Belgium; Hulshof and others, tions reflected the "shooting-direction" of inclined vents. Inclination of the

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vents of Laacher See Volcano has been postulated since Blenke (1880). contains nepheline and cancrinite (Worner and Schnincke, 1984a). The Today, elongate fallout fans produced by explosive volcanic eruptions are modal analysis of essential lapilli from different LST eruptive phases forms commonly interpreted in terms of prevailing paleo-wind directions at the a prerequisite for the discussion of heavy-mineral data. time of eruption (Eaton, 1963; Porter, 1981). In case of complex isopach The petrographic composition of essential lapilli was therefore de- patterns, however, interpretation of tephra deposits in terms of paleo-wind termined quantitatively by point counting and image analysis of thin sec- directions is beset with several major problems, as exemplified by the tions. The results (Fig. 5) show the following. Laacher See Tephra. Theoretically, the three main depositional lobes 1. The total phenocryst volume in essential clasts increases drasti- could represent: (1) three different wind directions at different altitudes cally from LLST (<2 vol%) to ULST (>45 vol%). throughout the eruption, (2) rapidly changing wind directions during a 2. The ratio of felsic versus mafic phenocryst phases decreases unsys- continuous eruption, or (3) the prevailing wind directions at three different tematically from LLST (7.5) to MLST C (3.5). This ratio shows maximum eruptive phases separated in time. We will attempt to solve this problem values within basal ULST lapilli (8.5), then decreases again toward the top by a study of the tephra composition. of the ULST section (3.2). Laacher See ash layers in the distal NE-fan have been described as 3. The sanidine/plagioclase ratio decreases almost by an order of white to light gray, those of the distal S-fan as darker gray to olive-brown- magnitude from LLST (26) to ULST (3). gray (Frechen, 1952; Hofmann, 1963; Martini, 1971), suggesting different 4. Clinopyroxene/amphibvole ratios in Laacher See magmas do not petrographic and chemical composition in different depositional areas. decrease toward deeper magma levels (toward the top of the tephra sec- Refractive indices of glass shards and the heavy-mineral compositions of tions, respectively) as postulated by Frechen (1953, 1976), but vary from the ash layers (their clinopyroxene/amphibole ratios) indicated that the 0.3 in LLST to 0.5 in MLST C up to 4.5 in uppermost ULST lapilli, thus southern ashes are more mafic than those in the NE-fan (Frechen, 1952; supporting increasing clinopyroxene concentrations in successively erupted Martini, 1971). Near-vent isopach maps of deposits from different eruptive Laacher See magmas, as suggested by Schmincke (1977). phases show that transport directions of LST ash clouds occasionally changed dramatically within the proximal fades (Bogaard and Schmincke, Composition of Ash Fractions 1984). It is, therefore, necessary to determine the transport directions independently of the near-vent isopach maps by analyzing the ash composi- Distal Laacher See Tephra consists exclusively of ash-sized particles tion in the different fans petrographically and chemically. (grain diameters <2 mm). Criteria to identify and correlate distal ash The Laacher See Tephra was erupted from a strongly composition- deposits from specific eruptive phases thus refer to compositional differ- ally zoned, phonolitic magma column (Worner and Schmincke, 1984a, ences within the matrix ash of proximal coarse LST deposits. 1984b). This pronounced zonation is preserved in near-vent tephra sec- Components. Sieve fractions 63-125 /xm of near-vent lapilli and ash tions and reflected in (a) the bulk rock chemistry of essential lapilli; layers contain lithic fragments, loose crystals, and vitric shards. Relative (b) type, composition, and amount of phenocryst phases within the lapilli; amounts were determined from grain mounts by point counting 300 to (c) the petrographic composition of the matrix ash; and (d) the chemical 700 gTains per sample. Relative frequencies of lithic fragments (Lower composition of the vitric groundmass of lapilli and vitric particles in the Devonian slates and Quaternary basalts) range from 3% to 17% in Plinian ash fraction. Due to vertical and lateral migration of the vent and strongly fallout pumice layers (LLST, MLST B, MLST C) but are considerably fluctuating eruptive mechanisms throughout the eruptive history, deposits higher in the phreatomagmatic surge and fallout deposits of the LLST*, from different eruptive phases contain varying types and amounts of acci- MLST A, and ULST eruptive phases (10% to 49%). dental lithic fragments, derived from the magma chamber's wall and roof. The crystal fraction consists of light and heavy mineral species. Sani- dine, plagioclase, and hauyne represent former phenocrysts of the LST Composition of Essential Lapilli magma, while abundant quartz is derived from the fragmentation of Lower Devonian sandstones. The heavy-mineral assemblage consists of Essential lapilli in Laacher See Tephra are moderately to highly clinopyroxene, amphibole, sphene, apatite, titanomagnetite, phlogopite, differentiated phonolites (Thornton-Tuttle differentiation index DI = 76 to and olivine, derived from the eruptive phonolite magma as well as older 93), comparable to phonolitic rocks from the French Massif Central, alkali basalts. The total amount of crystals is significantly higher in LLST* Vesuvius, and Gran Canaria. Early erupted LLST magmas are of agpaitic surge deposits (14%-21%) than in associated LLST fallout tephra

peralkalic composition, and strongly enriched in Na20 and MnO, volátiles (3%-5%), and it increases systematically toward ULST (up to 47%). (H20, F, CI), and magmatophile trace elements (Worner and Schmincke, The vitric shards consist of colorless (LLST to MLST C), pale olive- 1984a, 1984b). The degree of differentiation decreases systematically brown (MLST C to ULST) or dark brownish (ULST) glass, ranging from throughout MLST A, MLST B, and MLST C magmas, with the strati- vesicle-rich (LLST to basal ULST) to vesicle-poor (LLST*, MLST A, graphic boundary between MLST C and ULST deposits reflecting the MLST C, ULST) to finally vesicle-free (top ULST). Its relative amounts in transition from agpaitic LLST and MLST phonolites to predominantly the 63-125 /xm ash fraction vary from 70% to 96% in LLST and MLST miaskitic phonolites in ULST. Upper Laacher See Tephra shows increas- Plinian fallout layers, down to 45%-48% in LLST* and MLST A surge ingly higher concentrations in FeO, MgO, and CaO as well as compatible deposits, reaching some 9% in uppermost ULST surge deposits. trace elements (Cr, Ni, Sc, and Co). The last erupted, most mafic ULST In summary, the matrix of proximal LST pumice lapilli layers con- magmas are interpreted to be mixtures between basanite and phonolite sists mainly of vitric ash (LLST, MLST B, MLST C) or lithic-rich vitric magmas (Worner and Wright, 1984). In near-vent tephra profiles, this ash (LLST*, MLST A). The matrix ash of ULST lapilli beds is either chemical zonation is reflected in the color of essential lapilli, which are crystal-rich vitric ash or crystal-rich lithic ash or crystal ash (Fig. 6). white in LLST to MLST B deposits, change abruptly to greenish-gray at the MLST B / MLST C boundary, and become darker gray and finally Heavy Minerals black in the Upper Laacher See Tephra. Phenocryst phases in Laacher See phonolite are: sanidine, plagioclase, Differences in heavy-mineral composition of ash layers have been hauyne, clinopyroxene, amphibole, sphene, titanomagnetite, phlogopite- used to characterize and correlate deposits from discrete eruptions (Fre- biotite, and olivine. The most highly differentiated LLST phonolite also chen, 1952, 1953; Kittleman, 1973). The interpretation of heavy-mineral

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from 50-90 in LLST to 1.0 in uppermost ULST deposits, and postulated Glass that the heavy mineral composition of individual LST beds was constant A vitric ash over large distances (Frechen, 1962) or that it changed with dstance B lithic ash (Frechen, 1976). The clinopyroxene/amphibole ratio has since become C crystal ash the criterion most widely, but seldom successfully, used to correlate distal Laacher See ash deposits. We separated heavy minerals from sieve fractions +3 phi and +4 phi (125 to 250 /nm and 63 to 125 /xra) of LST matrix ash samples, using bromoform as a heavy liquid, and we counted the relative amounts of the different mineral phases in grain mounts. We found that clinopyroxene ranges from 37% in ULST to 94% in LLST samples, but that amphibole shows minimum values in LLST (4%) and maximum values in ULST (50%). Sphene ranges from less than 0.1% (LLST) to more than 11% in ULST samples. Among the accessory heavy minerals, phlogopite/biotite dominates in LLST and ULST, but apatite, in MLST B and MLST C ash. LLST and MLST A layers contain up to 4% olivine crystals. Clinopyrox- ene/amphibole frequency ratios range from 7 to 25 in LLST, 3 t3 12 in MLST A, 2 to 6 in MLST B, and 0.7 to 1.6 in MLST C and ULST Lithics Crystals samples. The data confirm that, within LLST matrix ash, amphibole becomes Figure 6. Pétrographie composition of matrix ash samples from enriched compared to clinopyroxene toward the top of the stratigraphic lapilli layers within the near-vent reference section of Laacher See section, as shown by Frechen (1953). Relatively high amounts of olivine in Tephra. Sieve fraction 63-125 fim. (1) LLST and MLST A surge the LLST and MLST A samples strongly suggest, however, that only part deposits, (2) LLST airfall deposits (3) MLST B airfall deposits, of the LST matrix ash crystals are essential components, as olivine c.oes not (4) MLST C airlall deposits, (5) ULST surge and airfall deposits. occur as phenocryst phase in LST essential lapilli except for the most mafic ULST magmas (Fig. 5)! Clinopyroxene/amphibole ratios in essential clasts, moreover, show exactly the opposite trend compared to matrix ash abundances in tephra is, however, not straightforward (Bogaard and data. This shows that the heavy-mineral composition of LST ash :>amples Schmincke, 1985). Heavy minerals in near-vent LST were analyzed by is strongly influenced by the influx of "accidental" mineral phases (that is, Frechen (1952,1962,1976), who argued that glass shards, pumice clasts, clinopyroxene and olivine) derived from the fragmentation and incorpora- and crystals were essential components of the Laacher See Tephra, and he tion of xenolithic leucitite, basanite, and tephrite lava and tuff, which are postulated that the mineral content of both essential lapilli and matrix ash rich in clinopyroxene and olivine phenocrysts (Bogaard and Schmincke, changed simultaneously with height in the tephra profiles. Frechen (1952, 1985). 1962) found that the clinopyroxene/amphibole ratio in LST ash decreased In order to use the heavy-mineral composition as a correlation tool,

Amphibole 20 30 i, 0 50 60 Figure 7. Relative frequencies of clinopyroxene, amphibole, and sphene heavy • ^ minerals, in matrix ash samples (+3 phi and +4 phi) from the near-vent reference section of Laacher See Tephra. Section of the ternary cpx- amph-sph diagram (s«!e index diagram). (A) Individual measurements (open circles = LLST, triangles = MLST A, squares = MLST B, crosses = MLST C, black dots = ULST). (b) Compositions of eruptive units LI,ST to ULST including absolute er- rors of measurement!..

70 60

Clinopyroxene cpx omph

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we prefer to use the relative frequencies of clinopyroxene, amphibole, and sphene rather than the clinopyroxene/araphibole ratio. This is much more diagnostic, as sphene is nearly absent from LLST deposits and not present among xenocrysts. The heavy-mineral compositions (cpx : amph : sphen) of the near-vent reference section samples are shown in Figure 7A. In addition, the absolute error of each percent value—depending on both the number of grains counted per sample, and the relative percentage of a component—was calculated according to Van der Plas and Tobi (1965). This transforms each discrete point in the ternary diagram into a hexagonal field which includes the analytical error. The marginal boundaries of the clusters of such hexagons are drawn in Figure 7B for the stratigraphic units and subunits of Laacher See Tephra. The diagram illustrates that heavy- mineral analysis in the case of the Laacher See Tephra is not a very suitable method for correlation purposes, as the fields for the different stratigraphic units are widely overlapping. It is impossible, furthermore, to distinguish LLST from MLST A, MLST A from MLST B, and MLST C from ULST if only clinopyroxene/amphibole ratios are used.

Physical Properties of Vitric Shards

Glass particles in volcanic-ash deposits are widely used for tephrostratigraphic purposes. Physical parameters, such as refractive indices of glass shards, were measured to characterize and correlate volcanic ash layers (Frechen, 1952; Ewart, 1963; Kittleman, 1973). The refractive index of natural glass, however, not only reflects the primary glass chemistry, it also depends on the state of hydration: increasing amounts of secondary H2O in silicic glass shards result in increasing refractive indices and thus may indicate chemical compositions which are "too mafic" (Ross and Smith, 1955). Refractive indices of LST glass particles determined by Frechen (1952) range from 1.515-1.520 (MLST) to 1.530-1.535 (ULST). Our measurements show refractive indices of 1.511-1.517 for LLST glass particles, 1.518-1.522 for MLST, and 1.523-1.526 for ULST glass shards. We do not use the refractive indices of glass particles for correlation purposes in this paper, because (a) differences between LST members and beds are very subtle and (b) the highly varying states of hydration of fine-grained distal ash material would be expected to cause inconclusive results. Matrix ash glass particles from different eruptive phases at Laacher See Volcano show highly variable clast morphologies, and they differ in external shape, color, and internal vesicle structure, resulting from the chemical composition and rheology of the erupted magma, as well as different (hydroclastic and pyroclastic) eruption and fragmentation processes, as discussed in more detail by Bogaard and Schmincke (1985b). At least eight morphological types of clasts can be distinguished in Figure 8. Glass shards in LST matrix ash samples 63-125 jum. grain mounts (Fig. 8). Vesicle-rich pumiceous clasts with spherical bubbles Figures 8a through 8g are transmission light photomicrographs of (type a) occur throughout the entire interval from LLST to the base of particle types. See text for detailed descriptions. ULST. Vesicle-rich pumiceous clasts with pipe-like elongated bubbles (type b2) are typical for LLST and MLST B deposits. The external shape ULST) are preferably vesicle-poor and/or equant, contrasting with more of these two, generally colorless glass shard types, is determined by densely irregular and vesicular shards formed during pyroclastic eruption mecha- packed, open (burst) vesicle cavities. Only very few, colorless, vesicle-rich nisms (LLST, MLST B, MLST C). clasts have spherical external shapes with smooth surfaces (type e). These are restricted to LLST deposits. Equant vesicle-poor clasts showing Major-Element Composition of Glass Shards strongly elongate vesicle-pipes with aspect ratios >50:1 (type bl, generally colorless) are abundant in LLST* and MLST A ash. Other vesicle-poor Major- and trace-element compositions of glass shards are fundamen- shards show small (type d; LLST*, MLST A, MLST C, ULST) or large tal criteria in modern tephrostratigraphy (Smith and Westgate, 1969; (type c; MLST C and ULST), spherical to oval vesicles in colorless to pale Sarna-Wojcicki, 1976). We measured the major-element composition of brownish glass; the external shapes of the clasts are determined by rela- LST glass shards from 15 layers throughout the entire near-vent reference tively thick broken vesicle walls. Equant clasts may also show very few or section, on polished thin sections of grain mounts, using an energy disper- no vesicles within colorless glass (type f; LLST*, MLST A, ULST), or pale sive electron microprobe. The number of analyses ranges from 30-50 to dark brown glass (type g; only ULST), so that the external shape of the grains per layer in LLST, MLST A, and MLST B to 20-30 grains per layer clasts is predominantly determined by conchoidal cracks. Thus, shards in MLST C and ULST deposits. Representative average glass composi- formed during phreatomagmatic eruptive phases (LLST*, MLST A, tions of LST members are shown in Table 1.

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TABLE 1. REPRESENTATIVE AVERAGE CHEMICAL COMPOSITIONS OF LST ESSENTIAL GLASS SHARDS

Highly evolved phonolite Mafic phonolite base LLST top MLST B base MLST C top ULST N (39)" (29)' (27)' (30)'

Si02 57.84 ± 0.69 56.37 59.55 ± 0.70 60.48 ± 0.55 59.86 ± 0.77 Ti02 bd 0.14 0.10 ± 0.07 0.18 ± 0.07 0.43 ± 0.23 A1203 22.85 ± 0.26 22.08 21.48 ± 0.28 21.00 ± 0.17 20.18 ± 0.46 FeO 1.56 ± 0.07 1.64 1.82 ± 0.14 1.92 ± 0.23 2.33 ± 0.33 MnO 0.45 ± 0.05 0.51 0.07 ± 0.08t 0.02 ± 0.05t 0.01 ± 0.04Î MgO bd 0.03 0.02 ± 0.05Î 0.06 ± 0.09t 0.20 ± 0.25t CaO 0.32 ± 0.03 0.39 1.04 ± 0.13 1.41 ± 0.27 1.80 ± 0.61 NajO 11.35 ± 0.75 13.08 8.59 ± 0.97 7.42 1 0.64 7.15 ± 0.80 KjO 5.37 ± 0.33 4.87 6.99 ± 0.42 7.21 ± 0.47 7.80 ± 0.46 P205 nd 0.01 nd nd nd h2o* nd 0.58 nd nd nd COj nd 0.08 nd nd nd s nd 0.01 nd nd nd a 0.26 ± 0.14 0.20 0.34 ± 0.06 0.29 ± 0.05 0.23 ± 0.08 100.00 100.00 100.00 100.00 100.00

itt 92.1 ± 2.3 99.2 94.6 ± 3.3 95.2 ± 2.3 96.1 ± 1.2

'microprobe analyses bd = below detection limits tmany analyses below detection limits • • XR F analysis nd = not determined ^normalized total N = number of analyses tf original total of microprobe analyses

The average Si02 content of glass shards increases systematically very steep gradients—or even indicate a compositional gap—at the strati- from basal LLST toward MLST C but varies only unsystematically in the graphic boundary between MLST B (uppermost white pumice lapilli) and MLST CI to ULST section. FeO, K2O, and CaO show a similar behavior. MLST C (first greenish-gray pumice lapilli) (Fig. 9). Glass compositions of TiC>2 and MgO concentrations are below detection limits in LLST glass eruptive phases LLST to MLST B are therefore termed "highly evolved shards but increase significantly from MLST CI to ULST. AI2O3 and phonolitic," those of MLST C and ULST "mafic phonolitic" from here on. Na20 show decreasing average concentrations from LLST to ULST. Standard deviations are generally high within mafic phonolite samples, MnO shows maximum values in LLST, decreases toward ULST, but falls while LLST to MLST B samples show more homogeneous populations— below detection limits from MLST CI on. Average totals of analyses range with one major exception: Glass compositions in LLST II are strongly from 92 to 94 wt% in LLST to 95 to 98 wt% in ULST, reflecting both high bimodal, including colorless sodium-rich highly differentiated phonolitic

concentrations of HzO (primary and secondary) and losses of Na during glass shards, and rare pale brownish vesicle-poor shards, whose mafic microprobe analysis. Most of the oxides (Na20, K20, and CaO) show phonolitic composition closely resembles ULST chemistry (Fig. 9). The

Stratigraphie Composition of Number of units and beds glass shards analyses —1—T I ! 1 1 1 1 1 1 1 1 1 1 1 1 XX 1 32 • XX * 30 XIX 1—•—1 h» — 27 • xvrn 1— 27 XVII. 1—•— 19 XVI 1—•—1 I-«—1 24 XV 1 •- 20 Figure 9. Chemical zonation of Laacher C2 XV 1 • 1 1—•—1 26 XIV See Tephra (ash-sized glass shard.'! in the H*—1 20 near-vent reference section). Mean values Ci XIV • 27 (dots) and standard deviations (bars) for 3 oxides and 17 samples throughout eruptive XIII • 1 • 1 29 phases LLST to ULST. Note bimodal composition of LLST II with highly differentiated B XIII • 1—•—1 h»H 33 XII - 1 • 1 l-H I* 44 (black dots) and mafic phonolithk (open dots) compositions.

A IX (-•H 48 V 1 • 1 h-*—( hH 34 L L S O T II CI IS h*H KO- l—O 18,13

I [wt.-%] _ • 39 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 7 8 9 10 II 6 7 8 À .8 1.2 1.6

Na20 K20 CaO

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and the data are compared in cartesian or ternary diagrams (Smith and Dato-points per Westgate, 1969). Statistical methods using concentrations of up to 20 unit area "E" different oxides simultaneously have also been applied (Borchardt and others, 1971; Sarna-Wojcicki, 1979; Taylor and others, 1981). Of the ten oxides measured in LST glass shards, T1O2, MgO, MnO, and CI are beyond detection limits in a wide stratigraphic range and are thus of little use. Of the other oxides measured, from the base (LLST I) to the top (ULST XX) of the LST section, the following changes were observed: Si02 increases by 2.8% absolute or by 5% relative FeO increases by 0.8% absolute or by 50% relative K2O increases by 3.1% absolute or by 57% relative CaO increases by 1.5% absolute or by 500% relative

Al203 decreases by 2.8% absolute or by 12% relative Na20 decreases by 4.7% absolute or by 42% relative

K20 and Na20 with pronounced absolute gradients, and CaO with a pronounced relative gradient, are thus most suitable for tephrostratigraphic correlation purposes. Relative percentages of these oxides were normalized to 100% and plotted in ternary diagrams. For each eruptive phase, the number of data points per unit area was counted, and areas of equal point density were constructed (Fig. 10). The contours enclose fields with more Figure 10. Na20-K20-Ca0 ratios of major-element analyses of than 2, 5, and 10 data points per unit area "E." Glass compositions of the glass shards from the near-vent reference section of LST. Frequency distal ash deposits will be compared to the "2-data-points reference fields" distributions of data points for (a) highly differentiated phonolitic in Figure 10. LLST (57 analyses), MLST A (82 analyses), and MLST B (106 analyses); and (b) mafic phonolitic MLST C (115 analyses) and ULST Composition of Distal Ash Deposits (142 analyses) samples. Contoured reference fields are for 2,5, and 10 datapoints per unit area (E). Samples of LST ash from distal NE-, S-, and SW-fans were treated

with H202 to remove organic matter, and the +4 phi grain size fraction latter are here interpreted as fragmented, mafic phonolite droplet inclu- (63-125 /um) was separated, using micromesh sieves. Bulk component sions, which form an accessory component of highly evolved LLST mag- analyses of samples (glass : lithics : crystals) were done on grain mounts. mas (Worner and Schmincke, 1984b). Heavy minerals were extracted with a centrifuge, using bromoform, Chemical correlations of tephra layers are commonly based on a mounted in smear slides, and point-counted. Polished thin sections from comparison of the concentrations of 2 or 3 oxides thought to be "critical," grain mounts of the light fractions served to estimate relative amounts of

Figure 11. Pétrographie composition of distal ash samples (63-125 fim from NE-, S- and SW-fans. Numbers are sample numbers. Additional data for samples (m) La Grande Buge, (k) Coinsins, and (j) Poisy are from southeastern France (Martini, 1971).

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Figure 12. Heavy-mineral analyses of distal LST ash samples. Polygonal reference fields show the near-vent, heavy-mineral compositio n of eruptive units LLST to ULST according to Figure 7B. Numbers are sample numbers. List of additional foreign data (site numbers iri parentheses): NE-fan: (a) = Luttersee (5), (b) = Giessen (1), (c) = Wallensen (6), (e) = Aschersleben (near site no. 9). All data by Frechen (1952). (0 = Rotes Moor (4) (Beug, 1957); (g) = Kirchhain-Amöneburg (3) (Lang, 1954). S-fan: (h) = Erlenbruckmoor (near sites no. 49, 52) (Frechen, 1952). (i) = Baulme, (k) = Coinsins, (1) = Genin, (m) = Poisy, (n) = Veigy. All data from southeastern France by Martini (1971). (o) = Erzwieler-Stammheimerriet, (p) = Wildert, (q) = Lützelsee (62). All data from eastern Switzerland. Mineral separates by F. Hofmann. SW-fsin: (r) = Boos Maar, (s) = Moosbrucher Maar, (t) Dreiser Weiher, (u) = Schalkenmeerener Maar, (v) = Miirmes, (w) = Hitsche (11)6), (x) = Hinkelsmaar. All data from West Eifel maars near site nos. 115,116 (Jungerius and others, 1968). (z) = Vance (Southern Belgium; Hulshof and others, 1968). (El) = Boos Maar (200-206 cm), (E2) = Boos Maar (213-220 cm), (E3) = Dreiser Weiher (203-205 cm), (E4) = Moosbrucher Maar, (E5) = Ellscheid, (E6) = Oberwinkeier Maar, (E7) = Immerater Risch, (E8) = Spriiker Maar, (E9) = Schalkenmerener Maar, (E10) = Mürmes (385 cm and 390-400 cm), (Ell) = Dürres Maar, (E12) = Hitsche (116), (El 3) = Strohner Maarchen, (E14) = Trauzberg, (E15) = Hinkelsmaar (-412 cm, 412-429 cm, and 432-433 cm). All data from West Eifel maars near site nos. 115,116 (Erlenkeuser and others, 1972).

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Figure 13. Major-element composition (Na20: K20: CaO) of glass shards from distal NE-fan deposits. Reference fields according to Figure 10. Numbers are sample numbers. Symbols represent individual analyses. Sample no. 170 = base, sample no. 175 = top of Kirchhain-Amoueburg section. See Figure 7 for location of samples/sites.

morphologically different glass shards and to analyze the major-element dark ash bed to at least 330-km distance and consists of a single, macro- composition of the glass shards with an electron microprobe. Where dif- scopically homogeneous ash layer at least 800 km from the volcano ferent beds within a distal tuff layer were macroscopically distinguishable (Frechen, 1952; Müller, 1959; Kleissle and Müller, 1969; Kliewe, 1969; by color or grain size, these were sampled separately. The correlations of Usinger, 1978). Finely dispersed ash material in marine, soft clays is traced distal LST ash deposits with certain eruptive phase (or stratigraphic units) up to Gotland (Sweden), 1,100 km distance from the vent. We analyzed of the near-vent reference section are chiefly based on the major-element LST samples from Kirchhain-Amöneburg (site no. 3; Fig. 1) Luttersee (site composition of vitric shards. The lithologic composition of the ashes no. 5), Pechsee and Tegeler Forst (Berlin; site nos. 12, 13), Vallensgârd (amounts of vitric shards, lithics, and crystals), heavy-mineral data, and Mose (Bornholm, Denmark; site no. 39), and Gotland (Sweden; site no. glass-shard morphologies are used as additional criteria. 40). The results are summarized in Figure 11 (lithologic compositions), Figure 12 (heavy-mineral compositions), Figure 13 (chemical composi- Ash Layers in the Distal NE-Fan tions), and Table B (see footnote 1 above concerning GSA Data Reposi- tory; glass-shard morphologies). Laacher See Tephra in the NE-fan forms complexly bedded profiles The northeastern, main fan of Laacher See Tephra consists of ash at least as far as 120 km distance. It is divided into a basal light and upper material from at least three chemically distinct eruptive phases, as the total

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population of glass compositions shows three frequency maxima in the more than 1/3 of the total LST thickness even at much greater ¿stances, ternary Na20-K20-Ca0 diagram (Fig. 13): two inside the fields of highly but ULST deposits (termed "LST-5" in Frechen, 1952, 1953) were not differentiated phonolitic compositions and one in the field of mafic phono- encountered in the distal NE-fan. litic eruptions. Data points cluster in or close to the near-vent reference The heavy-mineral diagram (Fig. 12) shows that none of t ie three fields of LLST, MLST B, and MLST C eruptive phases, but MLST A and eruptive phases can be unambiguously identified on the basis of i:he rela- ULST compositions are strongly underrepresented. The glass compositions tive frequencies of clinopyroxene, amphibole, and sphene, or even using therefore sugges : that the NE-fan was deposited by LLST, MLST B, and simple clinopyroxene/amphibole ratios. The data only confirm that the MLST C eruptions. Few data points located in the MLST A and ULST NE-fan is dominated by ash whose heavy-mineral composition is typical reference fields are interpreted in terms of analytical scatter and weathering of the highly differentiated phonolitic section (LLST to MLST B) of part of the glass shards (loss of Na). The lithologic compositions of the At 600 to 800 km from Laacher See, the NE-fan seems to show a ash layers fit our interpretation, as all deposits in the NE-fan are (crystal- bimodal composition, with LLST ash dominating at the north western bearing) vitric tuffs (Fig. 11). As much as 120 km from the volcano, all flank (Bornholm site), but MLST B ash dominating in the central axis and three eruptive pliases are represented within one chemically zoned LST at the southeastern flank (Berlin sections) of the entire fan (Figs. 1, 13). profile, condensed to a thickness of 15 cm (Kirchhain-Amöneburg section; Ash material from both eruptive phases is traceable up to 1,100 km samples no. 170-175). The LLST does not pinch out completely in the distance, although most of the glass particles in the Gotland sample were Westerwald area, as postulated by Frechen (1953, 1962) but makes up found to be strongly altered and thus not suitable for electron-microprobe

Figure 14. Major-element composition (Na20: K20: CaO) of glass shards from distal S-fan deposits. Reference fields according to Figure 10. Numbers are sample numbers. Symbols represent individual analyses.

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analysis. Rare mafic phonolitic glass shards that occur alongside highly ies, Martini (1971) correlated LST ash from the Lake Geneva area (Swit- differentiated ones within the Bornholm sample (sample no. 82; Fig. 13) zerland, France) with final ULST eruptions. In our study, LST samples are interpreted as fragments from mafic liquid inclusions in the LLST from the northern Black Forest (site no. 44), the southern Black Forest magma, as such clasts are also accessory components in the near-vent (site nos. 49,52), the Vosges (site no. 99), eastern and central Switzerland reference samples of LLST deposits. (site nos. 60, 75, 82, 76, 85), eastern France (site no. 97), and northern MLST C ash forms the uppermost dark colored part of the Italy (site no. 98) were analyzed by the methods described above. Kirchhain-Amoneburg and Luttersee sections, and also differs from the The S-fan of Laacher See Tephra shows a complex structure with ash below in heavy-mineral compositions with higher amphibole and principally different ash compositions east and west of the entire fan's sphene frequencies (samples no. 58, 173, 174; Fig. 12). It is not clear, symmetrical axis: LST in the eastern flank of the S-fan (samples 49,47,48, however, if MLST C ash participates significantly in NE-fan deposits at 50, 29, 54, and 206) consists of crystal-poor to crystal-bearing vitric tuff. distances greater than some 500 km. Deposits in the western flank (samples 80,199, and j, k, and m by Martini, In summary, the development of Laacher See Tephra, from com- 1971) are crystal tuffs (Fig. 11). Samples from both depositional areas posite compositionally zoned condensed profiles at distances up to 120 km contain glass shards with mafic phonolitic compositions. Samples in the to single homogeneous-looking ash beds at greater distances, seems to eastern flank also contain highly differentiated phonolitic glass shards be the result of (a) lateral displacement of depositional fans within the (Fig. 14). Heavy-mineral data of eastern ash samples accumulate inside the eruptive succession (LLST versus MLST B) and (b) rapid pinching-out MLST C reference field or in the overlapping regions of MLST C and of deposits from other eruptive stages (MLST C), as well. ULST or MLST C and MLST B, respectively (Fig. 12). A small sub-group of analyses plots in the LLST-MLST A overlapping area. Western ash Ash Layers in the Distal S-Fan samples show higher amphibole abundances but also plot within the MLST C-ULST overlapping region. Once again, the heavy-mineral data In the southern fan, too, Laacher See Tephra develops with distance give only a very crude impression of which eruptive phases of LST pro- from layered, condensed profiles (up to 100 km; Sonne and Stohr, 1959) duced the southern depositional fan. into bipartite ash beds (up to 300 km) and finally into macroscopically In summary, chemical compositions of LST ash from the distal S-fan homogeneous single ash layers (up to at least 600-km distance). According in the Na20-K20-Ca0 diagram (Fig. 13) cluster in exactly those reference to Frechen (1952), Laacher See Tephra in the Black Forest is related to fields that are barely covered by data from the NE-fan. One group of data MLST A and/or MLST CI eruptions, and, based on heavy-mineral stud- points concentrates inside or close to the MLST A contour line, with only

Figure 15. Mqjor-element composition (Na20: KjO: CaO) of glass shards from distal SW-fan deposits. Reference fields according to Figure 10. Numbers are sample numbers. Symbols represent individual analyses.

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minor scatter toward LLST and MLST B compositions. The highly differ- mineral assemblage (sanidine, plagioclase, hauyne, clinopyroxcne, amphi- entiated pho oolitic ash from the eastern flank of the S-fan is therefore bole, sphene, titanomagnetite, olivine, and apatite), and are intercalated in correlated to MLST A eruptions. Mafic phonolitic glass particles from undisturbed, post-maar-volcanic epiclastic and organic sediments. Two both the eastern and western flank plot within the overlapping region of samples of the West Eifel LST ash layer (Meerfelder Maar, site no. 115; MLST C and ULST reference fields (Fig. 14). Using the lithological Hitsche, site no. 116), and one sample from eastern Belgium (Konnerz- composition of the ash samples (Figs. 6, 11) as an additional criterion, venn, site no. 117) were available during our study. however, we interpret the eastern crystal-poor mafic ashes to be derived Both West Eifel samples are crystal tuffs (Fig. 11), and heavy-mineral from MLST C eruptions but the western crystal-rich mafic ashes as depos- compositions plot into or near to the ULST reference field, as do most of its from ULS T eruption clouds. This is also confirmed by abundance of the West Eifel samples analyzed by Jungerius and others (1968) and ULST-type glass shards (vesicle-poor to nonvesicular, often pale to dark Erlenkeuser and others (1972) (Fig. 12). Glass shards are predominantly brown) only in the western samples. It is difficult, however, to decide vesicle-poor, pale to dark brownish ULST-type clasts, and exclusively of

whether mafic glass shards in sites near the S-fan's symmetrical axis (sam- mafic phonolitic composition. Na20:K20: CaO values plot within the ples no. 26,135-139, and 206) can be related to either MLST C or ULST overlapping ULST and MLST C reference fields (Fig. 15). Lithologic eruptions. compositions, heavy-mineral contents, glass-shard morphologies, and chemical compositions unambiguously relate these ash deposits to the Ash Layers in the Distal SW-Fan ULST eruptive phase of Laacher See Volcano. Even though the chemical composition of the other West Eifel ash layers (summarized in Fig. 13) has The presence of Laacher See ash southwest of Laacher See Volcano not yet been studied, its uniform heavy-mineral compositions indicate that was first detested in post-volcanic sedimentary fillings of older maar cra- (a) all Allerod-dated phonolitic ash layers in West Eifel maars are derived ters in the West Eifel Volcanic Field (Jungerius and others, 1968), and late from Laacher See Volcano and (b) all are related to the ULST eruptive Quaternary sediments in South Belgium (site no. 106; Fig. 1), and recently phase, as suggested by Jungerius and others (1968). This would imply, in late glacial peat deposits in eastern Belgium (Pissart and Juvigne, 1980). however, a reinterpretation of the late Quaternary stratigraphy and pollen The correlation of Jungerius and others (1968) was challenged, however, record in the West Eifel area. by Erlenkeussr and others (1972) who interpreted these ashes as being Analytical results are somewhat contradictory for the ash layer from derived from the West Eifel volcanoes, even though the ash layers ap- eastern Belgium: the heavy mineral composition is typical for a mafic peared to be rich in phonolitic (?) glass particles, contained the typical LST phonolitic LST eruptive phase (MLST C or ULST; Fig. 12), but the

Eruptiv« Direction of Transport direction Relative phase isopach axes height of eruption Neor Medial Criterio for identifi- column -vent distance cation of distal osh

IO-2km) (15-20 km) m ÏhwI fiSl Imp! Low High

SS• E s S (D ? sw sw MLST C3 r>d (TI © © Figure 16. Relative height of eruption columns and transport directions of LST ash during eruptive phases LLST l:o ULST. MLST C2 SE Sor NE S ¿AC ~~<$> (a) Deduced from proximal isopuch maps. I* * (b) Inferred from ash composition in distal MLST CI ® NE Sor NE NE fans, (a+b) Combining both lines of evidence. 11 ML5T B NE NE NE © 0~~

~~MLST A SE • • S S CD © JL LLST ® NE NE NE LLST* SE nd (?) rp major ton u minor lobe strong, • moderate, and • minor evidencJLe~~

~~J uncertain nd not detected L lithology HM heavy minerals SM glass shard morphology MP imcroprobe analysis of gloss shards~~

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chemical composition of vitric shards (only 4 shards analyzed!) is highly differentiated phonolitic (Fig. 15). We therefore suspect the composition of the Konnerzvenn ash to have been postdepositionally modified either by epiclastic reworking or by selective weathering of mafic phonolitic glass components, especially because the sample consisted of up to 80% crystals (sanidine, plagioclase, clinopyroxene, amphibole, and sphene), apart from some 20% lithic fragments, but less than 1% glass particles. Its origin from Laacher See Tephra eruptions, however, as suggested by Pissart and Ju- vigne (1980), seems to be confirmed by the chemical and mineralogical composition. In summary, analyses of LST ash from the West Eifel Field and eastern Belgium indicate that ULST ash was transported not only to the south, but also up to at least 100 km to the southwest, thus possibly forming a discrete depositional fan extending southwestward from Laacher See Volcano.

~~TRANSPORT DIRECTIONS AND PALEOWIND DIRECTIONS~~

Transport directions of ash clouds during different eruptive phases of Laacher See Volcano (Fig. 16) and an over-all model of the general structure of the Laacher See Tephra Formation in central and northern Europe (Fig. 17) are inferred from (a) proximal transport directions as indicated by isopach maps (Bogaard and Schmincke, 1984; Bogaard and others, 1985) and (b) the petrographical and chemical composition of the distal ash deposits. The proximal isopach map of LLST shows an east-northeast- directed main fan, and a minor lobe directed to the southeast. This fits ash deposits of LLST composition which have been detected in 4 sites within the distal NE-fan up to 1,100 km from the vent. In the distal S-fan, however, ash of such highly differentiated composition has not been detected unambiguously so far. This may be due either to a restricted areal extent of these deposits to the south, or due to selective strong weathering or alteration of extremely sodium-rich glass shards in LST deposits in the Black Forest and eastern Switzerland. Figure 17. Transport directions of Laacher See Tephra and struc- Near Laacher See Volcano, MLST A eruptions resulted in the forma- tural model of the composition of the distal NE-, S- and S W-fans. tion of a south- to southeast-directed fallout fan. Vitric tuffs of MLST A composition form an important part of LST sections in the Black Forest and central Switzerland as well. (Fig. 17), however, strongly depends on the lack of ULST deposits in the Isopach maps of the major fallout pumice beds of MLST B indicate area between the S-fan and the SW-fan. If Laacher See ash will be found proximal transport to the east-northeast. A southern minor lobe was not in appropriate thickness in this area, for example in Lorraine, a model with developed during this eruptive stage. Consequently, distal ashes of MLST only one ULST fan directed to the south-southwest would be preferable. B composition are found only in the NE-fan up to 1,100 km distance, The origin of the complex internal structure of the Laacher See predominantly in the center and eastern flank of the fan. Deposits from Tephra deposit is discussed in terms of two principally different models, MLST C eruptive phases CI, C2, and C3 are difficult to correlate due to invoking either (a) wind directions changing with time and eruptive suc- minor differences in chemical and mineralogical compositions. Relatively cession, or (b) different but stable wind directions at different altitudes in high sodium concentrations in mafic phonolitic glass shards from the the atmosphere. One major reason argues against model a, but several Kirchhain-Amöneburg and Luttersee sections (sample nos. 173-175; 57 reasons favor model b: in analogy to recent monitored explosive eruptions and 58; Fig. 13) suggest that these deposits were derived from MLST CI and based on estimates of erupted magma volumes and discharge rates eruptions. This correlation is confirmed by proximal northeastern trans- (Bogaard, 1983), the duration of Laacher See Volcano's Plinian eruptions port directions in this specific eruptive phase (Fig. 16). Vitric tuffs of (LLST to MLST C) is estimated to have lasted —10 hr. It is very unlikely MLST C2 and/or MLST C3 composition dominate in the eastern flank of that within this short time interval wind directions could have shifted from the southern depositional fan. Judging from proximal isopach maps, how- southwest to north, then to southwest, and finally back to north again. ever, only MLST C2 eruption clouds were blown to the southeast; MLST Eruptive phases of Laacher See Volcano differ in eruptive mechanisms C3 tephra was transported to the east. MLST C3 tephra has not been (magmatic or phreatomagmatic) and eruption column heights inferred confirmed chemically or mineralogically in the distal NE-fan. from dispersal indices of proximal isopach maps (high altitude: LLST, The isopach map of proximal ULST deposits shows a major fallout MLST B, and MLST CI eruption columns; low altitudes: LLST*, MLST fan directed to the south-southeast, and an ill-defined minor fallout lobe A, MLST C2, MLST C3, and ULST eruption columns). The obvious directed to the south-southwest. Mafic phonolitic crystal tuffs of ULST correlation between eruptive phases with high eruption columns and composition form the western flank of the distal S-fan, and a discrete (?) northeastern transport, but southern transport during eruptive phases with fan directed to the southwest. Our model of two individual ULST fans low eruption column heights, suggests that the LST eruption columns were

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emplaced in an atmosphere which was vertically zoned with respect to SW-fan consists exclusively of ULST ash. Within the transport range wind directions and velocities. Southwesterly winds at high altitudes analyzed, the development from complex proximal profiles to single distal (probably near the tropopause level, some 10 km above sea level) resulted ash layers is chiefly due to the lateral displacement of depositions! fans in the formation of the strongly elongate NE-fan, but northerly to north- from one eruptive phase to the other. Selective pinching-out of S|3ecific easterly winds at lower atmospheric levels produced somewhat broader stratigraphic units (for example, MLST CI) appears to be subordinate. fans directed to ttie south and southwest. Additional evidence for this Only deposits from LLST* and MLST C3 eruptions were not de tected model is provided by transport directions of tephra in the very vicinity of unambiguously within the distal depositional fades. Laacher See Volaino, which reflect the behavior of the eruption columns The petrographic and chemical composition of the distal ash deposits when intersecting the lower atmosphere (Fig. 16): the isopach axes of corresponds remarkably well to proximal transport directions that were deposits that were finally transported to the northeast are inclined to the determined for the different eruptive phases by isopach maps. The correla- southeast in the vicinity of the vent before bending toward their ultimate tion between northeastern transport of ash and high eruption columns, and northeastern transport directions, indicating shifting of the eruption col- southern transport of ash and relatively low eruption columns, imiicates umns by northerly winds in the lower atmosphere. that LST eruption columns were emplaced in an atmosphere which was vertically zoned with respect to paleowind directions: northerly winds CONCLUSION« dominated at lower altitudes; southwesterly winds dominated at higher altitudes. This also explains that isopach axes, even of eruptive phases Explosive Plinian and phreatomagmatic eruptions of Laacher See which show distal northeastern transport, are directed to the southeast in Volcano resulted in a late Quaternary marker bed widespread in central the vidnity of Laacher See Volcano, before bending toward the northeast and northern Europe: the Laacher See Tephra Formation. Volcanic ash at some 5-10 km distance from the vent. partings are preseived in lake sediments and peat-bog deposits as far north as Sweden, as far south as Italy, and as far west as Belgium, indicating ACKNOWLEDGMENTS transport and deposition of ash in three main fans radiating to the northeast, south, and southwest of Laacher See Volcano. Laacher See ash is This paper is part of a Ph.D. thesis by the first author, financially easily distinguished from time-equivalent, andesitic-rhyolitic tuffs in north- supported by grants no. ET-4236-A (Bundesministerium für Forschung ern Europe, and basaltic or trachytic tuffs in southern Europe by (a) the und Technologie) and no. 230/77/EGD (European Community Energy phonolitic composition of its glass shards and (b) sanidine and plagioclase Commission), which we gratefully acknowledge. Samples and/or impor- occurring as dominant light mineral phases and clinopyroxene, amphibole, tant hints as to the location of distal LST sites were kindly provided by B. and sphene occurring as dominant heavy mineral phases. Based on near- Ammann, T.S.T. Andersen, E. Bibus, S. Björck, S. Bortenschlager, K.-H. vent sections, the chemically and mineralogically zoned Laacher See Ehrenberg, B. Engberg, M.-J. Gaillard, F. Hofmann, R. Huckriijde, E. Tephra is divided into deposits from three main eruptive phases: the Juvigné, M. Küttel, G. Lang, J. Merkt, J. Negendank, H.-J. Pachur, I. Lower, Middle, and Upper Laacher See Tephra. These differ in eruptive Páhlsson, T. Poetsch, C. Pöhlig, P. A. Riezebos, S. Schloss, R. Schneider, and transport mechanisms as well as chemical and mineralogical composi- B. Stay, W. T. Stöhr, H. Usinger, S. Wegmüller, and M. Welten. We tion, ranging from highly differentiated to mafic phonolite. LLST, MLST, gratefully acknowledge electron microprobe facilities provided by H. and ULST are members of the Laacher See Tephra Formation. Wänke (Max-Planck-Institut, Mainz) and kind assistance during mea- None of the more than 200 proximal tephra sections, the distal ash surements by J. Huth. We also thank H. Niephaus (XRF-analysis), A. sections, or the radiocarbon ages for both near-vent and distal LST depos- Fischer (photographs), and G. Olesch and D. Dettmar (thin sections). its indicate any significant pause in the eruptive history of this only known explosive eruption of Laacher See Volcano. "Ash layers" described to REFERENCES CITED overly ULST deposits some 6 km southwest of Laacher See Volcano, and Ahrens, W., 1929, Die Verbreitung des mittelrheinischen alluvialen Bimssteins und daraus folgende Rückschlüsse auf interpreted as teplira derived from a Younger Dryas-aged second eruption den Eruptionsmechanismus: Centralblatt Mineralogie, v. 7, p. 288-296. 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