The present study deals with the possibilities of applying the zircon saturation thermometry,which is based on the equilibrium between the zircon crystals and the melt,to strongly altered volcanic ashes—bentonites.It proposes an alternative to a widely used method of calcu-lating magma temperature from Zr content and major component composition(Boehnke in Chemn Geol 351:324-333,2013),that is not suitable for bentonites,as most of the major components have been largely altered in these rocks.For calculating source magma temperatures in strongly altered volcanic ashes,the exponential function from the Zr(ppm)/Al2O3(％)ratio with compositional corrections from the TiO2/Al2O3 ratio was found applica-ble.The idea to use the ratios of these elements is based on the low mobility of these elements in the earth's surface conditions.Temperatures of magma,forming in the partial melting process,are assessed from the bulk rock compo-sition.Pre-eruption temperatures were estimated from the composition of fine fractions of bentonites.The accuracy of the new method was established from comparison with the method by Boehnke et al.(Chem Geol 351:324-333,2013).The difference between the two methods was mostly less than±30° to±50°.The comparison with the magma temperature,estimated from the sanidine composition,revealed 13° lower values on average.Although the pro-posed method for estimating the source magma tempera-tures is less precise than the method of accounting for detailed rock compositions,it can be used in strongly altered rocks,where other methods are not usable.The new method still enables differentiation between felsic source magmas originating at low or high temperatures.Early Palaeozoic bentonites in the Baltic Basin can be divided,according to the source magma temperatures,into two types:(1)Low temperature(650-790℃),containing potassium-rich sanidine and abundant biotite(S type),(2)high temperature(770-850℃)with sodium-rich sanidine and scarce biotite(Ⅰ type).

Tarmo Kiipli

Department of Geology,Tallinn University of Technology,5 Ehitajate Road,19086 Tallinn,Estonia

地球化学学报（英文版）

[1]Tarmo Kiipli-.Using zircon saturation thermometry of source magma in strongly altered volcanic ashes)[J].地球化学学报（英文版）,2022(03):406-418

Boehnke,Chemn,Geol,sanidine,Palaeozoic

Using,zircon,saturation,thermometry,source,strongly,altered,volcanic,ashes,present,study,deals,possibilities,applying,which,equilibrium,between,crystals,bentonites,It,proposes,alternative,widely,used,from,Zr,content,major,that,not,suitable,components,have,been,largely,these,rocks,For,calculating,temperatures,exponential,function,ppm,Al2O3,compositional,corrections,TiO2,was,found,applica,idea,ratios,elements,mobility,earth,surface,conditions,Temperatures,forming,partial,melting,process,are,assessed,bulk,Pre,eruption,were,estimated,fine,fractions,accuracy,new,established,comparison,by,difference,two,methods,mostly,less,than,revealed,lower,values,average,Although,posed,estimating,precise,accounting,detailed,compositions,where,other,usable,still,enables,differentiation,felsic,magmas,originating,high,Early,Baltic,Basin,divided,according,into,types,Low,containing,potassium,rich,abundant,biotite,sodium,scarce

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