Contrib Mineral Petrol (1981) 76:229-233 Contributions to Mineralogy and Petrology �9 Springer-Verlag 1981

The Application of Spinel-Bearing Metapelites to P[ T Determinations: An Example from South India

Nigel Harris

Department of Earth Sciences, Open University, Milton Keynes, England

Abstract. Metasedimentary migmatites from the Archean charnockitic terrain of South India contain the five phase equilibrium assemblage spinel-cordierite-garnet-corundum-sil- limanite. The assemblages is a result of anatexis which has generated a silica-deficient anhydrous restite. Peak metamor- phic conditions are defined by the intersection of two di- variant reactions in the A120 3 - SiO 2 - F e O - M g O system at which the five phases coexist. These reactions are univariant and their intersection invariant if the Fe/Mg ratio of at least one femic phase is fixed.

The location of the invariant point in P/T space is derived from extracting standard stage thermodynamic data from published equilibria experiments in the system A120 3 - S i O 2 - F e O . Microprobe analyses of coexisting spinel, almandine and cordierite specify the Fe/Mg distributions between phases and allow the computation of the five phase invariant point for Pu2o=P ..... (770~ 5.9kb) and Pmo=O (740~ 4.8kb). A low Pn2o, implied by evidence of extreme anatexis, indicates a P/T field of T=740_+20~ C and Ptotat=4.8• kb which is consistent with the field of equilibration of interlayered char- nockites computed from garnet-hypersthene and garnet- plagioclase pairs.

Introduction

Spinel-bearing metapelites are exposed 15km south-east of Kodaikanal (10 ~ 15'N, 77 ~ 31'E) in Tamil Nadu, within the Archean terrain of South India. Kodaikanal is an upland area underlain by charnockites, with extensive spinel-bearing meta- pelites interlayered with charnockite gneisses on the eastern and southeastern flanks. The metapelites occur as stromatic migmatites which extend over several kilometres. The granitic leucosome is composed of coarse anhedral crystals of quartz, plagioclase and microperthite. The melanosome is a sili- ca-poor assemblage composed of cordierite, a green spinel and magnetite. Also present are rods of sillimanite, tabular corun- dum and skeletal garnet. Occasional flakes of biotite occur as inclusions within garnet. The melanosome is sheathed by equant plagioclase crystals which separates the silica-deficient phases from the free quartz in the leucosome. A similar texture and assemblage has been described from a mica schist in contact with a diorite in Glen Clova (Nockolds et al. 1978, p. 369) and is indicative of extreme fusion, the melanosome being a refractory remnant. The coexistence of the five phases cordierite, spinel, garnet, corundum and sillimanite in the

melanosome provides an invariant assemblage for a fixed Fe/Mg ratio in any femic phase, and therefore defines a unique PIT field of equilibration. This paper consitutes a study of the constraints placed by the spinel-bearing assem- blages on the P/T conditions of their formation.

Analytical Techniques

The application of thermodynamic equilibria equations to assem- blages including solid solution phases requires analysis of those phas- es. Garnet, cordierite, spinel, biotite and plagioclase were analysed from the metapelites and hypersthene, garnet and plagioclase from the charnockites using an energy dispersive electron microprobe at the Department of Earth Sciences, University of Cambridge. Accel- erating potential was 20 kV and live counting time 80 s. Peaks were processed and measured by iterative peak stripping (Statham 1976) and corrected using the method of Sweatman and Long (1969). Fe 3+ was determined in spinel and garnet by site allocation, and in hyper- sthene by charge balance.

Chemographic and Thermodynamic Constraints

Analyses of the green spinel (Table 1) indicate that the min- eral is predominantly from the FeA1204-MgA1204 solid solution series, with less than 4% Fe3§ substitution and less than 0.5% Cr/A1 substitution. The garnet is predom- inantly from the almandine-pyrope series with a maximum grossular content of 3 % and spessartine content of 0.7 %. The six phases cordierite, garnet, spinel, sillimanite, corundum and quartz may therefore be plotted in a tetrahedron with apices representing the four components A120 ~ - S i O 2 - F e O - M g O (Fig. 1).

Segregation has taken place in the metasediment between a silica and alkali-rich leucosome and an iron, magnesium and aluminium-rich melanosome. Plagioclase sheaths the melano- some from quartz implying disequilibrium between the quartz- bearing leucosome and silica-deficient melanosome. Such a segregation will result from extreme anatexis which is illus- trated in Fig. 1. Assuming biotite, sillimanite, quartz and plagioclase were present in the pelite prior to anatexis, the reaction

Biotite + Sillimanite + Quartz + Plagioclase + Microcline _+ H 20 -- Garnet + Cordierite + Liquid

is initiated at temperatures below 720~ (Holdaway and Lee 1977). The initial composition in the A 1 2 0 3 - S i O 2 - F e O - M g O tetrahedron lies within quartz-sillimanite-cordierite- garnet space. Anatexis reduces the concentrations of SiO 2, Na20, K20 and H20 in the restite such that its composition

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Table 1. Compositions of solid solution phases from spinel-bearing assemblages

Spinel Spinel Gnt. Gnt. Cord. Cord. Biot. Biot. Plag. Plag.

S i O 2 0.24 0.26 39.62 39.70 50.27 50.82 36.96 36.74 58.01 59.41 A120 3 59.02 59.89 22.58 22.46 33.66 34.12 15.75 16.32 25.65 26.15 TiO2 . . . . . . 5.37 6.44 - - CrzO 3 0.32 0.37 . . . . 0.14 - - - F%O 3 4.07 3.29 . . . . . . . . FeO 26.84 26.61 26.69 26.42 3.97 3.97 13.02" 12.06 0.33" 0.22" MnO - - 0.25 0.24 . . . . . . MgO 9.43 9.80 10.58 10.61 11.06 11.20 14.87 14.85 - - CaO - - 1.19 1.18 - - 0.11 0.12 7.73 8.21 Na20 . . . . . . . 0.39 6.84 6.76 Kz O . . . . . . 9.83 9.34 0.29 0.18

Total 99.92 100.22 100.90 100.61 98.96 100.10 96.06 96.26 98.86 100.93

Ions to 4 (0) Ions to 12 (0) Ions to 18 (0) Ions to 11 (0~ Ions to 8 (0)

Si 0.01 0.01 2.99 3.01 5.03 5.03 2.72 2.68 2.63 5.92 A1 1.90 1.92 2.01 2.00 3.97 3.98 1.37 1.40 1.37 3.07 Ti . . . . . . 0.30 0.35 - - Cr 0.01 0.01 . . . . 0.01 - - - Fe 3+ 0.08 0.07 . . . . . . . . Fe 2 + 0.62 0.60 1.69 1.67 0.33 0.33 0.80" 0.74 a 0.01" 0.02" Mn - - 0.02 0.01 . . . . . . Mg 0.38 0.40 1.19 1.20 1.65 1.65 1.63 1.61 - - Ca - - 0.10 0.10 - - 0.01 0.01 0.37 0.88 Na . . . . . . . 0.05 0.60 1.30 K . . . . . . 0.92 0.87 0.02 0.02

Mg/(Mg+ Fe z+ ) 0.38 0.40 0.41 0.42 0.83 0.83 0.67" 0.69 a - -

All Fe computed as FeO

AI203 3n

J Q

Fr SiO2 Fig. 1. AI203 - S i O 2 - F e O - MgO tetrahedron (MgO located on rear apex). G garnet; Cd cordierite; Sp spinel; S sillimanite; Cn corun- dum; Q quartz. Heavy lines represent tie-lines between coexisting mineral pairs. I1, 12 represent invariant 4 phase assemblages on restite composition path X - Y

in Fig. 1 (X) migra tes f rom the silica apex a long the line X - Y A t 11 the free qua r t z is los t a l toge the r a n d the com- pos i t i on lies on the cord ie r i t e -g~rne t - s i l l imani te plane. W i t h fu r ther me l t ing these th ree phases are j o i n e d by the SiO 2- u n d e r s a t u r a t e d phase spinel. The phases cord ie r i te -garne t - s i l - l imani te -sp ine l a re re la ted by the r eac t ion

G a r n e t + Si l l imani te = Cord ie r i t e + Spinel (1)

W h e n I z is r eached by the restite, the c o m p o s i t i o n lies on the p lane cordier i te -s i l l imani te-sp ine l . W i t h fu r ther mel t ing the

r eac t ion b e c o m e s

Biot i te + Si l l imani te + Plagioc lase _+ Mic roc l ine • H 2 0 -- Cord ie r i t e + Spinel + C o r u n d u m + Liquid .

The four phases cordier i te , c o r u n d u m , s i l l imani te and spi- nel are re la ted by the r eac t ion

Spinel + Si l l imani te = Cord ie r i t e + C o r u n d u m . (2)

R e a c t i o n s (1) and (2) are un iva r ian t in the S i O z - A l z O 3 - F e O system, and coexis t a long i sop le ths of c o n s t a n t F e / M g ra t io in the SiO 2 - A 1 2 0 3 - F e O - M g O system.

T h e coexis ten t o f the five phases c o r d i e r i t e - c o r u n d u m - garne t - s i l l imani te - sp ine l impl ies tha t m e t a m o r p h i c cond i t i ons are def ined by the in te r sec t ion o f r eac t ions (1) and (2), at the invar ian t p o i n t (I2). Such an invar ian t po in t will exist for all c o m p o s i t i o n s t ha t lie on the cord ie r i te -s i l l imani te -sp ine l p lane in Fig. 1, bu t its pos i t ion in PIT space is d e p e n d e n t on the F e / M g ra t ios of the femic phases . I t m a y there fore be re- ga rded as an invar ian t p o i n t only if the F e / M g ra t io of at

least one femic phase is specified. The re levant reac t ions m a y be ca lcu la ted in PIT space for

the A 1 / O 3 - S i O 2 - F e O sys tem by ca lcula t ing en t ropy and en tha lpy changes f rom expe r imen ta l reversa ls o f the reac t ions

Fe -C ord i e r i t e = A l m a n d i n e + Si l l imani te + Q u a r t z ( N e w t o n a n d W o o d 1979) (3)

Fe -C ord i e r i t e = He rcyn i t e + Q u a r t z ( R i c h a r d s o n 1968) (4)

Table 2. Standard state thermodynamic data from Robie et al. (1967, 1978)

V-molar volume (cal bar-l), S~ state third law entropy (cal mo1-1 ~ t), A:H~ state enthalpy of formation (cal mol 1)

V S O A :H ~

Quartz 0.54226 Corundum 0.61125 Sillimanite 1.1926 Almandine 2.7553 Hercynite 0.9739 Fe-Cordierite (anhydrous) 5.5468 a Fe-Cordierite (monohydrate) 5.7380

9.88 -217,663 12.17 -400,502 22.97 -618,011

a Calculated from the anhydrous Fe-cordierite value of Robie et al. (1967) plus the difference between anhydrous and monohydrate values of Helgeson et al. (1978)

4 t /

1

r 3.., :< Vo;:-.g. %

6 0 0 7 0 0 8 0 0 9 0 0 T ~

Fig.2. Univariant reactions in A1203-SiO2-FeO system between almandine (A), Fe-cordierite (Cd), hercynite (H), sillimanite (S), corun- dum (Cn) and quartz (Q) for Pn~o=Ptot~l. Solid lines represent re- actions used to define I~, [ 2

and from changes in molar volumes calculated from standard state thermodynamic data given in Table 2. These data are combined with standard state thermodynamic data of quartz, sillimanite and corundum to compute molar volume, entropy and enthalpy changes for reactions (1) and (2). Entropy and enthalpy values for the relevant femic phases are not ac- curately known and have therefore not been used in the calculation.

Reaction (4) has been experimentally determined only for PH2o=Ptotal and therefore reaction (3) has also been plotted for hydrous conditions in Fig. 2. Cordierite may take H20 into its structure which implies that reactions (1), (2), (3) and (4) are strictly divariant unless Pn~o is also specified. Reactions (3) and (4) are found to intersect at I 0. Hercynite from re- action (4) contains 0.11~ (molar) magnetite solid solution in Richardson's experiment and therefore I 0 has to be recalcu- lated for pure phases. If it is assumed that other phases in the experiment are pure end-members, the appropriate K n for the experimental reaction may be calculated. Using this value of K o, Richardson's experimental data and the molar volume change (A V) calculated from Table 2, entropy (AS ~ and en- thalpy (AyH ~ changes for the reaction may be calculated assuming that the heat capacities of reactants and products

231

are equivalent ( A @ = 0 ) and that AV, AS ~ and zJfH ~ are constants. For pure end-members ( K o = l ) reaction (4) in- tersects reaction (3) at 11. At this point (865~ 2.9 kb) Fe- cordierite, almandine, hercynite, sillimanite and quartz coexist for Pu2o=Ptola~. Since reaction (1) is the quartz-absent reaction of this invariant point, it is also constrained to pass through 11 .

The equilibrium equation for reaction (1) may be calculat- ed using standard state molar volumes from Table 2, and the entropy and enthalpy change of the reaction derived from solving reaction (1) for 0.11 ~o magnetite at I 0 and for pure phase at 11 simultaneously, assuming ACp=O and AV, AS ~ and A:H ~ are constants. The PIT conditions for the reaction given by

1358 +(lnK 1 +0.82) Tj (~ 1~ (bars) = 1 -t

0.7908

where agarnet

( Fe3Al2Si3Ot 2) 1 1 - - acordlerit e aSpineI

( ~o2A~4Si5O,,)( ~oAl~O3

The equilibrium equation for reaction (2) is calculated from combining thermodynamic data from reaction (4) with stan- dard state thermodynamic data for the reaction

Sillimanite = Corundum + Quartz

calculated from the published values of V, S o and A:H ~ for the constituent phases given in Table 2. PiT conditions for reaction (4) are given by

8720+(lnK2- 8.18 ) T2 (~ P2 (bars) = 1

0.4446

where spinel 2

/ ,I (aFeAl204) ~ 2 - - ~ "

(aFe2AI4Si5Ol8)

The intersecton of (1) and (2) at I a is defined by P~ =P2, 7-1 =T2 and is the invariant point at which almandine, Fe- cordierite, hercynite, sillimanite and corundum coexist for PH~o=Ptot~l. It is located at 645 ~ C, 2.7 kb.

Results

Microprobe analysis of relevant phases (Table 2) indicate that Fe/Mg variation for any phase is less than 2 ~ within a thin section, and zoning within individual crystals is less than 1 ~. This is consitent with a state of chemical equilibrium.

Assuming ideal Fe/Mg solid solution in spinel, cordierite and garnet

agarnet __ garnet 3 Fe3A12Si3012 - - ( X F e )

cordlerlte __/xeordierlte~2 aFe2A14Si~O18 - -k Fe )

spinel spa sp.ii 2 aFeAi204 = ( X F e ) ( X A I )

where spinel has the general formula

A i ( B z i ) 2 0 ,

Univariant reactions (1) and (2) are plotted for measured values of K 1 and K 2. in Fig. 3 which locates 12 at 770 ~ C and 5.9 kb for conditions of Pn2o=Ptotav In general, anatexis results in a decrease of Pn2o in the restite, and conditions of PHzo =0.2Ptolal are typical for pelitic restites (Harris and Jayaram, in press; Wells 1979). Since melting has been sufficiently extensive to cause silica depletion in the spinel-bearing res-

232

a_

4

§

_- ~ /-~ ~ L- / r

~ ' " 4 / : /

iS& /

, /-

/

7 0 0 8 0 0 T ~ 9 0 0 1,000

Fig. 3. Univariant reactions for garnet (G), cordierite (Cd), spinel (Sp), sillimanite (S), corundum (Cn) and quartz (Q) of natural compositions given in Table 2. Solid lines for Pn~o=Ptot~, dashed lines for PH~o=0. Dotted line gives path of invariant points for decreasing Pn~o

8 cG B% / I /

,> A I / 7 1 / I /

, ' - - - ' 1 - - I / / 6 /_% , I

_Q / / / 121 [ / /

n 1 / I" / 5 / ' 1 o~,I

I / I ~ I /

1 /

: 760 V ;C 8()0 900 Fig. 4. PIT field calculated from spinel-bearing assemblages (heavy dashed line defines field for unspecified Pn~o, heavy solid line defines field for Pn~o=0) and P/T conditions calculated from mineral pairs plagioclase-gamet (PG), orthopyroxene-garnet (OG), cordierite-garnet (CG) and biotite-garnet (BG 1 from Thompson 1976; BG 2 from Ferry and Spear 1978)

tites it is probable that PH20<0.2Ptotal during peak metamor- phic conditions. Conditions of Pn2o=0 reduce pressures of reaction (3) in excess of 1 kb (Newton and Wood 1979), which implies that I2 is a maximum pressure for the observed as- semblage.

Cordierite is the only hydrous phase in equilibrium at 12 and therefore conditions of reduced PH2O result in the mi- gration of I 2 down the cordierite-absent reaction curve

Garnet + Corundum = Spinel + Sillimanite. (5)

Similarly I~ migrates down the cordierite-absent reaction curve.

Garnet + Sillimanite = Spinel + Quartz. (6)

The gradient of both these curves may be derived from stan- dard state thermochemical data in Table 2 and the experi- mentally derived thermodynamic equations for reactions (3) and (4). The position o f I t for Pn2o=0 (11, Fig. 3) is located at the interesection of reaction (5) with the experimentally de- termined reaction (3) for PH2O~0 and for the appropriate cordierite composition (Newton and Wood 1979). Reaction (1) under anhydrous conditions can then be plotted, and its intersection with (5) gives the location of I a under anhydrous conditions (I~, Fig. 3) at 740~ and 4.8 kb. This is a mini- mum constraint on the observed assemblage.

In Fig. 3 I t occurs at higher temperatures than 12 al- though 11 occurs before 12 during prograde melting (Fig. 1). This implies 12 is metastable. However, it is observed that the five phase assemblage of 12 is a stable assemblage. This apparent inconsitency is a result of treating divariant re- actions as univariant for a fixed Fe/Mg ratio. The reactions which occurred during prograde metamorphism were con- tinuous with femic phases changing composition during pro- gressive melting. However, this does not affect the location in P/T space of i 2 for assemblages of known composition al-

though the position of I t in Fig. 3 is hypothetical since the femic phases in equilibrium with quartz at 11 would not have had the same compositions as those observed in the 12 assem- blage.

Discussion

The major source of error in determination of [2 and I~ lies in the experimental uncertainties of reactions (3) and (4). Experimental constraints on reaction (3) result in a P un- certainty of _+ 0.5 kb, on 12 and I~. Uncertainties of reaction (4) result in a maximum T error of _+20 ~ Therefore, the P/T conditions defined by these assemblages lie in the overall range of Ptotal = 5.4 kb _+ 1.0 kb, T= 755 4-_ 35 ~ C with unsepcified Pn,_o. Since extensive anatexis results in a reduced Pn2o in the restite, the probable field of equilibration is restricted to conditions of P~2o<0.2P~ot,1 whicb implies P~ot~=4.8+0.5kb and T= 740 +_ 20 ~ C. Uncertainties in standard state thermody- namic data would not have significant affect on these errors since enthalpy and entropy values of femic phases have not been used.

To check the accuracy of the implied P/T field of the spinel-bearing assemblages coexisting assemblages with pub- lished calibrations have been evaluated. Ca distribution be- tween placioclase and garnet, in the presence of sillimanite and absence of quartz, give a range of maximum pressures shown in Fig. 4, using the calibration of Harris and Jayaram (in press). The anorthite/grossular barometer of Ghent (1976) gives pressures 1 kb below this, but an activity coefficient for grossular in garnet in excess of unity as inferred by Ganguly and Kennedy (1974) could increase the computed pressure by up to 2 kb. Garnet-cordierite pairs give temperatures 30~ below the proposed field, using the calibration of Wells (1979). Since a 1% exchange of Fe/Mg during retrogression will reduce the temperature recorded by the geothermometer

Table 3. Composition of solid solution phases from charnockites

Hyp. Hyp. Gnt. Grit. Plag. Plag.

giG 2 48.64 49.22 39.43 39.35 56.83 56.56 A1203 7.11 6.65 22.17 22.14 27.46 27.73 Fe20 3 1.42 0.82 - 0.46 - - FeO 22.07 22.06 26.64 25.88 0.15" - MnO 0.33 0.32 0.49 0.46 - - MgO 20.31 20.71 9.94 10.68 - - CaO 0.12 - 1.52 1.72 10.09 10.17 Na20 . . . . 5.59 5.62 K20 . . . . 0.40 0.27

Total 100.00 99.78 100.19 100.69 100.52 100.35

Ions to 6(0) Ions to 12(0) Ions to 8(0)

Si 1.82 1.84 3.01 2.99 2.54 2.53 A1 iv 0.18 0.16 - - 1.45 1.46 A1 vl 0.14 0.13 2.00 1.98 - - Fe 3 + 0.04 0.02 - 0.02 - - Fe 2+ 0.69 0.69 1.70 1.65 0.01 a - Mn 0.01 0.01 0.03 0.03 - - Mg 1.13 1.15 1.13 1.21 - - Ca - - 0.12 0.14 0.48 0.49 Na . . . . 0.48 0.49 K . . . . 0.02 0.02

Mg/(Mg + Fe 2 +) 0.62 0.62 0.40 0.42 - -

All Fe computed as FeO

by abou t 30 ~ C, the implied tempera ture of 690 ~ C is consi tent with peak tempera tures of at least 720 ~ C. Such a retro- gression exchange will have a negligible affect on the spinel geothermometer . Garne t -b io t i t e pairs provide a geothermo- meter which indicates tempera tures in excess of 30~ above the field implied by the spinel-bear ing assemblages, but this is within the accuracy of the me thod (_+50 ~ C). Inconsis tencies between calibrat ions above 700~ suggest the garnet-bioti te geo the rmomete r is not reliable at higher tempera tures (Thompson 1976; Ferry and Spear 1978). Moreove r the h igh TiO 2 contents of these bioti tes may affect the geothermo- meter. P/T const ra in ts f rom these assemblages, using estab- lished P/T indicators, are therefore insufficiently specific to allow a satisfactory test of the spinel-bear ing assemblages as PIT indicators. The spinel-bear ing pelites are inter layered with charnocki tes which provide a more specific test of the spinel t he rmomete r /ba romete r . The A1 d is t r ibut ion of garnet- hypers thene pairs from the charnocki tes (Table3) give an independent ba romete r (Wood 1974) which is p lo t ted in Fig. 4 for the observed values and lies within 2 k b of the proposed P/T field. The quoted error on the ba romete r is 2 - 3 k b . Ca d is t r ibut ion between garnet-plagioclase pairs f rom the char- nockites give a m a x i m u m pressure, in the absence of silli- manite, and d is t r ibut ion coefficients lie within the range of garnet-plagioclase pairs f rom the spinel-bear ing assemblages (Fig. 4). The P/T field specified by the garne t -hypers thene and garnet-plagioclase geobarometers are consis tent with the field defined for the spinel-bear ing assemblages from the intersec- t ion of react ions (1) and (2).

C o n c l u s i o n s

The four phase assemblages of cordieri te-garnet-si l l imanite- spinel and cordier i te-corundum-si l l imani te-spinel each define a un ivar ian t react ion in the A120 3 - S i O 2 - FeO system for a

233

given Pn2o which intersect at an invar ian t poin t where the five phase assemblage cordier i te-garnet-corundum-si l l imani te-spi- nel coexist. In the absence of k n o w n Pn2o, F e / M g determi- na t ions of femic phases from the five phase assemblage allow a de te rmina t ion of P/T condi t ions within the limits + 3 5 ~ and + 1.0 kb. Inferred pressures are 0.4-0.7 kb below the cor- dierite breakdown. A knowledge of Pn2o reduces the uncer- ta inty in the de te rmina t ion of equi l ibr ium condit ions.

Spinel-bear ing assemblages f rom the metapeli tes of South India indicate tha t anatexis occurred under condi t ions of 4.8 + 0 . 5 k b and 740+_20~ assuming PH20<0.2Ptot,j, which is consistent with P/T condi t ions inferred by garnet-plagioclase pairs and garnet -hypers thene pairs f rom inter layered char- nockites.

Acknowledgements. Field work was supported by a Royal Society Scientific Investigations Grant and analytical work by a Research Consultancy at the Department of Earth Sciences, Open University. I am indebted to V. Gopal, V. Subramanian, P.V. Rajagopal, and K. Shanmugam of the Tamil Nadu Department of Mines and Geology for assistance in field work, to Drs. J.V.P. Long and P.J. Treloar Department of Earth Sciences, Cambridge for assistance with micro- probe analyses, and to Dr. G. Droop for invaluable discussion and constructive criticism of the manuscript.

R e f e r e n c e s

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Received November 7, 1980; Accepted February 27, 1981