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The age of the Permian-Triassic boundary
182
Earth and Planetary Science Letters, 105 (1991) 182 190
Elsevier Science Publishers B.V., Amsterdam [CHI
The age of the Permian-Triassic boundary J.C. Claour-Long
a, Z h a n g Z i c h a o
b,
Ma Guogan b and Du Shaohua b
Research School of Earth Sciences, A ustralian National University, GPO Box 4, Canberra, A. C. T. 2601, Australia h Yichang Institute of Geology and Mineral Resources, PO Box 502, Yichang 443003, China
Received November 8, 1990; revision accepted March 21, 1991
ABSTRACT The 5 cm boundary clay bed in the Chinese stratotype section through the Permian-Triassic boundary has been recognised as a bentonite. SHRIMP ion microprobe dating of zircons in the bentonite indicates a magmatic age of 251.2 _+3.4 Ma (20); this is the first direct constraint on the numerical age of the Permian-Triassic boundary. Future refinements of ages at this important, but poorly constrained, level of the Phanerozoic timescale may depend on re-analysis of this uniquely placed volcanic horizon, and other bentonites in the fossiliferous Chinese Upper Permian and Lower Triassic. The utility of defining the Permian Triassic boundary in the Chinese stratotype section, in the vicinity of known dateable horizons, should be considered.
1. Introduction T h e P e r m i a n - T r i a s s i c b o u n d a r y m a r k s one of the three m o s t p r o f o u n d biological revolutions in the P h a n e r o z o i c record, c o m p a r a b l e to the explosive evolution d e f i n i n g the onset of the C a m b r i a n a n d the g l o b a l e x t i n c t i o n s of the C r e t a c e o u s - T e r tiary b o u n d a r y . H o w e v e r , this transition has p r o v e d n o t o r i o u s l y difficult to s t u d y b e c a u s e m o s t areas of the world have a gap in the s e d i m e n t a r y r e c o r d at this point, or lack the fossil r e c o r d r e q u i r e d for b i o s t r a t i g r a p h i c correlation. In this c o n t e x t it is not surprising that the t i m i n g of events a b o u t the P e r m i a n - T r i a s s i c b o u n d a r y is p o o r l y u n d e r s t o o d : the recent time scale c o m p i l a tion of H a r l a n d et al. [1] lists n o reliable r a d i o m e t ric dates for the P e r m i a n stages u n d e r l y i n g the b o u n d a r y , a n d a single K - A r age in the L o w e r Triassic. This is u n f o r t u n a t e , b e c a u s e it is the rates of the s p e c t a c u l a r biological processes t a k i n g p l a c e that form the m a i n interest of the P a l a e o z o i c M e s o z o i c transition. T h e p u r p o s e of this c o n t r i b u t i o n is to r e p o r t the first direct c o n s t r a i n t on the age of the Permian-Triassic boundary, obtained by SHRIMP zircon d a t i n g of a b e n t o n i t e occurring at the b o u n d a r y in a Chinese c a n d i d a t e s t r a t o t y p e section. A m o n g the rare s e d i m e n t a r y sequences offer0012-821x/91/$03.50
© 1991 - Elsevier Science Publishers B.V.
ing n e a r - c o m p l e t e r e c o r d s t h r o u g h the P e r m i a n Triassic b o u n d a r y , those in S o u t h C h i n a have one of the richest L a t e P e r m i a n f a u n a s a n d c o n t r a s t with relatively b a r r e n s t r a t a elsewhere. These include the s t r a t o t y p e for the y o u n g e s t P e r m i a n Stage a n d c a n d i d a t e s t r a t o t y p e sections t h r o u g h the P e r m i a n - T r i a s s i c b o u n d a r y . It has recently b e e n recognised that thin clay b e d s i n t e r s p e r s e d t h r o u g h the Chinese sequence are b e n t o n i t e s , c l a y - a l t e r e d volcanic ashes, offering the p o s s i b i l i t y of direct r a d i o m e t r i c d a t i n g o f the s t r a t i g r a p h y . By r e p o r t i n g an a c c u r a t e n u m e r i c a l age for one of these, we d r a w a t t e n t i o n to the p o t e n t i a l for f u t u r e direct c o n t r o l on the L a t e P e r m i a n a n d E a r l y Triassic time scale b y reference to the u n i q u e c o n j u n c t i o n of fossiliferous a n d volcanic layers in the Chinese record.
2. The biostratigraphic markers T h e i m p o r t a n c e of the r a p i d c h a n g e s k n o w n as the P e r m i a n - T r i a s s i c b o u n d a r y are u n i v e r s a l l y agreed, b u t an i n t e r n a t i o n a l c o n v e n t i o n d e f i n i n g the f o r m a l p l a c i n g of the b o u n d a r y has yet to be resolved. This issue c u r r e n t l y is the subject o f an I U G S W o r k i n g G r o u p , whose i n t e r i m suggestion is to define the b o u n d a r y at the a p p e a r a n c e of the a m m o n o i d O t o c e r a s [2]. I n terms of c o n o d o n t
THE AGE OF THE PERMIAN-TRIASSIC BOUNDARY
zonation this is closely equivalent to the base of the Hindeodus parvus Zone [3]: H. parvus first appears within the Otoceras ammonoid Zone and is more geographically widespread than preserved specimens of Otoceras; on these grounds it has been suggested as a more practical indicator for worldwide correlation of the boundary [4]. Most workers consider that the youngest Permian faunas are typified by sediments defining the Changxingian Stage in China (named after Changxing Zhejiang Province), which is the uppermost Permian Stage [5,6]. The type area is in Meishan of Changxing where marine sequences are well exposed in a series of sections, designated Sections A - E , near Zhongxin Dadui quarry [6]. These include the stratotype for the Changxingian Stage and Chinese candidate stratotype for the Permian-Triassic boundary (see Fig. 1). The Changxing Formation is principally a carbonate sequence and contains typical Palaeozoic forms including fusulinids, rugose corals, and the uppermost limits of the Pseudotirolite, Pleuronodoceras
Bed
number
Grey-black marl/ mudstone
27~:'-'tIii!i!iii~iiiiii 8 ~:.;iii~il 6 ili ~iiii!i!l]
Clay
25
Grey dolomitic
24
marl
Bed
thickness (cm)
Ophiceras Claraia stachei Claraia wangi
7
*Mixed bed" 1 Otoceras? Hypophiceras Tompophiceras brachiopods
Grey-black 23 mudstone Grey-whiteyellow clay 22
*Mixed bed" 2 Geinitzina Hindeodus parvus
TRIASSIC PERMIAN ~
Grey limestone with chert 21 nodules
25
Palaeofusulina "~ ,~
Fig. 1. Stratigraphic column of Section D through the Permian-Triassic boundary in Meishan, South China. The Changxing Formation limestone in this section is the stratotype for the uppermost Permian Changxingian Stage [6]. The base of the Triassic is defined by the presence of Otoceras and Hindeodus parvus in the Mixed F a u n a Beds of the overlying Chinglung Formation. The boundary between the Permian and Triassic is therefore marked by the Bed 22 bentonite dated in this contribution. Adapted from [3,6].
183
and Rotodiscoceras ammonoid Zones, and is used with confidence to define the uppermost Permian. More equivocal is the attribution of the overlying Chinglung Formation. The contact is marked by lithological change from the Changxing Limestone to, successively, Mixed Fauna Bed 1, a mudstone about 10 cm thick; Mixed Fauna Bed 2, a dolomitic marl 20-30 cm thick; and Mixed Fauna Bed 3 comprising further mudstone of variable thickness; with thin clay beds intervening at the top of each carbonate layer [6] (Fig. 1). These mixed fauna beds contain new Triassic forms (e.g. bivalves, new ammonoids and new conodonts) together with some Permian-type fossils (principally brachiopods). Triassic fauna include the characteristic bivalve Claraia which is found in Mixed Bed 3 and above. The ammonoid Otoceras, is reported by Sheng et al. [6] from Mixed Bed 1, but Tozer [7] notes that preservation of specimens is poor and identification cannot be certain. As documented by Ding [3], the beginning of the Otoceras ammonoid Zone can readily be substituted by the beginning of the H. parvus conodont Zone, and H. parvus is definitely identified in the limestone of Mixed Fauna Bed 2 [3]. Sheng et al. [6] conclude that the Permian brachiopods in the mixed fauna beds are Palaeozoic survivors coexisting with the new Mesozoic fauna, and that there is a continuous sedimentary record through the Permian-Triassic transition. The boundary might then be placed anywhere between the first appearance of characteristic Triassic fossils (Otoceras? in Mixed Bed 1, Hindeodus parvus in Mixed Bed 2 which is 7 cm higher) and the last appearance of Permian types. These considerations lead to establishing a boundary at the base of Mixed Bed 1 of the Chinglung Formation, at its contact with the underlying Changxing Formation limestone. On the other hand, Tozer [2] points to the fact that the base of the bed containing Otoceras?, the distinguishing basal Triassic ammonoid, is a sharp bedding plane and lithological change. The mixed fauna beds can be interpreted to reflect erosion of the underlying Permian strata and consequent redeposition of Permian brachiopods in the earliest Triassic sediments. On this basis, the abrupt lower contact of the Chinglung Formation can be interpreted as unconformable and the Permian-Triassic boundary would then be defined within the unconformity.
184
J.c. CLAOUI~-LONG Eq- AL.
The Permian-Triassic boundary therefore lies between the top of the Changxing Formation limestone and the base of the overlying Chinglung Formation, regardless of whether unconformity or continuous sedimentation is interpreted, and the 5 cm clay layer recognised as a bentonite and dated in this contribution is the bed marking the boundary.
land et al. [1] prefer slightly younger ages near to 245 Ma. New dating reported by Gulson et al. [17] suggests an older age near to 255 Ma. In reality, the data are permissive and allow any number to be chosen within the very wide band between approximately 240 Ma and approximately 255-260 Ma, with preference naturally being accorded to the centre of the permitted range.
3. Existing numerical age constraints
4. Sampling rationale
No new data have become available since the review of Forster and Warrington [8] noted that " n o radiometric data are available from stratigraphic levels close to t h e . . . P e r m i a n - T r i a s s i c boundary", and "radiometric evidence for the age of upper Permian sequences is inconclusive". Faced with this inadequate database, it has been necessary to extrapolate the likely age of the boundary from uncertain age information in the Middle Triassic. The oldest ages obtained from undoubted Triassic sediments are from the Toogoolawah G r o u p in Australia, and the Puesto Viejo Formation in Argentina. Andesites in the Toolgoolawah G r o u p have informally reported K-Ar ages of 242 _+ 5 Ma (whole rock) and 239_+ 5 Ma (hornblende) [9], and palynology assigns these sediments as Middle Triassic (Anisian or Ladinian [10]). This suggests an age of approximately 240 Ma for the Middle Triassic. Fossil reptile assemblages in the Puesto Viejo Formation indicate Early to Middle Triassic strata (perhaps Scythian [11]) and intercalated volcanics have a mean K-Ar age of 237 + 4 Ma [12]; this may represent a minimum age for the Scythian. In Queensland, the KinKin Beds of the G y m p i e Basin, which contain Early Triassic (Smithian) ammonoids are intruded by the G o o m b o o r i a n Diorite which has an average K-Ar age of 236 _+ 7 Ma [13,14], again placing an approximate minimum constraint on the Early to Middle Triassic. N o n e of these ages are for samples with tight biostratigraphic constraints, but in general, the Permian-Triassic boundary has been believed to be 5 - 1 0 Ma older than this Triassic data. Forster and Warrington [8] suggested a boundary age close to 250 Ma, but suggested that an older age might be permitted. Based on essentially the same data, both Odin [15,16] and the recent review by Har-
Everywhere in South China a thin clay layer intervenes between the Permian Changxing Formation and the overlying Triassic Chinglung Formation [6]. This laterally continuous bed is one of m a n y clay layers interspersed throughout the Chinese Permian and Triassic sequences. Recent investigation has shown that some of these clays are in fact bentonites, or clay-altered volcanic ashes. This hitherto unsuspected association of volcanics with the most fossiliferous known sediments of the Late Permian and Early Triassic opens up, for the first time, the possibility of direct numerical age control of this part of the Phanerozoic time scale. As a first approach the bentonite occurring at the P e r m i a n - T r i a s s i c boundary was sampled for dating. This bentonite is well exposed in stratotype Section D of Changxing, which is the stratotype for the Changxingian Stage [6]. The volcanic layer is Bed 22, and according to the definition of the Permian-Triassic boundary discussed above, the bentonite is the bed marking the boundary: the 5 cm clay layer lies between the top of the Permian Changxing Formation limestone and the base of the overlying Mixed Fauna Bed 1 of the Chinglung Formation (Fig. 1). Approximately 30 kg of sample was obtained from the 5 cm thick bentonite in a fresh quarry face exposure of Section D. Care was taken to exclude from the sample any material from adjacent beds. The bentonite is greyish-white-yellow in colour and is a typical clay-altered ash now comprising mainly illite and smectite. Other phases present in trace amounts include the high temperature feldspar sanidine, zircon, and very small quantities of rutile and apatite. The sample was soaked in clean water to disperse the clay, the heavy mineral fraction was then separated using a Wilfley table, and the zircon separate was purified
THE AGE OF THE PERMIAN-TRIASSIC BOUNDARY
magnetically a n d b y h a n d picking. Sanidine was also o b t a i n e d for separate 4°Ar/39Ar dating which will be reported elsewhere. Zircons in the b e n t o n i t e (Fig. 2) have simple euhedral prismatic shapes a c c o m p a n i e d by concentric oscillatory growth zones consistent with m a g m a t i c crystallisation. N o cores or overgrowths are observed. All zircons have a b u n d a n t a n d large inclusions of other crystalline phases, a n d anhedral shapes filled with a m o r p h o u s material interpreted to be altered volcanic glass. These features indicate, respectively, late crystallisation of zircon seeded on other phases, a n d rapid skeletal growth, b o t h features consistent with crystallisation d u r i n g e r u p t i o n of the volcanic ash. The u b i q u i t o u s inclusions form up to 40% of some crystals, so conven-
185
tional analysis would u n a v o i d a b l y analyse a large p r o p o r t i o n of i n c l u d e d material. T h e ion microp r o b e m e t h o d of analysis was therefore chosen.
5. Shrimp zircon dating A few milligrams of zircon were o b t a i n e d from the 30 kg sample of the Bed 22 bentonite. Of these, 100 of the optically clearest zircons were h a n d picked a n d m o u n t e d o n the surface of a n epoxy plug, a n d the grains were sectioned approximately in half by polishing to expose grain interiors. U - T h - P b isotopic c o m p o s i t i o n s were measured for 25 /~m diameter areas of the sectioned zircons, using the S H R I M P ion microprobe. A cold trap was e m p l o y e d in the sample
Fig. 2. Photomicrographs of zircons in the Changxing Permian-Triassic boundary bentonite. Grains have been mounted in epoxy and sectioned in half to expose internal features. Trans = normal transmitted fight; refl = normal reflected light. Fields of view are approximately 300 × 250/Lm. The - 25 ~m wide/2 t~m deep analysis crater excavated by the ion microprobe is clearly visible in B and D, and illustrates the ability of the probe to avoid inclusions and target pure zircon for isotopic analysis, where conventional dissolution analysis would unavoidably incorporate the included material. A. (trans.) and B. (refl.) paired views of grain 21; C. (trans.) and D. (refl.) paired views of grain 1. Both zircons are of simple euhedral magmatic form and contain a high proportion of included material. The negative zircon crystal shape enclosing amorphous material at the centre of grain 21 appears to be a hollow enclosing altered volcanic glass, an indication of rapid, skeletal crystal growth. E. (trans.). Grain 10 which is sector zoned, a rare zoning pattern associated with rapid crystal growth. The pyramid sector has 1500 ppm U, a Th/U ratio of 1.9, and has leaked radiogenic Pb (see Table 1 and discussion in text); whereas the prism sector has a mildly elevated U content, a normal Th/U ratio less than 1, and a concordant Pb/U composition. F. (trans.). Grain 33 a Silurian xenocryst.
186
J.C. C L A O U E - L O N G
ET At.
c h a m b e r to m i n i m i s e h y d r i d e c o n t r i b u t i o n s , a n d a c h i p o f s t a n d a r d z i r c o n SL13 w a s m o u n t e d i n
initial Pb have been m a d e using average crustal Pb o f t h e s a m e a g e as t h e z i r c o n s [20] a n d c a l c u l a t e d
e p o x y w i t h t h e C h a n g x i n g z i r c o n s to m i n i m i s e a n y
u s i n g o n e o f t w o m e t h o d s as a p p r o p r i a t e . T h e Z°4pb m e t h o d c a l c u l a t e s t h e a m o u n t o f i n i t i a l P b
p o s s i b l e b i a s in t h e c o n d i t i o n s o f a n a l y s i s b e t w e e n the standard and the unknowns. Decay constants used
in this p a p e r
are
those recommended
by
S t e i g e r a n d J a e g e r [18].
b y direct r e f e r e n c e to the m e a s u r e d a b u n d a n c e of 2°4pb. T h i s a s s u m e s o n l y t h e c o m p o s i t i o n o f t h e common
Pb, b u t has low p r e c i s i o n o w i n g to the
E a c h a n a l y s i s is t h e m e a n o f five c y c l e s t h r o u g h
v e r y l o w a b u n d a n c e o f 2°4pb. M o r e p r e c i s e c a l c u -
t h e m a s s s t a t i o n s , a n d d a t a w e r e r e d u c e d in t h e
l a t i o n is p o s s i b l e f r o m t h e d i f f e r e n c e b e t w e e n t h e measured 2°spb/2°6pb ratio of each analysis and
m a n n e r d e s c r i b e d e l s e w h e r e [19]. C o r r e c t i o n s f o r
TABLE 1 Ion microprobe U-Th-Pb isotopic data for zircons in the Changxing boundary bentonite Grain
U
Th
-area
(ppm)
(ppm)
1.1 2.1 3.1 4.1 5.1 6.1 7.1 c 8.1 9.1 10.1 10.2 ~ 11.1 12.1 13.1 14.1 15.1 ~ 15.2 ~ 16.1 17.1 18.1 19.1 c 20.1 21.1 22.1 23.1 24.1 25.1 26.1 27.1 28.1 29.1 30.1 c 31.1 c 32.1 33.1 34.1 35.1 ~
473 484 311 209 421 212 318 349 367 785 1492 391 277 785 258 572 689 383 300 372 368 247 504 515 317 211 292 305 436 321 146 174 445 423 552 265 278
247 235 110 94 190 134 154 148 162 514 2856 178 138 291 170 278 513 137 112 156 273 97 194 335 135 85 182 163 21l 108 71 86 219 255 246 152 215
Th/U 0.52 0.49 0.35 0.45 0.45 0.64 0.48 0.42 0.44 0.65 1.91 0.46 0.50 0.37 0.66 0.49 0.74 0.36 0.37 0.42 0.74 0.39 0,39 0.65 0.42 0.40 0.62 0.53 0.49 0.34 0.49 0.50 0.49 0.60 0.45 0.58 0.77
2°4pb
fZO6pba
2orpb/23s U
2o7pb/235U
207pb/206pb
Apparent age b
(ppb)
(%)
( ± 1o)
( _+lo)
( _+lo )
(Ma) _+lo
3 3 3 4 8 3 4 8 3 21 123 5 4 2 5 8 16 8 4 4 4 10 4 5 3 11 30 5 0 6 2 2 2 4 3 7 12
0.31 0.33 0.56 1.16 0.92 0.78 0.70 1.18 0.53 1.48 5.56 0.64 0.73 0.16 0.92 0.77 1.32 1.02 0.69 0.62 0.58 2.17 0.39 0.46 0.55 2.57 5.26 1.00 0.00 0.95 0.67 0.53 0.30 0.46 0.14 1.37 2.60
0.0398+11 0.0403_+11 0.0401_+11 0.0386+11 0.0416 + 12 0.0403 -+ 12 0.0390_+11 0.0391_+11 0.0381-+11 0.0376_+11 0.0300_+ 9 0.0392-+11 0.0425 -+ 12 0.0411_+12 0.0404-+12 0.0392_+11 0.0378+12 0.0410_+12 0.0407_+13 0.0401_+13 0.0390_+13 0.0403_+13 0.0393 ± 13 0.0412_+13 0.0386_+13 0.0406_+13 0.0404_+13 0.0383 _+13 0.0401_+13 0.0404 _+13 0.0383_+13 0.0385_+13 0.0396_+13 0.0409_+13 0.0696 -+ 23 0.0412_+13 0.0359-+12
0.283_+14 0.285_+14 0.292+16 0.273_+19 0.291 _+15 0.276-+22 0.301_+23 0.263_+16 0.273-+14 0.253-+12 0.264-+21 0.290+16 0.304 _+20 0.308_+13 0.297_+22 0.26l_+20 0.259_+20 0.286_+16 0.282_+15 0.273_+14 0.294_+19 0.287_+18 0.264_+13 0.281_+15 0.266_+14 0.321_+21 0.274_+20 0.257_+15 0.289_+14 0.266 _+14 0.268_+19 0.266_+28 0.278_+17 0.298_+16 0.548_+23 0.304+19 0.220_+33
0.0516_+19 0.0512_+18 0.0528_+22 0.0513-+30 0.0508_+21 0.0498_+35 0.0560_+38 0.0488_+24 0.0520_+21 0.0489+17 0.0638-+46 0.0536+24 0.0519-+ 29 0.0544_+15 0.0532_+34 0.0483_+33 0.0498_+33 0.0507±22 0.0502_+19 0.0493_+17 0.0547_+29 0.0516_+26 0.0487_+15 0.0495_+18 0.0500_+19 0.0573_+29 0.0492_+31 0.0486_+23 0.0523_+17 0.0477_+18 0.0506+31 0.0501_+48 0.0510_+25 0.0530_+22 0.0571_+12 0.0535-+26 0.0444_+63
252+ 7 255_+ 7 254_+ 7 244_+ 7 263 _+ 7 255 _+ 7 247_+ 7 247-+ 7 241_+ 7 238_+ 7 191_+ 5 248-+ 7 268 _+ 7 259+ 7 256_+ 7 248+ 7 239_+ 8 259_+ 7 257_+ 8 253_+ 8 246_+ 8 255_+ 8 249 -+ 8 260_+ 8 244± 8 257± 8 255_+ 8 242 _+ 8 254-+ 8 256 _+ 8 243_+ 8 243_+ 8 250_+ 8 258_+ 8 434 _+14 260-+ 8 228_+ 7
a fZ06pb indicates the percentage of common 2°rpb in the total measured 2°6pb. b Apparent age is the 2°rPb/23Su age. c indicates 2°4pb correction substituted for 2°spb correction owing to alteration of the Th isotopic system.
THE AGE OF THE PERMIAN-TRIASSICBOUNDARY
187
the radiogenic 2°spb*/2°rpb* ratio appropriate to the age and T h / U of the zircon (2°8pb method [19]) and this has been applied where there has been no fractionation of Th and U in the zircons, and no differential movement of uranogenic and thorogenic Pb (Fig. 3). Uncertainties of common Pb correction are included in the errors quoted in Table 1. Radiogenic 2°7pb* is 20 times less abundant than 2°6pb* in zircons of this age and so has relatively poor measurement precision. As a consequence, calculated abundances of radiogenic 2°7pb* are very sensitive to correction for common 2°7pb, and this combines with the over-sensitivity of the 2°7pb*/z°rPb* chronometer in young zircons to frustrate definitive testing for concordance of the 2°Tpb*-235U and 2°6pb*-z38u systems. Pb loss a n d / o r inheritance is assessed instead from grouping of 2°6pb*/z38u data. Great significance is therefore attached to the reproducibility of P b / U ratios measured by the ion microprobe, which in turn is principally determined by the calibration of P b / U ratios to the known composition of a standard zircon. Ratios of P b / U in this sample are referenced to a value of 0.0928 for 2°6pb*/238U (equivalent to 572 Ma) in our standard zircon SL13. Forty S H R I M P analyses of this standard were alternated between each analysis of the unknowns throughout the two-day analytical period, and the data are compared with the S H R I M P calibration relationship in Fig. 4 which
.0
M.O~/
o. o
o
CY
e 0.20
~
~
0.10 Th/U 0.0o 0.0
~ 0.5
J 1.0
1.5
2.0
Fig. 3. Th/U-2°spb/2°6pb isochron diagram for the Changxing zircons showing the fit of data to the theoretical isochron for 251 Ma. Most zircons have seen no differential movement of Th and U, or of uranogenic and thorogenic Pb: initial Pb corrections can therefore be calculated accurately from difference between the measured 2°spb/2°6pb and that calculated from Th/U.
"~
(a) Day1
0.4
0.2
changXing~
0.0 6.5
t
i
7.5
8.5
UO/U 9.5
(b) D a y ~ 0.30
~
s~-~ " 0.20
~
° ~
Silurian xenocryst
0.10
0.00 I 5.5
,
J 6.5
,
,
UO/U
7.5
Fig, 4. Data for zircons in the Changxing boundary bentonite compared with the SHRIMP calibration curve and data collected for standard zircon SL13 at the same time. The calibration is a quadratic relationship between the measured ratios of P b / U and U O / U [21], and the data sets adhere closely to the expected curve. Scatter of data for the Changxing sample is similar to that observed for the standard, inferring a homogeneous composition within error.
shows the close adherence of analyses of the standard to the expected curve. This reproducibility-2.83 and 3.24% (lo), respectively, on the two days-is a direct measure of the reproducibility of a constant P b / U target and is included in the errors in P b / U quoted in Table 1. Additionally, the 0.47% ( l o ) uncertainty in the mean position of the calibration is included in the error on the mean age of the unknowns. Also shown in Fig. 4 are the measured compositions of the Changxing zircons. These adhere closely to a curve of the same form as that through the standard analyses, but at lower ratios of P b / U appropriate to their younger age. To first order, it can be seen directly from the raw data on the calibration graphs that most of the Changxing zircons have coherent 2°6pb*/238U with a reproducibility comparable to that obtained for the homogeneous standard zircon. One or two analyses
188
J.C. CLAOUI~-LONGET AL
have slightly lower P b / U ratios falling below the main group, and a single point is distinctively high with a P b / U ratio intermediate between the Changxing zircons and the 572 Ma standard. Plotted on a Concordia diagram in Fig. 5 these three groupings stand out clearly. The main population of 34 analyses clusters on Concordia at about 250 Ma. All of these analyses have the same composition, within error, and the weighted mean 2°6pb*/23SU ratio of the group is 0.03973+ 0.00057 (20), equivalent to an age of 251.2 + 3.4 Ma (20). Departing significanty from the main group are two analyses with younger 2°6pb*/238U ages. These are one of two analyses of grain 10, and a single analysis of grain 35. Both are zircons whose thorium isotopic compositions lie off the T h / U 0.08
==
(a)
.a
0.06
~,~
=o
Silurian xenocryst
0.04
~ 0.02
Pa°:~y~ts; n
I
Y
0.00 0.0
~
Zircons that have
lost radiogenicPb |
I
0.2
0.4
207pb / 235 U
0.6
0.08 1 (b) 0.04
25~¢
/ J ~ oooo11" 0.03 0.2
/
~
g
Permian-Triassic boundary 251.2 +_3.4 Ma (2~) " I 0.3
207pb / 235 U
0.4
Fig. 5. (a) Concordia diagram illustrating the range of individual zircon compositions (lo errors) found in the Changxing boundary bentonite. Apart from a single xenocryst and two zircons with lowered P b / U ratios, 32 of the 35 zircons analysed cluster on Concordia at approximately 250 Ma. (b) Expansion of the shaded area in (a) showing the precision and concordance of the weighted mean composition of the principal population of analyses (plotted as the 2o error box).
isochron in Fig. 3, indicating disturbance. Grain 10 is the highest U grain in the sample and analysis 2 has 1500 p p m U; it is therefore not surprising that it has been susceptible to Pb loss. Grain 35 has a more normal U content of 280 p p m U, but the evidence that its 2°6pb*/238U age is younger than the main group by more than the 95% confidence limits of its uncertainty, and that its Th system has been disturbed, combine to indicate that it, too, has lost radiogenic Pb. The compositions of grains 7, 15, 19, 30 and 31 also lie off the T h / U isochron but there is no indication that they have lost radiogenic Pb relative to the main group: post crystallisation T h / U fractionation is therefore inferred as the mechanism causing them to depart from T h / U isochron compositions. Lead loss from one area of grain 10 raises the question of Pb loss in the other analysed area of that grain. Analysis 10.1 also has an elevated U content (785 p p m U) and its apparent 2°6pb*/238U age is the youngest to fall statistically within the 95% confidence limits of the main group of compositions; it lies 1.99o below the mean. Deletion of this point from the main group would have the effect of slightly raising the mean age and uncertainty of the group from 251.2 + 3.4 Ma (20) to 251.7 _+ 3.6 Ma (20). However, the Th system of this area of the grain shows no evidence of disturbance, and, in the absence of statistical justification for treating this point as an outlier, it is retained as a respectable member of the principal population. The only other departure from the main group of compositions is grain 33. The Concordia diagram in Fig. 5 shows this zircon to be concordant and Silurian in age. Although selected as being similar in appearance to all the other grains, this grain must be either a xenocryst incorporated in the ash eruption, or a detrital zircon washed in with the water relain volcanic horizon. Clearly, the 251.2+ 3.4 Ma (20) population represents the crystallisation age of the volcanic ash. Equally clearly, the sample has been exposed to slight alteration, and areas of two of the 34 zircons have lost radiogenic Pb. It is therefore valid to ask whether the P b / U composition of the main group of zircons also includes a component of Pb loss. To answer this question, concordance testing of the U-Pb systems by reference to 2°7pb*/2°6pb* ratios is not useful because of the
THE AGE OF THE PERMIAN-TRIASSIC BOUNDARY
2OTpb Pb
Fig. 6. Histograms (gaussian summation) of the 2°6pb*/238U, 2°7pb*/235U and 2°7pb*/2°rpb* ages of zircons in the principal population, showing the concordance of the three independent chronometers and the close approach of the P b / U data to a normal distribution.
imprecision of 2°7pb*/2°6pb* ages of zircons of this age. Recourse must therefore be made to the grouping of P b * / U ratios. The most important feature expected of magmatic zircons is a single dominant age population. It is extremely unlikely that a Pb loss process will generate exactly the same degree of Pb loss in all areas of all zircons, so it is unlikely that zircons which have lost Pb will form a coherent group of P b / U ratios. Fig. 6 shows histograms of the 2°6pb/238U, 2°7pb//z35u and 2°7pb//z°rpb ages of the principal population of Changxing zircons calculated by gaussian summation of the individual analyses and uncertainties. This illustrates the close approach of the P b / U data to a normal distribution and the concordance (at the given level of uncertainty) of the 2°6pb*-238U and 2°7pb*-z35U chronometers. This consistency, and the agreement, within error, of 34 individual 2°6pb*/z38U age measurements, strongly suggests that the mean 2°6pb*//238U age is the primary crystallisation age and has not been biased to a younger level by loss of Pb. Accordingly, it is concluded that the age of deposition of the Bed 22 volcanic ash is 251.2 +__3.4 Ma (20).
6. Summary and conclusions (1) A 5 cm bentonite occurs within a sedimentary sequence at Meishan in South China which is a candidate for the world stratotype section for the Permian-Triassic boundary. This thin volcanic
189
layer separates the Changxing Formation limestone, stratotype for the uppermost Permian Changxingian Stage, from beds containing the Triassic ammonoid Otoceras? and conodont Hindeodus parous. As closely as can be defined with current biostratigraphy, therefore, this bentonite is the bed marking the Permian-Triassic boundary. (2) Zircons in the bentonite have been studied using the S H R I M P ion microprobe. Of 35 zircons analysed, one was found to be a Silurian xenocryst, two have areas that have leaked radiogenic Pb, and the remaining 34 analysed areas agree within error at a 2°6pb*/238U age of 251.2 + 3.4 Ma, supported by concordance of 2°7pb*/235U data. This age is interpreted as the age of eruption of the volcanic ash and is the first direct measure of the age of the Permian-Triassic boundary. Owing to the unique biostratigraphic position of the volcanic layer, the age of the boundary is exactly the age of the bentonite and needs no extrapolation. (3) Finally, we note that debate continues as to (a) where in the biostratigraphic succession to define the Permian-Triassic boundary, and (b) where in the world to define the stratotype section [2]. The presence of volcanic layers in the fossil-rich marine Upper Permian and Lower Triassic of South China offers a unique opportunity to match the biostratigraphic and numerical time scales in this section of the Phanerozoic. Certainly, future refinements of the age of the Permian-Triassic boundary are likely to depend on re-analysis of the uniquely placed bentonite reported in this contribution. In addition to the biostratigraphic merits which make the Changxing Permian-Triassic sequence a candidate for the world stratotype, the utility of defining the Permian-Triassic boundary in the vicinity of this known dateable horizon should be considered.
Acknowledgements This work was made possible through the support of the National Natural Science Foundation of China, and financial support of the Australian National University enabling Z.Z. to visit Australia. Wang Caihong, Wang Guihua and Huang Zhaoxian assisted with collection and preparation of the bentonite sample, and S.
190
Maxwell gave analytical assistance. We thank J.M. Dickens for comments and advice.
References 1 W.B. Harland, R.L. Armstrong, A.V. Cox, L.E. Craig, A.G. Smith and D.G. Smith, A geological time scale 1989, 263 pp, 1990. 2 E.T. Tozer, Towards a definition of the Permian-Triassic boundary, Episodes 11, 251-255, 1988. 3 M. Ding, Permian-Triassic boundary and conodonts in South China, Mere. Soc. Geol. Ital. 34, 263-268, 1986. 4 H. Yin, F. Yang, K. Z h a n g and W. Yang, A proposal to the biostratigraphic criterion of Permian/Triassic boundary, Mem. Soc. Geol. Ital. 34, 329-344, 1986. 5 J.-k. Zhao, J.-z. Sheng, Z.-q. Yao, X.-1. Liang, C.-z. Chen, L. Rui and Z.-t. Liao, The Changhsingian and Permian Triassic boundary of South China, Bull. Nanjing Inst. Geol. Palaeontol., Acad. Sinica 2, 58-72, 1981. 6 J.-z. Sheng, C.-z. Chen, Y.-g. Wang, L. Rui, Z.-t. Liao, Y. Bando, K.-i. Ishii, K. Nakazawa and K. Nakamura, Permian-Triassic boundary in middle and eastern Tethys, J. Fac. Sci. Hokkaido Univ. Ser. IV 21, 133-181, 1984. 7 E.T. Tozer, Definition of the Permian-Triassic (P T) boundary: the question of the age of the Otoceras beds, Mem. Soc. Geol. Ital. 34, 291-301, 1986. 8 S.C. Forster and G. Warrington, Geochronology of the Carboniferous, Permian and Triassic, in: The Chronology of the Geological Record, N.J. Snelling, ed., Geol. Soc. London Mem. 10, 99-113, 1985. 9 M.J. Irwin, Aspects of Early Triassic sedimentation in the Esk Trough, southeast Queensland, Pap. Dep. Geol. Univ. Qld. 7, 46-62, 1976. 10 N.J. De Jersey, Triassic miospores from the Esk Beds, Publ. Qld. Geol. Surv. 357, 1-21, 1972. 11 J.M. Anderson and A.R.J. Cruickshank, The biostratigraphy of the Permian and Triassic, Part 2. A preliminary review of the distribution of Permian and Triassic strata in time and space. Palaeontol. Afr. 21, 15-44, 1978. 12 D.A. Valencio, J.E. Mendia and J.F. Vilas, Palaeomagne-
J.¢. CLAOU E-LONG ET AL. tism and K-Ar ages of Triassic igneous rocks from the Ischigualasto-lschichuca basin and Puesto Viejo formation, Argentina, Earth Planet. Sci. Lett. 26, 319-330, 1975. 13 D.C. Green and A.W. Webb, Geochronology of the northern part of the T a s m a n Geosyncline, in: The T a s m a n G e o s y n c l i n e - - a Symposium, A.K. Dedmead, G.W. Tweedale and A.F. Wilson, eds., Geol. Soc. Aust. Qld. Div., pp. 275-291, 1974. 14 P.R. Murphy, H. Schwarzbodk, L.C. Cranfield, I.W. Withnall and C.G. Murray, Geology of the Gympie 1:250,000 sheet area, Rep. Geol. Surv. Qld. 96, 157 pp., 1976. 15 G.S. Odin and N.H. Gale, Numerical dating of Hercynian times (Devonian to Permian), in: Numerical Dating in Stratigraphy, G.S. Odin, ed., pp. 487-500, 1982. 16 G.S. Odin, C o m m e n t s on the geochronology of Carboniferous to Permian times, in: The Chronology of the Geological Record, N.J. Snelling, ed., Geol. Soc. London Mem. 10, 114 117, 1985. 17 B.L. Gulson, C.F.K. Diesel, D.R. Mason and T.E. Krogh, High precision radiometric ages from the northern Sydney Basin and their implications for the Permian time interval and sedimentation rates, Aust. J. Earth Sci. 37, 459-469, 1990. 18 R.H. Steiger and E. J~iger, Subcommission on Geochronology: convention on the use of decay constants in geo- and cosmochronology, Earth planet. Sci. Lett. 36, 359-362, 1977. 19 W. Compston, I.S. Williams and C. Meyer, U-Pb geochronology of zircons from lunar breccia 73217 using a sensitive high mass-resolution ion microprobe, J. Geophys. Res. 89, B525-534, 1984. 20 G.L. C u m m i n g and J.R. Richards, Ore lead isotope ratios in a continuously changing Earth, Earth Planet. Sci. Lett. 28, 155-171, 1975. 21 I.S. Williams and S. Claesson, Isotopic evidence for the Precambrian provenance and Caledonian m e t a m o r p h i s m of high grade paragneisses from the Seve Nappes, Scandinavian Caledonides. Contrib. Mineral. Petrol. 97, 205217, 1987.
Earth and Planetary Science Letters, 105 (1991) 182 190
Elsevier Science Publishers B.V., Amsterdam [CHI
The age of the Permian-Triassic boundary J.C. Claour-Long
a, Z h a n g Z i c h a o
b,
Ma Guogan b and Du Shaohua b
Research School of Earth Sciences, A ustralian National University, GPO Box 4, Canberra, A. C. T. 2601, Australia h Yichang Institute of Geology and Mineral Resources, PO Box 502, Yichang 443003, China
Received November 8, 1990; revision accepted March 21, 1991
ABSTRACT The 5 cm boundary clay bed in the Chinese stratotype section through the Permian-Triassic boundary has been recognised as a bentonite. SHRIMP ion microprobe dating of zircons in the bentonite indicates a magmatic age of 251.2 _+3.4 Ma (20); this is the first direct constraint on the numerical age of the Permian-Triassic boundary. Future refinements of ages at this important, but poorly constrained, level of the Phanerozoic timescale may depend on re-analysis of this uniquely placed volcanic horizon, and other bentonites in the fossiliferous Chinese Upper Permian and Lower Triassic. The utility of defining the Permian Triassic boundary in the Chinese stratotype section, in the vicinity of known dateable horizons, should be considered.
1. Introduction T h e P e r m i a n - T r i a s s i c b o u n d a r y m a r k s one of the three m o s t p r o f o u n d biological revolutions in the P h a n e r o z o i c record, c o m p a r a b l e to the explosive evolution d e f i n i n g the onset of the C a m b r i a n a n d the g l o b a l e x t i n c t i o n s of the C r e t a c e o u s - T e r tiary b o u n d a r y . H o w e v e r , this transition has p r o v e d n o t o r i o u s l y difficult to s t u d y b e c a u s e m o s t areas of the world have a gap in the s e d i m e n t a r y r e c o r d at this point, or lack the fossil r e c o r d r e q u i r e d for b i o s t r a t i g r a p h i c correlation. In this c o n t e x t it is not surprising that the t i m i n g of events a b o u t the P e r m i a n - T r i a s s i c b o u n d a r y is p o o r l y u n d e r s t o o d : the recent time scale c o m p i l a tion of H a r l a n d et al. [1] lists n o reliable r a d i o m e t ric dates for the P e r m i a n stages u n d e r l y i n g the b o u n d a r y , a n d a single K - A r age in the L o w e r Triassic. This is u n f o r t u n a t e , b e c a u s e it is the rates of the s p e c t a c u l a r biological processes t a k i n g p l a c e that form the m a i n interest of the P a l a e o z o i c M e s o z o i c transition. T h e p u r p o s e of this c o n t r i b u t i o n is to r e p o r t the first direct c o n s t r a i n t on the age of the Permian-Triassic boundary, obtained by SHRIMP zircon d a t i n g of a b e n t o n i t e occurring at the b o u n d a r y in a Chinese c a n d i d a t e s t r a t o t y p e section. A m o n g the rare s e d i m e n t a r y sequences offer0012-821x/91/$03.50
© 1991 - Elsevier Science Publishers B.V.
ing n e a r - c o m p l e t e r e c o r d s t h r o u g h the P e r m i a n Triassic b o u n d a r y , those in S o u t h C h i n a have one of the richest L a t e P e r m i a n f a u n a s a n d c o n t r a s t with relatively b a r r e n s t r a t a elsewhere. These include the s t r a t o t y p e for the y o u n g e s t P e r m i a n Stage a n d c a n d i d a t e s t r a t o t y p e sections t h r o u g h the P e r m i a n - T r i a s s i c b o u n d a r y . It has recently b e e n recognised that thin clay b e d s i n t e r s p e r s e d t h r o u g h the Chinese sequence are b e n t o n i t e s , c l a y - a l t e r e d volcanic ashes, offering the p o s s i b i l i t y of direct r a d i o m e t r i c d a t i n g o f the s t r a t i g r a p h y . By r e p o r t i n g an a c c u r a t e n u m e r i c a l age for one of these, we d r a w a t t e n t i o n to the p o t e n t i a l for f u t u r e direct c o n t r o l on the L a t e P e r m i a n a n d E a r l y Triassic time scale b y reference to the u n i q u e c o n j u n c t i o n of fossiliferous a n d volcanic layers in the Chinese record.
2. The biostratigraphic markers T h e i m p o r t a n c e of the r a p i d c h a n g e s k n o w n as the P e r m i a n - T r i a s s i c b o u n d a r y are u n i v e r s a l l y agreed, b u t an i n t e r n a t i o n a l c o n v e n t i o n d e f i n i n g the f o r m a l p l a c i n g of the b o u n d a r y has yet to be resolved. This issue c u r r e n t l y is the subject o f an I U G S W o r k i n g G r o u p , whose i n t e r i m suggestion is to define the b o u n d a r y at the a p p e a r a n c e of the a m m o n o i d O t o c e r a s [2]. I n terms of c o n o d o n t
THE AGE OF THE PERMIAN-TRIASSIC BOUNDARY
zonation this is closely equivalent to the base of the Hindeodus parvus Zone [3]: H. parvus first appears within the Otoceras ammonoid Zone and is more geographically widespread than preserved specimens of Otoceras; on these grounds it has been suggested as a more practical indicator for worldwide correlation of the boundary [4]. Most workers consider that the youngest Permian faunas are typified by sediments defining the Changxingian Stage in China (named after Changxing Zhejiang Province), which is the uppermost Permian Stage [5,6]. The type area is in Meishan of Changxing where marine sequences are well exposed in a series of sections, designated Sections A - E , near Zhongxin Dadui quarry [6]. These include the stratotype for the Changxingian Stage and Chinese candidate stratotype for the Permian-Triassic boundary (see Fig. 1). The Changxing Formation is principally a carbonate sequence and contains typical Palaeozoic forms including fusulinids, rugose corals, and the uppermost limits of the Pseudotirolite, Pleuronodoceras
Bed
number
Grey-black marl/ mudstone
27~:'-'tIii!i!iii~iiiiii 8 ~:.;iii~il 6 ili ~iiii!i!l]
Clay
25
Grey dolomitic
24
marl
Bed
thickness (cm)
Ophiceras Claraia stachei Claraia wangi
7
*Mixed bed" 1 Otoceras? Hypophiceras Tompophiceras brachiopods
Grey-black 23 mudstone Grey-whiteyellow clay 22
*Mixed bed" 2 Geinitzina Hindeodus parvus
TRIASSIC PERMIAN ~
Grey limestone with chert 21 nodules
25
Palaeofusulina "~ ,~
Fig. 1. Stratigraphic column of Section D through the Permian-Triassic boundary in Meishan, South China. The Changxing Formation limestone in this section is the stratotype for the uppermost Permian Changxingian Stage [6]. The base of the Triassic is defined by the presence of Otoceras and Hindeodus parvus in the Mixed F a u n a Beds of the overlying Chinglung Formation. The boundary between the Permian and Triassic is therefore marked by the Bed 22 bentonite dated in this contribution. Adapted from [3,6].
183
and Rotodiscoceras ammonoid Zones, and is used with confidence to define the uppermost Permian. More equivocal is the attribution of the overlying Chinglung Formation. The contact is marked by lithological change from the Changxing Limestone to, successively, Mixed Fauna Bed 1, a mudstone about 10 cm thick; Mixed Fauna Bed 2, a dolomitic marl 20-30 cm thick; and Mixed Fauna Bed 3 comprising further mudstone of variable thickness; with thin clay beds intervening at the top of each carbonate layer [6] (Fig. 1). These mixed fauna beds contain new Triassic forms (e.g. bivalves, new ammonoids and new conodonts) together with some Permian-type fossils (principally brachiopods). Triassic fauna include the characteristic bivalve Claraia which is found in Mixed Bed 3 and above. The ammonoid Otoceras, is reported by Sheng et al. [6] from Mixed Bed 1, but Tozer [7] notes that preservation of specimens is poor and identification cannot be certain. As documented by Ding [3], the beginning of the Otoceras ammonoid Zone can readily be substituted by the beginning of the H. parvus conodont Zone, and H. parvus is definitely identified in the limestone of Mixed Fauna Bed 2 [3]. Sheng et al. [6] conclude that the Permian brachiopods in the mixed fauna beds are Palaeozoic survivors coexisting with the new Mesozoic fauna, and that there is a continuous sedimentary record through the Permian-Triassic transition. The boundary might then be placed anywhere between the first appearance of characteristic Triassic fossils (Otoceras? in Mixed Bed 1, Hindeodus parvus in Mixed Bed 2 which is 7 cm higher) and the last appearance of Permian types. These considerations lead to establishing a boundary at the base of Mixed Bed 1 of the Chinglung Formation, at its contact with the underlying Changxing Formation limestone. On the other hand, Tozer [2] points to the fact that the base of the bed containing Otoceras?, the distinguishing basal Triassic ammonoid, is a sharp bedding plane and lithological change. The mixed fauna beds can be interpreted to reflect erosion of the underlying Permian strata and consequent redeposition of Permian brachiopods in the earliest Triassic sediments. On this basis, the abrupt lower contact of the Chinglung Formation can be interpreted as unconformable and the Permian-Triassic boundary would then be defined within the unconformity.
184
J.c. CLAOUI~-LONG Eq- AL.
The Permian-Triassic boundary therefore lies between the top of the Changxing Formation limestone and the base of the overlying Chinglung Formation, regardless of whether unconformity or continuous sedimentation is interpreted, and the 5 cm clay layer recognised as a bentonite and dated in this contribution is the bed marking the boundary.
land et al. [1] prefer slightly younger ages near to 245 Ma. New dating reported by Gulson et al. [17] suggests an older age near to 255 Ma. In reality, the data are permissive and allow any number to be chosen within the very wide band between approximately 240 Ma and approximately 255-260 Ma, with preference naturally being accorded to the centre of the permitted range.
3. Existing numerical age constraints
4. Sampling rationale
No new data have become available since the review of Forster and Warrington [8] noted that " n o radiometric data are available from stratigraphic levels close to t h e . . . P e r m i a n - T r i a s s i c boundary", and "radiometric evidence for the age of upper Permian sequences is inconclusive". Faced with this inadequate database, it has been necessary to extrapolate the likely age of the boundary from uncertain age information in the Middle Triassic. The oldest ages obtained from undoubted Triassic sediments are from the Toogoolawah G r o u p in Australia, and the Puesto Viejo Formation in Argentina. Andesites in the Toolgoolawah G r o u p have informally reported K-Ar ages of 242 _+ 5 Ma (whole rock) and 239_+ 5 Ma (hornblende) [9], and palynology assigns these sediments as Middle Triassic (Anisian or Ladinian [10]). This suggests an age of approximately 240 Ma for the Middle Triassic. Fossil reptile assemblages in the Puesto Viejo Formation indicate Early to Middle Triassic strata (perhaps Scythian [11]) and intercalated volcanics have a mean K-Ar age of 237 + 4 Ma [12]; this may represent a minimum age for the Scythian. In Queensland, the KinKin Beds of the G y m p i e Basin, which contain Early Triassic (Smithian) ammonoids are intruded by the G o o m b o o r i a n Diorite which has an average K-Ar age of 236 _+ 7 Ma [13,14], again placing an approximate minimum constraint on the Early to Middle Triassic. N o n e of these ages are for samples with tight biostratigraphic constraints, but in general, the Permian-Triassic boundary has been believed to be 5 - 1 0 Ma older than this Triassic data. Forster and Warrington [8] suggested a boundary age close to 250 Ma, but suggested that an older age might be permitted. Based on essentially the same data, both Odin [15,16] and the recent review by Har-
Everywhere in South China a thin clay layer intervenes between the Permian Changxing Formation and the overlying Triassic Chinglung Formation [6]. This laterally continuous bed is one of m a n y clay layers interspersed throughout the Chinese Permian and Triassic sequences. Recent investigation has shown that some of these clays are in fact bentonites, or clay-altered volcanic ashes. This hitherto unsuspected association of volcanics with the most fossiliferous known sediments of the Late Permian and Early Triassic opens up, for the first time, the possibility of direct numerical age control of this part of the Phanerozoic time scale. As a first approach the bentonite occurring at the P e r m i a n - T r i a s s i c boundary was sampled for dating. This bentonite is well exposed in stratotype Section D of Changxing, which is the stratotype for the Changxingian Stage [6]. The volcanic layer is Bed 22, and according to the definition of the Permian-Triassic boundary discussed above, the bentonite is the bed marking the boundary: the 5 cm clay layer lies between the top of the Permian Changxing Formation limestone and the base of the overlying Mixed Fauna Bed 1 of the Chinglung Formation (Fig. 1). Approximately 30 kg of sample was obtained from the 5 cm thick bentonite in a fresh quarry face exposure of Section D. Care was taken to exclude from the sample any material from adjacent beds. The bentonite is greyish-white-yellow in colour and is a typical clay-altered ash now comprising mainly illite and smectite. Other phases present in trace amounts include the high temperature feldspar sanidine, zircon, and very small quantities of rutile and apatite. The sample was soaked in clean water to disperse the clay, the heavy mineral fraction was then separated using a Wilfley table, and the zircon separate was purified
THE AGE OF THE PERMIAN-TRIASSIC BOUNDARY
magnetically a n d b y h a n d picking. Sanidine was also o b t a i n e d for separate 4°Ar/39Ar dating which will be reported elsewhere. Zircons in the b e n t o n i t e (Fig. 2) have simple euhedral prismatic shapes a c c o m p a n i e d by concentric oscillatory growth zones consistent with m a g m a t i c crystallisation. N o cores or overgrowths are observed. All zircons have a b u n d a n t a n d large inclusions of other crystalline phases, a n d anhedral shapes filled with a m o r p h o u s material interpreted to be altered volcanic glass. These features indicate, respectively, late crystallisation of zircon seeded on other phases, a n d rapid skeletal growth, b o t h features consistent with crystallisation d u r i n g e r u p t i o n of the volcanic ash. The u b i q u i t o u s inclusions form up to 40% of some crystals, so conven-
185
tional analysis would u n a v o i d a b l y analyse a large p r o p o r t i o n of i n c l u d e d material. T h e ion microp r o b e m e t h o d of analysis was therefore chosen.
5. Shrimp zircon dating A few milligrams of zircon were o b t a i n e d from the 30 kg sample of the Bed 22 bentonite. Of these, 100 of the optically clearest zircons were h a n d picked a n d m o u n t e d o n the surface of a n epoxy plug, a n d the grains were sectioned approximately in half by polishing to expose grain interiors. U - T h - P b isotopic c o m p o s i t i o n s were measured for 25 /~m diameter areas of the sectioned zircons, using the S H R I M P ion microprobe. A cold trap was e m p l o y e d in the sample
Fig. 2. Photomicrographs of zircons in the Changxing Permian-Triassic boundary bentonite. Grains have been mounted in epoxy and sectioned in half to expose internal features. Trans = normal transmitted fight; refl = normal reflected light. Fields of view are approximately 300 × 250/Lm. The - 25 ~m wide/2 t~m deep analysis crater excavated by the ion microprobe is clearly visible in B and D, and illustrates the ability of the probe to avoid inclusions and target pure zircon for isotopic analysis, where conventional dissolution analysis would unavoidably incorporate the included material. A. (trans.) and B. (refl.) paired views of grain 21; C. (trans.) and D. (refl.) paired views of grain 1. Both zircons are of simple euhedral magmatic form and contain a high proportion of included material. The negative zircon crystal shape enclosing amorphous material at the centre of grain 21 appears to be a hollow enclosing altered volcanic glass, an indication of rapid, skeletal crystal growth. E. (trans.). Grain 10 which is sector zoned, a rare zoning pattern associated with rapid crystal growth. The pyramid sector has 1500 ppm U, a Th/U ratio of 1.9, and has leaked radiogenic Pb (see Table 1 and discussion in text); whereas the prism sector has a mildly elevated U content, a normal Th/U ratio less than 1, and a concordant Pb/U composition. F. (trans.). Grain 33 a Silurian xenocryst.
186
J.C. C L A O U E - L O N G
ET At.
c h a m b e r to m i n i m i s e h y d r i d e c o n t r i b u t i o n s , a n d a c h i p o f s t a n d a r d z i r c o n SL13 w a s m o u n t e d i n
initial Pb have been m a d e using average crustal Pb o f t h e s a m e a g e as t h e z i r c o n s [20] a n d c a l c u l a t e d
e p o x y w i t h t h e C h a n g x i n g z i r c o n s to m i n i m i s e a n y
u s i n g o n e o f t w o m e t h o d s as a p p r o p r i a t e . T h e Z°4pb m e t h o d c a l c u l a t e s t h e a m o u n t o f i n i t i a l P b
p o s s i b l e b i a s in t h e c o n d i t i o n s o f a n a l y s i s b e t w e e n the standard and the unknowns. Decay constants used
in this p a p e r
are
those recommended
by
S t e i g e r a n d J a e g e r [18].
b y direct r e f e r e n c e to the m e a s u r e d a b u n d a n c e of 2°4pb. T h i s a s s u m e s o n l y t h e c o m p o s i t i o n o f t h e common
Pb, b u t has low p r e c i s i o n o w i n g to the
E a c h a n a l y s i s is t h e m e a n o f five c y c l e s t h r o u g h
v e r y l o w a b u n d a n c e o f 2°4pb. M o r e p r e c i s e c a l c u -
t h e m a s s s t a t i o n s , a n d d a t a w e r e r e d u c e d in t h e
l a t i o n is p o s s i b l e f r o m t h e d i f f e r e n c e b e t w e e n t h e measured 2°spb/2°6pb ratio of each analysis and
m a n n e r d e s c r i b e d e l s e w h e r e [19]. C o r r e c t i o n s f o r
TABLE 1 Ion microprobe U-Th-Pb isotopic data for zircons in the Changxing boundary bentonite Grain
U
Th
-area
(ppm)
(ppm)
1.1 2.1 3.1 4.1 5.1 6.1 7.1 c 8.1 9.1 10.1 10.2 ~ 11.1 12.1 13.1 14.1 15.1 ~ 15.2 ~ 16.1 17.1 18.1 19.1 c 20.1 21.1 22.1 23.1 24.1 25.1 26.1 27.1 28.1 29.1 30.1 c 31.1 c 32.1 33.1 34.1 35.1 ~
473 484 311 209 421 212 318 349 367 785 1492 391 277 785 258 572 689 383 300 372 368 247 504 515 317 211 292 305 436 321 146 174 445 423 552 265 278
247 235 110 94 190 134 154 148 162 514 2856 178 138 291 170 278 513 137 112 156 273 97 194 335 135 85 182 163 21l 108 71 86 219 255 246 152 215
Th/U 0.52 0.49 0.35 0.45 0.45 0.64 0.48 0.42 0.44 0.65 1.91 0.46 0.50 0.37 0.66 0.49 0.74 0.36 0.37 0.42 0.74 0.39 0,39 0.65 0.42 0.40 0.62 0.53 0.49 0.34 0.49 0.50 0.49 0.60 0.45 0.58 0.77
2°4pb
fZO6pba
2orpb/23s U
2o7pb/235U
207pb/206pb
Apparent age b
(ppb)
(%)
( ± 1o)
( _+lo)
( _+lo )
(Ma) _+lo
3 3 3 4 8 3 4 8 3 21 123 5 4 2 5 8 16 8 4 4 4 10 4 5 3 11 30 5 0 6 2 2 2 4 3 7 12
0.31 0.33 0.56 1.16 0.92 0.78 0.70 1.18 0.53 1.48 5.56 0.64 0.73 0.16 0.92 0.77 1.32 1.02 0.69 0.62 0.58 2.17 0.39 0.46 0.55 2.57 5.26 1.00 0.00 0.95 0.67 0.53 0.30 0.46 0.14 1.37 2.60
0.0398+11 0.0403_+11 0.0401_+11 0.0386+11 0.0416 + 12 0.0403 -+ 12 0.0390_+11 0.0391_+11 0.0381-+11 0.0376_+11 0.0300_+ 9 0.0392-+11 0.0425 -+ 12 0.0411_+12 0.0404-+12 0.0392_+11 0.0378+12 0.0410_+12 0.0407_+13 0.0401_+13 0.0390_+13 0.0403_+13 0.0393 ± 13 0.0412_+13 0.0386_+13 0.0406_+13 0.0404_+13 0.0383 _+13 0.0401_+13 0.0404 _+13 0.0383_+13 0.0385_+13 0.0396_+13 0.0409_+13 0.0696 -+ 23 0.0412_+13 0.0359-+12
0.283_+14 0.285_+14 0.292+16 0.273_+19 0.291 _+15 0.276-+22 0.301_+23 0.263_+16 0.273-+14 0.253-+12 0.264-+21 0.290+16 0.304 _+20 0.308_+13 0.297_+22 0.26l_+20 0.259_+20 0.286_+16 0.282_+15 0.273_+14 0.294_+19 0.287_+18 0.264_+13 0.281_+15 0.266_+14 0.321_+21 0.274_+20 0.257_+15 0.289_+14 0.266 _+14 0.268_+19 0.266_+28 0.278_+17 0.298_+16 0.548_+23 0.304+19 0.220_+33
0.0516_+19 0.0512_+18 0.0528_+22 0.0513-+30 0.0508_+21 0.0498_+35 0.0560_+38 0.0488_+24 0.0520_+21 0.0489+17 0.0638-+46 0.0536+24 0.0519-+ 29 0.0544_+15 0.0532_+34 0.0483_+33 0.0498_+33 0.0507±22 0.0502_+19 0.0493_+17 0.0547_+29 0.0516_+26 0.0487_+15 0.0495_+18 0.0500_+19 0.0573_+29 0.0492_+31 0.0486_+23 0.0523_+17 0.0477_+18 0.0506+31 0.0501_+48 0.0510_+25 0.0530_+22 0.0571_+12 0.0535-+26 0.0444_+63
252+ 7 255_+ 7 254_+ 7 244_+ 7 263 _+ 7 255 _+ 7 247_+ 7 247-+ 7 241_+ 7 238_+ 7 191_+ 5 248-+ 7 268 _+ 7 259+ 7 256_+ 7 248+ 7 239_+ 8 259_+ 7 257_+ 8 253_+ 8 246_+ 8 255_+ 8 249 -+ 8 260_+ 8 244± 8 257± 8 255_+ 8 242 _+ 8 254-+ 8 256 _+ 8 243_+ 8 243_+ 8 250_+ 8 258_+ 8 434 _+14 260-+ 8 228_+ 7
a fZ06pb indicates the percentage of common 2°rpb in the total measured 2°6pb. b Apparent age is the 2°rPb/23Su age. c indicates 2°4pb correction substituted for 2°spb correction owing to alteration of the Th isotopic system.
THE AGE OF THE PERMIAN-TRIASSICBOUNDARY
187
the radiogenic 2°spb*/2°rpb* ratio appropriate to the age and T h / U of the zircon (2°8pb method [19]) and this has been applied where there has been no fractionation of Th and U in the zircons, and no differential movement of uranogenic and thorogenic Pb (Fig. 3). Uncertainties of common Pb correction are included in the errors quoted in Table 1. Radiogenic 2°7pb* is 20 times less abundant than 2°6pb* in zircons of this age and so has relatively poor measurement precision. As a consequence, calculated abundances of radiogenic 2°7pb* are very sensitive to correction for common 2°7pb, and this combines with the over-sensitivity of the 2°7pb*/z°rPb* chronometer in young zircons to frustrate definitive testing for concordance of the 2°Tpb*-235U and 2°6pb*-z38u systems. Pb loss a n d / o r inheritance is assessed instead from grouping of 2°6pb*/z38u data. Great significance is therefore attached to the reproducibility of P b / U ratios measured by the ion microprobe, which in turn is principally determined by the calibration of P b / U ratios to the known composition of a standard zircon. Ratios of P b / U in this sample are referenced to a value of 0.0928 for 2°6pb*/238U (equivalent to 572 Ma) in our standard zircon SL13. Forty S H R I M P analyses of this standard were alternated between each analysis of the unknowns throughout the two-day analytical period, and the data are compared with the S H R I M P calibration relationship in Fig. 4 which
.0
M.O~/
o. o
o
CY
e 0.20
~
~
0.10 Th/U 0.0o 0.0
~ 0.5
J 1.0
1.5
2.0
Fig. 3. Th/U-2°spb/2°6pb isochron diagram for the Changxing zircons showing the fit of data to the theoretical isochron for 251 Ma. Most zircons have seen no differential movement of Th and U, or of uranogenic and thorogenic Pb: initial Pb corrections can therefore be calculated accurately from difference between the measured 2°spb/2°6pb and that calculated from Th/U.
"~
(a) Day1
0.4
0.2
changXing~
0.0 6.5
t
i
7.5
8.5
UO/U 9.5
(b) D a y ~ 0.30
~
s~-~ " 0.20
~
° ~
Silurian xenocryst
0.10
0.00 I 5.5
,
J 6.5
,
,
UO/U
7.5
Fig, 4. Data for zircons in the Changxing boundary bentonite compared with the SHRIMP calibration curve and data collected for standard zircon SL13 at the same time. The calibration is a quadratic relationship between the measured ratios of P b / U and U O / U [21], and the data sets adhere closely to the expected curve. Scatter of data for the Changxing sample is similar to that observed for the standard, inferring a homogeneous composition within error.
shows the close adherence of analyses of the standard to the expected curve. This reproducibility-2.83 and 3.24% (lo), respectively, on the two days-is a direct measure of the reproducibility of a constant P b / U target and is included in the errors in P b / U quoted in Table 1. Additionally, the 0.47% ( l o ) uncertainty in the mean position of the calibration is included in the error on the mean age of the unknowns. Also shown in Fig. 4 are the measured compositions of the Changxing zircons. These adhere closely to a curve of the same form as that through the standard analyses, but at lower ratios of P b / U appropriate to their younger age. To first order, it can be seen directly from the raw data on the calibration graphs that most of the Changxing zircons have coherent 2°6pb*/238U with a reproducibility comparable to that obtained for the homogeneous standard zircon. One or two analyses
188
J.C. CLAOUI~-LONGET AL
have slightly lower P b / U ratios falling below the main group, and a single point is distinctively high with a P b / U ratio intermediate between the Changxing zircons and the 572 Ma standard. Plotted on a Concordia diagram in Fig. 5 these three groupings stand out clearly. The main population of 34 analyses clusters on Concordia at about 250 Ma. All of these analyses have the same composition, within error, and the weighted mean 2°6pb*/23SU ratio of the group is 0.03973+ 0.00057 (20), equivalent to an age of 251.2 + 3.4 Ma (20). Departing significanty from the main group are two analyses with younger 2°6pb*/238U ages. These are one of two analyses of grain 10, and a single analysis of grain 35. Both are zircons whose thorium isotopic compositions lie off the T h / U 0.08
==
(a)
.a
0.06
~,~
=o
Silurian xenocryst
0.04
~ 0.02
Pa°:~y~ts; n
I
Y
0.00 0.0
~
Zircons that have
lost radiogenicPb |
I
0.2
0.4
207pb / 235 U
0.6
0.08 1 (b) 0.04
25~¢
/ J ~ oooo11" 0.03 0.2
/
~
g
Permian-Triassic boundary 251.2 +_3.4 Ma (2~) " I 0.3
207pb / 235 U
0.4
Fig. 5. (a) Concordia diagram illustrating the range of individual zircon compositions (lo errors) found in the Changxing boundary bentonite. Apart from a single xenocryst and two zircons with lowered P b / U ratios, 32 of the 35 zircons analysed cluster on Concordia at approximately 250 Ma. (b) Expansion of the shaded area in (a) showing the precision and concordance of the weighted mean composition of the principal population of analyses (plotted as the 2o error box).
isochron in Fig. 3, indicating disturbance. Grain 10 is the highest U grain in the sample and analysis 2 has 1500 p p m U; it is therefore not surprising that it has been susceptible to Pb loss. Grain 35 has a more normal U content of 280 p p m U, but the evidence that its 2°6pb*/238U age is younger than the main group by more than the 95% confidence limits of its uncertainty, and that its Th system has been disturbed, combine to indicate that it, too, has lost radiogenic Pb. The compositions of grains 7, 15, 19, 30 and 31 also lie off the T h / U isochron but there is no indication that they have lost radiogenic Pb relative to the main group: post crystallisation T h / U fractionation is therefore inferred as the mechanism causing them to depart from T h / U isochron compositions. Lead loss from one area of grain 10 raises the question of Pb loss in the other analysed area of that grain. Analysis 10.1 also has an elevated U content (785 p p m U) and its apparent 2°6pb*/238U age is the youngest to fall statistically within the 95% confidence limits of the main group of compositions; it lies 1.99o below the mean. Deletion of this point from the main group would have the effect of slightly raising the mean age and uncertainty of the group from 251.2 + 3.4 Ma (20) to 251.7 _+ 3.6 Ma (20). However, the Th system of this area of the grain shows no evidence of disturbance, and, in the absence of statistical justification for treating this point as an outlier, it is retained as a respectable member of the principal population. The only other departure from the main group of compositions is grain 33. The Concordia diagram in Fig. 5 shows this zircon to be concordant and Silurian in age. Although selected as being similar in appearance to all the other grains, this grain must be either a xenocryst incorporated in the ash eruption, or a detrital zircon washed in with the water relain volcanic horizon. Clearly, the 251.2+ 3.4 Ma (20) population represents the crystallisation age of the volcanic ash. Equally clearly, the sample has been exposed to slight alteration, and areas of two of the 34 zircons have lost radiogenic Pb. It is therefore valid to ask whether the P b / U composition of the main group of zircons also includes a component of Pb loss. To answer this question, concordance testing of the U-Pb systems by reference to 2°7pb*/2°6pb* ratios is not useful because of the
THE AGE OF THE PERMIAN-TRIASSIC BOUNDARY
2OTpb Pb
Fig. 6. Histograms (gaussian summation) of the 2°6pb*/238U, 2°7pb*/235U and 2°7pb*/2°rpb* ages of zircons in the principal population, showing the concordance of the three independent chronometers and the close approach of the P b / U data to a normal distribution.
imprecision of 2°7pb*/2°6pb* ages of zircons of this age. Recourse must therefore be made to the grouping of P b * / U ratios. The most important feature expected of magmatic zircons is a single dominant age population. It is extremely unlikely that a Pb loss process will generate exactly the same degree of Pb loss in all areas of all zircons, so it is unlikely that zircons which have lost Pb will form a coherent group of P b / U ratios. Fig. 6 shows histograms of the 2°6pb/238U, 2°7pb//z35u and 2°7pb//z°rpb ages of the principal population of Changxing zircons calculated by gaussian summation of the individual analyses and uncertainties. This illustrates the close approach of the P b / U data to a normal distribution and the concordance (at the given level of uncertainty) of the 2°6pb*-238U and 2°7pb*-z35U chronometers. This consistency, and the agreement, within error, of 34 individual 2°6pb*/z38U age measurements, strongly suggests that the mean 2°6pb*//238U age is the primary crystallisation age and has not been biased to a younger level by loss of Pb. Accordingly, it is concluded that the age of deposition of the Bed 22 volcanic ash is 251.2 +__3.4 Ma (20).
6. Summary and conclusions (1) A 5 cm bentonite occurs within a sedimentary sequence at Meishan in South China which is a candidate for the world stratotype section for the Permian-Triassic boundary. This thin volcanic
189
layer separates the Changxing Formation limestone, stratotype for the uppermost Permian Changxingian Stage, from beds containing the Triassic ammonoid Otoceras? and conodont Hindeodus parous. As closely as can be defined with current biostratigraphy, therefore, this bentonite is the bed marking the Permian-Triassic boundary. (2) Zircons in the bentonite have been studied using the S H R I M P ion microprobe. Of 35 zircons analysed, one was found to be a Silurian xenocryst, two have areas that have leaked radiogenic Pb, and the remaining 34 analysed areas agree within error at a 2°6pb*/238U age of 251.2 + 3.4 Ma, supported by concordance of 2°7pb*/235U data. This age is interpreted as the age of eruption of the volcanic ash and is the first direct measure of the age of the Permian-Triassic boundary. Owing to the unique biostratigraphic position of the volcanic layer, the age of the boundary is exactly the age of the bentonite and needs no extrapolation. (3) Finally, we note that debate continues as to (a) where in the biostratigraphic succession to define the Permian-Triassic boundary, and (b) where in the world to define the stratotype section [2]. The presence of volcanic layers in the fossil-rich marine Upper Permian and Lower Triassic of South China offers a unique opportunity to match the biostratigraphic and numerical time scales in this section of the Phanerozoic. Certainly, future refinements of the age of the Permian-Triassic boundary are likely to depend on re-analysis of the uniquely placed bentonite reported in this contribution. In addition to the biostratigraphic merits which make the Changxing Permian-Triassic sequence a candidate for the world stratotype, the utility of defining the Permian-Triassic boundary in the vicinity of this known dateable horizon should be considered.
Acknowledgements This work was made possible through the support of the National Natural Science Foundation of China, and financial support of the Australian National University enabling Z.Z. to visit Australia. Wang Caihong, Wang Guihua and Huang Zhaoxian assisted with collection and preparation of the bentonite sample, and S.
190
Maxwell gave analytical assistance. We thank J.M. Dickens for comments and advice.
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