Two 40Ar/39Ar isotopic dating methods, the total fusion and the incremental heating techniques, were employed to determine the age of the Suzhou granitic intrusion in eastern China. Agreement between the results of the two experiments is excellent; both yield an age of about 121.7 Ma. A generally uncomplicated and concordant incremental heating spectrum is seen, with a plateau age of 122.2 Ma. This general concordance suggests a valid age, most likely reflecting the time that the biotite cooled to about 300°C. Since the intrusion is not very large and is shallowly emplaced, it probably cooled quickly; thus, the age most likely represents the actual intrusive event. This Cretaceous age is supported by previous ages determined by Chinese geologists.No embarg

Chave, Glenn R.

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SENIOR THESIS 40Ar/39Ar AGE DETERMINATION OF THE SUZHOU GRANITE by Glenn R. Chave May, 1988 Submitted as partial fulfillment of the requirements for the degree of Bachelor of Science in Geology & Mineralogy The Ohio State University Approved by: Kt/.~ Dr. K. A. Foland ABSTRACT Two 40Ar/39Ar isotopic dating methods, the total fusion and the incremental heating techniques, were employed to determine the age of the Suzhou granitic intrusion in eastern China. Agreement between the results of the two experiments is excellent; both yield an age of about 121.7 Ma. A generally uncomplicated and concordant incremental heating spectrum is seen, with a plateau age of 122.2 Ma. This general concordance suggests a valid age, most likely reflecting the time that the biotite cooled to about 300°C. Since the intrusion is not very large and is shallowly emplaced, it probably cooled quickly; thus, the age most likely represents the actual intrusive event. This Cretaceous age is supported by previous ages determined by Chinese geologists. Introduction The purpose of this thesis is to determine the age of the Suzhou granitic intrusion in eastern China, using the 40Ar/39Ar variation of the K-Ar dating method. The Suzhou Granitic Intrusion The Suzhou granite is an elliptical stock about 10 km west of the city of Suzhou, in eastern China (see Fig. 1). The intrusion is in the Yangtze-Qiantang Paleozoic Depression Zone, west of the subduction zone of the Western Pacific Plate against the China Continental Plate, and borders on the southeastern flank of the huge Changjiang tension fault (Zhang Xinglong et al., 1987). The stock has an area of 40 km2, only 11 km2 of which is exposed due to Quaternary cover (Liu Yimao et al., 1987). The intrusion penetrated the axis of a north-northeastern-trending syncline composed of Silurian and Devonian sandstones and Permian shale. Faulting is prevalent in the area, which is I common in zones of magmatic intrusion. Lamprophyres, quartz porphyry, rhyolitic porphyry, dacite porphyry, and quartz veining are also observed in the area (Liu Yimao et al., 1987). Past studies have classified the Suzhou granitic intrusion into three stages, based on composition. The Ouyang Xinguei-Wang Xonggang study of 1985 is most representative of the general consensus. The three stages are: 1. Porphyritic-like hornblende biotite granite. 2. Porphyritic-like biotite and coarse-grained biotite granite. 3. Fine-grained biotite granite and albitized biotite granite. 1 FIGURE . , ... CO-tOZOIC HA ; Qo-1 .. ~Mod e J.rtlory'""rlod -b1~h-eit.M MESOZOIC ERA {' ~~~::..~ ....... ~ T"'.'""'='°:'flod g....;...; .... PAlfOIOIC fll:A mllppttl'ol•o.ralc H"""'"''~·k ~ M•tamorphic: Jt.ock PROTEROZOIC ANO AltCHEOZOtC flA l'fiJ ~!ols::. .. lr>tnn/.,..J:ock . 1 a: l ,, "''I I . \( FIGURE 1 b: LOCATION OF SUZHOU GRANITIC INTRUSION IN EASTERN CHINA LOCATION OF IN RELATION SUZHOU TO THE GRANITIC INTRUSION CITY OF SUZHOU The 40Ar/39Ar Dating Method - Principles and Procedures The K-Ar dating method is described in detail by Dalrymple and Lanphere (1969) a~d also by many standard treatises including Faure (1986). The following are basic assumptions and must hold true for any K-Ar date to be considered valid: 1. The system was closed. A. No 40Ar has escaped since formation. B. No 40Ar has entered since formation. C. The system was closed to K gain or loss since formation. 2. No 40Ar was initially present in the sample. 3. A proper correction is made to account for atmospheric 40Ar. 4. Isotopic K composition is normal and has not changed except by decay of 40K. 5. The decay constants are accurate. The 40Ar/39Ar method of dating can alleviate some of the problems inherent in the conventional K-Ar method. The problems of different subsamples I and difficulties of determining absolute K and Ar concentrations are avoided, since K and Ar are measured on the same subsample, and since only the isotopic ratio of Ar is needed. An added advantage to 40Ar/39Ar dating is that only a small amount of sample is required. This is attractive for use with rocks containing a small concentration of suitable dating mineral, or with small amounts of a very valuble sample, such as lunar rocks. The 40Ar/39Ar dating method depends upon the production of 39Ar by the irradiation of K-rich samples with fast neutrons in a nuclear reactor. The sample is sealed in a glass tube with flux monitors on either side (see below), placed in the reactor and irradiated for several days to allow for the formation of 39Ar. The nuclear reaction is: 39K (n,p) 39Ar 19 18 (Eq.1) 3 39Ar decays to 39K by beta emission, with a half-life of 269 years. Due to this slow decay rate, 39Ar can be considered stable during the analysis period. The ratio of 40Ar/39Ar can then be measured using a mass spectrometer, and the age of the rock can be calculated. When a K-rich sample is irradiated, several reactions produce isotopes of argon; only the n,p reaction (Eq.1) is used here. The starting point is Mitchell's (1968) formula which gives the number of 39Ar atoms formed during neutron irradiation: 39Ar = 39K ATJ ¢(E) u(E) dE (Eq.2) where: 39K = number of atoms in irradiated sample. AT = period of irradiation. ¢(E) = neutron flux density at energy E u(E) = capture cross-section of 39K for neutrons with energy E. The amount of radiogenic 40Ar (40Ar*) atoms from decay of 40K is: 40Ar* = Xe 40K (eXt-1) I x (Eq.3) where: Xe = decay constant of 40K for electron capture. X = total decay constant of 40K~ The ratio of 40Ar/39Ar after initial irradiation is then: 40Ar* = (eXt-1} (Eq.4) 39Ar J where: X 39K ¢(E) u(E) dE (Eq.5) J = - 7fUi( ATJ Xe K This is where use of flux monitors becomes crucial. Since the cross-sections of 39K for capturing neutrons of various energies and the energy spectrum of incident neutrons are not well known, the neutron flux density and capture cross-sections are difficult to ascertain. The use of flux monitors solves this dilemma. Samples of known age (flux monitors) are irradiated next to the unknown sample in the sealed glass tube, and by plotting their J-values 4 versus their position in the sample vial, J for the unknown sample can be interpolated. The age of the unknown sample can then be found by: 1 t = X ln (40Ar* ) \39Ar J + l (Eq.6) Both the K-Ar and 40Ar/39Ar require a correction for atmospheric 40Ar and also other Ar isotopes produced during irradiation. The incremental heating technique is a variation of the 40Ar/39Ar dating method. A series of ages can be calculated for one sample by releasing argon from it by increasing the temperature in successive steps. If the sample has neither lost nor gained potassium and argon since it cooled below the temperature at which argon is quantitatively retained (known as the blocking temperature), the date found for each heating increment will be constant. If radiogenic argon was lost after cooling, however, a range of dates will emerge from the va~ing 40Ar/39Ar ratios. Since the loss of radiogenic argon is often I attributed. to reheating by a metamorphic event, this method can prqvide much insight in to the regional history of the sample area. 5 ANALYTICAL METHODS Sample Preparation: The samples studied were collected by Professors K. A. Foland and Chen Jiang-Feng in China in December, 1987. A total of six Suzhou granite samples were available. These samples were cut into slabs and thin sections were made. From subsequent hand sample and thin section examinations (see Table 1), the most promising sample (Sample #5) for K-Ar dating was chosen. This decision was based upon high biotite content (>50% for Sample #5) and upon freshness. Part of the sample was then crushed into fragments using a jaw-crusher, and ground further using a disc grinder. A system of graded sieves was then employed to segregate the sample by grain size. Each size fraction was examined under magnification. The goal was to separate the biotite from the quartz and feldspar in the crushed granite sample, since biotite is a K-rich mineral and thus very useful for K-Ar analysis. In the +40 mesh fraction, the grains of biotite were still mostly joined with the feldspar and quartz. In the -40/+60·mesh fraction, most of the biotite flakes had separated, yet were still intact. In the -60/+80 mesh fraction, the biotite flakes were so.minute that separation would have been very difficult. Thus, fraction 2, the -40/+60 size fraction, was determined to be the best grade for separation of biotite for analysis. The biotite was separated out using the paper-shake method. Using this procedure, the sample was placed at the top of a large piece of stiff card paper, which was then tilted to about a 45° angle and gently shaken. This method relies upon the fact that the biotite grains are relatively large and 6 TABLE 1 . S r.r.1 PL E t!.Q. 8 7 SZ·I 6 7 SZ·Z 67 SZ•J 87 SZ·4 8 7 SZ·S 8 7 sz. 6 '· THIN SECTION ANALYSES FOR BIOTITE CONTENT AND GRAINS I ZE OF SUZHOU GRAN 1 TE SJ"d.IPLES. AMPHIBOLE BIOTITE GRANITE BIOTITE GRANITE BIOTITE GRANITE DIOTIZATION GRANITE BIOTITITE BIOTIZATION GRANITE L'§.IJ Hl\TFp fl I QT I H:. CONTENT 1 0 -t. I 5 ·~ 1 0 "!. 5 ~. 5 0 •:. I 5 ·~ VERY SMALL GnA1r~s 0 • I • 0 • 5 MM MEDIUM-SIZED GRAINS 0 • 5 • I • 5 MM VERY SMALL GRAINS 0 • I • 0 • 5 MM MEDIUM·SIZCD GRAINS 0 • 5 • I • S MM LARGE GRAINS 0.5-J.o MM MEDIUM SIZCO GRAINS 0 • ! ~ 1 . 5 MM C 0 N C t: N T R I\ T £: D I f~ I\ VEIN much flatter than the more spherical and blocky quartz and feldspar grains. As the paper is shaken, the quartz and feldspar grains separate out and roll down the paper, leaving only biotite flakes (with their lower centers of gravity) remaining on the paper. This leftover biotite is shaken off into a container, and the process is repeated many times. In this manner, enough biotite can eventually be accumulated for analysis; due to the relatively high biotite content of the sample, about 2 g of biotite was collected in the space of a few hours. The sample was then thoroughly washed in acetone, oven-dried, divided into two subsamples, and wrapped separately in Sn-foil in preparation for neutron irradiation. 8 40Ar/39Ar Analyses The two subsamples were irradiated as part of OSU Package No. 36 at the Ford Nuclear Reactor at the University of Michigan in April, 1988. The samples were irradiated for 100 hours, allowed to "cool'' in the reactor pool for two weeks, and were returned back at OSU in May, 1988. The smaller sample was for regular 40Ar/39Ar gas dating by total fusion, the larger one for use with the incremental heating method. The smaller sample was fused using RF induction heating, and the larger sample was incrementally heated using a double vacuum system. The gases released during heating were treated in a number of stages to purify the inert Ar. The gas was cleaned using liquid nitrogen cold-finger traps to remove water vapor, SAES getters to remove active gases, and turbo pumps to remove hydrogen. Once purified, each Ar gas sample was run in three subdivisions, using a Nuclide Model 4.5-60-RSS mass spectrometer automated with an HP9825A computer. The flux monitors used for analysis were OSU intra-lab monitor Mon-4, a biotite with an age 121.66 Ma. More details on the OSU I , procedures may be found in Foland (1983). 9 RESULTS AND DISCUSSION The agreement between the two 40Ar/39Ar dating experiments, total fusion and incremental heating, is excellent (see Table 2). The total fusion sample yielded an age of 121.8 Ma, compared to the weighted sum of all gas fractions from the incremental heating experiment, 121.9 Ma. This age is equivalent to a conventional K-Ar age. Figure 2 is a plot of the incremental heating data, and shows a generally concordant spectrum; the various gas fractions yield nearly the same apparent ages. Minor variations existing are only slightly larger than their analytical uncertainties. The plateau age (see Faure, 1986 for discussion) of 122.2 Ma is found for 83.5% of the released 39Ar. This internal agreement of the incremental heating experiment reflects a uniform K and 40Ar distribution in the biotite and implies that the date is valid. The age of the biotite js a cooling age, as is the case with most minerals, and represents the poirit at which the mineral cooled such that 40Ar loss by diffusion was negligible. This closure temperature is about 300°C for biotite. The age of 122.2 Ma thus represents the time of cooling of the Suzhou granite below 300°C. Since the intrusion is not extremely extensive, and intruded to a shallow depth in the crust, it probably cooled relatively rapidly. This means that the age found in this study should agree closely with the actual time of the magmatic intrusion. This age of 122.2 Ma for the Suzhou granite coincides with a Cretaceous age determined by Chinese geologists for the area (Liu Yimao et al., 1987). Many small, shallow intrusions occurred at about this time throughout eastern China (Chen, J-F, personal communication). 10 :TABLE 2:· AGE DETERMINATION DATA FOR TOTAL FUSION AND INCREMENTAL HEATING METHODS' " •; .··: r .i i \. tr• •. •· .. _, ·.·. I l ~ =~ .. . . , . ;_i, ,.i; . " i.·._' ';(r .. ,; ". ·.· .·. " i I ' , ~I , ;·: :' " " i I ' . ' . . Fr act i eor~ 3 '+ 5 6 7 a:. TOTAL FUSJON METHOD o~m; Dm Tm? l0/31 (Cl r.~u. 3b/3'i neu. 31m Corr. 36/J7 1 mr 1 40"r 1 3bAr Km AC: U'iCERi~l~TY 1~1. l Hm. VALUE TOrnL TOTP.L (C1,Cll (hi. I I cc::Zi. Jt/- 1.5 l FilSE l.OC2ot+Ol 2.410lH3 Y.3750E-OJ t.mlE-Ol 9.mo 100.C) ~2.bl 0.2~ 5.37£+01 UOE•Ol 121.M o.m 1.786 SUM l.0028EtGI 2.HOli:-03 U750H3 l.2124Hl 9.rno ICO.O 92.H l.ll 5.Sit:tOI l.2~Et0! 121.640 '· AGE 121 .a MA ., ib:., INCREMENTAL HEATING ' . METHOD ouTPuT im i lEttP ,: . 40/H Jb/H I <Cl ! ~ .. ~00 m -no· ·.m m m m : 1000 , FUSE I [·: CUK ; c: Kus. "e11, 5. 0070E +Ol !. 4061£-0 I 1.019BEtOI 2.16qE-OJ 9.5213E+OO U251HI um£100 l.1352H3 9.51JeE100 9.205BHI 9.ll79EIOQ 4.59HH\ · 9. 4SBSEtOO 4. 6585E-OI 9. 5577E tOO 5.H65H\ 9. SbSOE tOO. I. 2752E-03 1.0022E•OI 2.4185Hl PLATEAU AGE F value '3. 2':318':3':3 '3. C.~7 Bb '3 • 2 :Lc.~ 'i ·3 '3 ~i. ;;.:Lf 87 '3. 28 7':J l7/H. terr. b.QOJSE-02 I. OBJOE-02 4.SSB&E-03 8. I 170E-Ol B. 973BE-03 9. 7228£-03 I. I !BBE-02 1.m1E-02 2.2716E-02 9.0BJBE-Ol 38139 F 1 J'iAr l 40Ar l 3b~r K/C1 K/CI nus. ~ALUE TOTAL TOTAL IC1 1Cll I. t370E-OI B.1207 1.02 16.83 0.01 B. 70E•OO 4.59£101 l.M2HI v.ma 14.02 92.53 o.28 \.S~EtOl 4.32E +01 I. 22l3E-01 9.2m 20.80 Y7.59 Q.88 l.HE11)2 4.21E•Ol 1.2ll2E-OI 9.2786 13.21 H.IB 0.57 6. l4El·Jl UIEtOI I. 2353E-O I 9.2125 23.24 96.79 0.13 5.82E•l)I 4.23E•Ol 1.WlE-01 9.2l87 21.19 98.20 l.H 5. 71E+OI 1.18EtOI I. 2353E-OI um 5.08 98.19 1.55 4.67tt0l 4.23E•OI 1.1751E-OI 9.3636 0.19 97.97 2.14 l.1BE•OI 4.rn101 l.0167HI 9.1556 0.65 95.72 0.69 2.29Et01 1.m101 l.225BHI 9. ;m 100.0 92.51 0.28 5. rn101 4.2bEtOI 'Y- 2,'3Hr 21z1. B 13.;~:1 21.1'3 :5. IZIB AGE:i r.:.\!:.1!;;~ ( 11'1. y, J ;:::;::. l "I '30 l ~:: i~:'. • 1(..11 ilJ' / j;~~l.11 1 ;;:: l .. b::;;•?.1L1 1 i:'.: i~;:. u:: W9 AGE UllCERTAllHY (K1.1 ma. I J co:m. J11- u t 111.117 21.m 21.921 121.021 0.318 i.m 1£2.180 0. 2b3 t. n1" 122.0! 1 o.m I. 7il 121.169 o.m I. TiS 121.630 o.m I. 781 122.119 0.322 I. BOO 123.091 o. 250 I. go2 120.m o,m !. 796 121.m 1 2 t . 9 MA r.:: ~-" l t a. ( r11 • y • - • i~ ~:;iii; :i. ".:)~j,;S -. i=::tH 1M. 3!':5°. • ~'.i!.~1·:;t,:,1M1H . 'd" ';4;::"13b8i:::-1i.);~:: - •. 3·3·:1;:;-~ 1!""17 Tc•t i'\ .I arrtc:iunt of 390r = B3.5200l ~ AGE 122,2 MA FIGURE2: 130 125 -.--..... 0 ::2 "-.-/ Q) 01 <{ 120 -115 - I 0 20 '· 4 0AR/ 39 AR INCREMENTAL RELEASE SPECTRUM FOR A BIOTITE FROM A GRANITE FROM SUZHOU, CHINA ....... , ...... _ I. I r-I , I r I I I I 40 60 80 100 %39Ar Cumulative ACKNOWLEDGEMENTS Without the help of the following people and organizations, this paper would not have been possible: • Friends of Orton Hall Fund: Expenses for thin section preparation • Radiogenic Isotope Laboratory: Costs of isotopic analyses • Personnel of Ford Nuclear Reactor: Sample irradiation • Chen Jiang-Feng: Assistance with sample preparation and invaluable first-hand knowledge of the study area • Fritz Hubacher: Expertise and operation of the mass spectrometer and producing the computer graphics • Julie Schellhaas: Typing the paper and moral support • Tami Kay: The consumate secretary • Dr. Ken Foland, my thesis advisor: His vast knowledge, experience, and patience made this thesis a reality. I would like to thank everyone for bending over backward to accomodate me, for being patient, and for keeping a sense of humor! I value greatly the opportunity which has been afforded me. GRC 1988 13 REFERENCES Chiao-min Hsieh. 1973. Atlas of China. McGraw-Hill Publishing Co. Dalrymple, G. B., and Lanphere, M.A. 1969. Potassium-Argon Dating. W. H. Freeman and Co. San Francisco, 258 p. Faure, G. 1986. Principles of Isotope Geology. 2nd Ed. John Wiley and Sons, New York, N.Y. pp. 66-112. Foland, K. A. 1983. 40Ar/39Ar Incremental Heating Plateaus for Biotites with Excess Argon. Isotope Geoscience, Vol. 1, pp. 3-21. Liu Yimao, Zhang Yuxuei, and Zeng Ti. 1987. Geological-Geochemical Characteristics of Suzhou Granite and its Origin. International Symposium on Petrogenesis and Mineralization of Granitoids, Field Excursion Guide 7-10 December, 1987. Institute of Geochemistry, Academia Sinica & The Chinese Society of Mineralogy, Petrology, & Geochemistry. pp. 1-26. Zhang Xinglong, Wang Jiabing, and Shen Bochun. 1988. Chinese Journal of Geochemistry. Vol. 7, No. 1. 14 

40Ar/39Ar age determination of the Suzhou granite

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40Ar/39Ar age determination of the Suzhou granite

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