The objective of this thesis was to date a Precambrian rock from the basement of Ohio. The Precambrian sample came from a disposal well drilled in Perry Township in Lake County, Ohio. The rock, which was of a granitic composition with secondary dolomite, was dated by the Rb-Sr whole rock method. The date of the rock was found to be 566±45 million years.No embarg

Vargo, Robert T.

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S ~Vll~ r T ~e_s i5 /~7;;L v ().._, '{' Cj CJ Sr-Rb Age Determination of a Precambrian Rock from Lake County Ohio by Robert T. Vargo This thesis was done as a partial requirement for a- B.s. Degree. The Ohio State University May 31, 1972 Approved:f f ~ TABLE OF CONTENTS Page Abstract •.••...•.••.•.••....•..•....•..•.•.•.•..•...•••• 1 I. Inttoduc t ion • ...•.•...••.....•......•..•..•...•••••• • 1 II. Description of the core ••••••••••••••••••••••••••••• 1 III. Description of the specimen used for dating •••••••• 2 IV. Analytical procedures ••••••••••••••••••••••••••••••• ) v. Presentation and interpretation of the results ••••••• 6 VI. Conclusions ••••••••••••••••••••••••••••••••••••••••• 6 VII. Acknowledgments •••••••••••••••••••••••••••••••••••• 7 VIII. Figures and drawings •••••••••••••••• .••••••••••• 8-16 a) Map of Perry Township •••••••••••••••••••••• B b) Scale drawing of the core •••••••••••••••••• 9 c) X-ray identification of 29 angles ••••••••• 10 d) X-ray f l uores cence data sheets •• ~ •••••• 11-12 e) Graph of the Sr concentration •.••••••••••• 13 f) Graph of the Rb concentration ••••••••••••• 14 g) Graph of the Rb/Sr ratio •••••••••••••••••• 15 h) Graph of the Rb/Sr ratio •••••••••••••••••• 16 IX. References (i) Abstract The objective of this thesis was to date a Precambrian rock from the basement of Ohio. The Precambrian sample came from a disposal well drilled in Perry Township in Lake County, Ohio. The rock, which was of a granitic composition with secondary dolomite, was dated by the Rb-Sr whqle rock method. The da te of the rock was found to be 566%45 million years. I. Introduction The objective of this thesis was to date a Precambrian rock from the basement of Ohio. The Precambrian basement under Oh i o i s not e xposed at the surface, consequently little is known about the age of these rocks. Therefore any a bsolute ages determined from the basement under Ohio would be very valuable. The 4 inch diameter core wa s obtained by The Ohio State Geologic Survey from a disposal well drilled by Calhio Chem-icals Inc. under Permit No. 142. The well is l ocated in Perry Township of Lake County, Ohio. Figure 1 shows a map of Perry Township and the location of the ~ell. II. Description of the core The well wa s drilled to a total depth of 6072 feet, with the Precambrian section constituting the last 12 feet of the core. Overlying the basement unconformably is The Mount Simon Formati on, , a pure quartz sandstone of Cambrian age. _Inspection of the Precambrian section of the core revealed that its mineralogy and textural character varied widely. The first 4 feet of the core is primarily a fine grained chlorite schist. Chlorite makes up approximately 75% of the rock with a~ reddish tinted mineral, possible microcline, making up 15-20% of the rock. The reddish color may be due to finely disseminated iron opaques. The remainder of the rock is minor pyrite. This whole section is highly fractured and (2) marked by slickensides dipping JO to 60 degrees, assuming the core is vertical. The highly fractured zone grades into a non-fractured zone 1.5 feet thick where the sample for dating was obtained. Its mineralogy will be discussed in more detail later. This section grades into a 2 foot fine ~ grained zone with a mineralogy of approximately 45% feldspar, JO% dolomite, 20% quartz and 10% chlorite. The dolomite occurs in the rock as small grains as well as numerous veinlets cutting the rock. The next 2.5 feet is again a chlorite schist with its mineralogy and textural character being the same as t he previous chlorite zone. This zone grades into a one foot zone of a fine grained basic rock, probably a diorite or gabbro. Mineral identification is difficult due to the fine grained nature of the rock. Finally the last 2 feet of the core is a chlorite schist identical to the two zones previously mentioned. Figure 2 shows a sca le diagram of the core. III. Description of the specimen used for dating Hand specimen identificati Jn of the sample revealed t .hat a medium grained reddish mineral makes up 75% of the rock, the remaining 25% being fine grained dark minerals with only chlorite being identifiable. Thin section identification of the minerals present proved to be rather difficult, since much alteration of the rock was observed. Chlorite and dolomite were positively identified making up 10% and 15%, respectively. The chlorite (3) shows no apparent foliation, while the dolomite forms medium grained rhombs presumably of secondary origin. Iron opaques make up 5% of the rock, these being finely disseminated throughout the rock. The remaining 70% was tentatively identified as medium grained subhedral to anhedral feldspar and quartz containing many small inclusions. Simple twinning is observed in the feldspar as well as undulatory extinction in the quartz. Positive identification was attempted through the use of X-ray diffraction methods. A General Electric X-ray Diffractometer (XRD-6) was used to make positive identification of the minerals present. Figure J shows 2Q angles observed on the strip chart with values recorded in the litera ture (Chao, 1969). Quartz and microcline make up 40% and 30% respectively with dolomite and chlor1te 10% each. Opaques, sericite and zeblites were the other minor minerals present in the sample. IV. Analytical procedures The method used for dating the sample was the whole rock Rb-Sr method. A 1.5 inch slice of the sample section was cut out using a diamond saw. A piece of this section weighing approximately 50 grams was crushed to less than 200 mesh. The powder was then homogenized by rolling the powder on a sheet of paper. A J gram sample was taken and compressed into a pellet with a boric acid back ing. This pellet was . {4) placed in the X-ray fluorescence spectrometer f,or analysis of Sr and Rb concentrations. The spectrometer was scanned from 18 thru 39 degrees for the measurement and location of the Sr, Rb peaks and the baselines. The spectrometer was peaked on ZrK~radiation to obtain as accurate results as possible. The 0 first radiation measured was MoK-.C at 30.00. The Sr peak 0 • at 36.85 was surrounded by a symmetrical baselines at 35.40 and 36.50~ The net Sr values were calculated by the use of the following equation: Net Sr =Value of Sr peak -2 (B, -0 .40909 {B, -B2 )) where B,and B2 are the two baselines and o.40909 is the slope of the line connecting the two baselines. The Rb peak at 0 0 37.99 was surrounded by symmetrical baselines at 37.40 and 0 )8.58. Net Rb values were calcµlated by adding the two baselines together and subtracting the sum from the Rb value. MoK~C and all baseline radiations were counted for 100 seconds, with the Sr and Rb radiations being counted for 200 seconds. Four labratory standards were used in the analysis. The standards AVG-1, GSP-1, BCR-1 and G-2 were run a total of 13 times and the sample #528 was run 12 times to reduce the experimental error of the measurement. Figures 4 and 5 show the data of the X-ray fluorescence portion of the experiment. The net Sr and net Rb values were divided by the MoK C radiation values then plotted on a graph to obtain the Rb and Sr concen-trations in parts per million. Figures 6 and 7 show these (5) graphs. The Rb/S·r ratio of the sampJ.e was obtained by plotting on a graph the known Rb/Sr ratios of the standards against their known net RbK..Jnet SrK~, Since, the net RbK./net SrK~ of the sample is known through calculation the Rb/Sr ratio can be determined from the graph. Figures 8 and : 9 show the Rb/Sr ratios plotted against the net RbKocfnet SrK~ratios. The Sr8~sr16 ra t1o was determined by the use of a mass spectrometer. A 0.2 gram portion of the sample wa s dissolved in a concentrated mixture of HF and H2 SO"' acids. This was done . to form strontium salts. The solution wa s taken to dryness and a small amount of 2N HCL was added. This solution was then filtered and a Srn tracer added. The solution with the tracer was put on as ion exchange column 'powex 50) and the Sr solution wa s eluted with 2N HCL. The Sr fracti on wa s identified by the activity of the Sr99 tracer and was collected in a series of 15 ml. polyethylene beakers. Seven beakers of the solution were collected and tested for Sr~ radiation by the use of a Geiger Counter. The beaker with the highest counting rate was used for the mass spectrometer analysis, since it contained the highest concentration of ~, strontium. A small drop of the liquid was evaporated on a tantalum filament of the Nuclide Corp. Model 6-60-S mass spectrometer. The filament was loaded into the mass spectrometer and the run started with Sr isotopes 88, 87, and 86 being measured 1n the scanning procedure. The run lasted almost (6) two hours before the last trac~s of Sr isotopes were burned off • .. ,... ".:;,;-:·· "· .. ,~ V. Presentation and interpretation of the results The Sr and Rb concentrations determined by the X-ray fluorescence method were found to be 53.9 ±0.65 and J5.9±0.56 ppm for Sr and Rb respectively. The Rb/Sr ratio calculated to be 0~6?1-6 ±0.008. The Sr87/Sr 86 ratio wa s calculated by us ing eight groups of six peaks each from the mass spectrometer run . This was found to be 0.7188±0.0010. The main assumption made in the calculation of the date was that the initial Sr8J/Sr86 ratio ~ould be set at 0.7040. The following equation was used to calculate the date of the rock: t = _L_ 2. 303 /oo( sr-'['s..-''ue - sr'YSt-e<:.,,~,;o1 + 1 ] ~ v Rb8JSr8 (,, • r -11 where t equals time and /.is the~." '!-~cay constant s et a t 1.39 x 10 years. VI. Conclusions The date of the rock was found to be 566 ±45 million years. The date ~s younger than expected for rocks of the Grenville Province (approx. 900 million yrs.). The presence of the chlorite schist zones in the rock probably represented a shear zone. In a shearing event the Sr-Rb clock is many times reset along with metamorphic effects being induced in the I (7) I The whole 12 foot core showed signs of shearing and low grade metamorphism which would explain the anomalous younger age of the rock. The date points out that the Grenville Province was tectonically active into Cambrian time. VII. Acknowledgments I w:)uld like to thank Dr. Arie Janssens of The Ohio State Geologic Survey for his co-operation in making the core available and also Dr. Gunter Faure for his help and advice during the course of this project. I 6060 6072 { 9) Scale Drawing of the Core Chlorite Schist , highly fractured Granitic comp osition with seconda ry dolomite, samp le dated from here Granitic composition with dolomite veinlets cutting the roc k Chlorite Schist , highlt fractured Fine grained diorite or gabbro Chlorite Schist , highly fractured 0 1 2 Sea.le. liri = 2.f.'t. Figure 2 {10) X-ray Identification 29 angles in degrees (Cu) Actual 29(Cu) Mineral Observed 2Q(Cu (Chao,1969) % Correlation Chlorite 12.4 12.38 99 (Kammererite} 18. 7 18.59 98 24.9 24.81 99 37.4 37.68 98 Quartz 20.9 20.83 99 26.6 26.66 99 50 . 4 50.21 98 36.6 36.56 99 Dolomite Jo.7 30.84 99 (Ferroan) 41 . 0 41.04 99 50.6 50.36 98 50.8 50.96 99 23 .4 24.05 97 M1crocline 27.5 27 . 49 99 21.8 21.10 97 25.6 25.60 99 J0.4 30.15 98 Figure 3 .>nvr 17 0 f\"--:,~~~ ~r ,_ v-~~ "-"""-IS 't I I ;""t.l.. _.,,, ... ;:,r ·-~· ~ -· .;: ... ~~ l'dci=:=l'C!sl'Rl1"1'lt• '"'! -3_11.0o ,ss.s.r .. U .. O> J7.'lo .ll.'1~ 3KS9. c. ~ ...... Ko< •k<J.. 1'1ok. c. Ko1-I ""Rb kJ i'>lo I! (. IV~-f 5 r ?/'1 -Zoo 10-0 Zoo roa-~ --+-------I --t-·-f-----~ •.. -;,,,,,.JJ~ -too /00 7cn:r ,. ~ ---(i----2...--rs~-P=-"{D3 l-ll791'r 35-s-9·7 3770 ~-'-1-'lb 1'-!021 3) 3 2. -3"P/03+ '675-<I 3559/ 2<;8<13 07ao2 -.:;-578 I~ ~~ ~2)P, 0.277~,., ~-B 127004 '332-:<f 87;"82 -.29 SI Z~& --"27~ L'. 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"'S -= o:J.J~j-r~ A G.v-.i • ~: i-:~~ _s_ -r_ "-"'-.! , -~ 'IS c..e-i : Q._10~ 2 ------~ o. '?~.in. •r ------" .., 1-:::..,,.. 1.;,.-. -~ --,..,. "-b l\l<.i s~ -, .,.,.,,_--I I.? 9~ G'!.f'-1 : -= ·-~ --I. 301!>-,___ --1----<;-" = ----= i.?;l) 1.~1>9-~ ---------. k Sc.e..-! .= J ~ -f.--··-------• 700 600 500 • 300 200 100 .o .1 • .2 .3 (1J) ..q .S Ne± Sr MoK~C Figure 6 ·' 9 AGV-1 .r .8 .9 iO J.l Ko.vc..=G26-~ Co.lcu..ln+ed Sr~ 53.3 ppm ( 14) • 300 200 • Rb ppm 100 0 .1 .z .3 .4 .5 Ne± Rb k<Av<:.== 808. 2 • MoKO(C Colculo+c.d Rb-=3.3.8 ppm P1gure 1 • . • 1D . . 8 . 7 Rb/Sr. kno(Jt"'\ .6 .5 . .3 .. 2 .1 0 .1 .2 .3 (15) 528 .4 I I I I I 1 1 I l ' ' l .5 Net Rb Kpl. Ne+Sr Kcx. .6 0 GSP-1 .7 .8 . 9 . !.O CCA\c\.\lcd~d Rb/.Sr k~vc. -=1.2.CA~ #528: ,06.362 • Pigure 8' • . • • (16) f.O .CJ .8 Rb/Sr ·7 - - - - - - - - - - - sza know., .6 ..5 .4 .3 .2 .1 0 .1 .2 .3 CetlctAlo+c.d Rb/s..-# 528 = 0.6543 .s .6 NetRbKol.. Ne+SrK°'" Figure 9 .7 .8 .9 J.O k o.vc::. = 1.300 REFERENCES Chao G. Y., 2Q (Cu) Table for Comm.on Minerals, Geological Paper 69-2,Carleton University Dept of Geology, {1969). Hamilton E I., Applied Geochronology, Academic Press London & New York, (1965), 267 pages. The Grenville Problem, The Royal Society of Canada Specia l Publications No . 1, The University of Toronto Press, (1956) , 119 pa ges. Isotopic Age Map of Canada, Geological Survey of Canada, Department of Energy , Mines and Re sources, No.1256A. 

Sr-Rb age determination of a Precambrian rock from Lake County, Ohio

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Sr-Rb age determination of a Precambrian rock from Lake County, Ohio

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