Davenport, Thomas

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Ad.visor: The Effect of Dolomitization On the Strontium Concentration In the Columbus Limestone Columbus, Ohio By Thomas Davenport Geology 570 Gunter Faure Introduction: The Effect of Dolomitization On the Strontium Concentration In the Columbus Limestone Columbus, Ohib By Thomas Davenport In the diagenesis of limestone into dolomite there are three things which can happen to the concentration of the element strontium. These are 1.) the strontium concentration may increase as the percent of dolomite increases, 2.) it may remain constant as the percent of dolomite increases or 3.) it may decrease as the percent of dolomite increases. At the outset of this experiment a working hypothesis was formed. This hypothesis was that as the percent of dolomite increases the concentration of strontium should decrease. The reasoning behind this hypothesis was as follows. Strontium, calcium and magnesium have fairly similar 0 0 atomic radii; magnesium 1.60 A, calcium 1.97 A and strontium 0 2.15 A and the same oxidation states (+2). It is because of -these similarities that strontium is capable of occupying the same lattice sites as the calcium in limestone as noted by Graf (1960). lf the dolomite tested was formed by the replacement of magnesium for the calcium in the lattice sites then it is also possibiie that the strontium initially present would also be replaced by the magnesium• The same agent that brought in the magnesium and removed the calcium could, if the -2-previous assumptions were true, also remove the strontium providing it did not bring in any more strontium. If this is true then the more calcium which is replaced by magnesium ( the larger the percent of dolomite), the more strontium should also be replaced ( the lower the concentration of strontium). There are several assumptions made. One of these assumptions is that the concentration of strontium which was initially present in the limestone was, for the most part, constant throughout the total thickness of the formation sampled. The second assumption is that the greatest majority of the strontium initially present was in the calcium sites and not in accessory minerals such as celestite or strontianite. Several tests would have to be run to determine the percent of dolomite and the concentration of strontium present in the limestone formation sampled. The results would then have to be evaluated to see if an inverse relationship did indeed exist between the percent of dolomite and the concentration of strontium or, if not, what ratio'1 is present. The Columbus limestone: The Columbus limestone was chosen to test. There were twenty five samples of the Columbus limestone taken from two different locations. The first fourteen specimens were taken from an outcrop along Mill Creek one mile from where Mill Creek enters the Scioto River. Mill Creek is located in Delaware County west of the town of Bellepoint, Ohio -3-The remaining elevenspecimens were taken from an outcrop in Hayden Run. Hayden Run-, is in: Franklin County north west of Columbus, Ohio. The outcrop is just south of Hayden Run Road and one mile west of Scioto River Road. The section at Mill Creek is approximately fourty-five feet thick and is high in dolomite content. This section was sampled at approximately three feet intervals throughout it's entire thickness. The first specimen was taken just above a conglomerate bed, which marks the base of the Columbus limestone, and the last specimen at a bed containing colonial coral, probably a species of Stromatopora. The section at Hayden Run ls approximately sixty-five feet thick. In contrast to the Mill Creek outcrop, this section is almost entirely limestone, The Hayden Run outcrop was sampled at every major bed starting at the base of Hayden Run Falls where Stromatapora occurs, (this gives an approximate corralation between the two sections), to the top of the outcrop which is just below the base of the Delaware limestone. Although these two sections are seperated by approximately fifteen miles they include almost the total thickness of the Columbus limestone. According to Stout (1941) the ch~nge in the ~olomite content from very high in the lower part of the section to low in the upper part was to be expected. This is characteristic of the Columbus limestone and not a -4-result of the seperation of the two outcrops. In addition to this it was also noted by Stout (1941} that the highly calcereous part of the section is much more fossiliferous then the section which is rich in magnesium. Method: In order to determine the effect of dolomitization on the concentration of strontium two tests had to be made on each sample~. The first to determine the percent of dolomite by x-ray diffraction. The second to determine the concentration of strontium by x-ray fluorescence. Before these two tests could be run the specimens had to be prepared. The preparation was to grind each specimen, with a mortar and pestle, into a fine powder. The powder had to be fine enough to pass through a 140 mesh screen. After this was completed then the two x-ray tests could be run on the samples. Percent of Dolomite: The percent of dolomite present was determined by x-ray diffraction. The powdered sample was first mounted on a slide which was in turn mounted on a goniometer. The specimen was then exposed to x-radiation and rotated through an arc of from 25°2e to 35°20 and the x-radiation•;s counts per second were recorded on a chart. If any dolomite or calcite was present a seperate peak will appear for each in this ten degree arc. The calcite peak will come in at 29.4°20 and the 0 dolomite peak will come in at Jl 20. From these peaks it is possible to determine the percent of dolomite present. To -5-find the percentage of dolomite the height of the calcite and dolomite peaks are measured and each is multiplied by the scale on which the test was run. This will give the amount of calcite and dolomite in counts per second. The dolomite concentrat1on was then divided by the calcite concentration and the resulting figure is then used on the dolomite-calcite standardization curve to determine the percent of dolomite present in the sample assuming uniform dolomitization. The percent of calcite or limestone is found by subtracting the percent of dolomite from one hundred percent. In order to determine how accurate the readings were sample number eleven was tested ten times. From this data the limits of the readings were determined. The tests were run at 15 milliamperes and 45 KVP. The x-radiation was copper Ko< which was passed through a nickel filter. The dolomite-calcite standardization curve was taken from Habib (1968). The results of this test are listed in Table 1. Strontium Concentration: The concent~ation of strontium in the sample was determined by x-ray fluorescence. The principle is similar to that used in the x-ray diffraction test. The powdered specimen was mounted on a goniometer and exposed to x-radiation while being run through an arc of from 23° 20 to 27° 20. At 27° 20 the goniometer was reversed and the specimen was run through the same are back down to 23° 29. As in the x-ray diffraction the counts per second of the x-radiation was recorded on a chart. Any strontium present -- ; i __ _ ___ CF __ - . ~ ' - . ' - . L!l ~ ~~ -~~ ~-i ~~--~ ! ---.=-r'._:~:-i -~~~:-- r.~x.=-:s ~:~--~~\r·,s J r;~oor!:H~D:. BY T -• -1 1- •• '--1------------- --1--·--------~------·-L-----B F<.oiv• ;:,.,... "'··~ r R-,. --HAv-.s ---------------~ i ·• : . ---1- -='t I;:_ 196_~.F~~--: ,-~:.f-~~10':_~~-~ c i Table 1. Percentage of dolomite and calcite Sample 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1.5 16 17 18 19 20 21 22 23 24 25 Concentration (CPS) Dolomite Calcite 1892 0 1736 0 1760 206 1732 228 1710 340 1954 0 2120 110 1974 204 1970 48 1812 674 1622 588 2020 280 0 1964 0 1958 .58 2176 710 1632 232 1700 188 1862 128 1738 186 1818 98 2062 156 1780 0 1788 0 1934 0 1816 Dolomite/Calcite ~ Cl< 8.46 7.61 5.03 «.. 19.3 9.67 41.1 2.69 2.76 7.21 0 0 0.027 o.435 0.136 0.101 0.074 0.102 0.047 0.088 0 0 0 -6-Percentage z 1.2 Dolomite Calcite 100 0 100 0 87.5 12.5 85.5 14.5 79 21 100 0 100 0 89.8 10.2 100 0 72 28 72.2 27.8 82.5 17.5 0 100 0 100 0 100 33 67 7.8 92.2 0 100 0 100 0.5 99.5 0 100 0 100 0 100 0 100 0 100 Table 1. Percentage of dolomite and calcite Sample Concentration (CPS) Dolomite/Calcite Dolomite Calcite 11 1668 686 2.43 11 1606 606 2.65 11 1646 590 2.79 11 1658 570 2.90 11 1654 608 2.72 11 1656 570 2.91 11 1604 572 2.81 11 1660 604 2.75 11 1668 606 2.74 11 1660 622 2.67 Percentage :t 1.2 Dolomite Calcite 70.5 29.5 72.0 28. 0 72.6 27.4 72.9 27.1 72.2 27.8 72.9 27.1 72.5 27.5 72.3 27.7 72.3 27.7 72.0 2s.o -7-~a-would give a peak at 25.15 2e. From the intensity of this peak the concentration of strontium could be determined. The first step was to measure the height of the strontium peak and multiply by the scale the test was run on. Since two peaks were obtained for each specimen because the arc was run through twice an average intensity for the strontium in counts per second could be obtained. The intensity of the strontium was then used on figure 5~too4etermine the concentration of strontium in parts per million. As in the x-ray diffraction test the accuracy of the data had to be checked. To determine this a standard limestone sample (#309) was tested four times. From this the limits of the strontium concentration can be determined. In addition to determining the limits of the strontium concentration sample 309 was also used to construct the strontium concentration curve. The tests were run at 55 milliamperes and 65 KVP. A lithium fluoride crystal was used with the A E in. The readings were taken on the K 0( x-radiation. The standard limestone sample (309) was obtained from Dr. Gunter Faure and the strontium concentration curve from Habib (1968). The results of the tests for strontium concentration are shown in Table 2. Discussion: From the data acquired from the two tests which were run on the samples it was possible to show grahically, figure B, the relationship between the percent of dolomite and the concentration of strontium. From this graphical f I -l ! -1~ I I . ,, ( 000 __ _ -~ , I I ~ ~1· 1 f ao _ -------~ i I &oo '-i /co ~ Goo t { Joo ~ ~ ~ !fo.:> 3oo l<>o lo• /00 :2.r:> .:3<><' ·1 "J~ 1 p-ic.u,;£. S. .!i,. 0 '7 f 11.i "" I I I __ l_ ~ 0 °' "' !:: '' " --~ i.'''''''' '''' I! JI• ''' I I! l '' •I• t I !:: ~ ~ :: ~ ~ --:= °' " .., i '. rov G<Jo 700 ~ 9tJr> JIJcro 110"' 12<10 13<.1\) I.If-"' l.[c:n.-> n~ /7tN '~"' I I I T" -l·· --1--l· -,_ .-1--~ l-: :·~i~: ~h-1 ;. r ·. T Tr'. . T."i-' ~_::"1 ~-.':.-~II•':}_ P.1f":_' 1 I • I I ! t • , · • t t , • l • t· 1 • . + ,_ Cncen-lra-Pon W. 0fn1e,,.r,f ... ~~CPS:,__.... _l_J j .. .£." l ;Ji:;~ 0 ; ~ L'.~ I J I J I J J 11.,:1·1 1!1 1111 .·:j j;:: 11!! j: • ---~--' ·--• j. ~'. ..::~W~J.l1 ·I iii 11' •. j ·~!__'._ '.' -.L-~...-'-'. ~ ~ :: ~ ~ := °' .... "' "' "' -9-Table 2. Strontium concentration Sample Sr Concentration Scale Sr Concentration Sr Concentr-ation (inches) (CPS) (ppm) ± 270 309 6.025 5000 3012 8340 1 0.690 .5000 345 1170 2 1.945 2000 389 1330 3 1.920 2000 384 1310 4 1.965 2000 393 1340 5 1.690 2000 338 1130 6 1.820 2000 365 1240 7 1.830 2000 367 1250 8 1.805 2000 361 1230 309 5.770 5000 2885 7800 309 5.945 5000 2972 8200 9 1.710 2000 342 1160 10 1.855 2000 371 1260 11 2.125 2000 42.5 1460 12 2.035 2000 407 1390 13 1.265 2000 2.53 840 14 1.920 2000 384 1310 15 2.185 2000 437 1500 16 2.155 2000 431 1480 17 3.950 2000 790 -,.1800 18 2.385 2000 477 1640 19 J.880 2000 776 71800 20 1.830 2000 J66 1240 21 1.930 2000 386 1320 22 2.160 2000 438 1500 -10-Table 2. cont. Sample Sr Concentration Scale Sr Concentration Sr Concentration (inches) (CPS) {ppm) ± 270 23 i.775 2000 355 1200 24 1.965 2000 393 1350 25 2.235 2000 447 1540 309 5.800 5000 2900 8020 r.., 99 19 7 5 16 17 arcs to the Ccntin1ctcr -11-representation (the samples from the upper part of the formation are on the left of the graph) it appears that the hypothesis which stated that the strontium concentration might decrease as the percent of dolomite increased may be correct however before making a final judgement as to whether the working hypothesis is correct or not it will be necessary to consider several other factors. The first of these factors to be considered are the accessory minerals which contain strontium such as celesti te and strontiani,te. There are several ways in which these accessory minerals could effect the concentration of strontium in limestone and dolomite. One way would be if large amounts of these accessory minerals were initially present in the limestone. If this were the case then the dolomitization of the limestone would have no effect on the concentration of strontium. This possibility is very small since Graf (1960) found that most celestite is due to secondary redistribution. This however brings another problem to light, that of the secondary formation of accessory minerals. Even if most of the strontium was initially in the calcium sites the accessory minerals could still effect the strontium concentration. This is because the agent which brought in the magnesium did not remove all of the strontium which was removed from the calcium sites and this strontium formed the accesser,7y minerals. These accessory minerals would now be in the dolomite even though they were not initially present in the limestone. If this were the case then the concentration of strontium would not -12-decrease as the percent of dolomite increased. In the specimens tested the strontium concentration decreased and therefore the accessory minerals, in this instance, involving the strontium concentration"were not formed. Another source of strontium to be considered is that which is found in fossils. Strontium is found in modern clams and snails and therefore could also be present in the fossils which are found in the Columbus limestone. The concentration of strontium in the fossils are thought to be controlled by several factors. Graf (19601 seemed to think that the strontium content was dependant on the different genera while Kulp et al (1952) felt that the strontium content was a factor of the original strontium concentration in the water which· is in turn controlled by the salinity of that water. :n:t will be remembered that the c~olumbus limestone is much more fossiliferous ih the upper sixty-five feet than in the lower fourty feet. The strontium content is also higher in the Hayden Run section which corresponds to the upper sixty-five feet of the Columbus limestone. The fact that the Hayden Run section is more fossiliferous could account for the greater concentration af strontium rather then the dilution by dolomitization of limestone but until more is known about the salinity of the water durtng the deposition of the Columbus limestone and the strontium present in the fossils it cannot be said which plays the most important part in the variation of strontium concentration. -13-Another factor to be considered in the decrease in strontium~concentration is a stratigraphic o~e. It is possible that there iSSO"Me type of unconformity present in the Columbus limestone which could cause the decrease in strontium from the lower to the upper part of the section rather than the increase in dolomitization. ]n order to establish whether this is true more research must be done on-the environment of deposition of the Columbus limestone. Conclusion:: Although figure B would seem to indicate that the hypothesis conceJnl1mg a decrease in strontium with an increase in dolomite is correct a positive statement of it's validity cannot be made at this time. In order to make such a statement further analysis would have to be run on the samples. The content of strontium in the fossils would have to be st.u.Ldied to see if they played a significant role in the decreasing strontium concentration which was observed for the Columbus limestone. It may also be necessary to stµdy the role the accessory minerals play in the strontium concentration.and also to determine if stratigraphy plays a part in the decreasing strontium concentration. Although the dolomitization of the limestone cannot be proven to be the sole reason for the decrease in the strontium concentration it cannot be ruled out without further study. It may in fact, after the other factors are studied, turn out to be the primary cause for the change in the strontium concentration in the C-0lumbus limestone. -14-Acknowledgm.ents: ] would like to acknowledge the valuable assistance received from Dr. Gunter Faure. Dr. Faure supplied the limestone standard and gave instruction in x-ray diffraction and fluorescence. I am greatly indebted to him for his assistance. -15-References: Kulp, J.L., Turekian, K.K. and Boyd, D.w. 1952., Strontium content of limestone and fossils., Geol. Soc. Amer. Bull., Vol. 63., p 701-716. Graf, D.L. 1960., Geochemistry of carbonate sediments and sedimentary carbonate rocks., part 3., Illinois State Geol. Survey., circular 3., p 32-33· Stout, w. 1941., Dolomite and Limestone of Western Ohio., Geol. Survey of Ohio., t'ourth Series., Bull. 42., p 210-218. La Rocque, A. and Marple, M • .l.i'. 1963., Ohio Fossils., Bull. 54., p 75. Habib, M. 1968., Strontium distribution for the carbonates of the Flagstaff formation Ephraim, Utah., Senior Thesis. 

The effect of dolomitization on the strontium concentration in the Columbus limestone, Columbus, Ohio

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The effect of dolomitization on the strontium concentration in the Columbus limestone, Columbus, Ohio

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