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Elk Creek, Nebraska, carbonatite: Strontium geochemistry
Earth and Planetary Science Letters, 28 (1975) 79 82 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
ELK CREEK, NEBRASKA, CARBONATITE: STRONTIUM GEOCHEMISTRY
D.G. BROOKINS Department o f Geology, The University o f New Mexico, Albuquerque, N.M. (USA) S.B. TREVES Department o f Geology, The University o f Nebraska, Lincoln, Nebr. (USA)
and S.L. BOLIVAR Department o f Geology, The University of New Mexico, Albuquerque, N.M. (USA)
Received June 5, 1975 Revised version received August 8, 1975
Subsurface carbonatite at Elk Creek, Nebraska has been recognized in drill core taken from a depth interval of 630 to at least 950 ft. The core in this interval consists of carbonated breccia and phlogopite-bearing carbonate rock. Total REE, P2Os and Nb205 data are consistent with "average" values for carbonafite. 87Sr/S6Sr ratios from the carbonate fraction range from 0.7030 to 0.7055 for fifteen of eighteen samples (total Sr varies from 300 to 3500 ppm; .~= 1800 ppm); the remaining three samples have S7Sr/86Sr and total Sr values of 0.7085 : 40 ppm; 0.7064 : 92 ppm; 0.7067 : 252 ppm; these samples may be mixed with sedimentary carbonate and/or contaminated by other non-carbonatite material. The Elk Creek carbonatite is of special interest because of its position with respect to tectonic elements in basement rocks. It occurs in the center of gravity and magnetic anomalies over the approximate axis of the Nemaha anticline and is apparently aligned with the Riley County, Kansas, carbonatite-bearing kimberlites. It is far removed from the E-W-trending "38th parallel" lineament along which occur numerous kimberlites and carbonatites.
1. Introduction The Elk Creek, Nebraska, gravity and magnetic anomalies have been known since 1970 [1]. In brief, a positive gravity anomaly of approximately 6 mgal~ within an approximately circular feature of 12 square miles is coincident with a maximum positive magnetic anomaly of approximately 800 gammas. The location of these anomalies is: Sec. 33, T4N, R11E within one-quarter mile of the intersection of the boundary between Johnson and Pawnee Counties and Nebraska State Highway 50. The anomalies occur very nearly on the axis of the NNE-trending Nemaha anticline and on the Kansas extension of the Mid-Continent gravity high [2,3].
The subsurface geology of the area is fairly well k n o w n due to numerous exploratory petroleum drilling projects. In a normal vertical section one would expect to encounter some 75 ft of Quaternary deposits overlying approximately 550 ft of calcareous Pennsylvanian rocks resting on Precambrian basement rocks. The Precambrian basement sequence consists of average density granitic rocks that are first encountered in drill holes in the depth range of approximately 600 to 700 ft. Results of drilling over the anomaly were reported in 1972 [3]. Initial drilling in 1971 revealed 45 ft of Quaternary silts, clays and gravel overlying a normal interbedded sequence of Pennsylvanian limestones, shales, and sandstones that persisted to a depth of
80 630 ft. Silicate-bearing, iron-rich carbonate rocks, quite different from the normal Pennsylvanian sequence, were encountered from 630 to 703 ft [3]. Later drilling (1972) revealed similar iron-rich carbonate rocks to a depth of 952 ft where the hole was stopped due to lack of funds. Despite the uniqueness of the rocks in the 630-952 ft depth range, they do not appear to have the proper iron content nor density to account for the surface anomaly [3]. It is possible that the carbonatitic material is surrounded or underlain by more iron-rich rocks.
2. Description of silicate-bearing, iron-rich carbonate rocks Petrographic examination of the rocks from 630 to 952 ft depth reveals that they consist of carbonate (dolomite and ankerite), hematite, chlorite (some after phlogopite), phlogopite, barite, serpentine, and secondary (?) silica (quartz in places) with accessory pyrite, chalcopyrite, galena, sphalerite, feldspar, aparite, and fluorite. The carbonate rocks vary from coarse breccias to massive, coarsely crystalline dolomite into highly contorted, brecciated rock containing phlogopite and chlorite in dolomite-ankeritecalcite. Greenish clots noted in thin section appear to represent original olivine replaced by (and sometimes embayed by) serpentine and/or calcite. The most common textural relations suggest that olivine was replaced by serpentine which in turn was replaced by carbonate; however, serpentine after carbonate is also noted. Phlogopite and chloritized phlogopite commonly occur as phenocrysts. No evidence for serpentine and/or calcite after pyroxenes has been noted nor have ultramafic xenoliths been observed, though it should be emphasized that petrographic results are preliminary. Nevertheless, the preliminary rock descriptions coupled with rock chemistry (described later) indicate that these rocks should be classified as carbonatites [3].
3. Previous and present rock chemistry studies The chemical studies reported earlier [2,3] include data established by optical spectrography, Xray fluorescence spectrography, and neutron activa-
TABLE1A Sample No.
Rare earth oxide (%)
905 913 914 916 921.5 929.5 934.5 936 936.5 938.5 943 944 946 950 952
0.60 1.43 1.35 1.86 0.72 1.38 0.51 0.54 0.55 0.66 1.06 0.46 0.71 0.97 0.35
TABLE 1B Approximate sample No.
P20 s (%)
NbzO5 (%)
902-913 913-922.5 922.5-932.5 932.5-941.5
3.0 5.0 2.6 2.2
0.12 0.13 0.12 0.13
Notes to Table 1: REE determinations by neutron activation analysis [3]. Nb2Os and P205 data by optical spectrography [3]. The averagecarbonatite contains 2.06% P2Os, 0.2% Nb, and ~0.5% REE [7] whereas the averagelimestone contains extremely variable P205 but much lower values (i.e., <1 ppm <300 ppm) for Nb and REE, respectively [5,7].
tion analysis. Some of these data are shown in Table 1 which includes total rare earth elements (REE), P20s and Nb20s for the depth range 902-952 ft [3]. This depth range was chosen only because splits from these samples were available for our studies. The significance of the earlier reported data [3] will be discussed below. For the present study 0.5-g aliquots of powders were leached 2N HC1, filtered, and Sr separated by ion exchange chromatography. Dissolution was greater than 90% except for samples 913, 914, and 916 for which approximately 20% residue remained. The Sr isotopic composition was determined using a 12-inch, 90-degree mass spectrometer. The 87Sr /
81 TABLE 2 Sample*
87Sr/86Sr
Sr (ppm) 1
Sr (ppm) 2
Ba (ppm) 1
Ba (%)2
905 913 914 916 921.5 929 929.5A 929.5B 934.5 936 936.5 938.5 943A 943B 944 946 950 952
0.7039 0.7085 0.7064 0.7067 0.7033 0.7040 0.7030 0.7043 0.7034 0.7055 0.7045 0.7051 0.7040 0.7056 0.7048 0.7055 0.7043 0.7032
1480 40 92 252 1020 1290 535 286 1750 1440 2940 2440 1670 685 1140 870 565 3500
-
342 228 262 290 276 238 360 315 254 364 339 505 360 234 306 598 364 298
2.7 1.4 1.4 3.4 -
930 2700 2200 1200 -
* Number refers to depth in ft from where core was taken. Notes to Table 2: 87Sr/86Sr ratios normalized to 86Sr/88Sr = 0.1194. 1 Total Sr and Ba by atomic absorption spectrometry (Note: dissolution of carbonate material was not complete in all cases and some barite was identified in the residue; therefore, the Sr data are somewhat lower and the Ba data very much lower than values determined by optical spectrography [3]). 2 Optical spectrographic determinations [3].
6 Sr data are precise to -+0.0003 (~). Data for the E i m e r and A m e n d SrCOa analyzed during the course o f this investigation yielded a 7 Sr/S 6 Sr = 0.7080 + 0.0003 (o-). A separate split o f each sample was used for det e r m i n a t i o n o f total Sr and Ba by a t o m i c absorption s p e c t r o p h o t o m e t r y . As n o t e d with Table 2, the Sr and especially the Ba data are low due to the presence o f barite not dissolved during HC1 t r e a t m e n t . The powders were r e p o r t e d to be carbonate(s) w i t h some silicate, hence HC1 t r e a t m e n t was used; n o t until after dissolution was it learned that barite was present. Available optical spectrographic data for f o u r samples (Table 2) c o n f i r m that the a t o m i c absorption s p e c t r o p h o t o m e t r i c data are t o o low.
4. Results and discussion Sr isotopic and total Sr and Ba data are presented in Table 2. E x c e p t for the samples in the 9 1 3 - 9 1 6
ft d e p t h range the 8 7 Sr/a 6 Sr ratios are similar to those for the m a j o r i t y o f carbonatites f o u n d througho u t the world ( f r o m 0.703 to 0.705) and t e n d to con. firm the classification o f these rocks as carbonatites. The samples in the 9 1 3 - 9 1 6 ft d e p t h range are a n o m alous. T h e y possess a 7 Sr/86 Sr ratios close to (or in the range of) sedimentary carbonates [6] and lower total Sr relative to the o t h e r samples. We suspect cont a m i n a t i o n o f the carbonatite by sedimentary c a r b o n ate (or similar Sr-bearing material) but cannot unequivocally d e m o n s t r a t e this pending m o r e study. The data shown in Table 1A indicate high R E E cont e n t in this d e p t h interval which is inconsistent with a sedimentary c o n t a m i n a n t , however. A REE-rich, Sr-poor (with high a 7Sr/86Sr ) c o n t a m i n a n t is indicated but its nature u n k n o w n . F u r t h e r m o r e , ten o f the eighteen samples yield Sr values in excess o f 1000 p p m which is also typical o f carbonatites. The Ba data vary f r o m a p p r o x i m a t e l y 250 to 600 p p m but it is probable that these data are far t o o low for reasons m e n t i o n e d earlier. Optical spectrographic data
82 (Table 2) indicate several thousand ppm Ba and are probably more realistic. The only tentative conclusion reached from discussion of the total Sr and Ba data established in this study (Table 2) is that the bulk of the Sr is contained in carbonates and not in the small amounts of barite present. The mean 87Sr/86 Sr ratio of 0.7041 for fifteen of eighteen samples is significant in that it is not only a "typical" value common to many carbonatites but identical within the limits of experimental error to the carbonatitic parts of kimberlites from Riley County, Kansas. Furthermore, the Kansas kimberlites apparently lie on the same structural trend as does the Elk Creek carbonatites; i.e., the NE-trending MidContinent gravity high as opposed to the kimberlite and/or carbonatite and/or peridotite rocks aligned along the 38th parallel lineament. The tectonic implications of this relationship are currently being investigated in a separate study. The total REE, P2Os and Nb2Os (Table 1) data reported earlier [3] for the same range are also typical of carbonatites [3] and consistent with the Sr data. That the Sr data are very different for Pennsylvanian rocks in the general area has been demonstrated previously [4,5]. Typical 87Sr/S6Sr ratios range from 0.7075 to 0.7100 with total Sr contents ranging from very low values of 5 0 - 7 0 ppm (vein calcites in limestones) to values in the 5 0 0 - 6 0 0 ppm range. These sedimentary rocks further do not possess the high REE, P20s and Nb2 Os contents of carbonatites. Several of the many questions arising from study of the Elk Creek carbonatites are: (1) To what depth does the carbonatite persist? (2) How much kimberlite is associated with it? (3) Does this carbonatitic material represent part of an alkaline complex as yet not
discovered (i.e., in order to explain the gravity and magnetic anomalies)? (4) How does the Elk Creek carbonatite relate, or does it, to the Riley County, Kansas, kimberlites and possibly the Mansom Dome, Iowa? These questions are being evaluated and a more comprehensive chemical and petrographic study is underway.
Acknowledgements Partial financial support for this study was made possible by NSF Grant DES72-01655 A01 to the University o f New Mexico.
References 1 W.J. Burfeind, M.P. Carlson and R. Smith, The Elk Creek geophysical anomaly, Johnson and Pawnee Counties, Nebraska, Geol. Soc. Am. Programs with Abstracts 3 (1971) 254. 2 S.B. Treves, R. Smith and J. Rinehart, Petrography and mineralogy of the Elk Creek carbonatite, Nebraska, Geol. Soc. Am. Programs with Abstracts 4 (1972) 352-353. 3 S.B. Treves, R. Smith, M.P. Carlson and G. Coleman, Elk Creek carbonatite, Johnson and Pawnee Counties, Nebraska, Geol. Soc. Am. Programs with Abstracts 4 (1972) 297. 4 D.G. Brookins, The strontium geochemistry of carbonates in kimberlites and limestones from Riley County, Kansas, Earth Planet Sci. Lett. 2 (1967) 235-240. 5 D.G. Brookins. S. Chaudhuri and M.J. Lee, Unpublished data (manuscripts in preparation). 6 G. Faure and J.L. PoweU, Strontium Isotope Geology (Springer-Verlag New York, 1972). 7 E.W. Heinrich, The Geology of Carbonatites (Rand McNaUy, Chicago, 1966).
ELK CREEK, NEBRASKA, CARBONATITE: STRONTIUM GEOCHEMISTRY
D.G. BROOKINS Department o f Geology, The University o f New Mexico, Albuquerque, N.M. (USA) S.B. TREVES Department o f Geology, The University o f Nebraska, Lincoln, Nebr. (USA)
and S.L. BOLIVAR Department o f Geology, The University of New Mexico, Albuquerque, N.M. (USA)
Received June 5, 1975 Revised version received August 8, 1975
Subsurface carbonatite at Elk Creek, Nebraska has been recognized in drill core taken from a depth interval of 630 to at least 950 ft. The core in this interval consists of carbonated breccia and phlogopite-bearing carbonate rock. Total REE, P2Os and Nb205 data are consistent with "average" values for carbonafite. 87Sr/S6Sr ratios from the carbonate fraction range from 0.7030 to 0.7055 for fifteen of eighteen samples (total Sr varies from 300 to 3500 ppm; .~= 1800 ppm); the remaining three samples have S7Sr/86Sr and total Sr values of 0.7085 : 40 ppm; 0.7064 : 92 ppm; 0.7067 : 252 ppm; these samples may be mixed with sedimentary carbonate and/or contaminated by other non-carbonatite material. The Elk Creek carbonatite is of special interest because of its position with respect to tectonic elements in basement rocks. It occurs in the center of gravity and magnetic anomalies over the approximate axis of the Nemaha anticline and is apparently aligned with the Riley County, Kansas, carbonatite-bearing kimberlites. It is far removed from the E-W-trending "38th parallel" lineament along which occur numerous kimberlites and carbonatites.
1. Introduction The Elk Creek, Nebraska, gravity and magnetic anomalies have been known since 1970 [1]. In brief, a positive gravity anomaly of approximately 6 mgal~ within an approximately circular feature of 12 square miles is coincident with a maximum positive magnetic anomaly of approximately 800 gammas. The location of these anomalies is: Sec. 33, T4N, R11E within one-quarter mile of the intersection of the boundary between Johnson and Pawnee Counties and Nebraska State Highway 50. The anomalies occur very nearly on the axis of the NNE-trending Nemaha anticline and on the Kansas extension of the Mid-Continent gravity high [2,3].
The subsurface geology of the area is fairly well k n o w n due to numerous exploratory petroleum drilling projects. In a normal vertical section one would expect to encounter some 75 ft of Quaternary deposits overlying approximately 550 ft of calcareous Pennsylvanian rocks resting on Precambrian basement rocks. The Precambrian basement sequence consists of average density granitic rocks that are first encountered in drill holes in the depth range of approximately 600 to 700 ft. Results of drilling over the anomaly were reported in 1972 [3]. Initial drilling in 1971 revealed 45 ft of Quaternary silts, clays and gravel overlying a normal interbedded sequence of Pennsylvanian limestones, shales, and sandstones that persisted to a depth of
80 630 ft. Silicate-bearing, iron-rich carbonate rocks, quite different from the normal Pennsylvanian sequence, were encountered from 630 to 703 ft [3]. Later drilling (1972) revealed similar iron-rich carbonate rocks to a depth of 952 ft where the hole was stopped due to lack of funds. Despite the uniqueness of the rocks in the 630-952 ft depth range, they do not appear to have the proper iron content nor density to account for the surface anomaly [3]. It is possible that the carbonatitic material is surrounded or underlain by more iron-rich rocks.
2. Description of silicate-bearing, iron-rich carbonate rocks Petrographic examination of the rocks from 630 to 952 ft depth reveals that they consist of carbonate (dolomite and ankerite), hematite, chlorite (some after phlogopite), phlogopite, barite, serpentine, and secondary (?) silica (quartz in places) with accessory pyrite, chalcopyrite, galena, sphalerite, feldspar, aparite, and fluorite. The carbonate rocks vary from coarse breccias to massive, coarsely crystalline dolomite into highly contorted, brecciated rock containing phlogopite and chlorite in dolomite-ankeritecalcite. Greenish clots noted in thin section appear to represent original olivine replaced by (and sometimes embayed by) serpentine and/or calcite. The most common textural relations suggest that olivine was replaced by serpentine which in turn was replaced by carbonate; however, serpentine after carbonate is also noted. Phlogopite and chloritized phlogopite commonly occur as phenocrysts. No evidence for serpentine and/or calcite after pyroxenes has been noted nor have ultramafic xenoliths been observed, though it should be emphasized that petrographic results are preliminary. Nevertheless, the preliminary rock descriptions coupled with rock chemistry (described later) indicate that these rocks should be classified as carbonatites [3].
3. Previous and present rock chemistry studies The chemical studies reported earlier [2,3] include data established by optical spectrography, Xray fluorescence spectrography, and neutron activa-
TABLE1A Sample No.
Rare earth oxide (%)
905 913 914 916 921.5 929.5 934.5 936 936.5 938.5 943 944 946 950 952
0.60 1.43 1.35 1.86 0.72 1.38 0.51 0.54 0.55 0.66 1.06 0.46 0.71 0.97 0.35
TABLE 1B Approximate sample No.
P20 s (%)
NbzO5 (%)
902-913 913-922.5 922.5-932.5 932.5-941.5
3.0 5.0 2.6 2.2
0.12 0.13 0.12 0.13
Notes to Table 1: REE determinations by neutron activation analysis [3]. Nb2Os and P205 data by optical spectrography [3]. The averagecarbonatite contains 2.06% P2Os, 0.2% Nb, and ~0.5% REE [7] whereas the averagelimestone contains extremely variable P205 but much lower values (i.e., <1 ppm <300 ppm) for Nb and REE, respectively [5,7].
tion analysis. Some of these data are shown in Table 1 which includes total rare earth elements (REE), P20s and Nb20s for the depth range 902-952 ft [3]. This depth range was chosen only because splits from these samples were available for our studies. The significance of the earlier reported data [3] will be discussed below. For the present study 0.5-g aliquots of powders were leached 2N HC1, filtered, and Sr separated by ion exchange chromatography. Dissolution was greater than 90% except for samples 913, 914, and 916 for which approximately 20% residue remained. The Sr isotopic composition was determined using a 12-inch, 90-degree mass spectrometer. The 87Sr /
81 TABLE 2 Sample*
87Sr/86Sr
Sr (ppm) 1
Sr (ppm) 2
Ba (ppm) 1
Ba (%)2
905 913 914 916 921.5 929 929.5A 929.5B 934.5 936 936.5 938.5 943A 943B 944 946 950 952
0.7039 0.7085 0.7064 0.7067 0.7033 0.7040 0.7030 0.7043 0.7034 0.7055 0.7045 0.7051 0.7040 0.7056 0.7048 0.7055 0.7043 0.7032
1480 40 92 252 1020 1290 535 286 1750 1440 2940 2440 1670 685 1140 870 565 3500
-
342 228 262 290 276 238 360 315 254 364 339 505 360 234 306 598 364 298
2.7 1.4 1.4 3.4 -
930 2700 2200 1200 -
* Number refers to depth in ft from where core was taken. Notes to Table 2: 87Sr/86Sr ratios normalized to 86Sr/88Sr = 0.1194. 1 Total Sr and Ba by atomic absorption spectrometry (Note: dissolution of carbonate material was not complete in all cases and some barite was identified in the residue; therefore, the Sr data are somewhat lower and the Ba data very much lower than values determined by optical spectrography [3]). 2 Optical spectrographic determinations [3].
6 Sr data are precise to -+0.0003 (~). Data for the E i m e r and A m e n d SrCOa analyzed during the course o f this investigation yielded a 7 Sr/S 6 Sr = 0.7080 + 0.0003 (o-). A separate split o f each sample was used for det e r m i n a t i o n o f total Sr and Ba by a t o m i c absorption s p e c t r o p h o t o m e t r y . As n o t e d with Table 2, the Sr and especially the Ba data are low due to the presence o f barite not dissolved during HC1 t r e a t m e n t . The powders were r e p o r t e d to be carbonate(s) w i t h some silicate, hence HC1 t r e a t m e n t was used; n o t until after dissolution was it learned that barite was present. Available optical spectrographic data for f o u r samples (Table 2) c o n f i r m that the a t o m i c absorption s p e c t r o p h o t o m e t r i c data are t o o low.
4. Results and discussion Sr isotopic and total Sr and Ba data are presented in Table 2. E x c e p t for the samples in the 9 1 3 - 9 1 6
ft d e p t h range the 8 7 Sr/a 6 Sr ratios are similar to those for the m a j o r i t y o f carbonatites f o u n d througho u t the world ( f r o m 0.703 to 0.705) and t e n d to con. firm the classification o f these rocks as carbonatites. The samples in the 9 1 3 - 9 1 6 ft d e p t h range are a n o m alous. T h e y possess a 7 Sr/86 Sr ratios close to (or in the range of) sedimentary carbonates [6] and lower total Sr relative to the o t h e r samples. We suspect cont a m i n a t i o n o f the carbonatite by sedimentary c a r b o n ate (or similar Sr-bearing material) but cannot unequivocally d e m o n s t r a t e this pending m o r e study. The data shown in Table 1A indicate high R E E cont e n t in this d e p t h interval which is inconsistent with a sedimentary c o n t a m i n a n t , however. A REE-rich, Sr-poor (with high a 7Sr/86Sr ) c o n t a m i n a n t is indicated but its nature u n k n o w n . F u r t h e r m o r e , ten o f the eighteen samples yield Sr values in excess o f 1000 p p m which is also typical o f carbonatites. The Ba data vary f r o m a p p r o x i m a t e l y 250 to 600 p p m but it is probable that these data are far t o o low for reasons m e n t i o n e d earlier. Optical spectrographic data
82 (Table 2) indicate several thousand ppm Ba and are probably more realistic. The only tentative conclusion reached from discussion of the total Sr and Ba data established in this study (Table 2) is that the bulk of the Sr is contained in carbonates and not in the small amounts of barite present. The mean 87Sr/86 Sr ratio of 0.7041 for fifteen of eighteen samples is significant in that it is not only a "typical" value common to many carbonatites but identical within the limits of experimental error to the carbonatitic parts of kimberlites from Riley County, Kansas. Furthermore, the Kansas kimberlites apparently lie on the same structural trend as does the Elk Creek carbonatites; i.e., the NE-trending MidContinent gravity high as opposed to the kimberlite and/or carbonatite and/or peridotite rocks aligned along the 38th parallel lineament. The tectonic implications of this relationship are currently being investigated in a separate study. The total REE, P2Os and Nb2Os (Table 1) data reported earlier [3] for the same range are also typical of carbonatites [3] and consistent with the Sr data. That the Sr data are very different for Pennsylvanian rocks in the general area has been demonstrated previously [4,5]. Typical 87Sr/S6Sr ratios range from 0.7075 to 0.7100 with total Sr contents ranging from very low values of 5 0 - 7 0 ppm (vein calcites in limestones) to values in the 5 0 0 - 6 0 0 ppm range. These sedimentary rocks further do not possess the high REE, P20s and Nb2 Os contents of carbonatites. Several of the many questions arising from study of the Elk Creek carbonatites are: (1) To what depth does the carbonatite persist? (2) How much kimberlite is associated with it? (3) Does this carbonatitic material represent part of an alkaline complex as yet not
discovered (i.e., in order to explain the gravity and magnetic anomalies)? (4) How does the Elk Creek carbonatite relate, or does it, to the Riley County, Kansas, kimberlites and possibly the Mansom Dome, Iowa? These questions are being evaluated and a more comprehensive chemical and petrographic study is underway.
Acknowledgements Partial financial support for this study was made possible by NSF Grant DES72-01655 A01 to the University o f New Mexico.
References 1 W.J. Burfeind, M.P. Carlson and R. Smith, The Elk Creek geophysical anomaly, Johnson and Pawnee Counties, Nebraska, Geol. Soc. Am. Programs with Abstracts 3 (1971) 254. 2 S.B. Treves, R. Smith and J. Rinehart, Petrography and mineralogy of the Elk Creek carbonatite, Nebraska, Geol. Soc. Am. Programs with Abstracts 4 (1972) 352-353. 3 S.B. Treves, R. Smith, M.P. Carlson and G. Coleman, Elk Creek carbonatite, Johnson and Pawnee Counties, Nebraska, Geol. Soc. Am. Programs with Abstracts 4 (1972) 297. 4 D.G. Brookins, The strontium geochemistry of carbonates in kimberlites and limestones from Riley County, Kansas, Earth Planet Sci. Lett. 2 (1967) 235-240. 5 D.G. Brookins. S. Chaudhuri and M.J. Lee, Unpublished data (manuscripts in preparation). 6 G. Faure and J.L. PoweU, Strontium Isotope Geology (Springer-Verlag New York, 1972). 7 E.W. Heinrich, The Geology of Carbonatites (Rand McNaUy, Chicago, 1966).