Robison, Edward C.

Steele, Kenneth F.

English

UARK (University of Arkansas )

Journal of the Arkansas Academy of Science Volume 31 Article 44 1977 Chemical Weathering of Lamprophyric Rock, Central Arkansas Kenneth F. Steele University of Arkansas, Fayetteville Edward C. Robison University of Arkansas, Fayetteville Follow this and additional works at: https://scholarworks.uark.edu/jaas  Part of the Geomorphology Commons Recommended Citation Steele, Kenneth F. and Robison, Edward C. (1977) "Chemical Weathering of Lamprophyric Rock, Central Arkansas," Journal of the Arkansas Academy of Science: Vol. 31 , Article 44. Available at: https://scholarworks.uark.edu/jaas/vol31/iss1/44 This article is available for use under the Creative Commons license: Attribution-NoDerivatives 4.0 International (CC BY-ND 4.0). Users are able to read, download, copy, print, distribute, search, link to the full texts of these articles, or use them for any other lawful purpose, without asking prior permission from the publisher or the author. This General Note is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Journal of the Arkansas Academy of Science by an authorized editor of ScholarWorks@UARK. For more information, please contact scholar@uark.edu. Arkansas Academy of Science Proceedings, Vol.XXXI,1977119Arkansas Academy of ScienceJ. B. SHARP. N. GILBERT, and R. A. DE HON. Department ofPhysical Science, University of Arkansas at Monticello, Monticello. AR7,655.CHEMICALWEATHERING OF LAMPROPHYRIC ROCK, CENTRALARKANSASStudies of the weathering of mafic rocks are few, especially those considering trace transition metals. Wells (1959. Soil sequences frombasalt and graywacke. N.Z.Inst. Agr.Sci. Proc. 1959:40-44) reports the followingsequence of element removal frombasalts during weathering.Ca > Si > Mn,Co, Ni> Ti,V,Cr, FeBsequence is similar to that given by Tiller(1958. The geochemistry of basaltic materials and associated soils of southeastern South Australia,ilSci. 9:225-241) forbasalts.Ca > Co > Mn> Ni> V> Fe >TiWeathered and fresh samples of seven lamprophyric dikes (Table 1) were collected in Saline County, Arkansas. The weathered sampleswere ground with a mortar and pestle and then dried at 105°C for sixhours toremove excess moisture. The weathered and fresh material was then119Journal of the Arkansas Academy of Science, Vol. 31 [1977], Art. 44Published by Arkansas Academy of Science, 1977¦-120 Arkansas Academy of Science Proceedings, Vol. XXXI,1977General Notesground tominus 200 mesh and pressed intoboric acid-backed pellets forx-ray spectrographic analyses. The samples were analyzed for Si, Ca, Fe.Ti,Mn,Co,Cr, Niand Vinorder to determine the behavior of these elements during weathering.Because of volume changes and removal of some elements (whichleads to increase in weight percent of immobile elements) during weather-ing, itis convenient inweathering studies tonormalize elements data for fresh and weathered material to the number ofatoms ofan immobile ele-ment. The absolute numbers of these immobile atoms is assumed to remain constant, although the weight percent in the weathered materialgenerally increases due to the removal ofmobile elements and/or volume changes.Based on chemical behavior and other weathering studies (Wells, 1959; Tiller, 1958) Tihas been assumed to be immobile for this study andall data normalized to Ti.From Table 1 it is apparent that the increased weight percent of Tiinthe weathered sample is largely due to the loss ofCa. By plotting the absolute elemental change in abundance versus a weathering index, Fig. 1) it is possible to compare rates of change. Theweathering index used here is:Ti,,-Ti,Weathering Index (WI)= .—-Ti,Ti. is the number ofTiatoms in the weathered sample; Ti7 is the number of Tiatoms in the fresh rock and WIis increase ofTi in the weatheredsample relative to the fresh rock. The greater the WI value the more intense the weathering.Before plottingelemental change versus WI, the abundance of the element was normalized as follows:E* = X 10,,,ft)E., represents the number ofatoms of an element in the weathered sample; E, represents the number ofatoms ofan element in the fresh rock, n isa power of 10 used forgraphing purposes, and *Ethe change inabundance of the element inthe weathered sample relative to the fresh rock.Bycomparing the slopes of the lines inFigure 1,it is possible to make a comparison ofrelative element mobility.Although there are limiteddata available (7 samples) and there is considerable scatter ofpoints for several elements (Mn, and especially Co,Cr and Ni), the relative sequenceof element mobilitysuggested for lamprophyric rock isCa >Co > Si > Mn> Fe > V>Cr> Ni.This sequence is similar to those proposed for weathering of basalts, and any differences in the lamprophyre and basalt element- mobilityweathering sequences can be explained as a result of scatter inFigure 1and the lack ofdata. The differences inmineralogy between lamprophyreand basalt may also be a controlling factor.There are major changes in absolute element content of lamprophyre with weathering that may not be obvious innormal weight percenttabulation (e.g. compare Si in Table 1 and Figure 1). Effect of lamprophyre weathering on water composition can at least be qualitativelyestimated from the element mobilitysequence. Because of the intensive weathering ofmost lamprophyre outcrops, based on data in Figure 1 andbyanalyzing weathered lamprophyre samples, it should be possible to calculate the composition of the original rock which could be of value forregional igenous rock studies.This project was supported in part by funds from the University of Arkansas Research Reserve Fund and the Arkansas GeologicalCommission.SiO2 CaO Fe203* HO2 MnO Cr Co Ni VLS-270 42.00 15.00 9.70 3.85 0.160 698 39 264 670LS-270W 45.75 8.20 12.75 4. 74 0.204 726 66 304 870LS-70 42.50 14.70 12.55 4.74 O.263 <5 47 31 770LS-70W 42.75 1.00 15.50 7.81 0.078 650 90 600 1310AGS-104 46.50 6.80 13.85 3.73 0.177 136 52 52 622AGS-104W 42.25 1.00 24.25 6.78 0.395 228 155 228 1112AGS-55 44.25 10.40 11.75 4.08 0.230 100 32 87 688AGS-55W 42.50 8.20 14.25 4.65 0.189 48 69 76 796AGS-103 43.83* 10.60* 12.00 3.72 0.190 <5 49 27 562AGS-103W 44.00 7-50 14.95 4.65 0.199 136 61 76 850AGS-94 42.54* 12.40* 13.75 3.72 0.170 <5 68 17 640AGS-94W 44.50 1.00 12.50 9.12 0.097 512 109 455 1520AGS-74 45.25 14.00 10.75 3.23 0.200 70 43 39 562AGS-74W 40.00 12.80 13.50 3-89 0.255 102 54 75 688*Average valueTable 1.Chemical analyses of weathered and unweathered lamprophyre samples. Cr, Co, Niand Vinppm and all other values inweight percentFe»0i* is total iron calculated as ferric oxide. Samples AGS-74, AGS-94 and AGS-104 are ouachitite; sample LS-270 is a melamonchiquiatesamples AGS-103 and LS-70 are monchiquite and sample AGS-55 is a theralite.120Journal of the Arkansas Academy of Science, Vol. 31 [1977], Art. 44https://scholarworks.uark.edu/jaas/vol31/iss1/44Arkansas Academy of Science Proceedings, Vol.XXXI,1977 121Arkansas Academy of ScienceFigure 1. Change in element abundance in the weathered sample relative to the fresh sample calculated from data in Table 1. See text fordefinition of WI and *.In the *Cr and *Vplots the numbers correspond to sample numbers inTable 1. Circled points in the *Cr plot representminimum values.KENNETH F. STEELE and EDWARD C. ROBISON, Department ofGeology, University ofArkansas, Fayetteville, Arkansas 72701.THECHEMICAL COMPOSITION OF CARBONATITES INCONWAY ANDPERRY COUNTIES OF ARKANSASIThe geographical relationship of the Conway and Perry County Carbonatites to other igneous outcrops inArkansas is outlined by Stone andling (1%4) in their summary of igneous rock occurrences in Arkansas. The Conway County outcrops were first described by Croneis andngs in 1930. They have been called, proceeding historically, volcanic necks, sills, vein dikes, breccia and xenolithic dikes and classed asprophyres. The best and largest exposure, which is only a few square meters, in on the south side of the Arkansas River at Lock and Dam No.ar Morrilton, Arkansas. This material is a sillbetween shale and sandstone ofprobable Atokan age. With the help ofCharles G. Stone of theansas Geological Commission and R. R. Cohoon of Arkansas Technical University, Russellville, other exposures of similar looking material:located, and are summarized inTable 1.IThe purpose of this study was to compare the chemical composition of the groundmass of these six outcrops of igenous rocks. Samples ofleast weathered-looking material from each outcrop were obtained and a fresh, central portion of each, 1cm thick X 4 cm X 10 cm was madela water lubricated saw. Carefully avoiding large xenoliths which are plentiful,the dark gray groundmass was isolated and approximately 15 ge ground to 100 mesh bymortar and pestle. Samples were dissolved byHF and HC1 for chemical analyses. Samples were only partially solubleIC1. The solutions were analyzed by a Perkin Elmer Model 303 Atomic Absorption Spectrophotometer using standard techniques (Anony-is, 1973). Carbon dioxide was determined by the titration method ofShapiro (1975).121Journal of the Arkansas Academy of Science, Vol. 31 [1977], Art. 44Published by Arkansas Academy of Science, 1977

Chemical Weathering of Lamprophyric Rock, Central Arkansas

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Journal of the Arkansas Academy of ScienceVolume 31 Article 441977Chemical Weathering of Lamprophyric Rock,Central ArkansasKenneth F. SteeleUniversity of Arkansas, FayettevilleEdward C. RobisonUniversity of Arkansas, FayettevilleFollow this and additional works at: http://scholarworks.uark.edu/jaasPart of the Geomorphology CommonsThis article is available for use under the Creative Commons license: Attribution-NoDerivatives 4.0 International (CC BY-ND 4.0). Users are able toread, download, copy, print, distribute, search, link to the full texts of these articles, or use them for any other lawful purpose, without asking priorpermission from the publisher or the author.This General Note is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Journal of the ArkansasAcademy of Science by an authorized editor of ScholarWorks@UARK. For more information, please contact scholar@uark.edu, ccmiddle@uark.edu.Recommended CitationSteele, Kenneth F. and Robison, Edward C. (1977) "Chemical Weathering of Lamprophyric Rock, Central Arkansas," Journal of theArkansas Academy of Science: Vol. 31 , Article 44.Available at: http://scholarworks.uark.edu/jaas/vol31/iss1/44Arkansas Academy of Science Proceedings, Vol.XXXI,1977119Arkansas Academy of ScienceJ. B. SHARP. N. GILBERT, and R. A. DE HON. Department ofPhysical Science, University of Arkansas at Monticello, Monticello. AR7,655.CHEMICALWEATHERING OF LAMPROPHYRIC ROCK, CENTRALARKANSASStudies of the weathering of mafic rocks are few, especially those considering trace transition metals. Wells (1959. Soil sequences frombasalt and graywacke. N.Z.Inst. Agr.Sci. Proc. 1959:40-44) reports the followingsequence of element removal frombasalts during weathering.Ca > Si > Mn,Co, Ni> Ti,V,Cr, FeBsequence is similar to that given by Tiller(1958. The geochemistry of basaltic materials and associated soils of southeastern South Australia,ilSci. 9:225-241) forbasalts.Ca > Co > Mn> Ni> V> Fe >TiWeathered and fresh samples of seven lamprophyric dikes (Table 1) were collected in Saline County, Arkansas. The weathered sampleswere ground with a mortar and pestle and then dried at 105°C for sixhours toremove excess moisture. The weathered and fresh material was then119Journal of the Arkansas Academy of Science, Vol. 31 [1977], Art. 44Published by Arkansas Academy of Science, 1977¦-120 Arkansas Academy of Science Proceedings, Vol. XXXI,1977General Notesground tominus 200 mesh and pressed intoboric acid-backed pellets forx-ray spectrographic analyses. The samples were analyzed for Si, Ca, Fe.Ti,Mn,Co,Cr, Niand Vinorder to determine the behavior of these elements during weathering.Because of volume changes and removal of some elements (whichleads to increase in weight percent of immobile elements) during weather-ing, itis convenient inweathering studies tonormalize elements data for fresh and weathered material to the number ofatoms ofan immobile ele-ment. The absolute numbers of these immobile atoms is assumed to remain constant, although the weight percent in the weathered materialgenerally increases due to the removal ofmobile elements and/or volume changes.Based on chemical behavior and other weathering studies (Wells, 1959; Tiller, 1958) Tihas been assumed to be immobile for this study andall data normalized to Ti.From Table 1 it is apparent that the increased weight percent of Tiinthe weathered sample is largely due to the loss ofCa. By plotting the absolute elemental change in abundance versus a weathering index, Fig. 1) it is possible to compare rates of change. Theweathering index used here is:Ti,,-Ti,Weathering Index (WI)= .—-Ti,Ti. is the number ofTiatoms in the weathered sample; Ti7 is the number of Tiatoms in the fresh rock and WIis increase ofTi in the weatheredsample relative to the fresh rock. The greater the WI value the more intense the weathering.Before plottingelemental change versus WI, the abundance of the element was normalized as follows:E* = X 10,,,ft)E., represents the number ofatoms of an element in the weathered sample; E, represents the number ofatoms ofan element in the fresh rock, n isa power of 10 used forgraphing purposes, and *Ethe change inabundance of the element inthe weathered sample relative to the fresh rock.Bycomparing the slopes of the lines inFigure 1,it is possible to make a comparison ofrelative element mobility.Although there are limiteddata available (7 samples) and there is considerable scatter ofpoints for several elements (Mn, and especially Co,Cr and Ni), the relative sequenceof element mobilitysuggested for lamprophyric rock isCa >Co > Si > Mn> Fe > V>Cr> Ni.This sequence is similar to those proposed for weathering of basalts, and any differences in the lamprophyre and basalt element-mobilityweathering sequences can be explained as a result of scatter inFigure 1and the lack ofdata. The differences inmineralogy between lamprophyreand basalt may also be a controlling factor.There are major changes in absolute element content of lamprophyre with weathering that may not be obvious innormal weight percenttabulation (e.g. compare Si in Table 1 and Figure 1). Effect of lamprophyre weathering on water composition can at least be qualitativelyestimated from the element mobilitysequence. Because of the intensive weathering ofmost lamprophyre outcrops, based on data in Figure 1 andbyanalyzing weathered lamprophyre samples, it should be possible to calculate the composition of the original rock which could be of value forregional igenous rock studies.This project was supported in part by funds from the University of Arkansas Research Reserve Fund and the Arkansas GeologicalCommission.SiO2 CaO Fe203* HO2 MnO Cr Co Ni VLS-270 42.00 15.00 9.70 3.85 0.160 698 39 264 670LS-270W 45.75 8.20 12.75 4. 74 0.204 726 66 304 870LS-70 42.50 14.70 12.55 4.74 O.263 <5 47 31 770LS-70W 42.75 1.00 15.50 7.81 0.078 650 90 600 1310AGS-104 46.50 6.80 13.85 3.73 0.177 136 52 52 622AGS-104W 42.25 1.00 24.25 6.78 0.395 228 155 228 1112AGS-55 44.25 10.40 11.75 4.08 0.230 100 32 87 688AGS-55W 42.50 8.20 14.25 4.65 0.189 48 69 76 796AGS-103 43.83* 10.60* 12.00 3.72 0.190 <5 49 27 562AGS-103W 44.00 7-50 14.95 4.65 0.199 136 61 76 850AGS-94 42.54* 12.40* 13.75 3.72 0.170 <5 68 17 640AGS-94W 44.50 1.00 12.50 9.12 0.097 512 109 455 1520AGS-74 45.25 14.00 10.75 3.23 0.200 70 43 39 562AGS-74W 40.00 12.80 13.50 3-89 0.255 102 54 75 688*Average valueTable 1.Chemical analyses of weathered and unweathered lamprophyre samples. Cr, Co, Niand Vinppm and all other values inweight percentFe»0i* is total iron calculated as ferric oxide. Samples AGS-74, AGS-94 and AGS-104 are ouachitite; sample LS-270 is a melamonchiquiatesamples AGS-103 and LS-70 are monchiquite and sample AGS-55 is a theralite.120Journal of the Arkansas Academy of Science, Vol. 31 [1977], Art. 44http://scholarworks.uark.edu/jaas/vol31/iss1/44Arkansas Academy of Science Proceedings, Vol.XXXI,1977 121Arkansas Academy of ScienceFigure 1. Change in element abundance in the weathered sample relative to the fresh sample calculated from data in Table 1. See text fordefinition of WI and *.In the *Cr and *Vplots the numbers correspond to sample numbers inTable 1. Circled points in the *Cr plot representminimum values.KENNETH F. STEELE and EDWARD C. ROBISON, Department ofGeology, University ofArkansas, Fayetteville, Arkansas 72701.THECHEMICAL COMPOSITION OF CARBONATITES INCONWAY ANDPERRY COUNTIES OF ARKANSASIThe geographical relationship of the Conway and Perry County Carbonatites to other igneous outcrops inArkansas is outlined by Stone andling (1%4) in their summary of igneous rock occurrences in Arkansas. The Conway County outcrops were first described by Croneis andngs in 1930. They have been called, proceeding historically, volcanic necks, sills, vein dikes, breccia and xenolithic dikes and classed asprophyres. The best and largest exposure, which is only a few square meters, in on the south side of the Arkansas River at Lock and Dam No.ar Morrilton, Arkansas. This material is a sillbetween shale and sandstone ofprobable Atokan age. With the help ofCharles G. Stone of theansas Geological Commission and R. R. Cohoon of Arkansas Technical University, Russellville, other exposures of similar looking material:located, and are summarized inTable 1.IThe purpose of this study was to compare the chemical composition of the groundmass of these six outcrops of igenous rocks. Samples ofleast weathered-looking material from each outcrop were obtained and a fresh, central portion of each, 1cm thick X 4 cm X 10 cm was madela water lubricated saw. Carefully avoiding large xenoliths which are plentiful,the dark gray groundmass was isolated and approximately 15 ge ground to 100 mesh bymortar and pestle. Samples were dissolved byHF and HC1 for chemical analyses. Samples were only partially solubleIC1. The solutions were analyzed by a Perkin Elmer Model 303 Atomic Absorption Spectrophotometer using standard techniques (Anony-is, 1973). Carbon dioxide was determined by the titration method ofShapiro (1975).121Journal of the Arkansas Academy of Science, Vol. 31 [1977], Art. 44Published by Arkansas Academy of Science, 1977

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Chemical Weathering of Lamprophyric Rock, Central Arkansas

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