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Some observations on the uranium and thorium distributions in accessory zircon from granitic rocks
Geochimiea et Coemochimica Acta1965,vol. 20,pp. 711to 716. Pergamon Preea Ltd. Printedin NorthernIreland
Some observations on the uranium and thorium distributions in accessory zircon from granitic rocks L. H. AHRENS Department
of Geochemistry, University
of Cape Town
(Received 3 August 1964) Abstract-The uranium and thorium distributions in seventy two specimens of accessory zircon from granitic rocks have been examined. Frequency distributions of both elements are positively skewed and the distributions are clearly not normal; both distributions approximate lognormality. The dispersion of the thorium concentration is distinctly greater than that of uranium; ionic radii differences may be the cause. The ratio Th/U is also positively skewed and whereas the average ratio is 0.47, the modal (most frequent) value is -0.35. Though not well-developed, there appears to be a distinct correlation between uranium and thorium in the zircon under consideration.
IN an investigation on twenty granitic rocks, HURLEY and FAIRBAIRN (1957) demonstrated clearly that uranium and thorium are concentrated in the accessory minerals, monazite, zircon, allanite, epidote and apatite. Zircon is perhaps the most significant of the accessory minerals, because of its widespread use for estimating geological age by one or other variants of the lead method and our attention here will be on this mineral. The HURLEY and FAIRBAIRN study showed that the concentrations of both uranium and thorium varied widely in zircon from granite ; it was demonstrated also that the average ratio Th/U for zircon was distinctly lower than the average for the rock as a whole. In the present study it will be our main object to examine as quantitatively as possible, the extent and statistical nature of the variation of uranium and thorium in zircons from granitic rocks ; consideration will also be given to the uranium-thorium relationship and variation of the ratio Th/U. Aside from the data provided by HURLEY and FAIRBAIRN, data on zircons from granitic rocks have been culled from various other sources in the literature; the total is seventy two. Seventy two is not a large number for precise statistical work ; also, the samples cannot be regarded as truly representative. Nevertheless, these data should provide a reasonable but somewhat tentative idea of the extent and statistical nature of the variation of uranium and thorium in zircons from granitic rocks. Though a major proportion of the zircons are from rocks which have been described as granite or granodiorite, a few of the specimens are from granite-gneisses rather than granites. Zircons from pegmatites and from rocks from contact zones have not been included. DISCUSSION (i)
Distribution
of uranium and thorium
The uranium and thorium data in the seventy two zircon specimens are represented histogrammically in Fig. 1; the interval is 400 ppm for both elements. It is ‘ill
clear that both frequency distributions show decided positive skewnr:ss. Skewnc*xs o t the thorium distribution is distinctly greater than that of uranium. Because of the presence of distinct skewness. neither distribution is normal 01 approaches normalitv. As the skewness is positive. either or hot’11distributions c~oultl be lognormal or appr0ximat.e lognormality. It may be recalled that the distributions of many elements are either lognormal or approximatje lognormalit~r in srvcral specific geological materials. including individual minerals from igneous rocks.
Uranium, Pig.
ppm
Thorium.
ppm
1. Frequency distribution diagrams of U and Th in 72 zircons from granit’ic rocks. Note different degrees of positive skewness.
The possibility of lognormality is tested in Fig. 2 which shows cumulative frequency distributions, using logarithmic probability paper. Most of the points for both uranium and thorium are reasonably well accommodated by the fitted straight lines over a fairly large percentile range ; the greatest displacement tends to be at the very low and very high percentiles. It may be concluded therefore that for the seventy two zircons under consideration both uranium and thorium approximate lognormality over a considerable range of concentration and for the purpose further discussion, lognormality will be assumed. It is clear from Fig. 2 that the slopes of the two drawn straight lines are distinctly different ; that for thorium is greater than that for uranium. It follows therefore that the dispersion of the thorium concentration is greater than that of uranium. This could perhaps be inferred from the different degrees of skewness in Fig. 1, though the relationship might not be clear. Reconstructed frequency distribution diagrams based on the straight lines of Fig. 2, are shown in Fig. 3 (linear horizontal scale) and Fig. 4 (log horizontal scale). The different degrees of skewness (Fig. 3) and of dispersion (Fig. 4) may be readily compared.
of
Uranium
and thorium
distributions
in accessory
Cumulative
on probability Fig. 2. Test for lognormality over a considerable percentile
zircon from granitic
rocks
frequency
paper. Lognormality r is approximated range for both U and Th.
r 24 -
16 -
Th
average
(630 ppm) E 5 : r=
t9/ /
I
U
Fig.
3.
Reconstructed straight
or
U avemge (1330 ppm)
Th,
ppm
distribution diagrams of U and Th, based lines of Fig. 2. Abscissa1 scale is linear.
on tho fitted
713
One cause for the greater dispersion of thorium may be that of radius difference. Both UP+-(r = 6.97 A) and Th*+~(r = 1.02 _$) can evidently replace Zr4i-(r -:- 0.81) :i) in the zircon structure, but U*+ is perhaps preferential1.y accepted because its radius is closer to that of Zr*+. The fact that the ratio ThjU in zircons is invariably much smaller than in the rock as a whole (see below under (iii)) indicates that U4-t is pr(bferentially accepted into the zircon structure. In this respect, it. may be recalled tjhat in the triad K-Rb-Cs, the ions Rb+ and Cs+ substitute for K I-in potassium minerals. The dispersion of cesium is however, invariably greater than that of r~lbidi~lrn. h&t
U or Th ,
ppm
Fig. 4. Reconstructed distribution diagrams of U and Th, based on fitted straight, lines of Fig. 2. Abscissa1 scale is logarithmic.
in specific igneous rocks and in specific potassium minerals; the respective radii are K+( l-33 .II), Rb+( 1.44 A) and Cs+(ls67 8). Though a significant proportion of both uranium and thorium is presumably held within the zircon structure as the cations U4+ and Th4+, it should be borne in mind that significant amounts of these two elements might also be present in zircon as leachable uranothorite (SILVER and DEUTSCH, 1963). The average uranium and thorium concentrations in the seventy two zircons under consideration are 1330 and 630 ppm, respectively. Because of the presence of positive skewness in the uranium and thorium distributions, the above average values are distinctly greater than the estimated most frequent (modal) concentrations (Figs. 3 and 4) of *900 ppm U and ~300-400 ppm Th. (ii) ~i8tr~b~~o~ of the ratio ThjU The average ThjU value for the seventy two zircons considered here is 0.47, a value which may be compared with the frequently quoted magnitude of 3-4 as an average for different types of common igneous rock. It is of interest to note that the average of 0.47 is distinctly greater than the modal (most frequent or typicai) value of ~0.35 which has been estimated from a fitted curve (not shown) on another copy of the h~to~am in Fig. 5. The difference between typical and average values is due to the presence of distinct positive skewness in the Th/U distribution. Such a difference raises the question as to which estimate, the average or the most typical, should be used when comparing ratios? The same problem was encountered in a
Uranium and thorium distributions in accessory zircon from granitic rocks
20 -
Average
rotid (0471
/ I I 2 z g t! L
I I I I I I
IO-
-
i I f I
t
l-l
Ratio,
I I.20
090
0.60
0.30
0
I I.50
Th/U
Fig. 5. Frequency distribution diagram of the ratio Th/U in zircons from granitic rocks. Positive skewness is apparent. 5000 r 0
I
0 0
00 0
0°00
0
0
0
0 sol 100
t
t
I 500
trill
I
Uranium,
1
I
ItllrJ 5000
1000
woo
ppm
Fig. 6. The relationship between U and Th in granitic zircons. Though poorly developed, a trend is apparent, the “slope” of which is greater than unity.
715
discussion (BROOKS and BLHRENS, 1961) of the Itb/‘l’l ratio which in potassium minerals is also positively skewed. Problems of this type may also arise when comparing element concentrations; see for example ~~oLDSCHMII)T (1954, pp. 657-458). Iton~o~o\: (196a), TOLSTOI and OSTAFINHUK (1963) and A\~~~~~ (1964). (iii)
The relationship
between ,wranium and thor’um
The relationship between uranium and thorium in the seventy two zircons from granites is shown in Fig. 6 ; the co-ordinates are logarithmic. Both linear and Iogarithmic scales have been used, but the latter were chosen because the U-Th relationship was more clearly apparent. Though the relationship is not well-developed, a uranium-thorium correlation is nevertheless apparent for the zircon specimens under consideration. The trend of the plotted points indicates that there is a tendency for the ThjU ratio to increase with increasing uranium and thorium content; a line of unit slope (constant Th/Ii ratio) is shown for comparison. Tf this tendency is real, it is of interest to bear in mind that several workers (see HEIER and ROGERS (1963) for example, who refer to other work) have observed a tendency for the ratio ThjU to increase with differentiation in both basic and granitic rocks. REFERENCES AHRENS L. H. (1964) Element distributions in igneous rocks. Advanc. S’ci. (May) 7:3-78. BROOKS R. R. and AHRENS L. H. (1961) Some observations on the distribution of thallium. cadmium and bismuth in silicate rocks and t,he significance of covalency on their degree of association with other elements. Geo&~m. et Cosmochim.. Acta 23, 100-115. C:OLDSCHMIDTV. M. (1954) Geochemistry. Clarendon Press, Oxford. HEIEIS K. S. and ROGERS J. J. W. (1963) Radiometric determination of thorium, uranium and potassium in basalts and in two magmatic differentiation series. Geochim. et Cosmochim. Acta 27, 137.-154. HIJRLEY P. M. and FAIRBAIRN H. F. (1957) Abundance and distribution of uranium and thorium in zircon, sphene, apatite, cpidot,e and monazite in granitic rocks. Trans. A4mer. Geophya. Urj. 38, 939-944. RODIONOV D. A. (1962) Comparison of the average content,s of rock components having lognormal distributions. Geochemistry, So. 8, 846-851. SILVER L. ‘l’. and DEUTSCH SARAH (1963) Uranium-lead isotopic variations in zircons: a c(tso study. .J. Geol. 71, 721-758. TOLSTOI M. 1. and OSTAFIICHUK (1963) Some regularities of the distribution of the element,8 in rocks and their geochemical si@ficance. Geochemistry, NO. 10, 986-991.
Some observations on the uranium and thorium distributions in accessory zircon from granitic rocks L. H. AHRENS Department
of Geochemistry, University
of Cape Town
(Received 3 August 1964) Abstract-The uranium and thorium distributions in seventy two specimens of accessory zircon from granitic rocks have been examined. Frequency distributions of both elements are positively skewed and the distributions are clearly not normal; both distributions approximate lognormality. The dispersion of the thorium concentration is distinctly greater than that of uranium; ionic radii differences may be the cause. The ratio Th/U is also positively skewed and whereas the average ratio is 0.47, the modal (most frequent) value is -0.35. Though not well-developed, there appears to be a distinct correlation between uranium and thorium in the zircon under consideration.
IN an investigation on twenty granitic rocks, HURLEY and FAIRBAIRN (1957) demonstrated clearly that uranium and thorium are concentrated in the accessory minerals, monazite, zircon, allanite, epidote and apatite. Zircon is perhaps the most significant of the accessory minerals, because of its widespread use for estimating geological age by one or other variants of the lead method and our attention here will be on this mineral. The HURLEY and FAIRBAIRN study showed that the concentrations of both uranium and thorium varied widely in zircon from granite ; it was demonstrated also that the average ratio Th/U for zircon was distinctly lower than the average for the rock as a whole. In the present study it will be our main object to examine as quantitatively as possible, the extent and statistical nature of the variation of uranium and thorium in zircons from granitic rocks ; consideration will also be given to the uranium-thorium relationship and variation of the ratio Th/U. Aside from the data provided by HURLEY and FAIRBAIRN, data on zircons from granitic rocks have been culled from various other sources in the literature; the total is seventy two. Seventy two is not a large number for precise statistical work ; also, the samples cannot be regarded as truly representative. Nevertheless, these data should provide a reasonable but somewhat tentative idea of the extent and statistical nature of the variation of uranium and thorium in zircons from granitic rocks. Though a major proportion of the zircons are from rocks which have been described as granite or granodiorite, a few of the specimens are from granite-gneisses rather than granites. Zircons from pegmatites and from rocks from contact zones have not been included. DISCUSSION (i)
Distribution
of uranium and thorium
The uranium and thorium data in the seventy two zircon specimens are represented histogrammically in Fig. 1; the interval is 400 ppm for both elements. It is ‘ill
clear that both frequency distributions show decided positive skewnr:ss. Skewnc*xs o t the thorium distribution is distinctly greater than that of uranium. Because of the presence of distinct skewness. neither distribution is normal 01 approaches normalitv. As the skewness is positive. either or hot’11distributions c~oultl be lognormal or appr0ximat.e lognormality. It may be recalled that the distributions of many elements are either lognormal or approximatje lognormalit~r in srvcral specific geological materials. including individual minerals from igneous rocks.
Uranium, Pig.
ppm
Thorium.
ppm
1. Frequency distribution diagrams of U and Th in 72 zircons from granit’ic rocks. Note different degrees of positive skewness.
The possibility of lognormality is tested in Fig. 2 which shows cumulative frequency distributions, using logarithmic probability paper. Most of the points for both uranium and thorium are reasonably well accommodated by the fitted straight lines over a fairly large percentile range ; the greatest displacement tends to be at the very low and very high percentiles. It may be concluded therefore that for the seventy two zircons under consideration both uranium and thorium approximate lognormality over a considerable range of concentration and for the purpose further discussion, lognormality will be assumed. It is clear from Fig. 2 that the slopes of the two drawn straight lines are distinctly different ; that for thorium is greater than that for uranium. It follows therefore that the dispersion of the thorium concentration is greater than that of uranium. This could perhaps be inferred from the different degrees of skewness in Fig. 1, though the relationship might not be clear. Reconstructed frequency distribution diagrams based on the straight lines of Fig. 2, are shown in Fig. 3 (linear horizontal scale) and Fig. 4 (log horizontal scale). The different degrees of skewness (Fig. 3) and of dispersion (Fig. 4) may be readily compared.
of
Uranium
and thorium
distributions
in accessory
Cumulative
on probability Fig. 2. Test for lognormality over a considerable percentile
zircon from granitic
rocks
frequency
paper. Lognormality r is approximated range for both U and Th.
r 24 -
16 -
Th
average
(630 ppm) E 5 : r=
t9/ /
I
U
Fig.
3.
Reconstructed straight
or
U avemge (1330 ppm)
Th,
ppm
distribution diagrams of U and Th, based lines of Fig. 2. Abscissa1 scale is linear.
on tho fitted
713
One cause for the greater dispersion of thorium may be that of radius difference. Both UP+-(r = 6.97 A) and Th*+~(r = 1.02 _$) can evidently replace Zr4i-(r -:- 0.81) :i) in the zircon structure, but U*+ is perhaps preferential1.y accepted because its radius is closer to that of Zr*+. The fact that the ratio ThjU in zircons is invariably much smaller than in the rock as a whole (see below under (iii)) indicates that U4-t is pr(bferentially accepted into the zircon structure. In this respect, it. may be recalled tjhat in the triad K-Rb-Cs, the ions Rb+ and Cs+ substitute for K I-in potassium minerals. The dispersion of cesium is however, invariably greater than that of r~lbidi~lrn. h&t
U or Th ,
ppm
Fig. 4. Reconstructed distribution diagrams of U and Th, based on fitted straight, lines of Fig. 2. Abscissa1 scale is logarithmic.
in specific igneous rocks and in specific potassium minerals; the respective radii are K+( l-33 .II), Rb+( 1.44 A) and Cs+(ls67 8). Though a significant proportion of both uranium and thorium is presumably held within the zircon structure as the cations U4+ and Th4+, it should be borne in mind that significant amounts of these two elements might also be present in zircon as leachable uranothorite (SILVER and DEUTSCH, 1963). The average uranium and thorium concentrations in the seventy two zircons under consideration are 1330 and 630 ppm, respectively. Because of the presence of positive skewness in the uranium and thorium distributions, the above average values are distinctly greater than the estimated most frequent (modal) concentrations (Figs. 3 and 4) of *900 ppm U and ~300-400 ppm Th. (ii) ~i8tr~b~~o~ of the ratio ThjU The average ThjU value for the seventy two zircons considered here is 0.47, a value which may be compared with the frequently quoted magnitude of 3-4 as an average for different types of common igneous rock. It is of interest to note that the average of 0.47 is distinctly greater than the modal (most frequent or typicai) value of ~0.35 which has been estimated from a fitted curve (not shown) on another copy of the h~to~am in Fig. 5. The difference between typical and average values is due to the presence of distinct positive skewness in the Th/U distribution. Such a difference raises the question as to which estimate, the average or the most typical, should be used when comparing ratios? The same problem was encountered in a
Uranium and thorium distributions in accessory zircon from granitic rocks
20 -
Average
rotid (0471
/ I I 2 z g t! L
I I I I I I
IO-
-
i I f I
t
l-l
Ratio,
I I.20
090
0.60
0.30
0
I I.50
Th/U
Fig. 5. Frequency distribution diagram of the ratio Th/U in zircons from granitic rocks. Positive skewness is apparent. 5000 r 0
I
0 0
00 0
0°00
0
0
0
0 sol 100
t
t
I 500
trill
I
Uranium,
1
I
ItllrJ 5000
1000
woo
ppm
Fig. 6. The relationship between U and Th in granitic zircons. Though poorly developed, a trend is apparent, the “slope” of which is greater than unity.
715
discussion (BROOKS and BLHRENS, 1961) of the Itb/‘l’l ratio which in potassium minerals is also positively skewed. Problems of this type may also arise when comparing element concentrations; see for example ~~oLDSCHMII)T (1954, pp. 657-458). Iton~o~o\: (196a), TOLSTOI and OSTAFINHUK (1963) and A\~~~~~ (1964). (iii)
The relationship
between ,wranium and thor’um
The relationship between uranium and thorium in the seventy two zircons from granites is shown in Fig. 6 ; the co-ordinates are logarithmic. Both linear and Iogarithmic scales have been used, but the latter were chosen because the U-Th relationship was more clearly apparent. Though the relationship is not well-developed, a uranium-thorium correlation is nevertheless apparent for the zircon specimens under consideration. The trend of the plotted points indicates that there is a tendency for the ThjU ratio to increase with increasing uranium and thorium content; a line of unit slope (constant Th/Ii ratio) is shown for comparison. Tf this tendency is real, it is of interest to bear in mind that several workers (see HEIER and ROGERS (1963) for example, who refer to other work) have observed a tendency for the ratio ThjU to increase with differentiation in both basic and granitic rocks. REFERENCES AHRENS L. H. (1964) Element distributions in igneous rocks. Advanc. S’ci. (May) 7:3-78. BROOKS R. R. and AHRENS L. H. (1961) Some observations on the distribution of thallium. cadmium and bismuth in silicate rocks and t,he significance of covalency on their degree of association with other elements. Geo&~m. et Cosmochim.. Acta 23, 100-115. C:OLDSCHMIDTV. M. (1954) Geochemistry. Clarendon Press, Oxford. HEIEIS K. S. and ROGERS J. J. W. (1963) Radiometric determination of thorium, uranium and potassium in basalts and in two magmatic differentiation series. Geochim. et Cosmochim. Acta 27, 137.-154. HIJRLEY P. M. and FAIRBAIRN H. F. (1957) Abundance and distribution of uranium and thorium in zircon, sphene, apatite, cpidot,e and monazite in granitic rocks. Trans. A4mer. Geophya. Urj. 38, 939-944. RODIONOV D. A. (1962) Comparison of the average content,s of rock components having lognormal distributions. Geochemistry, So. 8, 846-851. SILVER L. ‘l’. and DEUTSCH SARAH (1963) Uranium-lead isotopic variations in zircons: a c(tso study. .J. Geol. 71, 721-758. TOLSTOI M. 1. and OSTAFIICHUK (1963) Some regularities of the distribution of the element,8 in rocks and their geochemical si@ficance. Geochemistry, NO. 10, 986-991.