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Is there a relationship between seismic velocity and heat production for crustal rocks?
Earth and Planetary Science Letters, 79 (1986) 145-150
145
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands [5]
Is there a relationship between seismic velocity and heat production for crustal rocks? D a v i d M. F o u n t a i n Program for Crustal Studies, Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071 (U.S.A.)
Received January 31, 1986; revised version received March 29, 1986 Several investigators proposed an empirical relationship between heat production and seismic velocity for rocks typical of the continental crust. These relationships could allow for indirect resolution of the distribution of heat-producing elements with depth. New heat production data for rock samples from the Pikwitonei granulite domain and the Sachigo granite-greenstone subprovince of the Superior Province do not follow these relationships indicating that heat production cannot be reliably predicted from seismic velocity. These two parameters are not likely to be related as velocity depends upon the major mineral constituents of rocks whereas heat production results from radiogenic components concentrated in both major and trace minerals in rocks.
I. Introduction Distribution of heat-producing elements in the continental crust is poorly constrained and the subject of considerable conjecture in the literature. Several models, such as the simple step function [1] and the exponential model [2], serve to explain the linear relationship between surface heat flow and surface heat production yet have not been adequately tested. Lachenbruch and Bunker [3] used heat production data from deep boreholes in an attempt to constrain the problem. Other efforts focused on heat production data from plutons emplaced at different levels in the crust [4,5], orogenic terrains with large structural and topographic relief [6] and an exposed section of the upper crust in the Vredefort structure [7,8]. These attempts have not fully resolved the variation of heat production in the crust. A novel approach to this problem was presented in a series of papers by Rybach and Buntebarth [9-12] and Allis [13]. Rybach [9] initially proposed a correlation between heat production and density for igneous rocks and, because density and velocity are related [14,15], heat production should be functionally related to seismic velocity. Allis [13] extended the relationship to metamorphic rocks. Rybach [10] proposed an exponential relationship between heat production 0012-821X/86/$03.50
© 1986 Elsevier Science Publishers B.V.
and seismic velocity based on a suggested dependence of both variables on a cation packing index. Rybach and Buntebarth [11,12] refined these relationships with new curves for Phanerozoic and Precambrian rocks. These curves are shown in Figs. 1, 2 and 3 where density is related to velocity by relationships presented by Nafe and Drake [15] and Birch [14]. The Nafe-Drake relationship, originally derived for sediments, is shown in Figs. 1, 2 and 3 as this relationship was used in the earlier Rybach papers. Other velocity-heat production relationsips have been proposed in the literature [16,17]. If these velocity-heat production curves hold, it would be a relatively simple matter to determine the variation of heat production with depth in continental regions with wellconstrained velocity models. Alfis [13] developed such a model for stable shield areas using this approach. Assessment of the applicability of these relationships are important because recently they have been used to estimate temperatures in the crust [17-19]. New heat production data for rocks from the Superior Province permit reevaluation of these velocity-heat production relationships. The suite of rocks studied was collected from the Sachigo and Pikwitonei subprovinces in Manitoba, a region regarded as a nearly intact exposed cross section through the continental crust of the
146
Archean Superior Province [20-22]. The suite includes a wide variety of metamorphic and igneous rocks with metamorphic grade ranging from greenschist to granulite facies. This suite is especially useful to study the proposed relationships as it includes many of the major rock types expected to constitute the continental crust. Rocks from the Pikwitonei domain are deep crustal in origin and Sachigo subprovince rocks represent mid to upper crustal levels. In this note I compare these data to the proposed velocity-heat production relationships in order to evaluate the feasibility of predicting heat production from seismic data.
facies equivalents of most of these rock types are found in the Pikwitonei subprovince which includes granulite facies rocks and the granuliteamphibolite facies transition zone. Enderbites, which are granulite facies equivalents of the lower-grade tonal±tic gneisses [27], constitute about 80% of the Pikwitonei subprovince.
3. Discussion The heat production and density data are plotted for individual samples from the Sachigo and Pikwitonei subprovinces in Figs. 1 and 2, respectively. Average values and their standard deviations for each lithologic category are shown in Fig. 3. Seismic velocities for these rocks can be estimated using one of the velocity-density relations [14,15]. In this case the Nafe-Drake curve [15] was used because this relationship was used in the earlier Rybach papers, but estimates for the relationships for mean atomic weight (m) of 21 and 22 [14] are also shown. This correlation is indicated in Figs. 1, 2 and 3 by the upper horizontal axes. Velocities for the Nafe-Drake relationship appear low because the relationship was originally derived for sediments and was extrapolated to high densities where it is somewhat similar to the m = 22 line [14]. It is evident from these figures that there is considerable scatter of data for individual samples around the various hypothetical relationships. There is a tendency for heat production values to lie above both the Phanerozoic and Precambrian curves [11,12] for the Sachigo subprovince rocks (Fig. 1) with marie rocks displaying the most striking deviation from the curves. At higher metamorphic grades (Fig. 2)
2. Heat production and density data Data for this discussion were originally collected as part of a study of heat production in exposed continental cross sections [22] and can be found in detail in Fountain and Salisbury [23]. Heat production was calculated from U, Th and K concentrations determined from standard gamma-ray spectrometry techniques [24,25] and calculated using the equation presented in Birch [26]. Errors in heat production are regarded to be about 6%. Density was determined from the volume determined with a pycnometer and the mass. A summary of data from the major rock units are presented in Table 1. Rock types in the greenschist and amphibolite domain of the Sachigo subprovince consist of greenstone belt lithologies (meta-gabbros, metabasalts, meta-andesites and meta-graywackes), various quartzo-feldspathic gneisses (primarily tonal±tic in composition) and silicic plutons (granites, granodiorites and tonalites). Granulite TABLE l
Summary of heat production and density data for samples from the Pikwitonei (P) and Sachigo subprovinces (S). Superior Province Rock type Silicic gneisses Marie gneisses Anorthosite Trondhjemites and garnet-bearing gneisses Silicic gneisses Silicic plutons Mafic volcanic and intrusive rocks Metasedimentary rocks Other metavolcanic rocks
Subprovince P P P P S S S S S
Number of samples 9 6 1 6 4 6 10 7 6
Mean heat production
Mean density
(~W/m 3)
(g/cm3)
0.37 + 0.13 0.14 ± 0.08 0,12 1.27 5:0.81 0.66 ± 0.16 1.46 ± 1.01 0.15 5:0,08 1.36 ± 0.68 0.94 _+0.59
2.68 + 0.03 3,08 5: 0.1l 2.71 2,70 ± 0,07 2.71 + 0,03 2.67 + 0.04 3.02 ;:t:0,08 2.78 + 0.06 2.87 ± 0.08
147 6.0 I
m =22 m =21
6.5 I
6'.5 6.0
5
70 I
710 6,5 ,
7.0 ic~
75 i
Z5 80
60 I
m =22 m =21 85 r
6.0
5
65 I
61.5
7.0 I
7'0 6 5
715
7.0
75
80
8.s
SACHIGO SUBPROVINCE •
4
Phonerozoic
'~
0
~-~-IX:E3~2 ~ 0 " :(: : : ; ::::::::.' 3"'::::::, _
0
Phanerozoic
'E
•
Ivrea
Data
?..:.:": :.
iiii ,//AHi~(,979)
c~ Other metovolcanic rocks +
(3,
Pikwitonei Data o Sachigo Data •
~"4
~ Marie meta-igneous rocks 0 Metasedimentary rocks
/
\/i!:O ~iii ! : / Alhs (1979) ....... 0
z3
Silicic gneisses
0 Siliclc plutons
5
Mylonite
": ,::.:[.
•
O0
"":;:: E3 •
W :E t
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:
~.. ".'...
•
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/Ryboch(78/79)
~ : ' . ~ : : : ~ " ~ o I
~
2.6
27
~
le
~-'~,-"~'-~ ~"~---~ 2.8
2.9
30
3.1
3.2
DENSITY (g.cm-3) F i g . 1. Density and velocity versus heat production for individual samples from the Sachigo subprovince, Superior Province, Manitoba. Also shown are various proposed relationships between heat production and density. The correlation with velocity is indicated by upper horizontal axis. Additional upper horizontal axes correspond to velocities calculated using relationships for mean atomic weight (m) of 21 and 22 at 1000 M P a [14].
the silicic granulite gneisses tend to fall well below the Precambrian curve. Average values scatter around the various relationships and can be construed to fit any one of the curves. Notable is the 6.0 I
m=22 m .21
6.5 I
615 6,0
70 I
7lOI 6 5
7.0
7.5
F
•
~5 8.0
65
i
PIKWITONEI @UBPROVINCE •
r~;" 4
Phonerozo(¢
'E
Enderbltll
v Marie gneissel ~, Anort h0|lt@
[!~i, Z
3
o
r~ Metandlmlntary gneliies
::::: ~Allil
(1979)
/
":;/::/:
{O
• garnet-bearing gnetsie.
•
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=
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0
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".:i;i/!i" / R y b o c h (1976)
~
~
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b
o
z,e
c
h
z,~
(78/7g)
so
~,
32
DENSITY (g,crn "3) Fig. 2. Density and velocity versus heat production for individual samples from the Pikwitonei subprovince. Superior Province, Manitoba. Curves and axes are the same as in Fig. 1.
2.6
z7
~ ' ; . " , :
:: , . - -
28 e9 30 DENSITY ( g e m -3)
,
3,
32
F i g . 3. Average density and velocity versus average heat production for various lithologic categories (Table 1) of Sachigo and Pikwitonei subprovince rocks. Bars represent + 1 standard deviation in density and heat production.
tendency of mafic rocks and tonalitic (enderbites and silicic gneisses) to lie close to the Precambrian curve, although other lithologic groups do not. Fig. 3 demonstrates that mean heat production values for low-density rocks can vary by at least a factor of two for a given density. This variation would lead to a variation in heat flow estimation by the same factor. Thus, use of one curve might adequately predict heat production for several categories yet would yield incorrect estimates for other groups. Thus, these results suggest that the proposed relationships are not reliable predictors of heat production from density and, by inference, not reliable predictors of heat production from velocity for crustal lithologies. Finally, use of these relationships requires selection of one velocitydensity relationship. It is evident in Figs. 1, 2 and 3 that there are several choices, suggesting considerable ambiguity in directly deriving heat production from velocity. Available data for other proposed crustal cross sections appear to corroborate these conclusions. In Table 2, I compare average heat production values [28] to average seismic velocities [29] for major lithologic units from the Ivrea zone, a Phanerozoic crustal cross section. Of particular interest here is the observation that heat production ranges from 0.16 to 1.3 / : t W / m 3 for rocks
148 TABLE 2
5.0
Average heat production (A) and seismic velocity (Vp) of Ivrea zone rocks [28,29] Rock type
Upper amphibolite facies felsic rocks Granulite facies felsic rocks Upper amphibolite facies mafic rocks Granulite facies mafic rocks
Average A (/~W/m 3)
Average Vp at 600 MPa (km/s)
Average Vp at 1000 MPa (km/s)
2.0 'E
1.9
6.39
6.49
1.3
7.29
7.40
0.43
7.22
7.37
0.16
7.30
7.39
g O
~" 1.0
13. O
which have compressional wave velocities in the narrow velocity range between 7.2 and 7.4 k m / s . These data are also plotted in Fig. 3 using the Nafe-Drake relationship. Heat production measurements for rocks from the Wawa-Kapuskasing crustal cross section in Ontario [30] suggest relatively uniform values of heat production for a wide compositional range and for several metamorphic grade zonations. Thus, the SachigoPikwitonei data do not appear particularly anomalous in their failure to follow the relationships presented in Figs. 1, 2 and 3. It might be argued, however, that these relationships were not intended for comparison with metamorphic rocks of various metamorphic facies and, indeed, Rybach's original relationship [9] was derived for igneous rocks. Allis [13] and Rybach and Buntebarth [11,12], however, used both igneous and metamorphic rocks in their derivation. Even if only igneous rocks were considered, there would still be severe discrepancies with the relationships. For instance, Tilling et al. [31] measured heat production for a variety of igneous rocks from a global distribution of magmatic suites. These data are summarized in Fig. 4, adapted from Fig. 4 in Tilling et al. [31]. Noticeable in this figure is that each suite seems to have a systematic variation of heat production relative to silica content and there are major differences between suites at any given silica content. This suggests that heat production does not directly depend on composition as implied in the velocity-heat production relation derivations. If these data were translated into velocity-heat production plots, there would
1-
40
50
60
Si02,
70
Wt
80
%
Fig. 4. Heat production-SiO 2 trends for various magmatic suites. 1 = Kamchatka, 2 = Mariana Islands, 3 = Mt. Garibaldi, 4 = Strawberry Mountains, 5 = Lassen, 6 = Southern California batholith, 7 = Modoc, 8 = Jemez Mountains, 9 = Boulder batholith, 10 = Dillsburg, Pennsylvania, 11 = Big Bend, Texas, 12=Virginia, and 1 3 = B e a r p a w Mountains. Figure adapted from Tilling et al. [31].
be scatter of the type observed in Figs. 1, 2 and 3. The reasons for the discrepancy between the Superior Province data and the proposed relationships rest in the fact that seismic velocity and density of rocks is related to the major minerals in rocks whereas heat production (or U, Th and K concentration) is related to both major and minor mineralogy. Potassium, in large part, is locked up in feldspars and phyllosilicates. As with potassium, U and Th are also contained in major minerals but, importantly, occur in large concentrations in trace minerals such as allanite, apatite, monazite, sphene and zircon [32,33] which occur in such small abundances in rocks that they have little or no influence on density and velocity but will strongly influence heat production. Furthermore, there are many well-documented processes which can enrich or deplete radiogenic components in a rock without having significant impact on the bulk mineralogy of the rock and,
149 therefore, n o t have m u c h affect o n density or velocity. F o r instance, it is well-known that granulite facies rocks are c o m m o n l y depleted in radiogenic c o m p o n e n t s b y various geochemical m e c h a n i s m s [34-37]. Thus, there are n o a priori reasons to suspect that density, velocity a n d heat p r o d u c t i o n should be i n t i m a t e l y related.
4. Conclusion Relationships b e t w e e n density a n d heat prod u c t i o n and, b y inference, seismic velocity are n o t s u b s t a n t i a t e d by n e w data from a suite of diverse crustal rock types form the Superior Province. This c o n c l u s i o n is supported b y data from other crustal cross sections a n d from suites of igneous rocks. U n f o r t u n a t e l y , this implies that heat prod u c t i o n in the deeper levels of the c o n t i n e n t a l crust c a n n o t be inferred from seismic velocity or d e n s i t y profiles leaving the quest for the heat p r o d u c t i o n - d e p t h relation unresolved.
Acknowledgements I wish to t h a n k M, Salisbury, W. Weber, employees of the M a n i t o b a Bureau of Geology a n d employees of Ellair for their assistance in the s a m p l i n g program. T. Ingersoll, J. Doerges, J. Goodge, D. M c D o n o u g h , M. Williams, J. Pujol, M. Young, T. Prichard a n d C. Bush provided valuable assistance a n d advice d u r i n g the laboratory phase of the study. C o m p u t i n g was performed on the U n i v e r s i t y of W y o m i n g P r o g r a m for Crustal Studies VAX 1 1 / 7 8 0 computer. This project was f u n d e d b y a U n i v e r s i t y of W y o m i n g Basic Research G r a n t a n d N a t i o n a l Science F o u n d a t i o n g r a n t s ISP-8111449 a n d E A R 8418350.
References 1 R.F. Roy, D.D. Blackwell and F. Birch, Heat generation of plutonic rocks and continental heat flow provinces, Earth Planet. Sci. Lett. 5, 1-12, 1968. 2 A.H. Lachenbruch, Crustal temperature and heat production: implications of the linear heat-flow relation, J. Geophys. Res. 75, 3291-3300, 1970. 3 A.H. Lachenbruch and C.M. Bunker, Vertical gradients of heat production in the continental crust, 2. Some estimates from borehole data, J. Geophys Res. 76, 3852-3860, 1971. 4 C.A. Swanberg, Vertical distribution of heat generation in the Idaho batholith, J. Geophys. Res. 77, 2508-2513, 1972.
5 R.F. Roy, D.D. Blackwell and E.R. Decker, Continental heat flow, in: The Nature of the Solid Earth, E.C. Robertson, ed., pp. 506-543, McGraw-Hill, New York, N.Y., 1972. 6 C.J. Hawkesworth, Vertical distribution of heat production in the basement of the eastern Alps, Nature 249, 435-436, 1974. 7 R.J. Hart, L.O., Nicolaysen and N.H. Gale, Radioelement concentrations in the deep profile through Precambrian basement of the Vredefort structure, J. Geophys. Res. 86, 10639-10652, 1981. 8 L.O. Nicolaysen, R.J. Hart and N.H. Gale, The Vredefort profile extended to supracrustal strata at Carletonville with implications for continental heat flow, J. Geophys. Res. 86, 10653-10661, 1981. 9 L. Rybach, Radiometric heat production in rocks and its relation to other petrophysical properties, Pure Appl. Geophys. 114, 309-318, 1976. 10 L. Rybach, The relationship between seismic velocity and radioactive heat production in crustal rocks: an exponential law, Pure Appl. Geophys. 117, 75-82, 1978/1979. 11 L Rybach and G. Buntebarth, Relationships between the petrophysical properties density, seismic velocity, heat generation, and mineralogic constitution, Earth Planet. Sci. Lett. 57, 367-376, 1982. 12 L. Rybach and C. Buntebarth, The variation of heat generation, density and seismic velocity with rock type in the continental lithosphere, Tectonophysics 103, 335-344, 1984. 13 R.G. Allis, A heat production model for stable continental crust, Tectonophysics 57, 151-165, 1979. 14 F. Birch, Fhe velocity of compressional waves in rocks to 10 kbar, 2, J. Geophys. Res. 66, 2199-2224, 1961. !5 J.E. Nafe and C.L. Drake, Physical properties of marine sediments, in: The Sea, 3, M. Hill, ed., pp. 794-815, Wiley-Interscience,New York, N.Y., 1963. 16 V.V. Gordienko, Comprehensive analysis of physical field in the region of the Carpathians and Dinarides, Acta Geol. Geogr. Montan. Hung. 19, 71-79, 1984. 17 L. Stegena and R. Meissner, Velocity structure and geothermics of the Earth's crust along the European Geotraverse, Tectonophysics 121, 87-96, 1985. 18 R. I-Iaenel,Estimation of temperature at great depths from heat-flow density values, Terra Cognita 5, 380-381, 1985. 19 L. Stegena and R. Meissner, Seismicvelocities and geothermal parameters in the Earth's crust along the European Geotraverse (a preliminary report), Terra Cognita 5, 394-395, 1985. 20 M.H. Salisbury and D.M. Fountain, Continent/continent obduction zones: geological and geophysical case studies, EOS, Trans. Am. Geophys. Union 57, 335, 1976. 21 W. Weber, The Pikwitonei granulite domain: Edge of the Superior Province craton, EOS, Trans. Am. Geophys. Union 61, 386-387, 1980. 22 D.M. Fountain and M.H. Salisbury, Exposed cross-sections through the continental crust: implications for crustal structure, petrology, and evolution, Earth Planet. Sci. Lett. 56, 263-277, 1981. 23 D.M. Fountain and M.H. Salisbury, A heat production model for Superior Province crust based on the
150
24
25
26 27
28 29
30
Pikwitonei-Sachigo continental cross section, Central Manitoba, submitted to Can. J. Earth. Sci. L. Salmon, Analysis of gamma-ray scintillation spectra by the method of least squares, Nucl. Instrum. Methods 14, 193-199, 1961. L. Rybach, Radiometric techniques, in: Modern Methods in Geochemical Analysis, R.E. Wainerdi and E.A. Uken, eds., pp. 271-318, Plenum, New York, N.Y., 1971. F. Birch, Heat from radioactivity, in: Nuclear Geology, H. Faul, ed., pp. 148-174, John Wiley, New York, N.Y., 1954. W. Weber and W.D. Loveridge, Geological relationships and Rb-Sr studies of the Cauchon Lake tonalite and the Pitwitonei enderbite in the northwestern Superior Province, Central Manitoba, Geol. Surv. Can. Pap. 81-1C, 115-121, 1981. D.A. Galson, Heat production of the Ivrea and Strona Ceneri zones, Terra Cognita 3, 14, 1983. D.M. Fountain, The Ivrea-Verbano and Strona-Ceneri zones, northern Italy: a cross-section of the continental crust--new evidence from seismic velocities of rock samples, Tectonophysics 33, 145-165, 1976. L.D. Ashwal, P. Morgan, S.A. Kelly and J.A. percival, An Archean crustal radioactivity profile: the Kapuskasing structural zone, Ontario, Geol. Soc. Am. Abstr. Prog. 16, 433, 1984.
31 R.I. Tilling, D. Gottfried and E.C.W. Dodge, Radiogenic heat production of contrasting magma series: Bearing on interpretation of heat flow, Geol. Soc. Am. Bull. 81, 1447-1462, 1971. 32 J.J.W. Rogers and J.A.S. Adams, Thorium, Abundance in rock forming minerals (I); Thorium minerals (II); phase equilibria (III), in: Handbook of Geochemistry, K.H. Wedepohl, ed., Vol. 2, Pt. 5, pp. 90-D-1 to 90-D-3, Springer, Berlin, 1969. 33 Rogers, J.J.W. and J.A.S. Adams, Uranium, Abundances in rock forming minerals (I); Uranium minerals (II), in: Handbook of Geochemistry, K.H. Wedepohl, ed., Vol. 2, Pt. 5, pp. 92-D-1 to 92-D-5, Springer, Berlin, 1969. 34 I. Lambert, The composition and evolution of the deep continental crust, Spec. Publ. Geol. Soc. Aust. 3, 419-428, 1971. 35 K.S. Heier, Geochemistry of granulite facies rocks and problems of their origin, Philos. Trans. R. Soc. London, Ser. A 273, 429-442, 1973. 36 J. Tarney and B.F. Windley, Chemistry, thermal gradients and evolution of the lower continental crust, J. Geol. Soc. London 134, 153-172, 1977. 37 K.D. Collerson and B.J. Fryer, The role of fluids in the formation and subsequent development of early continental crust, Contrib. Mineral. Petrol. 67, 151-167, 1978.
145
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands [5]
Is there a relationship between seismic velocity and heat production for crustal rocks? D a v i d M. F o u n t a i n Program for Crustal Studies, Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071 (U.S.A.)
Received January 31, 1986; revised version received March 29, 1986 Several investigators proposed an empirical relationship between heat production and seismic velocity for rocks typical of the continental crust. These relationships could allow for indirect resolution of the distribution of heat-producing elements with depth. New heat production data for rock samples from the Pikwitonei granulite domain and the Sachigo granite-greenstone subprovince of the Superior Province do not follow these relationships indicating that heat production cannot be reliably predicted from seismic velocity. These two parameters are not likely to be related as velocity depends upon the major mineral constituents of rocks whereas heat production results from radiogenic components concentrated in both major and trace minerals in rocks.
I. Introduction Distribution of heat-producing elements in the continental crust is poorly constrained and the subject of considerable conjecture in the literature. Several models, such as the simple step function [1] and the exponential model [2], serve to explain the linear relationship between surface heat flow and surface heat production yet have not been adequately tested. Lachenbruch and Bunker [3] used heat production data from deep boreholes in an attempt to constrain the problem. Other efforts focused on heat production data from plutons emplaced at different levels in the crust [4,5], orogenic terrains with large structural and topographic relief [6] and an exposed section of the upper crust in the Vredefort structure [7,8]. These attempts have not fully resolved the variation of heat production in the crust. A novel approach to this problem was presented in a series of papers by Rybach and Buntebarth [9-12] and Allis [13]. Rybach [9] initially proposed a correlation between heat production and density for igneous rocks and, because density and velocity are related [14,15], heat production should be functionally related to seismic velocity. Allis [13] extended the relationship to metamorphic rocks. Rybach [10] proposed an exponential relationship between heat production 0012-821X/86/$03.50
© 1986 Elsevier Science Publishers B.V.
and seismic velocity based on a suggested dependence of both variables on a cation packing index. Rybach and Buntebarth [11,12] refined these relationships with new curves for Phanerozoic and Precambrian rocks. These curves are shown in Figs. 1, 2 and 3 where density is related to velocity by relationships presented by Nafe and Drake [15] and Birch [14]. The Nafe-Drake relationship, originally derived for sediments, is shown in Figs. 1, 2 and 3 as this relationship was used in the earlier Rybach papers. Other velocity-heat production relationsips have been proposed in the literature [16,17]. If these velocity-heat production curves hold, it would be a relatively simple matter to determine the variation of heat production with depth in continental regions with wellconstrained velocity models. Alfis [13] developed such a model for stable shield areas using this approach. Assessment of the applicability of these relationships are important because recently they have been used to estimate temperatures in the crust [17-19]. New heat production data for rocks from the Superior Province permit reevaluation of these velocity-heat production relationships. The suite of rocks studied was collected from the Sachigo and Pikwitonei subprovinces in Manitoba, a region regarded as a nearly intact exposed cross section through the continental crust of the
146
Archean Superior Province [20-22]. The suite includes a wide variety of metamorphic and igneous rocks with metamorphic grade ranging from greenschist to granulite facies. This suite is especially useful to study the proposed relationships as it includes many of the major rock types expected to constitute the continental crust. Rocks from the Pikwitonei domain are deep crustal in origin and Sachigo subprovince rocks represent mid to upper crustal levels. In this note I compare these data to the proposed velocity-heat production relationships in order to evaluate the feasibility of predicting heat production from seismic data.
facies equivalents of most of these rock types are found in the Pikwitonei subprovince which includes granulite facies rocks and the granuliteamphibolite facies transition zone. Enderbites, which are granulite facies equivalents of the lower-grade tonal±tic gneisses [27], constitute about 80% of the Pikwitonei subprovince.
3. Discussion The heat production and density data are plotted for individual samples from the Sachigo and Pikwitonei subprovinces in Figs. 1 and 2, respectively. Average values and their standard deviations for each lithologic category are shown in Fig. 3. Seismic velocities for these rocks can be estimated using one of the velocity-density relations [14,15]. In this case the Nafe-Drake curve [15] was used because this relationship was used in the earlier Rybach papers, but estimates for the relationships for mean atomic weight (m) of 21 and 22 [14] are also shown. This correlation is indicated in Figs. 1, 2 and 3 by the upper horizontal axes. Velocities for the Nafe-Drake relationship appear low because the relationship was originally derived for sediments and was extrapolated to high densities where it is somewhat similar to the m = 22 line [14]. It is evident from these figures that there is considerable scatter of data for individual samples around the various hypothetical relationships. There is a tendency for heat production values to lie above both the Phanerozoic and Precambrian curves [11,12] for the Sachigo subprovince rocks (Fig. 1) with marie rocks displaying the most striking deviation from the curves. At higher metamorphic grades (Fig. 2)
2. Heat production and density data Data for this discussion were originally collected as part of a study of heat production in exposed continental cross sections [22] and can be found in detail in Fountain and Salisbury [23]. Heat production was calculated from U, Th and K concentrations determined from standard gamma-ray spectrometry techniques [24,25] and calculated using the equation presented in Birch [26]. Errors in heat production are regarded to be about 6%. Density was determined from the volume determined with a pycnometer and the mass. A summary of data from the major rock units are presented in Table 1. Rock types in the greenschist and amphibolite domain of the Sachigo subprovince consist of greenstone belt lithologies (meta-gabbros, metabasalts, meta-andesites and meta-graywackes), various quartzo-feldspathic gneisses (primarily tonal±tic in composition) and silicic plutons (granites, granodiorites and tonalites). Granulite TABLE l
Summary of heat production and density data for samples from the Pikwitonei (P) and Sachigo subprovinces (S). Superior Province Rock type Silicic gneisses Marie gneisses Anorthosite Trondhjemites and garnet-bearing gneisses Silicic gneisses Silicic plutons Mafic volcanic and intrusive rocks Metasedimentary rocks Other metavolcanic rocks
Subprovince P P P P S S S S S
Number of samples 9 6 1 6 4 6 10 7 6
Mean heat production
Mean density
(~W/m 3)
(g/cm3)
0.37 + 0.13 0.14 ± 0.08 0,12 1.27 5:0.81 0.66 ± 0.16 1.46 ± 1.01 0.15 5:0,08 1.36 ± 0.68 0.94 _+0.59
2.68 + 0.03 3,08 5: 0.1l 2.71 2,70 ± 0,07 2.71 + 0,03 2.67 + 0.04 3.02 ;:t:0,08 2.78 + 0.06 2.87 ± 0.08
147 6.0 I
m =22 m =21
6.5 I
6'.5 6.0
5
70 I
710 6,5 ,
7.0 ic~
75 i
Z5 80
60 I
m =22 m =21 85 r
6.0
5
65 I
61.5
7.0 I
7'0 6 5
715
7.0
75
80
8.s
SACHIGO SUBPROVINCE •
4
Phonerozoic
'~
0
~-~-IX:E3~2 ~ 0 " :(: : : ; ::::::::.' 3"'::::::, _
0
Phanerozoic
'E
•
Ivrea
Data
?..:.:": :.
iiii ,//AHi~(,979)
c~ Other metovolcanic rocks +
(3,
Pikwitonei Data o Sachigo Data •
~"4
~ Marie meta-igneous rocks 0 Metasedimentary rocks
/
\/i!:O ~iii ! : / Alhs (1979) ....... 0
z3
Silicic gneisses
0 Siliclc plutons
5
Mylonite
": ,::.:[.
•
O0
"":;:: E3 •
W :E t
{]
:
~.. ".'...
•
Rybach (19 76)
/Ryboch(78/79)
~ : ' . ~ : : : ~ " ~ o I
~
2.6
27
~
le
~-'~,-"~'-~ ~"~---~ 2.8
2.9
30
3.1
3.2
DENSITY (g.cm-3) F i g . 1. Density and velocity versus heat production for individual samples from the Sachigo subprovince, Superior Province, Manitoba. Also shown are various proposed relationships between heat production and density. The correlation with velocity is indicated by upper horizontal axis. Additional upper horizontal axes correspond to velocities calculated using relationships for mean atomic weight (m) of 21 and 22 at 1000 M P a [14].
the silicic granulite gneisses tend to fall well below the Precambrian curve. Average values scatter around the various relationships and can be construed to fit any one of the curves. Notable is the 6.0 I
m=22 m .21
6.5 I
615 6,0
70 I
7lOI 6 5
7.0
7.5
F
•
~5 8.0
65
i
PIKWITONEI @UBPROVINCE •
r~;" 4
Phonerozo(¢
'E
Enderbltll
v Marie gneissel ~, Anort h0|lt@
[!~i, Z
3
o
r~ Metandlmlntary gneliies
::::: ~Allil
(1979)
/
":;/::/:
{O
• garnet-bearing gnetsie.
•
g, 0-
=
i,i I T
0
2.6
".:i;i/!i" / R y b o c h (1976)
~
~
z.r
b
o
z,e
c
h
z,~
(78/7g)
so
~,
32
DENSITY (g,crn "3) Fig. 2. Density and velocity versus heat production for individual samples from the Pikwitonei subprovince. Superior Province, Manitoba. Curves and axes are the same as in Fig. 1.
2.6
z7
~ ' ; . " , :
:: , . - -
28 e9 30 DENSITY ( g e m -3)
,
3,
32
F i g . 3. Average density and velocity versus average heat production for various lithologic categories (Table 1) of Sachigo and Pikwitonei subprovince rocks. Bars represent + 1 standard deviation in density and heat production.
tendency of mafic rocks and tonalitic (enderbites and silicic gneisses) to lie close to the Precambrian curve, although other lithologic groups do not. Fig. 3 demonstrates that mean heat production values for low-density rocks can vary by at least a factor of two for a given density. This variation would lead to a variation in heat flow estimation by the same factor. Thus, use of one curve might adequately predict heat production for several categories yet would yield incorrect estimates for other groups. Thus, these results suggest that the proposed relationships are not reliable predictors of heat production from density and, by inference, not reliable predictors of heat production from velocity for crustal lithologies. Finally, use of these relationships requires selection of one velocitydensity relationship. It is evident in Figs. 1, 2 and 3 that there are several choices, suggesting considerable ambiguity in directly deriving heat production from velocity. Available data for other proposed crustal cross sections appear to corroborate these conclusions. In Table 2, I compare average heat production values [28] to average seismic velocities [29] for major lithologic units from the Ivrea zone, a Phanerozoic crustal cross section. Of particular interest here is the observation that heat production ranges from 0.16 to 1.3 / : t W / m 3 for rocks
148 TABLE 2
5.0
Average heat production (A) and seismic velocity (Vp) of Ivrea zone rocks [28,29] Rock type
Upper amphibolite facies felsic rocks Granulite facies felsic rocks Upper amphibolite facies mafic rocks Granulite facies mafic rocks
Average A (/~W/m 3)
Average Vp at 600 MPa (km/s)
Average Vp at 1000 MPa (km/s)
2.0 'E
1.9
6.39
6.49
1.3
7.29
7.40
0.43
7.22
7.37
0.16
7.30
7.39
g O
~" 1.0
13. O
which have compressional wave velocities in the narrow velocity range between 7.2 and 7.4 k m / s . These data are also plotted in Fig. 3 using the Nafe-Drake relationship. Heat production measurements for rocks from the Wawa-Kapuskasing crustal cross section in Ontario [30] suggest relatively uniform values of heat production for a wide compositional range and for several metamorphic grade zonations. Thus, the SachigoPikwitonei data do not appear particularly anomalous in their failure to follow the relationships presented in Figs. 1, 2 and 3. It might be argued, however, that these relationships were not intended for comparison with metamorphic rocks of various metamorphic facies and, indeed, Rybach's original relationship [9] was derived for igneous rocks. Allis [13] and Rybach and Buntebarth [11,12], however, used both igneous and metamorphic rocks in their derivation. Even if only igneous rocks were considered, there would still be severe discrepancies with the relationships. For instance, Tilling et al. [31] measured heat production for a variety of igneous rocks from a global distribution of magmatic suites. These data are summarized in Fig. 4, adapted from Fig. 4 in Tilling et al. [31]. Noticeable in this figure is that each suite seems to have a systematic variation of heat production relative to silica content and there are major differences between suites at any given silica content. This suggests that heat production does not directly depend on composition as implied in the velocity-heat production relation derivations. If these data were translated into velocity-heat production plots, there would
1-
40
50
60
Si02,
70
Wt
80
%
Fig. 4. Heat production-SiO 2 trends for various magmatic suites. 1 = Kamchatka, 2 = Mariana Islands, 3 = Mt. Garibaldi, 4 = Strawberry Mountains, 5 = Lassen, 6 = Southern California batholith, 7 = Modoc, 8 = Jemez Mountains, 9 = Boulder batholith, 10 = Dillsburg, Pennsylvania, 11 = Big Bend, Texas, 12=Virginia, and 1 3 = B e a r p a w Mountains. Figure adapted from Tilling et al. [31].
be scatter of the type observed in Figs. 1, 2 and 3. The reasons for the discrepancy between the Superior Province data and the proposed relationships rest in the fact that seismic velocity and density of rocks is related to the major minerals in rocks whereas heat production (or U, Th and K concentration) is related to both major and minor mineralogy. Potassium, in large part, is locked up in feldspars and phyllosilicates. As with potassium, U and Th are also contained in major minerals but, importantly, occur in large concentrations in trace minerals such as allanite, apatite, monazite, sphene and zircon [32,33] which occur in such small abundances in rocks that they have little or no influence on density and velocity but will strongly influence heat production. Furthermore, there are many well-documented processes which can enrich or deplete radiogenic components in a rock without having significant impact on the bulk mineralogy of the rock and,
149 therefore, n o t have m u c h affect o n density or velocity. F o r instance, it is well-known that granulite facies rocks are c o m m o n l y depleted in radiogenic c o m p o n e n t s b y various geochemical m e c h a n i s m s [34-37]. Thus, there are n o a priori reasons to suspect that density, velocity a n d heat p r o d u c t i o n should be i n t i m a t e l y related.
4. Conclusion Relationships b e t w e e n density a n d heat prod u c t i o n and, b y inference, seismic velocity are n o t s u b s t a n t i a t e d by n e w data from a suite of diverse crustal rock types form the Superior Province. This c o n c l u s i o n is supported b y data from other crustal cross sections a n d from suites of igneous rocks. U n f o r t u n a t e l y , this implies that heat prod u c t i o n in the deeper levels of the c o n t i n e n t a l crust c a n n o t be inferred from seismic velocity or d e n s i t y profiles leaving the quest for the heat p r o d u c t i o n - d e p t h relation unresolved.
Acknowledgements I wish to t h a n k M, Salisbury, W. Weber, employees of the M a n i t o b a Bureau of Geology a n d employees of Ellair for their assistance in the s a m p l i n g program. T. Ingersoll, J. Doerges, J. Goodge, D. M c D o n o u g h , M. Williams, J. Pujol, M. Young, T. Prichard a n d C. Bush provided valuable assistance a n d advice d u r i n g the laboratory phase of the study. C o m p u t i n g was performed on the U n i v e r s i t y of W y o m i n g P r o g r a m for Crustal Studies VAX 1 1 / 7 8 0 computer. This project was f u n d e d b y a U n i v e r s i t y of W y o m i n g Basic Research G r a n t a n d N a t i o n a l Science F o u n d a t i o n g r a n t s ISP-8111449 a n d E A R 8418350.
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150
24
25
26 27
28 29
30
Pikwitonei-Sachigo continental cross section, Central Manitoba, submitted to Can. J. Earth. Sci. L. Salmon, Analysis of gamma-ray scintillation spectra by the method of least squares, Nucl. Instrum. Methods 14, 193-199, 1961. L. Rybach, Radiometric techniques, in: Modern Methods in Geochemical Analysis, R.E. Wainerdi and E.A. Uken, eds., pp. 271-318, Plenum, New York, N.Y., 1971. F. Birch, Heat from radioactivity, in: Nuclear Geology, H. Faul, ed., pp. 148-174, John Wiley, New York, N.Y., 1954. W. Weber and W.D. Loveridge, Geological relationships and Rb-Sr studies of the Cauchon Lake tonalite and the Pitwitonei enderbite in the northwestern Superior Province, Central Manitoba, Geol. Surv. Can. Pap. 81-1C, 115-121, 1981. D.A. Galson, Heat production of the Ivrea and Strona Ceneri zones, Terra Cognita 3, 14, 1983. D.M. Fountain, The Ivrea-Verbano and Strona-Ceneri zones, northern Italy: a cross-section of the continental crust--new evidence from seismic velocities of rock samples, Tectonophysics 33, 145-165, 1976. L.D. Ashwal, P. Morgan, S.A. Kelly and J.A. percival, An Archean crustal radioactivity profile: the Kapuskasing structural zone, Ontario, Geol. Soc. Am. Abstr. Prog. 16, 433, 1984.
31 R.I. Tilling, D. Gottfried and E.C.W. Dodge, Radiogenic heat production of contrasting magma series: Bearing on interpretation of heat flow, Geol. Soc. Am. Bull. 81, 1447-1462, 1971. 32 J.J.W. Rogers and J.A.S. Adams, Thorium, Abundance in rock forming minerals (I); Thorium minerals (II); phase equilibria (III), in: Handbook of Geochemistry, K.H. Wedepohl, ed., Vol. 2, Pt. 5, pp. 90-D-1 to 90-D-3, Springer, Berlin, 1969. 33 Rogers, J.J.W. and J.A.S. Adams, Uranium, Abundances in rock forming minerals (I); Uranium minerals (II), in: Handbook of Geochemistry, K.H. Wedepohl, ed., Vol. 2, Pt. 5, pp. 92-D-1 to 92-D-5, Springer, Berlin, 1969. 34 I. Lambert, The composition and evolution of the deep continental crust, Spec. Publ. Geol. Soc. Aust. 3, 419-428, 1971. 35 K.S. Heier, Geochemistry of granulite facies rocks and problems of their origin, Philos. Trans. R. Soc. London, Ser. A 273, 429-442, 1973. 36 J. Tarney and B.F. Windley, Chemistry, thermal gradients and evolution of the lower continental crust, J. Geol. Soc. London 134, 153-172, 1977. 37 K.D. Collerson and B.J. Fryer, The role of fluids in the formation and subsequent development of early continental crust, Contrib. Mineral. Petrol. 67, 151-167, 1978.