Heat production in an Archean crustal profile and implications for heat flow and mobilization of heat-producing elements

We have measured concentrations of heat producing elements (Th, U, and K) in 58 samples representative of the main lithologies in a 100 km transect of the Superior Province of the Canadian Shield, from the Michipicoten (Wawa) greenstone belt, near Wawa, Ontario, through a domal gneiss terrane of amphibolite grade, to the granulite belt of the Kapuskasing Structural Zone, near Foleyet. This transect has been interpreted as an oblique cross section through some 25 km of crust, uplifted along a major thrust fault, and thus provides an opportunity to examine in detail a continuous profile into deep continental crust of Archean age. Mean heat production values for these terranes, based on aereal distribution of major rock types and calculated from their Th, U, and K concentrations are: Michipicoten greenstone belt = 0.72 μW m⁻³; Wawa domal gneiss terrane (amphibolite grade) = 1.37 μW m⁻³; Kapuskasing granulites = 0.44 μW m⁻³. Among the silicic plutonic rocks (tonalites, granites, and their derivative gneisses), the relatively large variation in heat production correlates with modal abundances of accessory minerals including allanite, sphene, zircon, and apatite. We interpret these variations as primary (pre-metamorphic). The relatively high weighted mean heat production of the domal gneiss terrane can be accounted for by the larger proportion there of late-stage Th-, U-, and K-rich granitoid plutons. These may have been derived from the underlying Kapuskasing granulite terrane, leaving it slightly depleted in heat producing elements. Transport of Th, U, and K, therefore, could have taken place in silicate melts rather than in aqueous or carbonic metamorphic fluids. This conclusion is supported by the lack of a statistically significant difference in heat production between tonalites, tonalite gneisses and mafic rocks of amphibolite versus granulite grade.The pre-metamorphic radioactivity profile for this crustal section is likely to have been uniformly low, with a mean heat production value less than 1 μW m⁻³. This result is distinctly different from measured profiles in more silicic terranes, which show decreasing heat production with depth. This implies fundamental differences in crustal radioactivity distributions between granitic and more mafic terranes, and may be an important factor in selective reactivation of lithologically different terranes, possibly resulting in preferential stabilization of basic terranes in the geological record. Our results indicate that a previously determined apparently linear heat flow-heat production relationship for the Kapuskasing area does not relate to the distribution of heat production with depth. Low, but significant heat production, 0.4–0.5 μW m⁻³, continues to lower crustal depths with no correlation to the depth parameter from the linear relationship. This low heat production may be a minimum average granulite heat production and suggests that, in general, heat flow through the Moho is 8–10 mW m⁻² lower than the reduced heat flow calculated from the heat flow-heat production regression.     

On the content and vertical distribution of K,Th and U in the continental crust

The content of K, Th and U in the continental crust is estimated based on the assumption that the concentration of these elements decreases with depth asA_x = A₀e^?x/D [11], withA_x andA₀ the heat production rates at depthx and at the surface, respectively. Taking the weighted mean heat production rate of the intrusive rocks of the upper crust asA₀ = 2.33 μWm^?3, that of the granulites representing the lower crust asA_x = 0.72 μWm^?3, and the mean scale heightD= 9.5km [1] the average vertical distancex = b between these intrusives and granulites is 11.2 km. Withb known and the average concentrations of K, Th and U in granulites and intrusive rocks of the upper crust the scale heights of the vertical distribution of these elements areD_K = 71km,D_Th = 9.5km,D_U = 5.8km. The knowledge of these parameters permits to calculate the average concentrations of these elements in a 33.3 km thick crust:K= 2.19%,Th= 4.43ppm,U= 0.66ppm; Th/U = 6.7 and K/U = 3.3 × 10⁴. The resulting heat flow is 23.0 mW m^?2 which is practically identical with the value deduced from heat flow measurements. Assuming that the Th/U ratio of the entire crust—including the sediments—is 3.9, the high ratio of 6.7 in the crystalline crust indicates that about 7.2 × 10¹² t U were extracted from it. All rocks with Th/U ratios <3.9 are possible sinks of this U. About half that amount is deposited in sedimentary rocks, mainly in black shales. The second important sink are the volcanic rocks of the continental margins.     

The heat flow/heat generation relationship: An interaction model of fluids with cooling intrusions

The linear heat flow/heat generation relationship observed for plutonic rocks has been interpreted by Lachenbruch as requiring an exponential depletion of radioactive elements with depth. The exponential depletion is difficult to interpret in terms of fractional crystallization of cooling magmas. Geochemical investigations and autoradiographic method have shown that an important part of the radioactive elements are in leachable sites in the rocks. A model is proposed in which the redistribution of these elements is achieved by fluid-rock interaction during the crystallisation and subsequent cooling of intrusions. A dissolution-precipitation process, mathematically similar to multipass zone refining will result in an exponential distribution of radioactive elements. The geothermal and geochemical implications of the model are discussed.     

Lewisian gneiss geochemistry and Archaean crustal development models

The geochemistry of Lewisian amphibolite-facies gneisses from northwest Scotland is described with particular reference to the rare earth elements (REE) and compared with the geochemistry of Lewisian granulite-facies gneisses. The results show that there are no significant differences between “Laxfordian” amphibolite-facies and “Scourian” granulite-facies gneisses in terms of REE and other immobile trace elements (at equivalent silica levels), although the mobile radioactive heat-producing elements, K, Rb, Th, U, are significantly lower in the granulites. In both types the basic gneisses have moderately fractionated REE patterns while the intermediate and acid gneisses have strongly fractionated REE patterns with low heavy REE abundances and decreasing levels of total REE with increasing SiO₂. The most silicic gneisses develop large positive europium anomalies.These gross chemical similarities between gneisses from intermediate (amphibolite-facies) and lower (granulite-facies) crustal levels constrain models for the evolution of the Archaean crust. The depletion of K, Rb, Cs, Th and U in granulites, but not other incompatible trace elements cannot be explained by magmatic processes. The positive Eu anomaly in the more siliceous gneisses of both facies is a function of the primary processes of crustal generation and not secondary processes such as intracrustal melting or fractional crystallisation. Fractionation of radioactive heat-producing elements from other trace elements is a result of granulite-facies metamorphism with these elements being removed by an active fluid phase. The apparent lack of partial melting in lower crustal granulites suggests a model for Archaean crustal growth largely through underplating by primary tonalitic magmas.     

华北地台岩石放射性元素与现代大陆岩石圈热结构和温度分布

在华北地台1.4×106km2的范围内系统采集了12193个岩石样,组合成1207件分析样,对放射性产热元素K,Th,U进行了测试,同时由国家一级岩石地球化学标准物质监控分析质量．根据岩石中K,Th,U的含量和地壳各结构层的岩石组成,计算了地壳各壳层放射性产热元素的含量,系统研究了华北地台岩石与各时代地层的平均热产生率及其变化,并建立了华北地台现代大陆岩石圈的热结构和温度分布．应用一维稳志热传导模型,求得了华北地台岩石圈的深度-平均地温分布曲线．地表热流密度分布和岩石圈深度-地温分布曲线研究表明,现今华北地台壳内熔融作用不明显。与世界上其他大的稳定克拉通区相比,华北地台地壳活动性较大,具有准地台的特点．     

Radioactive heat generation and heat flow in the Indian shield

Using a laboratory gamma-ray spectrometer, 160 rock samples from different heat flow sites in India have been analysed for their U, Th and K contents. Heat generation has been estimated for the significant rock types. Of the six regions covered by this study, correction due to heat generation contrast and geologic structure was found necessary and has been applied in two regions. Heat flow/heat generation plots for two regions in the southernmost part of the shield follow the line characteristic of a normal continental heat flow province. The plots for the other four regions indicate a heat flow province with an intercept of 0.92 HFU and a slope of 14.8 km. From data available at present, these parameters are higher compared to those from the other shields. The higher parameters imply a higher temperature and heat flow at the Moho, indicating a region of “hotter” upper mantle in the Indian shield.     

Uranium and thorium in paleozoic basalts of Nova Scotia (Canada)

In the Paleozoic basalts of Nova Scotia (Canada) metamorphosed to a greenschist facies grade, U and Th are closely associated with immobile elements (e.g. Zr and Nb). The coherence of these elements with K, typical of igneous rocks is, however, absent, U and Th are apparently not affected by greenschist facies metamorphism as are alkali and alkali earth elements and their variation thus reflects primary magmatic processes. It seems that during the early stages of metamorphism, U and Th were retained together with several other incompatible elements (Zr, Nb, La and Ce) in stable secondary phases or alternatively these elements may be held in primary accessory minerals such as zircon and apatite.     

Geochemistry of charnockites from Sa˜o Paulo State,Brazil

REE and Ba, Th, U, Au, Hf, Sb, Sc and Cs were determined by neutron activation techniques on samples belonging to an acidic high-K charnockitic formation outcropping along the coast of Sa?o Paulo State, Brazil. This formation was dated by the Rb-Sr method and gave whole-rock isochron ages of 546–558 m.y. and initial⁸⁷Sr/⁸⁶Sr ratios of 0.7098–0.7117. A mineral isochron gave an age of 479 m.y. Isotopic and geochemical data support the hypothesis that these rocks derive from the intrusion of a granitic magma produced by crustal anatexis. The source rocks were probably differentiated from the mantle 300–700 m.y. before the solidification of the charnockite. Th/U, K/Cs and Rb/Cs ratios and Au concentrations indicate that the source rock probably was of high metamorphic grade.     

Distributions and removal fluxes of trace metals in the water column of the Hung-Tsai Trough off southwestern Taiwan

Vertical profiles of dissolved and particulate Mn, Fe, Ni, Cu, Zn, Cd, Pb, and (234)Th were determined in the Hung-Tsai Trough off southwestern Taiwan during 19-23 November, 2004. Except in the case of Cd, the distribution coefficient (K(d)) of the trace elements showed a negative correlation with the suspended particle concentration. Based on the average K(d) values, the general sequence of particle affinities for the eight trace elements is, from highest to lowest, Fe>Mn≈Pb>Zn≈Th>Cd≈Cu≈Ni. The trace metal data was coupled with the particle removal flux estimated from (234)Th/(238)U disequilibrium to investigate metal removal by particle sinking from the euphotic layer. The residence time of trace elements with respect to particle removal from the euphotic layer was estimated. A negative correlation between the residence time and the distribution coefficient for the trace metals was found.     

Potassium, uranium, thorium radiogenic heat contribution to heat flow in the precambrian and younger silicic rocks of the Zuni and Florida mountains, New Mexico (U.S.A.)

High heat flow in the Zuni Mountains, New Mexico, U.S.A., has been explained by the possible presence of a buried felsic pluton. Alternately, high K, U, Th abundances have been proposed to account for part of the high heat flow. The mean radiogenic heat contribution for 60 samples of Precambrian core rocks is 7.23 μcal/gm-yr, which is slightly higher than the average for western U.S.A. granitic rocks and significantly higher than the average for continental “crust”; hence, the K, U, Th radiogenic contribution from Precambrian rocks to the overall heat flow is significant. Radiogenic K, U, Th heat for 32 samples for the Florida Mountains, New Mexico, U.S.A., yields a lower mean of 5.46 μcal/gm-yr. This value is somewhat anomalous in that the predominantly syenitic rocks commonly yield higher values. Furthermore, heat flow is higher in areas distant from the Floridas and the radiogenic heat contribution is considered small.     

Palaeomagnetism of Ketilidian metamorphic rocks of SW Greenland,and 1850-1600 m.y. apparent polar movements

Proterozoic supracrustal rocks of southwest Greenland and amphibolite dykes intruding the basement possess a thermal remanent magnetisation acquired during slow regional uplift and cooling between 1800 and 1600 m.y. following the Ketilidian mobile episode. Most samples from amphibolite dykes (mean palaeomagnetic pole 214°E, 31°N) possess a stable remanence associated with development of hematite during regional thermal metamorphism. Metavolcanics from the eastern part (eight sites, palaeomagnetic pole 230°E, 60°N, A₉₅ = 15°) and western part (twelve sites, 279°E, 59°N, A₉₅ = 17°) of Ars?k Island have magnetisations postdating folding and are related to KAr ages dating regional cooling (1700-1600 m.y.); magnetic properties are highly variable and partially stable remanence resides predominantly in pyrrhotite.These results agree in part with other palaeomagnetic results from the northern margin of the same craton, and currently available palaeomagnetic results assigned to the interval 1850-1600 m.y. are evaluated to define apparent polar wander movements. Two large polar movements are recognised during this interval with the possibility of a third at ca. 1800 m.y. It is concluded that apparent polar wander movements in Proterozoic times are most accurately described in terms of closed loops.     

The duration of volcanism in Mare Serenitatis

The crystallization ages of a suite of Apollo 17 basalts from four different stations have been measured using the³⁹Ar-⁴⁰Ar stepwise heating technique. The rocks analyzed include all principal petrographic types found at Apollo 17 landing site. A correlation between the ages and the petrographic type exists; plagioclase-poikilitic ilmenite basalts are youngest and olivine porphyritic ilmenite basalts are the oldest. The duration of volcanism in Mare Serenitatis is about 200 m.y., the same as observed for Mare Imbrium, and less than observed for Mare Tranquillitatis, 400 m.y. A relationship between duration of volcanism and gravity anomalies is noted. The cosmic ray exposure ages (in m.y.) for various locations range as: station 4, 58–315; station 5, 85–440; station 6, 110; station 8, 90–160.     

Destabilization of a 650 km chemical boundary layer and its bearing on the evolution of the continental crust

A mechanism for the production of a chemical change in the mantle, from primordial silicate compositions above the 650 km discontinuity to differentiated compositions below, is reviewed. Some consequences of this are the stabilization of two layer convection with a temperature contrast between the anhydrous mantle solidus and the geotherm which, at 650 km depth, is lower than any other location in the mantle. With thermal contributions from the concentration of the heat producing elements U, Th and K below the 650 km mantle boundary layer and the higher geotherms in the past, widespread or catastrophic melting may have taken place at this location. An episodic breach of this boundary layer by extensive heat and mass transport may have periodically destroyed any simple two-layer convection geometry in the mantle. Such episodic injections of mass and energy into the upper mantle from below may have been the mechanism responsible for episodes of enhanced surface tectonism and thermal activity which appear to be recorded in apparent polar wandering paths and radiometric ages of continental rocks.     

Palaeomagnetic study of the Swedish rapakivi suite: Proterozoic tectonics of the Baltic Shield

The major Proterozoic igneous intrusions in the Swedish sector of the Baltic Shield are the Ragunda complex (1293 m.y., palaeomagnetic pole 165°E, 54°N) and the Nordingrågabbro-granite-anorthosite complex (1385 ± 30 m.y.). The latter body has been partially remagnetised by later post-Jotnian dolerites (1254 m.y.), and sites influenced by the dolerites have a stable magnetisation with a mean direction D = 45°, I = ?39°, (α₉₅ = 4.3°). Elsewhere, the gabbro-anorthosite facies have a magnetisation of dual polarity predating the dolerite and recoverable at various stages of thermal and/or a.f. cleaning with a mean of D = 48°, I = 37° (α₉₅ = 5.3°); medium and high coercivity remanence resides in large magnetite grains and fine, predominantly hematite, rods in feldspar megacrysts. The Nordingrårapakivi granite yields a mean, also including dual polarities, of D = 221°, I = ?25° (α₉₅ = 13°), and the Gävle granite yields a mean of D = 26°, I = 17° (α₉₅ = 13°).New data define the a.p.w. path for the Baltic Shield after final uplift and cooling of the ca. 1800 m.y. Svecofennian mobile belt and prior to intrusion of the post-Jotnian dolerites at 1250 m.y.; this (ca. 1500–1200 m.y.) path defines a double loop similar in size and shape to the contemporaneous path for the Laurentian Shield and the paths can be superimposed to define relative positions of the shields. They were in juxtaposition prior to 1200 m.y. with the optimum reconstruction obtained by rotation of approximately 64° about a Euler pole at 1°E, 36°N. Pre-1500 m.y. palaeomagnetic data are also shown to fit this same unique reconstruction. The main geological correlations are an alignment of the Lower/Middle Proterozoic major strike-slip zones, the structural trends within the pre-1700 m.y. mobile belts, and the Grenville and Sveconorwegian (ca. 1100 m.y.) mobile belts. The anorogenic magmatism characteristic of Proterozoic times became gradually more restricted to one active margin of the continental reconstruction as temperature gradients decreased and the crust consolidated. All of these Proterozoic tectonic/magmatic trends are parallel to the long axis of the continental reconstruction.     

Estimate of influence of U-Th-K radiogenic heat on cooling process of granitic melt and its geological implications

The U-Th-~(40)K concentrations of granite are on 1―2 orders of magnitude greater than those of basal- tic-ultrabasic rocks. Radiogenic heat of a granitic melt has significant influence on the cool- ing-crystallization period of the melt. In this paper we derived a formula to calculate prolongation period (tA) of cooling-crystallization of a granitic melt caused by radiogenic heat. Calculation using this for- mula and radioactive element concentrations (U=5.31×10-6; Th=23.1×10-6; K=4.55%) for the biotite adamellite of the Jinjiling batholith shows that the tA of the adamellite is 1.4 times of the cooling period of the granitic melt without considering radiogenic heat from the initial temperature (Tm=960℃) to crystallization temperature (Tc=600℃) of the melt. It has been demonstrated that the radiogenic heat produced in a granitic melt is a key factor influencing the cooling-crystallization process of the granitic melt, and is likely one of the reasons for inconsistence between emplacement ages and crystallization ages of many Meso-Cenozoic granitoids.     

中国大陆地壳和岩石圈铀、钍、钾丰度的大地热流约束

根据实测大地热流数据，求出中国大陆主要构造单元的地壳平均生热率的上限值为1．3μWm^-3，中国大陆的地壳平均生热率不应高于该值；相应的铀、钍、钾丰度上限值分别为2．04（10^-6)、7．76（10^-6)和2．04（％）。据此对黎彤等近年发表的中国大陆及塔里木－华北板块地壳和岩石圈的铀、钍、钾元素丰度值进行检验。结果表明这些丰度值过高，不满足实测大地热流值的约束，这意味着黎彤等给出的铀、钍、钾的丰度值不可信。我们认为，根据出露地表的相应岩石的成分估计地壳和岩石圈地幔深部的微量元素丰度的研究方法，缺乏可靠的理论或实验基础。     

U–Th radioactive disequilibrium and ESR dating of a barite-containing sulfide crust from South Mariana Trough

A block of sulfide crust collected from an active hydrothermal mound in an Archaean site (12°56.4′N, 143°37.9′E; depth ca. 3000 m) of the South Mariana Trough was dated using both ²³⁰Th/²³⁴U disequilibrium and electron spin resonance (ESR) methods to establish the growth duration. Eight subsamples from the sulfide crust were separated further into magnetic and non-magnetic fractions using a Franz isodynamic separator. Thirteen sulfide samples, soluble in nitric acid, yielded ²³⁰Th/²³⁴U ages of 0.3–2.2 ka. The magnetic fractions had significantly lower Th/U ratios, which enabled age determinations as precise as ±2% (2σ). The age distribution obtained for the section of sulfide crust analyzed is consistent with deposition of sulfide minerals from the upper surface of the crust to the inner side. The ²³⁰Th/²³⁴U ages of the sulfide minerals were compared with ESR ages of barites separated from 12 subsamples of the same sulfide crust. ESR ages of 0.27–1.3 ka show a spatial pattern broadly resembling that observed in ²³⁰Th/²³⁴U dating method. While there are some significant offsets, these results illustrate the potential of the two methods for use in investigation of the evolutional history of a hydrothermal system.     

Terrestrial radioisotopes in black shale hosted Mn-carbonate deposit (Úrkút, Hungary)

Previously, little attention has been paid to terrestrial radioisotopes (U, Th, ⁴⁰K) occurring in manganese ores, despite the fact that the biogeochemical relationship between Mn and U is versatile. Occurrence of terrestrial radioisotopes in great amounts during mining on a long-term causes significant radiation exposure. It is important to inspect black shale-hosted manganese ores from this aspect, as black shales are typically potential U-rich formations. Despite the increased radon concentration in the mine, based on the detailed major elements, trace elements and gamma spectroscopy inspection of the rock types of deposit, the U, Th enrichment was undetectable. However, the U and Th content of about average terrestrial abundance of the great ore amount may be in the background of the increased radon concentration level. This Mn-carbonate ore deposit in spite of the low U content exhibit potential radon danger for miners, which can be eliminated with intensive air change only.     

Understanding airborne radiometric survey signals across part of eastern England

A low‐level airborne radiometric survey provides data on the concentrations of gammaemitting elements including potassium (K), thorium (Th) and uranium (U) in the upper half metre of the soil. Where weathering has not penetrated much beyond this depth, as in the young soils that cover much of England and Wales, the signal is likely to be related to the soil's clay content and its parent material. In these situations radiometric survey could be valuable for mapping soil digitally. We wished to understand how the radiometric signal relates to parent material and soil geochemistry, and to identify the spatial structure, if not the sources, of any unexplained variation. We analysed the joint spatial variation of the airborne gamma signal and high‐resolution soil geochemical survey data across part of eastern England by modelling their coregionalization. We also used reml to assess the joint effects of soil geochemistry and parent material on the radiometric signals of K and Th. The overall correlations of radiometric estimates with soil survey data for K and Th were large, as were the structural correlations for components of variation spatially dependent up to 49 and 16 km for K and Th respectively. This suggests that the radiometric signals for these two elements provide effective estimates of the amounts in the soil and their patterns of distribution. Although class of parent material accounted for a third of of the variance in the radiometric K signal, much of the variation within the classes is explained by geochemistry, suggesting that subtler changes can be detected. A larger proportion of the Th signal was accounted for by parent material. This supports our expectation that radiometric signals for K and Th provide information on parent material in the young landscapes of England and Wales. We are therefore confident that airborne radiometric surveys would be useful for making thematic maps of soil, particularly the soil's texture and closely related properties across England and Wales. Copyright © 2007 Natural Environment Research Council. Published in 2007 by John Wiley & Sons, Ltd.     

Distribution de l’uranium,du thorium et du potassium dans les roches volcaniques de l’Afar septentrional (Ethiopie)

Concentrations in uranium, thorium and potassium were determined by gamma spectrometry in more than 100 samples of volcanic rocks from the material obtained during the French-Italian expedition in Dankalia (Afar-Ethiopia) (December 1967 – February 1968). These samples were taken from two geographically distinct areas:

1. the danakile depression (Erta Ale range, Pierre Pruvost massif, Alayta, Afdera, Borawlu).
2. Ethiopian high lands (Adigrate — Axum).

The values we obtained give the following contents:

Dash

in the Erta Ale range, for basalts, K: 0,5%, U: 1,3 ppm, Th: 3,3 ppm, Th/U: 2,6.

for alkaline rhyolites, U: 4,1 ppm, K: 2,3%, Th: 15,2 ppm, Th/U: 3,7.

in the Pierre Pruvost massif, for acid rocks, K: 3,9%, U: 3,6 ppm, Th: 13,5 ppm, Th/U: 3,8.

in the Borawlu centre, K: 2,8%, U: 4,6 ppm, Th: 14,7, Th/U: 3,4 for the rhyolites, K: 0,6%, U: 0,9 ppm, Th: 3,1 ppm for the basalts.

on the Ethiopian high lands, for the Adigrate basaltic trap rocks, K: 0,6%, U: 0,6 ppm, Th: 1,1 ppm, Th/U: 1,7 and for the Axum zone of phonolitic pinnacles, K: 3,9%, U: 2,7 ppm, Th: 14,9 ppm, Th/U: 4,9.

The Th/K and U/K ratios are particularly high compared with the usual data, except for the Pierre Pruvost massif which seems to be different from the others. The difference of Th/U ratios for basic rocks and acid rocks is meaningful. For the magma which gave the volcanism of the studied area, one can think of an evolution more especially connected with transfers of volatile elements. The strong correlations existing between potassium, uranium and thorium probably indicate the presence of groups which are distinct from a geochemical point of view in the Dankalia lavas.