Distribution Of Heat Producing Elements Research Articles

Bounding the unknowns of martian crustal heat flow from a synthesis of regional geochemistry and InSight mission data

The thermal evolution of terrestrial planets is primarily modulated by the distribution of heat-producing elements (HPE) within the crust and mantle. Chemical data from Martian meteorites suggest that Mars differentiated early, which led to an early partitioning of incompatible heat-producing elements in the crust. Previous estimates of Martian crustal heat flow that used the bulk regolith abundances of HPEs from the Gamma-Ray Spectrometer (GRS) suite on Mars Odyssey spacecraft have further corroborated this view of Mars, albeit with poorly known crustal column representativeness. Here we couple the GRS-derived chemical maps of Mars with estimates of crustal thickness and density from the InSight lander to revise the estimated Martian crustal heat flow. The mean crustal heat flow values range from 3.0 to 13.9 mW m−2 for the endmember gravity-derived crustal thickness models anchored by constraints from InSight. We also estimate the crustal heat flow from other factors such as the distribution of HPEs with depth and the Uranium content of the crust. Our results suggest that the mean crustal heat flow varies substantially across models, with the highest mean values being associated with higher densities and an increased enrichment of HPEs with depth. Further work is needed to constrain the crustal thickness of Mars, as the largest uncertainties in the estimate of crustal heat flow stem from uncertainty in the crustal thickness estimates, not geochemical variability. The results from this work corroborate previous estimates of a strong fractionation of heat-producing elements into the Martian crust.

Heat‐Blanketed Convection and its Implications for the Continental Lithosphere

AbstractEarth's continental crust is characterized by a strong enrichment in long‐lived radioactive isotopes. Recent estimates suggest that the continental crust contributes to 33% of the heat released at the surface of the Earth, while occupying less than 1% of the mantle. This distinctive feature has profound implications for the underlying mantle by impacting its thermal structure and heat transfer. However, the effects of a continental crust enriched in heat‐producing elements on the underlying mantle have not yet been systematically investigated. Here, we conduct a preliminary investigation by considering a simplified convective system consisting in a mixed heated fluid where all the internal heating is concentrated in a top layer of thickness dHL (referred to as “heat‐blanketed convection”). We perform 24 numerical simulations in three dimensional Cartesian geometry for four specific set‐ups and various values of dHL. Our results suggest that the effects of the heated layer strongly depend on its thickness relative to the thickness of the thermal boundary layer (δTBL) in the homogeneous heating case (dHL = 1.0). More specifically, for dHL > δTBL, the effects induced by the heated layer are quite modest, while, for dHL < δTBL, the properties of the convective system are strongly altered as dHL decreases. In particular, the surface heat flux and convective vigor are significantly enhanced for very thin heated layers compared to the case dHL = 1.0. The vertical distribution of heat producing elements may therefore play a key role in mantle dynamics. For Earth, the presence of continents should however not affect significantly the surface heat flux, and thus the Earth's cooling rate.

JULOC: A local 3-D high-resolution crustal model in South China for forecasting geoneutrino measurements at JUNO

Geothermal energy is one of the keys for understanding the mechanisms driving the plate tectonics and mantle dynamics. The surface heat flux, as measured in boreholes, provides limited insights into the relative contributions of primordial versus radiogenic sources of the interior heat budget. Geoneutrino, electron antineutrino that is produced from the radioactive decay of the heat producing elements, is a unique probe to obtain direct information about the amount and distribution of heat producing elements in the crust and mantle. Cosmochemical, geochemical, and geodynamic compositional models of the Bulk Silicate Earth (BSE) individually predict different mantle neutrino fluxes, and therefore may be distinguished by the direct measurement of geoneutrinos. Due to low counting statistics, the results from geoneutrino measurements at several sites are inadequate to resolve the geoneutrino flux. However, the JUNO detector, currently under construction in South China, is expected to provide an exciting opportunity to obtain a highly reliable statistical measurement, which will produce sufficient data to address several vital questions of geological importance.However, the detector cannot separate the mantle contribution from the crust contribution. To test different compositional models of the mantle, an accurate a-priori estimation of the crust geoneutrino flux based on a three-dimensional (3-D) crustal model is important. This paper presents a 3-D crustal model over a surface area of 10° × 10° grid surrounding the JUNO detector and a depth down to the Moho discontinuity, based on the geological, geophysical and geochemical properties. This model provides a distinction of the thickness of the different crustal layers together with the corresponding Th and U abundances. We also present our predicted local contribution to the total geoneutrino flux and the corresponding radiogenic heat. Compared to previous studies where the surface layer is subdivided into a few geologic units and each of them is considered to have the same geochemical property, our method has provided an effective approach to reduce the uncertainty of geoneutrino flux prediction by constructing the composition of the surface layer through cell by cell which are independent to each other.

U, Th, and K partitioning between metal, silicate, and sulfide and implications for Mercury's structure, volatile content, and radioactive heat production

AbstractThe distribution of heat-producing elements (HPE) potassium (K), uranium (U), and thorium (Th) within planetary interiors has major implications for the thermal evolution of the terrestrial planets and for the inventory of volatile elements in the inner solar system. To investigate the abundances of HPE in Mercury’s interior, we conducted experiments at high pressure and temperature (up to 5 GPa and 1900 °C) and reduced conditions (IW-1.8 to IW-6.5) to determine U, Th, and K partitioning between metal, silicate, and sulfide (Dmet/sil and Dsulf/sil). Our experimental data combined with those from the literature show that partitioning into sulfide is more efficient than into metal and that partitioning is enhanced with decreasing FeO and increasing O contents of the silicate and sulfide melts, respectively. Also, at low oxygen fugacity (log fO2 < IW-5), U and Th are more efficiently partitioned into liquid iron metal and sulfide than K. Dmet/sil for U, Th, and K increases with decreasing oxygen fugacity, while DUmet/sil and DKmet/sil increase when the metal is enriched and depleted in O or Si, respectively. We also used available data from the literature to constrain the concentrations of light elements (Si, S, O, and C) in Fe metal and sulfide. We calculated chemical compositions of Mercury’s core after core segregation, for a range of fO2 conditions during its differentiation. For example, if Mercury differentiated at IW-5.5, its core would contain 49 wt% Si, 0.02 wt% S, and negligible C. Also if core-mantle separation happened at a fO2 lower than IW-4, the bulk Mercury Fe/Si ratio is likely to be chondritic. We calculated concentrations of U, Th, and K in the Fe-rich core and possible sulfide layer of Mercury. Bulk Mercury K/U and K/Th were calculated taking all U, Th, and K reservoirs into account. Without any sulfide layer, or if Mercury’s core segregated at a higher fO2 than IW-4, bulk K/U and K/Th would be similar to those measured on the surface, confirming more elevated volatile K concentration than previously expected for Mercury. However, Mercury could fall on an overall volatile depletion trend where K/U increases with the heliocentric distance if core segregation occurred near IW-5.5 or more reduced conditions, and with a sulfide layer of at least 130 km thickness. At these conditions, the bulk Mercury K/Th ratio is close to Venus’s and Earth’s values. Since U and Th become more chalcophile with decreasing oxygen fugacity, to a higher extent than K, it is likely that at an fO2 close to, or lower than, IW-6 both K/U and K/Th become lower than values of the other terrestrial planets. Therefore, our results suggest that the elevated K/U and K/Th ratios of Mercury’s surface should not be exclusively interpreted as the result of a volatile enrichment in Mercury, but could also indicate a sequestration of more U and Th than K in a hidden iron sulfide reservoir, possibly a layer present between the mantle and core. Hence, Mercury could be more depleted in volatiles than Mars with a K concentration similar to or lower than the Earth’s and Venus’s, suggesting volatile depletion in the inner solar system. In addition, we show that the presence of a sulfide layer formed between IW-4 and IW-5.5 decreases the total radioactive heat production of Mercury by up to 30%.

Pre-mission InSights on the Interior of Mars

Abstract The Interior exploration using Seismic Investigations, Geodesy, and Heat Trans- port (InSight) Mission will focus on Mars’ interior structure and evolution. The basic structure of crust, mantle, and core form soon after accretion. Understanding the early differentiation process on Mars and how it relates to bulk composition is key to improving our understanding of this process on rocky bodies in our solar system, as well as in other solar systems. Current knowledge of differentiation derives largely from the layers observed via seismology on the Moon. However, the Moon’s much smaller diameter make it a poor analog with respect to interior pressure and phase changes. In this paper we review the current knowledge of the thickness of the crust, the diameter and state of the core, seismic attenuation, heat flow, and interior composition. InSight will conduct the first seismic and heat flow measurements of Mars, as well as more precise geodesy. These data reduce uncertainty in crustal thickness, core size and state, heat flow, seismic activity and meteorite impact rates by a factor of 3–10× relative to previous estimates. Based on modeling of seismic wave propagation, we can further constrain interior temperature, composition, and the location of phase changes. By combining heat flow and a well constrained value of crustal thickness, we can estimate the distribution of heat producing elements between the crust and mantle. All of these quantities are key inputs to models of interior convection and thermal evolution that predict the processes that control subsurface temperature, rates of volcanism, plume distribution and stability, and convective state. Collectively these factors offer strong controls on the overall evolution of the geology and habitability of Mars.

Thorium distribution in the crust: Outcrop and grain-scale perspectives

The spatial distribution of heat producing elements (K, U, and Th) in the continental crust has long-term implications for the thermal and physical evolution of orogens. Heat producing elements, in particular Th, are most abundant in metasedimentary rock types. As such, these rock types have a significant control on the spatial distribution of heat production in the crust. The major host of the heat producing element thorium in pelitic metasedimentary rocks is the REE–Th phosphate monazite. We present in-field gamma ray spectrometry (in-field GRS) data integrated with grain-scale electron probe microanalysis data to reveal grain to terrane scale links in thorium distribution. In-field GRS data shows that thorium is not depleted in granulite facies residual rocks that have lost melt with respect to their subsolidus counterparts. Concurrently, the bulk thorium budget of monazite is approximately uniform within samples and if anything increases with increasing metamorphic grade. Monazite average grain size increases with metamorphic grade and prograde cores are largely preserved in granulite facies samples. Thorium is preserved in residual metasediments after melting and melt loss implying that even when melting and melt extraction is efficient it does not strip Th from granulite facies rocks.

Spectroscopy of geoneutrinos from 2056 days of Borexino data

We report an improved geo-neutrino measurement with Borexino from 2056 days of data taking. The present exposure is $(5.5\pm0.3)\times10^{31}$ proton$\times$yr. Assuming a chondritic Th/U mass ratio of 3.9, we obtain $23.7 ^{+6.5}_{-5.7} (stat) ^{+0.9}_{-0.6} (sys)$ geo-neutrino events. The null observation of geo-neutrinos with Borexino alone has a probability of $3.6 \times 10^{-9}$ (5.9$\sigma$). A geo-neutrino signal from the mantle is obtained at 98\% C.L. The radiogenic heat production for U and Th from the present best-fit result is restricted to the range 23-36 TW, taking into account the uncertainty on the distribution of heat producing elements inside the Earth.

Geo-neutrinos: recent developments

Radiogenic heating is a key component of the energy balance and thermal evolution of the Earth. It contributes to mantle convection, plate tectonics, volcanoes, and mountain building. Geo-neutrino observations estimate the present radiogenic power of our planet. This estimate depends on the quantity and distribution of heat-producing elements in various Earth reservoirs. Of particular geological importance is radiogenic heating in the mantle. This quantity informs the origin and thermal evolution of our planet. Here we present: currently reported geo-neutrino observations; estimates of the mantle geo-neutrino signal, mantle radiogenic heating, and mantle cooling; a comparison of chemical Earth model predictions of the mantle geo-neutrino signal and mantle radiogenic heating; a brief discussion of radiogenic heating in the core, including calculations of geo-neutrino signals per pW/kg; and finally a discussion of observational strategy.

A rock fragment related to the magnesian suite in lunar meteorite Allan Hills (ALHA) 81005

Among the lunar samples that were returned by the Apollo missions are many cumulate plutonic rocks with high Mg# [molar Mg/(Mg+Fe) in %] and abundances of KREEP elements (potassium, rare earth elements, phosphorus, U, Th, etc.) that imply KREEP-rich parental magmas. These rocks, collectively called the magnesian suite, are nearly absent from sampling sites distant from Imbrium basin ejecta, including those of lunar highlands meteorites. This absence has significant implications for the early differentiation of the Moon and its distribution of heat-producing elements (K, Th, U). Here, we analyze a unique fragment of basalt with the mineralogy and mineral chemistry of a magnesian suite rock, in the lunar highlands meteorite Allan Hills (ALH) A81005. In thin section, the fragment is 700 × 300 μm, and has a sub-ophitic texture with olivine phenocrysts, euhedral plagioclase grains (An97-70),and interstitial pyroxenes. Its minerals are chemically equilibrated. Olivine has Fe/Mn ~ 70 (consistent with a lunar origin), and Mg# ~80, which is consistent with rocks of the magnesian suite and far higher than in mare basalts. It has a rich suite of minor minerals: fluorapatite, ilmenite, Zr-armalcolite, chromite, troilite, silica, and Fe metal (Ni = 3.8%, Co = 0.17%). The metal is comparable to that in chondrite meteorites, which suggests that the fragment is from an impact melt. The fragment itself is not a piece of magnesian suite rock (which are plutonic), but its mineralogy and mineral chemistry suggest that its protolith (which was melted by impact) was related to the magnesian suite. However, the fragment’s mineral chemistry and minor minerals are not identical to those of known magnesian suite rocks, suggesting that the suite may be more varied than apparent in the Apollo samples. Although ALHA81005 is from the lunar highlands (and likely from the farside), Clast U need not have formed in the highlands. It could have formed in an impact melt pool on the nearside and been transported by meteoroid impact. Lunar highlands meteorites should be searched for rock fragments related to the magnesian-suite rocks, but the fragments are rare and may have mineral compositions similar to some meteoritic (impactor) materials.

Hot rocks in a cold place: high sub-glacial heat flow in East Antarctica

Numerical models are the primary predictive tools for understanding the dynamic behaviour of the Antarctic ice sheet. However, a key boundary parameter, sub-glacial heat flow, remains poorly constrained. We show that variations in abundance and distribution of heat-producing elements within the Antarctic continental crust result in greater and more variable regional sub-glacial heat flows than currently assumed in ice modelling studies. Such elevated heat flows would have a fundamental effect on ice sheet behaviour and highlight that geological controls on heat flow must be considered to obtain more accurate and refined predictions of ice mass balance and sea-level change. Supplementary materials: Heat flow and heat production definitions, heat production data, and details of the 2D model of the gross geometry of the continental lithosphere for a section through Prydz Bay are available at www.geolsoc.org.uk/SUP18690 .

Radiogenic heat production, thermal regime and evolution of continental crust

Abstract Heat flow and heat production data complement seismic information and provide strong constraints on crustal composition, thickness and evolution. They have helped understand the nature of the Mohorovicic discontinuity and the variations in seismic velocities below the Moho. Notably, heat flow studies have delineated the vertical distribution of heat producing elements throughout the crust and in the upper most mantle lithosphere. Analysis of global data sets on heat flow and crustal thickness demonstrate that there is no correlation between these two variables. This is due to the large spatial variations in crustal composition and heat production that exist within a single geological province. For a given crustal thickness, the Moho temperature varies within a wide range (≈ 300 K) depending on surface heat flux and crustal heat production. Thus one cannot use generic models based on a “type” crustal column to calculate crustal geotherms. In stable regions, lower crustal temperatures depend on the amount and vertical distribution of heat producing elements in the crust. These temperatures determine the conditions of crustal stability and impose a limit on the maximum thickness of a stabilized crust.

Highly reducing conditions during core formation on Mercury: Implications for internal structure and the origin of a magnetic field

Abstract The high average density and low surface FeO content of the planet Mercury are shown to be consistent with very low oxygen fugacity during core segregation, in the range 3–6 log units below the iron–wustite buffer. These low oxygen fugacities, and associated high metal content, are characteristic of high-iron enstatite (EH) and Bencubbinite (CB) chondrites, raising the possibility that such materials may have been important building blocks for this planet. With this idea in mind we have explored the internal structure of a Mercury sized planet of EH or CB bulk composition. Phase equilibria in the silicate mantle have been modeled using the thermodynamic calculator p-MELTS, and these simulations suggest that orthopyroxene will be the dominant mantle phase for both EH and CB compositions, with crystalline SiO 2 being an important minor phase at all pressures. Simulations for both compositions predict a plagioclase-bearing “crust” at low pressure, significant clinopyroxene also being calculated for the CB bulk composition. Concerning the core, comparison with recent high pressure and high temperature experiments relevant to the formation of enstatite meteorites, suggest that the core of Mercury may contain several wt.% silicon, in addition to sulfur. In light of the pressure of the core–mantle boundary on Mercury (∼7 GPa) and the pressure at which the immiscibility gap in the system Fe–S–Si closes (∼15 GPa) we suggest that Mercury’s core may have a complex shell structure comprising: (i) an outer layer of Fe–S liquid, poor in Si; (ii) a middle layer of Fe–Si liquid, poor in S; and (iii) an inner core of solid metal. The distribution of heat-producing elements between mantle and core, and within a layered core have been quantified. Available data for Th and K suggest that these elements will not enter the core in significant amounts. On the other hand, for the case of U both recently published metal/silicate partitioning data, as well as observations of U distribution in enstatite chondrites, suggest that this element behaves as a chalcophile element at low oxygen fugacity. Using these new data we predict that U will be concentrated in the outer layer of the mercurian core. Heat from the decay of U could thus act to maintain this part of Mercury’s core molten, potentially contributing to the origin of Mercury’s magnetic field. This result contrasts with the Earth where the radioactive decay of U represents a negligible contribution to core heating.

The thermal structure of stable continental lithosphere within a dynamic mantle

The thermal structure of stable continental lithosphere is determined by (1) the concentration and distribution of heat sources within the crust and (2) the amount of heat input from the convecting mantle. The self-consistent coupling of these two factors has not been included in thermal models of stable lithosphere to date. We conducted two suites of numerical simulations (one with variable crustal heat production and the other with a chemically distinct cratonic root) to explore the thermal coupling between stable continental lithosphere and the convecting mantle. The distribution of heat producing elements within the crustal column was found to play a significant role in determining the local thermal structure of the continental lithosphere. Concentrating heat producing elements in the lower crust lead to a thinner thermal lithosphere. Mantle heat flux into the base of stable continents was low relative to surface heat flux and did not vary significantly within the simulations regardless of the presence or absence of a thick cratonic root. A suite of simulations with variable root thickness indicated that although cratonic roots have a weak effect on surface heat flow patterns, relative to crustal heat source variations, they do have a pronounced effect on deeper thermal structure. Roots stabilized temporal variations of deep continental geotherms and were required to generate a thick thermal lithosphere. The ratio of thermal to chemical lithospheric thickness was found to decrease toward unity with increasing root thickness and thick cratonic roots limited small-scale mantle convection beneath themselves.

The thermal structure of stable continental lithosphere within a dynamic mantle

The thermal structure of stable continental lithosphere is determined by (1) the concentration and distribution of heat sources within the crust and (2) the amount of heat input from the convecting mantle. The self-consistent coupling of these two factors has not been included in thermal models of stable lithosphere to date. We conducted two suites of numerical simulations (one with variable crustal heat production and the other with a chemically distinct cratonic root) to explore the thermal coupling between stable continental lithosphere and the convecting mantle. The distribution of heat producing elements within the crustal column was found to play a significant role in determining the local thermal structure of the continental lithosphere. Concentrating heat producing elements in the lower crust lead to a thinner thermal lithosphere. Mantle heat flux into the base of stable continents was low relative to surface heat flux and did not vary significantly within the simulations regardless of the presence or absence of a thick cratonic root. A suite of simulations with variable root thickness indicated that although cratonic roots have a weak effect on surface heat flow patterns, relative to crustal heat source variations, they do have a pronounced effect on deeper thermal structure. Roots stabilized temporal variations of deep continental geotherms and were required to generate a thick thermal lithosphere. The ratio of thermal to chemical lithospheric thickness was found to decrease toward unity with increasing root thickness and thick cratonic roots limited small-scale mantle convection beneath themselves.

The long‐term thermal consequences of rifting: implications for basin reactivation

ABSTRACTThe attenuation of the continental crust during rifting and the subsequent filling of the rift‐related accommodation alter the long‐term thermal and mechanical state of the lithosphere. This is primarily because the Moho is shallowed due to density contrasts between the sediment fill and the crust, but also reflects the attenuation of the pre‐existing crustal heat production and its burial beneath the basin, as well the thermal properties of the basin fill. Moho shallowing and attenuation of pre‐existing heat production contribute to long‐term cooling of the Moho and thus lithospheric strengthening, as has been pointed out in many previous studies. In contrast, basin filling normally contributes to significant Moho heating allowing the possibility of long‐term lithospheric weakening, the magnitude of which is dependent on the thermal properties of the basin‐fill and the distribution of heat sources in the crust. This paper focuses on the thermal property structure of the crust and basin‐fill in effecting long‐term changes in lithospheric thermal regime, with particular emphasis on the distribution of heat producing elements in the crust. The parameter space appropriate to typical continental crust is explored using a formalism for the heat production distributions that makes no priori assumptions about the specific form of the distribution. The plausible parameter space allows a wide range in potential long‐term thermal responses. However, with the proviso that the accommodation created by the isostatic response to rifting is essentially filled, the long‐term thermal response to rift basin formation will generally increase average crustal thermal gradients beneath basins but cool the Moho due to its reduction in depth. The increase in the average crustal thermal gradient induces lateral heat flow that necessarily heats the Moho along basin margins, especially in narrow rift basins. Using coupled thermo‐mechanical models with temperature sensitive creep‐parameters, we show that such heating may be sufficient to localise subsequent deformation in the vicinity of major basin bounding structures, potentially explaining the offset observed in some stacked rift basin successions.

Early petrologic processes on the ureilite parent body

Abstract— We present a petrographic and petrologic analysis of 21 olivine‐pigeonite ureilites, along with new experimental results on melt compositions predicted to be in equilibrium with ureilite compositions. We conclude that these ureilites are the residues of a partial melting/smelting event. Textural evidence preserved in olivine and pigeonite record the extent of primary smelting. In pigeonite cores, we observe fine trains of iron metal inclusions that formed by the reduction of olivine to pigeonite and metal during primary smelting. Olivine cores lack metal inclusions but the outer grain boundaries are variably reduced by a late‐stage reduction event. The modal proportion of pigeonite and percentage of olivine affected by late stage reduction are inversely related and provide an estimation of the degree of primary smelting during ureilite petrogenesis. In our sample suite, this correlation holds for 16 of the 21 samples examined.Olivine‐pigeonite‐liquid phase equilibrium constraints are used to obtain temperature estimates for the ureilite samples examined. Inferred smelting temperatures range from ∼1150°C to just over 1300°C and span the range of estimates published for ureilites containing two or more pyroxenes. Temperature is also positively correlated with modal percent pigeonite. Smelting temperature is inversely correlated with smelting depth—the hottest olivine‐pigeonite ureilites coming from the shallowest depth in the ureilite parent body. The highest temperature samples also have oxygen isotopic signatures that fall toward the refractory inclusion‐rich end of the carbonaceous chondrite‐anhydrous mineral (CCAM) slope 1 mixing line. These temperature‐depth variations in the ureilite parent body could have been created by a heterogeneous distribution of heat producing elements, which would indicate that isotopic heterogeneities existed in the material from which the ureilite parent body was assembled.

Tectonic feedback and the ordering of heat producing elements within the continental lithosphere

The distribution of the heat producing elements within the lithosphere provides an important control on continental thermal regimes and the mechanical strength of the lithosphere. Moreover, the strong temperature dependence of lithospheric rheology suggests the possibility of an important feedback between deformation and the distribution of heat producing elements. Simple models for lithospheric rheology are used to illustrate how such feedback might serve as an important control on both the characteristic abundance of, and spatial variation in, the heat production elements in the crust. These models also imply that the organisation of heat producing elements is essential for the long-term tectonic stabilisation of the continental crust. This is particularly relevant to the evolution of cratons in early Earth history, wherein lies the most dramatic evidence for the role played by tectonic processes in achieving a stable ordering of the heat producing elements.

Long‐term thermal consequences of the redistribution of heat‐producing elements associated with large‐scale granitic complexes

AbstractCrustal thermal regimes are sensitive to both the amount and distribution of heat producing elements (HPEs). Since a significant proportion of the crustal complement of HPEs is contained within granites, granite generation and emplacement should lead to significant long‐term changes in the thermal structure of the crust. Using HPE concentrations appropriate to representative Australian Proterozoic granites we show that granite segregation leads to changes in the temperature field of the crust of up to c. 50 °C, producing long‐term cooling in the source regions and heating at emplacement levels, relative to the pre‐granite conductive thermal regime. Because of the intimate connection between thermal regime and lithospheric strength, granite‐assisted redistribution of HPEs is likely to be fundamental to cratonisation.

Tectonic feedback, intraplate orogeny and the geochemical structure of the crust: a central Australian perspective

AbstractThe geological record of intraplate deformation in central Australia implies that past tectonic activity (basin formation, deformation and erosion) has modulated the response of the lithosphere during subsequent tectonic activity. In particular, there is a correspondence between the localization of deformation during intraplate orogeny and the presence of thick sedimentary successions in the preserved remnants of a formerly widespread intracratonic basin. This behaviour can be understood as a kind of ‘tectonic feedback’, effected by the long-term thermal and mechanical consequences of changes in the distribution of heat producing elements induced by earlier tectonism. From a geochemical point of view, one of the most dramatic effects of intraplate orogeny in central Australia has been the exposure, in the cores of the orogens, of deep crustal rocks largely depleted in the heat producing elements. The geochemical structuring of the crust associated with the erosion of the heat-producing upper crust resulted in long-term cooling of the deep crust and upper mantle with associated lithospheric strengthening. This is illustrated here by mapping the consequences of deformation and associated tectonic responses onto the h-qc plane, where h is the characteristic length-scale for heat production distribution, and qc is the total crustal heat production. Because rates of intraplate deformation in central Australia appear to be much slower than that typical of plate margin orogens, it is possible that the ongoing geochemical structuring of the crust has played an important role in terminating intraplate orogeny in central Australia by providing a ‘thermal lock’. The diagnostic geophysical signature of this lock may be the extraordinary gravity anomalies of the central Australian intraplate orogens.

Studies of Mantle Convection in Japan.

The geodynamic studies carried out in Japan during the period of 1982-1992 have been reviewed. The studies include (1) estimates of mantle viscosity from the data of post glacial rebound, (2) numerical studies on the effects of heterogeneous distribution of heat producing elements on mantle convection, (3) numerical studies on the effects of heterogeneous distribution of major elements on mantle convection, (4) numerical studies on the influences of temperature-dependence and anisotropy of viscosity on mantle convection, and (5) numerical studies on the effects of the phase-transitions in the transition layer on mantle convection.