Thermal evolution and lifetime of intrinsic magnetic fields of Super Earths in habitable zones

We have numerically studied the thermal evolution of various-mass terrestrial planets in habitable zones, focusing on duration of dynamo activity to generate their intrinsic magnetic fields, which may be one of key factors in habitability on the planets. In particular, we are concerned with super-Earths, observations of which are rapidly developing. We calculated evolution of temperature distributions in planetary interior, using Vinet equations of state, Arrhenius-type formula for mantle viscosity, and the astrophysical mixing length theory for convective heat transfer modified for mantle convection. After calibrating the model with terrestrial planets in the Solar system, we apply it for 0.1--$10M_{\oplus}$ rocky planets with surface temperature of 300~\mbox{K} (in habitable zones) and the Earth-like compositions. With the criterion for heat flux at the CMB (core-mantle boundary), the lifetime of the magnetic fields is evaluated from the calculated thermal evolution. We found that the lifetime slowly increases with the planetary mass ($M_p$) independent of initial temperature gap at the core-mantle boundary ($\Delta T_{\rm CMB}$) but beyond a critical value $M_{c,p}$ ($\sim O(1)M_{\oplus}$) it abruptly declines by the mantle viscosity enhancement due to the pressure effect. We derived $M_{c,p}$ as a function of $\Delta T_{\rm CMB}$ and a rheological parameter (activation volume, $V^*$). Thus, the magnetic field lifetime of super-Earths with $M_p > M_{p,c}$ sensitively depends on $\Delta T_{\rm CMB}$, which reflects planetary accretion, and $V^*$, which has uncertainty at very high pressure. More advanced high-pressure experiments and first-principle simulation as well as planetary accretion simulation are needed to discuss habitability of super-Earths.Comment: 19pages, 15 figures, accepted for publication in Ap

The tectonic significance of the Cabo Frio Tectonic Domain in the SE Brazilian margin: A Paleoproterozoic through Cretaceous saga of a reworked continental margin

The Cabo Frio Tectonic Domain is composed of a Paleoproterozoic basement tectonically interleaved with Neoproterozoic supracrustal rocks (Buzios-Palmital successions). It is in contact with the Neoproterozoic-Cambrian Ribeira Orogen along the SE Brazilian coast. The basement was part of at least three continental margins: (a) 1.97 Ga; (b) 0.59 - 0.53 Ga; (c) 0.14 Ga to today. It consists of continental magmatic arc rocks of 1.99 to 1.94 Ga. Zircon cores show a 2.5 - 2.6 Ga inheritance from the ancient margin of the Congo Craton. During the Ediacaran, this domain was thinned and intruded by tholeiitic mafic dykes during the development of an oceanic basin at ca. 0.59 Ma. After the tectonic inversion, these basin deposits reached high P-T metamorphic conditions, by subduction of the oceanic lithosphere, and were later exhumed as nappes over the basement. The Cabo Frio Tectonic Domain collided with the arc domain of the Ribeira Orogen at ca. 0.54 Ga. It is not an exotic block, but the eastern transition between this orogen and the Congo Craton. Almost 400 m.y. later, the South Atlantic rift zone followed roughly this suture, not coincidently. It shows how the Cabo Frio Tectonic Domain was reactivated as a continental margin in successive extensional and convergent events through geological time

Studies of the Precambrian Geology of Iowa: Part 3. Geochronologic data for the Matlock drill holes

U-Pb ages have been determined for quartz monzodiorite (2,535 ± 5Ma) recovered from drill hole ClO and for keratophyre (1,782±10 Ma) recovered from drill hole C5 of the Matlock exploration project. Sm-Nd whole-rock analyses for several rock types from the Otter Creek layered mafic complex recovered from several drill holes of the Matlock project yield an isochron age of 2,890 ± 90 Ma, with an ϵNd(t) of -0.9 ± 2.4. These ages document crustal evolution in northwest Iowa, including the southern margin of the Superior Province, over more than one billion years from 2.9 to 1.8 Ga. Isotopic data also show that older crustal components were involved in genesis of the keratophyre, indicating that it was derived wholly or in part from older crust. Data are not sufficient to determine if the keratophyre was derived wholly from mafic crust like the Otter Creek complex or from a mixture of more felsic Archean crust and juvenile 1.8 Ga magmas

Noroeste da África - Nordeste do Brasil (Província Borborema) ensaio comparativo e problemas de correlação

A correlação geológica entre as porções Noroeste da África e Nordeste do Brasil enfrentam ainda uma série de problemas, de natureza virtual e real. A falta de um Projeto Internacional de Correlação Geológica (PICG) voltada para este tema é um aspecto crucial. O primeiro grupo de problemas compreende a heterogeneidade do conhecimento geológico (muitos trabalhos de circulação restrita) assim como a diversidade das escolas de pensamento científico envolvidas no tema. Progresso importante foi feito na década presente, incluindo mapas de reconstituição pré-deriva (para o Aptiano) e uma série de doutoramentos no Pré-Cambriano do Brasil e da África, fatos que motivaram os autores a fazer uma revisão no assunto. A correlação entre os crátons de S. Luis-África Ocidental (norte) e S. Francisco-Congo-Kasai-Angola (sul) que delimitam todo o contexto orogênico Brasiliano-Panafricano é relativamente bem documentada. Dos pontos de vista litoestratigráfico e tectônico, esta correlação é razoavelmente confirmada. Estes crátons/blocos devem ter sido fragmentos maiores de um mesmo supercontinente do Mesoproterozóico Superior, no qual as colagens orogênicas Paleoproterozóicas eram muito importantes. Igualmente, o cinturão de orogenias que perlonga e delineia a periferia destes crátons apresentam problemas de correlação de pequena monta, tanto no caso do Médio Coreaú/Dahomeides-Pharusiano (ao norte) como no caso do Sergipano/Oubanguides-Lindiano. Os principais problemas são quanto aos sistemas orogênicos distais dos crátons, mais para o interior da trama Brasiliano-Panafricana. Na parte mais ao norte da Borborema, ao norte do Lineamento Patos (Patos/Garoua) foi identificado um grande bloco de embasamento bastante retrabalhado no ciclo Brasiliano-Panafricano que foi considerado um terreno tectono-estratigráfico, fração do supercontinente Rodinia (como os crátons). Este bloco formado principalmente por rochas de alto grau contém diversos remanescentes esparsos de supracrustais proterozóicas ("schist belts") se estende do noroeste do Ceará à zona costeira da Borborema e daí para o interior do Niger. Predominam ortognaisses paleoproterozóicos (2,35; 2,15; 2,0 Ga) de variadas composições, com inserções locais de cálcio-silicáticas e rochas aluminosas, arranjados consoante complexo quadro lito-estrutural (Tansamazônico-Eburniano) que inclui alguns prévios núcleos arqueanos (3,45; 2,8/2,6 Ga). Os remanescentes xistosos sobrepostos ("schist belts") são compostos de contextos vulcano-sedimentares do Paleoproterozóico Superior (Estateriano) e do Neoproterozóico (ca. 820; 730, 640-600 Ma), todos eles trabalhados e/ou retrabalhados pelas fases mais posteriroes do ciclo Brasiliano/Panafricano. Ainda que a idade dos"schist belts" nigerianos seja tema controvertido, os resultados obtidos nos homólogos brasileiros podem provisoriamente ser estendidos para a África. Na parte mais central, na área entre os lineamentos Patos (Patos/Garoua) e Pernambuco/Nagaounderé, ou seja, o chamado"median shear corridor" a correlação permanece como um problema difícil. No lado brasileiro foi reconhecido uma faixa móvel Eo-neoproterozóica (Cariris Velhos), retrabalhada no Brasiliano, e que parece parte do cinturão mundial de orogenias da colagem Grenvilliana. Há poucas evidências da presença desta faixa móvel na África até o presente. Adicionalmente, a correlação de faixas de supracrustais neoproterozóicas (excelentes exposições no Brasil) é difícil, devido a exposição rarefeita e em níveis crustais mais profundos no lado da África.The geological correlation between Northwest Africa and Northeast Brazil faces a series of problems of both virtual and real natures. The lack of an IGCP (International Geological Correlation Program) on this subject is a crucial aspect. The first group of problems encompasses the heterogeneity backgrounds of geological knowledge (much of which is in papers of restricted circulation) as well as the diversity of scientific thought in the schools involved with this problem. Recent improvements in pre-drift (Aptian) paleogeographic maps, and a series of new PhD theses on the Precambrian shield in Brazil and Africa have stimulated the authors to review this theme. The correlation between the large cratonic blocks that confine the Brasiliano-Pan-African System - São Luís- and West African cratons in the north and the São Francisco and Congo-Kasai-Angola cratons in the south - is relatively well assigned and reasonably constrained from the litho-stratigraphic and tectonic points of view. These crátons/blocks seem to have been major fragments of the same Mesoproterozoic supercontinent, in which Paleoproterozoic orogenic collage was very important. Similarly, the set of Neoproterozoic orogenic belts that delimitate external outlines of these cratonic nuclei presents only minor correlation problems for discussion, such as those to the north, as in the case of the Médio Coreaú/Dahomeyides-Pharusian belts, or those to the south, as in the case of Sergipano/Oubanguides-Lindian belts. The main problems concern the branching systems of orogens of the internal Brasiliano/Pan-African domains, distal to cratonic nuclei. North of the Patos (Patos-Garoua) shear zone in the northern part of the Borborema Province, a large basement block has been identified that contains several vestigial supracrustal belts ("schist belts"). It is considered a tectonic-stratigraphic terrane, former part of Rodinia (like the cratonic nuclei) and considerably reworked during Brasiliano - Pan African events. This reworked landmass mostly composed of high-grade rocks Paleoproterozoic (2.35, 2.15, 2.0 Ga), orthogneisses of varied compositions, with minor insertions of calc-silicatic and aluminous gneisses, extends castward from NW Ceará to the Brazilian continental margin and, in Africa, from the coast far into the interior of Niger. This terrane displays a complex structural framework (Transamazonian/Eburnian) involving some small Archean nuclei (3.45; 2.8 / 2.6 Ga). Several scattered schist belts overlying this basement block are composed of volcano-sedimentary sequences of Late Paleoproterozoic (Statherian) and Neoproterozoic (ca. 820 Ma, 730, 640-600 Ma) ages, all of them reworked during the last phases of the Brasiliano collage. Although the radiometric data for the schist belts of Nigeria are controversial, the recent set of Neoproterozoic data from Ceará and Rio Grande do Norte (Brazil) may be extended to Africa. In the central part of the Borborema Province, between the Patos (Patos- Garoua) and Pernambuco/Ngaounderé shear zone systems ("Median Shear Corridor") correlation is still a difficult. In the Brazilian territory an Early-Neoproterozoic (1000-960 Ma"Cariris Velhos") fold belt has been identified, which seems to be part of the worldwide framework of the Grenvillian collage. There is little evidence for this event on the African side, however. Additionally, correlation of Neoproterozoic fold belts well-exposed in Brazil is rather difficult due to the scattered record and deeper crustal nature of these supposed African equivalents

Formation and Structure of Low Density Exo-Neptunes

Kepler has found hundreds of Neptune-size (2-6 R_Earth) planet candidates within 0.5 AU of their stars. The nature of the vast majority of these planets is not known because their masses have not been measured. Using theoretical models of planet formation, evolution and structure, we explore the range of minimum plausible masses for low-density exo-Neptunes. We focus on highly irradiated planets with T_eq>=500K. We consider two separate formation pathways for low-mass planets with voluminous atmospheres of light gases: core nucleated accretion and outgassing of hydrogen from dissociated ices. We show that Neptune-size planets at T_eq=500K with masses as small as a few times that of Earth can plausibly be formed core nucleated accretion coupled with subsequent inward migration. We also derive a limiting low-density mass-radius relation for rocky planets with outgassed hydrogen envelopes but no surface water. Rocky planets with outgassed hydrogen envelopes typically have computed radii well below 3 R_Earth. For both planets with H/He envelopes from core nucleated accretion and planets with outgassed hydrogen envelopes, we employ planet interior models to map the range of planet mass--envelope mass--equilibrium temperature parameter space that is consistent with Neptune-size planet radii. Atmospheric mass loss mediates which corners of this parameter space are populated by actual planets and ultimately governs the minimum plausible mass at a specified transit radius. We find that Kepler's 2-6 R_Earth planet candidates at T_eq=500--1000K could potentially have masses less than ~4 M_Earth. Although our quantitative results depend on several assumptions, our qualitative finding that warm Neptune-size planets can have masses substantially smaller than those given by interpolating the masses and radii of planets within our Solar System is robust.Comment: 17 pages, 9 figures, accepted for publication in Ap

Geo-neutrinos and Silicate Earth Enrichment of U and Th

The terrestrial distribution of U, Th, and K abundances governs the thermal evolution, traces the differentiation, and reflects the bulk composition of the earth. Comparing the bulk earth composition to chondritic meteorites estimates the net amounts of these radiogenic heat-producing elements available for partitioning to the crust, mantle, and core. Core formation enriches the abundances of refractory lithophile elements, including U and Th, in the silicate earth by ~1.5. Global removal of volatile elements potentially increases this enrichment to ~2.8. The K content of the silicate earth follows from the ratio of K to U. Variable enrichment produces a range of possible heat-producing element abundances in the silicate earth. A model assesses the essentially fixed amounts of U, Th, and K in the approximately closed crust reservoir. Subtracting these sequestered crustal amounts from the variable amounts in the silicate earth results in a range of possible mantle allocations, leaving global dynamics and thermal evolution poorly constrained. Terrestrial antineutrinos from {\beta}-emitting daughter nuclei in the U and Th decay series traverse the earth with negligible attenuation. The rate at which large subsurface instruments observe these geo-neutrinos depends on the distribution of U and Th relative to the detector. Geo-neutrino observations with sensitivity to U and Th in the mantle are able to estimate silicate earth enrichment, leading to a more complete understanding of the origin, accretion, differentiation, and thermal history of the planet.Comment: published version: 21 pages, 3 figures, 5 table

Piancó-Alto Brígida branching system of orogens (PE-PB-CE), Tectonic regionalization and geochronology

O Sistema de Dobramentos Piancó-Alto Brígida (SPAB) situa-se na porção central a centro-ocidental da Zona Transversal, da Província Borborema, nos Estados da Paraíba e do Pernambuco e em parte do Estado do Ceará. Trata-se do produto de um desenvolvimento orogenético na passagem Criogeniano-Ediacarano (630–580 Ma, acresção seguida de colisão), levando à inversão vigorosa de uma pretérita bacia de sedimentação marinha costeira. O limite ao norte é marcado pelo Lineamento de Patos e o ao sul é balizado pelo Lineamento Pernambuco. O contexto paleogeográfico (e litoestratigráfico) original é de reconstituição difícil devido aos eventos de deformação (pelo menos três fases) e metamorfismo, lateralmente variável de muito baixo, baixo a médio graus, seguidos de uma importante fase de tectônica extrusional tardia. O SPAB se encontra hoje geológico-geograficamente ramificado em diferentes faixas, separadas por alguns basement inliers, paleoproterozoicos/“terrenos”, expressivos em área e importantes partícipes dos eventos de deformação. Os principais litotipos presentes no SPAB se enquadram nas típicas assembleias quartzito-pelito-calcário (QPC) com a característica primordial da grande ritmicidade. Os gradientes metamórficos são de muito baixo, baixo e médio graus (nesse último caso, para o sul, faixa do interior de Pernambuco, “Supergrupo Salgueiro”). Nas zonas proximais dos contatos com o embasamento, consoante a linhas de falhas importantes, são registrados brechas e conglomerados polimícticos, arenitos imaturos, metagrauvacas e metacórseos (Formação Olhos d’Água). Embora haja várias contribuições, de diversas fontes, ao longo dos últimos 50 anos, em diferentes tempos e propósitos, uma discriminação da litoestratigráfica satisfatória ainda não foi possível. De sorte que todos os termos litoestratigráficos devem ser encarados como informais e provisórios. Múltiplos trabalhos de reconhecimento em geocronologia foram realizados aleatoriamente nesse sistema nas The Piancó-Alto Brígida branching system of orogens (SPAB) is in the central and center-western part of the Transversal Zone, of Borborema Province, in the states of Paraíba, Pernambuco and Ceará. It is a final tectonic product of an orogenetic development during Cryogenian-Ediacarian periods. The original paleogeographic (and lithostratigraphic) scenery is of difficult reconstitution due to tectonic events (at least three different phases of folding), with lateral variations of the metamorphism’s degree from very low to low grade. These events were followed by an important final phase of escape tectonics (extrusion). Today, the SPAB is geographically branched into different fold belts, bounded by some Paleoproterozoic basement inliers that are expressive both in area and participation in the regional deformation. The main lithological assemblages present in the SPAB belts may be classified as part of typical QPC (quartzite-pelite-carbonate) assemblages, with a striking characteristic of remarkable rhythmicity, as well as the metamorphic gradient very low to low grades up to medium grade (to the belt of the interior of Pernambuco “Supergurpo Salgueiro”). Along the zones near the contacts with the basement (following important fault lines) polymictic breccias and conglomerates, immature sandstones and arkoses (“Formação Olhos d´Água”) are recorded. Even though there are many sources of scientific contributions on this subject in the last 50 years, at different times and purposes, a satisfactory lithostratigraphic classification was not possible. So all the lithostratigraphic terminology (here discussed) should be faced as informal and provisory. Many reconnaissance projects of geochronology have been done along the two last decades. In the last years, a systematic program of geochronological research (Sm-Nd and U-Pb methods) has been developed trying to&nbsp

Thermal evolution and sintering of chondritic planetesimals III. Modelling the heat conductivity of porous chondrite material

The construction of models for the internal constitution and the temporal evolution of large planetesimals, the parent bodies of chondrites, requires information on the heat conductivity of the complex mixture of minerals and iron metal found in chondrites. It is attempted to evaluate the heat conductivity of a multi-component mineral mixture and granular medium from the heat conductivities of its mixture components. Random mixtures of solids with chondritic composition and packings of spheres are numerically generated. The heat conduction equation is solved in high spatial resolution for a test cube filled with such matter. From the heat flux through the cube the heat conductivity of the mixture is derived. The model results for porous material are consistent with data for compacted sandstone, but are at odds with measurements for H and L chondrites. The discrepancy is traced back to shock modification of the currently available meteoritic material by impacts on the parent body over the last 4.5 Ga. This causes numerous micro-cracks that act as additional barriers for heat transfer. The void structure in meteorites is different from that which probably existed in the pristine material of the parent bodies. The results obtained for the heat conductivity of the pristine material are used for calculating models for the evolution of the H chondrite parent body which are fitted to the cooling data of a number of H chondrites. The fit to the data good.Comment: 19 pages, 8 figures, accepted by Astronomy & Astrophysic

A zona tectônica Teixeira Terra Nova - ZTTTN: fundamentos da geologia regional e isotópica

A sigmoidal-shape linear tectonic zone occurs from Teixeira (Paraíba, to the north-northeast)- to Terra Nova (Pernambuco, to the south-southwest) as a line diagonal to the"Transversal Zone", which is the central tectonic portion of the Borborema Province, thus dividing this province into two different litho-structural and tectonic domains. To the north and northwest occur the Early Neoproterozoic Riacho Gravatá (RG) and the Neoproterozoic Piancó-Alto Brígida (PAB) terranes, both of them characterized by supracrustal rocks with low-grade of regional metamorphism. To the south-southwest occur the Alto Pajeú terrane (TAP, Early Neoproterozoic, orthognaisses and mesozonal supracrustal rocks) and the domain of its basement, the Alto Moxotó terrane (TAM, Paleoproterozoic in age), with high grade metamorphic rocks. This proposed"tectonic zone" is mostly composed by orthogneisses (from TAP and its Paleoproterozoic basement) pierced by an almost continuous series of intrusive granitic and syenitic rocks (plutons and stocks) of the Ediacaran period (ages between 630 and 570 Ma). During the evolutionary history of this linear portion of the plateau of the Borborema province there are some (direct and indirect) records of polycyclic geologic events of the Paleoproterozoic, Early Neoproterozoic, Ediacaran, Cambrian and Cenozoic era (a sedimentary veneer, evolution of a supergenic crust and regional uplift) with interesting implications for the regional geology, geography and social-economic development. In this paper the emphasis will be given to the isotopic data (Rb-Sr, Sm-Nd, mostly) along this special zone and the domains that it separates, as well its role on the regional tectonic framework. For this purpose, we take into consideration an expressive amount of available geological and isotopic data, obtained along the last decade (most of them not published) from different sources. Additionally, some first interpretative hypotheses will be postulated, where this tectonic zone (ZTTTN) would be formed as by-product of three parallel Neoproterozoic subduction zones.Uma zona tectônica linear, de forma sigmoidal, ocorre de Teixeira (Paraíba, ao norte-nordeste) a Terra Nova (Pernambuco, ao sul-sudoeste), com disposição diagonal ao domínio da Zona Transversal - ZT, que é a porção central da Província Borborema, de forma a subdividir a ZT em dois domínios lito-estruturais e geotectônicos distintos. Para norte e noroeste ficam os terrenos do Riacho Gravatá - RG (Eoneoproterozóico) e Piancó-Alto Brígida - PAB (Neoproterozóico), caracterizados por baixo grau de metamorfismo. Para o sul e sudeste fica o terreno Alto Pajeú (TAP, Eoneoproterozóico, ortognaisses e rochas supracrustais mesozonais) e seu embasamento, terreno Alto Moxotó - TAM (Paleoproterozóico), de grau médio a elevado de metamorfismo. Esta zona linear ou"alto" constituído de ortognaisses (do TAP e do embasamento paleoproterozóico que aflora esparsamente) é perlongado e marcado por uma série quase contínua de rochas intrusivas, plutões e stocks graníticos variados e sieníticos do tempo Ediacarano (idades entre 630 e 570 Ma, principalmente). Na história evolutiva desta porção linear particularmente soerguida do Planalto da Borborema há o registro (direto e/ou indireto) de eventos geológicos policíclicos, do Paleoproterozóico, do Eoneoproterozóico, do Ediacarano, do Eocambriano e do Cenozóico (sedimentação correlativa, elaboração de crostas e soerguimento regional), com implicações interessantes na geografia e sócio-economia regional. No presente trabalho será dada ênfase aos dados isotópicos (métodos Rb-Sr, Sm-Nd sobretudo) desta zona diferenciada, e nos contextos por ela separados, assim como no seu papel dentro do contexto tectônico regional, usufruindo para isto de expressivo acervo de dados geológicos e isotópicos, obtidos na última década (grande parte do qual ainda inédita). Além disso, hipóteses interpretativas serão postuladas para esta linha axial tectônica que subdivide a Zona Transversal em domínios geológicos distintos, considerando-a como conseqüência e subproduto de três processos paralelos de subducção no Neoproterozóico