exotic crust formation on mercury consequences of a shallow feo poor mantle the range in density and compressibility of mercurian melt compositions was determined to better understand the products of a possible mercurian magma ocean and subsequent volcanism our experiments indicate that the only mineral to remain buoyant with respect to melts of the mercurian mantle is graphite consequently it is the only candidate mineral to have composed a primary floatation crust during a global magma ocean this exotic result is further supported by mercury s volatile rich nature and inexplicably darkened surface additionally our experiments illustrate that partial melts of the mercurian mantle that compose the secondary crust were buoyant over the entire mantle depth and could have come from as deep as the core mantle boundary furthermore mercury could have erupted higher percentages of its partial melts compared to other terrestrial planets because magmas would not have stalled during ascent due to gravitational forces these findings stem from the feo poor composition and shallow depth of mercury s mantle which has resulted in both low melt density and a very limited range in melt density responsible for mercury s primary and secondary crusts the enigmatically darkened yet low feo surface which is observed today can be explained by secondary volcanism and impact processes that have since mixed the primary and secondary crustal materials

low altitude magnetic field measurements by messenger reveal mercury s ancient crustal field magnetized rocks can record the history of the magnetic field of a planet a key constraint for understanding its evolution from orbital vector magnetic field measurements of mercury taken by the mercury surface space environment geochemistry and ranging messenger spacecraft at altitudes below 150 kilometers we have detected remanent magnetization in mercury s crust we infer a lower bound on the average age of magnetization of 3 7 to 3 9 billion years our findings indicate that a global magnetic field driven by dynamo processes in the fluid outer core operated early in mercury s history ancient field strengths that range from those similar to mercury s present dipole field to earth like values are consistent with the magnetic field observations and with the low iron content of mercury s crust inferred from messenger elemental composition data

geochemistry geochronology and tectonic setting of early cretaceous volcanic rocks in the northern segment of the tan lu fault region northeast china abstract we present new geochemical and geochronological data for volcanic and related rocks in the regions of the jia yi and dun mi faults in order to constrain the late mesozoic tectonic evolution of the northern segment of the tan lu fault zircon u pb dating shows that rhyolite and intermediate mafic rocks along the southern part of the jia yi fault formed at 124 and 113 ma respectively whereas the volcanic rocks along the northern parts of the jia yi and dun mi faults formed at 100 ma the rhyolite has an a type granitoid affinity with high alkalis low mgo ti and p contents high rare earth element ree contents and ga al ratios enrichments in large ion lithophile liles e g rb th and u and high field strength element hfses e g nb ta zr and y and marked negative eu anomalies these features indicate that the rhyolites were derived from partial melting of crustal material in an extensional environment the basaltic rocks are enriched in light rees and liles e g rb k th and u and depleted in heavy rees hfses e g nb ta ti and p and sr these geochemical characteristics indicate that these rocks are calc alkaline basalts that formed in an intraplate extensional tectonic setting the dacite is a medium to high k calc alkaline i type granite that was derived from a mixed source involving both crustal and mantle components in a magmatic arc therefore the volcanic rocks along the jia yi and dun mi faults were formed in an extensional regime at 124 100 ma early cretaceous and these faults were extensional strike slip faults at this time

mercury s global contraction much greater than earlier estimates observations of compressional structures on mercury have fallen short of accommodating the global contraction that is required owing to cooling of the planet s interior mapping of folds and faults across mercury s surface using messenger spacecraft images reveals deformation consistent with a planet that has contracted radially as much as seven kilometres over its history

did 26al and impact induced heating differentiate mercury numerical models dealing with the planetary scale differentiation of mercury are presented with the short lived nuclide 26al as the major heat source along with the impact induced heating during the accretion of planets these two heat sources are considered to have caused differentiation of mars a planet with size comparable to mercury the chronological records and the thermal modeling of mars indicate an early differentiation during the initial 1 million years ma of the formation of the solar system we theorize that in case mercury also accreted over an identical time scale the two heat sources could have differentiated the planets although unlike mars there is no chronological record of mercury s differentiation the proposed mechanism is worth investigation we demonstrate distinct viable scenarios for a wide range of planetary compositions that could have produced the internal structure of mercury as deduced by the messenger mission with a metallic iron fe ni fes core of radius 2000 km and a silicate mantle thickness of 400 km the initial compositions were derived from the enstatite and cb bencubbin chondrites that were formed in the reducing environments of the early solar system we have also considered distinct planetary accretion scenarios to understand their influence on thermal processing the majority of our models would require impact induced mantle stripping of mercury by hit and run mechanism with a protoplanet subsequent to its differentiation in order to produce the right size of mantle however this can be avoided if we increase the fe ni fes contents to 71 by weight finally the models presented here can be used to understand the differentiation of mercury like exoplanets and the planetary embryos of venus and earth

nd and sr isotopic systematics of central australian granulites chronology of crustal development and constraints on the evolution of lower continental crust nd and sr isotopic data are reported for a granulite terrain in the proterozoic arunta block of central australia sm nd data from a wide range of rock types define a crust formation age of 2 070 125 ma and provide further evidence for voluminous crustal growth in the proterozoic an nd value of 1 5 0 8 indicates a depleted mantle source for this crustal segment and there is no evidence for a large component of significantly older sialic crust field relationships geochemistry and rb sr data for mafic and felsic granulites indicate that intracrustal differentiation and polyphase deformation were followed by granulite facies metamorphism rb depletion at 1 800 ma rb sr data for strongly retrogressed granulites define an age of 1 700 ma which is interpreted as the time of retrograde biotite growth partial melting at the presently exposed crustal level and anatexis at deeper crustal levels were broadly coeval with the retrograde metamorphism sm nd and rb sr isotopic systematics of minerals indicate that the terrain cooled slowly did not experience significant uplift until 1 000 ma and remained at temperatures above 320 c until the late palaeozoic the mineral data are consistent with geological relationships and petrological evidence for a prolonged period of isobaric cooling followed by uplift late in the metamorphic evolution of the terrain the granulite protoliths appear to have formed in a rift which closed within 280 ma of initial separation deformation and granulite facies metamorphism at 1 800 ma are interpreted to be a consequence of collision between the continental blocks which defined the rift regional retrogression and granitoid magmatism at 1 700 ma are attributed to underthrusting of the granulites by lower grade rocks in the final stages of collision subsequent events in the cooling and uplift history appear to have been controlled by the presence of long lived major faults in the crust and a prolonged history of episodic compression in the continental lithosphere the results of this study suggest that granulite terrains in general cannot be equated with lower continental crust but instead represent assemblages of mainly supracrustal rocks which in some instances have been involved in major collision events

skarn metallogeny through zircon record an example from the daye cu au fe mo district eastern china abstract the daye district in eastern china hosts more than 50 fe cu au mo porphyry skarn deposits that are associated with intermediate to felsic rocks of early cretaceous ages the origin of these ore related granitoids has been intensively investigated but the petrogenetic factors controlling variable metal associations e g fe cu mo or cu au fe in different skarn deposits remain not well understood here we present hf isotope and trace element data of zircons from ore related intrusions of four representative skarn deposits i e the lingxiang and chengchao fe skarn the tonglushan cu au fe skarn and the tongshankou cu mo porphyry skarn deposits to better understand controls of the source oxidation state and fractionation degree of the magmas on the polymetallic metallogeny in the daye district zircons from the ore related granodiorite at tongshankou quartz diorite at tonglushan and diorite at lingxiang have comparable ehf t values ranging from 11 3 to 1 1 which are similar to values of the early cretaceous mafic rocks ehf t 11 9 to 1 2 in the daye district and adjacent areas this similarity suggests that these ore related intrusions were largely derived from an enriched lithospheric mantle source in comparison zircons from the ore related diorite and quartz monzonite associated with the chengchao fe skarn have much lower ehf t values 11 6 to 25 3 indicating larger degrees of crustal contribution on average zircons from the cu related intrusions at tongshankou and tonglushan have higher ce ce 1100 method of zhong et al 2019 and ce4 ce3 170 method of ballard et al 2002 ratios which are interpreted reflecting higher oxidation state for the magmas in contrast zircons from fe related intrusions at chengchao and lingxiang have much lower average values of ce ce preliminary results presented here further reveal that zircon eu eu ratio can be a useful discrimination for intrusions associated with the various metal associations in the daye district zircon eu eu of 0 45 broadly separates the fe skarns and cu bearing skarns while eu eu of 0 65 can further distinguish the cu au fe skarn from the cu mo variety this study highlights zircon chemistry as a useful indicator for the nature evolution and metallogeny of the ore related intrusions

the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

the archaean gneiss complex in e hebei province north china geochemistry and evolution abstract the caozhuang and shuichang gneiss complexes from the western margin of the qianan granite gneiss dome eastern hebei north china were investigated for their isotopic and trace element geochemistry the oldest rocks so far recognized in china occur near caozhuang in the south of the area the caozhuang gneiss complex consists of supracrustal rocks and multiphase orthogneisses the supracrustals comprise a sequence of amphibolites quartzites felsic gneisses marble and bif which are thought to be laid down in a shallow water environment a ca 3 5 ga sm nd isochron has been obtained from the amphibolites and also from four felsic gneisses of the caozhuang supracrustals the orthogneisses from the caozhuang complex appear to have been generated in three main cycles at ca 3 5 ca 2 7 and ca 2 5 ga the earliest huangbaiyu gneiss was broken up and contaminated by the intrusions of granodiorites and late granitic gneisses the granodiorites have yielded a ca 2 7 ga rb sr isochron the relatively high isr 0 7057 2 2 for the rocks suggesting that the granodiorites were formed by anatexis of older continental crust late granitic gneisses have a u pb zircon age of 2 5 ga and they are remarkably similar in appearance and chemical composition to the huangbaiyu gneisses suggesting that the gneiss complex as a whole is a mixture of gneiss types of widely different ages the shuichang gneiss complex contains supracrustal rocks granitic rocks and charnockites the supracrustal rocks include greywackes semi pelites and bif which have been metamorphosed in the granulite facies and occur within a polyphase complex of deformed leucogranites microcline granites and charnockites the age of the supracrustals is not known except that they predate charnockites of 2 65 ga there is a close relationship between the production of the granites and the charnockites it is suggested that differences in mineral and chemical composition between granites and charnockites could result from partial melting processes occurring in the shuichang metasedimentary gneisses

major and trace element geochemistry of the archaean ntem plutonic complex south cameroon petrogenesis and crustal evolution the ntem complex crops out in south cameroon and belongs to the congo craton it corresponds to a 2 8 ga basement which in the northern part is made up of plutonic rocks 1 charnockites and 2 soo tonalites both have similar compositions and define a typical tonalite trondhjemite and granodiorite ttg suite the study of isotopes isr major elements and ree allows us to propose a petrogenetic model that may summarized as follows 1 1 partial melting of the mantle gave rise to tholeiites 2 2 tholeiites transformed into garnet bearing amphibolites underwent partial fusion thus generating the parental magma of the ttg suite 3 3 fractional crystallization mainly of hornblende and plagioclase produced the differentiated ttg suite this scheme is typical of archaean ttg suites and is supposed to have occurred in a subduction like geodynamic environment where because of the high archaean geothermal gradients the subducted oceanic crust underwent fusion before dehydration operated it is also possible to show that the ttg suite was not emplaced during the granulite facies metamorphism but that this occurred later affecting only some parts of the basement consequently the ntem complex appears to be an archaean high grade terrane where the granulitic assemblages may be early proterozoic in age characteristic that seems to be more widespread among archaean high grade terranes

global spherical harmonic models of the internal magnetic field of the moon based on sequential and coestimation approaches 1 three new models of the global internal magnetic field of the moon based on lunar prospector lp fluxgate magnetometer observations are developed for use in understanding the origin of the moon s crustal magnetic field and for modeling its interaction with the solar wind the models are at spherical harmonic degree 170 corresponding to 64 km wavelength resolution from 30 km mean altitude lp observations coverage is complete except for a few areas near each pole original signal amplitudes are best preserved in the sequential approach map whereas feature recognition is superior in the coestimation and harmonic wave number correlation maps spherical harmonic degrees less than 15 corresponding to 666 km wavelength are largely absent from the moon s internal magnetic field we interpret this bound in terms of the moon s impact history a derived magnetization map suggests magnetizations may locally exceed 0 2 a m in the lunar crust at the survey resolution if the magnetic crust is as thick as 40 km

structural interpretation of thrust fault related landforms on mercury using earth analogue fault models abstract the yakima fold province has often been referenced as an earth analogue to thrust fault related landforms on many solar system bodies including mars the moon and mercury the asymmetric ridges of deformed flood basalts in this region evoke the morphology and topographic expression of the landforms observed in the smooth plains units of mercury observed similarities have supported the interpretation that thrust faults and folds comprise both sets of landforms however this is an incomplete analogy recent modeling efforts have revealed that the terrestrial landforms developed over millions of years with vertical uplift associated with each landform accumulating on multiple faults and accommodated by large folds these studies highlight the important role that folding plays in accommodating crustal shortening work presented here uses the structural geometries and folding styles from the yakima fold province as a framework for constructing ten three dimensional models of thrust fault related landforms in smooth plains units near suisei planitia on mercury which traditionally would have been classified as wrinkle ridges based on appearance and some aspects of morphology forward modeling of topography revealed that multiple faults and folds are necessary to reproduce complex topographic features folding was found to account for 50 of vertical uplift associated with the landforms and without considering folding shortening accommodated by the landforms was underestimated by 61 to 88 studies which use thrust fault related landforms as proxies for stress and strain must therefore consider the effect of folding on these estimates and their associated effect on tectonic and thermal histories

lithification mechanisms for planetary regoliths the glue that binds it is understood how rocks are made on earth however on the moon mercury and to a lesser extent mars and venus there are distinct rock forming processes that we do not fully comprehend the surfaces and crusts of the inner planetary bodies may retain a history of disruption by hypervelocity impact resulting in the generation of disaggregated materials to several kilometers depth the uppermost component of this is called regolith typically 20 m thick on the moon which is part of a more extensive megaregolith that is up to tens of kilometers thick and which in places may pervade the entire crust of a planetary body it is from these pulverized materials that new rocks are reaggregated to form so called breccias this work reviews regolith and megaregolith structure for the inner planetary bodies and investigates how extraterrestrial breccias are produced three principal formation mechanisms are explored thermal sintering shock sintering and the dynamic interaction of impact generated melt w

a subdivision of the precambrian of china abstract precambrian rocks are widely distributed in china the precambrian is divided into two time units i e the archaean and proterozoic eon each of these is separated into three chronological intervals also with the status of eras with the prefixes early middle or late the time boundary between the archaean and proterozoic eon is placed at 2500 ma according to the present isotopic data the proposed subdivision for the archaean of china is two fold the age of the fuping group is younger than 2800 2900 ma and that of the qianxi group and the corresponding stratigraphic units of eastern liaoning are older than 2800 ma so that 2800 ma is selected as the boundary between the early middle and late archaean based on the representative stratigraphic units the wutai and huto groups and an intervening major unconformity formed by the wutaiian orogeny at 2200 2300 ma the early proterozoic is further divided into two periods with a time demarcation at 2200 ma a major episode of orogeny known as the luliangian movement occurred at the end of the early proterozoic at 1900 ma this disturbance was very extensive and is in a way responsible for the difference in geological conditions between the lower and middle upper proterozoic in china the boundary 1900 ma that relates to the luliangian movement is more important than the boundary corresponding to the age of 1600 ma which is recommended as the time boundary between proterozoic i and ii so we propose to use 1900 ma as the boundary between the early and middle proterozoic in china the time boundary between the middle proterozoic including the changcheng system and the jixian system and the late proterozoic which is composed of the qingbaikou and sinian systems is 1000 ma the age for the boundary between cambrian and precambrian based upon the recent isochron data is inferred to be 610 ma

geochemistry of ttg and ttg like gneisses from lushan taihua complex in the southern north china craton implications for late archean crustal accretion the late archean taihua complex mainly composed of amphibolite and ttg and ttg like gneisses is volumetrically most important metamorphic rock suites scattered along the southern margin of the north china craton ncc zircon shrimp u pb dating shows two episodes 2765 13 and 2723 9 ma of archean magmatism in the lushan area with distinct geochemical features the early 2765 13 ma suite ttg like gneisses has low sio2 52 5 66 1 wt high mg 0 47 0 68 low hree ybn 3 0 5 4 and y 8 07 13 9 ppm with low to moderate la yb n 6 7 37 1 and sr y 25 9 119 3 the younger 2723 9 ma suite ttg gneisses has high sio2 63 5 74 3 wt low mg 0 13 0 52 very low ree ybn 1 1 20 2 43 both suites show pronounced negative nb ta and ti anomalies on the primitive mantle normalized spidergram the ttg like gneiss suite has similar bulk rock nd and zircon hf model ages 3 0 ga with end t 0 0 26 1 46 and is interpreted as resulting from melt of mantle interactions with the melts derived from partial melting of subducted ocean crust with a residual assemblage of garnet clinopyroxene rutile amphibole which favors subducted slab model for the late archean ttg the ttg gneiss suite has abundant relic zircons 2 95 2 80 ga with end t 0 1 31 to 0 23 which is best interpreted as derived from partial melting of thickened lower continental crust with a garnet amphibolite residue garnet amphibole rutile significant high pressure fractional crystallization garnet amphibole and accumulation plagioclase are also required in the petrogenesis the lushan amphibolite with nearly flat primitive mantle normalized trace element pattern is interpreted to represent early ocean crust metamorphism these observations suggest a possible model of late archean crustal accretion from ocean crust to continental terrain in the southern north china craton

orbital eccentricity driven temperature variation at mercury s poles 1 due to mercury s extremely small obliquity variations in orbital eccentricity are likely to have been the dominant cause of changes in polar temperatures on mercury throughout the bulk of its history in the last 10 myr the eccentricity has varied between 0 1 and 0 3 over the past 100 myr eccentricity extremes of nearly zero to greater than 0 4 have likely been reached we calculate the temperature in the near polar region within 10 of the north pole as it varies in response to past eccentricities these calculations show periods with dramatically different temperatures diurnal amplitudes and long period thermal cycles all of which can have a dramatic effect on longitudinal variations in long term subsurface ice stability given forthcoming topographic data thermal models including long term eccentricity can help constrain when past ice might have been delivered to mercury

a magnetic disturbance index for mercury s magnetic field derived from messenger magnetometer data 1 we present a magnetic disturbance measure for mercury derived from messenger magnetometer data magnetic field fluctuations were computed in three period bands 0 1 2 s 2 20 s and 20 300 s from these we determined log average magnetic variability versus latitude and local time in mercury s magnetosphere the quietest regions are the southern tail lobe and the nightside poleward of 30 magnetic latitude and the most disturbed regions are near magnetopause boundaries and the magnetospheric cusp we used ratios at each location between the mean disturbance and that observed on each pass to compute normalized measures of magnetic disturbance for each orbit composite disturbance indices incorporate disturbance levels in all three bands percentile ranking of the composite indices provides a quantitative basis for selecting data from quiet to disturbed conditions

compositional heterogeneity of archean mantle estimated from sr and nd isotopic systematics of basaltic rocks from north pole australia and the isua supracrustal belt greenland abstract compositional variability found in modern mid ocean ridge basalt morb and ocean island basalt oib reflects differentiation processes associated with material recycling in the mantle crust system to investigate the timing at which this recycling system was established and how it transformed into the present day system we present geochemical analyses of the archean basalts from north pole np in the east pilbara craton western australia and the isua supracrustal belt isb southern west greenland these rocks represent archean accretionary complexes with ages of 3 5 ga and 3 7 3 8 ga respectively we analyzed the trace element contents including rare earth elements rees and sr and nd isotopic compositions of the basalts which may represent morbs and oibs from np and isb their trace element compositions are broadly similar but show distinct geochemical characteristics particularly with respect to rees that probably reflect differences in both the source mantle and degree of melting such differences are also evident in their initial nd isotopic compositions which were estimated based on equilibrium partitioning of rees and well defined isochron ages in contrast the sr isotopic compositions of the np and isb basalts are highly variable and their isochron ages are inconsistent with previous studies furthermore the partitioning of rb and sr in the np basalts indicates disequilibrium suggesting that the rb sr system has been disturbed by post igneous alteration and metamorphism based on these observations we propose the following model to explain the temporal variations in the geochemical composition of the archean mantle i 3800 ma recycling of plate material and melting occurred quite readily and therefore morbs and oibs were produced from differentiated mantle sources ii 3460 ma to 3800 ma mantle crust mixing occurred as the result of an extreme event such as mantle overturning reducing the compositional variation of the mantle and iii after 3460 ma mantle heterogeneity gradually developed in the material recycling system re establishing the compositional differences between morbs and oibs this model requires an extreme event to drive the homogenization during stage ii which may provide new insights into the evolution of the crust mantle system

paleo to eoarchean crustal evolution in eastern hebei north china craton new evidence from shrimp u pb dating and in situ hf isotopic study of detrital zircons from paragneisses abstract in the caozhuang complex in eastern hebei north china craton the paleo to eoarchean crustal evolution was earlier revealed by the preservation of detrital zircon grains older than or as old as 3 8 ga in fuchsite quartzite in order to test if the eoarchean antiquity is also preserved in rocks other than the fuchsite quartzite we collected two paragneisses a hornblende gneiss and a garnet biotite gneiss from huangbaiyu village and dated their detrital zircon grains the zircon dating of the hornblende gneiss yielded concordant 207pb 206pb ages ranging from 3684 to 3354 ma however an older date of 3782 ma with 18 discordancy was also obtained detrital zircon grains from the garnet biotite gneiss gave a similar 207pb 206pb age range from 3838 to 3342 ma the metamorphic domains of the zircon grains from both samples including the strongly recrystallized cores and rims recorded an overprinting metamorphism at ca 2 5 ga which correlates with the most widespread tectono thermal event in the north china craton in situ zircon hf isotope analyses on the dated zircon grains yielded a wide range of model ages tdm1 from 4 0 to 3 3 ga with corresponding ehf t from 36 0 to 4 8 this suggests that the evolution of the crustal segment in this area has involved multiple phases of juvenile crustal addition as well as recycling of older crustal rocks the new geochronological results imply the presence of a significant amount of eoarchean crustal fragments in the eastern hebei area the sedimentary protoliths of the paragneisses and other high grade metamorphic rocks in the caozhuang complex were probably deposited between 3 4 and 2 5 ga

isotopic data upb rbsr pbpb and smnd on mafic granulites from finnish lapland abstract a combined approach using several methods pb pb u pb rb sr sm nd on granulitic rocks selected from classic areas in northern finland is presented the results show that the emplacement of igneous rocks and the granulitic metamorphism occurred nearly coevally 1 9 2 0 ga this result which is supported by field observations means that these rocks are not ideal for comparing the response of the various parent daughter pairs to the high grade metamorphism or for checking the special geochronological possibilities of each method nevertheless because of the coherence of results from rocks of different origins a discussion is possible the bulk of the mafic granulite was certainly derived from upper mantle sources or short lived crustal mafic materials granulites of the granulitic belt and anorthosites are thought to have originated from two distinct mantle sources reflecting mantle heterogeneities at that time

nd isotopic constraints on crustal formation in the north china craton abstract recent tectonic analysis suggests that the north china craton consists of two archean continental blocks called the eastern and western blocks separated by the paleoproterozoic trans north china orogen although the published geochronological data are not sufficient to constrain the detailed tectonothermal evolution of the craton the available nd isotopic data show some important differences in nd model ages between the tectonic units the eastern block shows two main nd model age peaks one between 3 6 and 3 2 ga and the other between 3 0 and 2 6 ga limited nd isotopic data from the western block show a large range of model ages between 3 2 and 2 4 ga these differences are consistent with the recently proposed model the nd isotopic data from mantle derived mafic rocks indicate that the mantle beneath the north china craton was depleted in the archean consistent with major crustal growth during this period in the paleoproterozoic however the mantle derived mafic rocks show negative end t values implying crustal contamination this may have resulted from subduction and collision between the eastern and western block implying that the mechanisms of crustal formation and evolution may have been different between the archean and paleoproterozoic the north china craton was re activated by addition of mantle derived magma into the lower crust in the late mesozoic resulting in rejuvenation of the lower crust this indicates that underplating is also an important mechanism for continental addition although in this case it may not equate to crustal growth since it was preceded by removal of lithospheric mantle and possible some lower crust

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

extensional collapse and uplift in a polymetamorphic granulite terrain in the archaean and palaeoproterozoic of north china abstract the archaean granulite terrain in the datong huai an area north china comprises a basement complex of felsic and mafic granulite overlain by a sedimentary sequence dominated by metapelite and metapsammite both lithological associations are separated by a tectonic contact and structural textural and stratigraphic data indicate that the terrain was affected by two prominent tectono thermal cycles c a and c b associated with two granulite facies events m1 and m2 which probably occurred 2500 ma and 1800 ma ago respectively of these m1 events were dominant and associated with all major structures in the area in the basement gneiss m1 reached peak metamorphic conditions of 12 14 kbar and 800 900 c during a foliation forming event d 2ba which transposed an earlier s 1ba foliation of unknown age and origin in the allochthonous cover sediments m1 peak conditions of about 8 kbar and 800 c were reached after a prograde thickening event d 1co associated with the formation of a bedding parallel foliation while large amounts of s type granites were emplaced the localised d 2ba and d 1co structures are interpreted as spatially distinct responses at different crustal levels to one metamorphic event juxtaposition of basement and allochthonous cover along a large low angle normal shear zone occurred during d 3 this detachment zone accommodated extension after crustal thickening and allowed unloading of the footwall resulting in a rise of the basement rocks through 4 6 kbar as deduced from syn d 3 decompression textures coeval cooling textures in the allochthonous cover sequence indicate that lateral rather than vertical displacements occurred in the hanging wall of the detachment during c b m2 peak conditions of about 4 6 kbar and 650 700 c were reached coeval with the development of discrete left lateral strike slip zones d 4 that transect both basement and allochthonous cover and accommodated some uplift the deduced c a and c b p t paths have equivalents in many precambrian granulites in general decompressional paths like the one recorded in the basement rocks are linked to processes involving crustal thickening followed by extensional collapse in the datong huai an area it can be shown that low angle detachments may have played an important role in the uplift and cooling history of granulite belts as long ago as the archaean proterozoic boundary

the distribution and origin of smooth plains on mercury 1 orbital images from the messenger spacecraft show that 27 of mercury s surface is covered by smooth plains the majority 65 of which are interpreted to be volcanic in origin most smooth plains share the spectral characteristics of mercury s northern smooth plains suggesting they also share their magnesian alkali basalt like composition a smaller fraction of smooth plains interpreted to be volcanic in nature have a lower reflectance and shallower spectral slope suggesting more ultramafic compositions an inference that implies high temperatures and high degrees of partial melting in magma source regions persisted through most of the duration of smooth plains formation the knobby and hummocky plains surrounding the caloris basin known as odin type plains occupy an additional 2 of mercury s surface the morphology of these plains and their color and stratigraphic relationships suggest that they formed as caloris ejecta although such an origin is in conflict with a straightforward interpretation of crater size frequency distributions if some fraction is volcanic this added area would substantially increase the abundance of relatively young effusive deposits inferred to have more mafic compositions smooth plains are widespread on mercury but they are more heavily concentrated in the north and in the hemisphere surrounding caloris no simple relationship between plains distribution and crustal thickness or radioactive element distribution is observed a likely volcanic origin for some older terrain on mercury suggests that the uneven distribution of smooth plains may indicate differences in the emplacement age of large scale volcanic deposits rather than differences in crustal formational process

zircon u pb ages and hf isotopes of the huai an gneisses from the tianzhen xinpingbu area implications for the tectonic evolution of the trans north china orogen abstract one of the major advancements in understanding the formation and evolution of the north china craton was the recognition of a himalayan type collisional belt in the central part of the craton called the trans north china orogen tnco along which two micro continental blocks named the eastern block and western block joined together to form a coherent craton however controversy still remains about the pre collisional tectonic setting and architectures of the tnco in this contribution field based zircon u pb and lu hf isotopic investigations were carried out on the huai an gneisses from the tianzhen xinpingbu area in the northern part of the tnco the huai an gneisses consist predominantly of tonalitic trondhjemitic granodioritic ttg gneisses with minor gabbroic dioritic and granitic gneisses our zircon u pb results show that the plutonic protoliths of the huai an gneisses were emplaced at 2 51 2 40 ga and experienced a granulite facies metamorphic event at 1 86 1 80 ga the magmatic zircons of these samples yield e hf t values from 1 9 to 7 0 and t dm c ages of 2862 2619 ma these positive e hf t values associated with the lack of old inherited zircons suggest the derivation of their precursor magmas from a juvenile crust without contamination of old continental crust the t dm c model ages indicate that 2 8 2 6 ga is the best estimation of the mantle extraction time for the source rocks in the huai an gneisses revealing a major period of juvenile crustal growth thus it can be inferred that the partial melting of subducted oceanic crust slab produced the magmas of the huai an gneisses which subsequently interacted with overlying mantle wedge during the ascent suggesting an intra oceanic arc environment for the huai an gneiss complex taken together the development of the huai an gneiss complex was involved in a subduction related magmatic event in an intra oceanic arc at 2 5 ga followed by collision between the eastern and western blocks at 1 85 ga

gravity and tectonic patterns of mercury effect of tidal deformation spin orbit resonance nonzero eccentricity despinning and reorientation 1 we consider the effect of spin orbit resonance nonzero eccentricity despinning and reorientation on mercury s gravity and tectonic pattern large variations of the gravity and shape coefficients from the synchronous rotation and zero eccentricity values j2 c22 10 3 and b c a c 1 4 arise because of nonsynchronous rotation and nonzero eccentricity even in the absence of reorientation or despinning reorientation or despinning induces additional variations the large gravity coefficients j2 6 2 10 5 and c22 1 0 5 10 5 estimated from the mariner 10 flybys cannot be attributed to caloris alone since the required mass excess in this case would have caused caloris to migrate to one of mercury s hot poles similarly a large remnant bulge due to a smaller semimajor axis and spin orbit resonance can be dismissed since the required semimajor axis is unphysically small 0 1 au reorientation of a large remnant bulge recording an epoch of faster rotation without significant semimajor axis variations can explain the large gravity coefficients this requires initial rotation rates 20 times the present value and a positive gravity anomaly associated with caloris capable of driving 10 45 equatorward reorientation the required gravity anomaly can be explained by infilling of the basin with material of thicknesses 7 km or an annulus of volcanic plains emplaced around the basin with an annulus width 1200 km and fill thicknesses 2 km the predicted tectonic pattern due to these despinning and reorientation scenarios including some radial contraction is in good agreement with the lobate scarp pattern observed by mariner 10 we also predict that lobate scarps will follow a ne sw orientation in the eastern hemisphere and a positive gravity anomaly of a few hundred mgal associated with caloris

k rb sr ba u and th geochemistry of the lapland granulites fennoscandia lile fractionation controlling factors the lile geochemical patterns of the three main lithological units graywacke shale metasedimentary sequence tholeiitic metaigneous rocks and migmatitic rocks of the lapland granulite belt are described k ba sr and th concentrations in metasediments are nearly similar to average continental crust whereas rb and u are unevenly impoverished in particular graphitic metashales and calcsilicate rocks are not significantly depleted in uranium tholeiitic metaigneous rocks comprises metavolcanics which present k rb ratios similar to metasediments and metaplutonics with lile abundances close to those of the low k tholeiites migmatites show wide range in lile content metatexites and diatexites have higher k rb th and u concentrations and similar k rb ratios with respect to equivalent unmobilized rocks potassic pegmatoids are strongly enriched k rb ba and th but moderately in sr and u plagioclasic pegmatoids and ferromagnesian restites are rich in sr and poor in other lil elements

shrimp u pb zircon ages of the fuping complex implications for late archean to paleoproterozoic accretion and assembly of the north china craton the fuping complex is situated in the central part of the north china craton and consists of four major lithological assemblages fuping tonalitic trondhjemitic granodioritic gneisses longquanguan augen gneisses wanzi supracrustal assemblage and nanying granitic gneisses shrimp u pb geochronology combined with u th and cathodoluminescence cl imaging of zircon enables resolution of magmatic and metamorphic events that can be directed towards understanding the late archean to paleoproterozoic history of the fuping complex cl images reveal the coexistence of magmatic and metamorphic zircons in nearly all rock types of the fuping complex the metamorphic zircons occur as either single grains or overgrowth or recrystallization rims surrounding and truncating oscillatory zoned magmatic zircon cores and are all characterized by nebulous zoning or being structureless with extremely high luminescence and very low th contents these features make them distinct from magmatic zircons that are characterized by concentric oscillatory zoning comparatively low luminescence and high th and u contents shrimp u pb analyses on magmatic zircons reveal that the tonalitic trondhjemitic and granodioritic plutons of the fuping gneisses were emplaced at 2523 14 ma 2499 10 ma and 2486 8 ma respectively whereas the monzogranitic and granitic plutons of the longquanguan augen gneisses were intruded respectively at 2510 22 ma and 2507 11 ma prismatic and oscillatory zoned zircons dominate in the pelitic rocks of the wanzi supracrustal assemblage and are interpreted as detritus from igneous source rocks the concordant and discordant u pb ages of 2502 5 ma and 2507 14 ma obtained from two pelitic rock samples indicate these rocks must have been deposited no earlier than 2507 ma ago in addition a zoned zircon grain in one pelitic rock sample has a near concordant age of 2109 5 1 ma which may provide a maximum depositional age for the wanzi supracrustal rocks shrimp results also reveal that granitic magmatism assigned to the nanying granitic gneisses occurred over a protracted interval from 2077 13 ma to 2024 21 ma the nebulously zoned zircon grains and overgrowth recrystallization zircon rims from different rocks yielded similar concordant 207pb 206pb ages in the range 1875 to 1802 ma interpreted as approximating the age of regional metamorphism of the fuping complex timing of primary zircon crystallization and regional metamorphism of the fuping complex is in general agreement with recent u pb zircon ion probe results for the wutai and hengshan complexes that bound the fuping complex to the northwest these areas are characterized by the emplacement of major granitoid bodies at around 2 50 ga to 2 48 ga ago deposition of supracrustal rocks in the paleoproterozoic intrusion of paleoproterozoic granitic bodies at 2 1 to 2 0 ga and regional metamorphism at 1 875 to 1 802 ga these data indicate that the fuping and hengshan complexes do not represent an older crystalline basement to the wutai complex as suggested in previous tectonic models but together with the wutai complex represent elements of a single late archean to paleoproterozoic magmatic arc system that has been subsequently tectonically disrupted and juxtaposed during the collision of the eastern and western north china blocks at 1 85 ga which resulted in the final assembly of the north china craton

early tectonic evolution of china abstract at least four tectogenetic cycles are recognized in the early precambrian tectonic evolution of china they brought profound changes to the tectonic regime and delineate different evolutionary stages these are the qianxi an 3 1 2 9 ga and fuping ian 2 6 2 4 ga in the archaean and the wutai an 2 1 2 0 ga and luliang ian 1 8 1 7 ga of the early proterozoic time it is illustrated through the above four stages that the mode of tectonic processes has probably changed with time developing in a non uniformitarian fashion each stage of crustal evolution was characterized by specific tectonic regimes that reflect the response of the crust to the gradual changes of the physical conditions in the lithosphere and underlying mantle major crustal forming events occurred at the end of the archaean through which large crustal segments centering around north china and talimu came into existence these protocontinents were subsequently disrupted during extensive proterozoic reworking but by the end of early proterozoic time general consolidation of proto platforms of sub continental proportions was brought about and the main tectonic framework of chinese continental crust acquired its shape

geochemistry and petrology of the jequie granulitic complex brazil an archean basement complex the jequie granulitic complex is part of the extensive high grade metamorphic terrain located within the sao francisco craton of northeastern brazil some jequie rocks appear to have been formed in the middle archean 3 1 ga from preexisting sialic crust the dominant mineral composition of these rocks is quartz microcline plagioclase hyperstene and occurs over an extensive area 2 000 km2

thermo chemical evolution and global contraction of mercury the very limited amount of global contraction observed on mercury s surface poses severe constraints on models of the planet s thermo chemical evolution and current models rely on a very refractory thorium rich composition to slow planetary cooling however a refractory composition appears to be incompatible with evidence for pyroclastic eruptions which require a substantial amount of volatiles to be present in the planetary interior furthermore volcanic activity appears to have been ongoing for a considerable part of the planet s history while current models predict an early cessation of crustal production to address these inconsistencies we have reinvestigated the thermo chemical evolution of mercury using a non refractory compositional model taking the presence of a thermally insulating regolith layer into account we find that models with a stiff mantle rheology satisfy the observational constraints if the regolith layer is at least 2 km thick in these models inefficient mantle convection and thermal insulation significantly slow planetary cooling and prolong the phase of crustal production to 2 5 gyr after core formation allowing the volume increase associated with mantle differentiation to offset some of the radial contraction caused by planetary cooling models furthermore predict substantial core sulfur contents above 6 wt average crustal thicknesses between 10 and 40 km and secular cooling rates of 30 k gyr

la icp ms u pb zircon ages of the liaohe group in the eastern block of the north china craton constraints on the evolution of the jiao liao ji belt abstract as a major lithostratigraphic unit of the palaeoproterozoic jiao liao ji belt in the eastern block of the north china craton the liaohe group consists of an assemblage that is transitional from a lower arkose and volcanic rich sequence langzishan li eryu and gaojiayu formations through a middle carbonate rich sequence dashiqiao formation to an upper argillaceous sequence gaixian formation tectonically associated with the liaohe group are large volumes of granitoid rocks named the liaoji granite whose emplacement age has been dated at about 2 1 2 2 ga regionally the liaohe group and the associated liaoji granite extend along a nee trending belt that is flanked by 2 45 2 55 ga basement gneisses u pb zircon data and cathodoluminescence cl imaging are used to constrain the maximum depositional and metamorphic ages of the liaohe group which are critical in understanding the tectonic setting and history of the jiao liao ji belt cathodoluminescence images reveal that most detrital zircon grains from the liaohe group are characterized by concentric oscillatory zoning with comparatively low luminescence and high th u ratios suggesting magmatic origin but minor zircon grains have structureless overgrowth or recrystallization rims that are characterized by low th u ratio indicating metamorphic origin results of this study show that the detrital zircon grains of the langzishan formation the lowest sequence of the liaohe group have concordant u pb ages ranging from 2 05 to 2 24 ga suggesting that the liaohe group was deposited at some time after 2 05 ga although most overgrowth or recrystallization rims are too narrow to be analyzed some rims yield concordant or upper intercept ages around 1 93 ga interpreted as the peak metamorphic age of the liaohe group detrital zircon grains from the dashiqiao formation in the middle sequence of the liaohe group have concordant u pb ages ranging from 2 25 to 2 53 ga some of which are similar to those of the basement gneisses in the area this study indicates that the liaohe group was deposited after the emplacement of the liaoji granite and the latter may be a major component of the provenance for the langzishan formation in contrast the protoliths of the middle sequence of the liaohe group were most probably derived from the basement gneisses these new zircon ages for the liaohe group and recent data for the liaoji granite combined with lithological structural and metamorphic considerations support a rifting model for the evolution of the jiao liao ji belt

investigating sources of mercury s crustal magnetic field further mapping of messenger magnetometer data 106 low altitude passes of magnetometer data from the last two months of the messenger mission have been applied to produce a map of the crustal magnetic field at a constant altitude of 40 km covering latitudes of 35 75 n and longitudes of 270 90 e some anomalies correlate significantly with impact basins craters e g rustaveli and vyasa while other basins craters have no obvious anomalies a possible interpretation that is consistent with lunar evidence is that some impactors delivered more ferromagnetic fe ni metal to the interior subsurfaces and ejecta fields of the craters basins that they produced the amount of metallic iron that could plausibly be delivered is limited by the diameter and mass of an impactor that would yield a crater with observed diameters e g 200 km for rustaveli this in turn limits the maximum amplitude of anomalies that could be induced by impactor added iron in the present day mercury global field to relatively low values it is therefore concluded that if impactor added iron is the source of the observed crater associated anomalies then they must be almost entirely a consequence of ancient remanent magnetization a broad magnetic anomaly occurs over the northern rise a topographically high region with an associated strong free air gravity anomaly a possible interpretation of the latter anomaly is that an early major impact preconditioned the region for a later mantle uplift event

eocene k rich adakitic rocks in the central iran implications for evaluating its cu au mo metallogenic potential abstract the kal e kafi intrusive complex is located in the central iran magmatic zone and includes a series of granitoid rocks ranging in composition from gabbro to granite these granitoids show some affinities with adakites e g high sr y and la yb ratios coupled with low yb and y enrichment in lree and lile with no eu anomalies similar to products from all of which are compatible with derivation from slab melting however relatively their low end t values 0 21 to 2 35 and higher 87 sr 86 sr i 0 7041 0 7044 are inconsistent with an origin by slab melting with these rocks also showing shoshonitic affinities with high k 2 o na 2 o ratios geochemical characteristics indicate that the kal e kafi intrusive complex was probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thicknesses of 40 km generated near the boundary between garnet bearing amphibolite or amphibole bearing eclogite melting moreover the kal e kafi adakitic magmas must have interacted with the surrounding metasomatized mantle peridotite during their ascent which elevated not only their mgo cr and ni contents but also the oxygen fugacity o 2 of the magma the high o 2 could have induced oxidation of metallic sulfides in the mantle resulting in the mobilization of chalcophile elements which are required to produce porphyry cu au and mo mineralization

ree geochemistry and isotopic data of archean silicic volcanics and granitoids from the pilbara block western australia implications for the early crustal evolution eighteen silicic volcanic rocks of the warrawoona group and ten associated plutonic rocks from the pilbara block western australia have been chosen for geochemical and isotopic studies silicic volcanics of the unsb upper member of north star basalt are dated at 3 56 3 57 by both the rb sr and the sm nd methods the respective 1 initial isotopic composition values are 0 7005 5 sr and 0 50810 39 nd this age is consistent with the stratigraphic interpretation that the talgatalga subgroup in which the north star basalt occupies the lowermost position is overlain by the duffer formation whose age was earlier established at 3 45 by the zircon u pb method the new rb sr data on six silicic lava samples from the duffer formation yield an isochron of 3 23 0 28 2v though imprecise this age agrees with the zircon age within error limits rb sr ages of 2 3 2 4 obtained for the panorama rocks and the wyman formation do not correspond to their initial eruption ages chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2 3 2 4 ago geochemically most of these analyzed rocks volcanic and plutonic are of tonalite trondhjemitegranodiorite ttg composition a typical feature found in many other archean terrains they generally show fractionated ree patterns except the panorama formation rocks furthermore the wyman formation rhyolites and the post tectonic adamellites show significant negative eu anomalies suggesting a similar mode of magma generation and a probable genetic link theoretical considerations suggest that most of these ttg rocks could have been generated by partial melting of amphibolitic or basaltic sources followed by fractional crystallization although the archean granitic gneisses often possess mantle like isr values the trace element data indicate that they could not have been derived by direct melting of upper mantle materials the immediate tectonic implication is that in any archean terrain the formation of na rich continental crust of ttg suite must be preceded by the presence of basaltic crust the occurrence of this basaltic crust is a matter of controversy such crust might have been totally destroyed by repeated melting processes or its remnants are now represented by some of the mafic ultramafic enclaves within the tonalite trondhjemite batholiths

constraints on asteroid magnetic field evolution and the radii of meteorite parent bodies from thermal modelling abstract paleomagnetic measurements of ancient terrestrial and extraterrestrial samples indicate that numerous planetary bodies generated magnetic fields through core dynamo activity during the early solar system the existence timing intensity and stability of these fields are governed by the internal transfer of heat throughout their parent bodies thus paleomagnetic records preserved in natural samples can contain key information regarding the accretion and thermochemical history of the rocky bodies in our solar system however models capable of predicting these field properties across the entire active lifetime of a planetary core that could relate the processes occurring within these bodies to features in these records and provide such information are limited here we perform asteroid thermal evolution models across suites of radii accretion times and thermal diffusivities with the aim of predicting when fully and partially differentiated asteroids generated magnetic fields we find that dynamo activity in both types of asteroid is delayed until 4 5 5 5 myr after calcium aluminium rich inclusion formation due to the partitioning of 26al into the silicate portion of the body during differentiation and large early surface heat fluxes followed by a brief period

archean granulite gneisses from eastern hebei province china rare earth geochemistry and tectonic implications the granulite gneisses and their retrograded products of the qianxi group from eastern hebei province china have been investigated for their isotope and trace element geochemistry a consistent age of about 2 5 ae has been obtained by the rb sr and sm nd whole rock isochron methods in agreement with the zircon u pb data pidgeon 1980 d y liu unpubl geochemical arguments from initial isotopic ratios isr and ind and elemental distribution patterns have led us to conclude that this age of about 2 5 ae represents the time of granulite facies metamorphism which must have followed closely the primary emplacement of their protoliths previous claims for early archean ages 3 5 ae of these granulites are not substantiated the mineral isotope systematics register an important thermal event at about 1 7 ae roughly corresponding to the time of the widespread luliang orogeny ma and wu 1981 or chungtiao movement huang 1978

zircon u pb hf isotopes and geochemistry of two contrasting neoarchean charnockitic rock series in eastern hebei north china craton implications for petrogenesis and tectonic setting abstract the zunhua qinglong microblock in eastern hebei north china craton exposes two contrasting charnockitic rock series in the yuhuzhai taipingzhai and cuizhangzi areas charnockitic plagioclase gneiss series with chemical compositions of dioritic and tonalitic rocks and charnockite series showing tonalitic and granodioritic components petrographic observation reveals that two types of orthopyroxenes are preserved in these rocks medium fine grained anhedral orthopyroxenes are preserved in both series and coarse grained subhedral orthopyroxenes are preserved only in the charnockite series in situ trace element analyses reveal that the coarse grained subhedral orthopyroxenes exhibit typical chemical features of magmatic orthopyroxene whereas the medium fine grained anhedral orthopyroxenes display lower medium to heavy rare earth elements ree and no obvious eu anomalies resulting in left inclined ree patterns combined with petrographic features these new data indicate that the medium fine grained anhedral orthopyroxenes were formed by granulite facies metamorphism caused by dehydration reactions whereas the coarse grained subhedral orthopyroxenes were directly crystallized from their magmatic precursors laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb isotopic dating reveals that the magmatic precursors of the charnockitic plagioclase gneiss series were emplaced between 2530 17 and 2523 12 ma and those of the charnockite series were emplaced between 2527 28 and 2515 22 ma we conclude that these magmatic precursors are contemporaneous emplacements of two neoarchean granitoid magmatic events which were followed by regional granulite facies metamorphism at 2 45 ga the charnockitic plagioclase gneiss series and charnockite series exhibit similar hf t values of 2 4 to 5 3 and 3 2 to 5 5 indicating either depleted mantle or juvenile crust affinity and involvement of ancient crustal components the charnockitic plagioclase gneiss series exhibit relatively low sio2 contents 54 1 65 2 with various mgo contents 1 93 5 36 mg 0 36 0 55 and la yb n values 3 49 56 00 the charnockite series exhibit higher sio2 contents 60 1 69 2 with similar la yb n values 14 61 42 69 mgo contents 2 10 3 99 and mg 0 29 0 52 these geochemical and zircon lu hf isotopic features together with chemical modeling suggest that the magmatic precursors of the charnockitic plagioclase gneiss series may have been primarily derived from the partial melting of a depleted mantle that was metasomatized by slab derived melts fluids and experienced fractional crystallization processes with hornblende and plagioclase as primary fractionated phases the aforementioned features further suggest that the charnockite series originated from a partial melting of the subducted slab and it was strongly contaminated by mantle peridotites these new data combined with the previous studies of neoarchean basement rocks in the zunhua qinglong microblock in eastern hebei suggest that the magmatic precursors of these two contrasting charnockitic rock series were formed in a back arc related tectonic environment at a convergent plate margin

support of long wavelength topography on mercury inferred from messenger measurements of gravity and topography to explore the mechanisms of support of surface topography on mercury we have determined the admittances and correlations of topography and gravity in mercury s northern hemisphere from measurements obtained by nasa s mercury surface space environment geochemistry and ranging messenger spacecraft these admittances and correlations can be interpreted in the context of a number of theoretical scenarios including flexural loading and dynamic flow we find that long wavelength spherical harmonic degree l 15 surface topography on mercury is primarily supported through a combination of crustal thickness variations and deep mass anomalies the deep mass anomalies may be interpreted either as lateral variations in mantle density or as relief on compositional interfaces domical topographic swells are associated with high admittances and are compensated at 300 400 km depth in the lower reaches of mercury s mantle quasi linear topographic rises are primarily associated with shallow crustal compensation and are weakly correlated with positive mass anomalies in the mantle the center of the caloris basin features some of the thinnest crust on the planet and the basin is underlain by a large negative mass anomaly we also explore models of dynamic flow in the presence of compositional stratification above the liquid core if there is substantial compositional stratification in mercury s solid outer shell relaxation of perturbed compositional interfaces may be capable of creating and sustaining long wavelength topography

nature and evolution of the lower crust in the eastern north china craton a review abstract in this paper published data for granulite terrain rocks exposed at the surface lower crustal xenoliths and mesozoic intermediate felsic igneous rocks from the eastern north china craton ncc are integrated to constrain the nature and evolution of the lower crust in this area u pb zircon dating shows that the protolith ages for most of the granulite terrain rocks are 2500 to 2600 ma and that many of them experienced 1800 1900 ma metamorphism lower crustal xenoliths entrained in volcanic rocks with ages varying from 460 to 10 ma suggest that the lower crust is dominated by neoarchean rocks although there may be minor rocks with ages of meso to paleoarchean 3000 ma 45 ma and possibly 1900 ma locally the mesozoic intrusive rocks although varying from diorite to granite and spanning from triassic to cretaceous contain 2500 ma inherited zircons and have magmatic zircons with hf crust model ages t dm hf c ages of 2500 2700 ma and whole rock sr nd isotopic compositions falling within the field of the granulite terrain rocks pointing to their derivation by the melting of neoarchean lower crust the combined data for the granulite terrain rocks lower crustal xenoliths and mesozoic intermediate felsic igneous rocks indicate that the present lower crust is dominated by rocks with neoarchean ages and is intermediate to mafic in composition i e sio 2 87 sr 86 sr i e nd t and e hf t of the lower crust at 130 ma are considered to be 0 705 to 0 716 10 to 28 and 13 to 28 respectively the e nd t range is very different from that proposed previously 32 to 44 the large range of e hf t for the lower crust implies that significant e hf t variations for magmatic zircons from the mesozoic intermediate felsic igneous rocks do not necessarily reflect mixing of mantle and crustal magmas as commonly thought instead they may reflect heterogeneity in the ancient lower crust given that the voluminous mesozoic intermediate to felsic igneous rocks in the eastern ncc are derived dominantly by partial melting of the archean lower crust it requires that a large amount of archean lower crust be restitic a restite origin can explain some of the hannuoba granulite xenoliths having higher mg than the granulite terrains it may be applicable to other parts of the world

the lapland charnockitic complex ree geochemistry and petrogenesis abstract the lapland charnockitic complex is mainly composed of basic and intermediate granulites with more restricted types including ultramafic rocks and one high mg enderbite occurrence all these rocks are interbedded with or intrude the metasedimentary granulites khondalites the charnockites can be subdivided on the basis of their y and ree contents into three groups the first group is characterized by an increase in y content with progressive differentiation and by fractionated lree enriched patterns quite similar to those of some archean granulites the ree distribution patterns of these rocks suggests an origin by partial melting of a lree enriched mantle source metasomatism the other groups and the high mg enderbite are depleted in y and display strongly fractionated ree patterns either with u shaped hree and positive eu anomalies similar to archean ttg rocks despite more basic major element composition group ii and the high mg enderbite or with strong hree depletion group iii the high mg enderbite approximates closely high mg andesites with respect to major and some trace elements and its ree pattern is identical to those of early archean group iii komatiites of the onverwacht group south africa parental magmas for group ii rocks and the high mg enderbite are assumed to derive by partial melting of a garnet enriched mantle source metasomatized by a lree enriched fluid whereas those of group iii could be produced by melting of quartz eclogites or garnet amphibolites with fractionated ree patterns similar to those of group i the overall petrographical and geochemical features of the lapland charnockitic complex are consistent with an evolution of the belomorian fold belt according to a collision model

global resurfacing of mercury 4 0 4 1 billion years ago by heavy bombardment and volcanism analysis of craters on mercury s oldest most heavily cratered terrains shows that they were formed 4 0 4 1 billion years ago and that the planet s previous geological history was erased most probably by voluminous volcanism which may have been triggered by heavy asteroidal bombardment at that time the most heavily cratered terrains on mercury exhibit a lower density of craters smaller than about 100 km in diameter than on the moon a deficit that has been attributed to resurfacing by formation of ancient intercrater plains simone marchi et al used a crater areal density map based on data from the messenger spacecraft the colour coded foreground on cover with a global surface mosaic in the background to locate the oldest surfaces on mercury and interpret the crater populations in the framework of a recent lunar crater chronology they conclude that the oldest surfaces were emplaced just after the start of the late heavy bombardment 4 0 to 4 1 billion years ago the large impact basins not previously dated yield a similar surface age this agreement implies that resurfacing was global and due to volcanism perhaps aided by heavy bombardment as previously suggested the most heavily cratered terrains on mercury have been estimated to be about 4 billion years gyr old1 2 3 4 but this was based on images of only about 45 per cent of the surface even older regions could have existed in the unobserved portion these terrains have a lower density of craters less than 100 km in diameter than does the moon1 3 5 an observation attributed to preferential resurfacing on mercury here we report global crater statistics of mercury s most heavily cratered terrains on the entire surface applying a recent model for early lunar crater chronology6and an updated dynamical extrapolation to mercury7 we find that the oldest surfaces were emplaced just after the start of the late heavy bombardment lhb about 4 0 4 1 gyr ago mercury s global record of large impact basins8 which has hitherto not been dated yields a similar surface age this agreement implies that resurfacing was global and was due to volcanism as previously suggested1 5 this activity ended during the tail of the lhb within about 300 400 million years after the emplacement of the oldest terrains on mercury these findings suggest that persistent volcanism could have been aided by the surge of basin scale impacts during this bombardment

exposed cross sections through the continental crust implications for crustal structure petrology and evolution abstract the continental crust is exposed in cross section at numerous sites on the earth s surface these exposures which appear to have formed by obduction along great faults during continental collision may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle from an examination of five complexes which meet these criteria it is concluded that the most prominent layering in the crust is not compositional but metamorphic the lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks respectively post metamorphic granitic intrusions are common at intermediate to shallow levels position of discontinuities in refraction velocity where present commonly correspond to changes in composition or metamorphic grade with depth the continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by cocorp field observations coupled with geochemical data indicate a complex evolution of the lower crust which can include anatexis multiple deformation polymetamorphism and reworking of older crustal material the complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments

geochemical characteristics of igneous host rocks of lubin zardeh au cu deposit nw iran abstract the lubin zardeh au cu pb zn deposit is located in the azerbaijan western alborz zone 45 km ne of zanjan iran it is situated within the eocene oligocene volcanic plutonic belt of the tarom subzone of the alborz mountains range quartz monzonite and tuffs are the main host rock of the lubin zardeh au cu pb zn deposit granitoid rocks are predominantly high k calc alkaline with shoshonitic affinities they are i type metaluminous magnesian and are characterized by lile lree and th enrichment relative to hfse depletion in nb ta ti and weak depletion in hree and y these granitoids show some affinities with adakites e g high sr y and la yb ratios coupled with low yb and y and enrichment in lree and lile with weak eu anomalies their relatively low to moderate initial 87sr 86sr values 0 7047 0 7051 positive nd t 36 ma values 0 39 2 1 and tdm ages of 0 69 to 1 06 ga with pb isotopic ratios of initial 206pb 204pb 18 49 18 68 initial 207pb 204pb 15 58 15 61 and initial 208pb 204pb 38 33 38 77 are consistent with a phlogopite bearing amphibole rich and enriched lithospheric mantle source metasomatized by earlier subducted sediments the geochemical and isotopic compositions of the granitoids are similar to the host rocks of porphyry and epithermal deposits in the urumieh dokhtar zone

major and trace element geochemistry and crustal evolution of archaean granodioritic rocks from eastern finland in the eastern part of central finland vast areas of granodioritic gneisses surround the ca 2 65 ga greenstone belts these granodioritic gneisses can be divided into two chronological and lithological units 1 grey gneisses 2 86 ga old with a typical archaean ttg tonalite trondhjemite granodiorite composition 2 augen gneisses 2 51 ga old with a granodioritic to granitic composition the sr and nd isotopic data preclude derivation of the grey gneisses from very much older continental material the highly fractionated ree distribution patterns la yb n 37 103 with few or no eu anomalies militate against a direct mantle origin the data suggest that the grey gneisses have most likely been derived by partial melting of short lived basaltic crustal material garnet bearing amphibolite it is estimated that the basaltic protoliths were separated from a mantle source no more than 150 ma before the melting such an early basaltic crust might have been completely destroyed or might have left some remnants which are now represented by mafic inclusions in the grey gneisses the augen gneisses have a relatively high isr value 0 705 which suggests an origin by recycling of pre existing crustal material grey gneisses their ree patterns are less fractionated than those of the grey gneisses and sometimes have negative eu anomalies using the sr isotopic and trace element data a model calculation suggests that the augen gneisses could have been derived by partial melting 50 of mixed protoliths with ttg greenstone belt volcanics in a proportion of 70 30 by volume followed by fractional crystallisation

the thermal evolution of mercury s fe si core abstract we have studied the thermal and magnetic field evolution of planet mercury with a core of fe si alloy to assess whether an fe si core matches its present day partially molten state mercury s magnetic field strength and the observed ancient crustal magnetization the main advantages of an fe si core opposed to a previously assumed fe s core are that a si bearing core is consistent with the highly reduced nature of mercury and that no compositional convection is generated upon core solidification in agreement with magnetic field indications of a stable layer at the top of mercury s core this study also present the first implementation of a conductive temperature profile in the core where heat fluxes are sub adiabatic in a global thermal evolution model we show that heat migrates from the deep core to the outer part of the core as soon as heat fluxes at the outer core become sub adiabatic as a result the deep core cools throughout mercury s evolution independent of the temperature evolution at the core mantle boundary causing an early start of inner core solidification and magnetic field generation the conductive layer at the outer core suppresses the rate of core growth after temperature differences between the deep and shallow core are relaxed such that a magnetic field can be generated until the present also the outer core and mantle operate at higher temperatures than previously thought which prolongs mantle melting and mantle convection the results indicate that s is not a necessary ingredient of mercury s core bringing bulk compositional models of mercury more in line with reduced meteorite analogues

initial mapping of mercury s crustal magnetic field relationship to the caloris impact basin seventy eight low altitude passes of mercury surface space environment geochemistry and ranging magnetometer data from august to september of 2014 have been applied to produce an approximate map of the crustal magnetic field at a constant altitude of 80 km covering latitudes of 50 80 n and longitudes of 160 330 e the strongest anomalies are located in the westernmost part of the mapped region just north and northeast of the caloris impact basin the strongest single anomaly group lies over a smooth plains unit that extends northward from caloris a second anomaly lies on the odin formation interpreted as primary caloris ejecta a third broad anomaly also lies over a smooth plains unit suisei planitia although many smooth plains on mercury may have a volcanic origin a subset of these plains is concentrated around caloris and could have a dominantly impact origin it is therefore proposed that many of the mercurian anomaly sources consist of impact basin ejecta materials

magnetic classification of stony meteorites 2 non ordinary chondrites a database of magnetic susceptibility measurements on different non ordinary chondrites c e r and ungrouped populations is presented and compared to our previous similar work on ordinary chondrites it provides an exhaustive study of the amount of iron nickel magnetic phases essentially metal and magnetite in these meteorites in contrast with all the other classes cm and cv show a wide range of magnetic mineral content with a two orders of magnitude variation of whether this is due to primary parent body differences metamorphism or alteration remains unclear c34 and c2 yield similar values to the ones shown by ck and cm respectively by order of increasing the classes with well grouped are r co ck ci kak cr e ch cb based on magnetism eh and el classes have indistinguishable metal content outliers that we suggest may need to have their classifications reconsidered are acfer 202 co elephant moraine eet 96026 c45 meteorite hills met 01149 and northwest africa nwa 521 ck asuka a 88198 lapaz icefield lap 031156 and sahara 98248 r values can also be used to define affinities of ungrouped chondrites and propose pairing particularly in the case of cm and cv meteorites

pallasite paleomagnetism quiescence of a core dynamo recent paleomagnetic studies of two main group pallasites the imilac and esquel have found evidence for a strong late stage magnetic field on the parent body it has been hypothesized that this magnetic field was generated by a core dynamo driven by compositional convection during core solidification cooling models suggest that the onset of core solidification occurred 200 ma after planetary accretion prior to core solidification a core dynamo may have been generated by thermal convection however a thermal dynamo is predicted to be short lived with a duration of 10 ma to 40 ma after planetary accretion these models predict therefore a period of quiescence between the thermally driven dynamo and the compositionally driven dynamo when no core dynamo should be active to test this hypothesis we have measured the magnetic remanence recorded by the marjalahti and brenham pallasites which based on cooling rate data locked in any magnetic field signals present 95 ma to 135 ma after planetary accretion before core solidification began the cloudy zone a region of nanoscale tetrataenite islands within a fe rich matrix was imaged using x ray photoemission electron microscopy the recovered distribution of magnetisation within the cloudy zone suggests that the marjalahti and brenham experienced a very weak magnetic field which may have been induced by a crustal remanence consistent with the predicted lack of an active core dynamo at this time we show that the transition from a quiescent period to an active compositionally driven dynamo has a distinctive paleomagnetic signature which may be a crucial tool for constraining the time of core solidification on differentiated bodies including earth

ndsr isotope and ree geochemistry of alkali basalts from the massif central france abstract nd and sr isotopic compositions as well as trace element concentrations have been determined on a suite of alkali basalts from the massif central in france samples show a typical enrichment in incompatible elements in particular the ree patterns exhibit a strong fractionation characterized by a la yb n ratio of about 20 the ybn content is about 10 times chondrite the 143 nd 144 nd ratios exhibit a range from 0 512775 to 0 512989 values quite comparable to those from oceanic island basalts the 87 sr 86 sr ratios vary between 0 70338 and 0 70458 and are anti correlated with the nd isotopic ratio the isotopic and the trace element in particular ree data have been used in order to quantitatively model the genesis of the alkali basalts among the several types of models tested here the most likely one appears to be the model of mantle metasomatism a semi quantitative approach shows that the source of alkali basalts from the massif central was metasomatized prior to melting in such a model the basalts could be produced by rather high degrees of partial melting such as 10 or 15 of the metasomatically enriched mantle

a basaltic ferrobasaltic granulite association oonagalabi gneiss complex central australia magmatic variation in an early proterozoic rift extremely fractionated basaltic to ferrobasaltic amphibolites and granulites comprise two spatially associated mafic tholeiitic suites deformed sills within the early proterozoic oonagalabi basement gneiss complex harts range central australia the metatholeiites are characterised by high to very high feo tio2 and p2o5 contents and variable depletion in cao and al2o3 despite similar zr nb ratios the rocks from the two suites show different degrees of enrichment in lree and other immobile incompatible elements the basaltic melts which were parental to the two mafic suites were not comagmatic and the rocks cannot be related simply by fractionation of realistic assemblages of low pressure fractionating phases

the western border of the archaean province of the baltic shield evidence from northern sweden restricted areas of acid to intermediate gneisses in northernmost sweden are known to be archaean and several other gneissic granodioritic to tonalitic massifs have been suggested to be of this age to delimit the western border of the archaean province of the baltic shield and to characterize the swedish archaean we have studied the 2 83 ga old soppero gneiss and 8 of the proposed archaean massifs in northern sweden by sm nd isotopic analyses u pb zircon dating in two cases and geochemistry

microblock amalgamation in the north china craton evidence from neoarchaean magmatic suite in the western margin of the jiaoliao block abstract the archaean earth is considered to have been characterized by microcontinents that formed dominantly through the accretion of oceanic arcs and plateaus the north china craton ncc provides a typical case where at least seven ancient microcontinental nuclei with distinct lithological features and independent tectonic histories were amalgamated into the cratonic framework at the end of the archaean here we investigate a suite of magmatic rocks developed at the periphery of one of these microblocks the jiaoliao block that forms part of the composite eastern block of the ncc we present petrological geochemical and zircon u pb geochronological data from the taipingzhai charnockite suite and associated amphibolites metagabbros and orthogneisses from the qianxi complex geochemically the rocks show a wide range of sio 2 charnockite suite 52 57 75 50 wt metagabbro 43 71 wt amphibolite 50 24 wt garnet bearing biotite 63 73 wt and mgo charnockite suite 0 89 5 01 wt metagabbro 3 99 wt amphibolite 6 23 wt garnet bearing biotite 2 08 wt the composition of the felsic units straddle from diorite through syeno diorite to granite with both alkalic and subalkalic affinity with dominantly magnesian composition and arc related features their immobile trace element relationships suggest calc alkaline affinity they show positive pb ba la nd and gd and negative nb ta sr th and ti anomalies with slightly negative anomalies of ce and y attesting to arc related features in tectonic classification diagrams the rocks plot in the vag syn colg field or the vag area suggesting subduction related origin the dominant population of zircons in all these rocks displays magmatic crystallization features including high th u values with core rims textures indicating subsequent thermal events the zircon u pb data yield upper intercept ages of 2587 10 ma to 2543 17 ma and 207 pb 206 pb mean ages of 2578 7 3 ma to 2536 8 ma for the charnockite suite marking the timing of emplacement of the arc magmas the overgrowth rims as well as discrete neoformed grains are interpreted as dating subsequent metamorphism and yield 207 pb 206 pb ages between 2533 ma to 2490 ma zircons in the metagabbro preserve upper intercept ages of 2556 20 ma representing the crystallization age of this rock the younger ages of 2449 58 ma upper intercept age and 1845 25 ma 207 pb 206 pb spot age are interpreted to represent subsequent multiple thermal events in this area zircons in the amphibolite preserve the 207 pb 206 pb mean age of 2539 9 ma representing the crystallization age of this rock the garnet bearing biotite gneiss shows an upper intercept age of 2562 10 ma mswd 0 66 n 36 and the 207 pb 206 pb mean age of 2561 9 ma mswd 0 63 n 33 which is taken to represent the crystallization age of this rock some inherited zircons are also identified with 207 pb 206 pb ages of 2664 26 ma and 2628 26 ma zircon lu hf data show dominantly positive ehf t values and combined with crustal residence ages the results suggest mesoarchean to neoarchean juvenile crust formation in the ncc we interpret the data presented here to represent a phase of major late neoarchaean arc magmatism along the western margin of the jiaoliao block related to the birth of microcontinental nuclei within the ncc our data suggest that the western and eastern blocks might not have existed as discrete crustal blocks and that the construction of the ncc is a result of the assembly of several microblocks or terranes at the end of archaean similar archean cratonic nuclei in other regions of the world might have formed part of a primitive supercontinent in the early earth

thermochemical evolution of mercury s interior 1 a number of observations performed by the messenger spacecraft can now be employed to better understand the evolution of mercury s interior using recent constraints on interior structure surface composition volcanic and tectonic histories we modeled the thermal and magmatic evolution of the planet we ran a large set of monte carlo simulations based on one dimensional parametrized models spanning a wide range of parameters we complemented these simulations with selected calculations in 2 d cylindrical and 3 d spherical geometry which confirmed the validity of the parametrized approach and allowed us to gain additional insight into the spatiotemporal evolution of mantle convection core radii of 1940 km 2040 km and 2140 km have been considered and while in the first two cases several models satisfy the observational constraints no admissible models were found for a radius of 2140 km a typical thermal evolution scenario consists of an initial phase of mantle heating accompanied by planetary expansion and the production of a substantial amount of partial melt the evolution subsequent to 2 gyr is characterized by secular cooling that proceeds approximately at a constant rate and implies that planetary contraction should be ongoing today most of the models predict mantle convection to cease after 3 4 gyr indicating that mercury may be no longer dynamically active finally assuming the observed surface abundance of radiogenic elements to be representative for the entire crust we determined bulk silicate concentrations of 35 62 ppb th 20 36 ppb u and 290 515 ppm k similar to those of other terrestrial planets

intercrater plains on mercury insights into unit definition characterization and origin from messenger datasets abstract orbital observations by the mercury surface space environment geochemistry and ranging messenger spacecraft are used to re evaluate the nature and origin of the oldest mapped plains deposits on mercury the intercrater and intermediate plains units defined by mariner 10 investigators despite the large areal extent of these plains which comprise approximately one third of the planetary surface area viewed by mariner 10 their formation mechanism was not well constrained by mariner 10 imaging one hypothesis attributed plains formation to ponding of fluidized impact ejecta to create relatively smooth surfaces another hypothesis was that these plains are of volcanic origin to assess the origin of these older plains and the contribution of early volcanism to resurfacing on mercury we have used messenger data to analyze the morphology spectral properties impact crater statistics and topography of mariner 10 type areas of intercrater and intermediate plains on the basis of new criteria for the identification of intercrater and intermediate plains derived from these observations we have remapped 18 of the surface of mercury we find that the intercrater plains are a highly textured unit with an abundance of secondary craters whereas the intermediate plains are composed of both intercrater and smooth plains we suggest that the term intermediate plains not be used to map the surface of mercury henceforth but rather this unit should be subdivided into its constituent intercrater and smooth plains units we argue that a substantial percentage of the intercrater plains are composed of volcanic materials on the basis of 1 examples of areas where ejecta from a small number of superposed craters have transformed smooth plains deposits of volcanic origin into a unit indistinguishable from intercrater plains 2 the range in ages of intercrater plains deposits as interpreted from crater size frequency distributions and 3 the near global distribution of intercrater plains compared with the uneven distribution of impact basins and their associated ejecta deposits

recent tectonic activity on mercury revealed by small thrust fault scarps the planet mercury has contracted over its history the identification of small thrust fault scarps suggests the occurrence of tectonic activity on mercury within the past 50 million years and thus a slow cooling planetary interior large tectonic landforms on the surface of mercury consistent with significant contraction of the planet were revealed by the flybys of mariner 10 in the mid 1970s1 the mercury surface space environment geochemistry and ranging messenger mission confirmed that the planet s past 4 billion years of tectonic history have been dominated by contraction expressed by lobate fault scarps that are hundreds of kilometres long2 3 4 5 here we report the discovery of small thrust fault scarps in images from the low altitude campaign at the end of the messenger mission that are orders of magnitude smaller than the large scale lobate scarps these small scarps have tens of metres of relief are only kilometres in length and are comparable in scale to small young scarps on the moon6 7 8 their small scale pristine appearance crosscutting of impact craters and association with small graben all indicate an age of less than 50 myr we propose that these scarps are the smallest members of a continuum in scale of thrust fault scarps on mercury the young age of the small scarps along with evidence for recent activity on large scale scarps suggests that mercury is tectonically active today and implies a prolonged slow cooling of the planet s interior

time temperature relations for the remagnetization of pyrrhotite fe7s8 and their use in estimating paleotemperatures abstract paleotemperature controls the maturation of coal and hydrocarbons in sedimentary basins and is also important in determining paleogeothermal gradient and hence tectonic style in exhumed metamorphic terrains one method of estimating paleotemperature analyses the partial remagnetization of a rock due to heating in thick volcanic or sedimentary sequences over subcrustal heat sources such as plumes or at convergent plate margins the overprinted natural remanent magnetization nrm of a rock records both the age and the paleotemperature of remagnetization but a temperature correction from laboratory to geological time scales is required using theoretical time temperature relations time temperature relations are well known for magnetite fe 3 o 4 but are reported here for the first time for pyrrhotite fe 7 s 8 another common nrm carrier data for each mineral separately yield independent estimates of paleotemperature if geologically reasonable estimates of heating time can be made paleotemperature can be estimated without geological input if data for both minerals are combined together with the age of remagnetization determined from the paleomagnetic pole of the nrm overprint these paleotemperature estimates can be used to infer the history of heating and uplift following burial as a test case we examine thermally acquired nrm overprints carried by pyrrhotite fe 7 s 8 and magnetite fe 3 o 4 in the milton monzonite of southeastern australia these overprints record a heating event about 100 ma ago probably thermal doming prior to rifting of the tasman sea that upgraded coal rank in the sydney basin extrapolating from laboratory to geological times using the new time temperature contours for pyrrhotite we estimate that the presently exposed sydney basin in the vicinity of the milton monzonite was remagnetized by heating to 165 30 c for 100 ka assuming a paleogeothermal gradient of 70 c km appropriate for young or incipient rifts the depth of burial at the time of remagnetization is estimated to have been 2 3 0 4 km this figure is in excellent agreement with independent estimates based on reflectance data for the coal accessory mineral vitrinite

gravity field and internal structure of mercury from messenger radio tracking of the messenger spacecraft has provided a model of mercury s gravity field in the northern hemisphere several large gravity anomalies including candidate mass concentrations mascons exceed 100 milli galileos mgal mercury s northern hemisphere crust is thicker at low latitudes and thinner in the polar region and shows evidence for thinning beneath some impact basins the low degree gravity field combined with planetary spin parameters yields the moment of inertia c mr2 0 353 0 017 where m and r are mercury s mass and radius and a ratio of the moment of inertia of mercury s solid outer shell to that of the planet of cm c 0 452 0 035 a model for mercury s radial density distribution consistent with these results includes a solid silicate crust and mantle overlying a solid iron sulfide layer and an iron rich liquid outer core and perhaps a solid inner core

a review of the geodynamic setting of large scale late mesozoic gold mineralization in the north china craton an association with lithospheric thinning abstract abundant gold deposits are distributed along the margins of the north china craton ncc occurring throughout the precambrian basement and located in or proximal to mesozoic granitoids these deposits show a consistent spatial temporal association with late jurassic early cretaceous magmatism and are characterized by quartz lode or disseminated styles of mineralization with extensive alteration of wall rock their ages are mainly early cretaceous 130 110 ma and constrain a very short period of metallogenesis sr nd pb isotopic tracers of ores minerals and associated rocks indicate that gold and associated metals mainly were derived from multi sources i e the wall rocks precambrian basement and mesozoic granites and associated mafic rocks previous studies including high surface heat flow uplift and later basin development slow seismic wave speeds in the upper mantle and a change in the character of mantle xenoliths sampled by paleozoic to cenozoic magmas have been used to suggest that ancient cratonic mantle lithosphere was removed from the base of the ncc some time after the ordovician and replaced by younger less refractory lithospheric mantle the geochemistry and isotopic compositions of the mafic rocks associated with gold mineralization 130 110 ma indicate that they were derived from an ancient enriched lithospheric mantle source whereas the mafic dikes and volcanic rocks younger than 110 ma were derived from a relatively depleted mantle source i e asthenospheric mantle according to their age and sources relation to magmatism and geodynamic framework the gold deposits were formed during lithospheric thinning the removal of lithospheric mantle and the upwelling of new asthenospheric mantle induced partial melting and dehydration of the lithospheric mantle and lower crust due to an increase of temperature the fluids derived from the lower crust were mixed with magmatic and meteoric waters and resulted in the deposition of gold and associated metals

trm in low magnetic fields a minimum field that can be recorded by large multidomain grains abstract thermally acquired remanent magnetization is important for the estimation of the past magnetic field present at the time of cooling rocks that cool slowly commonly contain magnetic grains of millimeter scale this study investigated 1 mm sized magnetic minerals of iron iron nickel magnetite and hematite and concluded that the thermoremanent magnetization trm acquired by these grains did not accurately record the ambient magnetic fields less than 1 t instead the trm of these grains fluctuated around a constant value consequently the magnetic grain ability to record the ambient field accurately is reduced above the critical field trm acquisition is governed by an empirical law and is proportional to saturation magnetization m s the efficiency of trm is inversely proportional to the mineral s saturation magnetization m s and is related to the number of domains in the magnetic grains the absolute field for which we have an onset of trm sensitivity is inversely proportional to the size of the magnetic grain these results have implications for previous reports of random directions in meteorites during alternating field demagnetization or thermal demagnetization of trm extraterrestrial magnetic fields in our solar system are weaker than the geomagnetic field by several orders of magnitude extraterrestrial rocks commonly contain large iron based magnetic minerals as a common part of their composition and therefore ignoring this behavior of multidomain grains can result in erroneous paleofield estimates

a linear field dependence of thermoremanence in low magnetic fields abstract we tested a linear field dependence of thermoremanent magnetization trm to saturation isothermal remanent magnetization sirm ratio for magnetite containing natural samples the trm sirm shows a linear field dependence to very low field ranges

constraining the early history of mercury and its core dynamo by studying the crustal magnetic field esa research fellowship programme in space science anr project marmite french national research agency anr anr 13 bs05 0012 university of arizona nasa ddap region pays de la loire 2016 10982

probing the structure of the deep continental crust old buried deformed crystalline rocks apparently make up most of the 40 kilometer thick continental crust this part of the earth is poorly explored and constitutes a major frontier of modern earth science two techniques seismic reflection profiling and drilling which were developed by industry for other purposes offer special potential for such exploration seismic profiling of the deep crust by cocorp the consortium for continental reflection profiling has already produced important information including evidence for extensive thin skinned thrusting of older rocks over a continental margin as the corresponding ocean basin closed deep drilling of crystalline rocks of the continents for scientific purposes is so far relatively unexploited in the united states but is already being carried on elsewhere in general big science is likely to become more important in basic geology as this frontier is explored

lewisian gneiss geochemistry and archaean crustal development models abstract 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 sio 2 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