early differentiation of the earth and the problem of mantle siderophile elements a new approach the long standing problem of the excess abundances of siderophile elements in the mantle can be resolved by considering an equilibrium core mantle differentiation in the earth at 3000 to 3500 kelvin this high temperature differentiation results in mantle siderophile element abundances that closely match the observed values some lithophile light elements could enter the core in this process as is necessary to account for its low density the abundances of siderophile elements in the mantle are consistent with the conclusion derived from the recent physical models that the earth was molten during accretion

eoarchean within plate basalts from southwest greenland the majority of 3 ga metabasalts have chemical features such as high field strength element hfse depletions that are characteristic of modern island arc basalts these compositions have been interpreted as evidence for subduction of oceanic crust early in earth s history alternatively the apparent absence of archean mafic rocks with mid oceanic ridge basalt morb and ocean island basalt oib compositions and the ubiquitous occurrence of metabasalts with hfse anomalies suggest that these chemical features may instead be a widespread characteristic of the archean mantle related to early chemical differentiation and unrelated to modern style recycling of crust here we present major and trace element data for a suite of metabasalts from innersuartuut island southwest greenland which have a minimum age constraint of 3 75 ga and are likely as old as 3 85 ga samples from innersuartuut show no evidence for crustal contamination or subduction related magmatism and have a petrogenesis comparable to modern oib the new data demonstrate that a compositional range for volcanic rocks comparable to that seen in the phanerozoic existed in the eoarchean therefore rather than a global anomaly subduction related processes are the likely origin for the compositions of the most commonly preserved archean mafic rocks with island arc basalt characteristics

komatiites from west shandong north china craton implications for plume tectonics the mengyin komatiites are located at the base of the taishan complex 2 7 ga in the western shandong greenstone belt of the eastern block of north china craton the mengyin komatiites have high magnesian low titanium and aluminum undepleted chemical characteristics although the ultramafic rocks underwent amphibolite to greenschist facies metamorphism they preserve remnant igneous spinifex textures some komatiite samples even preserve rare fresh olivine according to the mass equilibrium between the olivine and melt the potential eruption temperature of the mengyin komatiites at 1 atm is about 1270 c which might indicate a plume tectonic setting it is inferred that komatiites in mengyin west shandong may be part of a plume related global magmatic event related to the break up of a supercontinent that included the eastern block of the north china craton 2006 international association for gondwana research published by elsevier b v all rights reserved

mg perovskite silicate melt and majorite garnet silicate melt partition coefficients in the system cao mgo sio2 at high temperatures and pressures we report the results of a mg perovskite silicate melt and a majorite garnet silicate melt elemental partitioning experiment conducted at high pressures and temperatures in the synthetic system cao mgo sio2 partition coefficients for some refractory lithophile elements e g sc sm deviate significantly from unity if mg perovskite segregated from a terrestrial magma ocean it would impart nonchondritic refractory lithophile element ratios such as sc sm to the primitive upper mantle of earth contrary to inference based on analyses of naturally occurring samples either earth was never substantially molten at the end of accretion or segregation of minerals from magmas was suppressed by vigorous convection it is difficult to avoid melting earth during accretion so the latter alternative seems more plausible

melting curve of mg fe sio3 perovskite to 96 gpa evidence for a structural transition in lower mantle melts the melting temperature of mg fe sio3 perovskite the dominant mineral phase of the earth s lower mantle has been measured to a pressure of 96 10 gpa below 60 gpa we find good agreement with the previous measurement of heinz and jeanloz 1987a and above 60 gpa the melting curve of silicate perovskite has a small positive slope of 19 5 5 5 k gpa the melting point of mg0 9fe0 1 sio3 perovskite is 3800 300 k at 96 gpa by extrapolation to 136 gpa it is 4500 500 k at the core mantle boundary these values provide an upper limit to the geotherm through the solid mantle and they are compatible with recent estimates of the temperature in the core being high 4500 k at the core mantle boundary our melting curve implies that the volume change on melting increases at 60 gpa from vm 0 0 0 2 cm mole to vm 0 16 0 03 cm mole we derive an equation of state for the melt from our data and find that a highly coordinated structure is required to explain the molar volume of liquid mg fe sio3 at pressures above 20 gpa

the reduction of aliasing in gravity anomalies and geoid heights using digital terrain data summary observations of gravity can be aliased by virtue of the logistics involved in collecting these data in the field for instance gravity measurements are often made in more accessible lowland areas where there are roads and tracks thus omitting areas of higher relief in between the gravimetric determination of the geoid requires mean terraincorrected free air anomalies however anomalies based only on the observations in lowland regions are not necessarily representative of the true mean value over the topography a five stage approach is taken that uses a digital elevation model which provides a more accurate representation of the topography than the gravity observation elevations to reduce the unrepresentative sampling in the gravity observations when using this approach with the australian digital elevation model the terrain corrected free air anomalies generated from the australian gravity data base change by between 77 075 and 84 335 mgal 0 193 mgal mean and 2 687 mgal standard deviation subsequent gravimetric geoid computations are used to illustrate the evect of aliasing in the australian gravity data upon the geoid the diverence between aliased and non aliased gravimetric geoid solutions varies by between 0 732 and 1 816 m 0 058 m mean and 0 122 m standard deviation based on these conceptual arguments and numerical results it is recommended that supplementary digital elevation information be included during the estimation of mean gravity anomalies prior to the computation of a gravimetric geoid model

optimal design of a gravity survey network and its application to delineate the jabera damoh structure in the vindhyan basin central india fractal dimension analysis was carried out for optimal designing of 2 d gravity survey network and to determine an optimum range of gridding interval to generate least aliased bouguer anomaly maps as a test case this method has been successfully applied to the jabera damoh region of the vindhyan basin which is considered as a potential hydrocarbon bearing area in particular we aim to delineate accurately the lateral extent of a possible hydrocarbon bearing structure to achieve this aim fractal dimension of survey network was computed using 2 d distributions of observation points in the planning phase of the survey so that the optimum station spacing for gravity survey can be obtained a range of optimum gridding interval for the gravity data set was suggested using the box counting method of fractal dimension determination bouguer anomaly maps of the region are prepared utilizing the optimum gridding interval for the first time these anomaly maps clearly outline the gravity evidence of an anomalous rifted structure which is bounded by parallel faults on either side this structure is interpreted as a favorable basin for the occurrence of hydrocarbons another finding of this study has been the delineation of an apparently small ridge like structure running east west dividing this basin in two parts a subsurface geological model along a profile across the jabera structure has also been presented

the equation of state of a molten komatiite 1 shock wave compression to 36 gpa the equation of state eos of an initially molten 1550 c komatiite 27 wt mgo was determined in the 5 36 gpa pressure range via shock wave compression shock wave velocity u s and particle velocity u p kilometers second follow the linear relationship u s 3 13 0 03 1 47 0 03 u p based on a calculated density at 1550 c 0 bar of 2 745 0 005 g cm 3 this u s u p relationship gives the isentropic bulk modulus k s 27 0 0 6 gpa and its first and second isentropic pressure derivatives k s 4 9 0 1 and k s 0 109 0 003 gpa 1 the calculated liquidus compression curve agrees within error with the static compression results of agee and walker 1988 to 6 gpa but is less dense than their extrapolated values at higher pressures we determine that olivine fo 94 will be neutrally buoyant in komatiitic melt of the composition that we studied near 8 2 gpa clinopyroxene would also be neutrally buoyant near this pressure liquidus garnet majorite may be less dense than this komatiitic liquid in the 20 24 gpa interval however pyropic garnet and perovskite phases are denser than this komatiitic liquid in their respective liquidus pressure intervals to 36 gpa liquidus perovskite may be neutrally buoyant near 70 gpa at 40 gpa the density of shock compressed molten komatiite would be approximately equal to the calculated density of an equivalent mixture of dense solid oxide components this observation supports the model of rigden et al 1989 for compressibilities of liquid oxide components using their theoretical eos for liquid forsterite and fayalite we calculate the densities of a spectrum of melts from basaltic through peridotitic that are related to the experimentally studied komatiitic liquid by addition or subtraction of olivine at low pressure olivine fractionation lowers the density of basic magmas but above 13 14 gpa this trend is reversed all of these basic to ultrabasic liquids are predicted to have similar densities at 13 14 gpa and this density is approximately equal to the density of the bulk preliminary reference earth model mantle in this pressure range this suggests that melts derived from a peridotitic mantle may be inhibited from ascending from depths greater than 400 km

os isotope constraints on the origin of the 2 7 ga boston creek flow ontario canada abstract the re os isotopic systematics of the ca 2 7 ga boston creek flow of the abitibi greenstone belt of ontario are examined the approximately 100 m thick fe rich komatiite flow became differentiated as it crystallized and ranges from an olivine cumulate at the base to a gabbrodiorite near the top the melt from which this flow crystallized was enriched in feo and was likely derived from an fe rich reservoir within the archean mantle the melt was also enriched in light and heavy rare earth elements zr hf and many other incompatible trace elements whole rock samples believed to be most representative of parental melt compositions indicate that the parental melt contained 15 17 wt mgo and that concentrations of re and os were approximately 0 6 and 0 2 ppb respectively the re and os concentrations are somewhat lower than concentrations estimated for most komatiitic and picritic liquids as is also common for differentiated komatiitic picritic and basaltic flows there is a positive correlation between os concentration and mgo there is a weak inverse correlation between re and mgo the re os systematics of most samples appear to have remained closed since crystallization seven samples define an isochron with an age of 2708 13 ma 2 which is within uncertainty of the zircon age previously established for the rocks three of the samples plot well above this isochron and most likely indicate late stage addition of radiogenic 187 os from mineralized rocks the initial ratio of the isochron is 0 1044 6 2 equivalent to an initial os value of 3 8 0 5 the very depleted initial ratio compared to current models for upper mantle evolution of os requires derivation of the os from a long term re depleted source the re depletion in the source probably occurred 500 to 1000 ma prior to the generation of the flow the depleted initial ratio is consistent with derivation of the parental melt from early archean melt depleted subcontinental lithospheric mantle the fe and incompatible trace element enrichment may have occurred during a metasomatizing event that caused renewed melting at 2 7 ga alternatively the source of the flow could represent a portion of the mantle that retained some characteristics of early earth formation such as majorite fractionation from a magma ocean or an initially higher feo content in the latter instance an archean melt depletion event is still probably necessary to explain the depleted os isotopic composition

anomalous crustal and lithospheric mantle structure of southern part of the vindhyan basin and its geodynamic implications abstract tectonically active vindhyan intracratonic basin situated in central india forms one of the largest proterozoic sedimentary basins of the world possibility of hydrocarbon occurrences in thick sediments of the southern part of this basin has led to surge in geological and geophysical investigations by various agencies an attempt to synthesize such multiparametric data in an integrated manner has provided a new understanding to the prevailing crustal configuration thermal regime and nature of its geodynamic evolution apparently this region has been subjected to sustained uplift erosion and magmatism followed by crustal extension rifting and subsidence due to episodic thermal interaction of the crust with the hot underlying mantle almost 5 6 km thick sedimentation took place in the deep faulted jabera basin either directly over the bijawar mahakoshal group of mafic rocks or high velocity high density exhumed middle part of the crust detailed gravity observations indicate further extension of the basin probably beyond nsl rift in the south a high heat flow of about 78 mw m2 has also been estimated for this basin which is characterized by extremely high moho temperatures exceeding 1000 c and mantle heat flow 56 mw m2 besides a very thin lithospheric lid of only about 50 km many areas of this terrain are thickly underplated by infused magmas and from some segments granitic gneissic upper crust has either been completely eroded or now only a thin veneer of such rocks exists due to sustained exhumation of deep seated rocks a 5 8 km thick retrogressed metasomatized zone with significantly reduced velocities has also been identified around mid to lower crustal transition

accretion of archean oceanic plateau fragments in the abitibi greenstone belt canada the arc arc collision model is often applied to the evolution of archean greenstone belts this model predicts concordant stratigraphic relations among the various lithologies evolving generally from komatiitic tholeiitic affinities toward calc alkalic affinities new geological data from the malartic composite block of the abitibi greenstone belt indicate that this volcanic pile rather than representing a conformable sequence is composed of four distinct mafic lithotectonic domains that are overlain by a calc alkalic sequence the mafic domains are thought to be examples of accreted archean oceanic plateau material that were deformed during their collage state the younger calc alkalic sequence represents extension related volcanism that erupted through the ac creted and deformed plateau material possibly as a consequence of ridge subduction in a regime of oblique convergence the relations described here may represent a common feature of archean terranes

silicate perovskite and majorite signature komatiites from the archean abitibi greenstone belt implications for early mantle differentiation and stratification three komatiite tholeiite sequences from the archean abitibi southern volcanic zone svz which are separated by major terrane boundaries show systematic differences in composition rare earth element ree patterns and high field strength element hfse ree fractionation komatiites from tisdale township are al undepleted with mgo 12 24 al2o3 tio2 13 17 cao al2o3 1 1 3 and high ni 722 1275 ppm and cr 1875 2820 ppm contents mg tholeiites and fe tholeiites are spatially associated with komatiites both komatiites and mg tholeiites have flat ree patterns la yb n 0 6 1 2 and nb nb zr zr and hf hf 1 calculated using primitive mantle normalized values and adjacent rees in the incompatibility sequence of elements in mid ocean ridge basalt morb suggesting that they may have formed by high degree partial melting of primitive mantle materials with olivine as the main liquidus phase at shallow depths 1 consistent with having been derived from mg perovskite enriched sources at depths 700 km below the transition zone whereas spatially associated tholeiites have la yb n 1 2 and nb nb zr zr and hf hf 1 suggesting a shallow undepleted mantle source distinct from the komatiites komatiites and tholeiites from boston township are characterized by mgo 11 29 al2o3 tio2 4 5 5 4 cao al2o3 l 4 2 5 high ni 203 3420 ppm and cr 194 1965 ppm contents al sc y and heavy rare earth element hree depletion and pronounced negative normalized zr and hf anomalies zr zr and hf hf 1 in accord with either partial melting of a majorite depleted mantle source or majorite fractionation during magma formation in the depth range of 300 600 km the fractionated hfses and rees of munro and boston area komatiites indicate that these archean komatiites may have preserved some signatures of early mantle differentiation which so far has only been inferred based on high pressure experimental studies and theoretical predictions further geochemical studies of archean komatiites especially precise multiple trace element analysis combined with improved measurements of partition coefficients over a wide pressure range will provide better understanding on early mantle differentiation stratification and the transition zone

the influence of alteration on the trace element and nd isotopic compositions of komatiites abstract to investigate the effects of hydrothermal alteration and metamorphism on the chemical and isotopic compositions of komatiites we studied samples from the alexo and texmont regions in the 2 7 ga abitibi belt of canada and from the weltevreden mendon and komati formations of the 3 2 3 5 ga barberton belt of south africa particular emphasis was placed on multiple samples from individual layered spinifex olivine cumulate flows the argument being that if these flows showed variations in the ratios of elements incompatible with olivine then these variations were most likely due to chemical mobility during alteration data for rare earth ree and high field strength elements hfse reveal anomalies non chondritic hfse ree ratios particularly in the most altered samples after taking into account the limits of analytical precision these anomalies are attributed to element mobility not to fractionation of high pressure minerals as suggested by other authors for abitibi komatiites in barberton komatiites three separate processes produced hfse anomalies element mobility during alteration crust assimilation in some samples and majorite fractionation the effects of the latter process were recognized from systematic relationships between hfse ree and al ti that coincided with calculated majorite fractionation trends initial nd isotopic compositions of the komatiites were determined by analysis of magmatic pyroxenes the initial nd of pyroxene from the 2 7 ga alexo komatiite is 3 8 slightly higher than in whole rocks 0 6 to 3 5 average 2 5 in the carbonatized texmont samples the range in initial nd is far greater 6 2 to 8 8 the variable initial nd values in the rocks are attributed to isotopic exchange of nd with surrounding rocks during early alteration and fractionation of sm nd during later events pyroxene from 3 3 ga barberton komatiitic basalt had initial nd of 2 3

chemical stratification of the mantle formed by melting in the early stage of the terrestrial evolution abstract the accretional growth of the earth necessarily involved a molten zone in the outer layer magma ocean of the growing earth resulting in downwards extraction of the molten iron alloy i e core formation the primary stratification resulted from melting in the accretional stage of the earth and consisted of an fe core a perovskite rich lower mantle and a partially molten ultrabasic upper mantle in which the melt was concentrated in two regions near the surface of the cooling gravitational differentiation and crystallization in the magma ocean should give rise further to a finer stratification the upper mantle composed of two layers the upper olivine rich layer and the lower majorite garnet rich layer and the lower mantle with a perovskite rich composition both the 400 and 670 km discontinuities of the present mantle may be the chemical boundary the geophysical constraints of the mantle i e the seismic wave velocity and density profiles favor such a stratified mantle

basin evolution and destruction in an early proterozoic continental margin the rinkian fold thrust belt of central west greenland in central west greenland early palaeoproterozoic siliciclastic and carbonate sequences of the karrat group shelf sequences of the rae craton margin were deposited in sedimentary basins controlled by nw and sw trending linked extensional fault systems the shelf basins were later filled and overtopped by turbidite systems filling a foredeep advancing ahead of a thrust system the karrat fjord thrust system that propagated west to east deformation culminated in emplacement of basement cored nappes progressive deformation and high grade metamorphism at c 1 87 ga reactivation of lower plate growth faults formed dome and basin like folds and related thrusts that refolded the structure of the karrat fjord thrust system and inverted the shelf basins the southern karrat fjord thrust system was reworked in a belt of intense ductile nw directed thrusting the nunaarsussuaq thrust system formed at c 1 84 ga at the northern limit of the nagssugtoqidian orogen kinematics of these events are at odds with the consensus view that the rinkian fold thrust belt is a northward extension of the nagssugtoqidian orogen resulting from north south convergence between the rae and north atlantic cratons application of structural restoration techniques to basin analysis of palaeoproterozoic rocks has potential to provide new insights into proterozoic orogenic processes worldwide

origin of komatiite at high pressures abstract experimental melting relations to 20 gpa demonstrate that the commonly accepted model of komatiite formation by non invariant melting olivine liquid is too restrictive komatiites could also have formed by pseudoinvariant melting olivine pyroxene garnet liquid modified spinel majorite liquid along the anhydrous solidus at high pressures the mgo content of komatiites cannot be used to infer the degree of partial melting in the mantle there would have been little change in mgo at a high pressure pseudoinvariant point even for degrees of partial melting that ranged from very small to over 80 instead the mgo content of komatiites may be a better indication of the depth of partial melting than it is the degree of partial melting late archean komatiites containing 20 28 mgo could have been generated at depths ranging from about 130 to 260 km early archean komatiites from the barberton mountain land could have formed at depths corresponding to the present day transition zone or at shallower levels from an unusually primitive mantle source komatiites which originated by partial melting in a lower mantle formed by majorite fractionation should have geochemical characteristics that have not yet been reported this indicates that either melting did not extend into the lower mantle or if it did the upper and lower mantles were isolated by a chemical and thermal boundary layer in archean times

neoarchean subduction a case study of arc volcanic rocks in qinglong zhuzhangzi area of the eastern hebei province north china craton abstract we report on volcanic rocks of the qinglong zhuzhangzi area in northeastern eastern hebei province north china craton the supracrustal rock associations in the study area consist of amphibole biotite plagioclase gneiss mica plagioclase gneiss schist biotite k feldspar gneiss pyroxene plagioclase amphibolite and amphibolite twenty five new mafic intermediate to felsic samples can be geochemically classified into four groups group 1 is similar to fore arc low ti tholeiite and is characterized by low sio 2 49 7 wt and tio 2 0 43 wt slightly depleted lree and mree pattern with la yb n ratio of 0 74 gd yb n ratio of 0 82 it most likely derived from depleted mantle in a subduction initiation environment group 2 is akin to primitive arc tholeiites with low sio 2 49 7 51 8 wt slight enrichments of lree la sm n 1 06 1 53 and negative nb anomalies nb la pm 0 50 0 76 group 3 resembles evolved arc tholeiites and displays pronounced enrichments of lree la yb n of 2 38 11 3 la sm n of 1 70 2 86 and negative nb ta anomalies nb la pm 0 14 0 34 except sample 11jd28 4 with that ratio of 1 00 group 2 and 3 formed by partial melting of sub arc depleted mantle wedge modified by differing quantities of slab flux addition group 4 comprises k rich andesite dacites which are distinguished by the highest sio 2 54 0 71 8 wt k contents up to 5 03 wt strong enrichments of lree la yb n ratios of 4 83 19 and negative nb ta anomalies analogous to shoshonitic rocks marking late stage of subduction la icpms zircon u pb and lu hf analyses of evolved arc basalts and k rich andesite dacites display crystallization ages of 2511 2604 ma and single stage hf model ages of 2534 2927 ma most focus on 2650 2800 ma in contrast to 2510 2560 ma arc associations 2604 ma shoshonitic dacites most plausibly represent an older orogenic event collectively supracrustal rocks in the qinglong zhuzhangzi area constitute a series of volcanic arc associations that correspond to an entire subduction cycle ranging from orogen to primitive arc evolved arc and back to an orogenic setting which further demonstrate that by the late archean plate tectonic and subduction processes already closely resembled modern counterparts

archean magmatism and deformation were not products of plate tectonics abstract the granite and greenstone terrains that dominate upper crust formed from about 3 6 to about 2 6 ga and record magmatic and tectonic processes very different from those of a younger time they indicate heat loss by the archean earth primarily by voluminous magmatism from a mantle much hotter than that of the present plate tectonic processes were not then operating the distinctive array of petrologic structural and stratigraphic features that characterize phanerozoic convergent plate systems ophiolites magmatic arcs accretionary wedges fore arc basins etc have no viable analogues in archean terrains purported archean plate tectonic indicators consist merely of rock types that even superficially resemble actual phanerozoic indicators only when considered in isolation from their association and structure archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite type successions they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic volcanic rocks and they require a mantle about 200 c hotter than now archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions and they lack the setting and disruption that characterize modern accretionary wedges the lithologic structural and stratigraphic assemblages that typify proterozoic and phanerozoic rifted and reassembled margins also lack archean analogues and no evidence has been found for archean rifting rotation and reassembly of continental plates conversely characteristic archean assemblages lack modern equivalents in any tectonic setting ultramafic lavas erupted at liquidus temperatures are voluminous granite and greenstone terrains have no modern analogues greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by and sinking between large rising diapiric elliptical composite batholiths the batholiths include both the products of new crustal melts and variably remobilized mid crustal gneisses the greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions and are not relics of successively formed volcanic chains little deformation generally preceded the diapirism and metamorphism was primarily of contact type the regionally uniform areal density and accordant crustal level of the diapiric batholiths their contacts primarily against the oldest strata of the synforms their general age 10 20 million years younger than most of the flanking stratiform rocks and considerations of high archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust producing the orientations of the batholiths rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it resulting in cooling of the deeper crust and subjacent mantle and thus cratonization the upper crust containing the granite and greenstone aggregates was decoupled from the gneissic middle crust which underwent flattening and extension subparallel to the elongation of the shallow batholiths this deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle the early earth was probably wholly molten the surface of the earth like that of the moon must have been wholly recycled by impacts before 3 9 ga and heavily modified by them until 3 8 ga zircons as old as 4 2 ga have been found as clastic grains in much younger archean quartzites and polycyclic migmatites last partly melted and reconstituted under hydrous conditions only after 3 6 ga contain relict zircons as old as 4 0 ga the lithologies of the early earth protoliths in which these zircons formed have not been established but impact melts and breccias must be represented and magma ocean fractionates may be the nature of the transition in tectonic style into the granite and greenstone mode is unknown plate tectonic rifting and convergence were operating by about 2 0 ga and were in an essentially modern mode by about 0 8 ga the nature of the transition from the granite and greenstone mode at about 2 6 ga to plate mode by about 2 0 ga has yet to be defined the change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense but hydrated delaminated archean crust sank into it

shallow crustal configuration of the narmada son lineament transition zone near the sahdol katni area of central india using simultaneous gravity and magnetic observations gravity and magnetic studies have been carried out to map the different depth formations of alluvium gondwana vindhyan mahakoshal and the crystalline basement in the narmada son lineament nsl near the sahdol katni area india higher elevations in the northern part of the study area have lower gravity the southern part of the study area however is moderately elevated and also has a higher gravity anomaly which justifies the isostatic adjustment this indicates the presence of high density material for example mahakoshal rocks in the upper crust which causes the higher anomaly value the mahakoshal rocks are widely exposed at mau chanaura nadawar khamaria umria and near the tala barhi area and also extend from sidhi to agoni village further east of the area the mahakoshal rocks are thinner between the river son and tikwa village which causes a different gravity anomaly pattern changing from the nne ssw direction to the n s direction however the trend of magnetic anomaly follows the same pattern toward the nne ssw direction mainly because of the effect of the crystalline basement two magnetic highs are prominent in the tikwa and amarpur regions 800 and 400 ntesla respectively because of the presence of the crystalline basement the depth of the crystalline basement obtained by 2 d gravity magnetic modelling varies from 2 7 to 2 9 km from spectral analysis the average depth of the crystalline basement varies from 2 83 to 3 05 km the different crustal depth sections obtained by 2 d simultaneous gravity magnetic modelling correlate well with other constrained data

the tectonic evolution of the abitibi greenstone belt of canada based on structural geochemical sedimentological and geochronological studies we have formulated a model for the evolution of the late archaean abitibi greenstone belt of the superior province of canada the southern volcanic zone svz of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large bimodal mafic felsic volcanic centres these volcanic rocks were erupted between approximately 2710 ma and 2700 ma in a series of rift basins formed as a result of wrench fault tectonics the svz superimposes an older volcanic terrane which is characterized in the northern volcanic zone nvz of the abitibi belt and is approximately 2720 ma or older the nvz comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic anorthositic plutonic complexes these volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 2 ma the tectonic model envisages the svz to have formed in a series of rift basins which dissected an earlier formed volcanic arc the nvz analogous rift environments have been postulated for the hokuroko basin of japan the taupo volcanic zone of new zealand and the sumatra and nicaragua arcs the difference between rift related submergent volcanism in the svz and emergent volcanism in the nvz resulted in the contrasting metallogenic styles the former being characterized by syngenetic massive sulphide deposits whilst the latter was dominated by epigenetic porphyry type cu au deposits

melting relations of hydrous pyrolite in cao mgo al2o3 sio2 h2o system at the transition zone pressures phase relations and melt compositions in cao mgo al2o3 sio2 pyrolite under hydrous 2 of h2o and anhydrous conditions have been determined at 13 20 gpa and 1600 2220 c liquidus and solidus temperatures for the hydrous system are about 50 100 c and 180 240 c lower than those for the dry system respectively majorite is a liquidus phase of the hydrous pyrolite from 13 to 20 gpa olivine is a liquidus phase at 13 gpa and both periclase and majorite are the liquidus phases at 20 gpa in the dry pyrolite we observed expansion of the stability field of anhydrous phase b in hydrous experiments compositions of partial melts at 13 20 gpa are generally similar in dry and hydrous systems but hydrous melts contain more sio2 at 13 17 gpa the melts formed by low degree of melting have al2o3 depleted and cao rich compositions trends of hydrous melt compositions are generally consistent with those of aluminum depleted komatiite magmas

the red lake greenstone belt superior province evidence of plume related magmatism at 3 ga and evidence of an older enriched source abstract the red lake greenstone belt of the superior province contains a number of volcano sedimentary assemblages that range in age from 2 99 ga to 2 74 ga the oldest of these is the 2 99 2 96 ga balmer assemblage which is part of the north caribou terrane one of the oldest portions of the superior province the balmer assemblage dominates the red lake greenstone belt and is significant in that it contains evidence for plume generated magmatism the assemblage is dominated by mafic volcanic flows with intercalated komatiitic lavas mafic and ultramafic sills and shallow water clastic and chemical sediments the basalts have 5 12 mgo and mantle normalized signatures which range from slightly light rare earth element ree depleted through essentially flat to light ree enriched some lavas display slight negative nb and ti anomalies indicating selective crustal contamination the komatiites have 12 14 mgo are the al undeplated type and are strongly depleted in light ree they have e nd values of ca 2 the geochemistry combined with sm nd isotope data indicates that the komatiites came from a long term enriched source that was probably generated at ca 3 4 ga that source underwent a melting event that depleted it in light ree prior to komatiite generation the presence of komatiites from a long term enriched source and the strong contrast in the ree profiles of the basalts and the komatiites are consistent with a plume model for the generation of the balmer assemblage the komatiites having originated in the plume core and the basalts from entrained upper mantle or mantle melted by the plume

icp ms a powerful tool for high precision trace element analysis in earth sciences evidence from analysis of selected u s g s reference samples abstract inductively coupled plasma mass spectrometry icp ms is a powerful analytical technique with considerable application to geochemistry in this paper we demonstrate the power of icp ms as a tool in petrogenetic studies data for 28 elements in seven u s g s basalt and andesite reference samples agv 1 bcr 1 bhvo 1 bir 1 dnc 1 w 1 and w 2 are presented accuracy and precision standard deviation for 26 of these elements is either excellent a detailed description of the icp ms technique we have developed is given a simple hf hno 3 dissolution in screwtop teflon bombs was used for sample preparation icp ms instrumental sensitivity was measured by external solutions with surrogate calibration for 4 elements matrix correction was made by use of standard addition naturally occurring internal standards rb y ce and pb were used to correct for instrumental drift between sample and spiked sample measurements we report data for the elements li cs rb sr ba zr hf nb ta y sc u th pb and all 14 of the ree in addition the procedure acquired data for be mo w tl and bi many elements in this array are crucial for the evaluation of petrogenetic models and source tracing herein lies the real power of icp ms one technique one dissolution small sample size good limits of detection good to excellent accuracy and good precision for many geochemically important elements

melting experiment on a model chondritic mantle composition at 25 gpa we have conducted melting experiments on a chondritic mantle composition in the five component system cao feo mgo al2o3 sio2 at 25 gpa the liquidus phase at this pressure is majorite garnet and the second crystallizing phase is mg silicate perovskite or magnesiowustite these phases seem to be consumed into the melt at almost the same temperature the present experiments indicate that majorite garnet fractionation might have occurred in the chondritic primitive mantle if the partial molten zone extended to a depth equivalent to the upper part of the lower mantle in the early stages of terrestrial evolution

anomalous sm nd ages for the early archean onverwacht group volcanics new sm nd isotopic data for eight samples of basalt and komatiite from the tjakastad subgroup lower onverwacht group of the barberton greenstone belt bgb of the kaapvaal craton in southern africa are reported they give new constraints on the interpretation of sm nd ages for the subgroup and highlight the petrogenesis of tjakastad volcanics although sm nd isotopic data earlier reported for volcanic rocks from the tjakastad subgroup yielded an isochron age of 3526 48 ma the new results give a much younger isochron date of 3269 84 ma the 3526 48 ma isochron age has been obtained in combining samples ranging in composition from felsic volcanics to ultrabasic komatiites and is thus considered suspect with regard to the pre requisite of geochronology that all the studied rocks must have had identical initial isotopic compositions the new isochron date of 3269 84 ma has been obtained in combining samples solely of basic ultrabasic composition it might thus represent a more correct age for the eruption of the tjakastad subgroup volcanism in fact owing to the potential problem of source heterogeneity and also in the light of geochronological and geochemical arguments we show that this date also has little chance to have any strict chronological meaning most likely the tjakastad volcanics were formed 3450 ma ago also most likely their source rocks were isotopically heterogeneous and the 3530 ma and 3270 ma linear arrays are not true but apparent isochrons based on the calculated nd 3450 values and other geochemical arguments we show that three possible sources might have been involved depleted mantle primitive mantle and older continental crust

paleo to mesoarchean basement recycling and terrane definition in the northeastern superior province quebec canada abstract the northeastern superior province nesp is a 500 km 700 km segment of plutonic archean crust generated by 1 2 ga of tectonomagmatic activity the nesp exhibits the classical plutonic series of archean terranes low k tonalite trondhjemite granodiorite ttg and high k granodiorite granite monzogranite ggm with unusually large volumes of px ttgs mostly enderbites and subordinate charnockites volcanic rocks constitute t 0 45 to 5 03 mafic to felsic volcanic rocks and mafic to ultramafic plutonic rocks t 0 40 to 6 90 these radiogenic signatures as well as inherited zircon cores record recycling of mesoarchean 3 8 2 9 ga crust either through direct melting or by coupled assimilation fractional crystallization processes in comparison the riviere arnaud terrane is underlain by younger t values 2 34 to 0 67 indicating either a juvenile character or recycling of fairly juvenile late mesoarchean early neoarchean crust overall geochemical and isotopic data of the late neoarchean

potential field power spectrum inversion for scaling geology we propose a method for inverting the power spectrum of gravity and magnetic data the method is demonstrated on aeromagnetic and bore well data from the german continental deep drilling project ktb density and susceptibility distributions in the earth s crust exhibit scaling behavior with power spectra proportional to f where f is the wavenumber and is the scaling exponent of the source distribution we model the sources of the potential field by a random function with scaling properties defined on a half space with its top at a specified depth beneath the observation plane comparing the theoretical power spectrum for this model with the power spectrum of the measured data we obtain the best values for the depth to source and the scaling exponent as a global minimum of the misfit function despite the simplicity of the model it offers a new understanding of the factors influencing the shape of the potential field power spectrum in particular the low wavenumber part of the power spectrum can be dominated by the scaling properties of the source distribution and not by the depth to some kind of basement the scaling exponent of the field varies with the type of surface geology the question of whether the scaling exponent can actually be used to identify different types of geology gives an interesting new aspect to power spectrum inversion

early evolution of the earth accretion atmosphere formation and thermal history atmospheric and thermal evolution of the earth growing by planetesimal impacts was modeled by taking into account the blanketing effect of an impact induced h2o atmosphere and the temperature dependence of h2o degassing when the water content of planetesimals is larger than 0 1 by weight and the accretion time of the earth is less than 5 107 years the surface of the accreting earth melts and thus a magma ocean forms and covers the surface the formation of a magma ocean will result in the initiation of core mantle separation and mantle differentiation during accretion once a magma ocean is formed the surface temperature the degree of melting in the magma ocean and the mass of the h2o atmosphere are nearly constant as the protoplanet grows further the final mass of the h2o atmosphere is about 1021 kg a value which is insensitive to variations in the model parameter values such as the accretion time and the water content of planetesimals that the final mass of the h2o atmosphere is close to the mass of the present oceans suggests an impact origin for the earth s hydrosphere on the other hand most of the h2o retained in planetesimals will be deposited in the solid earth free water within the proto earth may affect differentiation of the proto mantle in particular the mantle feo abundance and the incorporation of a light element in the outer core

pyrite deformation in stratiform lead zinc deposits of the canadian cordillera pyrite textures in five stratiform lead zinc deposits from lower to upper greenschist facies environment of the canadian cordillera are described and discussed in terms of deposition early diagenesis deformation metamorphism and hydrothermal alteration processes overgrowth is an important process during both diagenesis and deformation diagenetic and deformational overgrowths can be distinguished diffusive mass transfer involving pressure solution and oriented overgrowth of pyrite is the main deformation mechanism in pyrite deposits at low metamorphic grades although diffusive mass transfer favours fine grained mineral aggregates its effect on coarse pyrite grains has also been identified ore minerals dissolved by pressure solution may be transported with the assistance of pore fluids within fractures and grain boundaries over distances significantly greater than the scale of individual grains to give a range of pressure solution overgrowth textures the textural modification of pyritic ores from the early stages of diagnesis through metamorphism and deformation to post deformation thermal annealing has important implications for the distribution of trace elements and isotopic compositions in pyritic ores

magnetic data interpretation for the source edge locations in parts of the tectonically active transition zone of the narmada son lineament in central india the study has been carried out in the transition zone of the narmada son lineament nsl which is seismically active with various geological complexities upwarp movement of the mantle material into the crust through fault fractures lamination and upwelling nsl is one of the most prominent lineaments in central india after the himalaya in the indian geology the area of investigation extends from longitude 80 25 e to 81 50 e and latitude 23 50 n to 24 37 n in the central part of the indian continent different types of subsurface geological formations viz alluvial gondwana deccan traps vindhyan mahakoshal granite and gneisses groups exist in this area with varying geological ages in this study area tectonic movement and crustal variation have been taken place during the past time and which might be reason for the variation of magnetic field magnetic anomaly suggests that the area has been highly disturbed which causes the narmada son lineament trending in the ene wsw direction magnetic anomaly variation has been taken place due to the lithological variations subject to the changes in the geological contacts like thrusts and faults in this area shallow and deeper sources have been distinguished using frequency domain analysis by applying different filters to enhance the magnetic data various types of derivatives to identify the source edge locations of the causative source bodies the present study carried out the interpretation using total horizontal derivative tilt angle derivative horizontal tilt angle derivative and cos derivative map to get source edge locations the results derived from various derivatives of magnetic data have been compared with the basement depth solutions calculated from 3d euler deconvolution it is suggested that total horizontal derivative tilt angle derivative and cos derivative are the most useful tools for identifying the multiple source edge locations of the causative bodies in this tectonically active and transition zone area as this area is highly prone to hydrocarbon bearing zone hence the integrated interpretation could reliably image various thrusts and faults boundaries and the source edge locations with dip and strike orientation along with the basement lineation in encouraging exploration for better understanding of the geo scientific data

the evolution of the southern abitibi greenstone belt in light of precise u pb geochronology this paper summarizes u pb zircon ages for the archean southern abitibi greenstone belt and integrates them together with zircon and baddeleyite data for three new samples into a general framework for the evolution of the region new ages are reported for the ghost range gabbroic complex 2713 super 7 sub 5 ma and for a pyroclastic volcanic rock of the gauthier group 2700 or 3 ma a previously determined age of 2701 super 3 sub 2 ma for the felsic volcanic skead group is confirmed by magmatic zircons from a second sample that also contains a xenocrystic component or 2720 ma indicating that the skead volcano was built on top of an older volcanic sequence the compiled ages show that the southern abitibi greenstone belt developed between 2750 and 2670 ma the earliest ages date remnants of volcanic complexes that occur at the periphery of the belt major preorogenic magmatism was confined mainly to the period 2720 to 2700 ma in general each episode was characterized by the essentially coeval formation of komatiitic tholeiitic and calc alkalic igneous rocks taking into consideration the known geologic geochemical and isotopic data this association is consistent with evolution in paired active are and back are systems older crust was absent from the region but it may have interacted with these arcs and rift systems farther to the west the extensional regime was followed by a period of compression that caused folding and thrusting emplacement of calc alkalic plutons and deposition of turbidites between 2700 and 2688 ma the magmatism became predominantly alkalic during the main timiskaming period between 2681 and 2676 ma this event was accompanied by the deposition of alluvial fluvial sequences and was followed by renewed compression that caused polyphase folding and thrusting probably between approximately 2676 and 2670 ma lamprophyre dikes were associated with these final stages of deformation postorogenic activity includes faulting and episodes of hydrothermal activity locally associated with gold mineralization ages in the range 2630 to 2580 ma for hydrothermal minerals e g titanite and rutile camflo suggest a correlation of these events with lower crustal magmatism and metamorphism as recorded for example in the adjacent kapuskasing zone

the primodial terrestrial magma ocean and its implication for stratification of the mantle fractional crystallization behaviour of a magma ocean extending to lower mantle depths was deduced from estimations of melting relations for the deep mantle and the density relationships between ultrabasic liquid and mantle minerals the accretional growth of the earth necessarily involves a molten zone magma ocean in the outer layer of the growing earth the fractionation by melting during accretion results in primary stratification composed of a molten ultrabasic upper mantle magma ocean a perovskite rich lower mantle and an iron core a certain amount of al2o3 and cao was removed from the magma ocean and retained in the lower mantle due to eclogite fractionation in the early stage of accretion and the perovskite fractionation in the later stage of accretion models of the stratification of the upper mantle arising from fractional crystallization of the magma ocean and subsequent convective disturbance were deduced on the basis of estimations of melting relations for the deep mantle and the density relationships between the ultrabasic liquid and mantle minerals the stratification of the mantle which is consistent with geophysical constraints is as follows the upper mantle is composed of two layers the upper olivine rich layer and the lower garnet rich layer with a thickness around 200 km and the lower mantle with a perovskite rich composition in this model both the 400 and 650 km discontinuities are the chemical boundaries

comparative reos smnd and rbsr isotope and trace element systematics for archean komatiite flows from munro township abitibi belt ontario abstract a series of komatiite flows from munro township ontario have re and os concentrations that range from 0 545 to 1 27 ppb and 1 00 to 2 20 ppb respectively the187re 186os in the suite range from 14 05 to 52 37 and the187os 186os range from 1 49 to 3 22 giving a crystallization age of2726 93ma and an of initial187os 186os of0 873 0 035 this age is in good agreement with the crystallization ages that have been reported for these rocks using other isotopic systems the re os isochron indicates that the system was resistant to the low grade metamorphism that altered the rocks the initial os isotopic composition is consistent with the derivation of the melts from a mantle source s that originated with a chrondritic 187os 186os i and evolved with an approximately chondriticre os sm nd data for the same suite cluster about anend value at 2 72 ga of 2 to 3 rb sr isotope data indicate significant mobilization of these elements by the greenschist grade metamorphism that has altered the rocks comparisons of re and os concentrations with those of mgo feo ni cr and s suggests that re was incompatible d 1 os concentrations were likely controlled by the presence or precipitation of trace metallic or sulfide phases both during melting and crystallization and not by the modally predominant silicate minerals

likely mantle plume activity in the skellefte district northern sweden a reexamination of mafic ultramafic magmatic activity its possible association with vms and gold mineralization abstract most attention has been given to the geology of the extensive vms and subordinate precious metals mineralization in the skellefte district less attention has been given to indications of deep seated origins of felsic and mafic ultramafic volcanic rocks of vms and precious metals mineralizing fluids and the primary origins of these metals a holistic view of the significance of mafic ultramafic volcanic rocks to both the geotectonic evolution of the area and the existence of its important base and precious metals deposits has never been presented these subjects are discussed in this investigation primitive mantle normalized spider diagrams of rare earth elements ree distinguish two groups of mafic ultramafic volcanic rocks each with distinct geochemical characteristics a mid ocean ridge morb type and a geochemically unusual and problematic calc alkaline basalt cab type which is the main subject of this investigation the morb type mafic volcanic rocks are mostly older than the skellefte group felsic volcanic rocks hosting the vms deposits whereas the more primitive cab type mafic ultramafic volcanic rocks are mostly younger a common source for these cab type mafic mgo wt 14 volcanic rocks is suggested by their similar and distinctive geochemical features these are near chondritic al undepleted al 2 o 3 tio 2 ratios moderate to strong high field strength element hfse depletion light rare earth element lree enrichment and moderate heavy rare earth element hree depletion they outcrop throughout an area of at least 100 100 km gold mineralization is spatially associated with ultramafic volcanic rocks zr and hf depletion has been shown to be associated with al depletion in mafic ultramafic volcanic rocks elsewhere and has been attributed to deep seated partial melting in ascending mantle plumes zr and hf depletion in cab type al undepleted mafic ultramafic volcanic rocks is therefore unusual the solution to this dilemma is suggested to be contamination of an al depleted mantle plume by felsic crustal rocks whereby al depleted ultramafic magmas become al undepleted it will be argued that this model has the potential to explain previous observations of deep seated origins the spatial association of ultramafic volcanic rocks with occurrences of gold mineralization and even the primary origin of metals in vms deposits

polyphase deformation of the massive sulphide ore of the black angel mine central west greenland the massive fe zn pb sulphide sheets constituting the angel zone ore body of the black angel mine show evidence of three phases of deformation at greenschist facies metamorphic grade during an early phase an originally layered sulphide ore type was isoclinally folded subsequent thrusting parallel to the ore body transformed the layered ore into massive and porphyroclastic ore tectonites late open folds refolded the earlier structures and caused localized differential mobilization of the sulphides the microstructures of the layered ore tectonite indicate a period with static grain growth interpreted as the result of prograde metamorphism followed by a dynamic recrystallization under low stress and at low strain rates which is correlated with the early isoclinal folding the microstructures of the massive and the porphyroclastic ore tectonites indicate syntectonic recrystallization under high stress and at high strain rates corresponding to the thrusting of the ore bodies the microstructures of the mobilized sulphides show evidence of repeated plastic cataclastic deformation and recrystallization corresponding to highly variable strain and strain rate conditions during the mobilization post deformational annealing took place at elevated temperature and was largely controlled by inhibition dependent grain growth and to a minor extent by orientation dependent grain growth

majorite fractionation and genesis of komatiites in the deep mantle abstract melting experiments of the primitive mantle indicate that majorite fractionation is the key process to determine the geochemical nature of magmas generated in the deep upper mantle and the transition zone a model of petrogenesis of komatiite magma is presented by using the melting relations of the primitive mantle and the partitioning data of lithophile elements between majorite and the liquid alumina depleted komatiite generally observed in the early archean can be generated by any degree of partial melting at depths from 450 to 650 km this type of komatiite is also generated by a relatively low degree of partial melting at depths from 180 to 450 km alumina enriched komatiite can be generated by remelting of the residual mantle after separation of alumina depleted komatiite alumina undepleted komatiite generally observed in the late archean can not be formed by a low degree of partial melting of the primitive mantle at any depths this type of komatiite can be generated only by a high degree of partial melting of the mantle at the depths shallower than 450 km as an alternative model the alumina depleted komatiite can be formed by a high degree of partial melting of the archean upper mantle depleted in majorite such heterogeneity in the archean upper mantle was generated by a global melting event succeeding the accretion of the earth and erased by convection by the later archean hf isotope data seem to favour the former model of the petrogenesis of komatiite i e single stage melting of the primitive mantle at various depths

partitioning equilibrium temperature gradients and constraints on earth differentiation experimental determination of equilibrium partitioning relations is complicated when crystals grow in disequilibrium mode in environments of imposed thermal gradient contrary to claims in the literature neither dynamic growth nor thermal gradients produce systematic deviations of measured partition coefficients d s toward unity non systematic character in both effects arises from the composite nature of the processes involved if one can exclude the parts of the process caused by diffusion both chemical and thermal the deviations toward unity would be systematic however it is shown a priori on a quantitative basis and from qualitative internal evidence in reported high pressure experiments that neglect of diffusion leads to errors of interpretation these errors have a serious impact on our understanding of planetary differentiation perovskite fractionation in mantle petrogenesis is shown to remain possible based on rather than be excluded by reported experimental data

a post deformational post peak metamorphic timing for mineralization at the archaean big bell gold deposit western australia abstract previous workers were unable to find any suitable criteria to time gold mineralization at big bell mine an enigmatic low grade large tonnage deposit hosted in an upper amphibolite facies greenstone belt sequence in the murchison province of the archaean yilgarn craton western australia earlier studies suggested that the gold mineralization occurred by hydrothermal alteration either synchronous with volcanism or prior to peak metamorphism the following conclusions have been drawn from a detailed meso and microstructural analysis of the mine sequence and greenstone belt hosting the orebody 1 1 prior to mineralizaton the big bell area underwent five phases of deformation and m1 regional metamorphism to the staurolite andalusite zone of the amphibolite facies for metasediments 2 2 granite emplacement post dated d5 folding causing local m2 thermal metamorphism this peaked at the ms qtz breakdown reaction for pelites on or close to the and sil transition but below the onset of partial melting characterized by the narrow non degenerate divariant field involving seven kfmash phases qtz ms bt crd kfs als h2o this hornfelsing consumed cross cut and post dated s5 cleavage prograde qtz and sil veins replaced this hornfels assemblage and s5 cleavage and were caused by volatile escape at depth and hydraulic fracturing coincident with a reduction in bulk rock permeability resulting from post deformational hornfelsing 3 3 retrograde m3 assemblages and two stages of gold related mineralization cross cut and replaced all previous structures the retrograde gold bearing assemblages are both underformed stage 1 mineralization is typically found with widespread retrograde muscovite plates and consists of the following mineral associations py po ccp ilm mag au tur sch rt spn zrn ms bt ep czo qtz the second stage of mineralization is characterized by pervasive sericitic alteration that is locally developed to an extent where all pre existing mineralogies including quartz are totally destroyed within sericitic domains the stage 2 mineral associations are apy sbt sb mo sp au py po ilm mag ccp sericite chl carbonate cal ep czo qtz 4 4 gold mineralization at big bell was post deformational and post peak metamorphic unlike many archaean gold deposits there are no obvious shear zones controlling the formation of the big bell gold deposit at the structural level exposed either on the surface or in drill core big bell mine was in a low strain domain during the formation of late stage sinistral sub vertical d6 shear zones that are typically concentrated at the granitoid greenstone contacts the retrograde character of the mineralization has been deduced from detailed meso and microstructural observations and demonstrates the emplacement of this gold deposit was very late in the overall orogenic history of the murchison province

fractal characterization of the south australian gravity station network summary the spatial distribution of the south australian gravity station network over 65 000 stations can be approximated by a fractal point set of correlation dimension dc 1 4 the fractality is established over more than 2 decades of distance the fractal nature of the grid is possibly due to the multistage decisions involved in establishing a network in each step previously unexplored areas are dissected by geophysical traverses as in the classical fractal fragmentation process it is shown that we cannot observe the short wavelength components of the gravity field if the dimension of the network is less than two and any attempt to interpolate onto a regular grid could lead to spurious anomalies due to aliasing

mineralization at the golden grove cu zn deposit western australia ii deformation textures of the opaque minerals studies of the sulphide magnetite fabric at golden grove western australia metamorphosed under conditions of lower to middle greenschist facies indicate that pyrite and magnetite deformed in a b

geochemistry of two stratigraphically related ultramafic komatiite layers from the neoarchean sigegudda greenstone terrane western dharwar craton india evidence for compositional diversity in archean mantle plumes abstract two compositionally different ultramafic units are present in the neoarchean sigegudda greenstone terrane western dharwar craton india these ultramafic units occur in the same volcano sedimentary sequence and are separated by a fault bounded volcanic arc association because of deformation and amphibolite facies metamorphism the primary igneous textures have been extensively modified in both ultramafic units given their spatial and temporal association with sedimentary and volcanic rocks and high mgo contents 12 29 wt these ultramafic rocks are interpreted as metamorphosed komatiites and komatiitic basalts field relationships and rock types indicate that the lower section of the sigegudda volcano sedimentary sequence was deposited in a peri cratonic continental rift setting the lower ultramafic unit is characterized by 12 4 29 6 wt mgo mg 76 91 and 526 1150 ppm ni the upper ultramafic unit is overall more magnesian and compositionally restricted mgo 21 6 25 9 wt mg 86 89 and ni 610 1000 ppm the lower unit features a combination of relatively elevated tio 2 0 40 0 90 wt al 2 o 3 tio 2 the chondritic ratio of 21 and gd yb n ratios 1 these are ti enriched komatiites reported for the first time from greenstone belts in the dharwar craton the upper unit is compositionally comparable to al undepleted komatiites and characterized by al 2 o 3 tio 2 21 lree depletion coupled with near flat chondrite normalized hree patterns geochemical data indicate that the sigegudda komatiites and komatiitic basalts were variably contaminated by either continental crust or sub continental lithospheric mantle or both the geochemical differences between the two units are explained by variable depths and degrees of partial melting the ti enriched lower unit appears to have been formed through lower degrees of partial melting at depths 90 km whereas the al undepleted upper unit was generated by higher degrees of partial melting at depths 90 km field relationships and geochemical characteristics of the sigegudda komatiites can be explained by a geodynamic model in which the stratigraphically lower ti enriched komatiites were erupted from a mantle plume onto a rifting continental margin whereas the stratigraphically upper al undepleted komatiites originated from a younger mantle plume and erupted onto a volcanic arc sequence that accreted to the rifted continental margin

chemical stratification in the silicate earth abstract the largest density contrast between magmas and common ferromagnesian minerals in the mantle occurs at two locations these are nearest the earth s surface and at about 650 700 km depth where the lower mantle mineral assemblage becomes stable at solidus temperatures the large density contrast between magmas and olivine nearest the surface diminishes to zero somewhere between the middle and lower stratigraphic sections of the upper mantle it is also here where the composition of all anhydrous magmas is most similar to that of an undepleted garnet iherzolite composition for an earth that has differentiated by fractional crystallization and melting processes which depend on the separation of crystalline phases from silicate liquids having different densities the 650 km discontinuity could represent a chemical boundary layer the chemical change in the early archean could have been from primordial upper mantle compositions above the boundary layer to differentiated lower mantle compositions below a primary stratigraphic model is offered for the distribution of depleted enriched and primordial isotopic mantle components formed by the transport fate of magmas in the beginning of earth history depleted morb source regions were established in the uppermost part of the upper mantle primordial mantle was located at the middle to the deepest regions of an upper mantle which was processed by anhydrous melting and crystallization enriched and depleted components could have been formed at these levels also by igneous processing in the presence of h 2 o or co 2 and involving highly compressible magmas the lower mantle became heterogeneous with enrichments at the top and depletions elsewhere it is suggested that the present day isotopic structure of the mantle has inherited elements of this primary stratification primordial 143 nd 144 nd and 87 sr 86 sr observed in oceanic islands basalts are located in the middle of upper mantle convection cells enriched ratios found in both oceanic islands basalt and kimberlite could have originated near and on either side of the 650 km chemical boundary layer

the equation of state of a molten komatiite 2 application to komatiite petrogenesis and the hadean mantle new experimental data for the equation of state of a komatiitic liquid were used to model adiabatic melting in a peridotitic mantle if komatiites are formed by 30 partial melting of a peridotitic mantle then komatiites generated by adiabatic melting come from source regions that began their unmelted ascent in the lower transition zone 500 670 km or the lower mantle 670 km the great depth of incipient melting implied by this model suggests that komatiitic liquids may form in a pressure regime where they are denser than their coexisting crystals possibly the bulk mantle although komatiitic magmas are thought to separate from residual crystals in the mantle at a temperature 200 c greater than modem mid ocean ridge basalts morbs their ultimate sources are predicted to be diapirs that if adiabatically decompressed from initially solid mantle were more than 700 c hotter than the sources of morbs and were derived from great depth we also studied the evolution of an initially molten mantle i e a magma ocean our model considers the thermal structure of the magma ocean density constraints on crystal segregation and approximate phase relationships for a nominally chondritic mantle crystallization will begin at the core mantle boundary perovskite buoyancy at 70 gpa may lead to a compositionally stratified lower mantle with iron enriched magnesiowustite content increasing with depth large convective velocities in the magma ocean would prohibit crystal liquid fractionation by settling or flotation until quiescent boundary layers form such boundary layers could form when the crystal content of the magma reaches a critical value near 44 vol and in the late stages of crystallization around unmelted blocks of foundered protocrust matrix compaction and diapirism could also lead to fractionation effects the upper mantle could be depleted or enriched in perovskite components relative to the bulk mantle olivine neutral buoyancy may lead to the formation of a dunite septum in the upper mantle partitioning the ocean into upper and lower reservoirs but this septum must be permeable

melting and crystal liquid partitioning in the system mg2sio4 fe2sio4 to 25 gpa the melting relation of the mg2sio4 fe2sio4 system was studied at 8 5 gpa olivine and spinel melts incongruently into pyroxene and the liquid in the fe2sio4 rich portion of the system olivine melts congruently at least in the compositional range from mg0 5fe0 5 2sio4 to mg0 9fe0 1 2sio4 although the end member of mg2sio4 melts incongruently into periclase and the liquid at pressures above 8 gpa the melting relation of mg0 9fe0 1 2sio4 was also studied up to 25 gpa olivine and modified spinel of this composition melt incongruently into anhydrous phase b and liquid and into magnesiowustite and liquid at higher temperatures in the pressure range from 18 gpa to 23 gpa magnesiowustite is the liquidus phase at higher pressures at least up to 25 gpa in this composition the partition coefficients between crystal and liquid are dmg 1 19 0 06 dsi 0 88 0 04 dv 0 63 0 16 dcr 1 04 0 21 dmn 0 42 0 08 dfe 0 76 0 07 dco 1 24 0 44 dni 1 46 0 81 for modified spinel dmg 1 41 0 06 dsi 0 72 0 03 dv 0 44 0 11 dcr 0 94 0 12 dmn 0 30 0 07 dfe 0 56 0 06 dco 1 19 0 30 dni 1 65 0 87 for anhydrous phase b dmg 2 09 0 11 dv 0 64 0 06 dcr 1 01 0 05 dmn 0 7 0 04 dfe 1 45 0 08 dco 2 73 0 15 dni 4 72 0 25 for magnesiowustite the present results of melting relations and partitioning are applied for the fractionation in the mantles of the terrestrial planets such as mars and mercury

a new look at differentiation of the earth from melting experiments on the allende meteorite carbonaceous chondrites contain approximately solar abundances of the non volatile elements1 but are much more feo rich2 than upper mantle peridotites3 or hypothetical chondritic mantle compositions4 5 if the earth accreted from unfractionated primitive meteoritic debris similar to the carbonaceous chondrites how then did it become a fractionated layered body with a crust mantle and core here i report the results of high pressure melting experiments on the allende cv3 carbonaceous chondrite which address this question and provide a new look at the earth s earliest stage of differentiation multi anvil experiments at 24 26 and 26 5 gpa show that feo rich magnesiowustite is an abundant crystallizing phase at temperatures near the allende silicate liquidus if a chondritic earth experienced a high temperature molten stage then during cooling and crystallization feo rich magnesiowustite could be segregated to the deepest levels of the earth s interior magnesiowustite fractionation may thus have depleted the initial feo content of the primitive chondritic mantle and contributed to the formation and growth of the earth s core