sound velocities of majorite garnet and the composition of the mantle transition region the composition of the mantle transition region characterized by anomalous seismic wave velocity and density changes at depths of approximately 400 to 700 km has remained controversial some have proposed that the mantle transition region has an olivine rich pyrolite composition whereas others have inferred that it is characterized by pyroxene and garnet rich compositions piclogite because the sound velocities in pyrolite estimated from laboratory data are substantially higher than those seismologically observed although the velocities of the olivine polymorphs at these pressures wadsleyite and ringwoodite have been well documented those of majorite another significant high pressure phase in the mantle transition region with realistic mantle compositions have never been measured here we use combined in situ x ray and ultrasonic measurements under the pressure and temperature conditions of the mantle transition region to show that majorite in a pyrolite composition has sound velocities substantially lower than those of earlier estimates owing to strong nonlinear decreases at high temperature particularly for shear wave velocity we found that pyrolite yields seismic velocities more consistent with typical seismological models than those of piclogite in the upper to middle parts of the region except for the potentially larger velocity jumps in pyrolite relative to those observed at a depth of 410 km in contrast both of these compositions lead to significantly low shear wave velocities in the lower part of the region suggesting possible subadiabatic temperatures or the existence of a layer of harzburgite rich material supplied by the subducted slabs stagnant at these depths

obtaining diagenetic ages from metamorphosed sedimentary rocks u pb dating of unusually coarse xenotime cement in phosphatic sandstone the depositional age of nonfossiliferous metamorphosed sedimentary rocks is commonly bracketed between the age of the youngest detrital mineral and the age of the oldest metamorphic mineral the technique of dating diagenetic xenotime by ion microprobe can provide robust minimum ages for sediment deposition however in most cases xenotime is only a few microns m in size and rarely exceeds 10 m the minimum size for in situ ion microprobe analysis phosphatic sandstone in the greenschist facies mount barren group in southwestern australia contains unusually abundant xenotime occurring as exceptionally coarse 200 m pore filling cement that nucleated on detrital zircon grains the optimum environmental site for the formation of the cement was sand beds within a black shale condensed section analysis of xenotime by sensitive high resolution ion microprobe yields two age populations 1696 7 ma for cement adjacent to detrital zircon grains and 1646 8 ma for outer zones preserved textures show that initial xenotime growth was early diagenetic establishing the ca 1700 ma age as a proxy for the depositional age of the mount barren group the younger age ca 1650 ma is regarded as burial related the xenotime data reduce considerably the previous limits on the age of the succession i e between ca 1850 ma youngest zircon population and ca 1200 ma peak metamorphism the significant achievement of our results is establishing that early diagenetic xenotime retains its physical form and u pb isotopic age despite greenschist facies metamorphism and penetrative deformation

transmission electron microscope observation of the high pressure form of magnesite retrieved from laser heated diamond anvil cell abstract we applied focused ion beam fib technique to make a thin foil for transmission electron microscope tem observations of mgco3 magnesite and its new high pressure form magnesite ii recovered from laser heated diamond anvil cell lhdac both of which should play fundamental roles in the circulation of carbon in the lower mantle heterogeneous microstructures due to significant temperature gradients were observed along the axial direction of the disk sample 100 m in diameter and 30 m thick in spite of double sided laser heating and the use of thermal insulators between the diamond anvil and the sample the central portion of the mgco3 sample was found amorphous while the peripheral part remained magnesite on the basis of electron diffraction measurements with tem this and our earlier results based on in situ x ray diffraction measurements suggest that magnesite transformed to magnesite ii only at the central part where the temperatures should have been significantly higher than those near the pressure medium magnesite ii was amorphisized on release of pressure in contrast to our interpretation that it was converted back to magnesite based solely on in situ x ray diffraction measurements the new method of combining the fib technique with tem observation thus provides important information on phase transitions under very high pressure using lhdac which may be overlooked by in situ optical and x ray diffraction measurements

first principles investigations on the elasticity and phase stability of grossular garnet 1 in order to understand the fate of subducted continental materials by means of mechanisms such as tectonic erosion and sediment subduction it is important to understand the high pressure stability and elastic properties of grossular garnet we study these questions using the first principles computation method grossular garnet was found to dissociate into an assemblage of casio3 ca perovskite pv and al2o3 corundum cor at about 23 4 gpa accompanied by remarkable jumps of compressional wave 8 0 shear wave 11 6 bulk sound 5 7 velocities and density 12 1 although pv with the grossular garnet composition was suggested experimentally after the decomposition reaction this phase was found to be less stable than an assemblage of capv and cor at pressures higher than 20 gpa this indicates that observed pv with the grossular garnet composition is metastable but can be obtained because of the slow kinetics of the garnet decomposition reaction our results imply that the dissociation of grossular garnet as well as that of jadeite included in continental materials subducted into the deep mantle increases the complexity of the 660 km discontinuity and can explain seismically observed multiple reflections we have also investigated the anisotropy of elastic velocities of grossular garnet capv and cor and found that capv is the most anisotropic mineral this implies that capv could produce seismic anisotropy in the uppermost lower mantle

high pressure phase relations and thermodynamic properties of caal4si2o11 cas phase abstract phase relations in caal4si2o11 were examined at 12 23 gpa and 1000 1800 c by multianvil experiments a three phase mixture of grossular kyanite and corundum is stable below about 13 gpa at 1000 1800 c at higher pressure and at temperature below about 1200 c a mixture of grossular stishovite and corundum is stable indicating the decomposition of kyanite above about 1200 c caal4si2o11 cas phase is stable at pressure higher than about 13 gpa the triple point is placed at 14 7 gpa and 1280 c the equilibrium boundary of formation of cas phase from the mixture of grossular kyanite and corundum has a small negative slope and that from the mixture of grossular stishovite and corundum has a strongly negative slope while the decomposition boundary of kyanite has a small positive slope enthalpies of the transitions were measured by high temperature drop solution calorimetry the enthalpy of formation of caal4si2o11 cas phase from the mixture of grossular kyanite and corundum was 139 5 15 6 kj mol and that from the mixture of grossular stishovite and corundum was 94 2 15 4 kj mol the transition boundaries calculated using the measured enthalpy data were consistent with those determined by the high pressure experiments the boundaries in this study are placed about 3 gpa higher in pressure and about 200 c lower in temperature than those by zhai and ito zhai s ito e 2008 phase relations of caal4si2o11 at high pressure and high temperature with implications for subducted continental crust into the deep mantle phys earth planet inter 167 161 167 combining the thermodynamic data measured in this study with those in the literature dissociation boundary of cas phase into a mixture of ca perovskite corundum and stishovite and that of grossular into ca perovskite plus corundum were calculated to further constrain the stability field of cas phase the result suggests that the stability of cas phase would be limited at the bottom of transition zone and top of the lower mantle when sediments are subducted into the deep mantle it is also suggested that cas phase may be stable at the depth of the upper part of the lower mantle when partial melting of basalt occurs at the depth

phase relations for caal2si2o8 anorthite composition in the system caoal2o3sio2 at 14 gpa abstract multi anvil experiments at 14 gpa and 1500 c reveal that caal 2 si 2 o 8 anorthite composition undergoes subsolidus disproportionation into grossular ca 3 al 2 si 3 o 12 stishovite sio 2 and a calcium aluminosilicate phase of composition caal 4 si 2 o 11 this last phase cas was first synthesised by irifune et al 1994 earth planet sci lett 126 351 368 in a continental crust composition at p t conditions appropriate for the transition zone in the system cao al 2 o 3 sio 2 a new three phase assemblage grossular stishovite cas encountered at 14gpa replaces the grossular stishovite kyanite assemblage known to be stable at around 10gpa because of an univariant reaction between grossular and kyanite that yields stishovite and cas by 1600 c another univariant reaction between grossular and stishovite produces cas plus liquid schreinemakers analysis suggests that given ca al rich lithologies the cas phase should be stable at transition zone pressures

phase transformations of ca3al2si3o12 grossular garnet to the depths of the earth s mantle transition zone abstract high pressure and high temperature phase transformations of ca 3 al 2 si 3 o 12 grossular garnet were examined at 19 26 gpa and 700 2000 k using kawai type multi anvil apparatus coupled with in situ x ray diffraction xrd recovered samples were analyzed by a combination of micro focused x ray diffraction xrd and transmission electron microscopy tem the results show that grossular garnet gradually transforms to an al rich casio 3 perovskite at 22 26 gpa and 1000 1400 k the transition boundary can be expressed as p gpa 0 0082 t k 33 05 when the garnet completely disappears we observed orthorhombic casio 3 perovskite with a grossular composition at 20 24 gpa and temperatures above 1500 k the cas phase with the composition caal 4 si 2 o 11 appears to accommodate excess al from the perovskite along with two distinct populations of al bearing casio 3 perovskites with al content of 3 7 and 10 0 wt al 2 o 3 respectively the pressure and temperature of these transitions correspond to the lowermost part of the transition zone and therefore it suggests that ca rich aluminosilicates could provide alternative candidates to explain multiple seismic reflections near the 660 km depth discontinuity

phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600 800 km in the mantle abstract phase transformations in a representative oceanic crust composition morb have been studied at pressures up to 28 gpa at 1200 c and 1500 c using a multi anvil apparatus the assemblage of majorite casio 3 rich perovskite stishovite which is displayed by the morb composition at pressures above 20 gpa was found to transform further into an assemblage containing a new aluminous phase at pressures above 25 gpa this al phase has a complex composition and is also relatively sodium rich its x ray diffraction pattern is similar to that observed for mgal 2 o 4 calcium ferrite type and is tentatively indexed on the basis of an orthorhombic cell yielding a zero pressure density of 3 87 2 g cm 3 majorite garnet was found to remain as the most abundant phase at the highest pressure studied 28 gpa and does not appear likely to transform to perovskite until pressures exceed 29 30 gpa these results imply that the basaltic crust component of subducted oceanic lithosphere would be 0 1 0 2 g cm 3 less dense than surrounding mantle pyrolite between the depths of 660 800 km thereby confirming an earlier hypothesis of anderson moreover the underlying layer of harzburgite would be 0 05 g cm 3 less dense than pyrolite throughout most of this depth interval these buoyancy relationships contribute to a barrier which impedes and may sometimes prevent the entry of slabs into the lower mantle

monazite and xenotime petrogenesis in the contact aureole of the makhavinekh lake pluton northern labrador high temperature 700 900 c metamorphism in the contact aureole of the makhavinekh lake pluton mlp northern labrador led to the growth of monazite and xenotime during progressive replacement of regional garnet bearing assemblages m1 by lower pressure symplectitic coronas of orthopyroxene cordierite m2 in the inner aureole 500 m from the contact where m1 garnet is strongly resorbed high y hree monazite xy hree 0 14 0 18 occurs as small isolated grains and as discontinuous rims on partially resorbed pre m2 monazites that were liberated from garnet xenotime also occurs as small isolated grains within m2 coronas ion microprobe dating of thin high y rims indicates that new monazite growth occurred during m2 monazite xenotime miscibility gap temperatures are consistent with al solubility in orthopyroxene thermometry estimates indicating that peak temperatures in the inner aureole are accurately recorded and preserved by monazite m2 monazite records therefore the temperature and timing of m2 metamorphism two net transfer reactions modelled using singular value decomposition in the system p y hree lree are proposed to account for the growth of m2 phosphates 1 38 grt1 1 mnz1 1 13 mnz2 and 2 737 grt1 1 ap 1 mnz2 3 4 xno2 reaction 1 conserves p and gave rise to locally coronitic high y overgrowths on partially resorbed pre m2 monazite whereas reaction 2 accounts for the growth of small new monazite and xenotime grains both reactions were highly localized within individual m2 coronas due to slow intergranular diffusion accompanying fluid undersaturated metamorphism in the mlp aureole similar monazite forming reactions are expected in other polymetamorphosed granulites

arsenian monazite ce and xenotime y ree arsenates and carbonates from the tisovec rejkovo rhyolite western carpathians slovakia composition and substitutions in the ree y xo4 system x p as si nb s abstract a unique ree y th p as si nb s accessory assemblage was identified in a small body of lower triassic a type rhyolite of the silicic superunit near tisovec rejkovo central slovakia arsenian monazite ce phosphatian gasparite ce and arsenian xenotime y phosphatian chernovite y solid solutions in association with ree carbonates bastnasite parisite rontgenite synchysite and rare cerianite ce form anhedral to subhedral grains and aggregates 0 3 mm scattered in the groundass or as intergrowths with zircon and fe ti oxides compositions show a wide asp 1 substitution in monazite gasparite and xenotime chernovite solid solutions atom as as p 0 00 to 0 73 and 0 10 to 0 94 respectively the presence of s in monazite gasparite s s 0 14 apfu together with ca enrichment indicates clinoanhydrite substitution cas ree y 1 p as 1 rather than the incorporation of brabantite along the cath ree y 2 exchange vector moreover the th and nb rich members reveal thorite and fergusonite substitutions thsi y ree 1 p as 1 and nb p as 1 respectively the as rich ree phases originated probably during the post magmatic alteration of primary monazite ce and xenotime y by as rich high fo2 fluids whereas cerianite ce and the ree carbonates are products of a later hydrothermal overprint of the rhyolite by co2 rich solutions

monazite xenotime miscibility gap thermometry i an empirical calibration rare earth element ree and yttrium concentrations of coexisting monazite and xenotime were determined from a suite of seven metapelites from the variscan fold belt in ne bavaria germany the metapelites include a continuous prograde mainly low p 3 5 kbar metamorphic profile from greenschist c 400 c to lower granulite facies conditions c 700 c the lree la sm are incorporated preferentially in monoclinic monazite reo9 polyhedron whereas the hree plus y are concentrated in tetragonal xenotime reo8 polyhedron the major element concentrations of both phases in all rocks are very similar and do not depend on metamorphic grade monazite consists mainly of la ce and nd la0 20 0 23 ce0 41 0 45 nd0 15 0 18 po4 all other elements are below 6 mol likewise xenotime consists mainly of ypo4 with some dy and gd solid solutions y0 76 0 80 dy0 05 0 07 gd0 04 0 06 in contrast the minor hree concentrations in monazite increase strongly with increasing metamorphic grade y dy and gd increase by a factor of 3 5 from greenschist to granulite facies rocks monazite crystals often show zonation with cores low in hree and rims high in hree that is interpreted as growth zonation attained during prograde metamorphism similarly sm and nd in xenotimes increase by a factor of 3 4 with increasing metamorphic grade prograde zonation in single crystals of xenotime was not observed the xhree y in monazite and xlree in xenotime of the seven rocks define two limbs along the strongly asymmetric miscibility gap from c 400 c to 700 c the empirical calibration of the monazite miscibility gap limb coexisting with xenotime is appropriate for geothermometry due to its contents of u and th monazite has often been used for u pb age determination the combination of our empirical thermometer on prograde zoned monazite along with possible age determination of zoned single crystals may provide information about prograde branches of temperature time paths

the nature of the 660 kilometer discontinuity in earth s mantle from global seismic observations of pp precursors the 660 kilometer discontinuity which separates earth s upper and lower mantle has been detected routinely on a global scale in underside reflections of precursors to ss shear waves here we report observations of this discontinuity in many different regions using precursors to compressional pp waves the apparent absence of such precursors in previous studies had posed major problems for models of mantle composition we find a complicated structure showing single and double reflections ranging in depth from 640 to 720 kilometers that requires the existence of multiple phase transitions at the base of the transition zone the results are consistent with a pyrolite mantle composition

plastic deformation of garnets systematics and implications for the rheology of the mantle transition zone abstract plastic properties of materials with garnet structure have been studied under wide temperature conditions ranging from room temperature to 95 of the melting temperatures using uniaxial compression and hot microhardness tests garnets studied include single crystals of oxide garnets y 3 al 5 o 12 gd 3 ga 5 o 12 and y 3 fe 5 o 12 and silicate garnets various solid solutions including grossular almandine andradite pyrope spessartine and uvarovite both uniaxial compression and hot hardness tests indicate that there is a general trend in the plasticity of garnets when the data are compared at normalized conditions t t m and and that the resistance to plastic deformation in garnets is significantly higher than most of the other minerals in the earth s mantle based on both stress dip tests and microstructural observations it is proposed that the creep strength of garnet is largely controlled by the resistance to dislocation glide rather than by recovery processes this conclusion is consistent with the high peierls stress inferred from the hot hardness tests the high peierls stress in garnets is presumably due to the large unit cell i e long burgers vectors and or the bcc packing which are common to all garnets we postulate therefore that the present results can be applied to the strength of high pressure garnet majorite and suggest that garnet rich layers in the earth such as subducted oceanic crust in the transition zone or a possibly garnet rich bottom part of the transition zone will be considerably stronger than surrounding regions

accessory mineral behavior during differentiation of a granite suite monazite xenotime and zircon in the sweetwater wash pluton southeastern california u s a abstract compositional and textural characteristics of the accessory minerals monazite xenotime and zircon in the sweetwater wash granites and related aplites indicate that these phases not only controlled much of the trace element geochemistry of the suite but that they also record the melt compositional changes that occurred during magmatic differentiation fractionation of monazite due to decreasing saturation levels of its essential structural constituents with falling temperature was likely responsible for an ongoing trend of light rare earth element ree depletion this was accompanied by increasing heavy ree concentrations until xenotime joined the crystallizing assemblage subsequently combined monazite xenotime fractionation resulted in lowering of the entire ree budget zircon which contains inherited cores that were apparently resorbed and rounded during initial anatexis was saturated throughout the differentiation history of the sweetwater wash suite accessory phases in granite and aplite exhibit strong compositional differences at a variety of scales the differences are best displayed by monazite on average crystals in more differentiated rocks aplites are relatively depleted in light ree s and contain higher concentrations of the substituting elements u and th zircon displays complimentary increases in hf and y while xenotime shows a slight increase in th and in gd ho ratios both phases also exhibit higher u concentrations in aplites than in granites these differences in accessory mineral compositions are observed not only between granites and the more differentiated aplites but also within individual thin sections due to in situ fractionation on an even smaller scale strong compositional variations are present within single crystals possibly due to diffusion controlled melt compositional gradients in the regions a adjacent to growing major phases and b adjacent to the growing accessory crystal itself our observations indicate that compositional variations among accessory minerals are potentially useful for tracking of magmatic processes but that the scale of observed variations must be carefully considered

high pbehavior of anorthite composition and some phase relations of the cao al2o3 sio2 system to the lower mantle of the earth and their geophysical implications 1 multianvil experiments with long experimental durations have been made with the anorthite composition caal2si2o8at pressure temperature p t conditions of 14 25 gpa and 1400 2400 c at subsolidus conditions these experiments demonstrated three phase assemblages grossular gr kyanite ky stishovite st at 14 gpa gr calcium alumino silicate phase cas st at 18 gpa and cas casio3 perovskite capv st at above 20 gpa which are related by the reactions gr ky cas st and gr st cas capv following the method of schreinemakers we combined our data with the literature data to deduce ap tphase diagram for a portion of the cao al2o3 sio2system at subsolidus conditions which subsequently helped to solve some long lasting discrepancies in the high pbehavior of the compositions of anorthite and grossular the crystal chemistry of the cas and capv solid solutions was examined and new substitution mechanisms were firmly established along the solidus the melting reaction at 14 gpa is peritectic while that at 22 gpa is eutectic for both pressures st is the first phase to melt out and the melt is generally andesitic for the an composition its density starts to be significantly higher than the density of pyrolite at 2 5 gpa a much lower pressure than that for the or ab or qtz composition 7 5 10 gpa so that the an enriched continental crust material should readily plunge into the upper mantle

electron microscopy of high pressure phases synthesized from natural garnets in a diamond anvil cell implications for the mineralogy of the lower mantle abstract in order to identify the crystalline hosts for aluminium and calcium in the deep mantle the high pressure transformations of natural garnets pyrope grossular and fe rich garnet have been prepared at high pressures 40 and 50 gpa and temperatures between 1000 and 2500 k in a laser heated diamond anvil cell dac the recovered high pressure phases have been studied by analytical transmission electron microscopy atem the host minerals for al and ca appear to depend on temperature and iron content 1 at moderate temperature and without significant amounts of iron in the starting material the main host minerals for ca and al aluminium are casio 3 perovskite and a compound with an unknown structure and a ca mg al 2 sio 6 composition 2 at higher temperature the main host minerals for ca and al are respectively casio 3 perovskite and a new high pressure phase of al 2 sio 5 having the v 3 o 5 structure 3 when the iron content is sufficiently large in the starting material fe fe mg 15 the major host mineral for ca and al is a compound with a hollandite type structure and a ca mg fe al 2 si 2 o 8 composition whatever the temperature the solubility of al in mg perovskite was also studied it appears that this solubility strongly decreases with increasing pressure and slightly increases with temperature such behaviour of al in the deep mantle could lead to the occurrence of a progressive phase change in the lower mantle mineralogical models of the deep mantle are then discussed in the light of the above results

phase transition in casio3 perovskite abstract a phase transition in pure casio3 perovskite was investigated at 27 to 72 gpa and 300 to 819 k by in situ x ray diffraction experiments in an externally heated diamond anvil cell the results show that casio3 perovskite takes a tetragonal form at 300 k and undergoes phase transition to a cubic structure above 490 580 k in a pressure range studied here the transition boundary is strongly temperature dependent with a slightly positive dt dp slope of 1 1 1 3 k gpa it is known that the transition temperature depends on al2o3 content dissolved in casio3 perovskite kurashina et al phys earth planet inter 145 2004 67 74 the phase transition in casio3 3 wt al2o3 perovskite therefore could occur in a cold subducted mid oceanic ridge basalt morb crust at about 1200 k in the upper to mid lower mantle this phase transition is possibly ferroelastic type and may cause large seismic anomalies in a wide depth range

subduction of continental crust and terrigenous and pelagic sediments an experimental study abstract an important role for the recycling of terrigenous sediments in the mantle was proposed by armstrong 1 2 aspects of this hypothesis have been tested via an experimental study of phase equilibria in a composition similar to that of average upper continental crust over the pressure interval 6 24 gpa this composition is sufficiently close to those of the principal argillaceous and siliceous classes of pelagic sediments to use the results to interpret the behaviour of these latter sediments in addition to terrigenous sediments during subduction the subsolidus phase relationships displayed by lithologies derived from these materials are complex with major roles being played by k hollandite garnet na clinopyroxene and stishovite continentally derived lithologies achieve densities similar to or greater than surrounding mantle at depths greater than 200 km the geodynamical implications of these buoyancy relationships are discussed a reconnaissance study has also been made to assess the melting and element partitioning behaviours in the subducted continental crust lithologies during partial melting at relatively low pressures 5 10 gpa orthoclase wadeite and k hollandite are eliminated near the solidus whereas the stability field of na clinopyroxene extends to temperatures well above the solidus liquids formed by small to moderate degrees of partial melting of subducted continentally derived lithologies in this pressure interval have high k na ratios and high sio2 contents at higher pressures 16 24 gpa the stability fields of k hollandite and stishovite extend towards the liquidus whereas sodium bearing phases are eliminated much closer to the solidus resultant partial melts consequently possess relatively low k na ratios and sio2 contents lead possesses a high crystal liquid partition coefficient in k hollandite whereas uranium is excluded the broad stability field of k hollandite during partial melting of these lithologies at pressures above 15 gpa therefore has the capacity to cause fractionation of lead from uranium the petrological and geochemical implications of these results are discussed in the context of the partial melting of terrigenous materials during subduction and the interactions of the partial melts with the surrounding mantle

geologic constraints on the chaotic diffusion of the solar system the correlation of earth s orbital parameters with climatic variations has been used to generate astronomically calibrated geologic time scales of high accuracy however because of the chaotic behavior of the solar system two initially close calculations of earth s orbit diverge exponentially and have a large uncertainty beyond several million years in the past this chaotic behavior is related to a combination of angles in the precession motion of the orbits of earth and mars which currently is in resonance how long stays in libration critically depends on the dynamical model and initial conditions for the solar system here we show that geologic data can differentiate between astronomical solutions that do and do not exhibit a transition in since 40 ma and that sediments can thus provide a history for the evolution of we find that the chaotic transition of from libration to circulation did not occur after ca 30 ma we can thus constrain the chaotic diffusion of the solar system in the past and our results provide new and challenging constraints for astronomical models

a new natural high pressure na ca hexaluminosilicate caxna1 x al3 xsi3 xo11 in shocked martian meteorites a ca na hexaluminosilicate whose ca end member was previously synthesized in numerous high pressure experiments has been identified by raman spectroscopy in heavily shocked martian meteorites this mineral has a structural formula close to caxna1 x al3 xsi3 xo11 and is similar to the calcium aluminum silicate phase previously synthesized in high pressure experiments performed on anorthite and rocks of basaltic composition this new mineral occurs in shock melt pockets in two distinct settings and is intimately intergrown with sio2 stishovite the first setting encountered in zagami consists of idiomorphic equant crystals overgrown by acicular stishovite that crystallized from a melt of labradorite composition the second setting contains the na ca hexaluminosilicate phase intergrown with stishovite and hollandite and was formed during partial melting at high pressures the mineralogical association na ca hexaluminosilicate stishovite was observed in shock melt pockets which have distinct bulk compositions in seven martian shergottites this new mineral represents after majorite the second natural occurrence of a silicate mineral with silicon in both four and six coordination the assemblage stishovite na ca hexalummosilicate sets constraints on the pressure and temperature conditions that prevailed during shock in some of the studied meteorites the na ca hexaluminosilicate mineral is a potential carrier of al and na during subduction of oceanic crust in the lower mantle of the earth c 2003 elsevier b v all rights reserved

removal of oceanic ree by authigenic precipitation of phosphatic minerals abstract early diagenetic phosphatic minerals containing rare earth elements ree are widespread in australian shallow marine sandstones of all ages most common is the aluminophosphate florencite ree al 3 po 4 2 oh 6 but the isomorphous and chemically intergrading phases crandallite gorceixite and goyazite all with up to several percent ree substituted for their major cation ca ba and sr respectively may also be present together they comprise up to 0 144 wt average 0 0134 wt of their host rocks the authigenic phosphates xenotime and apatite are also widespread but less common they all apparently precipitated within the diagenetic zones of sulphate reduction and methanogenesis as a result of reverse weathering reactions after the release of ree and po 4 by reduction of fe mn oxyhydroxides and the decomposition of organic material coating detrital particles as the aluminophosphates and apatite are enriched in light ree whereas xenotime concentrates heavy ree they collectively deposit light ree at a much greater rate la 129 10 9 g cm 2 yr 1 than heavy yb 0 40 10 9 g cm 2 yr 1 the total burial flux of light ree by shallow marine phosphatic precipitation is about 50 times greater than previously determined for deep sea sediments whereas heavy ree are buried at comparable rates this new sink produces a major imbalance in the marine ree budget requiring the existence of a substantial but currently unknown source to restore balance its size has important implications for the use of ree abundances distributions and isotopes as monitors of marine geochemical processes

an experimental study of the garnet perovskite transformation in the system mgsio3mg3al2si3o12 phase transformations in the system mgsio3 mg3al2si3o12 have been studied at pressures between 22 and 28 gpa at 1500 c using a multi anvil apparatus majorite garnet with less than about 10 wt al2o3 was found to completely transform into an orthorhombic perovskite structure over a pressure interval between 24 and 27 gpa whereas pyrope garnet disproportionated into sub aluminous perovskite plus corundum solid solutions at pressures greater than 26 5 gpa alumina contents in the perovskite structure are dependent on temperature and increase from 11 wt at 1500 c to 17 wt at 1800 c at a pressure of 27 gpa the alumina content of perovskite also in creases with increasing pressure such that aluminous perovskite with the pyrope composition 25 mol al2o3 should be formed by about 30 40 gpa these observations are consistent with our recent experimental results on a pyrolite composition which demonstrated that no separate aluminous phase exists in the upper part of the lower mantle the entire inventory of al2o3 in pyrolite can be accommodated in solid solution in magnesium silicate perovskite

phase relations of caal4si2o11 at high pressure and high temperature with implications for subducted continental crust into the deep mantle abstract phase relations of the cas phase caal4si2o11 have been investigated at pressures of 10 23 gpa and at temperatures of 1000 1600 c by means of the quench method using the kawai type high pressure apparatus an assemblage of grossular gr corundum cor kyanite ky is stable up to about 10 gpa and at the temperatures examined the assemblage converts into the cas phase at 1600 c and higher pressures below 1450 c however the assemblage first changes to that of gr cor stishovite st corresponding to the dissociation of ky into cor st and then converts to the cas phase with increasing pressure the gr cor st cas boundary has a negative dp dt slope and the gr ky cor cas boundary has a near horizontal slope the triple point at which gr cor ky st and the cas phase coexist is located at around 11 5 gpa and 1450 c stability of the cas phase up to 23 gpa and 1600 c indicates that the cas phase is an important host of al and ca in the continental crust subducted into deep mantle comparison of the densities between the pyrolitic mantle and the subducted continental crust indicates that the continental crust provides a plunging force to the slab subducted into the upper mantle and transition zone whereas the continental crust in turn gives a buoyancy in the lower mantle as the consequence of the substantial changes in mineral assemblage in both the continental crust and pyrolitic mantle through the 660 km discontinuity the formation of the cas phase plays an important role in changing the density of the subducting continental crust

understanding geologic processes with xenotime composition chronology and a protocol for electron probe microanalysis abstract xenotime ypo 4 is iso structural with zircon group minerals and lies at one end of a compositional solid solution with monazite as with monazite and zircon naturally occurring xenotime may accommodate significant concentrations of rare earth elements as well as th and u making it an important repository for these valuable geochemical marker elements and a potential geochronometer xenotime grows in sediments during diagenesis in low to granulite grade metamorphic rocks in migmatites and is common in peraluminous igneous rocks xenotime can be a complement or indeed an alternative to monazite and or zircon in geochemical and geochronological studies to maximise the petrographic potential of xenotime a complete understanding of its composition and relationship to rock forming assemblages is essential electron beam techniques provide micron scale resolution of compositional and textural variation and yield precise dates for individual micro volumes in multi domain grains methods for collecting trace element data by electron probe microanalysis differ from those routinely employed to collect major element concentrations careful characterisation of background curvature and background interferences around the th u and pb peaks is required to avoid large overlap correction for y l 2 3 on pb m analysis of pb is made on the m peak a broad array of geochemical and geochronological applications can take advantage of the links between xenotime composition and texture and the evolution of silicate mineral assemblages in a diversity of environments one example of metasomatised xenotime from a pegmatite in sw norway demonstrates a practical application to dating geologic processes as well as the potential limitations that arise in compositionally heterogeneous xenotime

magmatic to hydrothermal crystallization in the w sn mineralized mole granite nsw australia part i crystallization of zircon and ree phosphates over three million years a geochemical and u pb geochronological study abstract zircon monazite and xenotime crystallized over a temperature interval of several hundred degrees at the magmatic to hydrothermal transition of the sn and w mineralized mole granite magmatic zircon and monazite thought to have crystallized from hydrous silicate melt were dated by conventional u pb techniques at an age of 247 6 0 4 and 247 7 0 5 ma respectively xenotime occurring in hydrothermal quartz is found to be significantly younger at 246 2 0 5 ma and is interpreted to represent hydrothermal growth from associated fluid inclusions it is concluded that it precipitated from a hydrothermal brine 600 c which is below the accepted closure temperature for u pb in this mineral these data are compatible with a two stage crystallization process precipitation of zircon and monazite as magmatic liquidus phases in deep crustal magma followed by complete crystallization and intimately associated sn w mineralization after intrusion of the shallow sill like body of the mole granite later hydrothermal formation of monazite in a biotite fluorite topaz reaction rim around a mineralized vein was dated at 244 4 1 4 ma which distinctly postdates the mole granite and is possibly related to a younger hidden intrusion and its hydrothermal fluid system obtaining precise age data for magmatic and hydrothermal minerals of the mole granite is hampered by uncertainties introduced by different corrections required for multiple highly radiogenic minerals crystallising from evolved hydrous granites including 230 th disequilibrium due to th u fractionation during monazite and possibly xenotime crystallization variable th u ratios of the fluids from which xenotime was precipitating elevated contents of common lead and post crystallization lead loss in zircon enhanced by the fluid saturated environment the data imply that monazite can also survive as a liquidus phase in protracted magmatic systems over periods of 10 6 years the outlined model is in agreement with prominent chemical core rim variation of the zircon