differentiation and crystallization conditions of basalts from the kerguelen large igneous province an experimental study phase relations of basalts from the kerguelen large igneous province have been investigated experimentally to understand the effect of temperature fo2 and fugacity of volatiles e g h2o and co2 on the differentiation path of lip basalts the starting rock samples were a tholeiitic basalt from the northern kerguelen plateau odp leg 183 site 1140 and mildly alkalic basalt evolved from the kerguelen archipelago mt crozier on the courbet peninsula representing different differentiation stages of basalts related to the kerguelen mantle plume the influence of temperature water and oxygen fugacity on phase stability and composition was investigated at 500 mpa and all experiments were fluid saturated crystallization experiments were performed at temperatures between 900 and 1 160 c under oxidizing log fo2 qfm 4 and reducing conditions log fo2 qfm in an internally heated gas pressure vessel equipped with a rapid quench device and a pt membrane for monitoring the fh2 in all experiments a significant influence of the fo2 on the composition and stability of the mg fe bearing mineral phases could be observed under reducing conditions the residual melts follow a tholeiitic differentiation trend in contrast melts have high mg mg2 mg2 fe2 and follow a calk alkalic differentiation trend at oxidizing conditions the comparison of the natural phenocryst assemblages with the experimental products allows us to constrain the differentiation and pre eruptive conditions of these magmas the pre eruptive temperature of the alkalic basalt was about 950 1 050 c the water content of the melt was below 2 5 wt h2o and strongly oxidizing conditions log fo2 qfm 2 were prevailing in the magma chamber prior to eruption the temperature of the tholeiitic melt was above 1 060 c with a water content below 2 wt h2o and a log fo2 qfm 1 early fractionation of clinopyroxene is a crucial step resulting in the generation of silica poor and alkali rich residual melts e g alkali basalt the enrichment of alkalis in residual melts is enhanced at high fo2 and low ah2o

petrologic and stable isotope constraints on the source and evolution of skarn forming fluids at pine creek california a detailed geochemical analysis of the well exposed multistage pine creek california tungsten deposit has constrained the roles of magmatic metamorphic and meteoric fluids in the development of the zoned scheelite ore and calc silicate skarn metamorphic conditions of p 2 kb t 600 degrees c and x sub co 2 0 25 are recorded in the metasedimentary rocks of the septum metamorphism yielded calcite diopside graphite and calcite dolomite periclase brucite forsterite assemblages in the variably dolomitic siliceous carbonate rocks of the pine creek septurn following intrusion of the morgan creek quartz monzonite metasomatism developed the zoned endoskarn exoskarn at the contact the massive garnet andradite sub 20 50 pyroxene hedenbergite sub 20 80 or quartz skarn assemblage carries most of the scheelite calculated t x sub co 2 equilibria corrected for solid solution effects restrict skarn formation to x sub co 2 0 2 at t 500 degrees or 50 degrees c calculated f sub o 2 f sub f 2 f sub s 2 diagrams indicate f sub o 2 values near quartz fayalite magnetite and f sub s 2 values well below pyrrhotite pyrite during skarn formation slightly falling temperatures were accompanied by increased f sub s 2 and hydration of the skarn assemblages stable isotope data for calcites show a mixing trend from unaltered marble delta 18 o 24ppm delta 13 c 3 5 degrees to values in equilibrium with magmatic fluids delta 18 o 8ppm delta 13 c 6ppm biotite quartz thermometry for the quartz monzonite yields temperatures of 410 degrees to 460 degrees c well below the solidus and suggesting later reequilibration pyroxene quartz analyses suggest that these minerals equilibrated at temperatures of 460 degrees to 560 degrees c the delta 18 o values measured for garnets suggest disequilibrium for the garnet quartz and garnet pyroxene pairs d h results for epidote and biotite suggest that meteoric water influx became more important with time sulfur isotope data are consistent with a reduced magmatic source

magnetite melt hfse partitioning abstract results from doped hydrous experiments on natural mafic to intermediate composition lavas at 2 5 kbar pressure were combined with existing 1 atm data to evaluate the effects of composition and temperature on the partitioning behavior of the high field strength elements hfse zr nb ta and hf between magnetite and natural silicate melts magnetite composition was found to be the strongest controlling factor on partitioning behavior the partition coefficients d for zr nb hf and ta correlate with d ti ti and al content of the magnetite temperature and pressure the partition coefficients for the hfse are similar to one another for any given magnetite melt pair but range from 2 in titanomagnetite in addition the relationship between ti and the hfse changes as a function of pressure and temperature with the hfse becoming more incompatible relative to ti at lower temperatures and or higher pressures this change in the relationship between d ti and d hfse with temperature and pressure means that the expressions presented in nielsen et al 1994 nielsen r l forsythe l m gallaghan w e fisk m r 1994 major and trace element magnetite melt partitioning chem geol 117 167 191 are not valid for hydrous aluminous systems expressions were derived to describe the relationship between d hfse and temperature pressure fe 2 mg exchange ti al ratio of the magnetite and d ti these expressions reproduce the input data within 35 50 1 over a range extending from highly incompatible to compatible

an experimental investigation of the influence of water and oxygen fugacity on differentiation of morb at 200 mpa crystallization experiments were performed at 200mpa in the temperature range 1150 950 c at oxygen fugacities corresponding to the quartz fayalite magnetite qfm and mno mn3o4 buffers to assess the role of water and fo2 on phase relations and differentiation trends in mid ocean ridge basalt morb systems starting from a primitive mgo 9 8 wt and an evolved morb mgo 6 49 wt crystallization paths with four different water contents 0 35 4 7 wt h2o have been investigated in primitive morb olivine is the liquidus phase followed by plagioclase clinopyroxene amphibole is present only at water saturated conditions below 1000 c but not all fluid saturated runs contain amphibole magnetite and orthopyroxene are not stable at low fo2 qfm buffer residual liquids obtained at low fo2 show a tholeiitic differentiation trend the crystallization of magnetite at high fo2 mno mn3o4 buffer results in a decrease of melt feo mgo ratio causing a calc alkaline differentiation trend because the magnetite crystallization temperature is nearly independent of the h2o content in contrast to silicate minerals the calc alkaline differentiation trend is more pronounced at high water contents residual melts at 950 c in a primitive morb system have compositions approaching those of oceanic plagiogranites in terms of sio2 and k2o but have ca na ratios and feo contents that are too high compared with the natural rocks implying that fractionation processes are necessary to reach typical compositions of natural oceanic plagiogranites

geothermometry using minor and trace elements in igneous and hydrothermal magnetite abstract the temperature t and oxygen fugacity fo2 dependence of minor and trace elements mg al mn cr ni in magnetite mag in equilibrium with water bearing silicate liquids varying in composition from basalt to rhyolite 49 77 wt sio2 is examined in a compilation of experimental data n 299 from 700 to 1050 c 50 900 mpa and five order of magnitude range in fo2 the xmg mg mg fetot of mag is strongly dependent on t and can be formulated into an empirical geothermometer tmg mag c 8344 320 lnxmg 4 1 0 28 273 r2 0 83 that reproduces 72 of the experimentas to 50 c and conventional fe ti oxide thermometry in tuffs and vitrophyres to better than 60 c for 85 of the samples the tmg mag thermometer may have potential application in igneous or hydrothermally altered rocks where conventional thermometry or oxybarometry is not possible application to natural mag n 476 compiled from several settings shows igneous rocks mostly record 700 c but with some lower temperatures in plutonic rocks due to slow cooling hydrothermal samples skarns porphyry deposits show consistently subsolidus conditions tmg mag of 700 350 c consistent with other mineral and o isotopic equilibria the element correlations in natural mag from various settings are explored using multivariate statistical analysis to identify which minor or trace elements most critically depend on t fo2 or fluid composition

paragenesis elemental distribution and stable isotopes at the pena colorada iron skarn colima mexico the pena colorada iron skarn is located in the southern part of the guerrero terrane it contains 150 million metric tons mt with a grade of 36 percent magnetite the deposit occurs at the contact of a 68 ma equigranular diorite with mid cretaceous volcano sedimentary rocks of the tepalcatepec formation volcanic units within this formation have tholeiitic affinity and ree patterns that are compatible with a primitive arc setting the pena colorada diorite and associated voluminous aplitic phases are part of a late cretaceous calc alkaline continental arc that subsequently intruded this basin northeast vergent deformation affected the region in the late cretaceous north south and east west reverse and northwest and northeast strike slip faults localize the mineralization the alteration halo around the diorite intrusion is 500 m wide within an inner 200 m halo the volcano sedimentary section was affected by an early metamorphic event with discontinuous bands of pyroxene di 68 hd 29 jo 3 hornfels and garnet gr 61 am 36 ad 3 disseminations metasomatic pyroxene garnet and plagioclase cut the early metamorphic event preferentially replacing carbonate and volcanic units up to and including a 35 m thick marl located 200 m above the intrusion metasomatic pyroxene and garnet have average compositions of di 77 hd 21 jo 2 and ad 76 gr 20 am 2 sp 1 py 1 respectively pyroxene compositions reach di 94 hd 5 jo 1 whereas garnet exhibits intragrain compositional zoning from ad 49 gr 46 in the core to ad 100 gr 0 in the rim albitization of preexisting protoliths modified igneous plagioclase compositions from ab 67 an 32 or 1 up to ab 96 an 3 or 1 albitization is followed by abundant fracture controlled fe epidote and cl bearing chlorite cha 84 cli 14 pen 2 that extend from the marl unit outward some 300 m this association is cut by a later assemblage of epidote chlorite prehnite that affects the entire 500 m width of the alteration halo and part of the intrusion chlorite cha 54 cli 45 pen 1 from this later event is richer in mg than that from the previous association igneous hornblende in the intrusion is altered to actinolite a potassic alteration event related to a hydrothermal breccia and aplitic dikes overprints the pre existing calc silicate associations alteration minerals in the breccia and the 200 m potassic halo around it evolve from early k feldspar biotite quartz to late jasperoid fluorapatite calcite veins mg rich biotite phl 77 ann 23 and ree rich apatite contain f cl oh ratios of 1 0 3 and 1 0 0 respectively massive magnetite 36 specular hematite 7 sulfide o c s and h stable isotope determinations on minerals from the calc silicate association are consistent with an igneous source for the initial fluids which mixed outward with an increasing component of trapped seawater or evaporite however water in equilibrium with magnetite exhibits unusually enriched 18 o values relative to other coexisting mineral derived fluids a solution that equilibrated with limestone at relatively high temperature is required to explain these heavy values

petrological and experimental constraints on the pre eruption conditions of holocene dacite from volcan san pedro 36 s chilean andes and the importance of sulphur in silicic subduction related magmas we present an experimental and petrological study aimed at estimating the pre eruptive conditions of a holocene dacitic lava from volcan san pedro 36 s chilean andes phase equilibrium experiments were performed at temperatures t from 800 to 950 c and mainly at 200 mpa but also at 55 150 and 406 mpa oxygen fugacity fo2 ranged from the ni nio buffer nno to 3 5 log units above nno 3 5 and water contents from 3 to 6 wt we also report several experiments where we added sulphur 0 1 1 wt s to the dacite the main mineral assemblage of the dacite hornblende orthopyroxene plagioclase is stable at 200 50 mpa 850 10 c with 4 5 5 5 wt h2o in the melt and at fo2 of nno 1 2 0 2 in accord with the crystallinity mineral proportions and t fo2 determination from fe ti oxides of the lava however biotite which is also present in the dacite is stable at these same t fo2 conditions only in experiments with 0 1 wt s added this result is in accord with the occurrence of pyrrhotite in the lava and with the presence of s in glass inclusions and biotite 300 ppm and up 170 ppm respectively moreover the zoning patterns and compositions of plagioclase phenocrysts together with the presence of high temperature minerals e g clinopyroxene in the lava suggest that the petrological history of the dacite is more complex than a single near equilibrium crystallization stage and could be explained by short lived 100 years temperature fluctuations 50 c in the magma reservoir

trace elements in magnetite as petrogenetic indicators we have characterized the distribution of 25 trace elements in magnetite mg al si p ca sc ti v cr mn co ni cu zn ga ge y zr nb mo sn hf ta w and pb using laser ablation icp ms and electron microprobe from a variety of magmatic and hydrothermal ore forming environments and compared them with data from the literature we propose a new multielement diagram normalized to bulk continental crust designed to emphasize the partitioning behavior of trace elements between magnetite the melt fluid and co crystallizing phases the normalized pattern of magnetite reflects the composition of the melt fluid which in both magmatic and hydrothermal systems varies with temperature thus it is possible to distinguish magnetite formed at different degrees of crystal fractionation in both silicate and sulfide melts the crystallization of ilmenite or sulfide before magnetite is recorded as a marked depletion in ti or cu respectively the chemical signature of hydrothermal magnetite is distinct being depleted in elements that are relatively immobile during alteration and commonly enriched in elements that are highly incompatible into magnetite e g si and ca magnetite formed from low temperature fluids has the lowest overall abundance of trace elements due to their lower solubility chemical zonation of magnetite is rare but occurs in some hydrothermal deposits where laser mapping reveals oscillatory zoning which records the changing conditions and composition of the fluid during magnetite growth this new way of plotting all 25 trace elements on 1 diagram normalized to bulk continental crust and elements in order of compatibility into magnetite provides a tool to help understand the processes that control partitioning of a full suit of trace elements in magnetite and aid discrimination of magnetite formed in different environments it has applications in both petrogenetic and provenance studies such as in the exploration of ore deposits and in sedimentology

trace elements in magnetite from porphyry cu mo au deposits in british columbia canada abstract this study examines trace elements in hydrothermal magnetite from five porphyry cu mo au deposits and two skarns in british columbia canada trace element concentrations vary several orders of magnitude both within and between magnetite from skarn and porphyry deposit settings the heterogeneous composition of hydrothermal magnetite may in part be due to the short duration low temperature and multiple fluid events that attend the formation of porphyry ore deposits principal component analysis shows two dominant patterns of trace element abundances in hydrothermal magnetite firstly positive correlations of ti al and v which account for nearly 40 of the total variation in magnetite are inferred to depend on temperature and oxygen fugacity secondly antithetic abundances of lower valence cations co mn with higher valence cations sn and mo may reflect variations in the redox potential acidity and metal speciation of hydrothermal fluids the cu fe and mn fe ratios calculated for fluids in equilibrium with the hydrothermal magnetite using experimental partitioning data are similar to those measured directly in brines trapped in quartz hosted fluid inclusions from porphyry cu mo au deposits

phase equilibria of the lyngdal granodiorite norway implications for the origin of metaluminous ferroan granitoids the proterozoic 950 ma lyngdal granodiorite of southern norway belongs to a series of hornblende biotite metaluminous ferroan granitoids hbg suite coeval with the post collisional rogaland anorthosite mangerite charnockite amc suite this granitoid massif shares many geochemical characteristics with rapakivi granitoids yet granodiorites dominate over granites to constrain both crystallization p t fo2 h 2o in melt and magma generation conditions we performed crystallization experiments on two samples of the lyngdal granodiorite with 60 and 65 wt sio2 at 4 2 kbar mainly at fo2 of nno nickel nickel oxide to nno 1 and under fluid saturated conditions with various h2o co2 ratios for each temperature comparison between experimental phase equilibria and the mineral assemblage in the lyngdal granodiorite indicates that it crystallized between 4 and 2 kbar from a magma with 5 6 wt h2 oa t an fo2 of nno to nno 1 these oxidized and wet conditions sharply contrast with the dry and reduced conditions inferred for the petrogenesis of the amc suite and many other rapakivi granites worldwide the high liquidus temperature and h2o content of the lyngdal granodiorite imply that it is not a primary magma produced by the partial melting of the crust but is derived by the fractionation of a mafic magma lyngdal type magmas appear to have volcanic equivalents in the geological record in particular our results show that oxidized high silica rhyolites such as the bishop tuff could be derived via fractionation of oxidized intermediate magmas and do not necessarily represent primary crustal melts this study underlines the great variability of crystallization conditions from anhydrous to hydrous and reduced to oxidized and petrogenetic processes among the metaluminous ferroan magmas of intermediate compositions granodiorites quartz mangerites quartz latites suggesting that there is not a single model to explain these rocks

a new fe mn geothermometer for hydrothermal systems implications for high salinity fluids at 13 n on the east pacific rise abstract field and experimental investigations demonstrate the chemistry of mid ocean ridge hydrothermal vent fluids reflects fluid mineral reaction at higher temperatures than those typically measured at the seafloor to account for this and in turn be able to better constrain sub seafloor hydrothermal processes we have developed an empirical geothermometer based on the dissolved fe mn ratio in high temperature fluids using data from basalt alteration experiments the relationship t c 331 24 112 41 log fe mn has been calibrated between 350 and 450 c the apparent fe mn equilibrium demonstrated by the experimental data is in good agreement with natural vent fluids suggesting broad applicability when used in conjunction with constraints imposed by quartz solubility associated sub seafloor pressures can be estimated for basalt hosted systems as an example this methodology is used to interpret new data from 13 n on the east pacific rise where high temperature fluids both enriched and depleted in chloride 339 646 mmol kg relative to seawater are actively venting within a close proximity accounting for these variable salinities active phase separation is clearly taking place at 13 n yet the fluid fe mn ratios and the silica concentrations suggest equilibration at temperatures less than those coinciding with the two phase region these data show the chloride enriched fluid reflects the highest temperature and pressure 432 c 400 bars of equilibration consistent with circulation near the top of the inferred magma chamber this is in agreement with the elevated co 2 concentration relative to the chloride depleted fluids the noted temperature derived from the fe mn geothermometer is higher than the critical temperature for a fluid of equivalent salinity this carries the important implication that despite being chloride enriched relative to seawater these fluids evolved as the vapor component of even higher salinity brine

dissolution reprecipitation process of magnetite from the chengchao iron deposit insights into ore genesis and implication for in situ chemical analysis of magnetite abstract magnetite formed in different environments commonly has distinct assemblages and concentrations of trace elements that can potentially be used as a genetic indicator of this mineral and associated ore deposits in this paper we present textural and compositional data of magnetite from the chengchao iron deposit daye district china to provide a better understanding in the formation mechanism and genesis of the deposit and shed light on analytical protocols for in situ chemical analysis of hydrothermal magnetite magnetite grains from the ore related granitoid pluton mineralized endoskarn magnetite dominated exoskarn and vein type iron ores hosted in marine carbonate intruded by the pluton were examined using scanning electron microscopy and analyzed for major and trace elements using electron microprobe back scattered electron images reveal that primary magnetite from the mineralized skarns and vein type ores were all partly reequilibrated with late stage hydrothermal fluids forming secondary magnetite domains that are featured by abundant porosity and have sharp contact with the primary magnetite these textures are interpreted as resulting from a dissolution reprecipitation process of magnetite which however are mostly obscure under optically primary magnetite grains from the mineralized endoskarn and vein type ores contain high sio 2 0 92 3 21 wt al 2 o 3 0 51 2 83 wt and low mgo 0 15 0 67 wt whereas varieties from the exoskarn ores have high mgo 2 76 3 07 wt and low sio 2 0 03 0 23 wt and al 2 o 3 0 54 1 05 wt this compositional contrast indicates that trace element geochemical composition of magnetite is largely controlled by the compositions of magmatic fluids and host rocks of the ores that have reacted with the fluids compared to its precursor mineral secondary magnetite is significantly depleted in most trace elements with sio 2 deceasing from 1 87 to 0 47 wt on average and al 2 o 3 from 0 89 to 0 08 wt in mineralized endoskarn and vein type ores and mgo from 2 87 to 0 60 wt in exoskarn ores on the contrary average content of iron is notably increased from 69 2 wt to 71 9 wt in the secondary magnetite grains the results suggest that the dissolution reprecipitation process has been important in significantly removing trace elements from early stage magnetite to form high grade high quality iron ores in hydrothermal environments the textural and compositional data confirm that the chengchao iron deposit is of hydrothermal origin rather than being crystallized from immiscible iron oxide melts as previously suggested this study also highlights the importance of textural characterization using various imaging techniques before in situ chemical analysis of magnetite as is the case for texturally complicated u th bearing accessory minerals that have been widely used for u pb geochronology study

experimental determination of coexisting iron titanium oxides in the systems fetialo fetialmgo fetialmno and fetialmgmno at 800 and 900 c 1 4 kbar and relatively high oxygen fugacity a synthetic low melting rhyolite composition containing tio2 and iron oxide with further separate additions of mgo mno and mgo mno was used in hydrothermal experiments to crystallize ilm hem and usp mt solid solutions at 800 and 900 c under redox conditions slightly below nickel nickel oxide nno to approx 3 log 10 f text o 2 units above the nno oxygen buffer these experiments provide calibration of the feti oxide thermometer oxygen barometer at conditions of temperature and oxygen fugacity poorly covered by previous equilibrium experiments isotherms for our data in roozeboom diagrams of projected usp vs ilm show a change in slope at 60 ilm consistent with the second order transition from feti ordered ilm to feti disordered ilm hem this feature of the system accounts for some but not all of the differences from earlier thermodynamic calibrations of the thermobarometer in rhyolite containing 1 0 wt mgo 0 8 wt mno or mgo mno usp mt crystallized with up to 14 of aluminate components and ilm hem crystallized with up to 13 geikielite component and 17 pyrophanite component relative to the fetialo system these components displace the ferrite components in usp mt and the hematite component in ilm hem as a result projected contents of ulvospinel and ilmenite are increased these changes are attributed to increased non ideality along joins from end member hematite and magnetite to their respective mg and mn bearing titanate and aluminate end members the compositional shifts are most pronounced in ilm hem in the range ilm50 80 a solvus region where the chemical potentials of the hematite and ilmenite components are nearly independent of composition the solvus gap widens with addition of mg and even further with mn the bacon hirschmann correlation of mg mn in usp mt and coexisting ilm hem is displaced toward increasing mg mn in ilmenite with passage from ordered ilmenite to disordered hematite orthopyroxene and biotite crystallized in experiments with added mgo and mgo mno their x fe varies with log 10 f text o 2 and t consistent with equilibria among ferrosilite annite and ferrite components and the chemical potentials of sio2 and orthoclase in the liquid experimental equilibration rates increased in the order opx bt ilm hem usp mag

postcumulus modification of magnetite grains in the upper zone of the bushveld complex south africa abstract a detailed electron microprobe study has been made of magnetite grains from magnetitite layer 1 of the upper zone of the bushveld complex in order to establish if there is any evidence for postcumulus processes having affected magnetite compositions representative composition profiles obtained from detailed step traverses are presented for touching magnetite grains and where magnetite is in contact with either ilmenite or plagioclase magnetite grains in all textural settings are compositionally heterogeneous touching magnetite grains display systematic compositional zonations the rims of grains show a marked decrease in al and mg towards the grain boundary the cores are characterised by a distinctive peak and trough pattern for these elements these variations can be related to the positions of exsolved phases exsolutions in the cores are either pleonaste or an fe 3 bearing al mg spinel whereas those at the grain boundaries are generally more al rich and include corundum similar exsolution related compositional variations for mg and al were found in magnetites in contact with ilmenite and plagioclase a model has been developed which illustrates the possible sequence of postcumulus events during the interval between accumulation of the magnetite and the cessation of subsolidus reactions the essential feature of the model is the delicate interplay between depletion of the host magnetite by corundum pleonaste and the fe 3 bearing al mg spinel exsolutions and diffusion of al and mg into the depleted areas from the surrounding magnetite data for cr contents in magnetites show that although this particular trace element is apparently resistant to redistribution by exsolution processes it is affected by subsolidus re equilibration between magnetite and ilmenite or magnetite and intercumulus liquid

fractional crystallization of primitive hydrous arc magmas an experimental study at 0 7 gpa differentiation of mantle derived hydrous basaltic magmas is a fundamental process to produce evolved intermediate to sio2 rich magmas that form the bulk of the middle to shallow continental and island arc crust this study reports the results of fractional crystallization experiments conducted in a piston cylinder apparatus at 0 7 gpa for hydrous calc alkaline to arc tholeiitic magmas fractional crystallization was approached by synthesis of starting materials representing the liquid composition of the previous higher temperature experiment temperatures ranged from near liquidus at 1 170 c to near solidus conditions at 700 c h2o contents varied from 3 0 to more than 10 wt the liquid line of descent covers the entire compositional range from olivine tholeiite 1 170 c to high silica rhyolite 700 c and evolves from metaluminous to peraluminous compositions the following crystallization sequence has been established olivine clinopyroxene plagioclase spinel orthopyroxene amphibole titanomagnetite apatite quartz biotite anorthite rich plagioclase and spinel are responsible for a marked increase in sio2 content from 51 to 53 wt at 1 040 c at lower temperatures fractionation of amphibole plagioclase and fe ti oxide over a temperature interval of 280 c drives the sio2 content continuously from 53 to 78 wt largest crystallization steps were recorded around 1 040 c and at 700 c about 40 of ultramafic plutonic rocks have to crystallize to generate basaltic andesitic liquids and an additional 40 of amphibole gabbroic cumulate to produce granitic melts andesitic liquids with a liquidus temperature of 1 010 c only crystallize 50 over an 280 c wide range to 730 c implying that such liquids form mobile crystal mushes 50 crystals in long lived magmatic systems in the middle crust allowing for extensive fractionation assimilation and hybridization with periodic replenishment of more mafic magmas from deeper magma reservoirs

contrasted mineralogy and textural relationships in the uppermost titaniferous magnetite layers of the bushveld complex in the bierkraal area north of rustenburg disseminated fe ti oxides and ore rich concentrations of these minerals in a 1 230 m vertical borehole section through the top of the upper zone of the bushveld complex are described the succession consists of a layered sequence of highly fractionated olivine rich rocks that range in composition from ferrogabbro to diorite the fe content of the olivine increases systematically from fa 65 at the base to a maximum of fa 94 at the top of the section fe ti oxides axe present throughout this interval and are divided into three distinct groups on the basis of their mineralogy and chemistry 1 an apatite poor high ti magnetite association 2 an apatite rich high ti magnetite association and 3 an apatite poor low ti magnetite association the ti magnetite in each of these associations exhibits a wide variety of ulvoespinel and or ilmenite microintergrowths the development of which can be interpreted in terms of the ambient f sub o 2 at various stages of subsolidus cooling certain ti magnetite crystals are characterized by the development of radiating eutectoidlike intergrowths of magnetite and ilmenite or ulvoespinel that appear to represent duplex cell decomposition products of magnetite ulvoespinel solutions the apatite poor high ti magnetite represents the dominant opaque oxide association and is present both as disseminations and distinct ore rich layers particularly toward the base of the study section the greatest concentration of this association occurs at a depth of between 1 033 to 1 060 m below the top of the intrusion the lowermost 14 m of this interval consist of a thick composite ti magnetite layer that contains numerous silicate inclusions and partings this is believed to represent the equivalent of the uppermost ti magnetite layer layer 21 in the eastern bushveld complex groups of minor ti magnetite layers are also present at depths of 412 and 225 m below the top of the intrusion the ti content of the magnetite increases with increasing stratigraphic height from 55 mole percent ulvoespinel at the base to 65 mole percent near the top of the study section where it gives way to the ti poor association the beginning of ti poor magnetite precipitation 225 m below the roof of the intrusion marks the onset of ti depletion in the residual liquid ilmenite is virtually the only fe ti oxide phase present in the uppermost 100 m of the complex both these associations formed in response to normal fractional crystallization processes the formation of discrete ore rich layers being triggered by episodic increases in f sub o 2 the apatite rich high ti magnetite association is present both as disseminations and in ore rich concentrations but is restricted to three relatively narrow horizons within the study section these are developed at the following depths below the top of the intrusion 233 to 283 m 677 to 700 m and 1 033 to 1 108 m discrete ore rich concentrations consisting almost entirely of ti magnetite ilmenite approximately 70 vol and apatite approximately 30 vol are present in the lower 2 m of the middle horizon the upper and lower horizons contain only disseminated opaque oxide apatite assemblages the formation of these assemblages is interpreted in terms of the separation of immiscible fe ti mn ca p ree enriched liquids from the late stage dioritic residual melt

effect of oxygen fugacity and water on phase equilibria of a hydrous tholeiitic basalt the influence of oxygen fugacity and water on phase equilibria and the link between redox conditions and water activity were investigated experimentally using a primitive tholeiitic basalt composition relevant to the ocean crust the crystallization experiments were performed in internally heated pressure vessels at 200 mpa in the temperature range 940 1 220 c the oxygen fugacity was measured using the h2 membrane technique to study the effect of oxygen fugacity three sets of experiments with different hydrogen fugacities were performed showing systematic effects on the phase relations and compositions in each experimental series the water content of the system was varied from nominally dry to water saturated conditions causing a range of oxygen fugacities varying by 3 log units per series the range in oxygen fugacity investigated spans 7 log units systematic effects of oxygen fugacity on the stability and composition of the mafic silicate phases cr spinel and fe ti oxides under varying water contents were recorded the mg of the melt and therefore also the mg of olivine and clinopyroxene changed systematically as a function of oxygen fugacity an example of the link between oxygen fugacity and water activity under hydrogen buffered conditions is the change in the crystallization sequence olivine and cr spinel due to a change in the oxygen fugacity caused by an increase in the water activity the stability of magnetite is restricted to highly oxidizing conditions the absence of magnetite in most of the experiments allows the determination of differentiation trends as a function of oxygen fugacity and water content demonstrating that in an oxide free crystallization sequence water systematically affects the differentiation trend while oxygen fugacity seems to have a negligible effect

hydrothermally reset magmatic spinels in retrograde serpentinites formation of ferritchromit rims and chlorite aureoles serpentinized harzburgites from southern tuscany italy host different kinds of spinels a relic magmatic al spinels b hydrothermally altered spinels occurring as ferritchromit rims c syn and post serpentinization magnetites the composition of relic al spinels suggests 5 15 partial melting of a fertile spinel lherzolite after the main serpentinization al spinels are progressively replaced by ferritchromit rims by a dissolution recrystallization process transmission electron microscopic investigation shows that ferritchromit actually consists of a complex nanometric association of cr magnetite mg0 03 fe0 972 al0 11 cr0 89 fe3 1 00 o4 chlorite and lizardite with 001 chl liz always parallel to 111 cr mag mg and al released during the al spinel ferritchromit replacement interact with mesh textured serpentine giving rise to chloritic aureoles i e randomly intergrown chlorite lizardite and septechlorite that overgrow and postdate mesh textures

comparing the efficiency of digital and conventional soil mapping to predict soil types in a semi arid region in iran abstract the efficiency of different digital and conventional soil mapping approaches to produce categorical maps of soil types is determined by cost sample size accuracy and the selected taxonomic level the efficiency of digital and conventional soil mapping approaches was examined in the semi arid region of borujen central iran this research aimed to i compare two digital soil mapping approaches including multinomial logistic regression and random forest with the conventional soil mapping approach at four soil taxonomic levels order suborder great group and subgroup levels ii validate the predicted soil maps by the same validation data set to determine the best method for producing the soil maps and iii select the best soil taxonomic level by different approaches at three sample sizes 100 80 and 60 point observations in two scenarios with and without a geomorphology map as a spatial covariate in most predicted maps using both digital soil mapping approaches the best results were obtained using the combination of terrain attributes and the geomorphology map although differences between the scenarios with and without the geomorphology map were not significant employing the geomorphology map increased map purity and the kappa index and led to a decrease in the noisiness of soil maps multinomial logistic regression had better performance at higher taxonomic levels order and suborder levels however random forest showed better performance at lower taxonomic levels great group and subgroup levels multinomial logistic regression was less sensitive than random forest to a decrease in the number of training observations the conventional soil mapping method produced a map with larger minimum polygon size because of traditional cartographic criteria used to make the geological map 1 100 000 on which the conventional soil mapping map was largely based likewise conventional soil mapping map had also a larger average polygon size that resulted in a lower level of detail multinomial logistic regression at the order level map purity of 0 80 random forest at the suborder map purity of 0 72 and great group level map purity of 0 60 and conventional soil mapping at the subgroup level map purity of 0 48 produced the most accurate maps in the study area the multinomial logistic regression method was identified as the most effective approach based on a combined index of map purity map information content and map production cost the combined index also showed that smaller sample size led to a preference for the order level while a larger sample size led to a preference for the great group level

early magnetite amphibole plagioclase alteration mineralization in the island copper porphyry copper gold molybdenum deposit british columbia the initial hydrothermal event in the evolution of the ca 377 mt 0 41 percent cu island copper porphyry cu au mo deposit northern vancouver island was the formation of an extensive alteration mineralization facies dominated by magnetite calcic amphibole and intermediate to sodic plagioclase early stage quasipervasive magnetite rich alteration and associated magnetite rich veinlets in this middle jurassic island arc hosted hydrothermal system developed in both an axial dacitic porphyry dike and contiguous basaltic flows and pyroclastic strata the magnetite rich alteration zone 500 to 700 m wide and originally extending over at least 450 m vertically exhibits an outward zonation from quartz magnetite albite an sub or 6 amphibole trace apatite trace through quartz magnetite amphibole albite an sub 6 10 or oligoclase andesine an sub 15 36 apatite trace or scapolite trace to amphibole magnetite oligoclase or sodic andesinc an sub 15 39 or quartz or apatite trace assemblages definition of early stage mineral assemblages impeded by unusually intense later intermediate argillic alteration and more local phyllic and advanced argillic alteration was facilitated by the use of novel petrographic techniques including incident light nomarski differential interference contrast microscopy early stage mineralization comprises several well defined vein veinlet types which were demonstrably emplaced prior to the main stage chalcopyrite pyrite stockwork and associated k silicate alteration minor chalco pyrite and pyrite occur in some early stage veins and magnetite is a constituent of many later main stage sulfide rich veins but the distinction between these two major vein systems is clear mass exchange calculations demonstrate that early alteration involved intense iron metasomatism of both felsic and mafic country rocks with an fe enrichment approaching 450 percent 20 g 100 cc in the dacite dike and lesser and more variable na enrichment with both ti and al behaving as mobile constituents paragenetically early fluid inclusions in the quartz phenocrysts of the dike are tentatively correlated with the magnetite rich alteration mineralization and imply that this event was initiated above 650 degrees c and at pressures of at least 1 kbar and was generated by fecl 2 rich brines at first single phase and moderately saline avg 15 wt nacl equiv but latterly boiling at ca 560 degrees to 645 degrees c and or ca 0 55 kbars the main stage potassic alteration zone has the configuration of an annulus ca 100 to 150 m in width which was entirely superimposed on the more extensive early alteration mineralization zone at ca 430 degrees to 575 degrees c and below 450 bars gold although correlated overall with cu and potassic alteration was probably extensively introduced in the early stage it is inferred that the scarcity of sulfides in the early alteration assemblages reflects the highly oxidized nature of the initial fluids in which so super 2 exceeded both h 2 s and so super 2 4 the deposition of abundant magnetite and the widespread and intense fe metasomatism which define the early stage record the high solubility of fe as fecl 2 in high temperature brines in equilibrium with magnetite bearing quartzofeldspathic rocks as well as the retrograde solubility of magnetite between ca 550 degrees and 750 degrees c whereas the high so 2 h 2 o so 4 ratios of the initial hydrothermal fluids at island copper and in other similar deposits may have resulted from the high inherent oxygen fugacity of the parental dacitic magmas they may have directly reflected the composition of supercritical fluids expelled on quenching of underplating s rich mafic melts the mineral assemblages of the early stage veins and associated quasi pervasive alteration cannot be assigned to the accepted alteration mineralization facies of porphyry systems such as a veins and potassic alteration and record specific and distinct conditions of fluid rock interaction

contact skarn formation at elkhorn montana i p t component activity conditions of early skarn formation black butte stock the initial stage of skarn development i is characterized by a variety of three phase and less often four phase assemblages which usually include garnet and pyroxene and occasionally epidote vesuvianite or clintonite the second stage of skarn development ii was marked by local replacement of garnet and pyroxene upon cooling by one or more of epidote amphibole clintonite chlorite calcite and quartz electron microprobe analyses indicate wide variations in pyroxene and garnet compositions in stage i application of experimental and calculated phase equilibria to observed skarn assemblages has allowed estimations of general physical chemical conditions of formation for stage i skarns p 1 0 or 0 3 kb t 525 or 35 degrees c 0 04

tidal creek extension and saltwater intrusion in northern australia low lying holocene coastal plains supporting freshwater wetlands are often close to or even below the level reached by the highest tides the vulnerability of such freshwater systems to saltwater intrusion will increase should the sea level rise as a consequence of global warming in areas with a large tidal range one of the main processes inducing a reversion to saltwater influence is likely to be the rapid extension of tidal creek systems this is demonstrated with an example from the coastal plains of the mary river in northern australia where tidal creek systems have extended more than 30 km inland in 50 years invading freshwater wetlands and destroying associated vegetation over an area of at least 17000 ha networks have grown at an exponential rate through a combination of headward extension along main channels and tributary development large tidal range very small elevational differences over the plains the presence of incompletely infilled paleochannels and uncontrolled feral buffalo have been major factors contributing to the rapid rate of expansion

geochemistry of magnetite from porphyry cu and skarn deposits in the southwestern united states a combination of petrographic observations laser ablation inductively coupled plasma mass spectrometry la icp ms and statistical data exploration was used in this study to determine compositional variations in hydrothermal and igneous magnetite from five porphyry cu mo and skarn deposits in the southwestern united states and igneous magnetite from the unmineralized granodioritic inner zone batholith japan the most important overall discriminators for the minor and trace element chemistry of magnetite from the investigated porphyry and skarn deposits are mg al ti v mn co zn and ga of these the elements with the highest variance for i igneous magnetite are mg al ti v mn zn for ii hydrothermal porphyry magnetite are mg ti v mn co zn and for iii hydrothermal skarn magnetite are mg ti mn zn and ga nickel could only be detected at levels above the limit of reporting lor in two igneous magnetites equally cr could only be detected in one igneous occurrence copper as mo ag au and pb have been reported in magnetite by other authors but could not be detected at levels greater than their respective lors in our samples comparison with the chemical signature of igneous magnetite from the barren inner zone batholith japan suggests that v mn co and ga concentrations are relatively depleted in magnetite from the porphyry and skarn deposits higher formation conditions in combination with distinct differences between melt and hydrothermal fluid compositions are reflected in al ti v and ga concentrations that are on average higher in igneous magnetite than in hydrothermal magnetite including porphyry and skarn magnetite low ti and v concentrations in combination with high mn concentrations are characteristic features of magnetite from skarn deposits high mg concentrations 1 000 ppm are characteristic for magnetite from magnesian skarn and likely reflect extensive fluid rock interaction in porphyry deposits hydrothermal magnetite from different vein types can be distinguished by varying ti v mn and zn contents titanium and v concentrations are highly variable among hydrothermal and igneous magnetites but ti concentrations above 3 560 ppm could only be detected in igneous magnetite and v concentrations are on average lower in hydrothermal magnetite the highest ti concentrations are present in igneous magnetite from gabbro and monzonite the lowest ti concentrations were recorded in igneous magnetite from granodiorite and granodiorite breccia and largely overlap with ti concentrations found in hydrothermal porphyry magnetite magnesium and mn concentrations vary between magnetite from different skarn deposits but are generally greater than in hydrothermal magnetite from the porphyry deposits high mg and low ti and v concentrations characterize hydrothermal magnetite from magnesian skarn deposits and follow a trend that indicates that magnetite from skarn calcic and magnesian commonly has low ti and v concentrations

separation of molybdenum and copper in porphyry deposits the roles of sulfur redox and ph in ore mineral deposition at bingham canyon the giant bingham canyon porphyry cu mo au deposit utah is associated with eocene subvolcanic intrusions it shows a distinct metal zonation above a barren core with dominantly shallow cu au mineralization cu stage following the early quartz monzonite porphyry qmp intrusion and spatially deeper mo mineralization mo stage occurring in a separate vein set exclusively after a late quartz latite porphyry qlp intrusion that truncates earlier cu au veins to understand this metal separation and the geochemical process of molybdenite mineralization we investigated fluid inclusions by microthermometry raman spectroscopy and laser ablation inductively couple plasma mass spectrometry la icp ms microanalysis in low and high grade quartz veins of both mineralization stages in deep low grade quartz veins interpreted to represent the root zone of the cu stage we found high concentrations of cu s and mo in the fluid inclusions whereas in low grade mo stage veins we found lower cu but similar concentrations of s and mo compared to the inferred input fluids to the cu stage sulfur and copper concentrations were similar in intermediate density type fluid inclusions in deep low grade cu stage samples whereas intermediate density type inclusions in low grade mo stage veins have s contents that exceed their cu contents in high grade mo stage vein we found large variations of mo concentrations in coexisting brine and vapor inclusions compared to the p t conditions of the cu precipitation stage 90 260 bars and 320 430 c the mo precipitating fluids were trapped at higher pressures and temperatures of 140 to 710 bars and 360 to 580 c mass balance calculation based on the compositions of intermediate density inclusions and brine vapor assemblages interpreted to be derived by phase separation during decompression of the ascending single phase intermediate density fluid indicate that the mass of vapor phase exceeded that of brine by about 9 1 in both mineralization stages combining this mass balance with the analyzed vapor brine partitioning data indicates that more than 70 of mo and s by mass in the deposit were deposited from the vapor phase earlier cu au deposition was similarly dominated by vapor but recently published data about postentrapment cu diffusion in and out of fluid inclusions cast doubt on previous quantifications suggesting that almost none of the copper was deposited by brine mo is less likely to be modified by selective diffusion and high mo contents max 0 0054 mo na in intermediate density 380 g g mo in brine in the hydrothermal fluids were maintained from the early cu stage to the late mo stage this indicates that mo concentration was not the decisive factor for separate precipitation of late mo ore at bingham canyon instead the metal separation may be explained by a reduction in redox potential and an increase in acidity in the evolving source region of the fluids i e a large subvolcanic magma reservoir this is indicated by the stoichiometry of chalcopyrite and molybdenite precipitation reactions a tentative difference in the fe mn ratio in fluids of both veining stages incipient muscovite alteration along high temperature molybdenite veins and an increasing tendency for mo to fractionate from brine to vapor we suggest that the early cu stage fluids were slightly more oxidized and neutral allowing cu fe sulfides to saturate first while molybdenite saturation was suppressed and mo was lost from the early ore stage by contrast during the later mo stage the fluids were more reduced and acidic thereby allowing selective saturation of molybdenite as the first precipitating sulfide in the cooling and expanding two phase fluid consistent with textural observations this interpretation may imply more generally that small differences in redox potential and acid base balance of the magmatic source of porphyry mineralizing systems may be decisive in the temporal and spatial separation of the two metals