diapiric transfer of melt in kilauea iki lava lake hawaii a quick efficient process of igneous differentiation kilauea iki lava lake formed in 1959 is a large pond of picritic basalt average mgo content 15 34 by weight which has cooled and crystallized as a small self roofed magma chamber repeated drilling of the upper crust of the lake down to its molten core and more recent 1981 drilling through the thermal maximum and part way through the lower crust have made it possible to monitor the differentiation processes in the lake in detail differentiation processes recognized as active in the lake include rather inefficient settling of the larger 2 10 mm olivine phenocrysts formation of segregation veins and formation of diapir like vertical olivine rich bodies all processes which occur in one or more of the other kilauean lava lakes as well in addition most of the central part of kilauea iki has been affected by diapiric melt transfer in this process relatively low density liquid present at 1145 1160 c rose from within the loose crystal mush at the base of the lens of melt and intruded the equivalent thermal horizon at the top of the lens of melt passing through the hotter but denser melt in the core of the lake without mixing the source volume from which the low density liquid was extracted is depleted in tio 2 and other incompatible elements and enriched in feo and cao the upper part of the lake shows the opposite effects the crystalline assemblage present was olivine augite minor plagioclase the crystallinity of both the source and receiving layers was low enough that no obvious textural imprint was left by the transfer process the principal evidence for its occurrence is the chemical zonation of the lake seen in core from depths of 13 to 80 m diapiric melt transfer was active from 1960 to 1971 and has affected most of the central part of the lake from 13 m to at least 80 m the process ran simultaneously with the other three main differentia tion processes but started and stopped independently of the others calculations suggest that between 21 and 42 wt liquid has been extracted from the depleted zone at 56 78 m in the center of the lake making this a very efficient process of chemical differentiation

a new cretaceous tertiary boundary site at flaxbourne river new zealand biostratigraphy and geochemistry abstract an exceptionally complete rock sequence across the cretaceous tertiary k t boundary has been discovered near the flaxboume river marlborough province south island new zealand the boundary is marked by a large ir anomaly 21 ng g on a decalcified basis with an integrated abundance of 134 ng cm2 after correction for background above the boundary there is a 30 cm transition zone in which a few cretaceous foraminiferal taxa such as hedbergella monmouthensis and guembelitria cretacea survived though with reduced abundance and size only ca 1 2 to 1 4 of normal apparently reflecting environmental stresses inaa and icp analyses show that in addition to ir the boundary clay is also enriched in cr and ni mainly from meteoritic material and as co cu sb and zn from terrestrial sources volcanic sources even when scaled to the 10 7 km 3 volume of the deccan basalts fail by three orders of magnitude to account for the ir and as at the k t boundary and by even larger factors for sb zn cu etc comparison of our data with those from six other k t boundary sites shows that the zn sb as sb and zn as ratios generally fall between crustal and oceanic values suggesting contributions from both sources mass balance calculations show that as and sb could be derived from only 300 500 m of ocean water or also from modest amounts 20 36 g cm 2 of average crustal rock copper and zn on the other hand can only be derived from crustal or mantle rock 5 15 g cm 2 presumably impact ejecta such an amount of ejecta is fairly close to the global fallout of boundary clay 2 5 g cm 2

the 2005 eruption of sierra negra volcano galapagos ecuador sierra negra volcano began erupting on 22 october 2005 after a repose of 26 years a plume of ash and steam more than 13 km high accompanied the initial phase of the eruption and was quickly followed by a 2 km long curtain of lava fountains the eruptive fissure opened inside the north rim of the caldera on the opposite side of the caldera from an active fault system that experienced an mb 4 6 earthquake and 84 cm of uplift on 16 april 2005 the main products of the eruption were an a a flow that ponded in the caldera and clastigenic lavas that flowed down the north flank the a a flow grew in an unusual way once it had established most of its aerial extent the interior of the flow was fed via a perched lava pond causing inflation of the a a this pressurized fluid interior then fed pahoehoe breakouts along the margins of the flow many of which were subsequently overridden by a a as the crust slowly spread from the center of the pond and tumbled over the pahoehoe the curtain of lava fountains coalesced with time and by day 4 only one vent was erupting the effusion rate slowed from day 7 until the eruption s end two days later on 30 october although the caldera floor had inflated by 5 m since 1992 and the rate of inflation had accelerated since 2003 there was no transient deformation in the hours or days before the eruption during the 8 days of the eruption gps and insar data show that the caldera floor deflated 5 m and the volcano contracted horizontally 6 m the total eruptive volume is estimated as being 150 106 m3 the opening phase tephra is more evolved than the eruptive products that followed the compositional variation of tephra and lava sampled over the course of the eruption is attributed to eruption from a zoned sill that lies 2 1 km beneath the caldera floor

the crystallization of lava lakes thermal convection driven by the kinetic undercooling of solidification can produce compositional and textural variations in an initially uniform lava flow that is cooled predominantly from above we show that such kinetic undercooling is sufficient to drive convection at high rayleigh numbers even in shallow lava lakes we then employ a theoretical model that includes the effects of convection at high rayleigh number to make predictions of the complete evolution of the lava we show that predictions of the rate of growth of solid crust at the surface of the lake do not differ much from predictions made by ignoring effects due to convection however convective motions do shorten the time for complete solidification because when they are driven by kinetic undercooling there is internal crystallization in addition to that occurring near the cooled upper boundary of the lava this important effect which is absent from purely conductive models predicts stratification and zonation of the lava in agreement with field observations we also examine the effect of taking into account the heat transfer to the country rock below the lava lake and determine how the evolution of the lava lake varies with the viscosity of the lava

magmatic fractionation by compositional convection in a sheet like magma body constraints from the nosappumisaki intrusion northern japan the mechanism of fractionation in a sheet like magma body was investigated based on observations from the nosappumisaki intrusion northern japan this is one of a number of late cretaceous shoshonite sills in the nemuro peninsula and has a compound structure comprising a bottom cumulate zone containing 50 vol cumulus crystals and a overlying middle zone free of such cumulus crystals these zones are sandwiched between porphyritic rocks of upper and lower chilled and marginal zones it is inferred that massive crystal settling or flotation from a magma initially laden with 20 vol of crystals primocrysts in a sheet like magma body formed a cumulus pile and an overlying crystal free melt layer after the formation of the compound structure with partial freezing of the upper and lower marginal zones the internal part of the intrusion slowly solidified during this period the melt in the central part of the intrusive sheet underwent fractional crystallization this is shown by patterns of vertical variation in the composition of the interstitial melt the overall patterns are inverted mirrorimage s shaped and s shaped for incompatible and compatible elements respectively the incompatible element abundances show maxima near the bottom of the middle zone which corresponds to the final solidification horizon they show minima in the cumulate zone which suggests discharge of an evolved melt from the crystal pile fractionation is inferred to have taken place via compositional convection without settling of crystals grown after the intrusion event or collapse of the upper boundary layer the melt in the central melt layer descended into the permeable crystal pile counterbalancing discharge of buoyant evolved melt formed by crystallization of the pore melt in the crystal pile this conclusion is based on the following observations 1 there is no accumulation of crystals grown after the intrusion of the magma above the cumulate zone 2 leucocratic pipe like structures which represent relict pathways for the evolved melt are developed in the cumulate zone these structures increase in diameter decrease in number density and increase in abundance of incompatible elements with height 3 cumulus augite in the cumulate zone has ca rich and al poor dissolved rims the extent of dissolution increases with stratigraphic height suggesting downwelling of an h2o rich melt from the overlying melt layer 4 there is no evidence for extensive compaction accompanying pore melt crystallization in the main part of the cumulate zone 5 the initial melt composition estimated from the groundmass of the chilled margin rocks and the average composition of the material interstitial to the primocrysts for the whole sill are statistically identical a mass balance model for compositional convection was constructed to quantify the melt transportation processes model parameters were optimized by fitting the observed vertical variation of the interstitial melt compositions the optimized results suggest that the evolved melts discharged from the crystal pile and rose through the central melt layer as plumes with minor mingling with the surrounding melt the initially formed compound structure is a highly preferential environment for compositional convection

perspectives on basaltic magma crystallization and differentiation lava lake blocks erupted at mauna loa volcano summit hawaii explosive eruptions at mauna loa summit ejected coarse grained blocks free of lava coatings from moku aweoweo caldera most are gabbronorites and gabbros that have 0 26 vol olivine and 1 29 vol oikocrystic orthopyroxene some blocks are ferrogabbros and diorites with micrographic matrices and diorite veins 2 cm cross cut some gabbronorites and gabbros one block is an open textured dunite the mgo of the gabbronorites and gabbros ranges 7 21 wt those with mgo 10 wt have some incompatible element abundances zr y ree positive eu anomalies lower than those in mauna loa lavas of comparable mgo gabbros mgo 10 wt generally overlap lava compositions olivines range fo83 58 clinopyroxenes have mg s 83 62 and orthopyroxene mg s are 84 63 all evolved beyond the mineral mg s of mauna loa lavas plagioclase is an75 50 ferrogabbro and diorite blocks have 3 5 wt mgo tio2 3 2 5 4 k2o 0 8 1 3 la 16 27 ppm and a diorite vein is the most evolved sio2 59 k2o 1 5 la 38 ppm they have clinopyroxene mg s 67 46 and plagioclase an57 40 the open textured dunite has olivine fo83 5 seven isotope ratios are 87sr 86sr 0 70394 0 70374 and 143nd 144nd 0 51293 0 51286 and identify the suite as belonging to the mauna loa system gabbronorites and gabbros originated in solidification zones of moku aweoweo lava lakes where they acquired orthocumulate textures and incompatible element depletions these features suggest deeper and slower cooling lakes than the lava lake paradigm kilauea iki which is basalt and picrite clinopyroxene geobarometry suggests crystallization at 1 kbar p highly evolved mineral mg s 75 are largely explained by cumulus phases exposed to evolving intercumulus liquids causing compositional shifts ferrogabbro and diorite represent segregation veins from differentiated intercumulus liquids filter pressed into rigid zones of cooling lakes clinopyroxene geobarometry suggests 300 bar p open textured dunite represents olivine melt mush precursor to vertical olivine rich bodies as in kilauea iki its fo83 5 identifies the most primitive lake magma as 8 3 wt mgo mass balancing and melts show that such a magma could have yielded both ferrogabbro and diorite by 50 fractional crystallization but under different fo2 fmq 250 bar led to diorite and fmq 250 bar yielded ferrogabbro these segregation veins documented as similar to those of kilauea testify to appreciable volumes of rhyolitic liquid forming in oceanic environments namely sio2 rich veins are intrinsic to all shields that reached caldera stage to accommodate various sized cooling differentiating lava lakes

convection and crystallization in magma cooled from above calculations are presented for the cooling from above of melts in the di an system in which the kinetics of crystallization are incorporated and play a dominant role the results indicate that even with no initial superheat whatsoever convection plays an important role in the cooling of magma chambers and allows substantial internal cooling crystallization and differentiation the calculations show in agreement with observations that in magma bodies hundreds of meters thick crystallization occurs predominantly in the interior or at the floor even though heat is lost only from the roof the ratio of the final thickness of the layer formed at the floor to that formed at the roof increases as the overall size of the chamber increases owing to the effects of convection

forearc structures and tectonic regimes at the oblique subduction zone between the hikurangi plateau and the southern kermadec margin the convergent southern kermadec margin is examined to study the structural development and mechanical parameters of the inner trench slope in response to the oblique subduction of the oceanic hikurangi plateau swath bathymetric data have been combined with other geophysical data to reveal that the south kermadec inner trench wall is segmented into distinct upper middle and lower margins the lower margin is a nongrowing compressionally unstable accretionary wedge that is dissected by transcurrent faults the wedge has been thinned and narrowed as a result of tectonic erosion the middle margin is stable by comparison and separated from the lower margin by the 220 km long awanui fault this fault is a landward dipping strike slip reverse fault the middle upper margin including east cape ridge ecr forms a deforming backstop with a sharp along strike change in tectonic regime south of 36 40 s transpression characterizes the ecr shear zone north of 36 40 s the ecr is marked by uplift by extension and farther north by subsidence it is suggested that subsidence and subsequent tectonic erosion of the margin north of 35 50 s in the wake of hikurangi plateau subduction have broken the link between once continuous segments of the ecr tonga platform the unstable nature of the southern kermadec wedge under compression together with a comparison of its taper characteristics with those of modern convergent margins implies that the wedge deforms under a relatively high internal pore pressure but high effective basal friction regime strike slip deformation in the wedge is attributable to a relatively strong interplate coupling related to the buoyancy and ruggedness of the hikurangi plateau

late cretaceous foraminifera from hikurangi plateau new zealand abstract rock samples dredged from the hikurangi plateau the large volcanic plateau currently being subducted beneath the north island of new zealand at 1 37 27 3 s 178 0 w 2 37 22 s 179 01 w 3 36 06 s 178 22 5 w include basalt volcanic breccia and rare fossiliferous sediment late cretaceous foraminifera occurring in two samples from a large volcanic feature at site 1 provide the only pre miocene paleontological ages ever obtained from the plateau sites 2 and 3 proved non fossiliferous one of the site 1 samples a calcareous volcanic breccia contains a dominantly planktic fauna with heterohelix globulosa h cf navarroensis globigerinelloides volutus and rugoglobigerina rugosa together with rare calcareous and agglutinated benthics it is dated with high confidence as early mid campanian to mid maastrichtian and more tentatively as mid maastrichtian and is correlated with the haumurian stage in new zealand the second sample studied only in thin section contains an apparently nearly identical fauna with undetermined species of globigerinelloides heterohelix and rugoglobigerina sp faunal similarity suggests that it is approximately the same age as the first sample both assemblages contain more than 95 planktic individuals indicating a fully oceanic depositional site low faunal diversity only 4 or perhaps 5 planktic taxa present and lack of tropical or subtropical forms strongly suggest a relatively high paleolatitude for the site

the boycott effect in magma chambers 1 we investigate the plausibility of the stratified boycott effect as a source of layering in magma chambers crystal settling within the magma chamber will generate buoyant fluid near the sloping sidewalls whose vertical ascent may be limited by the ambient stratification associated with vertical gradients in sio2 the resulting flow may be marked by a layered structure each layer taking the form of a convection cell spanning the lateral extent of the magma chamber using parameters relevant to magma chambers we estimate that such convection cells would be established over a timescale of a month and have a depth on the order of 4m which is roughly consistent with field observations of strata within solidified chambers

the impact of side wall cooling on the thermal history of lava lakes most simple models for cooling of sheet like intrusions the hawaiian lava lakes being one example neglect the effect of side wall heat loss on the overall thermal evolution in this paper we extend a conventional one dimensional 1d model for cooling of sheet like intrusions to account for lateral heat loss by either prescribing a fixed side wall heat flux or a heat flux controlled by heat transport in the surrounding wall rock in the first part of the study we analyze the general interplay between side wall cooling and the thermal evolution of the system the second part focuses on a comparison between our modeling results including models without and with lateral heat flux and the hawaiian lava lake data this comparison leads to the following three main conclusions 1 side wall cooling does have a significant impact on the cooling history of lava lakes 2 models assuming a time dependent temperature profile in the wall rock lead to a better fit with the measured temperature data 3 due to the sluggish conductive heat transfer in the mush its thermal evolution is significantly decoupled from the temperature evolution of the convecting bulk liquid

ground based thermal imaging of lava lakes at erebus volcano antarctica abstract mount erebus a large intraplate stratovolcano dominating ross island antarctica hosts the world s only active phonolite lava lakes the main manifestation of activity at erebus volcano in december 2004 was as the presence of two convecting lava lakes within an inner crater the long lived ray lake 1400 m2 in area was the site of up to 10 small strombolian eruptions per day a new but short lived 1000 1200 m2 lake formed at werner vent in december 2004 sourced by lava flowing from a crater formed in 1993 by a phreatic eruption we measured the radiative heat flux from the two lakes in december 2004 using a compact infrared ir imaging camera daily thermal ir surveys from the main crater rim provide images of the lava lake surface temperatures and identify sites of upwelling and downwelling the radiative heat outputs calculated for the ray and werner lakes are 30 35 mw and 20 mw respectively we estimate that the magma flux needed to sustain the combined heat loss is 250 710 kg s 1 that the minimum volume of the magma reservoir is 2 km3 and that the radius of the conduit feeding the ray lake is 2 m

crystal settling in a vigorously converting magma chamber there has been much debate concerning the mechanism of fractional crystallization in magma chambers the traditional hypothesis of crystal settling has been widely replaced by the concept of in situ crystallization coupled with compositional convection observations from layered intrusions however are equivocal1 4 doubts have been raised about crystal settling on theoretical grounds because convective velocities in magma chambers are often much greater than the crystal settling velocities predicted by stokes law5 but there has been no experimental study of crystal settling in such vigorous convection here we present physical considerations and laboratory experiments which show that the phenomenon of particle settling in these conditions can be accounted for by a simple theory application of this theory to crystal settling in magma chambers suggests that crystal settling may be an efficient differentiation mechanism at least in basaltic magma chambers despite large convective velocities

the giant ruatoria debris avalanche on the northern hikurangi margin new zealand result of oblique seamount subduction despite convergent margins being unstable systems most reports of huge submarine slope failure have come from oceanic volcanoes and passive margins swath bathymetry and seismic profiles of the northern hikurangi subduction system new zealand show a tapering 65 30 km wide by 65 km deep margin indentation with a giant 3150 630 km3 blocky debris avalanche deposit projecting 40 km out across horizontal trench fill and a debris flow deposit projecting over 100 km slide blocks are well bedded up to 18 km across and 1 2 km high the largest being at the avalanche deposit s leading edge samples dredged from them are mainly miocene shelf calc mudstones similar to those outcropping around the indentation cores from cover beds suggest that failure occurred 170 40 ka possibly synchronously with a major extension collapse in the upper indentation however the northern part of the indentation is much older the steep straight northern wall is close to the direction of plate convergence and probably formed around 2 0 0 16 ma as a large seamount subducted leaving in its wake a deep groove obliquely across the margin and an unstable triangle of fractured rock in the 60 angle between groove and oversteepened margin front the triangle collapsed as a blocky avalanche leaving a scalloped southern wall and probably causing a large tsunami tentative calculations of compacted volumes suggest that the indentation is over 600 km3 larger than the avalanche supporting a two stage origin that includes subduction erosion since failure convergence has carried the deposits 9 km back toward the margin causing internal compression the eventual subduction accretion of the ruatoria avalanche explains the scarcity of such features on active margins and perhaps the nature of olistostromes in fold belts

post emplacement melt flow induced by thermal stresses implications for differentiation in sills abstract we present the first steps of a new explanation model for differentiation in sills using a combination of geochemical data and field observations numerical modeling and dimensional analysis geochemical data from a saucer shaped dolerite sill intruded into the karoo basin south africa reveal a differentiation process which causes d shaped profiles the geometry name is based on the variation in whole rock mg number mg mg mg fe from floor to roof in a sill the d shaped geochemical profiles represent sheet intrusions with the most primitive composition i e high mg in its center and progressively more evolved composition i e low mg towards the upper and lower margins the differentiation is reversed compared to the normal differentiation produced by fractional crystallization c shaped profiles c shaped profiles are believed to be formed by segregation of crystals from the magma we propose that the opposite the d shaped profile may result from melt segregation from the crystal mush this is achieved by porous melt flow through a consolidated crystal network after the main phase of emplacement and before complete solidification we show that a significant flow is feasible under natural occurring conditions an underpressure of magnitude 108 pa develops at the cooling margins due to volume reduction of the crystallizing porous melt the resulting pressure gradient is the driving force for the melt flow towards cooling margins considered in this work as a result the margins will be enriched in the incompatible elements associated with the melt phase while the center will be depleted we show that the amount of flow is primarily a function of viscosity of the melt and permeability of the crystal network which in turn is a transient phenomenon dependent on a number of parameters diagrams have been constructed to evaluate the feasibility of substantial melt extraction in terms of these poorly constrained parameters data from the golden valley sill and many other natural occurrences of d and i shaped geochemical profiles show a reasonable agreement with our predictions of melt flow potential and are thus well explained by the presented model we conclude that in order to fully understand differentiation processes occurring in sheet intrusions we need to account for post emplacement segregation of melt from crystals and not only segregation of crystals from melt

towards a model for the in situ origin of pge reefs in layered intrusions insights from chromitite seams of the rum eastern layered intrusion scotland the current debate on the origin of platinum group element pge reefs in layered intrusions centres mostly on gravity settling of sulphide liquid from overlying magma versus its introduction with interstitial melt fluids migrating upward from the underlying cumulate pile here we show that pge rich chromitite seams of the rum eastern layered intrusion provide evidence for an alternative origin of such deposits in layered intrusions these laterally extensive 2 mm thick chromitite seams occur at the bases of several cyclic mafic ultramafic units and show lithological and textural relationships suggesting in situ growth directly at a crystal liquid interface this follows from chromitite development along the edges of steeply inclined culminations and depressions at unit boundaries even where these are vertically oriented or overhanging high concentrations of pge up to 2 3 ppm pd pt are controlled by fine grained base metal sulphides which are closely associated with chromitite seams the following sequence of events explains the origin of the pge rich chromitite seams a emplacement of picritic magma that caused thermal and mechanical erosion of underlying cumulate followed by in situ growth of chromite against the base b precipitation of sulphide droplets on chromite grains acting as favourable substrate or catalyst for sulphide nucleation c the scavenging of pge by sulphide droplets from fresh magma continuously brought towards the base by convection since the rate of magma convection is 105 107 times higher than that of the solidification km year to km day versus 0 5 1 0 cm year the in situ formed sulphide droplets can equilibrate with picritic magma of thousands to million times their own volume as a result the sulphide bearing rocks are able to reach economic concentrations of pge several ppm we tentatively suggest that the basic principles of our model may be used to explain the origin of pge rich chromitites and classical pge reefs in other layered mafic ultramafic intrusions

petrography and bulk chemistry of martian orthopyroxenite alh84001 implications for the origin of secondary carbonates abstract new petrologic and bulk geochemical data for the snc related martian meteorite alh84001 suggest a relatively simple igneous history overprinted by complex shock and hydrothermal processes alh84001 is an igneous orthopyroxene cumulate containing penetrative shock deformation textures and a few percent secondary extraterrestrial carbonates rare earth element ree patterns for several splits of the meteorite reveal substantial heterogeneity in ree abundances and significant fractionation of the rees between crushed and uncrushed domains within the meteorite complex zoning in carbonates indicates nonequilibrium processes were involved in their formation suggesting that co2 rich fluids of variable composition infiltrated the rock while on mars we interpret petrographic textures to be consistent with an inorganic origin for the carbonate involving dissolution replacement reactions between co2 charged fluids and feldspathic glass in the meteorite carbonate formation clearly postdated processes that last redistributed the ree in the meteorite

modeling lava lake heat loss rheology and convection 1 measurements at erta ale s lava lake and theoretical equations for lake rheology density driven convection and thermally driven plume ascent allow the constraint of lake dynamics cooling and crystallization expected from surface heat losses imply a viscosity increase from 150 pa s to 300 1800 pa s for cooled surface layers convection is expected to proceed vigorously under low viscosity conditions driving rapid 0 1 0 4 m s 1 surface motions and sluggishly under moderate to high viscosity conditions to drive slower motions 0 08 m s 1 convection is likely driven by small 6 kg m 3 density differences where surface cooling can influence lake rheology and explain variable rates of surface convective motion

the formation of plagioclase chains during convective transfer in basaltic magma the basaltic rock in the lower part of the thick holyoke lava flow in connecticut and massachusetts has been shown to have a remarkable texture with crystals of feldspar linked together in a continuous three dimensional network of chains1 heating experiments have revealed that this network persists to temperatures where the rock is 75 liquid2 and therefore the network was interpreted to have formed at an early stage of crystallization and to have played an important role in the compaction of crystal mush in the lower part of the flow3 4 despite the texture s importance to our understanding of how such basalt flows form the origin of the texture has remained uncertain1 2 3 here we show that although the network is present in the lower third of the flow it was actually formed in the upper solidification front and was transported down in plumes of dense crystal mush convection of this type has been postulated for intrusive magma chambers but corroborative field evidence has been equivocal especially in lava lakes and flows preservation of the roof generated texture in the lower part of a thick flood basalt flow therefore constitutes important evidence for the role of convection in the solidification and differentiation of a simple magma sheet

origin of modal and rhythmic igneous layering by sedimentation in a convecting magma chamber experimental investigations of convecting particle laden fluids show two regimes for convection driven by cooling from above1 in very dilute suspensions convection will maintain a homogeneous distribution of particles throughout the convecting layer provided that particle fall velocities are small compared with turbulent fluid velocities above a critical concentration convection is unable to keep the particles suspended so the particles settle leaving behind a layer of convecting fluid virtually free of particles here we apply these results to cooling magma chambers in which crystallization leads to an increase in suspended crystal content with time discrete sedimentation events are predicted each time the concentration exceeds the critical value for common igneous minerals critical concentrations are very small typically 0 002 0 03 wt and layers of the order of centimetres to a few metres thick will result because minerals of different density and size have different critical concentrations and settling velocities complex fluctuations in sedimentation rate and mineral proportions can occur in a multi component melt this may lead to either regular repetitive cycles or more complex fluctuations the process is confined to low viscosity magmas such as basalts in which the crystals are able to separate from the active thermal boundary layer during convection

magma dynamics crystallization and chemical differentiation of the 1959 kilauea iki lava lake hawaii revisited abstract using constraints from an extensive database of geological and geochemical observations along with results from fluid mechanical studies of convection in magma chambers we identify the main physical processes at work during the solidification of the 1959 kilauea iki lava lakes in turn we investigate their quantitative influence on the crystallization and chemical differentiation of the magma and on the development of the internal structure of the lava lake in contrast to previous studies vigorous stirring in the magma driven predominately by the descent of dense crystal laden thermal plumes from the roof solidification front and the ascent of buoyant compositional plumes due to the in situ growth of olivine crystals at the floor is predicted to have been an inevitable consequence of very strong cooling at the roof and floor the flow is expected to have caused extensive but imperfect mixing over most of the cooling history of the magma producing minor compositional stratification at the roof and thermal stratification at the floor the efficient stirring of the large roof cooling is expected to have resulted in significant internal nucleation of olivine crystals which ultimately settled to the floor additional forcing due to either crystal sedimentation or the ascent of gas bubbles is not expected to have increased significantly the amount of mixing in addition to convection in the magma circulation driven by the convection of buoyant interstitial melt in highly permeable crystal melt mushes forming the roof and the floor of the lava lake is envisaged to have produced a net upward flow of evolved magma from the floor during solidification in the floor zone mush convection may have caused the formation of axisymmetric chimneys through which evolved magma drained from deep within the floor into the overlying magma and potentially the roof we hypothesize that the highly evolved pipe like vertical olivine rich bodies vorbs bull volcanol 43 1980 675 observed in the floor zone of the lake are fossil chimneys in the roof zone buoyant residual liquid both produced at the roof solidification front and gained from the floor as a result of incomplete convective mixing is envisaged to have percolated or leaked into the overlying highly permeable cumulate displacing less buoyant interstitial melt downward the results from rayleigh fractionation type models formulated using boundary conditions based on a quantitative understanding of the convection in the magma indicate that most of the incompatible element variation over the height of the lake can be explained as a consequence of a combination of crystal settling and the extensive but imperfect convective mixing of buoyant residual liquid released from the floor solidification front the remaining chemical variation is understood in terms of the additional influences of mush convection in the roof and floor on the vertical distribution of incompatible elements although cooling was concentrated at the roof of the lake the floor zone is found to be thicker than the roof zone implying that it grew more quickly the large growth rate of the floor is explained as a consequence of a combination of the substantial sedimentation of olivine crystals and more rapid in situ crystallization due to both a higher liquidus temperature and enhanced cooling resulting from imperfect thermal and chemical mixing

fossil bacterial magnetite in deep sea sediments from the south atlantic ocean the magnetostratigraphy of deep sea sediment cores has proven to be a powerful and high resolution method of dating and comparing sedimentary records in contrast to this widespread application relatively few attempts have been made to analyse and identify the mineralogy grain size and origin of the carrier of the remanent magnetization1 2 it is widely assumed that dominant ferrimagnetic minerals in deep sea sediments are of lithogenic origin that is that they derive from oceanic or continental detritus volcanic ashfalls micrometeorites hydrothermal precipitation or chemical mineralization processes a very different view was taken by kirschvink and lowenstam3 who suggested that fossil remnants of magnetite mineralizing organisms in particular magnetotactic bacteria might play an important part in the magnetization of sediments deposited in certain detritus poor aquatic systems we have used rock magnetic diagnostic methods to characterize the magnetic phases in deep sea sediments from the south atlantic these phases were then extracted and studied with the electron microscope our results indicate that fossil bacterial magnetite is the main carrier of the natural remanent magnetization in sediments ranging in age from quarternary to eocene different types of bacterial magnetosomes were detected most of them occurring in chains of identical particles and nearly all of them falling into the theoretically determined4 size range of single domain magnetitite

crystal capture sorting and retention in convecting magma the mystery of producing strong compositional diversity among suites of comagmatic igneous rocks is investigated by considering the dynamic evolution of basaltic magma in a sheet like chamber a central conclusion is that inward progressing crystallization produces strong viscosity and temperature gradients that promote convection only near the leading edge of the upper thermal boundary layer convection is apparently confined to an essentially isoviscous isothermal region that hugs the downward growing roof zone strong changes in viscosity with crystallization divide the upper and lower thermal boundary layers into regions of decreasing viscosity and crystallinity n called rigid crust n 0 5 mush 0 5 n 0 25 and suspension n 0 25 the strong increase in viscosity near the mush suspension interface acts as a capture front that overtakes and traps slowly settling crystals initial phenocrysts mostly escape capture but crystals nucleated and grown in the suspension zone can escape only if the capture front slows to a critical rate attainable only in bodies thicker than about 100 m escaping crystals are redistributed and sorted by convection driven by the advance of the capture front itself crystal laden plumes traverse the central hot core of the body and deposit partially resorbed and sorted crystals within the lower suspension zone convection is never vigorous but is part of an overall intimate balance between roofward heat loss rigid crust growth crystallization kinetics and transport and sorting of sinking escaped crystals there is a strong similarity between these processes and those producing both varves and saline pan deposits it is clear that lavas lava lakes and sills are indeed examples of true magma chambers strongly exhibiting certain aspects of this over all process these aspects commonly also characterize the large mafic magmatic bodies because strong compositional changes in the residual melt occur largely outward that is at lower temperatures and higher crystallinities of the capture front which is immobile and mostly within rigid crust the possible range in comagmatic compositions available for eruption anywhere within the active magma is very limited this is in broad agreement with the compositional range observed in basaltic lava lakes sills and plutons like skaergaard the tuning of convection crystallization kinetics and phase equilibria in chambers of this type can produce a variety of textures and layering but not a diversity of compositions

crystal mush compaction and the origin of pegmatitic segregation sheets in a thick flood basalt flow in the mesozoic hartford basin connecticut

gas driven lava lake fluctuations at erta ale volcano ethiopia revealed by modis measurements the long lived lava lake of erta ale volcano ethiopia is remotely monitored by moderate resolution imaging spectroradiometers modis installed on satellites the normalised thermal index nti wright et al remote sens environ 82 135 155 2002 is shown to be proportional to the volume of the lava lake based on visual observations the lava lake s variable level can be plausibly related to a stable foam i e a mixture composed of densely packed non coalescing bubbles in suspension within a liquid this foam is trapped at the top of the magma reservoir and its thickness changes in response to the gas flux feeding the foam being successively turned on and off the temporal evolution of the foam thickness and the resulting variation of the volume of the lava lake is calculated numerically by assuming that the gas flux feeding the foam initially constant and homogeneous since december 9 2002 is suddenly stopped on december 13 2002 and not restarted before may 2003 the best fit between the theoretical foam thickness and the level of the lava lake deduced from the nti provides an estimate of both the reservoir radius 155 170 m and the gas flux feeding the foam 5 5 10 3 7 2 10 3 m 3 s 1 when existing this is in agreement with previous estimates from acoustic measurements bouche et al earth planet sci lett 295 37 48 2010 the very good agreement between the theoretical foam thickness and that deduced from modis data shows for the first time the existence of a regime based on the behaviour of a stable foam whose spreading towards the conduit wide conduit condition can explain the long lived activity our predictive model which links the gas flux at the vent to the foam spreading could potentially be used on any volcano with a long lived activity the underlying gas flux and the horizontal surface area of the magma reservoir can then be deduced by combining modelling to continuous measurements of gas flux the lava lake when high often shows regular rise and fall of its level we have recognised a minimum of 26 very well marked cycles between january 2001 and december 13 2002 corresponding to a typical return time of 10 8 2 3 days and a gas volume of 8 3 105 2 0 105 m 3 this corresponds to a gas volume fraction in the reservoir equal to 0 023 0 063 the yearly gas flux estimated between december 13 2002 and september 27 2004 varies between 2 3 10 6 and 5 9 10 6 m 3 s 1 at the depth of the reservoir the long time series provided by infra red sensors mounted on satellites could be used on any persistent volcano to detect potential periodic variations in the level of lava lakes or lava columns providing that the vent has a funnel shape as often and is sufficiently large

carbonate compositions in cm and ci chondrites and implications for aqueous alteration abstract carbonate minerals in fourteen cm chondrites and two ci chondrites have been analyzed by electron microprobe to provide a better understanding of the aqueous processes that affected carbonaceous chondrite parent bodies calcites in cm chondrites and dolomites and magnesites in ci chondrites display the compositions expected of stable phases formed at low temperatures dolomites in cm chondrites identified here for the first time in five members of the group have small amounts of excess ca which may reflect metastable growth the distribution of fe between dolomite and coexisting serpentine differs in the two chondrite groups if the distributions reflect an approach to chemical equilibrium then the difference implies higher alteration temperatures for the ci group than the cm group in agreement with the results of previously published oxygen isotope thermometry and mineral solubility modeling of the alteration process dolomite fe contents are relatively uniform in the two chondrite groups dolomite mn contents by contrast vary widely the variations may reflect transport controlled coprecipitation of mn resulting from a heterogeneous distribution of the element in the anhydrous precursor material if this interpretation is correct then the altering fluids were essentially immobile while hydration reactions proceeded on the meteorite parent bodies the near closed system character of the alteration process long known from bulk chemical analyses of the meteorites is a direct consequence of the limited mobility of dissolved species

on convection in a volatile saturated magma when a saturated basaltic magma cools and crystallises in a shallow magma reservoir gas bubbles may be exsolved from solution thereby influencing the density convective mixing and pressure evolution of the melt in wet basaltic magmas saturated with h2o the production of small bubbles may dominate the density evolution of the mixture causing a gradual decrease of the bulk density with cooling cool upper boundaries of the chamber may therefore become stable to convection while vigorous convection may develop as the bubbly mixture rises from cold lower boundaries the intensity of such bubble driven convection may be an order of magnitude greater than purely thermal or compositional convection which arises in unsaturated melt new laboratory experiments of such bubble driven convection suggest that after a transient an equilibrium bubble concentration is attained and subsequently bubble magma separation leads to build up of a layer of bubbles above the well mixed bubble laden melt 1 these results have some important implications for mixing when a volatile rich mafic magma is intruded below an evolved less dense body of silicic magma when the mafic magma becomes volatile saturated then owing to cooling at its upper boundary the bulk density in the upper boundary layer of the mafic magma will decrease owing to the exsolution of gas if the density falls below that of the overlying silicic magma then small plumes of buoyant bubble rich mafic magma may rise from the boundary into the upper silicic layer in addition on saturation of the mafic magma cooling at the lower boundary will begin to drive convection of bubble rich melt the mafic magma may eventually reach an equilibrium bubble concentration and subsequently the bubbles produced at the base of the layer will be supplied to the interface between the silicic and mafic layers large scale overturn of the mafic and silicic magmas can therefore only occur if at the equilibrium bubble concentration the bulk density of the bubbly mafic magma is smaller than that of the overlying magma otherwise mixing continues through small plumes of vesicular mafic melt rising from the upper boundary layer for basaltic magma in which co2 is the dominant volatile phase the generation of bubbles does not typically dominate the density evolution and the convective flows are similar to those in an unsaturated melt

addressing complexity in laboratory experiments the scaling of dilute multiphase flows in magmatic systems the kinematic and dynamic scaling of dilute multiphase mixtures in magmatic systems is the only guarantee for the geological verisimilitude of laboratory experiments we present scaling relations that can provide a more complete framework to scale dilute magmatic systems because they explicitly take into account the complexity caused by the feedback between particles crystal bubble or pyroclast and the continuous phase liquid or gas we consider three canonical igneous systems magma chambers volcanic plumes and pyroclastic surges and we provide estimates of the proposed scaling relations for published experiments on those systems dilute magmatic mixtures can display a range of distinct dynamical regimes that we characterize with a combination of average eulerian properties and instantaneous lagrangian variables the eulerian properties of the mixtures yield the reynolds number re which indicates the level of unsteadiness in the continuous phase the lagrangian acceleration of particles is a function of the viscous drag and gravity forces and from these two forces are derived the stokes number st and the stability number rt two dimensionless numbers that describe the dynamic behavior of the particles within the mixture the compilation of 17 experimental studies relevant for pyroclastic surges and volcanic plumes indicates that there is a need for experiments above the mixing transition ren10 4 and for scaling st and rt among the particle dynamic regimes present in surges and plumes some deserve special attention such as the role of mesoscale structures on transport and sedimentary processes or the consequences of the transition to turbulence on particle gathering and dispersal the compilation of seven experimental studies relevant to magma bodies indicates that in the laminar regime crystals mostly follow the motion of the melt and thus the physical state of the system can be approximated as single phase in the transition to turbulence magmas can feature spatially heterogeneous distributions of laminar regions and important velocity gradients this heterogeneity has a strong potential for crystals sorting in conclusion the re st rt framework demonstrates

mixing and compositional stratification produced by natural convection 1 experiments and their application to earth s core and mantle an extensive series of laboratory experiments is used to quantify the circumstances under which fluids can be mixed by natural convection at high flux rayleigh number a compositionally buoyant fluid was injected at a fixed rate into an overlying layer of ambient fluid from a planar horizontally uniform source the nature of the resulting compositional convection was found to depend on two key dimensionless parameters a reynolds number re and the ratio u of the ambient fluid viscosity to the input fluid viscosity increasing the reynolds number corresponded to increasing the vigor of the convection while the viscosity ratio was found to determine the spacing between plumes and whether buoyant fluid rose as sheets u 1 from measurements of the final density profile in the fluid after the experiments we quantified the extent to which buoyant liquid was mixed in terms of a thermodynamic mixing efficiency e the mixing efficiency was found to be high e 0 9 when either the reynolds number was large re 100 or the viscosity ratio was small u 200 the amount of mixing was related to whether ascending plumes generated a large scale circulation in the ambient fluid when our results are applied to the differentiation of the earth s core we suggest that the convection resulting from the release of buoyant residual liquid into the liquid outer core due to crystallization at the boundary between the inner and the outer core will probably lead to nearly complete mixing in the dynamically very different context of the mantle mantle plumes are predicted to ascend through the mantle and pond beneath the lithosphere whereas convection driven by the subduction of oceanic lithosphere is expected to produce moderate to extensive mixing of the mantle when the competing plate and plume modes of mantle convection are considered together we find that owing to a larger driving buoyancy flux the plate scale flow will destroy any stratification at the top of the mantle produced by mantle plumes applying our results to the stagnant lid style of thermal convection predicted to occur in the mantles of the moon mercury mars venus and pre archean earth we expect the respective flows to produce minor thermal stratification at the respective core mantle boundaries in part 2 of this study jellinek and kerr this issue we apply our results to the differentiation of magma chambers and komatiite lava flows

the role of large bubbles detected from acoustic measurements on the dynamics of erta ale lava lake ethiopia the activity at the surface of the lava lake on erta ale volcano ethiopia shows that large bubbles are regularly breaking at a fixed position on the lava lake this is also where the small lava fountains are sometimes produced since this location is likely to be directly above the volcanic conduit feeding the lava lake we have done continuous measurements between march 22 and 26 2003 to understand the degassing of a volcano in permanent activity the bubble size has been first estimated from videos which once combined with the acoustic pressure can constrain the source of the sound the gas volume and overpressure stayed roughly constant between 36 700 m3 and 4 103 1 8 104 pa respectively simultaneous thermal measurements showed regular peaks which occurred when the crust was broken by a large bubble hence gave a direct indication on the typical return time between the bubbles 1 h these spherical cap bubbles had a high reynolds number 4600 20000 therefore a wake periodically unstable formed and detached from the bubble bottom the bubbly wake if the detachment occurs close to the surface can explain the duration of lava fountains measured on the videos the periodic arrival of bubbly wakes which mostly detach from the driving spherical cap within the lava lake could explain the absence of cooling at erta ale erebus antartica villarica chile and nyiragongo democratic republic of congo without invoking a convective downflow of magma in the conduit as previously done

the magmatic rayleigh number and time dependent convection in cooling lava lakes estimates of the rayleigh number are necessary for determining the thermal and dynamic states of convecting magma calculation of a temperature based rayleigh number ra sub t is complicated by the thermal coupling that exists between an intrusion and its host however the typically greater thermal resistance of the host regime compared to that of convecting magma can allow the host to control heat flux from a magma chamber in a manner somewhat independent of the state of convection in the chamber in a system where heat flux is controlled rather than temperature a heat flux based rayleigh number ra sub f is a more natural parameter to employ although it has not been used in most magmatic convection models while the insulating properties of the host regime tend to reduce the value of the thermal rayleigh number whether temperature or flux based this value can be very large compared to any critical value of the rayleigh number that might exist the quantity ra sub f is used to obtain useful relationships for estimating the thermal and kinematic regime in magma conservative models of a cooling lava lake which include temperature dependent viscosity show that convection will be vigorous enough to circulate phenocrysts more however the time dependent nature of magmatic convection and the tendency of whole body convection to vanish before crystallization is complete severely complicate any attempt to infer convective amplitudes from the observed distribution of crystals frozen into the interior of the body copyright american geophysical union 1987 less

cooling and crystallization in sheet like magma bodies revisited abstract in order to characterize the interaction of convection and crystallization during the solidification of magmatic sills a one dimensional model for the solidification of magmatic sills cooling from above has been used the model is based on a stefan type solution for a mushy zone and takes into account the inward moving solidification front whose speed is exclusively controlled by crystal growth below the solidification front the magma cools by convection it may eventually cool below its liquidus and start to partially crystallize a kinetic model has been implemented to account for this crystallization the model calculations lead to three main conclusions 1 crystal growth is not the only factor controlling the advance of magmatic solidification fronts the advance appears to be mainly controlled by nucleation processes and only to a small extent by crystal growth 2 the longevity and vigor of convection inside a magma chamber is strongly dependent on the kinetics of crystallization of magma inside the chamber 3 immediately after the initiation of cooling the melt becomes laced with nuclei

water in snc meteorites evidence for a martian hydrosphere the shergotty nakhla chassigny snc meteorites purportedly of martian origin contain 0 04 to 0 4 percent water by weight oxygen isotopic analysis can be used to determine whether this water is extraterrestrial or terrestrial such analysis reveals that a portion of the water is extraterrestrial and furthermore was not in oxygen isotopic equilibrium with the host rock lack of equilibrium between water and host rock implies that the lithosphere and hydrosphere of the snc parent body formed two distinct oxygen isotopic reservoirs if mars was the parent body the maintenance of two distinct reservoirs may result from the absence of plate tectonics on the planet

numerical modelling of convection interacting with a melting and solidification front application to the thermal evolution of the basal magma ocean abstract melting and solidification are fundamental to geodynamical processes like inner core growth magma chamber dynamics and ice and lava lake evolution very often the thermal history of these systems is controlled by convective motions in the melt computing the evolution of convection with a solid liquid phase change requires specific numerical methods to track the phase boundary and resolve the heat transfer within and between the two separate phases here we present two classes of method to model the phase transition coupled with convection the first referred to as the moving boundary method uses the finite element method and treats the liquid and the solid as two distinct grid domains in the second approach based on the enthalpy method the governing equations are solved on a regular rectangular grid with the finite volume method in this case the solid and the liquid are regarded as one domain in which the phase change is incorporated implicitly by imposing the liquid fraction f l as a function of temperature and a viscosity that varies strongly with f l we subject the two modelling frameworks to thorough evaluation by performing benchmarks in order to ascertain their range of applicability with these tools we perform a systematic study to infer heat transfer characteristics of a solidifying convecting layer parametrized relations are then used to estimate the super isentropic temperature difference maintained across a basal magma ocean bmo labrosse et al 2007 which happens to be minute 0 1 k implying that the earth s core must cool at the same pace as the bmo

a seismic crustal section across the east cape convergent margin new zealand abstract crustal seismic reflection measurements were made along a 250 km profile across the pacific australian plate boundary north of new zealand data were recorded to 16 s two way time and have defined a major sedimentary basin corresponding to the southern end of the raukumara basin a fore arc basin reaching a depth of about 13 km this basin contains a lower generally conformable sequence of sediments and an upper sequence dominated by several major uncomformities the eastern part of this basin is cut by several recently active eastward dipping high angle thrust faults which are interpreted to be caused by the recent convergence at the adjacent pacific australian plate boundary the eastern margin of the basin is marked by a sharp change in character on the seismic data we interpret this feature as a terrane boundary coinciding with a major transcurrent fault which has juxtaposed a detached fragment of an island arc fore arc with a thin 15 km crust to the east against a passive margin sedimentary sequence to the west the southern raukumara basin is floored by cretaceous age oceanic crust fragments of which were obducted onto the north island about 22 ma ago the area has had a complex history a schematic model for the evolution of this margin during the past 22 ma is developed suggesting that the raukaumara basin has developed as a combination of fore arc basin processes and oblique crustal extension

distribution of orthocumulate textures in the jimberlana intrusion the p and u contents and the texture of cumulates have been used to document variations in residual porosity in the jimberlana intrusion the results show that rocks with low residual porosity adcumulate textures occur where heat loss through the floor of the chamber was low or where the viscosity of the melt from which they crystallized was low low residual porosity also correlates with the appearance of plagio clase as a cumulus phase and with zones of well developed rhythmic layering it is suggested that the initial porosity of cumulates is controlled by the balance between nucleation and crystal growth at the crystalliquid interface low initial porosity represents attainment of textural equilibrium and forms when adjacent crystals at the top of the cumulate pile continue to grow expelling the intercumulus magma in the process high initial porosities develop when new crystals nucleate at the top of the crystal pile before a high proportion of the intercumulus magma is expelled low viscosity in

polycyclic aromatic hydrocarbons pahs in antarctic martian meteorites carbonaceous chondrites and polar ice recent analyses of the carbonate globules present in the martian meteorite alh84001 have detected polycyclic aromatic hydrocarbons pahs at the ppm level mckay et al 1996 the distribution of pahs observed in alh84001 was interpreted as being inconsistent with a terrestrial origin and were claimed to be indigenous to the meteorite perhaps derived from an ancient martian biota we have examined pahs in the antarctic shergottite eeta79001 which is also considered to be from mars as well as several antarctic carbonaceous chondrites we have found that many of the same pahs detected in the alh84001 carbonate globules are present in antarctic carbonaceous chondrites and in both the matrix and carbonate druse component of eeta79001 we also investigated pahs in polar ice and found that carbonate is an effective scavenger of pahs in ice meltwater moreover the distribution of pahs in the carbonate extract of antarctic allan hills ice is remarkably similar to that found in both eeta79001 and alh84001 the reported presence of l amino acids of apparent terrestrial origin in the eeta79001 druse material mcdonald and bada 1995 suggests that this meteorite is contaminated with terrestrial organics probably derived from antarctic ice meltwater that had percolated through the meteorite our data suggests that the pahs observed in both alh84001 and eeta79001 are derived from either the exogenous delivery of organics to mars or extraterrestrial and terrestrial pahs present in the ice meltwater or more likely from a mixture of these sources it would appear that pahs are not useful biomarkers in the search for extinct or extant life on mars

convective crystal dissolution the effect of free and forced convection on crystal dissolution is examined both theoretically and experimentally well established relationships for heat and mass transfer are applied to obtain approximate expressions for the dissolution velocity and the associated thickness of the compositional boundary layer these expressions are found to be in good agreement with experimental observations of the dissolution of quartz crystals in basalt and nacl crystal in water when applied to light felsic crystals in basaltic magmas the expressions predict that forced convection will produce a boundary layer thickness of about 100 m and a dissolution velocity of order 10 6 cm s 1 these velocities are too slow for xenocrysts to be dissolved significantly during magma ascent in dykes but are sufficient for cm size crystals to dissolve in the interior of a convecting magma chamber larger crystals are likely to accumulate at the chamber s roof where free convection is predicted to dissolve them at velocities of order 10 7 cm s 1 in an appendix the dissolution of the chamber s walls is also considered and a velocity of order 10 8cm s 1 is predicted

hikurangi plateau a cretaceous large igneous province in the southwest pacific ocean the first dredge haul of basement rocks obtained from the hikurangi plateau in the southwest pacific ocean consists dominantly of volcanic and volcaniclastic rocks of probable pre late cretaceous age all samples have undergone extensive seafloor weathering to phyllosilicate and zeolite bearing assemblages petrography mineral chemistry and whole rock element concentrations and ratios of the least altered lavas e g tio2 1 3 1 6 wt cr 132 224 ppm zr y 2 9 3 6 zr nb 19 29 ti v 22 29 suggest a restricted igneous compositional range broadly comparable with normal to enriched mid ocean ridge basalt however isotopic compositions of leached samples 87sr 86sri 0 70361 0 70374 endi 5 7 6 2 are comparable with oceanic island basalt suites collectively these petrological characteristics are similar to rocks from large igneous provinces of cretaceous age in the western pacific ocean e g manihiki and ontong java plateaus consequently we interpret the hikurangi plateau as another of these early cretaceous basaltic oceanic plateaus speculatively parts of the obducted tangihua and matakaoa igneous complexes of the north island may have been derived from the hikurangi plateau as the plateau collided with the new zealand continent in the early miocene

in situ hydroclastic fragmentation of subaqueous ponded lavas new senator caldera abitibi greenstone belt quebec canada abstract subaqueous ponded lavas have been recognized within the structure of the new senator caldera in the blake river group of the abitibi greenstone belt quebec canada sub vertical to near vertical dips of extrusive volcanic facies within the southern sector of the caldera permit the identification of the internal architecture of an archean subaqueous volcanic complex detailed facies analyses of the dominantly mafic sequences have shown two localities with atypical primary volcanic structures and facies inconsistent with those observed at modern subaqueous mafic seamounts combined with the use of facies specific geochemical analyses a coherent model for the two localities accounting for all observed volcanic structures and features has been developed mapping completed at a scale of 1 100 at localities 1 and 2 reveals a series of sub parallel to parallel hyaloclastite rich horizons separating aphanitic to medium grained ponded units several primary volcanic structures are observed within the hyaloclastite horizons and are identified as pillowed forms cigar forms and v shaped structures ponded units range in thickness from 2 5 to 5 m with the exception of one unit which is approximately 30 m thick in central portions this thick unit is medium grained and has a sub ophitic texture units at locality 1 strike approximately nw while those at locality 2 strike ene several families of mafic dykes cross cut locality 1 and trend e w and nne both volcaniclastic and effusive facies have similar compositions affinities and trace and rare earth element signatures all facies have a tholeiitic affinity range in composition from basalt to andesite and have morb type trace and rare earth element signatures locality 1 appears slightly more evolved than locality 2 with later dykes and a late sill being the most evolved facies volcanic facies at localities 1 and 2 are consistent with subaqueous ponded lavas bounding synvolcanic structures and volcanic facies validate this interpretation and indicate these ponded lavas are hosted within a large synvolcanic depression at the summit of a mafic shield complex primary volcanic structures within hyaloclastite horizons formed via in situ fragmentation when water entered the system as ponded lavas began to cool water entered the system through synvolcanic fractures and encountered semi molten pockets of lava leading to hydroclastic fragmentation of the lavas v shaped and cigar structures formed during more energetic events and changed laterally into pillowed forms as energy dissipated from the system

compositional convection in a reactive crystalline mush and melt differentiation fractional crystallization and partial melting involve relative motion of liquid and solid phases and chemical and thermal interactions between them to elucidate some physical principles of thermosolutal convection in a reactive porous medium we describe experiments on the directional solidification of aqueous ammonium chloride solutions the addition of small amounts of a polymerizing agent permits variation of the solution viscosity independently of thermal conditions the phase diagram and permeability solutions were cooled from below and crystallization developed at the base of the tank generating at first a field of thin plumes of light residual fluid released from the boundary layer at the top of the mush the interstitial fluid within the mush also became unstable eventually the onset of convection occurring when the porous medium rayleigh number of the mush reached a critical value this threshold value was found to be 25 at low initial superheat and to decrease with increasing superheat local thermodynamic equilibrium between crystals and liquid within the mush coupled the evolution of temperature composition and porosity convective motions locally caused dissolution and precipitation and hence fluctuations of porosity developed dissolution occurred preferentially in the central parts of upwellings and the upflow gradually focused being ultimately channelized into narrow chimneys devoid of crystals all else being equal the areal density of chimneys was greater the lower the viscosity chimney diameter increased with increasing solution viscosity in the liquid above the growing mush the convective plumes were very similar to salt fingers depending on solution viscosity and temperature gradient they exhibited a phenomenon of collective instability such that vertical motion was disrupted by wave instabilities the base plate temperature chosen was above the eutectic and hence the total amount of crystals at the end of experiments with the same initial composition and the same final temperature was a constant fixed by the phase diagram the spatial distribution of crystals and the final porosity of the mush were however determined by the strength of compositional convection as measured by the porous medium rayleigh number when no convection occurred mush thickness finally became equal to the initial layer thickness and the system was homogeneous when compositional convection occurred the overlying reservoir underwent chemical evolution and mush growth slowed dramatically in experiments with progressively lower solution viscosity and hence more vigorous convection final mush thickness was progressively less and final porosity lower final mush thickness was found to scale with solution viscosity to the power 0 33 during fractional crystallization of magma the effects of compositional convection can be recorded in the chemical and mineralogical features of cumulate rocks we speculate that fossil chimney structures can be found in the lower zone of the bushveld ultramafic complex in the form of iron rich platinum bearing dunite pipes this could explain the adcumulate nature of the lower zone rocks and the chemical and mineralogical similarities between the pipes and the overlying merensky reef the merensky reef would be interpreted as a hiatus in mush growth the results may also have applications to the flow structure in regions of partial melting and in the earth s core

replenishment of magma chambers by light inputs magma chambers particularly those of basaltic composition are often replenished by an influx of magma whose density is less than that of the resident magma this paper describes the fundamental fluid mechanics involved in the replenishment by light inputs if denotes the uniform density of the resident magma and that of the input the situation is described by the reduced gravity g g the volume flux q and the viscosities of the resident and input magmas e and i respectively the nondimensional reynolds numbers ree g q3 1 5 e and rei g q3 1 5 i and chamber geometry then completely specify the system for sufficiently low values of the two reynolds numbers each less than approximately 10 the input rises as a laminar conduit for larger values of the reynolds numbers the conduit may break down and exhibit either a varicose or a meander instability and entrain some resident magma at still larger reynolds numbers the flow will become quite unsteady and finally turbulent the values of the reynolds numbers at which these transitions occur have been documented by a series of experiments with water glycerine and corn syrup if the input rises as a turbulent plume significant entrainment of the resident magma can take place the final spatial distribution of the mixed magma depends on the geometry of the chamber if the chamber is much wider than it is high the mixed magma forms a compositionally stratified region between the roof and a sharp front above uncontaminated magma in the other geometrical extreme the input magma is mixed with almost all of the resident magma if the density of the resident magma is already stratified the input plume may penetrate only part way into the chamber even though its initial density is less than that of the lowest density resident magma the plume will then intrude horizontally and form a hybrid layer at an intermediate depth this provides a mechanism for preventing even primitive basaltic magmas of minimum density from erupting at the surface by conducting an experiment using aqueous solutions we show that entrainment can lead to crystallization of the magma in the input plume by making it locally supersaturated all these effects are discussed and illustrated by photographs of laboratory experiments

laboratory modeling of convection in magma chambers crystallization against sloping floors crystallization against inclined floors in magma chambers commonly releases a light depleted fluid which drives compositional convection we describe the fundamental fluid mechanics of this process and derive equations describing the buoyant compositional boundary layer adjacent to the top of the crystal pile the question of whether magma chambers become zoned as a result of compositional convection from sloping floors as has been previously suggested is addressed in a laboratory study because crystal sizes in magma chambers are usually smaller than the thickness of the compositional boundary layer an appropriate laboratory model is thermal convection from a smooth sloping heat exchanger we investigate the circumstances under which this configuration leads to thermal stratification and find that only when the experimental tank has an extension with an unheated floor does a filling box process lead to significant stratification this geometry however is geologically unrealistic requiring that crystallization be confined to only part of the floor we conclude that compositional convection from inclined floors will not lead to compositional stratification in magma chambers experiments investigating the same problem by cooling and crystallizing aqueous solutions are misleading partly because the size of crystals grown from aqueous solutions is extremely large compared with the thickness of the compositional boundary layer whereas this is not usually true in magma chambers this difference has a major influence on the nature of convection and we suggest that caution should be applied in extending the results of aqueous solution experiments involving crystallization to magma chambers the petrological consequences of this work are 1 the fluid mechanics of crystallization at an inclined floor and horizontal floor are similar and suggestions made elsewhere concerning the geological implications in the latter case apply equally to the sloping floor case 2 large volumes of homogeneous magma can exist in a magma chamber when crystallization is taking place against an inclined floor and 3 models describing the crystallization of layered intrusions which involve the interaction of crystallization against an inclined floor with compositionally stratified magma require the stratification to be established by an external process

what can crystal size distributions and olivine compositions tell us about magma solidification processes inside kilauea iki lava lake hawaii abstract lava lakes offer the opportunity to investigate magma solidification and can be considered as a proxy for small magma chambers here we present olivine compositions and crystal size distributions csds from scoria and drill core samples from kilauea iki lava lake which formed during the 1959 eruption of kilauea volcano hawaii three chemically distinct olivine populations were distinguished in the basis of their forsterite fo content 1 a high fo population fo86 90 2 an intermediate fo population fo78 82 and 3 a minor low fo population fo74 78 populations 1 and 2 both have deformed and undeformed crystals the third population may be the result of rejuvenation olivine in the lower 60 m of lake has a less fo rich composition and more crystals are deformed the csd analysis yields estimates of the average olivine residence time 1 60 years the shape of the olivine csds is fairly uniform with respect to depth curved csds are considered to be evidence of hybrid populations partly or totally involving crystal or magma mixing the turndown at the smallest sizes of most foundered crust and lake csds may be the result of coarsening making this process active both before and after eruption our csd modelling does not support significant crystal settling and overall convection in the lava lake although small advective currents are known to have occurred the olivine vertical stratification cannot be an original feature which is consistent with supposed strong stirring of the lake magma due to intense activity over the 17 eruptive phases it is also possible that independent basal feeding of the lake during the eruption may be needed to explain fully features of the chemical and mineralogical stratification

carbonates in martian meteorite alh84001 a planetary perspective on formation temperature we have re examined the formation temperature of carbonates in alh84001 accounting for the prior evolution of the stable isotopes used in the analysis oxygen and carbon in the atmosphere likely have become isotopically fractionated due to various nonthermal loss mechanisms operating throughout geologic time taking this previous evolution into account we derive a formation temperature from oxygen isotope thermometry of 40 250 c this range of temperatures represents a lower limit an increase in groundwater salinity could further raise the 18o of the groundwater as seen in terrestrial hydrothermal systems allowing for an even higher temperature of formation derived from the isotopic data

quantitative field constraints on the dynamics of silicic magma chamber rejuvenation and overturn the catastrophic eruption of large volume crystal rich silicic magmas is often proposed to be a consequence of reheating melting and overturn of partially molten buoyant silicic material following repeated injection of dense hot mafic magma to test this rejuvenation hypothesis we analyze at high spatial resolution 33 examples of deformed interfaces between intrusive mafic and silicic layers in two plutons of the coastal maine magmatic province usa these deformed interfaces are thought to record the buoyant overturn of silicic crystal mush layers apparently in response to the injection and cooling of hot dense mafic magmas we use spectral analysis and scaling theory along with petrologic and textural data to identify characterize and understand periodic deformations from the scale of individual crystals 1 cm to the thicknesses of mafic and silicic layers deformations at the largest scale lengths 100 m are at wavelengths comparable to or greater than silicic layer thicknesses and support a conjecture that mafic recharge can cause large scale rayleigh taylor type overturning of silicic mushy layers by contrast the smallest scales of individual crystals probably record effects related to production and buoyancy driven rise of melt from the tops of silicic mushes in contact with overlying hot basalt whereas intermediate scales are explained by compaction our results constrain the evolution of a thermal rejuvenation event and potentially identify a condition for a large scale overturn of the magma chamber that may lead to eruption this work provides the first quantitative field based constraints on some of the key physical processes inherent to the rejuvenation hypothesis

rates of thermal and chemical evolution of magmas in a cooling magma chamber a chronological and theoretical study on basaltic and andesitic lavas from rishiri volcano japan rates of magmatic processes in a cooling magma chamber were investigated for alkali basalt and trachytic andesite lavas erupted sequentially from rishiri volcano northern japan by dating of these lavas using u th radioactive disequilibrium and c dating methods in combination with theoretical analyses we obtained the eruption age of the basaltic lavas to be 29 3 0 6 ka by c dating of charcoals the eruption age of the andesitic lavas was estimated to be 20 2 3 1ka utilizing a whole rock isochron formed by u th fractionation as a result of degassing after lava emplacement because these two lavas represent a series of magmas produced by assimilation and fractional crystallization in the same magma chamber the difference of the ages i e 9 kyr is a timescale of magmatic evolution the thermal and chemical evolution of the rishiri magma chamber was modeled using mass and energy balance constraints as well as quantitative information obtained from petrological and geochemical observations on the lavas using the timescale of 9 kyr the thickness of the magma chamber is estimated to have been about 1 7 km the model calculations show that in the early stage of the evolution the magma cooled at a relatively high rate 40 18c year and the cooling rate decreased with time convective heat flux from the main magma body exceeded 2w m when the magma was basaltic and the intensity diminished exponentially with magmatic evolution volume flux of crustal materials to the magma chamber and rate of convective melt exchange compositional convection between the main magma and mush melt also decreased with time from 0 1m year to 10 3 m year and from 1m year to 10 2 m year respectively as the magmas evolved from basaltic to andesitic compositions although the mechanism of the cooling i e thermal convection and or compositional convection of the main magma could not be constrained uniquely by the model it is suggested that compositional convection was not effective in cooling the main magma and the magma chamber is considered to have been cooled by thermal convection in addition to heat conduction

magnetostratigraphic dating of shallow water carbonates from san salvador bahamas magnetostratigraphic results are reported here from a sequence of late neogene quaternary shallow water carbonate sediments from a continuous core drilled on the island of san salvador bahamas on the basis of the remanent magnetism of 136 samples from a 91 m measured section of core the polarity sequence can be correlated with the magnetic polarity time scale from the gilbert chron early pliocene through the late brunhes chron late pleistocene holocene magnetic polarities were determined on the basis of relative up down direction in the unoriented core extraction studies of the magnetic particles reveal the presence of single domain crystals of magnetite resembling those produced by the magnetotactic bacteria and algae the sequence of reversals provides a minimum of six new major chronostratigraphic markers for the pliocene pleistocene of the bahamas it confirms and refines the local timing of both the lithologic change from skeletal to nonskeletal sediments and the disappearance of coral and molluscan species from the bahamas as upper late pliocene between 2 6 and 2 7 ma that the primary magnetic remanence is preserved in shallow water carbonates including replacement dolomites suggests that this technique could be used to date similar tertiary and possibly even older carbonate sequences the establishment of a reliable magnetostratigraphy provides refined dating of shallow water carbonates and regional faunal appearances or disappearances sediment accumulation rates subsidence and depositional events

formation of carbonates in the tatahouine meteorite the tatahouine meteorite in southern tunisia shows terrestrial contamination that developed during 63 years of exposure on earth s surface samples collected on the day of the fall in 1931 contained fractures with no secondary minerals whereas samples collected in 1994 contain calcite aggregates 70 to 150 micrometers and rod shaped forms 100 to 600 nanometers in length and 70 to 80 nanometers in diameter on the fractures carbon isotope analysis of the carbonates within the tatahouine meteorite delta13c 2 0 per mil pee dee belemnite standard pdb and the underlying ground delta13c 3 2 per mil pdb confirm their terrestrial origin

ar ar chronology of the martian meteorite alh84001 evidence for the timing of the early bombardment of mars abstract alh84001 a cataclastic cumulate orthopyroxenite meteorite from mars has been dated by ar ar stepped heating and laser probe methods both methods give ages close to 3 900 ma the age calculated is dependent on assumptions made about 39ar recoil effects and on whether significant quantities of 40ar from the martian atmosphere are trapped in the meteorite if as suggested by xenon and nitrogen isotope studies martian atmospheric argon is present then it must reside predominantly in the k rich phase maskelynite independently determined 129xe abundances in the maskelynite can be used to place limits on the concentration of the atmospheric 40ar these indicate a reduction of around 80 ma to ages calculated on the assumption that no martian atmosphere is present after this correction the nominal ages obtained are 3940 50 3870 80 and 3970 100 ma by stepped heating and 3900 90 ma by laser probe 1 statistical errors giving a weighted mean value of 3 920 ma ambiguities in the interpretation of 39ar recoil effects and in the contribution of martian atmospheric 40ar lead to uncertainties in the ar ar age which are difficult to quantify but we suggest that the true value lies somewhere between 4 050 and 3 800 ma this age probably dates a period of annealing of the meteorite subsequent to the shock event which gave it its cataclastic texture the experiments provide the first evidence of an event occurring on mars coincident with the time of the late heavy bombardment of the moon and may reflect a similar period of bombardment in the southern highlands of mars whether the age determined bears any relationship to the time of carbonate deposition in alh84001 is not known such a link depends on whether the temperature associated with the metasomatic activity was sufficient to cause argon loss from the maskelynite and or whether the metasomatism and metamorphism were linked in time through a common heat source

the relative roles of boundary layer fractionation and homogeneous fractionation in cooling basaltic magma chambers abstract in a cooling magma chamber magmatic differentiation can proceed both by fractionation of crystals from the main molten part of the magma body homogeneous fractionation and by mixing of the main magma with fractionated melt derived from low temperature mush zones boundary layer fractionation in this study the relative roles of boundary layer fractionation and homogeneous fractionation in basaltic magma bodies were examined using a thermodynamics based mass balance model model calculations show that boundary layer fractionation cannot be a dominant fractionation mechanism when magma chambers are located at low pressures 2 wt such as arc basalt and the magma chambers are located at depth 100 mpa because the melt derived from mush zones is enriched in alkalis and h 2 o crystallization from the main magma is suppressed by mixing with the mush melt as a consequence of depression of the liquidus temperature therefore homogeneous fractionation is more effectively suppressed in magma chambers in which boundary layer fractionation is more active if magmatic differentiation proceeds primarily by boundary layer fractionation magmas can remain free of crystals for long periods during magmatic evolution

the hikurangi plateau abstract recently collected geophysical data reveal the basement and sedimentary structure of the hikurangi plateau presently being subducted beneath the north island of new zealand we have subdivided the plateau into 6 areas on the basis of basement structure two basins separated by a basement high trend subparallel to the chatham rise probably filled with mesozoic sediments another is parallel with the present plate margin along the east coast of the north island and is filled with late cenozoic sediments the northern part of the plateau is characterised by shallow volcanic basement the southeast part of the region is complex but has several kilometers of sediments deformed by volcanic intrusions at least two episodes of volcanism have occurred the older probably mesozoic episode produced numerous large isolated edifices some exposed as seamounts and others buried by sediments activity resumed in the neogene resulting in the emplacement of smaller knolls and widespread intrusions variation in crustal thickness north of the chatham rise supports the hypothesis of a convergent margin along the north flank of the chatham rise in the mesozoic the structure of the hikurangi plateau may explain the origin of the postulated physiographic variations and segmentation of the downgoing slab along the active kermadec hikurangi subduction margin

magma differentiation rates from 226ra 230th and the size and power output of magma chambers we present a mathematical model for the evolution of the 226ra 230th activity ratio during simultaneous fractional crystallization and ageing of magma the model is applied to published data for four volcanic suites that are independently known to have evolved by fractional crystallization these are tholeiitic basalt from ardoukoba djibouti morb from the east pacific rise alkali basalt to mugearite from vestmannaeyjar iceland and basaltic andesites from miyakejima izu bonin arc in all cases 226ra 230th correlates with indices of fractional crystallization such as th and the data fall close to model curves of constant fractional crystallization rate the best fit rates vary from 2 to 6 10 4 yr 1 consequently the time required to generate moderately evolved magmas f 0 7 is of the order of 500 to 1500 yrs and closed magma chambers will have lifetimes of 1700 to 5000 yrs these rates and timescales are argued to depend principally on the specific power output i e power output per unit volume of the magma chambers that are the sites of fractional crystallization equating the heat flux at the epr to the heat flux from the sub axial magma chamber that evolves at a rate of ca 3 10 4 yr 1 implies that the magma body is a sill of ca 100 m thickness a value which coincides with independent estimates from seismology the similarity of the four inferred differentiation rates suggests that the specific power output of shallow magma chambers in a range of tectonic settings covers a similarly narrow range of ca 10 to 50 mw km 3 their differentiation rates are some two orders of magnitude slower than that of the basaltic makaopuhi lava lake hawaii that cooled to the atmosphere this is consistent with the two orders of magnitude difference in heat flux between makaopuhi and the east pacific rise 226ra 230th data for magma suites related by fractional crystallization allow the magma differentiation rate to be estimated and from this the thermal budget of the magma chamber addressed and where an independent measurement of heat flux exists to place constraints on the size of the magma chamber such results have the potential to constrain the likely timescale and size of future eruptions of evolved magmas

quantifying the building blocks of igneous rocks are clustered crystal frameworks the foundation most phenocryst populations in volcanic rocks and those preserved in shallow level igneous intrusions are clustered variously referred to as clots clumps or glomerocrysts these clusters of crystals are the building blocks that accumulate to form the high porosity touching crystal frameworks from which igneous cumulates form examination of touching crystal frameworks in olivine komatiite cumulates and experimental charges and plagioclase dominant crystal populations holyoke flood basalt connecticut usa reveal complex high porosity clustered crystal arrangements olivine touching frameworks in komatiite flows are interpreted to form in hundreds of days plagioclase frameworks are calculated to have formed in less than 17 years for a crystal growth rate of 1 x 10 10 mm s to less than 3 years for a growth rate of 5 x 10 10 mm s based on crystal size distributions the origin of crystal clusters is likely to involve either or a combination of heterogeneous nucleation remobilization of cumulate mushes or crystals sticking together during settling and or flow the spatial distribution pattern of clustered crystal frameworks from both natural and experimental examples constrains fields on spatial packing diagrams that allow the identification of touching and non touching crystal populations and further improve our understanding of crystal packing arrangements and cluster size distributions

dynamics of evolving magma chambers textural and chemical evolution of cumulates at the arrival of new liquidus phases abstract basaltic magma chambers are best exemplified by layered intrusions fossilized natural laboratories that historically constrain most fundamental principles of igneous petrology progressive fractional crystallization of basaltic melts in layered intrusions results in successive appearance of 5 10 liquidus minerals most minerals appear in the stratigraphy of layered intrusions via two distinct stages they first emerge as oikocrysts large irregularly shaped and interstitial crystals that form in a crystal liquid framework and then as primocrysts relatively small idiomorphic and cumulus crystals that crystallize from a main magma body the transition from oikocrysts to primocrysts is commonly marked by the following textural and chemical features a a similarity in chemical composition of the last forming oikocrysts and the first forming primocrysts b slow arrival modal overabundance and finer than normal size of the first primocrysts c a low abundance of trapped liquid in rocks with the first forming primocrysts d a step like increase in mineral dihedral angle in rocks slightly below the level of the primocryst arrival and e the rims on plagioclase primocrysts that are identical in composition to cores of plagioclase primocrysts at the first appearance of new liquidus phases higher up in the section the phase equilibria control on the cumulus stratigraphy of layered intrusions require large long lived and largely molten magma chambers and therefore their origin is not compatible with several recent concepts that deny the existence of such reservoirs in the earth s crust we therefore suggest that the systematic changes associated with the arrival of cumulus phases in layered intrusions are best explained by a traditional concept that considers layered intrusions as large initially crystal free melt bodies that gradually lose heat and crystallize from margins inwards through thin solidification fronts the entire magma body during its internal evolution is thought to be kinetically supercooled i e kept slightly below its liquidus temperature with respect to cumulus phases that crystallize at the margins each new cumulus phase arrives on the liquidus with some kinetic delay and nucleates heterogeneously against pre existing crystals at the floor roof and sidewalls of basaltic magma chambers this implies that internal differentiation of basaltic magma chambers mostly occurs through convective separation of evolved liquid from in situ growing crystals and its mixing with the main magma body

mechanisms of differentiation in the skaergaard intrusion thanks to its magnificent exposures and extraordinary petrologic development the skaergaard intrusion provides an excellent test for three basic mechanisms of crystal liquid fractionation diffusive exchange compaction convective exchange 1 diffusive exchange at the solidification front does not seem to have played a major role for no obvious difference is seen in the behavior of components of widely differing diffusivities the effects of diffusion were probably masked by porous flow of the interstitial liquid 2 compaction and 9filter pressing9 of residual liquids can be tested by comparing concentrations of incompatible elements in parts of the intrusion that crystallized in different orientations with respect to gravity the contrast between the large concentrations of these elements in the upper border series and consistently smaller abundances in the layered series long thought to be due to contamination of the upper levels with crustal rocks is more likely a result of compaction that took place at the floor but not under the roof 3 convective exchange driven by differences in the compositional densities of the main magma and interstitial liquids seems to have been important at the steep walls and to a lesser degree under the roof it may also have been responsible for a large compositional anomaly near the centre of the layered series where an iron rich liquid percolated down through parts of lower and middle zones at later stages of crystallization when density relations were reversed a buoyant liquid rich in volatiles and incompatible elements rose from the layered series and permeated the upper part of the intrusion a liquid of this latter kind could have caused the pervasive metasomatic alteration seen in much of the layered series in both compaction and convective exchange re equilibration and reactions between migrating liquids and early formed crystals augmented permeabilities and brought about extensive compositional and textural changes although inhomogeneities in the initial magma could account for some of the compositional variations the main trend of differentiation can be ascribed to a combination of compaction and convective fractionation the convective flow was of two kinds one of descending iron rich liquids and another of more evolved rising liquids and fluids with low densities and large concentrations of incompatible elements in the early stages the proportions of iron rich liquids were much greater than those of the silica rich variety and until the crystallization fronts converged at the sandwich horizon the former made the greatest contribution to evolution of the main body of magma as crystallization advanced however light silica rich liquids became more important they permeated the upper border series and much of the upper part of the layered series this late stage liquid continued to differentiate after the crystallization fronts met a secondary sandwich horizon was formed where incompatible elements reach maximum concentrations in the lower part of the upper border series

the role of laboratory experiments in volcanology experiment is central to scienti c methodology within science in general experiments are used for four primary purposes i e as a tool to explore novel phenomena and to provide systematic observations of processes to determine the values of key parameters to test hypotheses and theoretical models and to validate computational models which themselves can be considered as numerical experiments the study of volcanic eruptions and magmatic processes is becoming increasingly systematic quantitative and rigorous the drive is towards providing detailed quantitative descriptions and models of the ow phenomena in terms of fundamental physico chemical processes this is motivated in no small way by an increasing need for reliable and quantitative predictions of volcanic eruptions on which to base rational hazard and risk management decisions during volcanic crises a natural consequence of this development of the scienti c study of volcanic ow phenomena is that rigorous laboratory experimentation is becoming an increasingly important feature within volcanic research the contribution of experimental research to our understanding of volcanic processes is di cult to overstate the determination of the values of key parameters has traditionally taken place in the laboratory however more recently experiments have become much more widely applied to the problem of understanding complex dynamical processes and their underlying mechanisms without models experimental theoretical numerical we are limited to qualitative interpretations of eld observations and remote sensing field data are of course essential and provide the measure against which ultimately the applicability of observations from other sources must be assessed however there are several reasons why the scienti c study of volcanic phenomena cannot rely solely on eld data direct observations of eruption processes in the eld are limited to those parts of an eruption that are accessible this restricts detailed observations to a limited range of above surface processes moreover logistical constraints mean that the systematic collection of data will usually only be possible for volcanic eruptions near an established observatory and for activity for which there has been some forewarning as a result of these constraints direct eld based observations of natural volcanic eruption processes are generally incomplete and uncertain another source of natural data derives from observations made on volcanic deposits in principle processes can be inferred from the features

triggering basaltic volcanic eruptions by bubble melt separation understanding the processes by which volcanic eruptions are triggered is crucial for volcanic hazard prediction and assessment in intermediate and silicic magmas where water is the dominant volatile phase1 2 second boiling provides an effective eruption trigger1 as the magma cools and crystallizes volatiles are increasingly concentrated in the residual melt where they eventually become saturated and are exsolved thereby raising the magma pressure to the point of eruption1 2 in contrast in basaltic magma chambers in which carbon dioxide is the dominant volatile phase the effect of second boiling is to decrease the chamber pressure but basaltic magmas are also relatively inviscid so volatile bubbles can separate from the turbulently convecting melt to produce a foam layer at the chamber roof3 4 decompression of the rising bubbles can increase the chamber pressure and so trigger an eruption5 6 here we model the complex competition between the processes of magma cooling and bubble ascent we find that in basalt of very low viscosity 1 10pas bubble separation dominates and can increase the reservoir pressure by as much as 10 mpa sufficient to trigger an eruption7 on timescales of 1 10 years comparable to the duration of eruptive episodes at kilauea volcano hawaii4 8

density changes during the fractional crystallization of basaltic magmas fluid dynamic implications the dynamical behaviour of basaltic magma chambers is fundamentally controlled by the changes that occur in the density of magma as it crystallizes in this paper the term fractionation density is introduced and defined as the ratio of the gram formula weight to molar volume of the chemical components in the liquid phase that are being removed by fractional crystallization removal of olivine and pyroxene whose values of fractionation density are larger than the density of the magma causes the density of residual liquid to decrease removal of plagioclase with fractionation density less than the magma density can cause the density of residual liquid to increase during the progressive differentiation of basaltic magma density decreases during fractionation of olivine olivine pyroxene and pyroxene assemblages when plagioclase joins these mafic phases magma density can sometimes increase leading to a density minimum calculations of melt density changes during fractionation show that compositional effects on density are usually greater than associated thermal effects in the closed system evolution of basaltic magma several stages of distinctive fluid dynamical behaviour can be recognised that depend on the density changes which accompany crystallization as well as on the geometry of the chamber in an early stage of the evolution where olivine and or pyroxenes are the fractionating phases compositional stratification can occur due to side wall crystallization and replenishment by new magma with the most differentiated magma tending to accumulate at the roof of the chamber when plagioclase becomes a fractionating phase a zone of well mixed magma with a composition close to the density minimum of the system can form in the chamber the growth of a zone of constant composition destroys the stratification in the chamber a chamber of well mixed magma is maintained while further differentiation occurs unless the walls of the chamber slope inwards in which case dense boundary layer flows can lead to stable stratification of cool differentiated magma at the floor of the chamber in a basaltic magma chamber replenished by primitive magma the new magma ponds at the base and evolves until it reaches the same density and composition as overlying magma successive cycles of replenishment of primitive magma can also form compositional zonation if successive cycles occur before internal thermal equilibrium is reached in a chamber in a chamber containing well mixed plagioclase saturated magma the primitive magma can be either denser or lighter than the resident magma in the first case the new magma ponds at the base and fractionates until it reaches the same density as the evolved magma mixing then occurs between magmas of different temperatures and compositions in the second case a turbulent plume is generated that causes the new magma to mix immediately with the resident magma

origin of carbonate magnetite sulfide assemblages in martian meteorite alh84001 a review of the mineralogical isotopic and chemical properties of the carbonates and associated submicrometer iron oxides and sulfides in martian meteorite alh84001 provides minimal evidence for microbial activity some magnetites resemble those formed by magnetotactic microorganisms but cubic crystals 50 nm in size and elongated grains 25 nm long are too small to be single domain magnets and are probably abiogenic magnetites with shapes that are clearly unique to magnetotactic bacteria appear to be absent in alh84001 magnetosomes have not been reported in plutonic rocks and are unlikely to have been transported in fluids through fractures and uniformly deposited where abiogenic magnetite was forming epitaxially on carbonate submicrometer sulfides and magnetites probably formed during shock heating carbonates have correlated variations in ca mg and 18o 16o magnetite rich rims and they appear to be embedded in pyroxene and plagioclase glass carbonates with these features have not been identified in carbonaceous chondrites and terrestrial rocks suggesting that the alh84001 carbonates have a unique origin carbonates and hydrated minerals in alh84001 like secondary phases in other martian meteorites have o and h isotopic ratios favoring formation from fluids that exchanged with the martian atmosphere i propose that carbonates originally formed in alh84001 from aqueous fluids and were subsequently shock heated and vaporized the original carbonates were probably dolomite magnesite siderite assemblages that formed in pores at interstitial sites with minor sulfate chloride and phyllosilicates these phases like many other volatile rich phases in martian meteorites may have formed as evaporite deposits from intermittent floods

crystal settling and in situ crystallization in aqueous solutions and magma chambers abstract laboratory experiments are described in which aqueous solutions of potassium nitrate are cooled from above depending on the viscosity of the solution and the rate at which it is cooled the mechanism of crystallization varies in the least viscous most slowly cooled experiment after an initial period crystals nucleate and grow on pre existing crystals on the floor of the tank driving vigorous compositional convection in more viscous or more rapidly cooled experiments in addition to crystallization on the floor crystals nucleate and grow at the top of the fluid these crystals settle through the solution adding to the crystals on the floor the crystallization mechanism adopted by the laboratory experiments depends on the degree to which the solutions become supersaturated at low cooling rates and viscosities the critical supersaturation for homogeneous nucleation is not exceeded crystals do not form within the fluid and crystallization occurs only on the floor at higher cooling rates and viscosities the critical supersaturation is exceeded in this case crystals are able to form at the top of the fluid and crystal settling operates in addition to crystallization on the floor applying a similar argument to magma chambers it is suggested that in situ crystallization may occur on its own in the most slowly cooled least viscous mafic magma chambers in other chambers crystal settling is expected to accompany in situ crystallization a quantitative assessment of the crystallization of an olivine saturated mafic magma is presented for in situ crystallization to occur alone i e without any superimposed crystal settling in a chamber which is cooled by heat conduction through the country rock supercoolings of between 1 and 20 c must be maintained in the chamber without homogeneous nucleation occurring if crystal settling does occur then the crystals may not form in the thermal boundary layer because although it is the coldest part of the magma it is not necessarily the most supersaturated part because of the effect of pressure on the liquidus