controls of tor formation cairngorm mountains scotland tors occur in many granitic landscapes and provide opportunities to better understand differential weathering we assess tor formation in the cairngorm mountains scotland by examining correlation

a reevaluation of crystal size distributions in chromite cumulates although studies have shown that igneous cumulates can form by in situ crystallization without requiring crystal settling it has not been demonstrated that crystal size distributions csds are consistent with such a process plots of crystal size fractions per unit volume vs crystal size for chromite grains from the stillwater complex show a log linear distribution with negative slope at larger sizes and a concave down distribution at smaller sizes the log linear portion of these trends is similar to previously reported trends for other silicate and oxide phases in crystallizing magmas however the lack of smaller grain sizes indicates that the original population was altered during a postnucleation crystalaging period that resulted in the loss of the smaller size fractions an interpretation consistent with textural and volume balance evidence the csd trends imply that cr was not concentrated by settling of chromite but was brought to the site of the growing chromite grains similarities between the stillwater data and csd trends for garnet in metapelites suggest that such trends are a characteristic feature of any geologic system undergoing crystal aging after an initial period of nucleation and crystal growth

evidence for a crustal root beneath the paleoproterozoic collision zone in the northern ordos block north china abstract recent studies have shown that paleoproterozoic rocks called the khondalite belt crop out along a nearly ew trending zone in the northern edge of the ordos block north china this widely distributed khondalite rock suggests that they originated in the margin of a passive continent during the paleoproterozoic era seismic stations deployed in the northern ordos block and its adjacent regions provided abundant information on crustal thicknesses and vp vs ratios which were determined by use of receiver function h stacking the h stacked images show an approximately ew trending anomalous zone with thickened crust 46 5 km and a higher poisson s ratio 0 28 beneath the khondalite belt in the paleoproterozoic collision zone based on the long wavelength information of gravity anomalies a two dimensional crustal density model indicated that a crustal root with a density contrast of 130 kg m3 between the root and surrounding mantle is required for the isostatic gravity compensation the ratio of surface elevation to crustal root thickness was 0 02 suggesting that the exhumation of the crustal root in response to surface erosion continued to occur over time scales of hundreds of millions of years this crustal thickening model is evidence for the crustal root beneath the paleoproterozoic collision zone in the northern ordos block as the density contrast becomes smaller the amount of isostatic rebound in response to erosion also decreases and therefore the crustal root is maintained

crystal size distribution in jurassic ferrar flows and sills victoria land antarctica evidence for processes of cooling nucleation and crystallisation jurassic ferrar rocks in victoria land antarctica occur predominantly as basaltic or andesitic flows and sills both show characteristic petrographical and chemical variations which can be related to in situ differentiation processes such characteristics have been investigated at one flow colonnade flow and one sill thumb point sill in the prince albert mountains central victoria land based on a statistical grain size analysis and the application of the crystal size distribution theory a third magma body hiti unit which in previous literature was described as a flow does not show clear similarities to either the flow or sill sill and flow are in situ differentiated with accumulation of silicic residual melt in the latest cooled parts for the flow this part is located in the lower half and for the sill in the upper third of their thickness thumb point sill additionally shows an accumulation zone of olivine in the lower third the position of the residual melt accumulation zone is an indicator for an origin as flow or sill the hiti unit by contrast exhibits only moderate petrographical and chemical variations growth and nucleation rates have been determined using with some modifications the csd theory introduced by marsh 1988 growth rates of plagioclase for the flow vary from 10 11 to 10 10 cm s and are comparable with literature values for basaltic lava lakes on hawaii nucleation rates vary from 10 4 to 10 3 cm 3s 1 which are generally smaller than for hawaiian basalts growth and nucleation rates for the sill cover a large range from 10 13 to 10 11 cm s and 10 9 to 10 3 cm 3s 1 respectively systematic variations of these parameters with vertical position were obtained for the sill with its extensive differentiation history nucleation and growth rates are dependant on the mode of cooling nevertheless small but significant differences between the flow and sill exist growth and nucleation rates of hiti unit 10 12 to 10 11 cm s and 10 6 to 10 5 cm 3s 1 are intermediate between the flow and sill and thus do not allow a distinction of emplacement mode

geochemical and strontium isotopic characteristics of parental aleutian arc magmas evidence from the basaltic lavas of atka eighteen flows from a basal stratigraphic sequence on the aleutian island of atka were analyzed for major elements trace elements and initial 87sr 86sr ratios petrographically these lavas contain abundant plagioclase 24 45 and lesser amounts of olivine 7 magnetite and clinopyroxene phenocrysts compositionally the lavas are high alumina 20wt basalts 48 51 wt sio2 with low tio2 1 and mgo 5 within the section compositional variations for all major elements are quite small while mgo content correlates with olivine phenocryst contents no such relationship exists between the other oxides and phenocryst content these lavas are characterized by 8 10 ppm rb high sr 610 669 ppm 308 348 ppm ba and very constant zr 23 29 ppm and sc 23 29 ppm abundances ni and cr display extremely large compositional ranges 12 118 ppm and 12 213 ppm respectively no correlation exists between trace element concentrations and phenocryst contents strontium isotopic ratios show a small but significant range 0 70314 0 70345 and are slightly elevated with respect to typical morb no systematic correlation between stratigraphic position and petrography or geochemistry is evident ree abundances measured on six samples are lree enriched la yb n 2 20 2 81 and display similar chondrite normalized patterns one sample has a slight positive eu anomaly but the other lavas do not compared to other aleutian basalts of similar silica content these lavas are less lree enriched and have lower overall abundances the geochemical characteristics of these basalts suggest they represent true liquid compositions despite their highly porphyritic nature published phase relations indicate fractionation of a more mgo rich magma could not have produced these lavas the high al2o3 and low mgo and compatible element abundances suggest a predominantly oceanic crustal source for parental high alumina basalts

calculation of time dependent nucleation and growth rates from quantitative textural data inversion of crystal size distribution magmatictexturesprovideaninsufficientlyexploredtooltointerpretphysical processes and environmental variables that drive differenti ation and crystallization in magma chambers we derive a newmethod which utilizes the crystal size distribution csd to re trieve the ratesof nucleation and growth from natural igneous rocks however a single csd resultsfrom an arbitrary numberofcombin ations of nucleation and growth rates ifadditional parameters suchas crystallinity evolution are employed the solution for rates becomesunique interpretation of representative log linear csd trendsshows that the nucleation rates are sensitive to even minor featuresof the csd whereas the growth rate functions are mainly related tothe crystallinity evolution the reconstructed growth rates becomeminimal at intermediate crystallinities but diverge to very highvalues at the beginning and the end of crystallization this generalresult is related to the small effective area of the solid liquid inter face close to the liquidus few small grains and near the solidus largely solidified with diminishing melt pools assuming thatgrowth rate is related to environmental variables such as magmaundercooling these results suggest that the solid fraction in manymagmas increases in a sigmoidal manner over their crystallizationinterval and indicate the tendency for largest undercooling at the be ginning and at the end of crystallization for crystallization timesconstrained by conductive cooling models thegrowth ratescalculatedfor representative csds are of the order of10

groundmass crystallization of mount st helens dacite 1980 1986 a tool for interpreting shallow magmatic processes the 1980 1986 eruption of mount st helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale groundmass plagioclase size measurements are coupled with measured changes in matrix glass plagioclase and fe ti oxide chemistry to document increasing groundmass crystallinity and thus to better constrain proposed physical models of the post may 18 1980 magmatic reservoir measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth approximately 10 9 cm s of groundmass plagioclase occurred immediately subsequent to may 18 relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10 11 cm s plagioclase growth rates then decreased to 1x10 11 cm s through 1986 changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst rim chemistry although the matrix glass composition appears to have remained approximately constant from 1981 1986 after a rapid compositional change from may 18 until the end of 1980 plagioclase microphenocrysts show increasingly more complex zoning patterns with time microphenocryst core compositions are commonly positively correlated with crystal size both of these observations indicate continuous groundmass plagioclase growth through 1986 magmatic temperatures estimated from fe ti oxide pairs are approximately constant through 1981 at eruption temperatures of 930 c and at log fo2 of 10 8 by 1985 1986 oxide temperatures decreased to 870 c chemical and textural changes can be explained by 1 rapid degassing and crystallization in response to the intrusion of magma into a shallow 4 5 km reservoir toward the end of the may 18 1980 eruption 2 continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir growth rates and consequent chemical changes appear to decrease at the end of 1980 this is coincident with the change in eruption style from explosive eruptions sometimes followed by dome growth to solely extrusive dome building events and can be explained by the expected viscosity increase of both degassing and increasing crystallinity the model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies casadevall et al 1983 gerlach and casadevall 1986 a b patterns of crater deformation chadwkck et al 1988 and post 1980 seismicity endo et al 1990 although it does not explain the experimental data of hill and rutherford 1989 on the growth rate of amphibole reaction rims textural measurements on mount st helens dacite can also be used to evaluate crystallization kinetics in silicic magmas systems for which experimental data is almost non existent plagioclase growth rates are 5 10 times slower than estimated plagioclase growth rates in basaltic systems a result consistent with the higher viscosity of a more silicic melt furthermore patterns of textural change both average crystal size and number density are similar to those observed during the 1984 mauna loa eruption by lipman and banks 1987 suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates such that the rate of crystallization scales with the melt viscosity

an image of the columbia plateau from inversion of high resolution seismic data the authors use a method of traveltime inversion of high resolution seismic data to provide the first reliable images of internal details of the columbia river basalt group crbg the subsurface basalt sediment interface and the deeper sediment basement interface velocity structure within the basalts delineated on the order of 1 km horizontally and 0 2 km vertically is constrained to within plus minus 0 1 km s for most of the seismic profile over 5 000 observed traveltimes fit their model with an rms error of 0 018 s the maximum depth of penetration of the basalt diving waves truncated by underlying low velocity sediments provides a reliable estimate of the depth to the base of the basalt which agrees with well log measurements to within 0 05 km 165 ft the authors use image blurring calculated from the resolution matrix to estimate the aspect ratio of images velocity anomaly widths to true widths for velocity features within the basalt from their calculations of image blurring they interpret low velocity zones lvz within the basalts at boylston mountain and the whiskey dick anticline to have widths of 4 5 and 3 km respectively within the upper 1 5 km of the model at greater depth the widths of these imaged lvzs thin tomore approximately 2 km or less they interpret these linear subparallel low velocity zones imaged adjacent to anticlines of the yakima fold belt to be brecciated fault zones these fault zones dip to the south at angles between 15 to 45 degrees less

support of high elevation in the southern basin and range based on the composition and architecture of the crust in the basin and range and colorado plateau toexplorethenatureofhowthestructureandphysicalpropertiesofthecrustvaryfromextendedtorelativelyunextendeddomains we present teleseismic receiver functions which measure crustal thickness shear wavespeed structure and the vp vs ratio at 12 seismic stations in eastern arizona the crustal thickness is 28 km increases slightly eastward and remains nearly uniform beneath the varying elevations in the southern basin and range the observed vp vs ratio in the basin and range 1 78 exceeds the global average the southern colorado plateau exhibits thicker crust 40 km and a slightly greater observed vp vs 1 81 a discrete region in the colorado plateau generates an unusually high vp vs ratio 1 90 and contains low wavespeed zones which serve as evidence of partial melt related to quaternary volcanism the metamorphic core complexes in the southern basin and range likewise exhibit anomalously high vp vs values 1 79 1 87 and lack locally compensating crustal roots density models show that 85 kg m 3 lighter crust or 35 kg m 3 lighter mantle than that of the surrounding basin and range helps these metamorphic core complexes maintain their high elevation compositional modeling of intrusive bodies exposed throughout the catalina rincon metamorphic core complex indicates that the observed high vp vs ratio and modeled low density could result from substantial amounts of a plagioclase rich quartz poor rock these vp vs data are evidence that significant compositional heterogeneity of the crust can occur over a short distance and provide a clue as to how these areas that underwent significant tertiary extension may have been preconditioned for orogenic collapse

magmatism as an essential driving force for formation of active metamorphic core complexes in eastern papua new guinea the d entrecasteaux islands in eastern papua new guinea are composed of a number of active metamorphic core complexes which have been intruded by granodiorite plutons during their formation the plutons do not appear to have been intruded by diapiric processes as previously suggested late relatively undeformed plutons form flat lying bodies which crosscut structural boundaries and are strongly discordant to core complex shear zones granodiorite magmatism and the development of the metamorphic core complexes have occurred in a linear zone which coincides with a zone of thick crust and rugged topography it is proposed that plutonism facilitated deformation in ductile extensional shear zones which resulted in tectonic exhumation of deep crustal rocks and formation of the metamorphic core complexes the source of the plutons is thought to be related to a linear zone of mantle upwelling beneath the islands related to the propagation of the woodlark seafloor spreading center into continental crust it is suggested that a localized heat source of this type which can provide heat and magmatic material to the crust is essential for the development of metamorphic core complexes

ultrasonic p and s wave attenuation and petrophysical properties of deccan flood basalts india as revealed by borehole studies petrophysical properties and ultrasonic p and s wave attenuation measurements on 35 deccan basalt core specimens recovered from killari borehole site in western india provide unique reference data sets for a lesser studied deccan volcanic province these samples represent 338 m thick basaltic column consisting four lava flows each of ambenali and poladpur formations belonging to wai subgroup of the deccan volcanic sequence these basalt samples are found to be iron rich average feot 13 4 wt but relatively poor in silica content average sio2 47 8 wt the saturated massive basalt cores are characterized by a mean density of 2 91 g cm3 range 2 80 3 01 g cm3 and mean p and s wave velocities of 5 89 km s range 5 01 6 50 km s and 3 43 km s range 2 84 3 69 km s respectively in comparison saturated vesicular basalt cores show a wide range in density 2 40 2 79 g cm3 as well as p wave 3 28 4 78 km s and s wave 1 70 2 95 km s velocities based on the present study the deccan volcanic sequence can be assigned a weighted mean density of 2 74 g cm3 and a low vp and vs of 5 00 and 3 00 km s respectively such low velocities in deccan basalts can be attributed mainly to the presence of fine grained glassy material high iron contents and hydrothermally altered secondary mineral products besides higher porosity in vesicular samples the measured q values in saturated massive basalt cores vary enormously qp 33 1960 and qs 35 506 while saturated vesicular basalt samples exhibit somewhat lesser variation in qp 6 46 as well as qs 5 49 in general high porosity rocks exhibit high attenuation but we observed the high value of attenuation in some of the massive basalt core samples also in such cases energy loss is mainly due to the presence of fine grained glassy material as well as secondary alteration products like chlorophaeite that could contribute to intrinsic attenuation dominance of weekly bound secondary minerals might also be responsible for the generation of microcracks which may generate squirt flow in saturated samples hence we argue that the deccan basalts attenuate seismic energy significantly where its composition plays a major role

origin of megacrysts in granitoids by textural coarsening a crystal size distribution csd study of microcline in the cathedral peak granodiorite sierra nevada california microcline megacrysts in the cathedral peak granodiorite and other parts of the tuolumne intrusive suite were formed by textural coarsening ostwald ripening of earlier formed crystals the early formed crystals nucleated and grew in an environment of increasing undercooling probably during the ascent of the magma emplacement of the magma into warm host rocks promoted textural coarsening crystals smaller than a certain size the critical size dissolved in the interstitial melt whilst large crystals grew microcline was most sensitive to this effect as the magma temperature was buffered close to its liquidus for a long period by the release of latent heat of crystallization positive feedback between textural coarsening and magma permeability channelled the flow of interstitial melt to produce a heterogeneous distribution of megacrysts megacryst growth was halted when cooling resumed at the end of the intrusive cycle k feldspar nucleation was then renewed and k feldspar crystals grew to form part of the groundmass it was the particular thermal history of this pluton that promoted textural coarsening chemically similar plutons that lack megacrysts probably did not have the pause during cooling that was necessary for the development of this texture

crustal structure along mehmadabad billimora profile in the cambay basin india from deep seismic soundings abstract deep seismic sounding dss investigations were carried out along a 220 km long n s profile from mehmadabad 22 50 n 72 46 e to billimora 20 46 n 72 58 e in the cambay basin india for studying the detailed crustal structure in the region seismic refraction as well as narrow and wide angle reflection travel time data were obtained for various crustal boundaries starting from the sedimentary section down to the moho by continuous profiling analysis of reversed reflection and refraction travel time data has yielded crustal velocity depth functions for the mehmadabad broach and broach navsari sections along the profile which are significantly different from each other the average velocities at depths in the magmadabad broach section have been found to be considerably lower than those in the broach navsari section by about 25 at 5 km 17 at 10 km 7 at 20 km and 2 at the moho depth and even deeper down in the upper mantle the presence of a relatively thicker sedimentary section especially in the jambusar broach area as revealed by the reversed refraction data analysis is responsible for this gross difference in the average velocity functions for the two parts of the profile and the same have therefore been used for obtaining the detailed crustal depth along the profile the cambay basin which is well known to be bounded by step faults on the eastern and western margins is also dissected into seven major crustal blocks in the n s direction from mehmadabad to billimora the various crustal blocks identified are from the north 1 i mehmadabad block 2 ii tarapur block 3 iii petlad katana block 4 iv jambusar broach block 5 v kosamba block 6 vi surat block and 7 vii navsari billimora block these blocks are displaced up or down relative to the adjacent blocks due to movements along the faults bounding them most of the faults have been found to extend quite deep and to affect the moho boundary considerably as is quite evident from the relief of the moho found along the profile further the seismic activity observed in the region during historical as well as modern times suggests that these deep faults may even be currently active the jambusar broach block which is well known as a broad regional syncline with the maximum thickness of cenozoic sediments has been found to be a major graben revealing a maximum crustal thickness of 38 40 km the granitic basement in this graben is also the deepest at 6 0 6 5 km depth towards the north and south of this major graben the moho as well as the granitic basement are found to be at relatively shallower depths in various crustal blocks the boundary velocity at the moho has been found to be 8 1 8 2 km sec whereas at the granitic basement level there seems to be a lateral velocity variation from 5 7 to 6 3 km sec the crustal depth section obtained in the present study reveals a close relationship between the deep and shallow structural features in this region the deccan traps which form the basement of the overlying tertiary sediments over most part of the profile vary in thickness from 1 0 to 1 8 km however the trap surface rises to very shallow depths on the southern part of the profile the boundary velocity in the deccan traps has been found to vary between 4 8 and 5 2 km sec in the anklesvar kosamba area there is also evidence for the presence of a 1 2 km thick layer of mesozoic sediments with a velocity of about 4 0 km sec underlying the 1 1 km thick deccan traps layer

nature of the moho beneath the scottish highlands from a receiver function perspective abstract the geology of scotland documents a protracted geological history from precambrian basement formation through the caledonian orogeny and to the tertiary opening of the atlantic a temporary deployment of 22 closely spaced broadband seismic stations in scotland traverses many of the major terrane boundaries in the region and provides a unique opportunity to place spatial and temporal constraints on variations in crustal structure with teleseismic p wave receiver functions we use the h method to determine variations in bulk crustal parameters crustal thickness h and v p v s ratio mean crustal thickness is 28 km varying from 23 km in the ne highlands and increasing to 30 km near the highland boundary faultin the southern part of the study area mean v p v s values of 1 76 show no significant variation across the study area an abrupt increase in crustal thickness of 4 5 km nw across the moine thrust is not easily linked to tertiary recent tectonic activity instead it appears that scotland s crust has retained features for hundreds of millions of years since it was first formed despite abundant volcanic activity in tertiary times our work favours the view that the moho is a compositional boundary rather than a mineralogical one defined by a reaction in p t space

imaging of subvolcanic mesozoics in the saurashtra peninsula of india using traveltime inversion of wide angle seismic data low velocity mesozoic sediments exposed in the northeastern part of the saurashtra peninsula of western india are assumed to be below the high velocity volcanic rock cover conventional seismic refraction and reflection profiling experiments often fail to map hidden low velocity formations caused by inherent limitations of these methods however wide angle seismics involving both refractions and reflections is an effective tool for imaging in such a geological setting inversion of wide angle seismic traveltime data from a 45 km long profile reveals a three layered velocity structure above the basement with mesozoic sediments sandwiched between exposed volcanic rock and an underlying basalt flow the derived velocity model closely matches the lithology of the nearby deep lodhika well demonstrating the potential of wide angle seismics in mapping formations hidden below volcanic rock cover the volcanic rock cover is almost flat 1 4 km thick with an average velocity of 5 1 km s 1 the mesozoic sediments have a velocity of 4 3 km s 1 and thickness variations from 0 9 to 1 6 km the underlying basalt flow has a velocity of 5 15 km s 1 and varies in thickness from 1 to 2 km the basement with a velocity of 5 80 5 95 km s 1 is at an average depth of 3 9 km shear wave analysis yielded velocities varying between 2 65 2 75 and 3 30 3 40 km s 1 for the exposed volcanic rock and the subsurface basement respectively corresponding to poisson ratios of 0 31 and 0 26 the high poisson ratio in the volcanic rock cover is a result of rock fracturing as indicated by the fast decay of seismic energy with offset

combining csd and isotopic microanalysis magma supply and mixing processes at stromboli volcano aeolian islands italy integrating isotopic microanalysis with other analytical techniques creates powerful new methodologies for understanding the evolution of rock samples at the sub grain scale here we present crystal size distribution csd data for a 26 000 year old sample from stromboli volcano and accompanying isotopic microanalysis of the phenocrysts a technique called the icsd plot is introduced which given stated assumptions allows the integration of both sets of data to generate timelines of isotopic evolution through the volcanic system the combined approach is powerful allowing investigation of the magma supply mixing crystallisation and contamination processes prior to eruption of a volcanic sample for stromboli volcano the combined analysis suggests that the change in magma type following a cone collapse took roughly five years to complete similar to the timescale of changes seen in recent decades

basement configuration of the jhagadia rajpipla profile in the western part of deccan syneclise india from travel time inversion of seismic refraction and wide angle reflection data abstract 2 d velocity structure up to the basement is derived by travel time inversion of the first arrival seismic refraction and wide angle reflection data along the sw ne trending jhagadia rajpipla profile located on the western part of deccan syneclise in the narmada tapti region the study region is mostly covered by alluvium inversion of refraction and wide angle reflection data reveals four layered velocity structure above the basement the first two layers with p wave velocities of 1 95 2 3 km s 1 and 2 7 3 05 km s 1 represent the recent and quaternary sediments respectively the thickness of these sediments varies from 0 15 km to 3 4 km the third layer with a p wave velocity of 4 8 5 1 km s 1 corresponds to the deccan volcanics whose thickness varies from 0 5 km to 1 0 km presence of a low velocity zone lvz below the high velocity volcanic rocks in the study area is inferred from the travel time skip and amplitude decay of the first arrival refraction data and the wide angle reflection from top of the lvz present immediately after the first arrival refraction from deccan trap layer the thickness of the low velocity mesozoic sediments varies from 0 3 km to 1 7 km the basement with a p wave velocity of 5 9 6 15 km s 1 lies at a depth of 4 9 km near jhagadia and shallows to 1 2 km towards northeast near rajpipla the results indicate presence of low velocity mesozoic sediments hidden below the deccan trap layer in the western part of the deccan syneclise

crust and mantle structure across the basin and range colorado plateau boundary at 37 n latitude and implications for cenozoic extensional mechanism we present new evidence on the seismic velocity and density of the crust and upper mantle along a 200 km long transect across the eastern basin and range and western colorado plateau at 37 n latitude receiver functions computed from the p waveforms recorded with 10 portable broadband stations deployed along the transect were used to estimate crustal thickness variations the crust is 30 35 km thick within the eastern basin and range and increases over a distance of 100 km at the western edge of the colorado plateau reaching a maximum of approximately 45 km east of the hurricane fault the timing of crustal multiples within the reciever functions were used to estimate the v p v s of the crust along the profile and we found that the western colorado plateau crust is characterized by a high poisson s ratio 0 28 0 29 indicative of a crust with an average mafic composition we estimated the upper mantle lid thickness along our profile based on teleseismic p wave travel times and constraints provided by gravity data our data and available geophysical constraints are most consistent with a lithosphere that thickens from an average thickness of 60 km beneath the basin and range to 100 km beneath the western colorado plateau although the basin and range lithosphere may have significant thickness variations the thick strong mafic crust and thicker mantle lid under the colorado plateau can account for the relative geologic stability and subdued magmatism of the plateau during laramide compression and cenozoic extension compared to surrounding regions the crustal and lithospheric thinning across the tectonic boundary occurs over a short distance 100 km suggesting it is a geologically young feature produced by a predominantly mechanical response to late cenozoic extension our new lithosphere model at 37 n latitude is consistent with the existence in early cenozoic time of a flat subducted slab at 100 km depth and a relict sevier laramide 50 60 km thick crustal welt and 60 100 pure shear extension values of 1 6 2 0 during the late cenozoic

detachment and rotation of a metamorphic core complex during extensional deformation palaeomagnetic study of the catalina rincon core complex basin and range province arizona abstract the catalina rincon complex exposes a metamorphic core of mid crustal rocks exhumed by extensional tectonism along a major detachment exposed along the southeast side of the catalina rincon mountains near tucson arizona the juxtaposition of core complexes against high level crust segmented into rotated and tilted fault blocks is an important component to models for crustal extension and key questions surround the way s that cores are exhumed we report palaeomagnetic results from rocks beneath the detachment in the catalina rincon core and additional data from lavas and tuffs in adjoining hanging wall rocks immediately above the detachment magnetic remanence in the metamorphic core is carried predominantly by magnetite with a dominant wnw ese dual polarity direction a1 d i 288 50 n 59 components r 55 60 95 4 6 this remanence is interpreted to have been acquired through a broadened blocking temperature spectrum during protracted exhumation and cooling in oligocene times since the magnetic inclination accords with the mid cenozoic field it is difficult to explain this magnetization in terms of large later tilting and it appears to be the record of near vertical axis ccw rotation of 63 5 relative to cratonic north america although magnetic inclination identifies no significant horizontal axis rotation this could be fortuitous and hide the influence of magnetic anisotropy and resultant inclination shallowing to assess this contribution both anisotropy of magnetic remanence amr and anisotropy of magnetic susceptibility ams have been investigated ams resolves only weak fabrics comparable with the top ne mylonite deformational fabric near the detachment which probably dates from a mid eocene high temperature event amr shows that deflections of magnetic remanence of up to several tens of degrees into the foliation may be anticipated although weak alignment of the magnetite is likely to reduce this effect the full importance of horizontal axis tilting remains unresolved although a possible relict component in the core a2 d i 269 9 n 24 components r 22 91 95 6 6 is rotated 20 more ccw than a1 and also of presumed mid cenozoic cooling related origin it suggests that segments of middle crust comprising the exposed core might have been rotated by large amounts into their present position highly faulted early oligocene pantano formation volcanic rocks in the hanging wall immediately above the detachment zone appear to have rotated by a much smaller amount ccw on the south west side d i 161 45 6 sites and by larger variable amounts on the northeast side movements of the upper plate immediately above the detachment are difficult to resolve with precision due to complex fault block rotations although the underlying core complex has clearly undergone large differential slip relative to these cover rocks the magnitude of inferred vertical axis rotation of the catalina rincon core implies that it is not rooted into adjoining crust as proposed in classical simple shear models instead the core exposed below the detachment may be part of a fault carapace that has decoupled and rotated over a ductile lower crust

interpretation of first arrival travel times in seismic refraction work the necessary condition for the seismic refraction method to succeed is that the refracted first arrivals from each layer in a multilayered earth system should be detected on a seismogram as first arrivals and this is possible only when velocities of all underlying layers are successively greater the usual procedure to interpret the refraction travel times is to fit such a data set with several intersecting straight lines by employing a visual technique which may lead to errors of subjective judgment as the velocity model depends on the selection of various line segments through the data to remove the visual fit we propose here a layer stripping method based on minimum intercept time apparent velocity rms residual and maximum data points by least squares fitting to yield several intersecting straight lines once data are segmented out the conventional equations can be used to determine the velocity structure

evidence for a local crustal root beneath the santa catalina metamorphic core complex arizona we used data from the tucson arizona broad band seismic station to analyze crustal and mantle structure under and adjacent to the catalina metamorphic core complex the data can be modeled with a simple two layer crust having an average moho depth of 30 km and seismic velocities consistent with a felsic composition the lower crust thickens under the catalina mountains depressing the moho by 4 2 km the crustal root can isostatically compensate the average topography of the catalina mountains indicating that the depth of isostatic compensation is at or below the moho and that the catatina mountains act as a coherent block bounded by high angle normal faults the presence of the crustal root is not consistent with models that predict upward bowing of the moho under the catalina metamorphic core complex the presence of the root cannot rule out models of lower crustal flow during the formation of the core complex however the presence of a root favors a model of metamorphic core complex development described by crustal thickening in shortening episodes crustal welt model followed by collapse of the crustal welt through extension

crystallization calculations for a binary melt cooling at constant rates of heat removal implications for the crystallization of magma bodies abstract we report on thermodynamic non equilibrium crystallization calculations for a unit volume of a binary melt subject to a constant prescribed rate of heat loss crystallization histories and crystal size distributions for both melt components were calculated by accounting for the nucleation and growth of crystals the crystal sizes were found to decrease with increasing rates of heat loss the crystallization time defined as the time to crystallize 99 of the unit volume also decreased strongly with increasing rates of heat loss up to a critical rate the critical rate was found to be somewhat smaller than the heat loss rate for the beginning of glass formation at larger than critical rates crystallization time increased again and for rates larger than the glass formation rate crystallization time became infinite the residual melt composition was found to increasingly deviate from the equilibrium composition with increasing rates of heat loss but as long as the loss rate was less than the critical rate the crystallization path reverted to the eutectic composition during the final crystallization for supercritical rates no such reversion was observed we compared the critical rate with estimates of the rates of heat loss in magmatic intrusions based on the stefan solution for a freezing half space it was found that rates of heat loss should be supercritical at distances of up to 0 5 m from the margin of an intrusion in this region non equilibrium effects are expected to dominate and the texture of the crystallized rock should be characterized by small crystals and by glass the glass and the crystals should be of non equilibrium composition non equilibrium effects should be negligible only at distances of more than 5 m from the margin where the rates of heat loss are less than 10 2 times critical at these distances the crystallized rock should have an equigranular texture and an equilibrium composition

dissolution crystallization kinetics recorded in the 2002 2003 lavas of stromboli italy on 28 december 2002 new vents opened on the flanks of stromboli just below the summit craters interrupting the persistent activity of the volcano with a 7 month long effusive eruption we here report on the plagioclase size distribution plgsd in lava samples collected following the chronology of the 2002 2003 eruption data reveal a linear plgsd similar to that found in samples of normal stromboli activity indicating that the switch from strombolian explosive to effusive activity is not associated with changes in texture nevertheless the crystal size distribution slopes and intercepts exhibit slight sinusoidal temporal variations that are here ascribed to a magma supply mechanism able to induce resonance in the crystal size distribution with an amplitude that depends on the supply rate

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

kinetics of crystallization of igneous rocks the geochemistry of igneous rocks is discussed with the primary objectives of bringing together the theories underlying the kinetics of crystallization of igneous rocks and illustrating the use of these theories in understanding experimental and observational data the primary purpose of the chapter is to introduce current thinking about the kinetics of igneous rocks and to provide a basis for understanding other work a basic assumption made in the discussion is that the rate of any chemical reaction including the crystallization of igneous rocks is zero at equilibrium and proceeds at a finite rate only at a finite deviation from equilibrium as such an understanding of the processes operating in igneous rocks requires an understanding of how deviation from equilibrium affects the rates and mechanisms of the processes occurring during crystallization these processes are detailed with special emphasis given to nucleation and crystal growth jmt

crustal velocity structure beneath the eastern flank of the ruby mountains metamorphic core complex results from normal incidence to wide angle seismic data abstract p wave velocity structure along the eastern side of the ruby mountains metamorphic core complex determined using 2 d ray inversion modeling shows that 1 the crust can be divided into three layers corresponding to the upper middle and lower crust 2 from east of ruby dome to the northern end of the seismic profile velocities of the upper crust range from 6 12 0 1 to 6 20 0 1 km s and near surface velocities range from 1 90 0 1 to 4 80 0 1 km s from this point to the southern end of the profile upper crustal velocities range from 5 80 0 1 to 6 25 0 1 km s and near surface velocities range from 3 01 0 1 to 4 80 0 1 km s 3 middle crustal velocities range from 6 35 0 15 to 6 45 0 15 km s 4 lower crustal velocities range from 6 60 0 15 to 6 80 0 15 km s and 5 depths to the moho vary irregularly between 30 5 and 33 5 km from interpretation of these results we conclude that 1 the transition in metamorphic grade from deep upper crustal rocks south of harrison pass to middle crustal rocks north of harrison pass does not correlate with an increase in seismic velocity near surface basement seismic velocities increase in the central part of the range near ruby dome well north of the major metamorphic transition 2 depths to the moho do not reflect local surface relief isostatic balance is achieved predominantly in the crust rather than by formation of a local crustal root and 3 the crust has been modified during extension by intracrustal processes rather than by large scale magmatic underplating

a numerical experiment of crystallization for a binary eutectic system with application to igneous textures numerical experiments of crystallization in a binary eutectic system were carried out to understand the origin of textural variation in igneous rocks in this study the number density of crystals was intensively examined as a parameter of texture the constant cooling rate experiment no latent heat release yields a higher number density of crystals by 1 order of magnitude than the constant heat loss experiment full latent heat release differences in growth mechanism diffusion limited with capillary effect versus reaction limited without capillary effect result in the different dependence of cooling rate on number density of crystals for diffusion limited growth with the capillary effect for spherical crystals the cooling rate dependence of number density of olivine clinopyroxene plagioclase and opaque minerals seems to be consistent with the experimental data with superliquidus initial condition for plagioclase experimental data with subliquidus initial temperature and the observational data are not consistent with the present numerical results for both growth processes suggesting that the plagioclase growth is governed by a different mechanism of nucleation and growth from homogeneous nucleation and both diffusion limited growth with capillary effect and simple reaction limited growth without capillary effect assuming the conductive cooling of a dike it is predicted that for the diffusion limited growth the number density of crystals is proportional to y 3 where y is distance from the cooling boundary this is in good agreement with some of the observational data by gray 1978 except for plagioclase the consistency among numerical natural and experimental data highly suggests that at the nucleation stage with superliquidus initial condition crystals grow by the diffusion limited process taking the spherical shape for olivine pyroxene plagioclase and some oxides the difference between volcanic and plutonic textures is caused by the difference of cooling rate by 2 orders of magnitude the variation from intergranular ophitic to poikilitic textures results from the difference in cooling rate dependence on number density between the plagioclase and the other minerals the formation of porphyritic texture must satisfy the limited condition the very small efficiency of latent heat production nearly constant cooling rate small values of interfacial energy of the first precipitating phase and smaller value of interfacial energy of the second phase

a flower like glomerophyric diorite porphyry from central china constraints on the unusual texture abstract an unusual texture with plagioclase phenocrysts clustered as flower like glomerocrysts has been discovered in the wulong diorite porphyry at the southern margin of north china craton the rock consists mainly of plagioclase and amphibole with the former as dominant phenocryst two groups of the plagioclase phenocrysts have been distinguished flower like glomerocrysts fg type and single isolated phenocrysts sp type chemical compositions of both fg and sp type plagioclases are similar in anorthite contents an35 45 which are slightly higher than the matrix an25 35 calculation of the plagioclase crystallization time based on crystal size distribution csd program shows that the fg type crystals have been formed within a longer timescale 226 2782 years than the sp type crystals 98 1910 years the concave down csds and lack of small crystals for fg type plagioclases suggest that coarsening may have been involved in contrast a concave up csd within the size of 3 10 mm plagioclase indicating that a rejuvenation kinetic process after coarsening a possible formation history for the unusual flower like plagioclase glomerocrysts was proposed as follows 1 initial nucleation and growth of single crystals in a confined environment 2 ascent of early crystals with magma upwelling and radial coarsening at the roof layer of magma chamber 3 remobilization of dense crystal mush and final emplacement at shallow level of crust 4 rapid cooling and extensive nucleation and growth of matrix 5 post magmatic alteration and metamorphism it is likely that the morphologic diversity of plagioclase glomerocrysts are largely influenced by variation of undercooling degrees the second step is the most important for the formation of the flower like glomerophyric texture which requires an environments of relatively low undercooling degree

first in situ observation of crystallization processes in a basaltic andesitic melt with the moissanite cell the evolution of texture and crystal size distribution of igneous rocks is mainly determined by processes such as nucleation grain growth and ostwald ripening however the role played by these processes on the final crystal size distribution is difficult to quantify with conventional methods here we present the first direct in situ observation of a crystallizing supercooled basaltic andesitic liquid in order to directly assess the role of grain scale processes occurring during magma solidification the experiments were conducted at 900 c and 1 bar with a new moissanite cell specifically designed for long term high temperature experiments the method allowed the direct continuous observation of the crystallizing melt until the crystal fraction approached 0 7 investigation of the evolving texture revealed that individual crystals followed distinct growth histories characterized by intervals of relatively uniform free growth abrupt size increase due to grain coalescence and growth reduction due to impingement the latter played a dominant role at higher crystal fractions despite this growth dispersion average growth rates calculated for single grains showed only little variation 4 9 10 6 to 1 9 10 6 mm s no clear relation between growth rate and crystal size was observed crystal coarsening through a combination of overgrowth and coalescence can also account for the counterclockwise rotation and the drop off at smaller size fractions of the recovered crystal size distributions in our experiments these results suggest that crystal size distributions of natural rocks do not record the original nucleation rates

use of traveltime skips in refraction analysis to delineate velocity inversion1 first arrival refraction data does not normally provide any indication of the velocity inversion problem however under certain favorable circumstances when the low velocity layer lvl is considerably thicker than the overlying higher velocity layer hvl the velocity inversion can be seen in the form of a traveltime skip model studies show that in such cases the length of the hvl traveltime branch can be used to determine the thickness of the hvl and the magnitude of the traveltime skip in order to determine the thickness of the lvl this is also applicable in the case of field data

large explosive source wide recording aperture seismic profiling on the columbia plateau washington clear subsurface seismic images have been obtained at low cost on the columbia plateau washington the columbia plateau is perhaps the most notorious of all bad data areas because large impedance contrasts in surface flood basalts severely degrade the seismic wavefield this degradation was mitigated in this study via a large explosive source wide recording aperture shooting method the shooting method emphasizes the wide angle portion of the wavefield where fermat s principle guarantees reverberation will not interfere with the seismic manifestations of crucial geologic interfaces the basalt diving wave normally discarded in standard common midpoint cmp seismic profiling can be used to image basalt velocity structure via traveltime inversion maximum depth penetration of the diving wave tightly constrains basalt sediment interface depth an arrival observed only at shot receiver offsets greater than 15 km can be used to determine the velocity and geometry of basement via simultaneous inversion

a complex multi chamber magmatic system beneath a late cenozoic volcanic field evidence from csds and thermobarometry of clinopyroxene from a single nephelinite flow djbel saghro morocco we used quantitative textural measurement electron microprobe microanalysis and thermobarometry on clinopyroxene from a cenozoic pyroxene nephelinite flow located along the northern boundary of the west african craton to decipher magma differentiation processes in underlying magma chambers the crystal size distributions of clinopyroxene phenocrysts show straight but also curved and kinked patterns and the clinopyroxene show large compositional variations in a single flow mg number 48 88 these observations are strong evidence for magma mixing between a nephelinite magma and a more differentiated phonolitic melt at depth detailed thermobarometry on these clinopyroxene shows that at least three magma chambers are present below saghro and that they are emplaced at the main physical interface within the lithosphere 1 at the crust mantle boundary where the mantle derived nephelinite has been mixed with a pre existing phonolitic magma chamber 2 at the lower upper crust boundary 3 close to the surface in a sub volcanic magma chamber some high pressure phenocrysts up to 14 kbar have also probably crystallized within the upper lithospheric mantle the high clinopyroxene proportion in samples from the base of the flow is thought to reflect crystal settling during cooling of the nephelinite flow at the surface the geological society of london 2008

a review of mesoscopic magmatic structures and their potential for evaluating the hypersolidus evolution of intrusive complexes abstract the perception that plutonic bodies are structurally simple is disappearing with the recognition of an array of magmatic structures useful for constraining hypersolidus temporal histories evolving rheologies strain fields flow directions growth and cooling patterns tilting and syn emplacement tectonism these histories provide a powerful means of testing an array of growth emplacement chamber evolution and tectonic models important points include that 1 many structures form in hydrogranular or congested magma slurries during magma mush avalanching local convection and late hypersolidus strain challenging the notion that magmas must have 55 crystals to convect fractionate and form compositional diversity in upper crustal magma chambers 2 magmatic fabrics reflect transient strain in these slurries rather than flow directly the latter must be inferred by linking geometries with temporal and kinematic information 3 caution is needed when using anisotropy of magnetic susceptibility and similar quantitative tools that characterize through a single ellipsoid preferred orientations of mineral grains given the increasing recognition of multiple fabrics in plutons and 4 that future studies are particularly needed in plutons focusing on the distribution and styles of compositionally defined structures magmatic folding shear zones and faults multiple fabrics and of the physical chemical behaviors in hydrogranular slurries

igneous microstructures from kinetic models of crystallization during crystallization a variety of competing kinetic processes determine the evolution of the igneous microstructure yet the relative contribution of each process remains elusive to this end a stochastic algorithm is developed to yield a detailed spatial representation of the igneous microstructure during progressive crystallization this algorithm is used to test a variety of kinetic models for nucleation and crystal growth that yield realistic igneous microstructures the algorithm itself relies on a theoretical model of crystallization the avrami method and can therefore be internally validated in addition to being verified against natural microstructures using crystal size distributions csds the most realistic simulated igneous microstructure in which the csds remain approximately log linear throughout the crystallization interval is produced using a kinetic model involving exponential nucleation rate and constant growth rate an extension of the algorithm is used to simulate multiply saturated mineral growth emulating basalt crystallization through simultaneous nucleation and growth of plagioclase and clinopyroxene a fundamentally important feature of this procedure is the numerical production of detailed 3d representations of the microstructure that can be interrogated to study many physical and chemical processes for example the critical crystallinity necessary for the onset of a finite yield strength and the interstitial melt flow within the igneous microstructure depend on the percolation of the solid and melt phases respectively bounds on the percolation thresholds the critical crystallinity at which the phase of interest is connected or contiguous such that it extends across the full microstructure from one face to the opposite face are determined for the simulated igneous microstructure and are comparable with previously published results for the percolation threshold of both the melt phase and solid network the simulated igneous microstructure can also be used as input into other physical models to ascertain the physical properties of partially molten magmas that are otherwise difficult to estimate by experiment in a real sense these computed microstructures are equivalent to 3d microtomographic images of partially molten basalt within a solidification front

investigation into the petrogenesis of apollo 14 high al basaltic melts through crystal stratigraphy of plagioclase abstract the petrogenesis of apollo 14 high al basaltic melts was studied using crystal stratigraphy which involves textural crystal size distributions csds and chemical analyses electron microprobe and laser ablation inductively coupled plasma mass spectrometry the samples studied here include pristine basalt 14072 and basaltic clasts from breccia 14321 and impact generated crystalline samples 14073 14276 and 14310 plagioclase was the focus of this study because of its relatively high modal abundances and because it was on the liquidus for much of the melt cooling histories plagioclase crystals were analyzed core to rim compositions where possible to test and refine petrogenetic models based upon whole rock compositions groups a b and c designations and to investigate basalt 14072 and impact melt crystallization textural studies have shown that each basalt group has distinctive plagioclase csds which are in turn distinctive from those of the impact melts evolution of the individual basaltic melts was studied by comparing the equilibrium melt compositions calculated from plagioclase compositions using relevant partition coefficients to fractional crystallization fc and assimilation and fractional crystallization afc models petrogenetic modeling of trace elements in group a basalts revealed that petrogenesis continued beyond 40 total crystallization required to model whole rock compositions and that there were open system processes that affected the magma during plagioclase crystallization petrogenetic modeling of pristine high al basalts 14072 and groups a b and c using trace elements shows that the equilibrium melt compositions do not fall on a single afc or fc trajectory this is consistent with fluctuating degrees of assimilation i e variable r values and or variable assimilant compositions during petrogenesis petrogenetic modeling reveals that the impact melts experienced only closed system fractional crystallization this work demonstrates the importance of crystal stratigraphy in revealing the intricacies of lunar basalt petrogenesis

interpretation of a vertical seismic profile conducted in the columbia plateau basalts seismic reflection data are often of poor quality when recorded in areas where volcanic rocks are present at or near the surface in order to investigate this phenomenon a vertical seismic profiling vsp experiment was conducted in the columbia plateau basalts so that the behavior of seismic energy in subsurface volcanic rocks could be observed directly thus giving insight into data acquisition in volcanic terrains the lithologic section at the vsp site consists of low velocity 400 m s to 900 m s alluvium in the uppermost 50 m beneath which are layers of high velocity about 5800 m s high density basalts interbedded with clay layers with much lower velocities about 1700 m s and densities these large velocity and density contrasts dramatically influence wave generation and propagation in spite of the small source borehole offset 61 m large amplitude s waves are generated by the downgoing p waves when they reach a shallow 250 m clay basalt boundary these s waves in turn generate strong

relationship between plagioclase crystallization and cooling rate in basaltic melts rock textures commonly preserve a record of the near surface crystallization history of volcanic rocks under conditions of simple cooling without convection or mixing textures will reflect sample cooling rate the temperature at which crystallization was initiated and the distribution of mineral phase precipitation across the crystallization interval compilation of plagioclase size and number density data on natural dike sill and lava lake and experimental samples suggests that 1 growth and nucleation rates of plagioclase in natural basaltic samples are a predictable function of cooling rate and 2 the observed crystallization rate dependence on cooling rate is similar to that observed in experiments initiated at subliquidus temperatures comparison of natural and experimental samples thus suggests that most basalts crystallize under conditions of heterogeneous nucleation with the number density of preexisting nucleii partially controlling textural responses to cooling rate changes time scales of crystallization and cooling in magmatic systems are intimately linked through a balance between heat removal from the system and heat evolved through crystallization evaluation of textural data in the context of recent numerical models of crystallization in simple one and two component systems provides new insight into regularities in the crystallization behavior of basaltic magmas for example the rate of change in crystal size and number density as dictated by mass balance has been used as a measure of the relative importance of time scales of crystallization and cooling in numerical models of crystallizing systems in natural samples plagioclase size scales with the length scale of cooling such that a logarithmic plot of grain size as a function of normalized distance across the dike has a slope that appears approximately independent of dike width solidification time comparison with available textural data for other phenocryst phases suggests that the same may be true for pyroxene and magnetite crystallization with each phase having a characteristic slope probably controlled by the thermodynamic properties of the crystallizing phase measured crystal size distributions are unimodal and show maximum frequencies in the smaller size classes distributions broaden and the grain size at peak frequency increases with increasing crystallization times decreasing cooling rates in contrast partially crystallized makaopuhi lava lake samples have crystal size distributions that decrease exponentially with increasing crystal size measured size distributions in dikes can be explained by late stage modification of makaopuhi type distributions through loss of small crystals possibly the consequence of growth without nucleation finally this compilation of the textural response of basaltic magmas to changes in cooling rate suggests that empirical calibrations of crystallization rate dependence on cooling rate from natural samples provide a reasonable model for plagioclase crystallization in near surface basaltic systems predicted growth rates will be slow and relatively constant 10 10 10 11 cm s for crystallization times expected in most shallow volcanic systems 1000 years

fission track analysis of the footwall of the catalina detachment fault arizona tectonic denudation magmatism and erosion new apatite and zircon fission track ages obtained from the footwall of the catalina metamorphic core complex record a complicated cooling history associated with mid tertiary extension zircon fission track ages record the progressive unroofing of the catalina metamorphic core complex along the catalina detachment fault zircon fission track ages range from 31 9 to 19 4 ma generally decrease in the hanging wall slip direction and yield slip rates along the catalina detachment fault ranging from 1 2 to 12 km myr 1 in contrast apatite fission track ages increase in the hanging wall slip direction samples from the main range of the santa catalina mountains yield apatite fission track ages of 20 5 to 14 6 ma samples from the santa catalina mountain forerange located closer to the detachment fault yield apatite fission track ages of 21 4 to 18 8 ma rapid cooling 40 to 60 c myr 1 related to detachment faulting is best recorded by zircon fission track ages and higher temperature thermochronometers in the main range and by nearly concordant zircon and apatite fission track ages in the forerange slower cooling 3 7 c myr 1 of the footwall is recorded by shortened mean confined fission track lengths 14 m and is related to erosional unroofing approximately 2 km of late tertiary erosion played a significant role in the unroofing of the footwall of the catalina metamorphic core complex in contrast to metamorphic core complexes in western arizona where detachment faulting is the dominant unroofing mechanism

magma rheology variations in sheet intrusions of the ardnamurchan central complex scotland inferred from gabbro inclusion characteristics entrainment of xenoliths and their consequent assimilation are key processes in modifying the crystallization kinetics and magma dynamics of conduit systems here an integrated textural and mineral chemical study of the evolution of a suite of gabbroic inclusions within a set of sheet intrusions from the ardnamurchan central complex nw scotland is presented the key findings are as follows 1 the host magma sheets and inclusions are not cognate 2 there are microstructural and mineral chemical similarities between the gabbroic inclusions and the textures and mineralogy of the major hypersthene gabbro on ardnamurchan 3 orientations of magnetic fabrics within the host sheet groundmass and within the gabbroic inclusions are virtually identical field observations suggest that the inclusions were derived from the hypersthene gabbro and were entrained in a few laterally restricted magma segments that subsequently coalesced with inclusion free segments into continuous sheets using stokes law and adaptations thereof we calculate that the magma within the inclusion free segments behaved as a newtonian fluid with a potential settling velocity of50 028 m s in contrast the presence of gabbro inclusions probably modified the magma dynamics to bingham like behaviour we infer that this variation in the magma rheology of separate segments continued after coalescence and internally partitioned the magma sheet preventing lateral mixing and inclusion transport

heterogeneous nucleation as the predominant mode of crystallization in natural magmas numerical model and implications for crystal melt interaction crystallization of natural magmas is inherently a disequilibrium process which involves nucleation and growth kinetics melt crystal mechanical interactions and subsolidus modifications which are all recorded in the resulting rock texture we use a new high resolution three dimensional numerical model to address the significance and consequences of homogeneous versus heterogeneous crystal nucleation in silicate magmas with increasing amount of heterogeneous nuclei during crystallization initially equigranular textures evolve to porphyritic bimodal and spherulitic types the corresponding crystal size distributions csds become concave up curved the clustering index progressively decreases and the grain contact relationships record increased clustering concave up curved csds previously interpreted as resulting from multistage crystallization mixing of crystal populations grain agglomeration or size dependent growth are now predicted consistently with other size spatial and clustering parameters to form by heterogeneous crystal nucleation correlation relationships between various textural parameters and the fraction of heterogeneous nuclei are calibrated and used on representative volcanic and plutonic rocks including cumulate rocks to deduce the fraction of heterogeneous nuclei the results indicate that 60 to 99 of all nuclei are heterogeneous for plutonic and cumulate rocks the estimate of the heterogeneous nuclei fraction based on the clustering index is significantly lower than other estimates such discrepancies in general point to the occurrence of other processes and here the results imply that crystal mush compaction and interstitial melt extraction were involved during the magma solidification formation of crystals in clusters implicit for heterogeneous nucleation implies that greater efficiency of crystal melt separation is expected in these situations

textural effects of steady state behaviour of the stromboli feeding system abstract the exceptionally persistent activity of stromboli volcano has lasted for at least 1400 years the normal strombolian activity is periodically interrupted by more energetic explosions 1 2 per year and by sporadic effusive episodes every 10 20 years normal activity and effusive episodes are characterized by crystal rich high k to shoshonitic basalts issuing from a volatile poor shallow system crystal poor pumice are emitted only during more violent explosions and are thought to derive from deep pulses of volatile rich magma shallow level degassing induces massive crystallization of deep pulses of feeding magma that continuously mixing with the resident one produces the crystal rich shoshonite of the persistent activity we examined the crystallization history of the crystal rich shallow reservoir using plagioclase crystal size distribution csd analysis of scoriae and lavas emitted in the past twenty years csds show a linear dependence from crystal size in the size interval 0 06 1 2 mm number density of larger crystals is biased by right hand truncation effects csds slopes and intercepts are quite constant during the whole considered time span revealing a system that is close to the equilibrium also from a kinetic point of view the linear crystal size distribution are reached by the system through episodes of growth and resorption respectively occurring in the degassed and undegassed magma during the continuous mixing in the feeding system plagioclase net growth rate 2 10 11 cm s results from a balance of growth 10 10 cm s and resorption episodes which induce spectacular zoning and resorption textures in crystals larger than 200 m csds of mafic phases cannot be accurately acquired on each single sample due to poor counting statistics the evaluation of pyroxene and olivine csd on the whole data set however confirms the conclusions acquired from plagioclase csds

shallow velocity structure along the hirapur mandla profile using traveltime inversion of wide angle seismic data and its tectonic implications summary in order to investigate the velocity structure and hence shed light on the related tectonics across the narmada son lineament traveltimes of wide angle seismic data along the 240 km long hirapur mandla profile in central india have been inverted a blocky laterally heterogeneous three layer velocity model down to a depth of 10 km has been derived the first layer shows a maximum thickness of the upper vindhyans 4 5 km s1 of about 1 35 km and rests on top of normal crystalline basement represented by the 5 9 km s1 velocity layer the anomalous feature of the study is the absence of normal granitic basement in the great vindhyan graben where lower vindhyan sediments 5 3 km s1 were deposited during the precambrian on high velocity 6 3 km s1 metamorphic rock the block beneath the narmada son lineament represents a horst feature in which high velocity 6 5 km s1 lower crustal material has risen to a depth of less than 2 km south of the lineament the deccan traps were deposited on normal basement during the upper cretaceous period and attained a maximum thickness of about 800 m

crystallization history of the 1984 mauna loa lava flow during a 3 week eruption in 1984 mauna loa produced vent lavas that increased in crystallinity from 1 to 30 and 27 km long flows that increased in crystallinity as they moved downstream we examined the crystallization history of these lavas using crystal size distribution csd analysis to study the rates of crystallization viscosity increase and latent heating typical average growth and nucleation rates were 5 10 9 cm s 1 and 5 cm 3 s 1 for microphenocrysts 20 to 500 m size crystals nucleated in the rift zone and 5 10 8 cm s 1 and 5 104 cm 3 s 1 for microlites 1 to 20 m size crystals nucleated in the channel these crystallization rates are high compared with those found in other csd studies of igneous rocks probably due to highly nonequilibrium conditions brought on by rapid degassing in the rift zone and cooling in the lava channel growth and nucleation rates decreased with time at the vent and with distance downstream the maximum downstream total crystallinity measured is 39 25 microlites 14 microphenocrysts in a quenched sample 14 km from the vent growth and nucleation rates cannot be calculated for postemplacement samples but they place upper limits of 53 58 on the amount of crystallization in the channel 9 20 km from the vent crystallization could have been mostly responsible for the 105 fold downstream increase in apparent viscosity although degassing and increasing incorporation of solid lava fragments also contributed another effect of crystallization on the lava flow was the sizeable latent heating 0 01 j g 1 s 1 over the first half of the flow length if the crystallinity of downstream quench samples is representative of the hot fluid core which may have been counteracted by entrainment of cooler material measurements of crystallization are shown to be crucial in the study of lava flow emplacement dynamics

magma dynamics beneath kameni volcano thera greece as revealed by crystal size and shape measurements size distributions of plagioclase crystals in series of recent porphyritic dacite lavas from kameni volcano greece can be modelled by mixing two populations of crystals each with overlapping linear crystal size distributions csd termed microlites and megacrysts the magmas bearing the microlites and megacrysts started to crystallise 6 13 and 24 96 years respectively before each eruption the dates of initiation of crystallisation of the megacrysts indicate that they are left overs of earlier injections of new magma into a shallow chamber some magma remains after each eruption and continues to crystallise new magma with few or no crystals is then introduced and the microlites crystallise from the mixed magma eruption followed 6 13 years after mixing such a model would suggest that some porphyritic magmas are products of a shallow magma chamber that is never completely emptied just topped up from time to time

the duration of prograde garnet crystallization in the uhp eclogites at lago di cignana italy abstract the distinct core to rim zonation of different rees in garnet in metamorphic rocks specifically sm relative to lu suggests that sm nd and lu hf isochron ages will record different times along a prograde garnet growth history therefore ree zonations in garnet must be measured in order to correctly interpret the isochron ages in terms of the garnet growth interval which could span several m y new ree profiles garnet crystal size distributions and garnet growth modeling combined with previously published sm nd and lu hf geochronology on a uhp eclogite of the zermatt saas fee zsf ophiolite lago di cignana italy demonstrate that prograde garnet growth of this sample occurred over a 30 to 40 m y interval relative to peak metamorphism at 38 to 40 ma garnet growth is estimated to have begun at 11 to 14 kbar pressure at 70 to 80 ma although such a protracted garnet growth interval is surprising this is supported by plate tectonic reconstructions which suggest that subduction of the liguro piemont ocean occurred through slow and oblique convergence these results demonstrate that ree zonations in garnet coupled to crystal size distributions provide a powerful means for understanding prograde metamorphic paths when combined with sm nd and lu hf geochronology

constraints on crustal structure and complex moho topography beneath pinon flat california from teleseismic receiver functions abstract we use teleseismic p waves recorded at pinon flat observatory pfo to constrain the three dimensional crustal and upper mantle velocity structure beneath the station by forward modeling radial receiver function waveforms we construct a one dimensional crustal model that includes a significant shear velocity inversion at 9 km in depth arrivals on the tangential components indicate dip of at least the uppermost discontinuity complicated moho topography deepening to the northwest of pfo is suggested by azimuthal dependence of travel times and amplitudes of the receiver functions and travel times of crustal p wave reverberations although fine details cannot be resolved each of those sets of observations plus mislocation vectors provide strong indications of abrupt moho topography possibly including step offsets of several kilometers this is not only consistent with gravity data in implying airy isostasy with compensation at moho depth but extends that model to a much finer length scale than had been resolved

quartz crystal size distribution of the ordovician millbrig k bentonite abstract crystal size distribution csd analysis has been applied to quartz crystals of the ordovician millbrig k bentonite which represents one of the largest known fallout ash deposits in the phanerozoic era to establish crystal growth histories and conditions in the magma chamber prior to eruption specific csds were examined for crystal growth conditions all millbrig samples exhibit lognormal concave down shapes in agreement with previously reported csds on large silicic systems but in contrast to more mafic systems characterized by linear csds concave down csds are possibly an intrinsic property of phenocryst growth in large volume silicic magmas as documented in the literature bindeman i n 2003 crystal sizes in evolving silicic magma chambers geology 31 367 370 quartz crystals of the millbrig must have been produced by surface controlled size dependent growth with a minor ostwald ripening component which could have easily changed to supply controlled growth due to a limited supply of nutrients as the crystals grew larger csds were also used to examine the possible origin of multiple ash beds in the millbrig these ash beds appear to be a product of a series of separate eruptions that represent separate magma layers or batches each with slightly different crystal growth conditions

recrystallized impact glasses of the onaping formation and the sudbury igneous complex sudbury structure ontario canada abstract the origin of the sudbury structure and of the associated heterolithic breccias of the onaping formation and the sudbury igneous complex have been controversial while an impact origin of the structure has gained wide acceptance over the last 15 years the origin of the recrystallized onaping formation glasses and of the igneous complex is still being debated recently the interpretation of the breccias of the onaping formation as suevitic fall back impact breccias has been challenged the igneous complex is interpreted either as a differentiated impact melt sheet or as a combination of an upper impact melt represented by the granophyre and a lower impact triggered magmatic body consisting of the norite sublayer formations the onaping formation contains glasses as fluidal and nonfluidal fragments of various shapes and sizes they are recrystallized and our research indicates that they are petrographically heterogeneous and span a wide range of chemical compositions these characteristics are not known from glasses of volcanic deposits this suggests an origin by shock vitrification an interpretation consistent with their association with numerous and varied country rock clasts that exhibit microscopic shock metamorphic features the recrystallized glass fragments represent individual solid state and liquid state vitrified rocks or relatively small melt pods the basal member lies beneath the gray and black members of the onaping formation and where not metamorphic has an igneous matrix igneous textured melt bodies occur in the upper two members and above the basal member a comparison of the chemical compositions of recrystallized glasses and of the matrices of the basal member and the melt bodies with the components and the bulk composition of the igneous complex is inconclusive as to the origin of the igneous complex basal member matrix and melt bodies on average are chemically similar to the granophyre of the sudbury igneous complex suggesting that they are genetically related our chemical results allow interpretation of the entire igneous complex as a differentiated impact melt however they are also consistent with the granophyre alone being the impact melt and the norite and quartz gabbro beneath it representing an impact triggered magmatic body this interpretation is preferred as it is consistent with a number of field observations a re evaluation and extension of structural field studies and of geochemical data as well as a systematic study of the contact relationships of the various igneous phases of the igneous complex are needed to establish a sudbury impact model consistent with all data and observations

crustal structure of the gujarat region india new constraints from the analysis of teleseismic receiver functions abstract in this study receiver function analysis is carried out at 32 broadband stations spread all over the gujarat region located in the western part of india to image the sedimentary structure and investigate the crustal composition for the entire region the powerful genetic algorithm technique is applied to the receiver functions to derive s velocity structure beneath each site a detail image in terms of basement depths and moho thickness for the entire gujarat region is obtained for the first time gujarat comprises of three distinct regions kachchh saurashtra and mainland in kachchh region depth of the basement varies from around 1 5 km in the eastern part to 6 km in the western part and around 2 3 km in the northern part to 4 5 km in the southern part in the saurashtra region there is not much variation in the depth of the basement and is between 3 km and 4 km in gujarat mainland part the basement depth is 5 8 km in the cambay basin and western edge of narmada basin in other parts of the mainland it is 3 4 km the depth of moho beneath each site is obtained using stacking algorithm approach the moho is at shallower depth 26 30 km in the western part of kachchh region in the eastern part and epicentral zone of the 2001 bhuj earthquake large variation in the moho depths is noticed 36 46 km in the saurashtra region the crust is more thick in the northern part it varies from 36 38 km in the southern part to 42 44 km in the northern part in the mainland region the crust is more thick 40 44 km in the northern and southern part and is shallow in cambay and narmada basins 32 36 km the large variations of poisson s ratio across gujarat region may be interpreted as heterogeneity in crustal composition high values of 0 30 at many sites in kachchh and few sites in saurashtra and mainland regions may be related to the existence of high velocity lower crust with a mafic ultramafic composition and locally to the presence of partial melt the existing tectono sedimentary models proposed by various researchers were also examined

tomographic imaging of sub basalt mesozoic sediments and shallow basement geometry for hydrocarbon potential below the deccan volcanic province dvp of india summary a shallow 2d tomographic p wave velocity image is obtained in the deccan volcanic province dvp of india using first arrival seismic data along a 90 km long n s trending profile in the deccan syneclise region the tomographic image depicts smooth velocity variations of quaternary 2 0 2 5 km s sediments basalt 5 0 5 5 km s and sub basalt mesozoic sediments 4 0 4 5 km s as well as basement 5 8 6 0 km s configuration down to a maximum depth of 5 0 km due to late cretaceous volcanism with outpouring of basaltic lava flows this region is affected by numerous dyke intrusions and thick basaltic trap forming sills with the presence of horst and graben structures due to deep basinal faults although imaging below the basalt is a major challenge for the oil industry with the help of tomographic imaging technique of long offset seismic refraction data we are able to image sporadic sub and intra trappean mesozoic sediments 0 75 km deposited below thick columns 0 3 km of basalt flows above the basement the imaged mesozoic sediments are expected to contain hydrocarbon due to their wide extension and suitable trapping mechanism of basaltic sills the robustness of the velocity image is assessed through numerous tests like velocity perturbations 2 estimates rms residuals of traveltime fit resolution and uncertainty estimates through computation of ray density or hits

flowage differentiation in an andesitic dyke of the motru dyke swarm southern carpathians romania inferred from ams csd and geochemistry abstract two dykes of different thickness 5 5 m for tj31 and 23 m for tj34 from the late pan african calc alkaline motru dyke swarm s carpathians romania have been studied by electron microprobe mineral chemistry crystal size distribution csd anisotropy of magnetic susceptibility ams and whole rock geochemistry all the physical and chemical variations observed across the dyke s width point to concordant results and show that the variations of both modal abundance and size of the amphibole and biotite microphenocrysts inside the dykes deduced from the classical csd measurements are the result of a mechanical segregation of suspended crystals during magmatic transport despite a pene contemporaneous regional tectonic the flow induced differentiation in the thicker dyke is characterized by the concentration of pre existing ti rich pargasite tschermakite clinopyroxene and plagioclase crystals in the core of the dyke and of the extracted differentiated liquid near the walls this mechanical differentiation induces a chemical differentiation with a basaltic andesite composition for the core of the dyke whereas the margins are andesitic thus the chilled margins appear as a slightly more evolved liquid with a newtonian behaviour when compared to the average composition of the dyke the localization of the liquid on both sides of the dyke has certainly facilitated the ascent of the central part of the dyke that behaved as a binghamian mush

understanding conspicuous gravity low over the koyna warna seismogenic region maharashtra india and earthquake nucleation a paradigm shift the continued seismicity in koyna warna region of the western part of maharastra india and its relationship with subsurface structures concealed below thick volcanic sequences are studied in detail using gravity field along with newly available deep scientific drilling results this seismically active zone is marked by a large conspicuous negative gravity anomaly the causes of which are yet to be fully understood recent findings from the boreholes drilled in the koyna gupta et al in int j earth sci 104 1511 1522 2015 and killari seismic zones both of which penetrated the thick deccan volcanic cover and the underlying archean crystalline basement have motivated us to revisit the bouguer gravity field over this region using a newly developed finite element method of regional residual separation our study reveals the presence of two thick low density low velocity crustal zones below the koyna warna region the shallower one between 5 and 13 km depth and the deeper one between 35 and 43 km depth just above the moho both of these zones appear to contain mantle metasomatised and fractionated magmatic material respectively interestingly the hypocenters of all m 5 koyna earthquakes occur within the upper low velocity low density zone we also suggest high order crustal exhumation below this region which led to the removal of the entire sedimentary and granitic upper crustal column this process has brought denser mid crustal lithological facies close to the surface quaternary uplifting and movement of fault blocks along the old as well as newly created fault planes seem to be still continuing a paleo rift may have existed beneath this region below which moho temperatures 600 c and mantle heat flow 31 mw m2 are still high

mapping the sub trappean mesozoic sediments in the western part of narmada tapti region of deccan volcanic province india abstract deccan traps spread over large parts of south west and central india possibly hiding underneath sediments with hydrocarbon potential here we present the results of seismic refraction and wide angle reflection experiments along three profiles and analyze them together the results from all other refraction profiles executed earlier in the western part of narmada tapti region of the deccan volcanic province dvp we employ travel time modelling to derive the granitic basement configuration including the overlying trap and sub trappean sediment thickness if any travel time skips and amplitude decay in the first arrival refraction data are indicative of the presence of low velocity sediments mesozoic which are the low velocity zones lvz underneath the traps reflection data from the top of lvz and basement along with the basement refraction data have been used to derive the mesozoic sediment thickness in the middle and eastern parts of the study region between narmada and tapti the mesozoic sediment thickness varies between 0 5 and 2 0 km and reaches more than 2 5 km south of sendhwa between narmada and tapti rivers thick mesozoic sediments in the eastern parts are also accompanied by thick traps the mesozoic sediments along the present three profiles may not be much prospective in terms of its thickness except inside the cambay basin where the subtrappean sediment thickness is about 1000 1500 m in the eastern part of the study area the deepest section 4 km has thick 2 km mesozoic sediments but with almost equally thick deccan trap cover results of the present study provide important inputs for future planning for hydrocarbon exploration in this region

steady state volcanism paleoeffusion rates and magma system volume inferred from plagioclase crystal size distributions in mafic lavas dome mountain nevada abstract the dome mountain dm lavas located in the moat of the timber mountain caldera of the sw nevada volcanic field cap a thick sequence of welded tuffs massive and bedded tuffs and mafic and rhyolitic lava flows these 10 9 m y old flows comprise a pile of some twenty flows with a total thickness of about 300 m individual flows range in thickness from 3 to 20 m and are characterized by a basal vesicular zone a massive central region and an upper vesicular region containing in most cases cooling fractures the flows dip gently away from the summit of dm a prominent topographic feature of the region with an elevation of 2065 m silica contents range from 48 8 wt at the base of the pile to 59 4 wt at the top although most of the flows are andesitic crystal size distributions csds for plagioclase in these comagmatic lavas have been determined to provide information on magma storage times these times when coupled with phase equilibria can be used to infer cooling rates which are indicative of the relative dimensions of magmatic bodies and the vigor of various magmatic cooling processes most of the csds show no sign of crystal fractionation or accumulation assuming a growth rate of 10 10 cm s calculated mean crystal residence times for most lavas cluster in the range of 1 5 to 4 years two of the lower lavas have residence times in the range of 4 to 9 years the residence times correlate inversely with nucleation density and broadly correlate inversely with silica content high residence time low silica lavas occur at the base of the pile whereas low residence time high silica lavas occur at the top of the pile one interpretation of the residence time data is that the lavas are from an open system of more or less constant residence time but varying spatially in system locations e g a magma chamber in which steady state recharge and eruption may have been reached alternatively these lavas may represent small samples closely spaced in time of a slowly cooling large batch or reservoir of magma the greater residence times associated with the lower flows may indicate the increased amount of time necessary to form the conduit of the magma volcano system by the oldest magma or simply that part of the system with the highest liquidus temperature a minimum volume of 1 4 to 2 5 km 3 for the magma chamber is estimated for the dm system using the estimated residence times in a simplified cooling model of a sheet of aspect ratio r the actual system volume depends on r which is strictly unknown and thus the actual system volume may be much larger e g 200 km 3 a mean paleoeffusion rate of 0 05 km 3 yr is obtained for the lavas by assuming that the residence time of each lava corresponds to the amount of time elapsed since the eruption of the underlying flow

crystal size distributions and scaling laws in the quantification of igneous textures this study demonstrates the value of the combined use of scaling of the chemical evolution of cooling magmas have investigated the detailed relationships between temanalysis and crystal size distribution csd measurements in the interpretation of igneous textures and outlines applications of these perature phase abundance and phase composition the reaction series bowen 1922 and the cipw norm cross methods to other fundamentally important problems theoretical calculations and measurements of natural samples are used to et al 1903 have been supplemented by modern thermodynamic models that predict the compositions and abundcharacterize the relationship between igneous texture and cooling history the total number nt and mean length l of crystals ances of minerals in equilibrium with melt of a given composition and temperature e g ariskin et al 1993 in a sample are correlated through a scaling relationship of the form n t l this proportionality depends on the mineralogy ghiorso sack 1995 although there are a few areas of uncertainty the chemistry of igneous petrogenesis is of the rock and so a modal normalization factor c is introduced the csd slope s and intercept ln n are uniquely related to fairly well understood but chemical models cannot address the central probeach other through the equation ln n 4ln s ln c where s l this overall relationship allows the texture of a rock lem of igneous crystallization which is the physical process of generating a set of crystals from magma two to be related to the local duration of cooling through an arbitrary crystal growth model quantification of the link between the texture magmas with the same composition will normally produce the same minerals in the same proportions however of a rock and its cooling history makes it possible to predict spatial variations in texture we show an example of this method using depending on their thermal histories these two magmas can solidify into rocks with drastically different physical the sudbury igneous complex canada additional applications include relating the textures of volcanic rocks to spatial and temporal appearances e g basalt and gabbro the set of minerals is the same but what distinguishes them is the abundance variations in the magma chamber and extracting kinetic parameters from suites of comagmatic rocks and size of the constituent crystals the fine grained rocks in the chilled margins of most basic intrusions cooled rapidly and the coarse grained rocks in the interiors of plutons cooled slowly this has

on bimodal differentiation by solidification front instability in basaltic magmas part 1 basic mechanics extensive coarse grained silicic lenses are common in the upper portions of large diabase sills lava lakes and gabbroic intrusions these lenses which often contain up to 10 wt more silica than their host rocks record a clear process of bimodal differentiation that may play a fundamental role in generating significant quantities of silicic magma in predominantly basaltic systems the spatial compositional textural and contact relations suggest that these silicic segregations form by infilling of tears formed during gravitational instability of the upper solidification front this process is investigated in some detail by exploring the thermal and mechanical conditions leading to internal failure of the front and segregation formation as the upper solidification front grows inward and thickens the local strength of the developing crystalline network must increase upward through the front sufficiently fast to offset the increasing weight of the front itself this necessary local variation in strength dictates how crystallinity must vary to prevent failure but the actual variation in crystallinity is determined by phase equilibria which in general varies in a fashion that is noncovariant with that required for mechanical stability it is this mismatch that apparently leads to failure and formation of silicic segregations the rate of filling of the tears by porous flow of interstitial melt from below regulates the rate of tear opening and the overall size of the tears is regulated by the local rate of advance of the solidification front segregations therefore thicken downward as the growth of the front slows and then die out altogether with approach of the lower solidification front this process irreversibly introduces siliceous noise into basaltic systems the silicic signal can be enhanced through subsequent reprocessing of the host rock to form volumetrically important masses of silicic magma this process may be fundamental to achieving the strong compositional diversity of the igneous rocks to yielding continental masses and to the differentiation of earth itself