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

origin of ultramafic xenoliths containing exsolved pyroxenes from hualalai volcano hawaii hualalai volcano hawaii is best known for the abundant and varied xenoliths included in the historic 1800 kaupulehu alkalic basalt flow xenoliths which range in composition from dunite to anorthosite are concentrated at 915 m elevation in the flow rare cumulate ultramafic xenoliths which include websterite olivine websterite wehrlite and clinopyroxenite display complex pyroxene exsolution textures that indicate slow cooling websterite olivine websterite and one wehrlite are spinel bearing orthopyroxene olivine cumulates with intercumulus clinopyroxene plagioclase two wehrlite samples and clinopyroxenite are spinel bearing olivine cumulates with intercumulus clinopyroxene orthopyroxene plagioclase two pyroxene geothermometry calculations based on reconstructed pyroxene compositions indicate that crystallization temperatures range from 1225 to 1350 c migration or unmixing of clinopyroxene and orthopyroxene stopped between 1045 and 1090 c comparisons of the abundance of k2o in plagioclase and the abundances of tio2 and fe2o3in spinel of xenoliths and mid ocean ridge basalt and a single 87sr 86sr determination indicate that these hualalai xenoliths are unrelated to mid ocean ridge basalt similarity between the crystallization sequence of these xenoliths and the experimental crystallization sequence of a hawaiian olivine tholeiite suggest that the parental magma of the xenoliths is hualalai tholeiitic basalt xenoliths probably crystallized between about 4 5 and 9 kb the 155 230 c of cooling which took place over about 120 ka the age of the youngest hualalai tholeiitic basalt yield maximum cooling rates of 1 3 10 3 1 91 10 3 c yr hualalai ultramafic xenoliths with exsolved pyroxenes crystallized from hualalai tholeiitic basalt and accumulated in a magma reservoir located between 13 and 28 km below sealevel we suspect that this reservoir occurs just below the base of the oceanic crust at about 19 km below sealevel

dissipation of fast strike slip faulting within and beyond northeastern tibet structural patterns global positioning system gps velocities and quaternary fault slip rates in northeastern tibet indicate a transfer of left lateral slip from the kunlun fault northeast to the haiyuan fault and minor crustal shortening and rotation within a 200 km wide stepover zone related deformation also continues at least a few hundred kilometers north of the haiyuan fault into a region of diffuse extensional shear or rotation underlain by average thickness crust fast localized slip along the central kunlun fault transforms into distributed deformation across a 500 km wide zone where the lower crust is weak the distribution of fault parallel gps velocities across this region suggests a decrease in fault slip toward eastern fault tips and progressive dissipation of slip to the north rather than east of the tibetan plateau as previously suggested

petrology of lavas from the puu oo eruption of kilauea volcano iii the kupaianaha episode 1986 1992 the puu oo eruption has been remarkable in the historical record of kilauea volcano for its duration over 13 years volume 1 km3 and compositional variation 5 7 10 wt mgo during the summer of 1986 the main vent for lava production moved 3 km down the east rift zone and the eruption style changed from episodic geyser like fountaining at puu oo to virtually continuous relatively quiescent effusion at the kupaianaha vent this paper examines this next chapter in the puu oo eruption episodes 48 and 49 and presents new icp ms trace element and pb sr and nd isotope data for the entire eruption 1983 1994 nearly aphyric to weakly olivine phyric lavas were erupted during episodes 48 and 49 the variation in mgo content of kupaianaha lavas erupted before 1990 correlates with changes in tilt at the summit of kilauea both of which probably were controlled by variations in kilauea s magma supply rate these lavas contain euhedral olivines which generally are in equilibrium with whole rock compositions although some of the more mafic lavas which erupted during 1990 a period of frequent pauses in the eruption accumulated 2 4 vol olivine the highest forsterite content of olivines 85 in kupaianaha lavas indicates that the parental magmas for these lavas had mgo contents of 10 wt which equals the highest observed value for lavas during this eruption the composition of the puu oo lavas has progressively changed during the eruption since early 1985 episode 30 when mixing between an evolved rift zone magma and a more mafic summit reservoir derived magma ended the normalized to 10 wt mgo abundances of highly incompatible elements and cao have systematically decreased with time whereas ratios of these trace elements and pb sr and nd isotopes and the abundances of y and yb have remained relatively unchanged these results indicate that the hawaiian plume source for puu oo magmas must be relatively homogeneous on a scale of 10 20 km3 assuming 5 10 partial melting and that localized melting within the plume has apparently progressively depleted its incompatible elements and clinopyroxene component as the eruption continued the rate of variation of highly incompatible elements in puu oo lavas is much greater than that observed for kilauea historical summit lavas e g ba y 0 09 a 1 vs 0 03 a 1 this rapid change indicates that puu oo magmas did not mix thoroughly with magma in the summit reservoir thus except for variable amounts of olivine fractionation the geochemical variation in these lavas is predominantly controlled by mantle processes

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

mechanism of subsidence of ancient cratonic rift basins cratonic basins commonly occur over ancient rift zones these inactive rift basins are recognizable by a positive linear bouguer gravity anomaly that may correspond to the axial gravity high found in modern rift valleys many of these basins undergo discrete periods of increased subsidence rates or reactivations long after the mass excess responsible for the linear gravity high was emplaced furthermore the reactivation of many cratonic basins occurs simultaneously with large scale compressional tectonics it is suggested that the driving force for subsidence is the isostatically uncompensated ancient mass excess the subsidence of these basins is modelled by a lithospheric flexure model with a nonlinear maxwell viscoelastic rheology solutions to this model indicate that basins may experience a low subsidence rate throughout geologic time the subsidence of a basin will stop only when isostatic compensation of the mass excess is achieved since ancient rift mass excesses may be uncompensated over long geologic time intervals the early thermal and structural evolution of rifts may not significantly influence later basin subsidence the models suggest that basins may be reactivated by any mechanism which lowers the effective viscosity of the lithospheric material allowing the uncompensated basin to settle toward an isostatic compensation depth faster than normal since viscosity is a strong function of temperature reactivation by a world wide increase in heat flow is suggested as a possible mechanism for the synchroneity of basin subsidence throughout a continent an increase of 15 in the geothermal gradient for example from 16 5 18 9 k km will cause about a 5 increase in subsidence this increase in heat flow however seems unlikely of producing by itself the magnitude of basin subsidence during a reactivation phase that is observed in the geologic record where up to 100 increase in subsidence might occur since the rheology of the lithosphere is nonlinear effective viscosity is also a strong nonlinear function of stress the presence of a regional compressive stress during periods of tectonism of 1 1 108 pa about 2 8 of the buckling strength of the lithosphere produces a short period of reactivated subsidence 105 yr during the reactivated subsidence the newly imposed regional stress relaxes sufficiently in the lower lithosphere to restore the effective viscosity to values similar to that before reactivation this suggests that reactivated subsidence caused by regional compression can be maintained as long as the stress level remains high in the lower lithosphere this may be accomplished by an intermittent application of the regional stress over time

the ninole basalt implications for the structural evolution of mauna loa volcano hawaii lava flows of the ninole basalt the oldest rocks exposed on the south side of the island of hawaii provide age and compositional constraints on the evolution of mauna loa volcano and the southeastward age progression of hawaiian volcanism although the tholeiitic ninole basalt differs from historic lavas of mauna loa volcano in most major element contents e g variably lower k na si higher al fe ti ca ree and other relatively immobile minor elements are similar to historic and prehistoric mauna loa lavas and the present major element differences are mainly due to incipient weathering in the tropical environment new k ar whole rock ages from relatively fresh roadcut samples suggest that the age of the ninole basalt is approximately 0 1 0 2 ma although resolution is poor because of low contents of k and radiogenic ar originally considered the remnants of a separate volcano the ninole hills are here interpreted as faulted remnants of the old south flank of mauna loa deep canyons in the ninole hills eroded after massive landslide failure of flanks of the southwest rift zone have been preserved from burial by younger lava due to westward migration of the rift zone landslide induced depressurization of the southwest rift zone may also have induced phreatomagmatic eruptions that could have deposited widespread basaltic ash that overlies the ninole basalt subaerial presence of the ninole basalt documents that the southern part of hawaii island had grown to much of its present size above sea level by 0 1 0 2 ma and places significant limits on subsequent enlargement of the south flank of mauna loa

gravity anomalies and flexure of the lithosphere at mountain ranges the bouguer gravity anomaly over the himalayan alpine and appalachian mountains is characterized by a generally asymmetric gravity low which spans the mountains and associated foreland basins the minimum of the gravity low is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology in addition the alps and appalachians are associated with a generally symmetric gravity high that is unrelated to the topographic relief together the gravity low and high form a characteristic positive negative anomaly couple the steep gravity gradient between the positive and negative couple correlates with the insubric line and its equivalents in the alps and the brevard zone in the southern appalachians this gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface buried and surface topographic loading the magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography by combining these loads the flexure of the lithosphere and the associated bouguer gravity anomaly are calculated for different assumed values of the elastic thickness te of continental lithosphere the best fitting value of te for the himalayas is 80 100 km for the alps 25 50 km and for the appalachians 80 130 km the model calculations suggest that the main contribution to the gravity couple in the alps and appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads however in the case of the himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low when subsurface loads do exist therefore we would not expect a close correlation between mountain topographic relief and depth to the m discontinuity as required by classical models of isostasy rather the m discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere the surface and subsurface loads inferred from the gravity anomaly play a major role in the development of mountains and foreland basins the emplacement of large subsurface loads represents the primary event in the alps and appalachians while in the himalayas the emplacement of the surface load is the primary event the emplacement of the subsurface load probably marks the initiation of foreland basin development the regional characteristics of the basin are controlled by the primary load while local variations are controlled by the secondary thrust sheet loads subsequent movement of either load will produce migration of the basin depocenter the origin of the subsurface load is not clear but its association with the insubric line in the alps and the brevard zone in the appalachians suggests that it may be related to the obduction of crustal blocks flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate

evolution of mauna kea volcano inferences from lava compositions recovered in the hawaii scientific drilling project the lower 776 m of core recovered during the initial phase of the hawaii scientific drilling project hsdp contains lavas erupted from mauna kea volcano tholeiitic and alkalic basalts including an fe ti rich flow are intercalated in the upper 58 m of mauna kea lavas similar basaltic sections are subaerially exposed on the lower east flank of mauna kea the fe ti rich lavas reflect large amounts of clinopyroxene plagioclase and olivine fractionation within the crust and upper mantle but the range from tholeiitic to alkalic compositions reflects variable extents of melting of a garnet bearing source based on abundances of incompatible elements the extent of melting for a basanitoid was a factor of 2 less than that for nearly coeval tholeiitic lavas all flow units in the lower 718 m of the hsdp core are tholeiitic lavas their variability in major element compositions reflect variable accumulation of olivine incompatible element abundance ratios in these lavas reflect a complex temporal variation in extent of melting within the tholeiitic part of the core lavas from 800 m to 950 m formed by the largest extent of melting whereas tholeiitic lavas from the bottom of the core and from just below the tholeiitic to alkalic transition formed by lower degrees of melting inferred melt compositions at 16 mgo show that the 200 to 400 ka mauna kea lavas from the hsdp core and the 250 ka subaerial exposures define an inverse correlation between sio2 and feo contents based on experimental studies this correlation is caused by differing pressures of melt segregation furthermore abundances of nb and sio2 are also inversely correlated in these calculated melts in general the younger lavas are relatively enriched in feo and incompatible elements but are depleted in sio2 these trends are interpreted to reflect an overall trend of increasing pressure of melt segregation and decreasing extent of melting with decreasing eruption age there are however geochemical variations which indicate short term reversals in this long term trend previously the geochemical trends accompanying the transition from tholeiitic to alkalic volcanism at hawaiian volcanoes have been interpreted as reflecting the effects of increasing distance from the plume axis the long term geochemical trends of tholeiitic lavas in the hsdp core also reflect migration of mauna kea away from the hawaiian plume

geodynamic models of convergent margin tectonics transition from rifted margin to overthrust belt and consequences for foreland basin development a quantitative geodynamic model for overthrusting of a passive continental margin during attempted continental subduction demonstrates the mechanical and thermal coupling between overthrust loads the lithosphere and the associated foreland basin the model treats the lithosphere as a two dimensional nonuniform elastic plate whose strength is controlled thermally the thermal and flexural evolution of a margin is followed from initial rifting and passive margin development through overthrusting and foreland basin deposition to postdeformational erosion our models indicate the following 1 foreland basin geometry is relatively insensitive to the age of the preexisting passive margin prior to overthrusting sensitivity decreases the farther the overthrust load is empl ced onto the craton 2 foreland basin depth is strongly influenced by the size of the overthrust load maximum depths in excess of 10 km 6 mi can occur given overthrust topography similar to the present day himalayas 3 a few tens of kilometers of postdeformational erosion can occur across topographically high orogenic belts before moderate topography is attained very little of this eroded material is deposited in the foreland basin most must bypass the basin because it also is subjected to erosion and associated flexural uplift 4 thick overthrusts as interpreted from cocorp profiling are consistent with low topographic expression because they were emplaced on preexisting oceanic and stretched continental lithosphere

a technique for incorporating geothermal gradients and nonlinear creep into lithospheric flexure models a beam flexure model for the lithosphere which employs a nonlinear viscous creep constitutive relation has been developed for time dependent flexure caused by surficial loads the lithosphere is buoyantly supported by the underlying asthenosphere which is modeled as an elastic winkler foundation creep is highly dependent on temperature and melting point which in turn vary with depth these factors are incorporated into the beam lithospheric model by using weertman s constitutive relation for creep of rock at elevated temperature a typical value of the creep power n in the weertman equation is 3 for rock but the value of n may be varied in the model the governing nonlinear partial differential equations for moment and deflection are solved by an analytical procedure for the spatial coordinate and by a numerical marching procedure in the time coordinate stresses in the lithosphere are also evaluated with special attention given to an iterative technique for locating the neutral axis which will not be at the centroid because of the nonlinearity and the geothermal gradient

thrust loads and foreland basin evolution cretaceous western united states two dimensional modeling of loading during the formation of the idaho wyoming thrust belt shows that regional isostatic compensation by flexure of an elastic lithosphere is sufficient to control the formation of a foreland basin the flexural rigidity of the lithosphere is inferred to have been approximately 1023 nm 1030 dyne cm on the basis of palinspastic comparison of predicted downwarping due to the thrust plate loads to the shape of the sedimentary wedge on the west side of the cretaceous western interior seaway erosion of part of the uplifted thrust plates redistributed the load depositing it farther to the east thereby causing subsidence over a much wider area than could have been accomplished only by the loading by thrust plates paleotopography after major cretaceous thrust events was calculated the resulting mountainous terrain gentle alluvial plain and flat sea floor correspond well with the topography of the modern foreland thrust belt and basin system in the andes of south america and to paleogeographic reconstructions in the western united states thrust belt topography is controlled by the subsurface geometry of the thrust faults particularly the positions of ramp zones and by isostatic subsidence

hawaiian magma reservoir processes as inferred from the petrology of gabbro xenoliths in basalt kahoolawe island gabbro xenoliths in a tholeiitic lava of kahoolawe island hawaii a 1 3 1 4 ma shield volcano are 1 3 cm in size and comprised of plagioclase clinopyroxene and orthopyroxene gabbro textures while intergranular and in part subophitic are open due to 28 48 vol of vesicular basalt occupying xenolith space vesicles in and around the xenoliths are lined or filled with rhyolitic glass segregation vesicles the host is evolved tholeiite mgo 6 1 wt with phenocrysts microphenocrysts and glomerocrysts of olivine clinopyroxene orthopyroxene and plagioclase and megacrysts 1 cm of plagioclase the sr isotope ratio of one xenolith is 0 70489 the host basalt ratio is 0 70460 xenolith isotope composition grain resorption and clinopyroxene fs12 5 15wo38 35 5 orthopyroxene fs19 5 24wo4 1 and plagioclase an68 65or0 8 1 2 compositions suggest that these gabbros crystallized from kahoolawe tholeiitic magma of essentially the same composition as the host basalt but pre dating the magma represented by the host based on the absence of intergranular fe ti oxide phases from the pl cpx opx assemblages and the open vuggy textures we envision crystallization on a reservoir roof at temperatures 1100 c entrainment of gabbro assemblages and plagioclase megacrysts from a roof mush suspension zone occurred during convection associated with replenishment of the magma reservoir these open textured gabbro xenoliths are therefore not fragments of preexisting coarse grained bodies such as sills or segregation veins rhyolitic glass in vesicles represents a gas effervescence filtration process that forced fractionated residual liquids from the groundmass into voids associated with the xenoliths

koolau shield basalt as xenoliths entrained during rejuvenated stage eruptions perspectives on magma mixing rejuvenated stage tuff cones honolulu volcanics on koolau volcano oahu hawaii contain xenoliths of koolau shield basalt because koolau subaerial shield lavas represent a hawaiian geochemical end member and submarine shield lavas have compositions with some affinities to mauna loa and kilauea we analyzed 28 xenolithic basalts from salt lake and koko head cones to determine how these seemingly random samplings of the koolau profile compare to established koolau geochemistry analyses reveal that 24 are shield tholeiitic basalt the focus of this study and 4 are rejuvenated stage basaltic rocks the tholeiitic xenoliths represent largely upper koolau shield lavas as these samples 8 3 to 5 8 wt mgo have with one exception overall major and trace element compositions that overlap those of koolau subaerial shield lavas secondary processes however created some distinctions namely enrichments depletions in k ba sr sio2 and feo and due to zeolitization chabazite with attending okenite and apophyllite elevated cao one xenolithic basalt with 8 2 wt mgo has higher ti zr nb and sc and lower zr nb than subaerial lavas and appears to represent relatively early deeper shield thereby reinforcing that the koolau shield source varied temporally olivine orthopyroxene and plagioclase are the phenocrysts clinopyroxene is rare and their core compositions range widely across the suite fo87 8 72 orthopyroxene mg s 85 72 and an74 60 several xenolithic basalts have both normally and reversely zoned orthopyroxene and plagioclase with a variety of core compositions e g orthopyroxene core mg s 82 77 and 72 all in one sample these compositions and zonations record evidence for wide compositional ranges of replenishment mgo 13 8 wt and reservoir mgo 7 to 5 wt magmas mixing in varying proportions however extreme mgo lavas 13 and 5 wt are not observed as either subaerial or xenolithic basalt but are indicated by phenocryst cores of fo87 8 and orthopyroxene mg 72 the koolau magma mixing history resembles that of kilauea and is unlike the steady state mixing known for mauna loa finally these basalt samples show that any xenolithic occurrence of koolau lava is subject to the zeolitization prevalent in the tuff cone hosts

axisymmetric finite element analysis of an elastic lithosphere subjected to thermal loading summary an axisymmetric finite element was used to model the lithosphere as an infinite 40 km thick circular elastic plate on an elastic foundation asthenosphere subjected to concentrated thermal loading thermal loading is produced by a point heat source of 4 184 lo8 watts located 60 km beneath the plate within the asthenosphere increased nodal temperatures within the lithospheric finite element mesh were obtained from a solution to the steady state heat conduction equation for a point source in a semi infinite half space vertical as well as radial thermal expansion coefficients of 1 33 x lo k were utilized in the solutions thermoelastic loads as well as isostatic uplift forces are applied as nodal loads in the finite element lithosphere model thermoelastic loads are produced from uneven heating of the lithosphere and differential expansion isostatic uplift forces result from radial expansion and the diminished density of heated lithosphere columns uplift of the lithosphere due to the point heat source and vertical expansion is reduced because of radial expansion and flexure this occurs because material at the base of the lithosphere is closer to the point heat source and experiences greater radial expansion for sediment infdl thermoelastic loading reduces the surface uplift produced by isostatic uplift forces from 3 11 to 192m thermoelastic lithosphere models yield significantly higher radial stress profiles than density models without thermoelastic stresses the neutral plane for radial stress is displaced towards the surface in the thermoelastic models due to greater radial expansion of basal lithosphere material which is closer to the point heat source the finite element model has several advantages over the classical thin plate approach stresses and strains can be evaluated at any point within the lithosphere this capability was used in this study to determine vertical lithosphere expansion thickening non linear variation of stress with depth and the stress neutral plane position

intracrustal controls on the coexistence of tholeiitic and calc alkaline magma series at aso volcano sw japan aso caldera sw japan was formed incrementally over a period of 300 kyr by the eruption of four basaltic to dacitic pyroclastic flow and fall deposits aso 1 4 these units are collectively referred to as the aso pyroclastic flow deposit apfd in which tholeiitic and calc alkaline magmas have been erupted in close temporal and spatial proximity detailed petrological and geochemical studies indicate that a major shift in magmatic composition occurred between aso 1 and 4 as the caldera forming stage evolved changing from a predominantly tholeiitic to a calc alkaline signature the mineralogical and geochemical characteristics of aso 1 4 suggest that magmatic evolution within each unit is controlled by fractional crystallization associated with magma mixing crustal assimilation by altering the fractional crystallization mineral assemblage as well as the relative amounts of fractional crystallization magma mixing and crustal assimilation the systematic shifts in differentiation trends and magmatic series observed between the four eruptive units can be reproduced this suggests that the coexistence of tholeiitic and calc alkaline magma series can be attributed to shallow level intracrustal processes

geochemistry of the 1984 mauna loa eruption implications for magma storage and supply the 1984 eruption of mauna loa volcano hawaii produced about 220 106 m3 of lava of remarkably homogeneous composition no significant compositional changes occurred during the 22 day eruption and along the 20 km vent system even though crystallinity increased substantially accompanying a decline in the eruption rate such homogeneity in lava composition is typical of many historic mauna loa eruptions the 1984 lava along with most other homogeneous mauna loa lavas is multiply saturated crystallizing olivine clinopyroxene and plagioclase bulk compositions are perched at the intersection of an olivine control trend with a trend for more differentiated lavas there is no evidence to indicate that the 1984 lava is a differentiate of the 1975 magma or that it results from mixing a new batch of magma with differentiated melt remaining in the magmatic plumbing system after the 1975 eruption it is proposed that these lavas are erupted directly from a small continuously replenished magma reservoir located about 3 4 km beneath the summit caldera successive eruptions therefore provide a sampling of magma chamber processes and small but discernible changes in lava composition reflect the competing processes of magma supply differentiation and eruption

apophyllite 40ar 39ar and rb sr geochronology potential utility and application to the timing of secondary mineralization of the kirkpatrick basalt antarctica apophyllite kca 4 si 8 o 20 f oh 8h 2 o which formed as a low temperature secondary mineral in the kirkpatrick basalt has been examined to evaluate its potential for geochronology using the k ar by the 40 ar 39 ar technique and rb sr methods the 40 ar 39 ar total gas ages for apophyllites from the kirkpatrick basalt are 114 133 ma for the central transantarctic mountains 95 114 ma for south victoria land and 76 100 ma for north victoria land within individual hand samples apophyllite 40 ar 39 ar apparent ages show variations of up to 24 m y and within single large up to 2 cm crystals differences of up to 14 m y are observed the 40 ar 39 ar incremental heating spectra are generally flat but have low and high temperature discordances attributed to 39 ar recoil rb sr model ages for the apophyllites range from 94 to 144 ma and vary from concordant with the 40 ar 39 ar dates to as much as 14 m y older the dates are consistent with other temporal indicators of low temperature alteration in the basalts and are interpreted to broadly reflect the time of apophyllite precipitation they document a geographically widespread early to middle cretaceous secondary mineralization which is inferred to be caused by a major period of groundwater movement associated with the initiation of uplift and denudation of the transantarctic mountains the results suggest that apophyllite can produce geologically meaningful ages by both rb sr and k ar methods although further work is required to fully understand the variations in age that may be attributed to ar loss alkali mobility or an extended history of mineral precipitation

petrology of gabbroic xenoliths in 1960 kilauea basalt crystalline remnants of prior 1955 magmatism the 1960 kapoho lavas of kilauea s east rift zone contain 1 10 cm xenoliths of olivine gabbro olivine gabbro norite and gabbro norite textures are poikilitic ol sp cpx in pl and intergranular cpx pl ol opx poikilitic xenoliths which we interpret as cumulates have the most primitive mineral compositions fo82 5 cpx mg 86 5 and an80 5 many granular xenoliths ol and noritic gabbro contain abundant vesicular glass that gives them intersertal hyaloophitic and overall open textures to suggest that they represent mush and crust of a magma crystallization environment their phase compositions are more evolved fo80 70 cpx mg 82 75 and an73 63 than those of the poikilitic xenoliths associated glass is basaltic but evolved mgo 5 wt tio2 3 7 5 8 wt the gabbroic xenolith mineral compositions fit existing fractional crystallization models that relate the origins of various kilauea lavas to one another feo mgo crystal liquid partitioning is consistent with the poikilitic ol gabbro assemblage forming as a crystallization product from kilauea summit magma with 8 wt mgo that was parental to evolved lavas on the east rift zone for example least squares calculations link summit magmas to early 1955 rift zone lavas 5 wt mgo through 28 34 crystallization of the ol sp cpx pl that comprise the poikilitic ol gabbros the other ol gabbro assemblages and the olivine gabbro norite assemblages crystallized from evolved liquids such as represented by the early 1955 and late 1955 lavas 6 5 wt mgo of the east rift zone the eruption of 1960 kapoho magmas then scoured the rift zone reservoir system to entrain portions of cumulate and solidification zones that had coated reservoir margins during crystallization of prior east rift zone magmas