formation of lava tubes and extensive flow field during the 1991 1993 eruption of mount etna detailed mapping during the 1991 1993 eruption of mount etna has shown that there is a relationship between tumuli ephemeral vents lava tubes and their parent lava flows during this eruption many tubes formed in stationary inflated a a lava flows ephemeral vents at the fronts of these stationary flows and above lava tubes fed secondary lava flows many of which subsequently developed new tubes the resulting complex network of tubes ephemeral vents and secondary flows was responsible for most of the widening thickening and lengthening of the 1991 1993 etna lava flow field the supply of relatively uncooled lava via tubes to distal parts of this flow field allowed lava to flow 3 km farther from the vent than the longest channel fed lava flow our observations suggest that lava tubes play a more important role in the formation of extensive a a flow fields on etna than has previously been recognized

correlation of plate motions with continental tectonics laramide to basin range some of the major tectonic and magmatic events of the last 150 ma in the western cordillera can be correlated with a new model for the displacement histories between western north america and adjacent oceanic plates sierra nevada plutonism ended and laramide compression began during increasingly rapid convergence of the farallon plate with north america and during a moderate increase in the westward motion of north america in the hotspot reference frame the end of the laramide and beginning of widespread arc magmatism and extension correlates with slowing of both of these motions the spectacular slowing of faralion north america convergence is attributed to the decreasing age of the farallon plate entering the trench and thus to a change from negative to positive buoyancy the transition from widespread arc related magmatism and rapid extension to basaltic volcanism and moderate extension in the basin and range province finds no ready explanation in the plate motions of the pacific basin a change to oblique spreading in the basin and range province accompanied growth in length of the san andreas fault as the mendocino triple junction progressed northward

global spherical harmonic models of the internal magnetic field of the moon based on sequential and coestimation approaches 1 three new models of the global internal magnetic field of the moon based on lunar prospector lp fluxgate magnetometer observations are developed for use in understanding the origin of the moon s crustal magnetic field and for modeling its interaction with the solar wind the models are at spherical harmonic degree 170 corresponding to 64 km wavelength resolution from 30 km mean altitude lp observations coverage is complete except for a few areas near each pole original signal amplitudes are best preserved in the sequential approach map whereas feature recognition is superior in the coestimation and harmonic wave number correlation maps spherical harmonic degrees less than 15 corresponding to 666 km wavelength are largely absent from the moon s internal magnetic field we interpret this bound in terms of the moon s impact history a derived magnetization map suggests magnetizations may locally exceed 0 2 a m in the lunar crust at the survey resolution if the magnetic crust is as thick as 40 km

an early lunar core dynamo driven by thermochemical mantle convection although the moon currently has no internally generated magnetic field palaeomagnetic data combined with radiometric ages of apollo samples provide evidence for such a magnetic field from 3 9 to 3 6 billion years gyr ago1 possibly owing to an ancient lunar dynamo1 2 but the presence of a lunar dynamo during this time period is difficult to explain1 2 3 4 because thermal evolution models for the moon5 yield insufficient core heat flux to power a dynamo after 4 2 gyr ago here we show that a transient increase in core heat flux after an overturn of an initially stratified lunar mantle might explain the existence and timing of an early lunar dynamo using a three dimensional spherical convection model6 we show that a dense layer enriched in radioactive elements a thermal blanket at the base of the lunar mantle can initially prevent core cooling thereby inhibiting core convection and magnetic field generation subsequent radioactive heating progressively increases the buoyancy of the thermal blanket ultimately causing it to rise back into the mantle the removal of the thermal blanket proposed to explain the eruption of thorium and titanium rich lunar mare basalts7 plausibly results in a core heat flux sufficient to power a short lived lunar dynamo

asymmetric thermal evolution of the moon 1 the moon possesses a clear dichotomy in geological processes between the nearside and farside hemispheres the most pronounced expressions of this dichotomy are the strong concentration of radioactive heat sources on the nearside in a region known as the procellarum kreep terrane pkt and the mare basaltic lava flows that erupted in or adjacent to this terrane we model the thermochemical evolution of the moon using a 3 d spherical thermochemical convection code in order to assess the consequences of a layer enriched in heat sources below the pkt on the moon s global evolution we find that in addition to localizing most of the melt production on the nearside such an enriched concentration of heat sources in the pkt crust has an influence down to the core mantle boundary and leaves a present day temperature anomaly within the nearside mantle moderate gravitational and topographic anomalies that are predicted in the pkt but not observed may be masked either by crustal thinning or gravitational anomalies from dense material in the underlying mantle our models also predict crystallization of an inner core for sulfur concentrations less than 6 wt

relative motions between oceanic plates of the pacific basin appendix tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 012 2 50 payment must accompany order relative motion poles describing the displacement histories between the pacific plate and once adjacent oceanic plates farallon kula izanagi i izanagi ii and phoenix were derived for the late mesozoic and cenozoic eras because fracture zone and magnetic anomaly data are generally available from the pacific plate but not from adjacent plates a new method of analysis for onesided data was required this analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions the following are the main conclusions drawn from our analysis 1 for time intervals of the order of 107 years termed stages relative motion poles for plate pairs remained nearly fixed between stages shifts in poles were commonly both large and abrupt within stages rates of plate motion were commonly observed to change markedly indicating that plates changed speed more frequently than they changed direction 2 the relative motions of all of the plates analyzed changed at about chron m11 135 ma chron 34 85 ma and chron 25 56 ma 3 during the early cretaceous there were five oceanic plates in the pacific basin rather than the four recognized by previous workers 4 to determine the number of farallon plates that existed to the east of the pacific plate during the time interval from chron 34 85 ma to chron 25 56 ma fracture zones and magnetic anomalies that record pacific farallon spreading from the northern central and southern pacific plate were analyzed separately and collectively the analysis shows that a single pacific farallon relative motion pole and a single rate are consistent with all of the data 5 spreading rates along the pacific kula ridge decreased markedly between chrons 32b and 25 72 56 ma probably in response to the arrival of buoyant kula lithosphere at a subduction zone northwest of the bering sea 6 soon after chron 25 56 ma a major reorganization is recorded along the pacific farallon boundary the kula farallon boundary is also thought to have changed during this reorganization 7 kula pacific anomalies north of anomaly 25 are modeled by assuming continued kula pacific spreading after chron 25 56 ma 8 probably by chron 18 43 ma pacific kula spreading had ceased and the kula farallon ridge system had evolved into alignment with the pacific farallon spreading direction however the timing of these events is uncertain because pacific kula data younger than chron 25 56 ma are sparse

cenozoic structural deformation and dynamic processes of the bohai bay basin province china abstract cenozoic structures in the bohai bay basin province can be subdivided into eleven extensional systems and three strike slip systems the extensional systems consist of normal faults and transfer faults the normal faults predominantly trend nne and ne and their attitudes vary in different tectonic settings paleogene rifting sub basins were developed in the hanging walls of the normal faults that were most likely growth faults neogene quaternary sequences were deposited in both the rifting sub basins and horsts to form a unified basin province the extensional systems were overprinted by three nne trending right lateral strike slip systems fault zones although the principal displacement zones pdz of the strike slip fault zones are developed only in the basement and lower basin sequences in some cross sections the structural deformation characteristics of the upper basin sequences also indicate that they are basement involved right lateral strike slip fault zones according to the relationships between faults and sedimentary sequences the extensional systems were mainly developed from the middle paleocene to the late oligocene whereas the strike slip systems were mainly developed from the oligocene to the miocene strike slip deformation was intensified as extensional deformation was weakened extensional deformation was derived from horizontal tension induced by upwelling of hot mantle material whereas strike slip deformation was probably related to a regional stress field induced by plate movement

magmatic structures in the tuolumne intrusive suite california a new model for the formation and deformation of ladder dikes understanding the growth and differentiation of silicic magma chambers is a central issue in volcanology we use field observations of meter scale ladder dikes from the tuolumne intrusive suite tis california usa together with analogue experiments and scaling theory to demonstrate that prior to solidification these features are deformed and broken by shearing motions in the magma chamber from field observations the majority of the ladder dikes are 1 exposed perpendicular to gravity in a nearly horizontal slice through a solidified magma chamber 2 located near a contact specifically where they occur in large clusters with either host rock or another intrusive unit in the tis and 3 oriented roughly perpendicular to that contact the ladder dikes show evidence of breakup at the meter scale as well as at the centimeter scale mafic felsic banding we hypothesize that the ladder dikes are buoyant plumes in a stirred magma chamber in particular their orientation and deformation are related to their rise or descent velocity the stirring velocity within the chamber and the rheological contrast between the plume and ambient material the comparison between field data and laboratory experiments suggests that most of the ladder dikes deformed in a regime dominated by breakup from yielding under tension

magnetic field generation in the lunar core the role of inner core growth abstract the source of the magnetic field recorded in the lunar crust remains an unresolved problem the field was most likely produced by a self sustaining dynamo in the moon s electrically conducting metal core but heat flux across the core mantle boundary was probably insufficient to power a dynamo for the field s currently known duration from 4 2 to 3 56 ga since seismic measurements indicate the existence of a solid iron inner core in addition to a still liquid iron alloy outer core inner core solidification and its associated thermochemically driven convection in the outer core could have been responsible for extending the dynamo s lifetime even in the absence of superadiabatic heat flux here we present a coupled mantle core thermal evolution model of the moon and show that core solidification could explain the onset and shutoff of the lunar dynamo consistent with the global magnetic field inferred from the paleomagnetic record

the lunar dynamo background it is unknown whether the moon has a fully differentiated and melted structure with a metallic core or retains a partially primordial unmelted interior the differentiation history of the moon is manifested by its record of past magnetism paleomagnetism although the moon today does not have a global magnetic field the discovery of remanent magnetization in lunar rocks and in the lunar crust demonstrated that there was a substantial lunar surface field billions of years ago however the origin intensity and lifetime of this field have been uncertain as a result it has been unclear whether this magnetization was produced by a dynamo in the moon s advecting metallic core or by fields generated externally to the moon establishing whether the moon formed a core dynamo would have major implications for understanding its interior structure thermal history and mechanism of formation as well for our understanding of the physics of planetary magnetic field generation the interior structure of the moon and the lunar dynamo new magnetic measurements of lunar rocks have demonstrated that the ancient moon generated a dynamo magnetic field in its advecting liquid metallic core innermost red shell this dynamo may have been driven by convection possibly powered by crystallization of the core innermost red sphere and or stirring from the solid mantle thick green shell the magnetic field was recorded as magnetization by rocks on the lunar surface image created by hernan canellas the interior structure of the moon and the lunar dynamo new magnetic measurements of lunar rocks have demonstrated that the ancient moon generated a dynamo magnetic field in its advecting liquid metallic core innermost red shell this dynamo may have been driven by convection possibly powered by crystallization of the core innermost red sphere and or stirring from the solid mantle thick green shell the magnetic field was recorded as magnetization by rocks on the lunar surface image created by hernan canellas advances a new generation of laboratory magnetic studies of lunar rocks and spacecraft measurements of lunar crustal magnetic fields have produced major advances in our understanding of the evolution of ancient magnetic fields on the moon it has now been established that a dynamo magnetic field likely existed on the moon from at least 4 5 billion to 3 56 billion years ago with an intensity similar to that at the surface of earth today the field then declined by at least an order of magnitude by 3 3 billion years ago the early epoch of high field intensities may require an exceptionally energetic power source such as mechanical stirring from mantle precession the extended history of the lunar dynamo appears to demand long lived power sources such as mantle precession and core crystallization outlook measurements of the intensity of the ancient lunar dynamo have shown that it was surprisingly intense and long lived the next phase of lunar magnetic exploration will be to obtain more accurate measurements of field paleointensities and to determine when the dynamo initiated and finally disappeared this will be coupled with the continued development of magnetohydrodynamic models for characterizing mechanical and other unusual dynamo mechanisms and further investigations into the thermal structural and geodynamical history of the lunar core and mantle the eventual availability of absolutely oriented samples and in situ spacecraft measurements of bedrock should enable the first measurements of the paleo orientation of lunar magnetic fields such directional data could determine the lunar field s geometry and reversal frequency as well as constrain ancient local and global scale tectonic events

toward a robust normalized magnetic paleointensity method applied to meteorites abstract we propose a new paleointensity method based on normalization by isothermal remanent magnetization derivative vs alternating field rem method it provides an estimate of the absolute paleointensity with an uncertainty of about a factor two contrary to methods using normalization by total isothermal remanent magnetization or anhysteretic magnetization it is applicable to multicomponent magnetizations artificial or natural isothermal remanent magnetizations can be recognized and relaxation of natural remanent magnetization can be taken into account it is applicable for magnetizations carried by magnetite feni alloys and pyrrhotite the rem method is of particular interest to estimate paleointensities in meteorites as these materials are often characterized by complex multicomponent magnetization of various origins impact magnetization thermoremanence isothermal remanent magnetization and metastable magnetic minerals not suitable for thellier experiments however like other paleointensity methods the rem method underestimates the true paleointensity for meteorite samples with magnetizations that are heterogeneous on spatial scales below the sampling scale for l ordinary chondrites an upper limit of 1 t is proposed for the paleofield tentative paleofield estimates in the 0 05 0 5 t range are proposed for ll ordinary chondrites which is much lower than previous results the first paleofield estimates for rumuruti chondrites indicate the presence of magnetic fields around 6 t during the last major impact on their parent body aubrites and hed achondrites paleofields of at least 10 t may suggest the existence of a dynamo field during the cooling of their parent body results from martian meteorites are scattered between 1 and 24 t and may represent the crustal magnetic field of the planet after dynamo shutdown carbonaceous chondrites provide contrasting results with possible evidence of strong field mt processes that clearly require additional studies

mapping of crustal magnetic anomalies on the lunar near side by the lunar prospector electron reflectometer lunar prospector lp electron reflectometer measurements show that surface fields are generally weak in the large mare basalt filled impact basins on the near side but are stronger over highland terranes especially those lying antipodal to young large impact basins between the imbrium and nectaris basins many anomalies correlate with the cayley and descartes formations statistical analyses show that the most strongly magnetic nearside terranes are cayley type light plains terra materials and pre imbrian craters light plains and terrae include basin impact ejecta as a major component suggesting that magnetization effects from basin forming impacts were involved in their formation the magnetization of pre imbrian craters however may be evidence of early thermal remanence relatively strong small scale magnetic anomalies are present over the reiner gamma feature on western oceanus procellarum and over the rima sirsalis rille on the southwestern border of procellarum both apollo subsatellite and lp data show that the latter anomaly is nearly aligned with the rille though lp magnetometer and reflectometer data show that the anomaly peak is actually centered over a light plains unit this anomaly and the reiner gamma anomaly are approximately radially aligned with the center of imbrium suggesting an association with ejecta from this basin

an impact driven dynamo for the early moon recent palaeomagnetic and seismological studies have strengthened the suggestion that the moon once possessed a core dynamo despite its importance as a constraint on lunar evolution there is currently no consensus on how such a dynamo was driven two groups working independently have arrived at the idea that the lunar dynamo was powered by mechanical stirring of the liquid core dwyer et al investigate the mechanism of a dynamo driven by continuous mechanical stirring arising from the differential motion between the solid silicate mantle and the liquid core beneath they show that the fluid motions and the power required to drive a dynamo operating continuously for more than 1 billion years are readily obtained by such mechanical stirring le bars et al propose a model whereby the dynamo action comes from impact induced changes in the moon s rotation rate they show that basin forming impact events are energetic enough to have unlocked the moon from synchronous rotation and that the subsequent large scale fluid flows in the core excited by the tidal distortion of the core mantle boundary could have powered a lunar dynamo the origin of lunar magnetic anomalies1 2 3 4 5 remains unresolved after their discovery more than four decades ago a commonly invoked hypothesis is that the moon might once have possessed a thermally driven core dynamo3 but this theory is problematical given the small size of the core and the required surface magnetic field strengths6 an alternative hypothesis is that impact events might have amplified ambient fields near the antipodes of the largest basins7 but many magnetic anomalies exist that are not associated with basin antipodes here we propose a new model for magnetic field generation in which dynamo action comes from impact induced changes in the moon s rotation rate basin forming impact events are energetic enough to have unlocked the moon from synchronous rotation8 and we demonstrate that the subsequent large scale fluid flows in the core excited by the tidal distortion of the core mantle boundary9 could have powered a lunar dynamo predicted surface magnetic field strengths are on the order of several microteslas consistent with palaeomagnetic measurements5 and the duration of these fields is sufficient to explain the central magnetic anomalies associated with several large impact basins

demise of the rapid field change hypothesis at steens mountain the crucial role of continuous thermal demagnetization abstract rapid continuous thermal demagnetization in a triaxe vibrating sample magnetometer disproves an earlier hypothesis that an episode of extraordinarily fast field change is recorded in the remanent magnetization of a single lava flow that cooled during a geomagnetic polarity reversal the original evidence for this hypothesis was the distinctive pattern of variation in direction with vertical position after stepwise demagnetization to 500 c but continuous thermal demagnetization now proves that this flow actually carries consistent high temperature remanence parallel to that of the flow immediately below why does continuous thermal demagnetization succeed when conventional thermal and alternating field demagnetization fail to isolate this direction detailed investigation of the demagnetization trajectories and underlying rock magnetic properties suggest that normal polarity secondary remanent magnetization acquired both at room temperature in today s polarity chron and during modest reheating in a normal field during cooling of the overlying flow is responsible for the misleading pattern of directions alteration during ordinary thermal demagnetization evidently promotes the overprint to higher blocking temperatures so that it masks the primary component whereas the rapid heating 40 c min during continuous demagnetization is fast enough to demagnetize the normal overprint before this masking can happen

a long lived lunar dynamo powered by core crystallization the moon does not possess an internally generated magnetic field at the present day but extensive evidence shows that such a field existed between at least 4 2 and 3 56 ga ago the existence of a metallic lunar core is now firmly established and we investigate the influence of inner core growth on generating a lunar core dynamo we couple the results of a 3 d spherical thermochemical convection model of the lunar mantle to a 1 d thermodynamic model of its core the energy and entropy budget of the core are computed to determine the inner core growth rate and its efficiency to power a dynamo sulfur is considered to be the main alloying element and we investigate how different sulfur abundances and initial core temperatures affect the model outcomes for reasonable initial conditions a solid inner core between 100 and 200 km is always produced during its growth a surface magnetic field of about 0 3 t is generated and is predicted to last several billion years though most simulations predict the existence of a core dynamo at the present day one way to stop magnetic field generation when the inner core is growing is by a transition between a bottom up and top down core crystallization scheme when the sulfur content becomes high enough in the outer core according to this hypothesis a model with about 6 to 8 wt sulfur in the core would produce a 120 160 km inner core and explain the timing of the lunar dynamo as constrained by paleomagnetic data

apparent polar wander of the mean lithosphere reference frame summary apparent polar wander in the mean lithosphere no net rotation no net torque uniform drag reference frame is compared with apparent polar wander in the hotspot reference frame over the past 100 myr palaeomagnetic poles and plate rotations previously used to determine an apparent polar wander path for the hotspot reference frame are here used to determine an apparent polar wander path in the mean lithosphere reference frame we find that the two paths are similar especially for late cretaceous time when a 10 20 shift of the pole occurred to first order the hotspots and lithosphere as a whole moved in unison relative to the palaeomagnetic axis during late cretaceous time a non dipole field explanation for the apparent shift can probably be excluded however either motion of the time averaged geomagnetic axis relative to the spin axis or polar wandering could have caused this shift the latter being the more likely explanation

the density and porosity of lunar rocks 1 accurate lunar rock densities are necessary for constructing gravity models of the moon s crust and lithosphere most apollo era density measurements have errors of 2 5 or more and few include porosity measurements we report new density and porosity measurements using the bead method and helium pycnometry for 6 apollo samples and 7 lunar meteorites with typical grain density uncertainties of 10 30 kg m 3 0 3 0 9 and porosity uncertainties of 1 3 comparison between igneous grain densities and normative mineral densities show that these uncertainties are realistic and that the helium fully penetrates the pore space basalt grain densities are a strong function of composition varying over at least 3270 kg m 3 high aluminum basalt to 3460 kg m 3 high titanium basalt feldspathic highland crust has a bulk density of 2200 2600 kg m 3 and porosity of 10 20 impact basin ejecta has a bulk density of 2350 2600 kg m 3 and porosity of 20

global mapping of lunar crustal magnetic fields by lunar prospector abstract the lunar prospector electron reflectometer has obtained the first global map of lunar crustal magnetic fields revealing that the effects of basin forming impacts dominate the large scale distribution of remanent magnetic fields on the moon the weakest surface magnetic fields 40 nt are diametrically opposite to these same basins this pattern is present though less pronounced for several other post nectarian impact basins larger than 500 km in diameter the reduced strength and clarity of the pattern for older basins may be attributed to 1 demagnetization from many smaller impacts which erases antipodal magnetic signatures over time 2 superposition effects from other large impacts and 3 variation in the strength of the ambient magnetizing field the absence of fringing fields stronger than 1 nt around the perimeter of the imbrium basin or associated with craters within the basin implies that any uniform magnetization of the impact melt must be weaker than 10 6 g cm3 g 1 this limits the strength of any steady ambient magnetic field to no more than 0 1 oe at the lunar surface while the basin cooled for tens of millions of years following the imbrium impact 3 8 billion years ago

a komatiite component in apollo 16 highland breccias implications for the nickel cobalt systematics and bulk composition of the moon the lunar crust at the apollo 16 landing site contains substantial amounts of a primitive component in which the ferromagnesian group of elements is concentrated the composition of this component can be retrieved via an analysis of mixing relationships displayed by lunar breccias it is found to be a komatiite which is compositionally similar to terrestrial komatiites both in major and minor elements the komatiite component of the lunar crust is believed to have formed by extensive degrees of melting of the lunar interior at depths greater than were involved in the formation of the lunar magma ocean which was parental to the crust after formation of the anorthositic crust it was invaded by extensive flows and intrusions of komatiite magma from these deeper source regions the komatiites became intimately mixed with the anorthosite by intensive meteoroid impacts about 4 5 b y ago thereby accounting for the observed mixing relationships displayed by the crust the compositional similarity between lunar and terrestrial komatiites strongly implies a corresponding similarity between the compositions of their source regions in the lunar interior and the earth s upper mantle the composition of the lunar interior can be modelled more specifically by combining the komatiite composition with its liquidus olivine composition as determined experimentally in proportions chosen so as to produce a cosmochemically acceptable range of mg si ratios for the bulk moon except for higher feo and lower na2o the range of compositions thereby obtained for the bulk moon is very similar to the composition of the earth s upper mantle the effects of meteoritic contamination on the abundances of cobalt and nickel in lunar highland breccias were subtracted on the assumption that the contaminating projectiles were chondritic the cobalt and nickel residuals thereby obtained were found to correlate strongly with the mg fe content of the breccias demonstrating that the co and ni are associated with the ferromagnesian component of the breccias and are genuinely indigenous to the moon the lunar highland co and ni residuals also display striking ni co versus ni correlations which follow a similar trend to those displayed by terrestrial basalts picrites and komatiites the lunar trends provide further decisive evidence of the indigenous nature of the co and ni residuals and suggest the operation of extensive fractionation controlled by olivine liquid equilibria in producing the primitive component of the lunar breccias indigenous nickel abundances at the apollo 14 15 and 17 sites are much lower than at the apollo 16 site although rocks from all sites follow the same ni co versus ni trends it is suggested that the primitive component at the apollo 14 15 and 17 sites was generally of basaltic composition in contrast to the komatiitic nature of the apollo 16 primitive component

the scientific work 1957 1987 of allan cox during the summer of 1951 allan cox then a 25 year old chemistry major at the university of california berkeley became curious about geology and worked as a geology field assistant to clyde wahrhaftig thus did allan cox begin a momentous career in geomagnetism paleomagnetism and plate tectonics which left its indelible mark in the sciences themselves in the workings of earth sciences through his leadership roles in the american geophysical union the geological society of america and the national academy of sciences and most importantly in the lives and works of many junior scientists in the united states and throughout the world twenty three years ago i was one of those junior scientists whose life was touched and transformed by this unique individual over the course of ensuing years it is quite possible that but for a chance meeting with allan at the 1965 iaga assembly in pittsburgh i would have not seen for a long time the important ways by which basic rock magnetism can and does affect progress in geomagnetism and paleomagnetism

a preliminary global map of the vector lunar crustal magnetic field based on lunar prospector magnetometer data processing and mapping of the lunar prospector magnetometer data is supported by the nasa discovery data analysis program

magnetic alteration of zero age oceanic basalt the youngest sampled submarine lava flow which erupted june 1993 on the juan de fuca ridge provides the basis for a tight constraint on the initial or zero age magnetization state of morb detailed profiles of magnetic hysteresis parameters curie temperatures and unblocking temperatures of nrm with respect to the chilled margin of a pillow fragment show evidence of significant oxidation which preferentially affected the finest grain size fraction and principal remanence carrier of the titanomagnetite magnetic mineralogy the oxidation must have occurred during or immediately after initial cooling implying that morb is already appreciably magnetically altered before aging nevertheless successful results of thellier paleointensity experiments on the basalt sample lend support to the idea that crustal magnetization represented by morb preserves a record of geomagnetic intensity variations that may be reflected in small scale magnetic anomalies

antipodal effects of lunar basin forming impacts initial 3d simulations and comparisons with observations abstract 3d simulations of basin scale lunar impacts are carried out to investigate a the origins of strong crustal magnetic fields and unusual terrain observed to occur in regions antipodal to young large basins and b the origin of enhanced magnetic and geochemical anomalies along the northwest periphery of the south pole aitken spa basin the simulations demonstrate that a basin forming impact produces a massive hot partially ionized cloud of vapor and melt that expands thermally around the moon converging near the basin antipode approximately 1 h after the impact for typical impact parameters in agreement with previous work analytic calculations of the interaction of this vapor melt cloud with an initial ambient magnetic field predict a substantial temporary increase in field intensity in the antipodal region the time of maximum field amplification coincides with a period when impacting ejecta also converge near the antipode the latter produce antipodal shock stresses within the range of 5 25 gpa where stable shock remanent magnetization srm of lunar soils has been found experimentally to occur calculated antipodal ejecta thicknesses are only marginally sufficient to explain the amplitudes of observed magnetic anomalies if mean magnetization intensities are comparable to those produced experimentally this suggests that pre existing ejecta materials which would also contain abundant metallic iron remanence carriers may be important anomaly sources a possibility that is consistent with enhanced magnetic anomalies observed peripheral to spa the latter anomalies may be produced by amplified secondary ejecta impact shock waves in the thick spa ejecta mantle occurring near the antipodes of the imbrium and serenitatis impacts together with converging seismic compressional waves these antipodal impact shocks may have produced especially deep fracture zones along the northwest edge of spa near the imbrium antipode allowing the ascent of magma with enhanced kreep concentrations

magnetic signature of the lunar south pole aitken basin character origin and age 1 a new magnetic map of the moon based on lunar prospector magnetometer observations sheds light on the origin of the south pole aitken basin spa the largest and oldest of the recognized lunar basins a set of wnw trending linear to arcuate magnetic features evident in both the radial and scalar observations covers much of a 1000 km wide region centered on the nw portion of spa the source bodies are not at the surface because the magnetic features show no first order correspondence to any surface topographic or structural feature patchy mare basalts of possible late imbrian age are emplaced within spa and are inferred to have been emplaced through dikes directly from mantle sources we infer that the magnetic features represent dike swarms that served as feeders for these mare basalts as evident from the location of the thomson mare ingenii van de graaff and leeuwenhoek mare basalts on the two largest magnetic features in the region modeling suggests that the dike zone is between 25 and 50 km wide at the surface and dike magnetization contrasts are in the range of 0 2 a m we theorize that the basaltic dikes were emplaced in the lunar crust when a long lived dynamo was active based on pressure temperature and stress conditions prevalent in the lunar crust dikes are expected to be a dominantly subsurface phenomenon consistent with the observations reported here

strength of the geomagnetic field in the cretaceous normal superchron new data from submarine basaltic glass of the troodos ophiolite 1 we present here new paleointensity data from 39 sampling sites collected from the quenched margins of pillow lavas and dikes exposed within the troodos ophiolite similar to 92 ma formed during the cretaceous normal superchron cns a period of approximately 40 million years when the geomagnetic field reversed extremely infrequently if at all monte carlo simulations suggest that a minimum of 25 estimates are necessary for a reasonably robust estimate for the average field strength our data suggest a dipole strength equivalent to the present field or nearly twice the post cns average the mean and standard deviation of the dipole moment 81 43 zam 2 z 10 21 from the 57 data points compiled here agree remarkably well with those predicted from the long paleointensity record derived from dsdp site 522 the new data set for the cns suggests a picture of a strong and stable field during the period of time when it stopped reversing moreover the similarity of the cns data with the present geomagnetic field suggests that it is presently in a state of unusual polarity stability

structure and evolution of the lunar procellarum region as revealed by grail gravity data gravity data show a rectangular pattern of narrow linear anomalies bordering the lunar procellarum region that are interpreted to be the frozen remnants of lava filled rifts and underlying feeder dykes the procellarum region is a broad area on the nearside of the moon characterized by low elevations and thin crust and largely covered by dark basalts that can be seen from earth with the unaided eye this structure has been interpreted as an ancient impact basin jeffrey andrews hanna et al use data from nasa s gravity recovery and interior laboratory grail mission to examine the subsurface structure of the procellarum they find that a pattern of narrow linear anomalies border the region interpreted to be the frozen remnants of lava filled rifts and underlying feeder dikes the discontinuous surface structures that were earlier interpreted as remnants of an impact basin rim are a part of this continuous set of quasi rectangular border structures with angular intersections the authors conclude that the procellarum was formed in response to thermal stresses produced by the differential cooling of the province relative to its surroundings coupled with magmatic activity driven by elevated heat flux the procellarum region is a broad area on the nearside of the moon that is characterized by low elevations1 thin crust2 and high surface concentrations of the heat producing elements uranium thorium and potassium3 4 the region has been interpreted as an ancient impact basin approximately 3 200 kilometres in diameter5 6 7 although supporting evidence at the surface would have been largely obscured as a result of the great antiquity and poor preservation of any diagnostic features here we use data from the gravity recovery and interior laboratory grail mission8 to examine the subsurface structure of procellarum the bouguer gravity anomalies and gravity gradients reveal a pattern of narrow linear anomalies that border procellarum and are interpreted to be the frozen remnants of lava filled rifts and the underlying feeder dykes that served as the magma plumbing system for much of the nearside mare volcanism the discontinuous surface structures that were earlier interpreted as remnants of an impact basin rim are shown in grail data to be a part of this continuous set of border structures in a quasi rectangular pattern with angular intersections contrary to the expected circular or elliptical shape of an impact basin9 the spatial pattern of magmatic tectonic structures bounding procellarum is consistent with their formation in response to thermal stresses produced by the differential cooling of the province relative to its surroundings coupled with magmatic activity driven by the greater than average heat flux in the region

alteration and dissolution of fine grained magnetite and its effects on magnetization of the ocean floor abstract scanning and transmission electron microscopy stem of ocean floor pillow basalts with ages between 0 and 70 ma reveals progressive alteration of submicrometer titanomagnetite to phases such as goethite and clays in contrast larger titanomagnetite grains 1 m oxidize to titanomaghemite without apparent change in crystal morphology rock magnetic experiments are consistent with a selective removal of the submicrometer grains as the basalts age and show good correlations between the ranges of grain sizes observed by stem and those indicated by hysteresis properties remanence contributions from the larger pseudo single domain and multi domain titanium iron oxides are inferred to decrease only slightly as the ocean floor becomes older whereas the overall remanence decays with age as the substantial contribution from stable single domain titanomagnetite grains diminishes greatly due to their alteration to other phases although more work on many more samples is required to verify our conclusions the current data imply that this alteration is one of the reasons that amplitudes of marine magnetic anomalies diminish with age over time scales of tens of millions of years

graphite oxidation in the apollo 17 orange glass magma implications for the generation of a lunar volcanic gas phase abstract an apollo 17 picritic orange glass composition has been used to experimentally investigate the conditions at which graphite would oxidize to form a co rich gas and ultimately produce lunar fire fountain eruptions isothermal decompression experiments run above the a17 orange glass liquidus temperature 1350 c suggest that the initial co rich gas phase produced by graphite oxidation would be generated during magma ascent at a pressure of 40 mpa 8 5 km beneath the lunar surface additional experiments with 2000 ppm s and 1000 ppm cl showed that the presence of these dissolved gas species would not affect the depth of graphite oxidation verifying that the first volcanic gas phase would be generated by the oxidation of graphite a simple ideal chemical mixing model for calculating melt feo activity in a fe metal silicate melt system was tested with a series of 0 1 mpa controlled oxygen fugacity experiments agreement between the model and experiments allows the model to be used to calculate oxygen fugacity in picritic lunar glass compositions such as the a17 orange glass using this model in a reanalysis of chemical equilibria between the natural a17 orange glass melt and the metal spherules fe85ni14co1 trapped within the glass beads indicates a log oxygen fugacity of 11 2 0 7 log units more oxidized than previous estimates at the a17 orange glass liquidus temperature 1350 5 c this fo2 corresponds to a minimum pressure of 41 mpa on the graphite c o surface the fact that the same critical graphite oxidation pressure was determined in decompression experiments and from the fe feo activity model for the natural a17 orange glass metal assemblage strongly supports this pressure 8 5 km depth for volcanic gas formation in lunar basalts generation of a gas by oxidation of c in ascending magma is likely to have been important in getting dense lunar magmas to the surface as well as in generating fire fountain eruptions the vesicles common in many lunar basalts and the ubiquitous fe metal in these rocks are also likely generated by the oxidation of carbon the presence of carbon in the lunar basalts and the recent discovery of ppm levels of water in lunar basalts indicate that at least parts of the lunar interior still contained volatiles at 3 9 bybp

controlled atmosphere thermal demagnetization and paleointensity analyses of extraterrestrial rocks we describe a system for conducting thermal demagnetization of extraterrestrial rocks in a controlled atmosphere appropriate for a wide range of oxygen fugacities within the stability domain of iron thermal demagnetization and thellier thellier paleointensity experiments on lunar basalt synthetic analogs show that the controlled atmosphere prevents oxidation of magnetic carriers when combined with multidomain paleointensity techniques this opens the possibility of highly accurate thermal demagnetization and paleointensity measurements on rocks from the moon and asteroids

geodetic evidence for en echelon dike emplacement and concurrent slow slip during the june 2007 intrusion and eruption at kīlauea volcano hawaii 1 a series of complex events at k lauea volcano hawaii 17 june to 19 june 2007 began with an intrusion in the upper east rift zone erz and culminated with a small eruption 1500 m3 surface deformation due to the intrusion was recorded in unprecedented detail by global positioning system gps and tilt networks as well as interferometric synthetic aperture radar insar data acquired by the envisat and alos satellites a joint nonlinear inversion of gps tilt and insar data yields a deflationary source beneath the summit caldera and an ene striking uniform opening dislocation with 2 m opening a dip of 80 to the south and extending from the surface to 2 km depth this simple model reasonably fits the overall pattern of deformation but significantly misfits data near the western end of an inferred dike like source three more complex dike models are tested that allow for distributed opening including 1 a dike that follows the surface trace of the active rift zone 2 a dike that follows the symmetry axis of insar deformation and 3 two en echelon dike segments beneath mapped surface cracks and newly formed steaming areas the en echelon dike model best fits near field gps and tilt data maximum opening of 2 4 m occurred on the eastern segment beneath the eruptive vent although this model represents the best fit to the erz data it still fails to explain data from a coastal tiltmeter and gps sites on k lauea s southwestern flank the southwest flank gps sites and the coastal tiltmeter exhibit deformation consistent with observations of previous slow slip events beneath k lauea s south flank but inconsistent with observations of previous intrusions slow slip events at k lauea and elsewhere are thought to occur in a transition zone between locked and stably sliding zones of a fault an inversion including slip on a basal decollement improves fit to these data and suggests a maximum of 15 cm of seaward fault motion comparable to previous slow slip events