indicators of aqueous alteration in cm carbonaceous chondrites microtextures of a layered mineral containing fe s o and ni a petrographic and transmission electron microscopy study of the mighei murchison and murray cm carbonaceous chondrites shows that much of the cm matrix material was probably produced by aqueous alteration of olivine pyroxene sulfide and metal the amount of cm matrix appears to be proportional to the degree of alteration as suggested by mcsween 1979 and microtextures of pcp poorly characterized phase provide evidence of the progressive alteration pcp is divided into two major types one occurs in chondrules and aggregates and consists largely of an fe ni s o phase type i and the other occurs in matrix and consists of the fe ni s o phase and cronstedtite in various proportions type ii microtextures of pcp suggest that it resulted from a three stage alteration process 1 1 type i was produced by alteration of kamacite in chondrules and aggregates presumably early in the parent body regolith 2 2 as the alteration advanced olivine and pyroxene were converted to serpentine type i pcp separated from chondrules and aggregates into the matrix during regolith gardening simultaneously the fe ni s o phase reacted with si released by alteration of olivine and pyroxene producing well formed platy cronstedtite and coherent intergrowths of the fe ni s o phase and cronstedtite the fe ni s o phase also recrystallized into platy and pod like crystals fe s ni cr and p were leached out of type i pcp and were deposited as small grains of fe ni sulfides magnetite chromite and a mineral unidentified containing fe ni cr and p as a result pcp came to consist primarily of the fe ni s o phase and cronstedtite i e type ii pcp 3 3 during continued alteration the well formed crystals of the fe ni s o phase cronstedtite and their intergrowths in type ii pcp were replaced by poorly formed fibers in comparison to other cm chondrites mighei murchison and murray are relatively unaltered their matrices retain abundant amounts of the fe ni s o phase and cronstedtite commonly as distinctive pcp grains which account for a large proportion of the fe in these meteorite matrices in more altered cm chondrites much of the fe ni s o phase was probably consumed to produce cronstedtite magnetite and sulfides with further alteration cronstedtite itself reacted with the serpentine to form ferroan serpentine thus the cm matrix was increasingly enriched in mg with alteration and pcp was increasingly degraded and intimately mixed with the magnesian phyllosilicates

differences in the degree of weathering between antarctic and non antarctic meteorites inferred from infrared diffuse reflectance spectra abstract infrared diffuse reflectance spectra were measured for antarctic and non antarctic meteorites to compare the degree of terrestrial weathering by using the integrated intensity of absorption bands near 3 m caused by hydrous minerals and the intensity of absorption bands near 7 4 m 1350 cm 1 probably caused by hydrous carbonates non antarctic ordinary chondrite finds usually show large values of the integrated intensity of the 3 m band whereas non antarctic ordinary chondrite falls show small intensities some antarctic ordinary chondrites are as fresh as non antarctic ordinary chondrite falls in the sense of the integrated intensity of the 3 m band some antarctic eucrites show large values of the integrated intensity of the 3 m band although the degree of weathering on the a b c scale is reported to be a weathering produced hydrous carbonates seem to be ubiquitous in antarctic ordinary chondrites because the spectra of all the antarctic ordinary chondrites measured show the presence of the 7 4 m band non antarctic ordinary chondrite falls show very weak 7 4 m bands suggesting only little weatheringproduced hydrous carbonates diffuse reflectance spectra were also measured for the mixtures of akaganeite goethite artinite or calcite with the nuevo mercurio h5 ordinary chondrite to examine spectral changes and to compare them with those of meteorites absorption bands near 7 m of diffuse reflectance spectra are sensitive to the presence of carbonates

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

influence of secular variation of the geomagnetic field on production rates of in situ produced cosmogenic nuclides abstract scaling factors are presented that allow a full description of the cosmic ray flux modulation during secular variation of the geomagnetic field these scaling factors are analogous to those determined in a similar work dunai t j earth planet sci lett 2000 176 157 169 that were accurate for field strengths equivalent to present day field strength but cannot accommodate field strength variations strategies to derive the relevant geomagnetic field parameters are discussed and are tested with existing production rate calibrations for 3 he a production rate of 103 4 2 atoms g olivine yr sea level and for zero cutoff rigidity was derived from calibration sites from hawaii western usa and canary islands covering the time range of 2 281 kyr

helium and neon isotopes in the imnaha basalt columbia river basalt group evidence for a yellowstone plume source abstract helium neon and argon isotopic compositions were measured in two flows of the columbia river flood basalt the imnaha basalt has a 3 he 4 he ratio of 11 4 times atmospheric and 20 ne 22 ne and 21 ne 22 ne ratios characteristic of a plume component the measured 3 he 4 he is a lower limit due to possible preferential 3 he loss and or addition of radiogenic 4 he a wanapum basalt flow erupted approximately 2 ma later in the waning stages of volcanism has more morb like noble gases the he nd and sr isotopic compositions of these lavas suggest that the columbia river basalts were derived from the yellowstone plume head which contained both high helium plume material and entrained depleted mantle as the eruptions progressed the plume component in the melting region was gradually diluted or replaced

non nebular origin of dark mantles around chondrules and inclusions in cm chondrites abstract our examination of nine cm chondrites that span the aqueous alteration sequence leads us to conclude that compact dark fine mantles surrounding chondrules and inclusions in cm chondrites are not discrete fine grained rims acquired in the solar nebula as modeled by metzler et al accretionary dust mantles in cm chondrites evidence for solar nebula processes geochim cosmochim acta 56 1992 2873 2897 nebular processes that lead to agglomeration produce materials with porosities far higher than those in the dark mantles we infer that the mantles were produced from porous nebular materials on the cm parent asteroid by impact compaction a process that produces the lowest porosity adjacent to chondrules and inclusions compaction was followed by aqueous alteration that formed tochilinite serpentine ni bearing sulfide and other secondary products in voids in the interchondrule regions metzler et al reported a correlation between mantle thickness and the radius of the enclosed object in yamato 791198 we find no correlation when all sizes of central objects and dark lumps are included but a significant correlation r2 0 44 if we limit consideration to central objects with radii 35 m a moderate correlation is also found in que 97990 we suggest that impact induced shear of a plum pudding like precursor produced the observed mantles these were shielded from comminution during impact events by the adjacent stronger chondrules and inclusions some mantles in cm chondrites with low degrees of alteration show distinct layers that may largely reflect differences in porosity typically a gray uniform inner layer is surrounded by an outer layer consisting of darker silicates with bse bright speckles the cm chondrite objects characterized as primary accretionary rocks by metzler et al did not form in the nebula but rather on the parent body the absence of solar flare particle tracks and solar wind implanted rare gases in these clasts reflect their lithified nature and low surface volume ratios during the period when they resided in the regolith and were subject to irradiation by solar particles the clasts are analogous to the light colored metamorphosed clasts in ordinary chondrite regolith breccias which also lack solar flare particle tracks and solar wind gas

a new extraction technique and production rate estimate for in situ cosmogenic 14c in quartz the potential of in situ cosmogenic 14c in situ 14c for surficial process studies is widely recognized yet a reliable means of isolating it has proved difficult to develop we present here a new method for extracting in situ 14c from quartz that overcomes difficulties encountered with earlier techniques yielding more reliable production rate estimates comparison of 14c thermal release patterns from surficial and deeply shielded quartz samples lifton 1997 demonstrated that contaminant 14c is released at or below 500 c and that 14c released between 500 and 1500 c is essentially all in situ produced the new technique builds on this key result using resistance heating of samples in the presence of a lithium metaborate libo2 flux and collection of all evolved carbon as co2 between 500 c and 1100 to 1200 c our improved method has four distinct advantages over other extraction methods 1 we can identify and quantitatively eliminate atmospheric organic 14c contamination 2 we can identify the in situ 14c component unambiguously without assumptions of 14co 14co2 production proportions within the rock or equilibria on extraction 3 in situ 14c is reliably extracted from quartz at lower temperatures and in less time than earlier methods and 4 blank 14c levels are consistently low 2 3 0 1 105 14c atoms 1 our new extraction procedures should thus enable researchers to use in situ 14c in diverse applications without reservation we developed our new procedures using samples of wave cut quartzite benches from the well dated bonneville 17 4 0 3 cal ky and provo 16 8 0 3 cal ky shorelines of pleistocene lake bonneville utah and from underlying deeply shielded locations in situ 14c was extracted from quartz separated from 2 bonneville shoreline samples 6 aliquots and 1 provo shoreline sample 2 aliquots results demonstrate that our new procedures can effectively isolate the in situ 14c fraction with replicate analytical precision better than 2 1 n 5 while remaining consistent with earlier results this level of precision and accuracy is comparable to or exceeds those currently obtainable with in situ cosmogenic 10be 26al 3he 21ne and 36cl resulting weighted mean in situ 14c site production rates for the bonneville and provo shorelines are 52 9 1 7 14c atoms g sio2 y and 48 7 2 8 14c atoms g sio2 y 1 respectively consistent with earlier production rate estimates current and previously published in situ 14c site production rate estimates were then scaled to sea level and high geomagnetic latitude using the latitude altitude scaling models of lal 1991 and dunai 2000 results indicate that both models yield sea level high latitude production rates consistent with independent estimates our new in situ 14c data yield integrated late quaternary production rate estimates at sea level and high latitude of 15 1 0 5 14c atoms g sio2 y using the lal 1991 model and 15 8 0 5 14c atoms g sio2 y with that of dunai 2000 until significant uncertainties in these models are addressed however we prefer the value from the widely used lal 1991 model as our best estimate of the integrated late quaternary production rate for in situ 14c

compositions and taxonomy of 15 unusual carbonaceous chondrites abstract we used instrumental neutron activation analysis and petrography to determine bulk and phase compositions and textural characteristics of 15 carbonaceous chondrites of uncertain classification acfer 094 type 3 0 ungrouped cm related belgica 7904 mildly metamorphosed anomalous cm like chondrite possibly a member of a new grouplet that includes wisconsin range wis 91600 dhofar 225 and yamato 86720 dar al gani dag 055 and its paired specimen dag 056 anomalous reduced cv3 like dag 978 type 3 ungrouped dominion range 03238 anomalous magnetite rich co3 1 elephant moraine 90043 anomalous magnetite bearing co3 graves nunataks 98025 type 2 or type 3 ungrouped grosvenor mountains gro 95566 anomalous cm2 with a low degree of aqueous alteration hammadah al hamra hah 073 type 4 ungrouped possibly related to the coolidge loongana c l 001 grouplet lewis cliff lew 85311 anomalous cm2 with a low degree of aqueous alteration northwest africa 1152 anomalous cv3 pecora escarpment pca 91008 anomalous metamorphosed cm queen alexandra range 99038 type 2 ungrouped sahara 00182 type 3 ungrouped possibly related to hah 073 and or to c l 001 and wis 91600 mildly metamorphosed anomalous cm like chondrite possibly a member of a new grouplet that includes belgica 7904 dhofar 225 and y 86720 many of these meteorites show fractionated abundance patterns especially among the volatile elements impact volatilization and dehydration as well as elemental transport caused by terrestrial weathering are probably responsible for most of these compositional anomalies the metamorphosed cm chondrites comprise two distinct clusters on the basis of their 17o values approximately 4 for pca 91008 gro 95566 dag 978 and lew 85311 and approximately 0 for belgica 7904 and wis 91600 these six meteorites must have been derived from different asteroidal regions

paleoseismology and seismic hazards evaluations in extensional volcanic terrains extensional structures in volcanic terrains are the surface expression of shallow dike intrusion and can be misinterpreted as structures associated with major tectonic faults dike induced structures can be distinguished from their tectonic counterparts by their association with cogenetic volcanic rocks and by several geometric relationships tensile fissures with little or no vertical displacement fissure swarms flexural monoclines and normal faults are commonly symmetrically distributed about a central eruptive fissure sometimes forming a graben above shallow dikes the structures typically occur within broad zones not narrow belts near a main fault zone reflecting their origin by repeated dike injection colluvial wedges containing records of single large earthquakes generally do not form instead fault scarps with several meter vertical displacements in volcanic bedrock may reflect the cumulative effects of many decimeter scale displacement events from several dike injection episodes the mechanism of dike intrusion and the nature of cointrusive seismicity have important implications for determination of the maximum magnitude and recurrence of earthquakes in extensional volcanic terrains observational seismicity from volcanic rift zones worldwide suggests the maximum magnitudes of dike induced earthquakes are 3 8 0 8 earthquakes are generally small to moderate because downdip extents of faults and fissures are controlled by the depth to the top of the associated dike usually 5 km permitting only small rupture areas also rupture and displacement on faults and fissures migrate incrementally at about the velocity of propagating dikes 0 5 m s as dike dilation stresses the zone above and ahead of the dike earthquake recurrence is tied to recurrence of volcanic cycles based on the geochronology of the associated volcanic materials based on these concepts innovative approaches have led to estimates of the maximum magnitude and recurrence for magma induced seismicity in eastern snake river plain esrp volcanic rift zones the most conservative approach uses surface length fault width downdip extent and rupture areas of normal faults and fissures produced by dike injection to estimate a maximum m5 5 earthquake for an episode of rift zone volcanism chronology of volcanic rocks suggests annual probabilities of 10 4 to 10 5 for volcanic rift zones near idaho national engineering laboratory facilities probabilistic assessments show that ground motion hazard due to volcanic rift zone seismicity is lower than the hazard from other earthquake sources this is because of the relatively low magnitudes and long recurrence intervals of cointrusive earthquakes in volcanic rift zones

cosmogenic 3he in himalayan garnets indicating an altitude dependence of the 3he 10be production ratio to investigate the potential of using cosmogenic helium in garnet as a dating tool 3hec and 10bec have been measured within garnets and coexisting quartz respectively sampled in the himalayan range at various elevations comparison and correlation of 3hec and 10bec concentrations demonstrate that cosmogenic 3he is well retained in himalayan garnets and can thus be used in this mineral to quantify precisely earth s surface processes this in the case of recently cooled orogenic samples which contain limited amounts of non cosmogenic helium for these high elevation samples the exposure ages derived from 3hec using classical scaling factors lead however to an overestimation of 3hec ages compared to 10bec ages 0 1 to 28 ky by up to a factor of 2 additionally over the samples altitude range 3000 4622 m the theoretically invariant polar 3hec 10bec production ratio 22 5 increases with increasing elevation challenging classical models of production rate evolution to explain this observation we propose a new 3hec production mechanism based on the altitudinal dependence of the energy spectrum of cosmic rays and according to which neutrons or protons resulting from a first spallation reaction within the rock tertiary particles may have sufficient energy to induce an additional in situ spallation reaction producing a second 3h or 3he but insufficient energy to produce 10be the probability of such a mechanism increasing with altitude according to these observations we propose for future and ongoing studies to use the empirically determined attenuation length of 121 g cm2 for helium production at high altitude

neon diffusion kinetics in olivine pyroxene and feldspar retentivity of cosmogenic and nucleogenic neon abstract we performed stepwise degassing experiments by heating single crystals of neutron or proton irradiated olivine pyroxene and feldspar to study diffusion kinetics of neon this is important in evaluating the utility of these minerals for cosmogenic 21 ne measurements and potentially for ne thermochronometry degassing patterns are only partially explained by simple arrhenius relationships most samples do not exhibit a precisely determined activation energy in an individual diffusion domain regardless we find clear differences in diffusion kinetics among these minerals based on sub selected data our estimates for neon diffusion kinetics activation energy e a and pre exponential factor d o assuming the analyzed fragments approximate the diffusion domain in each mineral are as follows for the feldspars e a ranges from 65 to 115 kj mol and d o from 3 9 10 3 to 7 1 10 2 cm 2 s 1 for the pyroxenes e a ranges from 292 to 480 kj mol and d o from 1 6 10 2 to 2 9 10 11 cm 2 s 1 for the olivines e a ranges from 360 to 370 kj mol and d o from 1 5 10 6 to 5 0 10 6 cm 2 s 1 differences in these parameters are broadly consistent with the expected effect of structural differences between feldspar and olivine and pyroxene these results indicate that cosmogenic 21 ne will be quantitatively retained within olivine and pyroxene at earth surface temperatures over geological timescales the diffusion kinetics for feldspars on the other hand predicts that 21 ne retention at earth surface temperatures will vary significantly with domain size crystal microtexture surface temperature and exposure duration quantitative retention is expected only in favorable conditions this conclusion is reinforced by additional measurements of cosmogenic 21 ne in coexisting quartz and feldspar from naturally irradiated surface samples sanidine from a variety of rhyolitic ignimbrites exhibits quantitative retention whereas alkali feldspar from several granites does not

the formation and failure of natural dams of the numerous kinds of dams that form by natural processes dams formed from landslides glacial ice and late neoglacial moraines present the greatest threat to people and property landslide dams form in a wide range of physiographic settings the most common types of mass movements that form landslide dams are rock and debris avalanches rock and soil slumps and slides and mud debris and earth flows the most common initiation mechanisms for dam forming landslides are excessive rainfall and snowmelt and earthquakes landslide dams can be classified into six categories based on their relation with the valley floor type i dams 11 of 184 landslide dams from around the world that we were able to classify do not reach from one valley side to the other type ii dams 44 span the entire valley floor in some cases depositing material high on opposite valley sides type iii dams 41 move considerable distances both upstream and downstream from the landslide failure type iv dams 1 are rare and involve the contemporaneous failure of material from both sides of a valley type v dams 1 also are rare and are created when a single landslide sends multiple tongues of debris into a valley and forms two or more landslide dams in the same reach of river type vi dams 3 involve one or more failure surfaces that extend under the stream or valley and emerge on the opposite valley side many landslide dams fail shortly after formation in our sample of 73 documented landslide dam failures 27 of the landslide dams failed less than 1 day after formation and about 50 failed within 10 days over topping is by far the most common cause of failure the timing of failure and the magnitude of the resulting floods are controlled by dam size and geometry material characteristics of the blockage rate of inflow to the impoundment size and depth of the impoundment bedrock control of flow and engineering controls such as artificial spill ways diversions tunnels and planned breaching by blasting or conventional excavation glacial ice dams can produce at least nine kinds of ice dammed lakes the most dangerous are lakes formed in main valleys dammed by tributary glaciers failure can occur by erosion of a drainage tunnel under or through the ice dam or by a channel over the ice dam cold polar ice dams generally drain supraglacially or marginally by downmelting of an outlet channel warmer temperate ice dams tend to fail by sudden englacial or subglacial breaching and drainage late neoglacial moraine dammed lakes are located in steep mountain areas affected by the advances and retreats of valley glaciers in the last several centuries these late neoglacial dams pose hazards because 1 they are sufficiently young that vegetation has not stabilized their slopes 2 many dam faces are steeper than the angle of repose 3 these dams and lakes are immediately downslope from steep crevassed glaciers and near vertical rock slopes and 4 downstream from these dams are steep canyons with easily erodible materials that can be incorporated in the flow and increase flood peaks the most common reported failure mechanism is overtopping and breaching by a wave or series of waves in the lake generated by icefalls rockfalls or snow or rock avalanches melting of ice cores or frozen ground and piping and seepage are other possible failure mechanisms natural dams may cause upstream flooding as the lake rises and downstream flooding as a result of failure of the dam although data are few for the same potential energy at the dam site ownstream flood peaks from the failure of glacier ice dams are smaller than those from landslide moraine and structed earth fill and rock fill dam failures moraine dam failures appear to produce some of the largest downstream flood peaks for potential energy at the dam site greater than 1011 1012 joules differences in flood peaks natural dam failures appear to be controlled by dam characteristics and failure mechanisms

cosmogenic 3he and 21ne dating of biotite and hornblende stable cosmogenic isotopes such as 3he and 21 ne are useful for dating of diverse lithologies quantifying erosion rates and ages of ancient surfaces and sediments and for assessing complex burial histories although many minerals are potentially suitable targets for 3he and 21 ne dating complex production systematics require calibration of each mineral isotope pair we present new results from a drill core in a high elevation ignimbrite surface which demonstrates that cosmogenic 3he and 21 ne can be readily measured in biotite and hornblende 21 ne production rates in hornblende and biotite are similar and are higher than that in quartz due to production from light elements such as mg and al we measure 21 ne hbl 21 ne qtz 1 35 0 03 and 21 ne bio 21 ne qtz 1 3 0 02 which yield production rates of 25 6 3 0 and 24 7 2 9 at g 1 yr 1 relative to a 21 ne qtz production rate of 19 0 1 8 at g 1 yr 1 we show that nucleogenic 21 ne concentrations produced via the reaction 18 o n 21 ne are manageably small in this setting and we present a new approach to deconvolve nucleogenic 21 ne by comparison to nucleogenic 22 ne produced from the reaction 19 f n 22 ne in f rich phases such as biotite our results show that hornblende is a suitable target phase for cosmogenic 3he dating but that 3he is lost from biotite at earth surface temperatures comparison of 3he concentrations in hornblende with previously measured mineral phases such as apatite and zircon provides unambiguous evidence for 3he production via the reaction 6li n 3h 3he due to the atypically high li content in the hornblende 160 ppm we estimate that li produced 3he represents 40 of total 3he production in our samples and must be considered on a sample specific basis if 3he dating in hornblende is to be widely implemented

cosmic ray produced 21ne in terrestrial quartz the neon inventory of sierra nevada quartz separates the study of cosmic ray produced radioactive and stable nuclides on the surface of the earth can provide relevant geomorphological and glaciological information at present the cosmic ray production rates of stable 21ne are not well known this study attempts to remedy the situation by determining the production rate ratio of 21ne and 26al p21p26 in quartz 26al concentrations and p26 rates have previously been investigated for quartz separates of sierra nevada rocks which were brought to the surface by glacial scouring during the tioga period at the end of the last ice age 1 we used splits of the same samples for our studies and found that ne in these rocks represents a mixture of several components trapped ne nucleogenic 21ne and 22ne produced by n reactions in oxygen and fluorine respectively as well as cosmic ray produced ne which is the component of interest in this study the trapped component was substantially lost in one sample w86 12 by crushing and by a density separation of the grain sizes 38 90 m and 90 125 m permitting the resolution of the in situ produced 21ne into cosmic ray spallation and n produced components and the determination of a lower limit to p21p26 in a second sample w86 8 one split contained small enough amounts of nucleogenic 21ne to permit the determination of a reasonable upper limit to p21p26 the two ratio determinations are consistent within error limits and the value adopted 0 65 0 11 2 agrees with ratios observed in extraterrestrial matter apparently p21p26 is thus not very sensitive to the neutron spectrum however the observed production rate ratio is substantially larger than theoretical estimates for si targets reflecting poorly known neutron excitation functions the above p21p26 value coupled to the observed 26al production rate 1 corresponds to a 21ne production rate of p21 21 atoms g 1 a 1 in quartz or to p21 45 atoms g si 1 a 1 at sea level and high latitudes this rate is based on an adopted exposure age of 11 000 yr for our quartz samples

iron rich aureoles in the cm carbonaceous chondrites murray murchison and allan hills 81002 evidence for in situ aqueous alteration iron rich aureoles in cm carbonaceous chondrites are previously unidentified domains of aqueously altered matrix material whose feo content may exceed that of the surrounding matrix by up to more than 15 wt we describe the petrography and mineralogy of these objects in the cm chondrites murray murchison and allan hills alh 81002 the size of fe rich aureoles ranges from a few hundred microns to several millimeters in diameter and appears to be a function of the degree of alteration of the host chondrite the origin of fe rich aureoles is related to the alteration of large metal grains that has resulted in the formation of characteristic pcp rich reaction products that are frequently observed at the centers of the aureoles this suggests that fe rich aureoles in cm chondrites are the result of the mobilization of fe from altering metal grains into the matrix the fact that fe rich aureoles enclose numerous chondritic components such as chondrules calcium aluminum rich inclusions cais and mineral fragments as well as their radial symmetric appearance are strong evidence that they formed in situ and that significant directional fluid flow was not involved in the alteration process this and additional constraints such as the distribution of s and other elements as well as the inferred alteration conditions are consistent with in situ parent body alteration the observations are however entirely incompatible with preaccretionary alteration models in which the individual cm chondrite components have experienced diverse alteration histories the presence of numerous intact aureoles in the brecciated cm chondrites murray and murchison further suggests that the alteration occurred largely after brecciation affected these meteorites therefore the progressive aqueous alteration of cm chondrites may not be necessarily coupled to brecciation as has been previously proposed

a cosmic trip 25 years of cosmogenic nuclides in geology terrestrial cosmogenic nuclides produced by secondary cosmic ray interactions in the atmosphere and in situ within minerals in the shallow lithosphere are widely used to date surface exposure of rocks and sediments to estimate erosion and weathering rates and to date sediment deposition or burial their use has transformed geomorphology and quaternary geology for the first time allowing landforms to be dated and denudation rates to be measured over soil forming time scales the application of cosmogenic nuclides to geology began soon after the invention of accelerator mass spectrometry ams in 1977 and increased dramatically with the measurement of in situ produced nuclides in mineral grains near earth s surface in the 1980s the past 25 yr have witnessed the development of cosmogenic nuclides from their initial detection to their prevalence today as a standard geochronological and geochemical tool this review covers the major developments of the past 25 yr by comparing the state of the field in 1988 with that of today and by identifying key advances in that period that moved the field forward we emphasize the most commonly used in situ produced nuclides measured by ams for geological applications but we also discuss other nuclides where their applications overlap our review covers ams instrumentation cosmogenic nuclide production rates the methods of surface exposure dating measurement of erosion and weathering and burial dating and meteoric 10 be in memoriam devendra lal 1929 2012 whose vision inspired the field

the meteoritical bulletin no 82 1998 july abstract meteoritical bulletin no 82 lists information for 974 new meteorites including 521 finds from antarctica 401 finds from the sahara 21 finds from the nullarbor region of australia and 7 falls ban rong du burnwell fermo jalanash juancheng monahans 1998 and silao many rare types of meteorites are reported counting pairing groups as one these include one cr chondrite two ck chondrites two co chondrites four cv chondrites one ch chondrite or bencubbin like six c2 unclassified chondrites two eh chondrites two el chondrites three r chondrites thirty unequilibrated ordinary chondrites one un grouped chondrite three eucrites six howardites one diogenite eleven ureilites nine iron meteorites one mesosiderite two brachinites one lodranite one winonaite and two lunar meteorites dar al gani 400 and eet 96008 all italicized abbreviations refer to addresses tabulated at the end of this document

late pleistocene ages for the most recent volcanism and glacial pluvial deposits at big pine volcanic field california usa from cosmogenic 36cl dating the big pine volcanic field is one of several quaternary volcanic fields that poses a potential volcanic hazard along the tectonically active owens valley of east central california and whose lavas are interbedded with deposits from pleistocene glaciations in the sierra nevada range previous geochronology indicates an 1 2 ma history of volcanism but the eruption ages and distribution of volcanic products associated with the most recent eruptions have been poorly resolved to delimit the timing and products of the youngest volcanism we combine field mapping and cosmogenic 36cl dating of basaltic lava flows in the area where lavas with youthful morphology and well preserved flow structures are concentrated field mapping and petrology reveal approximately 15 vents and 6 principal flow units with variable geochemical composition and mineralogy cosmogenic 36cl exposure ages for lava flow units from the top middle and bottom of the volcanic stratigraphy indicate eruptions at 17 27 and 40 ka revealing several different and previously unrecognized episodes of late pleistocene volcanism olivine to plagioclase pyroxene phyric basalt erupted from several vents during the most recent episode of volcanism at 17 ka and produced a lava flow field covering 35 km2 the late pleistocene 36cl exposure ages indicate that moraine and pluvial shoreline deposits that overlie or modify the youngest big pine lavas reflect tioga stage glaciation in the sierra nevada and the shore of paleo owens lake during the last glacial cycle

cosmogenic neon in recent lavas from the western united states the absolute cosmogenic production of 21ne 21nec and 3he 3hec in olivine fo81 is measured for the well dated tabernacle hill flow values scaled to sea level and high latitude 60 are 45 atoms g yr for 21nec and 109 atoms g yr for 3hec for the past 17 800 years production rates for 21nec are about 50 higher than the theoretical values given by lal 1991 a ratio of 21nec 3hec of 0 41 0 05 is measured in olivines that have a range of 25 in cosmogenic exposure there is no evidence of 3hec loss in samples older than 100 000 years volcanic plagioclase is shown to be a suitable mineral for cosmogenic 21nec dating of surfaces 21nec in co existing plagioclase and olivine phenocrysts have a constant ratio of 0 42 04 in samples with more than 106 atoms of 21nec using all of the 21nec data and the 3hec as a cosmic ray flux monitor production of 21nec in ab36 plagioclase is calculated at 16 8 1 7 atoms g yr for the past 17 800 years the production of 21nec from 23na is estimated at 160 50 atoms g yr at 60 and sea level or equivalent to that on 24mg

multiple constraints on the age of a pleistocene lava dam across the little colorado river at grand falls arizona the grand falls basalt lava flow in northern arizona was emplaced in late pleistocene time it flowed 10 km from its vent area to the little colorado river where it cascaded into and filled a 65 m deep canyon to form the grand falls lava dam lava continued 25 km downstream and 1 km onto the far rim beyond where the canyon was filled subsequent fluvial sedimentation filled the reservoir behind the dam and eventually the river established a channel along the margin of the lava flow to the site where water falls back into the preeruption canyon the ca 150 ka age of the grand falls flow provided by whole rock k ar analysis in the 1970s is inconsistent with the preservation of centimeter scale flow top features on the surface of the flow and the near absence of physical and chemical weathering on the flow downstream of the falls the buried little colorado river channel and the present day channel are at nearly the same elevation indicating that very little if any regional downcutting has occurred since emplacement of the flow newly applied dating techniques better define the age of the lava dam infrared stimulated luminescence dating of silty mudstone baked by the lava yielded an age of 19 6 1 2 ka samples from three noneroded or slightly eroded outcrops at the top of the lava flow yielded 3he cosmogenic ages of 16 1 ka 17 1 ka and 20 1 ka a mean age of 8 19 ka was obtained from averaging four samples using the 40ar 39ar step heating method finally paleomagnetic directions in lava samples from two sites at grand falls and one at the vent area are nearly identical and match the curve of magnetic secular variation at ca 15 ka 19 ka 23 ka and 28 ka we conclude that the grand falls flow was emplaced at ca 20 ka

cosmogenic 3he in terrestrial rocks the summit lavas of maui we have identified terrestrial cosmic rayproduced 3he in three lava flows on the crest of haleakala volcano on maui 3 km above sea level and 0 5 million years old although these lavas like all oceanic basalts contain primordial 3he from the mantle the cosmogenic component 3hec can be identified unambiguously because it is extractable only by high temperature vacuum fusion in contrast a large fraction of the mantle helium resides in fluid inclusions and can be extracted by vacuum crushing leaving a residual component with 3he 4he ratios as high as 75 those in the atmosphere which can be liberated by melting the crushed grains cosmogenic 3he is present in both olivines and clinopyroxenes at 0 8 1 2 10 12 ml stp g and constitutes 75 5 of the total 3he present the observed 3hec levels require a cosmic ray exposure age of only some 64 000 years much less than the actual age of the lavas if there is no erosion using a model that includes effects of uplift or submergence as well as erosion we calculate an apparent erosion rate of the order of 8 5 m 106 years for the western rim of the summit crater as an example of the application of measurements of cosmogenic rare gases to terrestrial geological problems

terrestrial weathering of antarctic stone meteorites formation of mg carbonates on ordinary chondrites abstract white efflorescences of weathering origin occur superposed on fusion crusts or along fractures in the interiors of approximately 5 of all meteorites in the us antarctic collection efflorescences from equilibrated ordinary chondrites consist of the hydrous mg carbonates nesquehonite hydromagnesite x ray diffraction and scanning electron microscope studies of efflorescences from lew 85320 h5 show abundant elongate prismatic crystals of nesquehonite idiomorphic not pseudomorphous after lansfordite with minor local encrustations of hydromagnesite abundances of na k ca and rb in efflorescences from lew 85320 suggest that the observed contents of these elements would require only modest fractionation of chondritic composition whereas extensive fractionation would be required to derive the observed cation ratios from terrestrial sea salts therefore cations in evaporite minerals on antarctic meteorites are most likely not products of contamination by terrestrial marine salts the mg in the efflorescences probably originated from weathering of meteoritic olivine other cations in the efflorescences are also of meteoritic provenance thermodynamic analysis of the reaction forsterite water carbon dioxide nesquehonite silica at antarctic temperatures and pco 2 indicates spontaneity for all water activities greater than 0 65 compatible with the presence of liquid water as brines and or thin films

cosmetic isotope analyses applied to river longitudinal profile evolution problems and interpretations the use of cosmogenic isotopes to determine surface exposure ages has grown rapidly in recent years the extent to which cosmogenic nuclides can distinguish between mechanistic hypotheses of landscape evolution is an important issue in geomorphology we present a case study to determine whether surface exposure dating techniques can elucidate the role knickpoint propagation plays in longitudinal profile evolution cosmogenically produced 10be 26al 36cl 3he and 21ne were measured in olivines collected from 5 2 ma basalt flows on kauai hawaii several obstacles had to be overcome prior to the measurement of in situ produced radionuclides including removal of meteoric 10be from the olivine grains discrepancies between the radionuclide and noble gas data may suggest limits for exposure dating approximate surface exposure ages calculated from the nuclide concentrations indicate that large boulders may remain in the hawaiian valley below the knickpoint for hundreds of thousands of years the ages of samples collected above the knickpoint are consistent with estimates of erosion based on the preservation of palaeosurfaces although the exposure ages can neither confirm nor reject the nickpoint hypothesis boulder ages downstream of the knickpoint are consistent with a wave of incision passing upvalley the long residence time off the coarse material in the valley bottom further suggests that knickpoint propagation beneath a boulder pile is necessary for incision of the bedrock underlying the boulders to occur 1997 by john wiley sons ltd

climate forcing of volcano lateral collapse evidence from mount etna sicily in this study we present evidence for early holocene climatic conditions providing circumstances favourable to major lateral collapse at mount etna sicily the volcano s most notable topographic feature is the valle del bove a 5 x 8 km cliff bounded amphitheatre excavated from the eastern flank of the volcano its origin due to prehistoric lateral collapse is corroborated by sturtzstrom deposits adjacent to the amphitheatre s downslope outlet but the age nature and cause of amphitheatre excavation remain matters for debate cosmogenic 3 he exposure ages determined for eroded surfaces within an abandoned watershed flanking the valle del bove support channel abandonment ca 7 5 ka bp as a consequence of its excavation in a catastrophic collapse event watershed development was largely dictated by pluvial conditions during the early holocene which are also implicated in slope failure a viable trigger is magma emplacement into rift zones in the eastern flank of a water saturated edifice leading to the development of excess pore pressures consequent reduction in sliding resistance detachment and collapse such a mechanism is presented as one potential driver of future lateral collapse in volcanic landscapes forecast to experience increased precipitation or melting of ice cover as a consequence of anthropogenic warming

geochemical discrimination of five pleistocene lava dam outburst flood deposits western grand canyon arizona abstract pleistocene basaltic lava dams and outburst flood deposits in the western grand canyon arizona have been correlated by means of cosmogenic 3he 3hec ages and concentrations of sio2 na2o k2o and rare earth elements these data indicate that basalt clasts and vitroclasts in a given outburst flood deposit came from a common source a lava dam with these data it is possible to distinguish individual dam flood events and improve our understanding of the interrelations of volcanism and river processes at least five lava dams on the colorado river failed catastrophically between 100 and 525 ka subsequent outburst floods emplaced basalt rich deposits preserved on benches as high as 200 m above the current river and up to 53 km downstream of dam sites chemical data also distinguishes individual lava flows that were collectively mapped in the past as large long lasting dam complexes these chemical data in combination with age constraints increase our ability to correlate lava dams and outburs

correlated petrologic and geochemical characteristics of co3 chondrites many properties of co3 chondrites have been shown previously to have resulted from thermal metamorphism petrologic subtypes 3 0 3 7 have been assigned to members of the group additional properties that correlate with the metamorphic sequence but seem to have resulted from hydrothermal alteration include the modal abundance of amoeboid olivine inclusions aoi chondrule size the types of refractory inclusions and whole rock o isotopic composition the percentage of rimmed aoi increases with petrologic subtype the rims most likely formed during hydrothermal alteration the previously reported correlation between aoi abundance and chondrite subtype is probably an artifact due to the difficulty in recognizing small unrimmed aoi in the least metamorphosed co3 chondrites because large 200 micron size porphyritic chondrules have nearly the same mean size in all co3 chondrites it seems likely that the correlation between chondrule size and subtype is due to alteration of the smallest chondrules to the point of unrecognizability as complete objects in the more metamorphosed co3 chondrites the previously reported decrease in the proportion of melilite rich refractory inclusions with increasing petrologic subtype may have resulted from more extensive hydrothermal alteration in co3 4 3 7 chondrites that converted primary melilite into ca pyroxene andradite and nepheline alteration probably caused the preferential occurrence of o 16 poor oxygen isotopes in the more metamorphosed whole rock samples

accretionary dust mantles in cm chondrites evidence for solar nebula processes abstract the texture and modal composition of fourteen carbonaceous chondrites of the cm group were studied in detail to unravel the origin of their various components and the evolution of their parent body ies the analysis and mapping of complete thin sections by scanning electron and optical microscopy revealed textural units in most samples in which all coarse grained primordial chondritic components chondrules chondrule fragments refractory inclusions particles rich in hydrous silicates pcp and various mineral fragments are coated by fine grained serpentine and tochilinite rich dust the dust mantled particles have spheroidal shapes and form a special lithology in which they are densely aggregated and compacted without any interstitial clastic material the dust mantles termed accretionary dust mantles and their host lithology called primary accretionary rock are interpreted as products of accretion processes in the solar nebula an origin by processes on the parent body is excluded for textural and mineralogical reasons most cm chondrites are affected by secondary impact induced brecciation of variable intensity except for yamato 791198 which is composed entirely of a primary accretionary rock obviously unaltered by secondary parent body processes such as aqueous alteration or brecciation the cm chondrites y74662 haripura cold bokkeveld kivesvaara mighei pollen nogoya y793321 alh 83100 murray and murchison consist of clasts of primary accretionary rock embedded in a fine grained clastic matrix derived from primary rock material the volume percentage of the clastic matrix ranges from 5 y74662 to 76 murchison bells and essebi are brecciated and affected by aqueous alteration to an extent that fragments of primary rock are not preserved the evaluation of literature data indicates that solar wind implanted noble gases are absent in the primary accretionary rock component of cm chondrites they are only present in brecciated samples it must be concluded from the relation between the content of trapped solar wind and degree of brecciation that 1 the solar noble gases are residing exclusively in the clastic matrix and 2 some cm chondrites are regolith breccias and others are fragmental or monomict breccias both types were formed on the parent body although the latter were never exposed to the solar wind based on the results of textural chemical and mineralogical investigations we conclude that a major portion of the water bearing phases in cm chondrites were produced by the hydration of refractory phases prior to the formation of dust mantles and prior to the formation of the recent cm parent body this process requires special physicochemical conditions which allow the crystallization of oh bearing silicates e g higher density of the nebula gas or special precursor planetesimals of the cm chondrite parent body

image processing and roughness analysis of exposed bedrock fault planes as a tool for paleoseismological analysis results from the campo felice fault central apennines italy abstract morphologic investigations along the campo felice cf fault central apennines italy have been made in order to develop a procedure for the paleoseismological analysis of bedrock fault scarps the cf fault has been responsible for the formation of an impressive limestone fault scarp geomorphologic work on the cf basin and related fault indicated that the scarp originated from tectonic fault displacements three morphologic units have been distinguished along the fault scarp and defined as morphosome m1 lowest part of the scarp m2 and m3 the uppermost part these units display different karstic features which are the result of their different duration of exposure to weathering micromorphologic analyses focused on the morphosome m1 along which the cf fault plane is exposed for a height ranging between 4 and 7 m these analyses were aimed at defining differently weathered bands located at various heights and parallel to the fault scarp top and base the presence of these bands suggests repeated fault movements the exposed fault surface displays a low grade biokarstic weathering due to the action of epilithic and endolithic organisms the biokarst distribution is however inhomogeneous and conditioned by the presence of nourishing elements moisture and by light intensity an area preferentially affected by the biokarstic processes develops as a band at the bedrock soil contact at the base of the scarp roughness and colour analyses were made to identify uplifted bands which previously formed at the bedrock soil contact the roughness analysis was made using a microroughness meter along 20 cm long horizontal transects repeated each 20 cm of fault height for the entire morphosome m1 at various sites along the scarp the roughness variance data plotted vs the fault height failed to identify differently weathered bands of paleoseismological interest this result is probably due to the complex distribution of biokarst along the investigated fault plane more reliable results have been obtained by areal analysis of the variation of the colour rendering of the rocks exposed along the fault plane at different sites photographic images of large portions of fault surfaces have been processed with standard graphic computer programs the variations of colour indicated the presence of bands at various heights along the fault plane two uplifted bands have been recognised at all the investigated sites suggesting two displacement events e1 and e2 a preliminary chronological framework for these two events the youngest of which affected the cf fault can be derived from the paleoseismological data available for the southernmost branch of the regional fault system that includes the cf fault according to these data e1 may have occurred between 860 and 1300 ad while e2 may have occurred at about 1900 bc work is in progress to define surface exposure ages of different parts of the fault plane by means of in situ produced cosmogenic 36 cl this procedure will give further chronological constraints for the age of e1 and e2 and will also permit to test the validity of the micromorphologic analysis of bedrock fault scarps for paleoseismological aims

the cosmic ray produced he 21ne ratio in ultramafic rocks although production of noble gases in meteorites are well studied since many years e g bogard and cressy 1973 the first analysis of cosmic ray produced helium in terrestrial rocks were performed only in 1986 by kurz and by craig and poreda later marti and craig 1987 reported cosmic ray produced neon in summit lavas from maui more data on cosmogenic helium and neon were reported by staudacher and allegre 1991 on ultramafic nodules from asia the absolute production rate of cosmic ray produced helium has been determined by kurz 1986 1987 in olivine rich basalts from hawaii with known k ar or 14c ages the resulting hec production rate is 3 72 10 18cm stp g a this allows cosmogenic he to be used to determine cosmic ray exposure ages of surface samples for neon no such absolute production rate has yet been determined in order to develop a complementary dating method using cosmogenic 21ne staudacher and allegre 1991 measured the cosmogenic he 21ne ratio in ultramafic nodules their value was 1 4 which is significantly smaller than the value of 3 5 obtained in this work here we discuss the cosmogenic he 21ne ratio in view of new noble gas data on olivine separates of oceanites from reunion island and on ultramafic nodules from mongolia

cosmogenic 14c in carbonate rocks abstract the rate of in situ production of cosmogenic 14 co in limestone is 18 3 atom g caco3 year corrected to high latitude and sea level based on measurements of 14 co in limestone associated with the bonneville provo shoreline complex in utah in situ cosmogenic 14 co was separated cryogenically from co2 using 14 c dead co as a carrier for in situ cosmogenic 14 co this rate of production was used to estimate an age of 4360 1220 cal year b p for the last movement on the south oquirrh mountain fault zone in northern utah and an age of at least 10 600 900 years for the penultimate event of the pleasant valley fault in northern nevada

nwa 1152 and sahara 00182 new primitive carbonaceous chondrites with affinities to the cr and cv groups we have investigated the mineralogy petrography bulk chemistry and light element isotope composition of the ungrouped chondrites north west africa nwa 1152 and sahara 00182 nwa 1152 contains predominantly type 1 porphyritic olivine po and porphyritic olivinepyroxene pop chondrules chondrule silicates are magnesium rich fo98 8 1 2 n 36 fs2 3 2 1 wo1 2 0 3 n 23 matrix comprises 40 vol of the sample and is composed of a micron sized silicate groundmass with larger silicate sulfide magnetite and fe ni metal ni 50 wt grains phyllosilicates were not observed in the matrix refractory inclusions are rare 0 3 vol and are spinel pyroxene aggregates or amoeboid olivine aggregates melilite is absent from the refractory inclusions sahara 00182 contains predominantly type 1 po chondrules pop chondrules are less common most chondrules contain blebs of and are often rimmed with fe ni metal and sulfide chondrule phenocrysts are magnesium rich fo92 2 0 6 n 129 fs4 4 1 8 wo1 3 1 1 n 16 matrix comprises 30 vol of the meteorite and is predominantly sub micron silicates with rare larger silicate gains matrix fe ni metal mean ni 5 8 wt and sulfide grains are up to mm scale no phyllosilicates were observed in the matrix refractory inclusions are rare 1 1 vol and melilite is absent the oxygen isotope composition of nwa 1152 falls within the range of the cv chondrites with 17o 3 43 18o 0 70 and is similar to sahara 00182 17o 3 89 18o 0 19 grossman and zipfel 2001 based on mineralogical and petrographic characteristics we suggest nwa 1152 and sahara 00182 show many similarities with the cr chondrites however oxygen isotopes suggest affinity with the cvs thus neither sample can be assigned to any of the currently known carbonaceous chondrite groups based on traditionally recognized characteristics both samples demonstrate the complexity of inter and intra group relationships of the carbonaceous chondrites whatever their classification nwa 1152 and sahara 00182 represent a source of relatively pristine solar system material

possible impact induced refractory lithophile fractionations in el chondrites abstract literature data show that refractory lithophile elements in most chondrite groups are unfractionated relative to ci chondrites the principal exception is the el chondrite group whose observed falls all of which are type 6 are depleted in ca and light ree in contrast literature data and our new inaa data on el3 pca 91020 el3 mac 88136 and el4 grein 002 show that some replicates of these samples have nearly flat ree patterns unlike those of el6 chondrites other replicates exhibit fractionated ree patterns similar to those of el6 chondrites petrographic examination shows that many el6 and some el3 and el4 chondrites are impact melt breccias or contain impact melted portions we suggest that the same impact processes that formed these breccias and produced melt are responsible for the observed bulk compositional fractionations in refractory lithophile elements i e el6 chondrites were produced from initially unequilibrated el3 material when large amounts of impact heat were deposited plagioclase and or oldhamite cas the major ree carriers in enstatite chondrites may have been melted and then transported appreciable 10 cm distances el6 chondrites represent the residuum that is depleted in ree particularly in lree and ca unlike the case for el chondrites our new inaa data on alh 84170 eet 87746 and sah 97096 all eh3 show some scatter but are consistent with the eh group having uniform refractory lithophile abundances

noble gases in mafic phenocrysts and xenoliths from new zealand we have determined the elemental and isotopic compositions of noble gases in young subduction related phenocrystic olivine and clinopyroxene samples from the taupo volcanic zone central north island new zealand and in behind arc intraplate phenocrystic and xenolithic olivine samples from the northland and auckland volcanic provinces northern north island new zealand helium isotopic ratios range from morb like 3he4he values of about 11 10 6 to lower values of about 6 10 6 consistent with previous measurements of helium isotopic ratios in subduction related samples 40ar36ar and 21ne22ne ratios range from atmosphere like compositions to maximum values of about 700 and 0 033 respectively in contrast most of the 21ne22neratios are generally indistinguishable from atmospheric values the variations in helium neon and argon isotopic ratios are interpreted as resulting from mixing of 1 a a primordial 3he rich component derived from the upper mantle and characterised by morb like 3he4he ratios of about 12 10 6 2 b a radiogenic 4he rich component derived from crustal materials and characterised by a 3he4he ratio of less than 2 10 7 and 3 c a helium poor atmosphere derived component which dominates the heavier noble gases by combining the helium and argon isotopic results it is possible to estimate the relative contribution of each of these three components to the total 40ar observed in each sample based on the present understanding of the origin and evolution of arc magmas a simple qualitative model of the mechanisms which introduce noble gases to the parent magmas of the samples is developed however it remains uncertain whether the atmospheric and crustal derived noble gas components are introduced to the mantle source regions of the magmas by subduction or alternatively are introduced by interactions between the ascending parent magmas and the overlying crust

evolution of a landslide creating a temporary lake successful prediction the anzola landslide is a complex landslide that failed on the left side of the ceno valley northern apennines italy it was studied using a multidisciplinary approach in which geomorphology geology and engineering geology were combined to understand the kinematics evolution the main rock mass characteristics the behavior of the landslide and its morphologic evolution the anzola landslide in 1996 was probably a neo activation it was affected by at least three re activations recently always in connection with heavy rainfall which twice caused the blockage of the valley and formed a small lake the possibility of river blockage as a consequence of the re activation of the landslide was investigated using a grass gis based shell script that simulates the landslide material descent and computes the dam and lake sizes the simulations allowed assessment of the amount of the depleted material coupled with the morphological situation characterized by a very narrow riverbed that can lead easily to the formation of consistent damming and lakes this situation of course can represent a serious risk in case of a dam break and the ensuing flooding could affect downstream infrastructures and small villages

dating geomorphologic surfaces using cosmogenic 3he cosmogenically produced 3he in rocks can be used to date geomorphologic surfaces using the late pleistocene bonneville flood event the subsequent development of the provo shoreline and associated volcanic rocks a production rate of 432 atoms g yr in olivine is determined for 38 56 n latitude present geomagnetic latitude 46 5 at 1445 m altitude integrated over the last 14 400 years measurements of cosmogenic helium in flood deposits on river scoured surfaces and lava flows show that it is possible to date directly large scale events such as volcanic eruptions and catastrophic floods olivine and pyroxene are both satisfactory minerals for cosmogenic dating by the 3he method however in most cases plagioclase and quartz are unsuitable because they do not quantitatively retain helium isotopes in their lattices

cosmic ray exposure dating with in situ produced cosmogenic 3he results from young hawaiian lava flows in an effort to determine the in situ production rate of spallation produced cosmogenic 3 he and evaluate its use as a surface exposure chronometer we have measured cosmogenic helium contents in a suite of hawaiian radiocarbon dated lava flows the lava flows ranging in age from 600 to 13 000 years were collected from hualalai and mauna loa volcanoes on the island of hawaii because cosmic ray surface exposure dating requires the complete absence of erosion or soil cover these lava flows were selected specifically for this purpose the 3 he production rate measured within olivine phenocrysts was found to vary significantly ranging from 47 to 150 atoms g 1 yr 1 normalized to sea level although there is considerable scatter in the data the samples younger than 10 000 years are well preserved and exposed and the production rate variations are therefore not related to erosion or soil cover data averaged over the past 2000 years indicate a sea level 3 he production rate of 125 30atoms g 1 yr 1 which agrees well with previous estimates the longer record suggests a minimum in sea level normalized 3 he production rate between 2000 and 7000 years 55 15atoms g 1 yr 1 as compared to samples younger than 2000 years 125 30 atoms g 1 yr 1 and those between 7000 and 10 000 years 127 19atoms g 1 yr 1 the minimum in production rate is similar in age to that which would be produced by variations in geomagnetic field strength as indicated by archeomagnetic data however the production rate variations a factor of 2 3 0 8 are poorly determined due to the large uncertainties in the youngest samples and questions of surface preservation for the older samples calculations using the atmospheric production model of o brien 1979 35 and the method of lal and peters 1967 11 predict smaller production rate variations for similar variation in dipole moment a factor of 1 15 1 65 because the production rate variations archeomagnetic data and theoretical estimates are not well determined at present the relationship between dipole moment and production rate will require further study precise determination of the production rate is an important uncertainty in the surface exposure technique but the data demonstrate that it is feasible to date samples as young as 600 years of age providing that there has been no erosion or soil cover therefore the technique will have important applications for volcanology glacial geology geomorphology and archaeology

cosmogenic 3he age estimates of plio pleistocene alluvial fan surfaces in the lower colorado river corridor arizona usa abstract plio pleistocene deposits of the lower colorado river lcr and tributary alluvial fans emanating from the black mountains near golden shores arizona record six cycles of late cenozoic aggradation and incision of the lcr and its adjacent alluvial fans cosmogenic 3he 3hec ages of basalt boulders on fan terraces yield age ranges of 3 3 2 2 ma 2 2 1 1 ma 1 1 ma to 110 ka

using in situ produced cosmogenic isotopes to estimate rates of landscape evolution a review from the geomorphic perspective the application of in situ produced cosmogenic isotopes to problems in geomorphology has increased rapidly over the past decade at least 57 papers and numerous abstracts have been published since the mid 1980s when the first mass spectrometric measurements of terrestrially produced cosmogenic isotopes were made taken at face value these studies provide quantitative information about rates of landscape evolution and landform age however the significance of calculated erosion rates and exposure ages depends strongly on the models used to interpret isotopic data the validity of assumptions inherent to these models and the geologic surroundings in which the samples were collected this paper attempts to place cosmogenic isotope studies in a geomorphic context by reviewing fundamentals of the method and evaluating the validity of assumptions under which these data have been interpreted at present the establishment of high precision cosmogenically based glacial and alluvial chronologies is stymied by the evolution of geomorphic surfaces the erosion of rock from sampled boulders the potential for isotope inheritance from previous exposure and the uncertainty of isotopic measurements uncertainties in isotope production rates and the observed variability of exposure ages on individual geomorphic surfaces limit the confidence with which cosmogenic ages can be correlated reliably with those obtained by other techniques estimation of erosion rates at single points on the landscape gives useful small scale information extrapolation of these rates over longer time and larger spatial scales is less sure and most likely biased toward lower erosion rates by die inadvertent selection of resistant sample sites however because erosion rates are so poorly constrained at present even estimates to within a factor of 2 may be of significant value to geomorphologists and tectonicists

glacial chronology and production rate cross calibration of five cosmogenic nuclide and mineral systems from the southern central andean plateau abstract glacial deposits on the high altitude arid southern central andean plateau cap the puna in northwestern argentina document past changes in climate but the associated geomorphic features have rarely been directly dated this study provides direct age control of glacial moraine deposits from the central puna 24 s at elevations of 3900 5000 m through surface exposure dating with cosmogenic nuclides our results show that the most extensive glaciations occurred before 95 ka and an additional major advance occurred between 46 and 39 ka the latter period is synchronous with the highest lake levels in the nearby pozuelos basin and the minchin inca huasi wet phase on the altiplano in the northern cap none of the dated moraines produced boulder ages corresponding to the tauca wet phase 24 15 ka additionally the volcanic lithologies of the deposits allow us to establish production ratios at low latitude and high elevation for five different nuclide and mineral systems 10be 21ne and 26al from quartz 11 or 12 samples and 3he and 21ne from pyroxene 10 samples we present production ratios for all combinations of the measured nuclides and cross calibrated production rates for 21ne in pyroxene and quartz for the high sub tropical andes the production rates are based on our 10be normalized production ratios and a weighted mean of reference 10be production rates calibrated in the high tropical andes 4 02 0 12 at g 1 yr 1 these are 21neqtz 18 1 1 2 at g 1 yr 1 and 21nepx 36 6 1 8 at g 1 yr 1 en 88 94 scaled to sea level and high latitude using the lal stone scheme with 1 uncertainties as 3he and 26al have been directly calibrated in the tropical andes we recommend using those rates finally we compare exposure ages calculated using all measured cosmogenic nuclides from each sample including 11 feldspar samples measured for 36cl and a suite of previously published production rates

thermally metamorphosed carbonaceous chondrites from data for thermally mobile trace elements we report radiochemical neutron activation analysis rnaa data for u co au sb ga rb cs se ag te zn in bi tl and cd ordered by increasing ease of vaporization and loss from the murchison cm2 chondrite during open system heating in nine antarctic c2 and c3 chondrites these meteorites exhibit properties obtained by reflectance spectroscopy o isotopic mass spectrometry and or mineralogy petrology suggesting thermal metamorphism in their parent bodies five of these meteorites asuka a 881655 yamato y 793495 y 790992 pecora escarpment pca 91008 and y 86789 paired with y 86720 exhibit significant depletion of the most thermally mobile 1 5 trace elements which is consistent with open system loss during extended parent body heating under conditions duplicated by week long heating of the murchison c2 chondrite heated at 500 700 c in a low pressure initially 10 5atm h2 atmosphere from earlier data three other c3 chondrites allan hills alh 81003 alh 85003 and lewis cliffs lew 85332 show significant cd depletion nine additional c2 and c3 chondrites show no evidence of mobile trace element depletion including y 793321 which by all other criteria was mildly metamorphosed thermally either metamorphism of these nine meteorites occurred under closed conditions and or alteration took place under such mild conditions that even cd could not be lost the rnaa data suggest that 10 of the 46 antarctic carbonaceous chondrites including 4 of 37 from victoria land and 6 of 9 from queen maud land exhibit open system loss of at least some thermally mobile trace elements by heating in their parent bodies whereas none of the 25 non antarctic falls experienced this these results are consistent with the idea that the antarctic sampling of near earth material differs from that being sampled today

cosmogenic 3he ages and frequency of late holocene debris flows from prospect canyon grand canyon usa abstract lava falls rapid which was created and is maintained by debris flows from prospect canyon is the most formidable reach of whitewater on the colorado river in grand canyon and is one of the most famous rapids in the world debris flows enter the colorado river at tributary junctures creating rapids the frequency of debris flows is an important consideration when management of regulated rivers involves maintenance of channel morphology we used cosmogenic 3 he 14 c and historical photographs to date 12 late holocene and historic debris flows from prospect canyon the highest and oldest deposits from debris flows on the debris fan yielded a 3 he date of about 3 ka which indicates predominately late holocene aggradation of one of the largest debris fans in grand canyon the deposit which has a 25 m escarpment caused by river reworking crossed the colorado river and raised its base level by 30 m for an indeterminate although likely short period we mapped depositional surfaces of 11 debris flows that occurred after 3 ka two deposits inset against the highest deposit yielded 3 he ages of about 2 2 ka and at least two others followed shortly afterwards at least one of these debris flows also dammed the colorado river the most recent prehistoric debris flow occurred no more than 0 5 ka the largest historic debris flow which constricted the river by 80 occurred in 1939 five other debris flows occurred after 1939 these debris flows constricted the colorado river by 35 80 assuming the depositional volumes of late holocene debris flows can be modeled using a lognormal distribution we calculated recurrence intervals of 15 to more than 2000 years for debris flows from prospect canyon

cosmic ray produced neon in antarctic rocks we studied three antarctic rocks and quartz separates thereof for their records of cosmic ray produced 21ne all three samples reveal an excess of 21ne and in two of them a cosmic ray origin is documented by the spallation ratios 22ne 21ne 3he is shown to be incompletely retained in quartz 10 previous studies have shown the suitability of quartz as a monitor for cosmic ray produced radionuclides 10be and 26al and therefore as a tracer for geological processes 26al 10be ratios allow the calculation of minimum exposure ages and maximum erosion rates and measured ratios 21ne 10be and 21ne 26al add significant information to exposure histories especially in the case of a complex exposure this information is necessary for an evaluation of erosion rates and to define geological processes such as uplift and variations in the antarctic ice cover

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

cosmic ray produced neon and helium in the summit lavas of maui a 3he component attributed to cosmic ray induced spallation reactions 3hec in silicate minerals has recently been identified in lava flows on the summit of haleakala volcano on maui1 3 this3hec is observed in the helium extracted by fusion of olivine and pyroxene phenocrysts after first removing almost all of the mantle derived helium by vacuum crushing a procedure which releases the volatiles trapped in fluid inclusions within the phenocrysts cosmic ray produced 3he was found in the high altitude 3 000 m summit lavas which are 500 000 years old but was not observed in sub surface samples which were overlain by younger basalts2 or in a 200 yr old lava flow at sea level on maui1 2 we report here the identification of cosmic ray produced 21nec in addition to 3hec in gases extracted by fusion of olivines and clinopyroxenes after vacuum crushing the observed 3he 21ne c ratios and the ratio of 21nec in olivine to that in clinopyroxene are consistent with an in situ origin of 3hec and 21nec by cosmic ray spallation reactions this component could be important for interpreting helium isotopic data in terrestrial reservoirs geophysical applications could include determinations of erosion rates and exposure histories of terrestrial rocks

an isochron approach to 21ne cosmic ray exposure dating by activation with deuteron deuteron fusion neutrons abstract this paper describes an alternative approach to cosmogenic 21 ne 21 ne cos exposure dating that entails irradiating mg bearing samples in a deuteron deuteron d d neutron generator to produce 22 ne by the reaction 25 mg n 22 ne mg following irradiation and provided that the 22 ne mg 22 ne cos ratio is greater than 10 3 one can identify cosmogenic 21 ne using a 21 ne 20 ne vs 22 ne 20 ne isochron the method is analogous to identifying radiogenic 40 ar using a conventional 40 ar 36 ar vs 39 ar k 36 ar isochron with knowledge of the 21 ne production rate and the relationship between 22 ne mg and mg content determined by co irradiating a standard of known mg concentration the slope of the isochron 21 ne cos 22 ne mg can be translated into an exposure age in this way a 21 ne exposure age is calculated without making assumptions regarding the isotopic composition of the trapped or cosmogenic components this technique if proven to be viable will be applicable to minerals that contain a stoichiometric abundance of mg e g orthopyroxene clinopyroxene and olivine

helium isotopic composition of the tabar lihir tanga feni island arc papua new guinea helium abundances and isotopic ratios have been measured in samples of geothermal gases submarine ultramafic xenoliths and subaerial mafic phenocrysts and xenoliths from the tabar lihirtanga feni tltf arc in the bismarck archipelago of papua new guinea these unusual volcanoes are produced by adiabatic decompression melting of subduction modified upper mantle and apparently carry an exceptionally large slab derived component they therefore provide an ideal setting for studying the role of subducted helium in arc volcanoes helium isotopic ratios in the geothermal fluids submarine xenoliths and most helium rich subaerial samples indicate the mantle source regions beneath the tabar island group and lihir island have 3he 4he ratios of about 7 2 times the atmospheric ratio r a whereas the mantle source for ambitle island feni group in the more southern tltf arc has a lower ratio of about 6 6 r a these ratios are only slightly lower than typical depleted upper mantle values of about 8 5 r a indicating that even severely slab modified mantle wedge carries a relatively minor slab derived helium component the systematically lower 3he 4he ratios of the ambitle samples are interpreted to reflect a slightly greater slab derived helium component helium isotopic ratios as low as 2 r a which are correlated with low total helium abundances were observed in subaerial augitic clinopyroxene phenocrysts coexisting olivine and sodium rich acmitic clinopyroxene have higher helium abundances and isotopic ratios this isotopic disequilibrium between coexisting mineral phases is interpreted as crustal level addition of a low 3he 4he component to an actively outgassing magma from which different mineral phases trap helium sequentially

siderophile element anomalies in ck carbonaceous chondrites implications for parent body aqueous alteration and terrestrial weathering of sulfides abstract ck chondrites constitute the most oxidized anhydrous carbonaceous chondrite group most of the fe occurs in magnetite and in feo rich mafic silicates the two observed ck falls karoonda and kobe along with thirteen relatively unweathered ck finds have unfractionated siderophile element abundance patterns in contrast a sizable fraction of ck finds 9 of 24 investigated shows fractionated siderophile abundance patterns including low abundances of ni co se and au the most extreme depletions are in ni 0 24 of normal ck and au 0 14 of normal ck this depletion pattern has not been found in other chondrite groups out of the 74 ck chondrites listed in the meteoritical bulletin database 2006 excluded considerably paired specimens see http tin er usgs gov meteor metbull php we analyzed 24 and subclassified the ck chondrites in terms of their chemical composition and sulfide mineralogy sl siderophiles low six samples for large depletions in ni co se and au 50 of sulfides lost sm siderophiles medium two cks for moderately low ni and co abundances sulfides are highly altered or partly lost sh siderophiles high one specimen for enrichments in ni co se and au normal for unfractionated samples 13 samples the sole sh sample may have obtained additional sulfide from impact redistribution in the parent asteroid we infer that these elements became incorporated into sulfides after asteroidal aqueous processes oxidized nebular metal thermal metamorphism probably also played a role in their mineral siting the siderophile losses in the sl and sm samples are mainly the result of oxidation of pentlandite pyrite and violarite by terrestrial alteration followed by leaching of the resulting phases some antarctic ck chondrites have lost most of their sulfides but retained ni co se and au presumably as insoluble weathering products

simulation of the parma river blockage by the corniglio landslide northern italy abstract near the village of corniglio in the northern apennines about 40 km sw of parma a big landslide was recently 1994 1996 subjected to reactivation damaging many buildings and partially obstructing the parma river in this work we have carried out a quantitative analysis of the recent evolution of the landslide with the help of a geographic information system gis in order to predict the most plausible future developments then various simulations of landslide descent were carried out and above all to evaluate the possibility of blockage formation to this purpose two computer programs that simulate the material descent through a cellular automata model and define blockage characteristics were written many different simulations were carried out 16 of which are presented here for each of them the computer output consists of a table defining the quantity of displaced material area and volume of the dam and lake and of a map showing the areas involved in the landslide movement and upstream flooding in order to assess the probability of blockage formation and dam stability we applied the simulation results to some empirical rules defined by preceding authors through statistical analysis carried out on a high number of analogous phenomena these rules suggest that it is very unlikely that blockage takes place as the result of very slow landslide velocity riverbed width and high erodibility of the material if it were to occur the size and shape of the dam seem sufficient to guarantee a considerable degree of stability making a sudden failure improbable

the meteoritical bulletin no 84 2000 august meteoritical bulletin no 84 lists information for 1341 newly classified meteorites comprising 842 from antarctica 341 from africa 66 from australia 48 from asia including 42 from the arabian peninsula 38 from north america 4 from europe and 2 from south america information is provided for 11 recent falls bilanga devri khera djoumine guangmingshan kitchener kobe leighlinbridge sabrum songyuan tagish lake and vissannapeta 4 martian meteorites dar al gani 670 735 dhofar 019 los angeles and sayh al uhaymir 005 008 3 lunar meteorites northwest africa 032 dhofar 025 and 026 an ungrouped enstatite rich meteorite zaklodzie 11 iron meteorites and a wide variety of other interesting stony meteorites including ck cm co cr cv r enstatite and unequilibrated ordinary chondrites primitive achondrites hed achondrites ureilites and aubrites

late pleistocene basaltic ash and volcanic eruptions in the bonneville basin utah microprobe analyses of glass and radiocarbon dating of samples associated with three basaltic volcanic ashes in the deposits of lake bonneville make the ashes useful stratigraphic markers for correlation within the bonneville basin two of the ashes were derived from tuff cones pavant butte and tabernacle hill that erupted into lake bonneville in what is now the black rock desert of south central utah near kanosh utah the pavant butte ash is interbedded with barrier beach lagoon marl that is slightly less than 16 000 yr old five radiocarbon dates on carbonate materials stratigraphically associated with the ash elsewhere in the sevier desert average 15 300 yr b p lake bonneville was in its transgressive phase about 15 m below the bonneville shoreline when the pavant butte eruption began at the tabernacle hill tuff cone a basalt flow that has pillows wave rounded cobbles and tufa on its outer margins was erupted into lake bonneville shortly after the tuff cone at or near the provo shoreline the tabernacle hill tuff cone and basalt flow are older than a radio carbon date of about 14 300 yr b p on tufa collected from the outer margin of the basalt flow but younger than the bonneville flood at about 14 500 yr b p the third ash informally termed the thiokol ash has been found interbedded with lake bonneville deposits in two exposures in northern utah and in sediment cores from the great salt lake it is about 25 000 yr old and may have been erupted from a volcanic field on the northwest shore of great salt lake or from the snake river plain microprobe analyses of glass from samples of the black rock desert ashes show them to have similar chemical compositions but glass from tabernacle hill can be distinguished from pavant butte glass by its higher concentrations of cao and p 2 o 5 systematic differences in chemical composition between samples of pavant butte glass can be explained by comagmatic processes thiokol glass has less sio 2 ai 2 o 3 mgo and na 2 o but greater total iron and p 2 o 5 than do the black rock desert glasses

cosmogenic neon and helium at reunion measurement of erosion rate abstract we report analyses of both helium and neon for olivine separates from a drill core at an altitude of 2330 m in oceanite at piton de la fournaise volcano we show for the first time that cosmogenic 21 ne like cosmogenic 3 he decreases exponentially with core depth we obtain an attenuation length of 165 6 g cm 2 1 for cosmogenic 21 ne identical to the value found previously for cosmogenic 3 he alone 1 for each depth in the core the measured amount of cosmogenic 21 ne is about 13 lower than predicted based on the high precision k ar age of 65 200 2000 yrs 1 2 and this result is attributed to erosion assuming a constant erosion rate we derive a value of 3 5 1 7 mm yr 1 from the 21 ne data the same order of magnitude as the value obtained from cosmogenic 3 he at haleakala 1 3 this work highlights the important potential of both cosmogenic helium and neon for measuring in situ exposure ages and erosion rates

geomagnetic field control of 14c production over the last 80 ky implications for the radiocarbon time scale we have used a recent estimate of the geomagnetic dipole field strength over the last 80 ky to examine the extent to which changes in geomagnetic field intensity have affected the radiocarbon time scale the magnetic record was converted into a 14c production signal which was then used to produce a synthetic atmospheric 14c record using a two box model for the carbon cycle this calibration suggests that the geomagnetic field has been the main factor governing the production of cosmogenic 14c atoms over the period of time considered and that 14c changes due to other causes such as climatic changes have been of lesser importance the results indicate that the radiocarbon ages are 2 to 3 ky younger than true age in the interval 18 40 ky b p in good agreement with recent u th and 14c age comparisons moreover the results predict that a negligible difference between true and 14c ages should be observed around 45 50 ky

the production rate of cosmogenic 38ar from calcium in terrestrial pyroxene abstract pyroxene separated from dolerite surfaces which were exposed to cosmic ray irradiation for millions of years in the antarctic dry valleys contains considerable amounts of cosmogenic 38ar as evident from 38ar 36ar ratios up to 0 2887 air 0 1880 in five out of eight mineral separates the concentrations of k ti and cl are low enough to enable us to deduce the terrestrial production rate of 38ar from ca the remaining relevant target element by relating cosmogenic 38ar concentrations to 3he and 21ne exposure ages correlations of cosmogenic 38ar normalized to ca to total 3he and to cosmogenic 21ne normalized to mg respectively are excellent however various currently used methods to derive the 3he and 21ne production rates in pyroxene lack consistency depending on the method chosen the 38ar production rate derived here ranges between 191 21 atoms g ca 1 a 1 and 254 28 atoms g ca 1 a 1 at sea level standard atmospheric pressure and high latitude a more accurate value may be calculated from our data as soon as the he and ne production rates from pyroxene are known more reliably our production rate determination is largely independent of past geomagnetic field variations and of the scaling method used

cosmogenic 3he production rates from holocene lava flows in iceland abstract we measured cosmogenic 3 he production rates in olivine phenocrysts from four radiocarbon dated postglacial basaltic lava flows in the western volcanic zone of iceland these measurements provide important new calibrations of cosmogenic nuclide production rates near sea level at high latitudes calibration sites from lambahraun 4040 250 cal yr bp n 4 leitahraun 5210 110 cal yr bp n 5 burfellshraun 8060 120 cal yr bp n 6 and ingvallahraun 10 330 80 cal yr bp n 4 yield a mean production rate of 132 5 atoms 3 he g 1 yr 1 1 normalized to sea level at high latitudes with the standard atmosphere mean production rates from the four flows agree within uncertainty indicating that these calibrations establish a reproducible local 3 he production rate that will significantly increase the accuracy of exposure dating in iceland the 3 he production rate in iceland is 17 higher than the mean of normalized holocene values previously determined in the western usa the high production rates in iceland are attributed to the influence of persistent low atmospheric pressure over iceland the icelandic low through the holocene which yielded higher cosmic ray fluxes the icelandic calibrations thus support previous suggestions that cosmogenic isotope production rates should be adjusted for regional variations in long term atmospheric pressure by extending the latitudinal extent of previous calibration studies these results are also useful for evaluating scaling models

cosmogenic chlorine 36 from calcium spallation calcium is a major target element for cosmogenic 36cl production consequently 36cl rapidly reaches detectable levels in minerals such as calcite and calcium feldspar exposed at the earth s surface spallation of calcium isotopes typically accounts for 80 90 of 36cl production in these minerals with subsidiary contributions from negative muon capture by 40ca and thermal neutron capture by 35cl to provide a basis for surface exposure dating we have calibrated cosmogenic 36cl production in calcium feldspar from the 17 300 year old tabernacle hill basalt at an altitude of 1445 m and an effective geomagnetic latitude of 40 9 the calcium spallation rate is 152 11 atoms g ca 1 a 1 the corresponding rate at sea level and high latitude is estimated at 48 8 3 4 atoms g ca 1 a 1 the muon capture rate used to derive these values is 8 8 2 2 atoms g ca 1 a 1 at the tabernacle hill site scaled from a value of 4 8 1 2 atoms g ca 1 a 1 at sea level and high latitude the calcium spallation rate determined in this study is in excellent agreement with previous whole rock calibration measurements at tabernacle hill when these are recalculated with respect to the absolute timescale the calibration of 36c1 production from calcium underpins development of an exposure dating technique for calcite due to its high calcium content the 36cl production rate in calcite is higher than in any other common rock forming mineral measurement of 36cl in calcite with an accelerator mass spectrometric detection limit of 5 103 atoms per gram allows dating of limestone surfaces exposed for periods ranging from 102 106 years alternatively erosion rates from less than 1 to greater than 1000 m per year can be determined in the case of eroding karst surfaces though the 36cl production rate is lower in calcium feldspar than in calcite measurements on this mineral will provide a useful means of dating young basalt lavas