controls on the 87sr 86sr composition of seawater from the middle eocene to oligocene hole 689b maud rise antarctica a 87sr 86sr isotope curve of the middle eocene to oligocene was produced from analysis of foraminifera in ocean drilling program hole 689b maud rise near the coast of antarctica sediments from the hole are well preserved with no evidence of diagenetic alteration the sequence is nearly complete from 46 3 to 24 8 ma with an average sampling interval of 166 kyr excellent magnetostratigraphy in hole 689b allows calibration to the geomagnetic polarity time scale of cande and kent 1992 marine strontium isotopic ratios were nearly stable from 46 3 to 35 5 ma averaging near 0 70773 after which they began to increase a slow increase began after 40 4 ma rising at a rate of only about 8 10 6 m y from base values of 0 707707 from 35 5 ma to 24 8 ma the average slope increased to 40 10 6 m y the slope remained constant at least until 24 8 ma when the record becomes discontinuous owing to unconformities we evaluate several possible controls on the marine strontium isotope curve that could have led to the observed growth in 87sr 86sr ratios near the eocene oligocene boundary three mechanisms are considered including the onset of antarctic glaciation increased mountain building in the himalayan tibetan region and decreased hydrothermal activity none of the mechanisms alone seems to adequately explain the increased 87sr 86sr ratios during the oligocene glaciation as a weathering agent was too episodic and probably began too late to explain the upturn in marine 87sr 86sr ratios there is evidence that uplift in the himalayan tibetan region began in the miocene much too late to control oligocene strontium isotope ratios lastly hydrothermal flux changes since the eocene were apparently not great enough alone to account for the rise in marine 87sr 86sr ratios we suggest that a combination of causes such as decreased hydrothermal activity perhaps followed by increased glaciation and mountain building might best explain the growth of the marine 87sr 86sr curve during the oligocene

surface area control of shallow cratonic to deep marine carbonate accumulation sites of calcium magnesium carbonate accumulation have been shifting from shallow shelf to deep marine settings since the late cretaceous during this time total area of flooded epicratonic shelf has decreased about 50 and areas of shelf carbonate deposition have declined from 27 106 km to present areas of 1 4 106 km while the maximum extent of platform carbonate accumulation has decreased from about 50 to 28 north latitude in terms of cratonic carbonate this decrease in tropical shelf area has resulted in a sixfold reduction in shoal water limestone accumulation from a cretaceous rate of 0 6 km m y to a present value of about 0 1 106 km m y over the same time period the global carbonate compensation depth has deepened by about 1 5 km attendant with a threefold areal increase of calcite ooze depositionn 40 to 130 106 km this expansion has resulted in an eightfold increase in pelagic limestone accumulation from a cretaceous rate of 0 05 106 km m y to a present rate of about 0 4 106 km m y transfer of accumulation from shallow cratonic to deep marine depositional settings records the central role of sedimentary environment surface area in predicating the differential partitioning of finite carbonate fluxes to phanerozoic global oceans

geochemical signature of the cenomanian to eocene rocks in israel a palaeoenvironmental indicator distinctive trace metal concentrations characterize cenomanian to eocene marine carbonates from israel the cenomanian turonian platform carbonates including clayey formations exhibit low average values ranging between 2 and 29 ppm for zn cr v ni cu u and co the santonian campanian and early to middle eocene marine chalks show higher average concentrations of these trace metals ranging between 3 and 56 ppm the highest average concentrations of these metals 5 118 ppm are found in the maastrichtian and in the palaeocene marine chalks and marls the possible relationship between these metal background levels and the lithology the biogenic productivity the organic matter content the iron oxide concentration the rate of sedimentation of the studied time rock units as well as the palaeogeographical changes are discussed the extent of the exposed palaeo landmasses due to tectonics the intensity of weathering conditions and the detritus supply into the basin control primarily the iron and trace metal content in the studied sediments

retrodiction of secular variations in deep sea caco 3 burial during the cenozoic abstract deep sea sediments record changes in oceanic carbonate chemistry and caco 3 sedimentation rate through temporal variations in the total burial of caco 3 and the position of the carbonate snowline i e the ocean depth at which caco 3 free sediments are first recorded this paper links mathematically secular changes in snowline to those in the burial rate through a set of relatively simple equations when the available cenozoic deep sea burial records are employed to predict secular variations in snowline the process fails at some time in the past indicating that these records are not consistent with each other the burial records are more likely the source of this problem as they involve far more uncertainties than the snowline records as a consequence we introduce a method for estimating carbonate burial through the use of a canonical caco 3 depth profile which can respond dynamically to secular changes in carbonate sedimentation and the positions of both the snowline and the carbonate saturation horizon the resulting synthetic caco 3 burial record is consistent with snowline records and indicates that the burial rates offered by davies and worsley 1981 are generally too high with highly questionable maxima at 25 and 47 ma bp our estimates of burial are more consistent with the range advanced by mackenzie and morse 1992 nevertheless our results differ from the latter with respect to timing and magnitude of the variations our approach allows simultaneous calculation of the mean carbonate ion concentration of the deep sea we find that carbonate ion levels fell through the cenozoic and are similar to those calculated by tyrrell and zeebe 2004 using a different model secular variations in caco 3 burial are found to be primarily driven by changes in the ca 2 co 3 2 ion product within the bottom waters with an increase in the sedimentation rate of caco 3 of secondary importance over the cenozoic

pacific ocean and cenozoic evolution of climate 1 the pacific ocean has played a major role in climate evolution throughout the cenozoic 65 0 ma it is a fundamental component of global heat transport and circulation the dominant locus of primary productivity and consequently the largest reservoir for carbon exchange between the oceans and the atmosphere a satisfactory understanding of the cenozoic evolutionary history of the pacific and its impact on global climate is currently data limited nevertheless the large dynamic range of cenozoic conditions sets the stage to greatly expand our understanding of global climate and biogeochemical cycles past earth experiments are particularly useful to understand interactions between climate and geosystems under different greenhouse gas loads we highlight in this review four important problems in which the pacific played a major role the effect of changing geographic boundary conditions on ocean circulation interactions between the carbon cycle and climate the pacific ocean s influence on north american climate and its water cycle and the gradual evolution of climate systems

phosphorus in foraminiferal sediments from north atlantic ridge cores and in pure limestones samples of foraminiferal ooze from two north atlantic cores were cleaned and progressively dissolved most of the p fe and mn was released during the first of two reductive cleaning steps most of the remainder of these elements and most of the ca were released during the final acid dissolution step the p in these samples is present largely in fe and mn rich coatings not as a constituent of the foraminiferal shells themselves our results are consistent with those of earlier studies the concentration of p in carbonate oozes in which it is clearly associated with coatings is similar to that of modern calcareous sediments in general and with that of reasonably pure limestones of all ages phosphorus is apparently associated with fe mn oxide coatings in many carbonate sediments the rate of removal of p from the oceans as a constituent of such sediments depends on the rate of formation of fe mn coatings not on the rate of incorporation of p into calcium carbonate

lithium in foraminiferal shells implications for high temperature hydrothermal circulation fluxes and oceanic crustal generation rates the lithium content of planktonic foraminiferal calcite has been determined to evaluate temporal variability of seawater li concentrations over the past 116 m y mean foraminiferal calcite lithium calcium in each time interval is no more than 16 greater nor 25 less than the mean li ca of all samples li ca minima are observed for samples from 50 60 m y and 80 90 m y with li ca about 25 lower than in adjacent time intervals at no time during the past 40 m y does mean li ca appear to be higher than that at present subject to the limitations imposed by sample coverage and diagenesis a similar conclusion holds for the past 116 m y coupled with an oceanic mass balance model for li these data suggest that 1 oceanic li concentrations and therefore high temperature hydrothermal circulation fluxes during the past 40 m y and perhaps the past 100 m y have not been more than perhaps 30 40 greater than at present for intervals any longer than a million years at most and 2 these fluxes werenot a factor of two higher 100 m y ago by inference variations in oceanic crustal generation rates over these time periods are similarly limited decreases in hydrothermal circulation fluxes and crustal generation rates or fluctuations up to 20 in these rates of a few million years duration arenot necessarily ruled out by the li ca data the lack of variability in li ca over time is not unequivocal evidence that hydrothermal fluxes have not varied because the rates of removal processes may be linked to changes in input fluxes

global chemical erosion during the last glacial maximum and the present sensitivity to changes in lithology and hydrology geographically based calculations for 18 000 years ago 18 ka and today were made to examine the potential effects on terrestrial chemical erosion of changes in lithology and hydrology on glacial interglacial timescales runoff fields were derived from general circulation model predictions of precipitation minus evaporation then global lithologic maps were prepared so that empirical relationships for runoff versus bicarbonate flux for different rock types could be used to calculate global chemical erosion rates assuming that significant chemical erosion does not occur beneath ice sheets we find that chemical erosion in ice free areas at 18 ka was only slightly greater than today this result arises because the amount of land covered by ice sheets is roughly compensated for by exposed shelf areas and because there is little difference in global runoff the small 20 increase in the global chemical erosion rate during glacial conditions is due to exposure on the shelves of a relatively high proportion of carbonates which weather faster than average data on glacial interglacial movements of the calcite compensation depth ccd are inconclusive but seem to indicate little change whereas even a 20 increase in the riverine bicarbonate flux should cause an observable deepening if the ccd did not indeed change then our predicted increase in the bicarbonate flux from land during glaciations would have to be accommodated by other means such as increased carbonate productivity about half of the observed decrease in atmospheric pco2 at 18 ka could be explained if increased silicate chemical erosion accompanied increased total chemical erosion in ice free areas consideration of the maximum possible effect of meltwater at ice margins leads to an 80 increase in the global chemical erosion rate at 18 ka such an increase would lead to a larger and faster drop in atmospheric pco2 but also to an excessive deepening of the global ccd this scenario seems unrealistic our results are contrary to interpretations of a much higher silicate chemical erosion rate during glaciations based on recently published records of quaternary marine sr isotopic ratios therefore if these records are indeed globally representative and entirely due to increased silicate chemical erosion as opposed to changes in source areas or hydrothermal inputs then other factors such as a shift in the global weathering regime which we do not explicitly consider here may be involved sensitivity tests show that considering spatial heterogeneities in lithology and runoff leads to lower predictions of global chemical erosion rates than what one would obtain by considering only global averages in a non spatially resolved calculation

from source to surface tracking magmatic boron and chlorine input into the geothermal systems of the taupo volcanic zone new zealand the magmatic contribution into geothermal fluids in the central taupo volcanic zone tvz new zealand has been attributed to either andesitic arc type fluids or rhyolitic rift type fluids to explain the compositional diversity of discharge waters however this model relies on outdated assumptions related to geochemical trends associated with the magma at depth of typical arc to back arc settings current tectonic models have shown that the tvz is situated within a rifting arc and hosts magmatic systems dominated by distinct rhyolite types that are likely to have evolved under different conditions than the subordinate andesites therefore a new appraisal of the existing models is required to further understand the origin of the spatial compositional diversity observed in the geothermal fluids and its relationship to the structural setting here we use volatile concentrations i e h2o cl b from rhyolitic and andesitic mineral hosted melt inclusions to evaluate the magmatic contribution to the tvz geothermal systems the andesite and two different types of rhyolites r1 and r2 are each distinct in cl h2o and b cl which will affect volatile solubility and phase separation vapor vs hydrosaline liquid of the exsolved volatile phase ultimately these key differences in the magmatic volatile constituents will play a significant role in governing the concentration of cl discharged into geothermal systems we estimate bulk fluid compositions b and cl in equilibrium with the different melt types to show the potential contribution of parent fluids to the geothermal systems throughout the tvz the results of this analysis show that the variability in fluid compositions partly reflects degassing from previously unaccounted for distinct magma source compositions we suggest the geothermal systems that appear to have an arc type andesitic fluid contribution are actually derived from a rhyolite melt in equilibrium with a highly crystalline andesite magma this model is in better agreement with the current understanding of magma petrogenesis in the central tvz and its atypical rifted arc tectonic setting and show that the central tvz records an arc not back arc fluid signature

87sr 86sr ratios in hydrothermal waters and deposits from the east pacific rise at 21 n abstract 87 sr 86 sr ratios of three hydrothermal waters collected on the east pacific rise at 21 n define a mixing line between seawater and a hydrothermal end member at 0 7030 which is derived by seawater basalt interaction at ca 350 c and water rock ratio of about 1 5 sr concentrations are not affected in the process while mg uptake from seawater is almost complete up to2 3 of this hydrothermal component is involved in anhydrite precipitation while the sr isotopic ratio in sulfides chalcopyrite sphalerite cannot be distinguished from that of sulfate it is estimated that ca 1 10 10 moles of strontium are yearly cycled in the hydrothermal systems of mid oceanic ridges thereby affecting the 87 sr 86 sr budget of seawater mass balance between river runoff limestone precipitation and ridge basalt alteration suggests that the 87 sr 86 sr ratios of the river runoff are in the range 0 7097 0 7113 and are largely dominated by limestone alteration

hydrothermal water rock interaction and the redistribution of li b and cl in the taupo volcanic zone new zealand abstract lithium b and cl occur as trace elements in the dominant protoliths rhyolite andesite and greywacke underlying high temperature hydrothermal systems in the taupo volcanic zone tvz the fluid mobility of li decreases substantially at 200 c 20 c with the pervasive deposition of quartz chlorite regardless of protolith and altering fluid compositions boron and cl are more fluid mobile than li at all temperatures with mobility affected by the rock type composition of circulating aqueous solutions and to a limited extent by incorporation of the trace elements in hydrothermal minerals uptake of b by clays decreases its mobility at 320 c occur compared to volcanic rocks the mobility of b and cl in greywackes is very low and thus the b cl signature of greywacke in hydrothermal aqueous solutions of the tvz is overwhelmed by the effects of rhyolite and andesite dissolution temperature dependent transitions from mg fe clays and phyllosilicates to k bearing illitic clays tend to decrease the mg li ratios in aqueous solutions at 200 c

a comparison of early paleogene export productivity and organic carbon burial flux for maud rise weddell sea and kerguelen plateau south indian ocean 1 marine biological productivity has been invoked as a possible climate driver during the early paleogene through its potential influence on atmospheric carbon dioxide concentrations however the relationship of export productivity the flux of organic carbon c from the surface ocean to the deep ocean to organic c burial flux the flux of organic c from the deep ocean that is buried in marine sediments is not well understood we examine the various components involved with atmosphere to ocean c transfer by reconstructing early paleogene carbonate and silica production using carbonate and silica mass accumulation rates mars export productivity using biogenic barium bio ba mars organic c burial flux using reactive phosphorus p mars redox conditions using uranium and manganese contents and the fraction of organic c buried relative to export productivity using reactive p to bio ba ratios our investigations concentrate on paleocene eocene sections of sites 689 690 from maud rise and site 738 from kerguelen plateau in both regions export productivity organic c burial flux and the fraction of organic c buried relative to export productivity decreased from the paleocene early eocene to the middle eocene a shift is indicated from an early paleogene two gyre circulation in which nutrients were not efficiently recycled to the surface via upwelling in these regions to a circulation more like the present day with efficient recycling of nutrients to the surface ocean export productivity was enhanced for kerguelen plateau relative to maud rise throughout the early paleogene possibly due to internal waves generated by the plateau regardless of gyre circulation

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

paleogene stable isotope events the oxygen isotope record in paleogene benthic foraminifera shows that at the base of the paleogene the ocean deep waters had a temperature of about 10 c rising to about 12 c at the base of the eocene and cooling between 51 ma and 49 ma to about 9 c the most dramatic event occurred just after the eocene oligocene boundary at about 35 8 ma when ocean deep waters cooled by several degrees within 104 105 yr probably in association with temporary glaciation in the antarctic region another more intense glacial event in antarctica may have occurred later in the oligocene at about 31 ma and a third near the top of the oligocene at 24 ma in the marine carbon isotope record a very rapid negative excursion occurred precisely at the cretaceous tertiary boundary a recovery to unusually positive values in the late paleocene was followed by a second negative excursion close to the paleocene eocene boundary that was even more extreme in magnitude although it was not as rapid these major carbon isotope events permit very accurate stratigraphic correlation there are many other smaller features in the carbon isotope record that will also prove useful for this purpose

enhanced cenozoic chemical weathering and the subduction of pelagic carbonate the observed trend of increasing oceanic 87sr 86sr ratios during the late cenozoic led raymo et al 1 to propose that chemical weathering rates increased at this time as a result of enhanced weatherability of silicate rocks they suggested that this was due in turn to continential uplift primarily in the himalayas and the andes because weathering involves the reaction of silicates with atmospheric carbon dioxide considerations of changes in weathering rates must take into account the need to balance the global carbon cycle to maintain this balance on timescales greater than 106yr enhanced weathering requires an increased flux of co2 into the atmosphere2 without such an increased flux weathering rates could not increase and one is then forced to search for other explanations for the observed 87sr 86sr trend such as a changing riverine strontium isotope composition3 6 here i assume that the strontium isotope record does indeed reflect enhanced weathering in the late cenozoic but propose that its cause may have been an increased co2 flux arising from the recycling of pelagic sedimentary carbon at subduction zones since the jurassic sedimentary carbonate accumulation has shifted from primarily cratonic to primarily pelagic environments7 11 because sea floor spreading transports pelagic carbonate to and recycles it through metamorphic environments in subduction zones this shift to pelagic carbonate accumulation would provide the increased co2 flux needed to increase weathering rates12 13

possible evidence for a large decrease in seawater strontium calcium ratios and strontium concentrations during the cenozoic article i nfo few constraints exist on the major element chemistry of ancient oceans turritellid marine snails precipitate aragonitic shells and are abundant in the cenozoic fossil record and therefore may be ideal for reconstructing past seawater chemistry here we report strontium to calcium sr ca ratios of modern and early cenozoic turritellid shells that still retain an aragonitic mineralogy by applying partition coefficients determined from modern specimens to data for paleocene and eocene shells we calculate paleo seawater sr ca values the high sr ca values recorded in fossil shells may imply that seawater sr ca ratios varied from at least 9 5 15 2 mmol mol much higher than modern average seawater values 8 6 mmol mol high paleo seawater sr ca ratios may indicate that substantial changes in the biogeochemical cycling of sr have occurred over the past 37 million years a decrease in seawater strontium concentrations could have arisen from an increase in the proportion of weathered radiogenic silicate rocks relative to carbonates reducing the riverine flux of strontium to the oceans or changes in the exposure and erosion of aragonitic carbonates on tropical shelves due to variations in sea level

an apparent contradiction in the role of phosphorus in cenozoic chemical mass balances for the world ocean little is known about the fluxes to and from the ocean during the cenozoic of phosphorus p a limiting nutrient for oceanic primary productivity and organic carbon burial on geologic timescales previous studies have concluded that dissolved river fluxes increased worldwide during the cenozoic and that organic carbon burial decreased relative to calcium carbonate burial and perhaps in absolute terms as well to examine the apparent contradiction between increased river fluxes of p assuming p fluxes behave like the others expected to drive increased organic carbon burial and observations indicating decreased organic carbon burial we determined p accumulation rates for equatorial pacific sediments from ocean drilling program leg 138 sites in the eastern equatorial pacific and leg 130 sites on the ontong java plateau in the western equatorial pacific although there are site specific and depth dependent effects on p accumulation rates there are important features common to the records at all sites p accumulation rates declined from 50 to 20 ma showed some variability from 20 to 10 ma and had a substantial peak from 9 to 3 ma centered at 5 6 ma these changes in p accumulation rates for the equatorial pacific are equivalent to substantial changes in the p mass balance however the pattern resembles neither that of weathering flux indicators 87sr 86sr and ge si ratios nor that of the carbon isotope record reflecting changes in organic carbon burial rates although these p accumulation rate patterns need confirmation from other regions with sediment burial significant in global mass balances e g the north pacific and southern ocean it appears that p weathering inputs to the ocean are decoupled from those of other elements and that further exploration is needed of the relationship between p burial and net organic carbon burial

degassing of the h2o rich rhyolites of the okataina volcanic center taupo volcanic zone new zealand the taupo volcanic zone tvz new zealand is the most active region of rhyolitic volcanism on earth with 50 rhyolitic eruptions and 780 km3 of magma erupted in the past 60 k y here we use analyses of quartz hosted melt inclusions from eight eruptions in the okataina volcanic center ovc of the tvz to constrain magmatic volatile contents pressures and temperatures of quartz crystallization and degassing of h2o cl and minor co2 from the rhyolitic magmas the ovc melt inclusions trapped volatile rich melts with 6 wt h2o and 0 27 wt cl our data indicate that vapor saturated crystallization of quartz occurred at low temperatures 760 805 c over a wide range of pressures 50 200 mpa for some eruptions variations in volatiles and major and trace elements provide evidence for simultaneous crystallization and partial loss of h2o cl and co2 into a vapor phase using the combination of melt inclusion and pumice glass volatile contents we calculate minimum volatile emissions of 3 1011 to 8 1012 kg h2o and 7 109 to 4 1010 kg cl during the ovc eruptions we estimate that emissions from the smaller volume 100 of the global arc h2o flux and as much as 90 of the global arc cl flux these results underscore the importance of individual magmatic provinces in creating large temporal variations in global arc volatile fluxes to earth s hydrosphere

δ13c values of miocene pacific benthic foraminifera correlations with sea level and biological productivity 13 c 12 c ratios of miocene benthic foraminifera from 22 pacific ocean sites vary with time but are similar at almost all sites in any restricted interval 13 c values are correlated with sea levels inferred from onlap offlap curves reflecting the deposition of greater amounts of organic matter on the continental shelves during transgressions differences in 13 c between sites are correlated with local differences in biological productivity in the overlying surface waters 13 c 12 c values of benthic foraminifera show promise as indicators of marine paleoproductivity

the marine phosphorus cycle phosphorus introduced to the ocean must ultimately be removed if a steady state is to be maintained over geologic time aeolian input is insignificant and there is evidently no hydrothermal contribution rivers dominate p input to the ocean the fluvial p flux is poorly quantified due to the interactions of two factors 1 man s influence on river nutrient fluxes and 2 inherent complexities in understanding the biological and inorganic associations of p in estuarine and coastal waters our best estimate of the natural or pre man p input flux is about 10 x 10 sup 9 moles p cm sup 2 ocean surface year of the identified flux of phosphorus to the sediments about 90 percent is in the form of biogenic debris or its regeneration products the burial of phosphatic fish debris is negligible p removal by direct reaction of phosphate in seawater with sea floor basalts during mid ocean ridge hydrothermal circulation is insignificant p removal by processes such as sorption on clay surfaces or formation of authigenic fe and al phosphate phases is presently unquantified and remains a serious uncertainty in the phosphorus cycle the identified sources and sinks approximately balance yielding a residence time for p in the oceans of about 10 sup 5 yrs

hornblende cummingtonite and biotite hydrogen isotopes direct evidence of slab derived fluid flux in silicic magmas of the taupo volcanic zone new zealand abstract hornblende cummingtonite and biotite d values from rhyolitic and mingled mixed mafic rocks from the central taupo volcanic zone were determined in an effort to characterize the isotopic composition of the dissolved fluid of magmas in this predominantly rhyolitic volcanic zone hand samples were obtained from pyroclastic flow airfall and lava flow deposits as well as co eruptive cognate plutonic blocks mineral d values range from 88 to 35 smow standard mean ocean water cummingtonite phenocryst d overlaps with hornblende from the same rocks while biotite values are consistently 7 more negative than hornblende phenocrysts from the same hand sample calculated volcanic steam d values ca 46 to 25 are consistent with an island arc crustal source ca 40 to 15 with a negligible to absent mantle ca 100 to 60 component earlier researchers suggested that the taupo volcanic zone fluid flux included dilute concentrations of slab derived phases i e co 2 cl similar to back arc volcanic systems at other convergent margins however the island arc type fluid composition estimated from the minerals we studied is similar to hornblende and biotite compositions from pyroclastic deposits and volcanic fumaroles from the kamchatka and japanese arcs the silicic rocks from the 2 0 1 0 ma mangakino caldera the most inland caldera system we studied have the lowest d hornblende and biotite suggesting that silicic volcanism distal to the subduction trench was derived from a back arc like i e mixture of mantle and slab fluid however we found that the currently active more proximal locus of silicic volcanism reflects typical arc like fluid compositions our results further suggest that the composition of volcanic steam migrating through the upper crust was similar in composition to regional meteoric waters ca 45 to 23

a chemical and isotopic study of the tokaanu waihi geothermal area new zealand abstract the tokaanu waihi geothermal area lies on the northeast flanks of the tongariro volcanic centre an andesitic massif which forms part of the taupo volcanic zone arc subduction system geothermal activity consists of hot springs pools a geyser and an area of steaming ground fumaroles and acidic springs at tokaanu the near neutral ph boiling waters are notable for a high mineral content 3000 mg kg cl relative to other taupo volcanic zone geothermal systems a high b cl ratio and high li alkali ratio the waihi waters and bore waters are lower in mineral content but higher in hco3 so4 mg and ca and represent mixing of deep hot waters with steam heated groundwaters and dilution with cold surface water the sulphate water geothermometer indicates that the deep hot water is at 250 c higher temperatures 350 400 c are indicated for greater depths by the h2s so4 s isotope ratios and the ch4 co2 carbon isotope measurements a minimum residence time of 0 25 ma for co2 and ch4 below the reservoir is proposed but h2s and so4 in the reservoir cannot have coexisted for more than 200 a isotopic analyses of the tokaanu hot spring waters show a significant 180 shift up to 4 from the local meteoric waters some samples show further 180 d and cl enrichment as a result of steam loss at depth and surface evaporation

geothermal waters from the taupo volcanic zone new zealand li b and sr isotopes characterization in this study we report chemical and isotope data for 23 geothermal water samples collected in new zealand within the taupo volcanic zone tvz we analyzed major and trace elements including li b and sr and their isotopic compositions delta7li delta11b 87sr 86sr in high temperature geothermal waters collected from deep boreholes in different geothermal fields ohaaki wairakei mokai kawerau and rotokawa geothermal systems lithium concentrations are high from 4 5 to 19 9 mg l and lithium isotopic compositions delta7li are very homogeneous being comprised between 0 5 and 1 4 in particular it is noteworthy that except the samples from the kawerau geothermal field having slightly higher delta7li values 1 4 the other geothermal waters have a very constant delta7li signature around a mean value of 0 0 6 2s n 21 boron concentrations are also high and relatively homogeneous for the geothermal samples falling between 17 5 and 82 1 mg l boron isotopic compositions delta11b are all negative and display a range between 6 7 and 1 9 these b isotope compositions are in agreement with those of the ngawha geothermal field in new zealand li and b isotope signatures are in a good agreement with a fluid signature mainly derived from water rock interactions involving magmatic rocks with no evidence of seawater input on the other hand strontium concentrations are lower and more heterogeneous and fall between 2 and 165 g l 87sr 86sr ratios range from 0 70549 to 0 70961 these sr isotope compositions overlap those of the rotorua geothermal field in new zealand also confirming that some geothermal waters with more radiogenic strontium have interacted with bedrocks from the metasedimentary basement each of these isotope systems on their own reveals important information about particular aspects of either water source or water rock interaction processes but considered together provide a more integrated understanding of the geothermal systems from the tvz in new zealand

phase equilibrium geobarometers for silicic rocks based on rhyolite melts part 2 application to taupo volcanic zone rhyolites constraining the pressure of crystallisation of large silicic magma bodies gives important insight into the depth and vertical extent of magmatic plumbing systems however it is notably difficult to constrain pressure at the level of detail necessary to understand shallow magmatic systems in this study we use the recently developed rhyolite melts geobarometer to constrain the crystallisation pressures of rhyolites from the taupo volcanic zone tvz as sanidine is absent from the studied deposits we calculate the pressures at which quartz and feldspar are found to be in equilibrium with melt now preserved as glass the quartz 1 feldspar constraint of gualda and ghiorso contrib mineral petrol 168 1033 doi 10 1007 s00410 014 1033 3 2014 we use glass compositions matrix glass and melt inclusions from seven eruptive deposits dated between 320 and 0 7 ka from four distinct calderas in the central tvz and we discuss advantages and limitations of the rhyolite melts geobarometer in comparison with other geobarometers applied to the same eruptive deposits overall there is good agreement with other pressure estimates from the literature amphibole geobarometry and h2o co2 solubility models one of the main advantages of this new geobarometer is that it can be applied to both matrix glass and melt inclusions regardless of volatile saturation the examples presented also emphasise the utility of this method to filter out spurious glass compositions pressure estimates obtained with the new rhyolite melts geobarometer range between 250 to 50 mpa with a large majority at 100 mpa these results confirm that the tvz hosts some of the shallowest rhyolitic magma bodies on the planet resulting from the extensional tectonic regime and thinning of the crust distinct populations with different equilibration pressures are also recognised which is consistent with the idea that multiple batches of eruptible magma can be present in the crust at the same time and can be tapped simultaneously by large eruptive events

two competing effects of volatiles on heat transfer in crystal rich magmas thermal insulation vs defrosting the build up and eruption of crystal rich ignimbrites commonly referred to as monotonous intermediates has been attributed to the incremental addition of new magma batches and the episodic partial melting and reactivation of rheologically locked crystal rich magma bodies mushes in this study we explore the role of volatiles exsolved from a hot intrusion underplating a crystal mush on the thermal evolution of the coupled mush intrusion system we solve the enthalpy conservation equation for the mush and the intrusion and investigate the exsolution of volatiles from the intrusion and their transport through the mush in one dimension our calculations span a range of pressures from 1 to 4 kbar mush composition andesite to rhyolite and initial water contents from 1 5 to 6 wt the mobility of volatiles in the mush is controlled by the volume fraction of the pore space they occupy and as a consequence by the amount of melting and the injection rate of volatiles from the intrusion to the mush we find that volatiles affect the melting of the mush or defrosting in two opposite ways depending on pressure and the initial water content of the intrusion when the intrusion volatile content is high and pressure relatively low 44 wt h2o at 2 kbar the mass transfer of volatiles from the intrusion to the mush carries enthalpy beyond the melting front and can thus enhance defrosting and possibly remobilization of the mush for lower initial volatile contents 54 wt h2o in the intrusion and or higher pressure 3 4 kbar volatiles stall at the interface between the two magma bodies and prevent defrosting as they thermally insulate the mush from the intrusion we propose that the dual role played by volatiles during the thermal evolution of crystal rich magmas reheated by an underplating intrusion can explain the presence of crystal rich ignimbrites in arcs and their absence in drier hotspots or extensional regions

high resolution stratigraphy with strontium isotopes the isotopic ratio of strontium 87 to strontium 86 shows no detectable variation in present day ocean water but changes slowly over millions of years the strontium contained in carbonate shells of marine organisms records the ratio of strontium 87 to strontium 86 of the oceans at the time that the shells form sedimentary rocks composed of accumulated fossil carbonate shells can be dated and correlated with the use of high precision measurements of the ratio of strontium 87 to strontium 86 with a resolution that is similar to that of other techniques used in age correlation this method may prove valuable for many geological paleontological paleooceanographic and geochemical problems

underlying causes for long term global ocean δ13c fluctuations over the last 1 20 myr pleistocene stable carbon isotope 13c records from surface and deep dwelling foraminifera in all major ocean basins show two distinct long term carbon isotope fluctuations since 1 00 ma the first started around 1 00 ma and was characterised by a 0 35 decrease in 13c values until 0 90 ma followed by an increase of 0 60 lasting until 0 50 ma the subsequent fluctuation started with a 0 40 decrease between 0 50 and 0 25 ma followed by an increase of 0 30 between 0 25 and 0 10 ma here we evaluate existing evidence and various hypotheses for these global pleistocene 13c fluctuations and present an interpretation where the fluctuations most likely resulted from concomitant changes in the burial fluxes of organic and inorganic carbon due to ventilation changes and or changes in the production and export ratio our model indicates that to satisfy the long term stability of the pleistocene lysocline the ratio between the amounts of change in the organic and inorganic carbon burial fluxes would have to be close to a 1 1 ratio as deviations from this ratio would lead to sizable variations in the depth of the lysocline it is then apparent that the mid pleistocene climate transition which apart from the glacial cycles represents the most fundamental change in the pleistocene climate was likely not associated with a fundamental change in atmospheric pco2 while recognising that high frequency glacial interglacial cycles are associated with relatively large 100 ppmv changes in pco2 our model scenario with burial changes close to a 1 1 ratio produces a maximum long term variability of only 20 ppmv over the fluctuation between 1 00 and 0 50 ma

lithologic controls on calcite recrystallization in cenozoic deep sea sediments abstract interpretations of calcite strontium calcium records in terms of ocean history and calcite diagenesis require distinguishing the effects on deep sea calcite sediments of changes in ocean chemistry of different mixes of calcite depositing organisms as sediment contributors through time and space and of the loss of sr during diagenetic calcite recrystallization in this paper sr ca and 18 o values of bulk calcium carbonate sediments are used to estimate the relative extent of calcite recrystallization in samples from four time points core tops 5 6 9 4 and 37 1 ma at eight ocean drilling program sites in the equatorial atlantic ceara rise and equatorial pacific ontong java plateau and two eastern equatorial pacific sites the possibility that site to site differences in calcite sr ca at a given time point originated from temporal variations in ocean chemistry was eliminated by careful age control of samples for each time point with sample ages differing by less than the oceanic residence times of sr and ca the sr ca and 18 o values of 5 6 and 9 4 ma samples from the less carbonate rich eastern equatorial pacific sites and ceara rise site 929 appear to be less diagenetically altered than the sr ca and 18 o values of contemporaneous samples from the more carbonate rich sites it is evident from these data that both sr ca and 18 o in bulk calcite have been diagenetically altered in some samples 5 6 ma and older these data indicate that noncarbonate sedimentary components like clay and biogenic silica have partially suppressed recrystallization at the lower carbonate sites sr ca data from the less altered carbonate poor sites indicate higher oceanic sr ca relative to today at 5 6 and 9 4 ma

rapid remobilization of magmatic crystals kept in cold storage we lack thermal histories for magma reservoirs but here the magma under mount hood oregon usa is shown to have been too cold to mobilize for most of the time it has been stored which implies that magma mobilizes at which point it can be imaged geophysically very quickly prior to eruption this paper presents a novel approach to constraining the thermal history of the subsurface magma bodies that feed eruptions by combining temporal information from a variety of sources kari cooper and adam kent combine timescales derived from uranium series disequilibria textural information and trace element zoning in crystals they show that at mount hood oregon only a small fraction of the total storage duration has been spent at temperatures above the critical crystallinity the point at which magma is easily mobilized they conclude that largely liquid bodies that could be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption the processes involved in the formation and storage of magma within the earth s upper crust are of fundamental importance to volcanology many volcanic eruptions including some of the largest result from the eruption of components stored for tens to hundreds of thousands of years before eruption1 2 3 although the physical conditions of magma storage and remobilization are of paramount importance for understanding volcanic processes they remain relatively poorly known4 5 eruptions of crystal rich magma are often suggested to require the mobilization of magma stored at near solidus conditions6 7 8 however accumulation of significant eruptible magma volumes has also been argued to require extended storage of magma at higher temperatures7 8 9 what has been lacking in this debate is clear observational evidence linking the thermal and therefore physical conditions within a magma reservoir to timescales of storage that is thermal histories here we present a method of constraining such thermal histories by combining timescales derived from uranium series disequilibria crystal sizes and trace element zoning in crystals at mount hood oregon usa only a small fraction of the total magma storage duration at most 12 per cent and probably much less than 1 per cent has been spent at temperatures above the critical crystallinity 40 50 per cent at which magma is easily mobilized partial data sets for other volcanoes also suggest that similar conditions of magma storage are widespread and therefore that rapid mobilization of magmas stored at near solidus temperatures is common magma storage at low temperatures indicates that although thermobarometry calculations based on mineral compositions may record the conditions of crystallization they are unlikely to reflect the conditions of most of the time that the magma is stored our results also suggest that largely liquid magma bodies that can be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption

soil column depth dependent seismic site coefficients and hazard maps for the upper mississippi embayment abstract the presence of deep unconsolidated deposits in the upper mississippi embayment ume has an important impact on surface motions and has been the subject of several studies including a study by two of us who integrated probabilistic seismic hazard analysis psha with nonlinear site effects nl this study extends the psha nl procedure to 1 incorporate a finite fault model capable of generating near source motions instead of using point source models only 2 better approximate the range of available attenuation relations for the central and eastern united states ceus 3 examine the hazard at very high levels of shaking and 4 propagate generated rock motions through randomized soil profile properties to achieve a more consistent probabilistic procedure the extended psha nl procedure is used to develop updated depth dependent site coefficients for the ume which generally provides a lower hazard at short periods but a higher hazard at longer periods than current approaches proposed probabilistic seismic hazard maps at selected spectral accelerations that incorporate the proposed site effects are also presented for the ume and are systematically compared with seismic hazard maps proposed by others

the himalayas organic carbon burial and climate in the miocene cooling ages of rock in the himalayas imply that rapid exhumation between the main central thrust system and the south tibetan detachment system occurred between 21 and 17 ma the generation of relief and enhanced weathering which followed this event may have resulted in a pronounced increase in the delivery of dissolved strontium carbon phosphorus and other chemical weathering products to the ocean richter et al 1992 the increased supply of nutrients stimulated productivity in oceanic upwelling zones and expansion of the oxygen minimum zone leading to enhanced burial and preservation of organic matter in the monterey formation and other deposits from this interval a downdraw of atmospheric co2 associated with enhanced chemical weathering rates and organic matter burial may have led to global cooling and the expansion of the antarctic ice sheet by 15 ma the above scenario differs from the monterey hypothesis of vincent and berger in that co2 downdraw is primarily via silicate weathering rather than organic carbon burial and that organic carbon burial is driven by increased delivery of nutrients to the ocean rather than by stronger upwelling a carbon mass balance calculation which assumes that river fluxes have been increasing over the last 40 ma predicts that absolute organic carbon burial increased over this interval while at the same time the fraction of carbon buried as organic matter versus carbonate decreased this implies that the organic carbon cycle has acted as a net source of co2 to the atmosphere over the late cenozoic

water solubility and chlorine partitioning in cl rich granitic systems effects of melt composition at 2 kbar and 800 c abstract experiments have been conducted in the system na2o k2o al2o3 sio2 h2o cl2o 1 at 2 kbar and 800 c to determine the effects of melt composition on the solubility of water in haplogranite melt and on the distribution of cl between aqueous fluid and haplogranite melt the melts studied include peraluminous subaluminous and peralkaline haplogranites and quartz albite and quartz orthoclase compositions the solubility of water at 2 kbar and 800 c in water saturated haplogranite melts varies strongly as a function of melt composition water solubility in melt decreases in the order strongly peralkaline haplogranite quartz albite composition moderately peralkaline haplogranite peraluminous haplogranite subaluminous haplogranite quartz orthoclase composition d cl wt cl in fluid wt cl in melt is a strong function of system composition d cl increases as the concentration of cl in the system increases a comparison of the distribution of cl between haplogranite melts of subaluminous peraluminous and peralkaline compositions and aqueous fluids shows that with similar concentrations of cl in the system d cl is largest for subaluminous melts a nk 1 having relatively high k2o concentrations as the a nk of melt increases or decreases from 1 or as the n nk of the melt increases both d cl and the concentration of cl in the fluid decrease the experimental data imply that complexes involving cl and na in hydrous subaluminous granite melts are dominant furthermore the data imply that in hydrous peraluminous granite melts cl complexes with network modifying a13 and in hydrous peralkaline granite melts cl complexes with network modifying na k on a molar basis cl apparently complexes with network modifying a13 in peraluminous melts as strongly as it does with na and k in peralkaline melts experimental studies of cl partitioning in granitic systems suggest that the strongest enrichment of magmatic hydrothermal fluids in cl and ore metals complexed with cl will most likely occur in fluids exsolved from subaluminous granite magmas that are relatively enriched in k2o sio2 and cl and relatively depleted in f co2 and na2o

molybdenum and boron isotopic compositions of porphyry cu mineralization related adakitic rocks in central eastern china new insights into their petrogenesis and crust mantle interaction

concentrations of arsenic antimony and boron in steam and steam condensate at the geysers california abstract studies at the geysers geothermal field california indicate that under some circumstances elements that are transported in the vapor phase can become enriched in the liquid phase waters from two condensate traps steam traps on steam lines at the geysers are enriched with arsenic antimony and boron compared to the concentrations of these elements in coexisting steam concentrations of boron in condensate trap waters were as high as 160 mg l arsenic as high as 35 mg l and antimony as high as 200 g l enrichment of arsenic antimony and boron is at least partially controlled by the partitioning of these elements into the liquid phase according to their vapor liquid distribution coefficients after they are transported in steam several of the elements that are most soluble in steam including arsenic and antimony are part of the trace element suite that characterizes precious metal epithermal ore deposits

neogene growth of the sedimentary organic carbon reservoir we develop a recycling model using 13c 12c mass balance for net growth loss of the sedimentary organic carbon corg reservoir and apply it to the neogene bulk marine carbonate 13c record the model allows for variations in photosynthetic fractionation factors carbon cycling rates and the isotopic composition of riverine carbon inputs to the oceans the sign of the net flux term is controlled by the difference between fractional corg burial and fractional corg weathering independent of any variations in carbon cycling rate these terms are in turn estimated from the carbon isotope mass balance of newly deposited and weathered sediments respectively the magnitude of the net flux is sensitive to the global carbon cycling erosion deposition rate which may be estimated by the use of the records of radiogenic isotopic variations nd sr in paleoseawater a key observation and input to the model is that photosynthetic carbon isotope fractionation by both marine algae and terrestrial plants has decreased during the cenozoic incorporating time dependent carbon isotope fractionation into the model shows that the sedimentary corg reservoir has grown throughout most of the neogene even as marine 13c values fell after 14 ma a similar result is obtained if the variation in the marine 13c record is largely caused by changes in the carbon isotopic composition of river fluxes to the oceans rather than changes in the organic inorganic ratio of output to the burial sink the growth of the sedimentary organic carbon reservoir requires that the neogene sedimentary carbon cycle was a net source of o2 and a net sink of co2 to the atmosphere at least until the plio pleistocene as a consequence neogene co2 consumption by silicate weathering cannot be balanced by oxidation of sedimentary corg placing a significant constraint on global carbon balance models a related prediction of our model is that atmospheric o2 levels rose during the neogene

exsolution of immiscible vapor and liquid phases from a crystallizing silicate melt implications for chlorine and metal transport abstract the compositional variation of a cl bearing aqueous fluid exsolving from a crystallizing silicate melt is evaluated for the system haplogranitic melt na k c1 h 2 o with attention to the effect of vaporliquid immiscibility on the aqueous fluid above critical pressures for the aqueous fluid p 1 5 kb at 800 c where the fluid is single phase the cl concentration in the aqueous fluid c cl aq gradually decreases with crystallization of a silicate melt under rayleigh fractionation as demonstrated by previous studies however the variation of ca is quite different below critical pressures where vapor liquid immiscibility occurs in particular at low pressures p cl h 2 o ratio in a melt is small and ca will increase with crystallization a high cl h 2 o ratio at low pressure results in liquid exsolution and decreasing ca with crystallization eventually both vapor and liquid phases will exsolve simultaneously regardless of the initial exsolving phase during this simultaneous exsolution the composition of the exsolving vapor and liquid phases the mass ratio of these phases and the cl concentration of the silicate melt will all remain constant comparison of the model and published experimental data indicates that the simultaneous exsolution of vapor and liquid may occur from a normal rhyolite crystallized at pressures below the critical pressure for the aqueous fluid therefore simultaneous exsolution of the immiscible aqueous phases is likely to be a common phenomenon in magmatic hydrothermal systems at conditions for the vapor liquid immiscibility model calculations at 0 6 and 1 2 kb indicate that major amounts of cl in the system will be distributed to the liquid phase particularly at the lower pressure this suggests that the majority of chloridecomplexed metals exsolving from a magma may be transported in the liquid phase under shallow crustal conditions

experimental study of chlorine behavior in hydrous silicic melts chlorine solubility in silicate melts has been investigated at 830 850 plus minus 5c and at pressures ranging from 50 to 200 mpa using both natural pantellerite rhyolite phonolite and synthetic sio sub 2 al sub 2 o sub 3 k sub 2 o na sub 2 o compositions and a stated h sub 2 o nacl kcl fluid phase near 4 molal at 100 mpa cl contents in pantelleritic melts reach a solubility plateau at initial aqueous phase molality near 2 this plateau coincides with a large immiscibility gap between aqueous and chloride rich fluids with the coexisting cl saturated aqueous phase cl ranges from 2 720 plus minus 120 ppm in rhyolite to 8 960 plus minus 85 ppm in pantellerite and reaches 6 270 plus minus 170 ppm in phonolite at 100 mpa between 50 and 200 mpa the cl content in pantelleritic melt decreases from 9 640 plus minus 200 ppm to 5 040 plus minus 150 ppm although cl solubility increases with increasing feo sup in high sio sub 2 melts it is mainly controlled by the al si and na k al molar ratios of the melt with a minimum at na k al 1 in a series of synthetic rhyolitic to pantelleritic melts the experimental results suggest that chlorine occurs as alkali chloride complexes in high sio sub 2 more melts they also indicate that cl is concentrated in the aqueous fluids in equilibrium with sio sub 2 rich melts the exact value of d depending on melt composition and melt chlorine concentration volcanic degassing will create chlorine rich hydrothermal fluids and decrease chlorine melt content less

conditions of melt generation beneath the taupo volcanic zone the influence of heterogeneous mantle inputs on large volume silicic systems many arc silicic igneous provinces exhibit compositional variability defined by oscillation between dry and wet rhyolites the origins of this variability are often uncertain due to the poor constraints on the compositions of the mantle derived inputs to the lower crustal hybridization zones the taupo volcanic zone tvz in new zealand the most productive of modern silicic igneous provinces exhibits variability of rhyolite compositions but small volume coeval basaltic eruptions also occur making it an ideal location to study the mantle contributions to these distinctive types of rhyolite here we present major and trace element data from 12 magmatic centers that reveal that mafic units from the northern portion of the central tvz are enriched in large ion lithophile elements lile in comparison to the southern tvz the results of models quantifying slab derived contributions to the mantle suggest that the observed heterogeneity in lile concentrations and volatile fugacities can be explained by variable amounts of subduction derived fluid within the melting region of the basalts these new data and modeling results provide the first direct evidence that the spatial diversity in the flux of mantle derived basalts and associated volatile elements into the lower crustal differentiation system of the tvz are coincident with wet to dry rhyolite compositional variability

authigenic apatite formation and burial in sediments from non upwelling continental margin environments abstract evidence for precipitation of authigenic carbonate fluorapatite cfa in long island sound and mississippi delta sediments suggests that formation of cfa is not restricted to environments of active coastal upwelling we present porewater data suggestive of cfa formation in both these areas application of a sequential leaching procedure designed specifically to separate authigenic carbonate fluorapatite from other phosphorus containing phases including detrital apatite of igneous or metamorphic origin provides strong supporting evidence for authigenic apatite formation in these sediments the size of the authigenic apatite reservoir increases with depth indicating continued formation of cfa during early diagenesis this depth increase is mirrored by a decrease in solid phase organic p at both sites suggesting that cfa is forming at the expense of organic p mass balance considerations application of diagenetic models to interstitial water nutrient data and the saturation state of the interstitial water are consistent with this interpretation diagenetic redistribution of phosphorus among the different solid phase reservoirs is observed at both sites and results in near perfect retention of p by these sediments over the depth intervals sampled formation of cfa in continental margins which do not conform to the classically defined regions of phosphorite formation renders cfa a quantitatively more important sink than has previously been recognized including this reservoir as a newly identified sink for reactive p in the ocean the residence time of p in the modern ocean must be revised downward the implication for ancient oceans of cfa formation in continental margin sediments other than phosphorites is that phosphorite formation may be less a representation of episodicity in removal of reactive p from the oceans than of localized concentration of cfa in phosphatic sediments by secondary physical processes

carbonate preservation history in the peru basin paleoceanographic implications we studied preservation dissolution cycles and paleoproductivity in eight sediment cores from the peru basin south of the highly productive surface waters of the eastern equatorial pacific stratigraphy is based on stable oxygen isotopes and on combined magnetostratigraphy and biostratigraphy sediment cores which span the last 8 m y were retrieved during cruise 79 with rv sonne close to the carbonate compensation depth ccd in general sediments show pacific type carbonate cycles we interpret a pronounced carbonate peak between 6 and 7 ma as the result of a western and northern extension of the highly productive peru current decreased carbonate contents from the late miocene to the late pliocene might be associated with a slow contraction of the latitudinal extent of the high productivity belt north of the study areas during the pliocene carbonate variations showed 400 kyr cycles indicating the growth and decay of ice sheets which should have been associated with pulsations of the antarctic ice cap an abrupt collapse of the carbonate system occurred at 2 4 ma higher frequency variations of the carbonate record indicate the major increase of the northern hemisphere glaciation during the quaternary carbonate fluxes are high during glacials and low during interglacials large amplitude variations with long broad minima and maxima associated with small migrations of the lysocline and the ccd 200 m are indicative of the preservation dissolution history in the peru basin during the early pleistocene climatic forcing by the 41 kyr obliquity cycle is not observed in the carbonate record during the last 800 kyr variability in the carbonate record was dominated by the 100 kyr eccentricity cycle fluxes of biogenic material calcium carbonate organic carbon opal and barium were greatest during glacials which imply higher productivity and export production of the peru current during cold climatic periods dissolution was greatest during interglacials as inferred from the relatively poor preservation of planktonic foraminifera and from the low accumulation rate of carbonate after the mid brunhes event 400 ka we observe a plateaulike shift to enhanced dissolution and to intensified productivity

variations in the velocities of the major plates since the late cretaceous abstract we present models for the relative and absolute velocities of the major plates from 80 m y ago to the present as tests of several alternative plate reconstructions and as a guide in the evaluation of the hypothesis that pronounced changes in plate speeds or directions arise directly from changes in the configuration of plate boundary forces digitized plate boundaries were rotated backwards in time using published rotations to determine positions at 5 m y intervals back to the late cretaceous the oceanic plate boundaries were then corrected so as to properly match magnetic anomalies relative plate velocities were obtained by differencing finite rotations absolute velocities were determined by assuming that no net torque is exerted on the lithosphere rms absolute speeds of each plate were calculated as functions of time as a conceptually simple measure of plate velocities and accelerations we have tested several possible reconstructions for the antarctic and pacific plates proposed to explain the failure of simple rigid antarctic and or pacific plate models to match paleomagnetic data hotspot tracks and equatorial crossings defined by pelagic sediment deposition we find that 1 1 plate reconstruction models with either two antarctic or two pacific plates in the early tertiary are distinguishable from models with single antarctic and pacific plates principally on the basis of faster speeds for the kula plate and the northern pacific greater areas for the kula and farallon plates and faster subduction at the aleutians and along the west coast of south america for that time 2 2 oceanic plates as a group have moved faster than those with continental lithosphere only since 50 m y ago active subduction appears to be a sufficient condition for rapid plate motion regardless of the fractional plate area comprised of continental lithosphere 3 3 the absolute motion of india in the late cretaceous was anomalously fast 15 20 cm yr compared to the speeds of the present plates 4 4 the rms velocities of a number of plates and of the lithosphere as a whole are much higher in the late cretaceous than through most of the cenozoic 5 5 the predicted apparent polar wander for north america is in fairly good agreement with paleomagnetic data with the slight discrepancy attributable to a net contribution to the torque balance from plate boundary forces

isotopic signature of burial diagenesis and primary lithological contrasts in periplatform carbonates miocene great bahama bank the stable isotope geochemistry of miocene sediments from the leeward margin of the great bahama bank was examined to investigate burial diagenetic processes in periplatform carbonates data indicate that in addition to differences in bulk proportions of neritic and pelagic carbonate along the slope rhythmic variation in primary carbonate content has controlled patterns of burial diagenesis and associated geochemical signatures throughout much of the succession examined the present study focuses on ocean drilling program sites 1006 and 1007 the most distal of five sites drilled from marginal to deep basin environments during leg 166 these miocene sections are characterized by their cyclic appearance manifest as decimeter to meter scale alternations between light colored ooze chalk limestone and dark colored marl marlstone the section at site 1006 contains a high proportion of pelagic carbonate and is unlithified to a subbottom depth of 675 m fluctuations in 18 o and 13 c values at this site are independent of lithological variation and reflect primary conditions at site 1007 located at the toe of slope and composed of a mixture of bank derived and pelagic carbonate limestones are densely cemented show little evidence of compaction and have 18 o values up to 2 higher than coeval sediments at site 1006 marlstones at site 1007 are poorly cemented exhibit an increase in compaction related features with depth and have lower and more variable 18 o values that are similar to those of coeval sediments at site 1006 isotopic and petrographic characteristics of limestone interbeds result from cement precipitation from cold sea water during the first 100 m of burial higher proportions of insoluble materials and pelagic carbonate seem to have inhibited diagenetic alteration in adjacent marlstones in spite of significant compaction and pressure solution during burial original isoto pic compositions appear to be best preserved in these intervals at site 1007 the documented contrasts in petrographic and isotopic patterns illustrate the role of primary sediment composition in controlling lithification processes in periplatform carbonates and stress the importance of consid ering such factors when interpreting geochemical data from ancient shelf and slope limestones

the source of halogens in geothermal fluids from the taupo volcanic zone north island new zealand abstract forty seven samples of deep geothermal well fluids and hot springs from 14 geothermal sites in the taupo volcanic zone tvz north island new zealand were analyzed for cl br and li concentrations and 37cl 7li 18o and d values the main purpose of this study was to identify the sources of salinity in the tvz fluids and to assess the processes that control stable chlorine isotopic fractionation in the geothermal waters the samples were obtained from deep wells in the mokai ohaaki wairakei rotokawa kawerau ngatamariki and tauhara geothermal fields and from the hot springs at rotorua waiotapu lake taupo waimangu waikite orakei korako and tokaanu two main meteoric recharge areas were identified in the central tvz one located to the ne with 18o and d values of 5 5 and 35 respectively and the other to the sw with a 18o value of 6 7 and d value of 43 water rock interactions in most of the geothermal fluids resulted in a 18o shift to more positive values up to 1 for mokai and over 3 for kawerau the 7li values range from 2 9 to 2 0 and suggest that the deep geothermal fluids have interacted with rocks with low 7li values the highest 7li values 1 4 in the reservoir fluids were found at kawerau where the low quartz content of the greywacke may result in the depletion of 6li the 37cl values of the samples ranged from 1 0 to 0 8 the 37cl values of 0 2 0 1 and 0 0 were measured in samples from orakei korako waiotapu and waimangu respectively and these are interpreted to have been affected by surface evaporation and distillation processes excluding these data most waters with high cl br ratios had positive 37cl values the cl br molar ratios in well samples ranged from 659 to 1664 and the 37cl values from 1 0 to 0 8 the hot springs had cl br molar ratios ranging from 858 to 1611and 37cl values from 0 8 to 0 7 overall the cl br molar ratios of the fluids are comparable with published data on basaltic magmas and these magmas are inferred to be the main source of halogens in the geothermal fluids however the geothermal fluids can be separated into two main groups one associated with rhyolitic magmas characterized by positive 37cl values and high cl br molar ratios and other related to andesitic magmas with negative 37cl values and lower cl br molar ratios we suggest therefore that the isotopic fractionation of the stable cl isotopes is related to the difference in water contents of andesitic and rhyolitic magmas and that the fluid phase is enriched in 37cl during magmatic evolution there is no evidence of isotopic fractionation during the transit of the geothermal fluids from the reservoir to the surface but evaporation and distillation processes at the surface can affect the 37cl values

tertiary paleoceanic chemical variability unintended consequences of simple geochemical models five lines of concrete quantitative evidence about oceanic chemical composition during the past 65 m y can provide insights into the controlling processes and their temporal variations but the system constraints are not yet fully understood the calcium carbonate compensation depth mean carbon isotopic ratio and strontium isotopic ratio have all changed significantly during this time while changes in lithium to calcium and strontium to calcium ratios have been much smaller when these tracers are considered separately simple scenarios of changes in geochemical and geological factors can be used to explain the historical records these factors include 1 increases in continental chemical weathering rates and net continental fluxes to the oceans 2 changes in the isotopic composition of material weathered from continents to the oceans 3 decreases in the proportion of calcium carbonate deposited on continental shelves 4 changes in the deposition or weathering rates of certain types of sediments and 5 changes in hydrothermal circulation because these geochemical cycles are linked to each other and to others such as nutrients and global atmospheric carbon dioxide an explanation invoked for one tracer must satisfy the direction magnitude and timing of the patterns for all the tracers several simple scenarios for geochemical variations have been evaluated for their consequences both intended and unintended and compared to the historical records despite the attractiveness and possible justifications of some simple hypotheses for example decreases in organic carbon burial rates increases in carbonate weathering rates on the continents or increases in all continental weathering rates no one simple scenario appears to be consistent with both the timing and magnitude of the changes observed during the past 65 m y for all tracers considered the contradictions uncovered can be useful in defining what knowledge is crucial to further understanding of oceanic chemical history better knowledge is needed of absolute deep ocean sediment accumulation rates and their temporal variability of the extent and time scale of variability in the net carbon isotope input to the ocean from continental weathering of steady state fluxes for oceanic mass balances and the extent to which these mass balances approach steady state behavior and of the links and interactions between the geochemical cycles of various elements

miocene benthic foraminiferal isotope records a synthesis abstract 18 o 16 o and 13 c 12 c ratios of miocene benthic foraminifera from a number of atlantic pacific and indian ocean dsdp sites 71 77b 206 208 238 279 289 296 329 357 and 366a have been compiled these provide a rather detailed history of miocene deep water especially in the pacific ocean bottom water temperatures rose during the early miocene and then declined rapidly during the middle miocene this decline was accompanied by an increase in antarctic glaciation late miocene bottom temperatures and antarctic ice volumes are inferred to be similar to today s but exhibited some fluctuation the early miocene ocean was less thermally stratified at intermediate and abyssal depths while the late miocene deep ocean had a thermal structure generally similar to the modern ocean foraminiferal carbon isotope ratios at most of the sites varied quasi sympathetically throughout the miocene these variations must reflect comparable variations in the mean 13 c 12 c of marine hco 3 however the causes of such variations are not yet clear

isotopic and elemental partitioning of boron between hydrous fluid and silicate melt the fractionation of b and its isotopes between aqueous fluid and silicate melt has been studied from 550 to 1100 c and 100 500 mpa fluid melt partition coefficients are 1 for rhyolite melt this shows that b is not always strongly extracted from melts into hydrous fluids boron isotopic fractionation is large compared with the carbon and oxygen stable isotopic systems especially at high t and is most simply explained by differences in coordination trigonal vs tetrahedral among coexisting phases combined with earlier measurements on illite water 300 350 c b isotopic fractionation defines a temperature dependent trend from 300 to 1100 c because of the large magnitude and apparent low sensitivity to bulk composition b isotopic fractionation can be readily applied to studies of diagenesis hydrothermal alteration of planetary bodies subduction zone processing and arc magma generation and magma chamber evolution

strontium and its isotopes in canadian rivers fluxes and global implications systematic sampling of the 39 largest canadian rivers shows that the weighted average 87sr86sr ratio in the dissolved load is 0 7111 similar to previous measurements on such large rivers as the amazon and mississippi consequently we believe that the above estimate is likely representative of the global average this imposes a limit of 6 5 1011g yr 1 on the amount of sr exchanged with basalts in hydrothermal cells on mid oceanic ridges evaluation of geological information suggests that sr from silicate sources is of considerable importance for all but the largest canadian rivers the latter have chemical and isotopic composition consistent with 4 1 carbonate vs silicate derivation of sr but such interpretation is not unique in terms of their water discharge the 39 canadian rivers studied account for 4 2 of the world total and their weighted average concentrations for other dissolved solutes are tds 176 ppm ca 18 ppm cl 6 8 ppm and sr 84 ppb

phosphorus accumulation in marine sediments and the oceanic phosphorus cycle ideas about key factors in the oceanic mass balance of dissolved reactive phosphate have changed substantially i present an integrated overview of these here with an emphasis on evaluating the burial sinks for p and defining areas needing further research the major source of reactive p to the ocean is river input reactive p is delivered to the oceanic sediment water interface primarily in particulate organic matter p scavenged by hydrothermal iron rich oxyhydroxide particles with uptake in proportion to deep water phosphate concentrations represents a substantially smaller flux to the sediment water interface diagenetic transformations are important influences on the form of reactive p burial in marine sediments p burial occurs with organic carbon burial and as p associated with iron rich oxyhydroxide particles and coatings formation of authigenic p rich phases presumably apatite at the expense of organic p and oxide associated p is significant in open ocean marine sediments the authigenic p sink may represent a substantially larger portion of the sedimentary burial than indicated by previous estimates focused on p burial in organic rich continental margin sediments

geochemical consequences of increased late cenozoic weathering rates and the global co2 balance since 100 ma large imbalances in the relative net co2 flux over the last 100 m y are obtained from independently derived estimates of co2 uptake by weathering and organic carbon burial and of co2 outgassing assumed proportional to the mean mid ocean ridge mor spreading and mantle plume production mpp rates increasing the river flux of ca mg p and c by 50 since 30 ma to parallel the proposed increase in the sr river flux needed to explain the sr isotopic evolution of seawater richter et al 1992 has a large impact on the global carbon cycle by reducing the excess net co2 imbalance in the late cenozoic co2 uptake is calculated by assuming that 19 of the ca river flux and 60 of the mg river flux are derived from weathering silicates and that mg is removed in the oceans by 1 1 molar exchange reactions with calcsilicates an increase in the mg river flux combined with an overall decrease in the mor spreading rate predicts as much as a factor of 2 increase in the seawater mg concentration since 100 ma the net organic carbon burial flux burial minus weathering largely reflects changes in the bulk carbonate 13c record and to a lesser extent variations in the net carbonate burial flux and mean 13c value of the carbonate weathering flux organic carbon burial efficiency is markedly less than that of p from the middle middle miocene to the present because of early diagenetic transfer of organic p to francolite and oxidation of organic carbon during reworking of margin sediments by sea level fluctuations the large co2 imbalance of the late cretaceous requires significantly less carbonate subduction or greater weathering rates a co2 balance can be achieved in the miocene by decreasing the proposed increase in river fluxes by up to half or by variably increasing the amount of carbonate subducted variations in shallow water carbonate burial and hence the global carbonate record are currently too poorly known to differentiate between the above possibilities the decrease in the excess co2 flux generally parallels the trend to cooler climate inferred from the 18o record but periods of large co2 imbalance generally precede 18o shifts by several million years

a simple model for the caco3 saturation state of the ocean the strangelove the neritan and the cretan ocean 1 a simple model of the caco3 saturation state of the ocean is presented it can be solved analytically and is intended to identify the fundamental controls on ocean carbonate ion concentration it should also attract researchers unfamiliar with complex biogeochemical models despite its limitations the model calculated caco3 saturation state of today s ocean agrees well with observations in general the model reveals three distinctly different modes of operation the strangelove ocean of high supersaturation which is dominated by inorganic caco3 precipitation 2 the neritan ocean of indefinite saturation dominated by biogenic shallow water caco3 precipitation and 3 the cretan ocean of low saturation dominated by biogenic pelagic caco3 precipitation in the latter mode the deep ocean co32 is remarkably stable provided that the biogenic production of caco3 exceeds the riverine flux of ca2 and co32 this explains the overall constancy of the saturation state of the ocean documented over the last 100 ma the model is then used to address diverse questions one important result is that the recovery of the oceanic carbonate chemistry from fossil fuel neutralization in the future will be accelerated due to expected reduced biogenic calcification

causes and consequences of the relation between area and age of the ocean floor the distribution of area of the ocean floor with age t is approximately described by da dt c0 1 t tm where c0 is the rate of crustal generation and tm the maximum age a linear differential area versus age relation can be explained by a balance between crustal generation and consumption where consumption is uniformly distributed with age the present distribution of consumption with age was estimated from the isochron map used to derive the area age relation and a recently published set of angular velocity vectors describing present plate motions the rate of consumption per unit age shows considerable scatter about a mean value equal to c0 tm 1 7 10 8 km2 yr2 but consumption is sufficiently close to being uniformly distributed with age to explain the observed degree of approximation to a linear differential area age relation the trenches appear to be distributed randomly with respect to age provinces in the oceans it was not possible to discern any trend of mean subduction velocities with age apart from the smaller velocities observed for the two youngest age provinces 0 9 ma changes in the rate of plate generation and the distribution of consumption with age result in shifts in the area age distribution in turn these shifts produce changes in the plate driving forces which act to restore the rate of plate generation and distribution of consumption to their initial states the ridge pushing force for example decreases as the rate of crustal generation increases this coupling between driving forces and the area age distribution provides a feedback mechanism limiting the extent of any changes a measure of the magnitude of shifts in the area age distribution is given by global changes in sea level if long term changes in sea level are less than 150 m as suggested by recent estimates then only small changes 12 in the rate of plate generation and distribution of consumption with age are required sea level changes as large as 300 m require about a 50 change in the rate of crustal generation the above area age relation can be combined with simple expressions for depth and heat flow versus age to obtain an empirical hypsometric distribution parameterized in terms of age and exact expressions for the heat loss from the ocean floor the latter is estimated to be 2 9 1013 w with 25 of this heat being lost by seafloor younger than 5 ma of the total heat loss from the mantle 26 is estimated to be transported by small scale convection

sr isotopes in interstitial waters of marine sediments from deep sea drilling project cores sr isotopic compositions of interstitial sea waters have been documented in detail with cores from 37 deep sea drilling project sites the 87sr 86sr ratios are sensitive indicators of diagenetic reactions involving alteration of volcanic matter dispersed in sediments and recrystallization of carbonates effects of basalt alteration reactions except possibly for hydrothermal signals and of alteration of continental debris are minimal carbonate recrystallization has a significant effect on the geochemical mass balance of sr but diffusion of unradiogenic strontium from interstitial waters has only a small effect on the 87sr 86sr ratio of ocean water

miocene benthic foraminiferal cd ca records south atlantic and western equatorial pacific cadmium to calcium records in benthic foraminifera interpreted in conjunction with carbon isotope records reflect oceanic circulation patterns and oceanic inventories of cd carbon 13 carbon 12 and indirectly phosphorus cd ca ratios have been determined in benthic foraminifera cibicidoides spp for the late middle and early miocene from a site in the south atlantic deep sea drilling project dsdp site 525 leg 74 2500 m walvis ridge and from a site in the western equatorial pacific dsdp site 289 leg 30 2200 m ontong java plateau both sites have well established carbon isotope records for benthic foraminifera typical of the global record dominated by inventory changes cd ca ratios and carbon isotope values were averaged for the miocene in time intervals substantially longer than estimated oceanic residence times of cd c or p oceanic mean cd ca and carbon isotopes were estimated as weighted means of the values from both sites using weighting factors based on replicating modern oceanic means this may tend to underestimate oceanic inventories earlier than the late miocene because of changing water mass dominance at the south atlantic site from these means the oceanic cd inventory appears relatively stable during the miocene while there are large changes in carbon isotope inventories miocene cd inventory estimates are at least 20 lower than published estimates for quaternary stages 1 and 2 increasing oceanic cd inventories over the past 10 m y are consistent with hypotheses of increased river fluxes in the recent geologic past cd ca differences between sites may be indicative of the extent of basin basin fractionation from bottom water circulation although the validity of this comparison depends on how representative each of these two sites is through time for its respective ocean basin the pacific atlantic cd ca difference based on these two sites was larger in the late miocene compared to the middle and early miocene the relationship between cd ca and carbon isotope inventories and intersite gradients for these sites suggests that either oceanic phosphorus inventories or parameters related to marine organic matter composition must have been different in the miocene than at present large changes in the carbon isotope balance reflected as both positive and negative excursions in the carbon isotope inventories occurred with little effect on cd inventories changes in phosphorus fluxes associated with these carbon isotope flux changes may have been compensated for by changing deposition fluxes to other phosphorus sinks phosphorus inventories may have increased from the middle to late miocene with increasingly negative oceanic carbon isotope inventories however confirmation of these suggestions depend on collecting more cd ca and 13c data throughout the miocene oceans and on developing the application of other constraints for these interpretations

processes controlling the distribution of boron and chlorine in natural waters in iceland abstract the concentrations of b and cl in natural waters in iceland lie in the range 0 001 10 and 1 20 000 ppm respectively lowest concentrations occur in surface and groundwaters in the central highlands highest cl concentrations are found in high temperature waters on the reykjanes peninsula and highest b concentrations in well waters from high temperature geothermal systems in the axial zones of the active volcanic belts the b and cl contents of icelandic basalts are 0 1 6 6 and 75 750 ppm respectively these large variations are considered to result from variable degassing during consolidation as well as concentration variations in the initial magma tholeftes are lowest in both elements av 1 2 ppm b and 170 ppm ci transitional basalts intermediate av 2 1 ppm b and 240 ppm ci and alkali basalts highest av 3 8 ppm b and 340 ppm cl cl b ratios in tholeiites are most often within 25 50 but somewhat lower in alkali basalts most geothermal waters contain b and cl within 0 05 1 and 10 100 ppm respectively these rather low values are attributed to the low content of b and cl in the basaltic rock the b and cl distribution in the icelandic natural waters indicate that these elements act as essentially incompatible their sources of supply include 1 the atmosphere seawater spray and aerosols 2 the rock with which the water interacts 3 seawater groundwater and 4 magma intrusions in surface water and nonthermal groundwater the dominant source of supply of cl is seawater spray and aerosols but the soil and rock contribute often significantly to the b in these waters their cl b ratio is similar to that of seawater 1330 or somewhat lower boron and cl concentrations generally increase with water temperature warm waters cl b ratios intermediate between those of seawater and basalt with increasing temperature this ratio decreases and gradually approaches that of the rock some high temperature geothermal waters have as low cl b ratios as 1 and b concentrations as high as 10 ppm the cause is either b degassing of the magmatic intrusive heat source or phase separation in the producing aquifer some low temperature waters

lithospheric structure of an active backarc basin the taupo volcanic zone new zealand seismic data from both explosive and earthquake sources have been used to model the crustal and upper mantle velocity structure beneath the taupo volcanic zone tvz an active backarc basin in central north island new zealand volcanic sediments with p wave velocities of 2 0 3 5 km s 1 reach a maximum thickness of 3 km beneath the central tvz underlying these sediments to 16 km depth is material with velocities of 5 0 6 5 km s 1 interpreted as quartzo feldspathic crust east and west of the tvz crust with similar velocities is found to depths of 30 and 25 km respectively beneath the tvz material with p wave velocities of 6 9 7 3 km s 1 is found from 16 to 30 km depth and is interpreted as heavily intruded or underplated lower crust the base of the crust at 30 km depth under the tvz is marked by a strong seismic reflector interpreted as the moho modelling of arrivals from deep 40 km earthquakes near the top of the underlying subducting pacific plate reveals a region with low mantle velocities of 7 4 7 8 km s 1 beneath the crust of the tvz this region of low mantle velocities is best explained by the presence of partially hydrated upper mantle resulting from dehydration of hydrous minerals e g serpentinite carried down by the underlying subducting plate within the lower crust beneath the tvz a region of high 0 34 poisson s ratio is observed indicating the presence of at least 1 per cent partial melt this melt probably fractionates and assimilates crustal material before some of it migrates into the upper crust where it provides a source for the voluminous rhyolitic magmas of the tvz

the behavior of metals pb zn as mo cu during crystallization and degassing of rhyolites from the okataina volcanic center taupo volcanic zone new zealand tthe taupo volcanic zone tvz new zealand is a region of voluminous and frequent rhyolitic volcanism and widespread geothermal activity additionally the hydrothermal systems of the tvz contain relatively high concentrations of base and precious metals here we present an extensive dataset of major element volatile and trace element including pb zn as mo cu abundances in melt inclusions pumice glasses and minerals from eight eruptions within the okataina volcanic center ovc of the tvz to investigate the behavior of metals during melt evolution the high sio2 rhyolites of the ovc contain high concentrations of volatiles 6 wt h2o 0 25 wt cl and underwent significant degassing prior to and during eruption the ovc melts contain moderate concentrations of metals 11 24 ppm pb 20 50 ppm zn 2 7 ppm as 2 5 ppm mo 5 ppm cu ferromagnesian minerals amphibole biotite and orthopyroxene in the ovc pumice have high concentrations of zn 1500 ppm and plagioclase and biotite contain moderate amounts of pb 11 ppm the melt inclusion and pumice glass trace element data reveal complex histories of magma mixing and mingling prior to eruption however discrete melt batches are easily identified based on trace element geochemistry variations in incompatible trace elements within these melt batches suggest that the ovc rhyolites underwent at least 20 25 fractional crystallization during quartz crystallization and melt inclusion entrapment at pressures of 100 200 mpa and little to no crystallization 5 during ascent and eruption comparison of melt inclusion metal and incompatible element e g u concentrations reveals that melt pb mo and as increase whereas melt zn decreases during fractional crystallization at depth 100 200 mpa these observations can be explained by minor partitioning of the metals pb mo and as into the fractionating minerals and stronger partitioning of zn into the ferromagnesian phases supported by calculated metal d values and analyzed metal concentrations in ovc minerals interestingly throughout both deep vapor saturated crystallization and during extensive degassing during magma ascent and eruption as recorded by pumice glasses the metals analyzed here do not appear to partition appreciably into the vapor we propose that the lack of volatility of the metals analyzed in this study can be attributed to a combination of several factors first vapor melt partitioning requires the presence of ligands commonly cl s and oh with which the metals may complex given the low cl h2o ratios in the ovc melts and the extensive degassing of h2o compared with cl it seems likely that the rhyolites would have exsolved h2o rich vapor with insufficient cl to transport metals in particular pb and zn into the vapor phase either at depth or during magma ascent second the overall small volumes of vapor present during crystallization at pressures of 100 200 mpa would have impeded significant vapor melt partitioning of the metals finally the estimated very rapid ascent of the ovc melts from depths of 4 8 km suggests that there was insufficient time at low pressure for diffusion of metals out of the melt these results imply that there may be an indirect connection between the rhyolites and the metals of the hydrothermal systems of the tvz as the metals and other species such as cl remain in the rhyolitic magmas upon eruption they are available in the large volumes of rhyolite emplaced in the upper crust of the tvz for leaching by heated meteoric waters

a possible link between the seawater osmium isotope record and weathering of ancient sedimentary organic matter the os isotopic composition of leachable os from a north pacific pelagic clay sequence pegram et al 1992 and bulk sediment samples of metalliferous carbonates deposited near the east pacific rise ravizza 1993 display similar patterns of temporal variation fig 1 this similarity indicates that the composite record of t87os 186os ratio variations reflects changes in the os isotopic composition of seawater temporal changes in the sr isotopic composition of seawater record changes in the relative rates of continental weathering and oceanic crustal alteration the os isotopic composition of seawater should be influenced by similar processes in both the rb sr and re os systems the continental crust is characterized by large time integrated parent daughter ratios compared to the deep earth imparting relatively radiogenic sr and os signatures to continental material however the seawater sr and os records are decoupled from one another over the past 27 ma from 27 to 15 ma 87sr 86sr increased rapidly while the 18vos 186os ratio of seawater remained nearly constant at 15 ma the 87os ts6os ratio of seawater began to increase rapidly but the rate