Bulk Partition Coefficients Research Articles

Subduction-modified subcontinental mantle in South China: Trace element and isotope evidence in basalts from Hainan Island

Cenozoic lavas from Hainan Island, South China, comprise quartz tholeiite, olivine tholeiite, alkali basalt, and basanite and form a continuous, tholeiite-dominated, compositional spectrum. Highly incompatible elements and their relationships with isotopes in these lavas are shown to be useful in evaluating mantle-source composition, whereas modeling suggests that ratios of elements with bulk partition coefficients significantly larger than those of Nb and Ta may be sensitive to partial melting. Th/Ta and La/Nb ratios of alkali basalts are lower than those of tholeiites, and they are all lower than those of the primitive mantle. These ratios correlate positively with207Pb/204Pb and87Sr/86Sr ratios. Such relationships can be explained by mixing of depleted and enriched source components. A depleted component is indicated by alkali basalt compositions and is similar to some depleted OIB (PREMA). The enriched component, similar to sediment compositions, is indicated by tholeiites with high LILE/HFSE,207Pb/204Pb, and87Sr/86Sr ratios.

The role of hydrothermal fluids in the production of subduction zone magmas: Evidence from siderophile and chalcophile trace elements and boron

In order to evaluate the processes responsible for the enrichments of certain siderophile/ chalcophile trace elements during the production of subduction-related magmas, representative lavas from seven subduction zones have been analyzed for Pb, As, Sb, Sn, W, Mo, Tl, Cu, and Zn by inductively coupled plasma-mass spectrometry (ICP-MS), radiochemical epithermal neutron activation analysis (RENA), and atomic absorption (AA). The siderophile/chalcophile elements are compared to the highly fluid-mobile element B, the light rare earth elements (LREEs), U, and Th in order to place constraints on their behavior in subduction zones. Boron, As, Sb, and Pb are all enriched in arc lavas and continental crustal rocks more so than expected assuming normal magmatic processes (melting and crystallization). Tin, W, and Mo show little evidence of enrichment. Correlations of Pb/Ce, As/Ce, and Sb/Ce with B/La are statistically significant and have high correlation coefficients (and, more importantly, slopes approaching one) suggesting that Pb, As, and Sb behave similarly to B (i.e., that they are fluid-mobile). In addition, across-arc traverses show that B/La, As/Ce, Pb/Ce, and Sb/Ce ratios decrease dramatically with distance towards the back-arc basin. W/Th, Tl/La, Sn/Sm, and Mo/Ce ratios and Cu and Zn concentrations have much less systematic across-arc variations and correlations with B/La are not as strong (and in some cases, not statistically significant) and the regression lines have much lower slopes. Mixing models between upper mantle, slab-derived fluid, and sediment are consistent with a fluid-derived component in the arcs displaying extra enrichments of B, Pb, As, and Sb. These observations imply efficient mobilization of B, Pb, As, Sb, and possibly Tl into arc magma source regions by hydrothermal fluids derived from metamorphic dehydration reactions within the slab. Tin, W, and Mo show little, if any, evidence of hydrothermal mobilization. Copper appears to be slightly enriched in arc lavas relative to mid-ocean ridge basalts (MORBs) whereas Zn contents of arc lavas, MORB, ocean island basalts (OIBs), and continental crustal samples are similar suggesting that the bulk partition coefficient for Zn is approximately equal to one. However, Zn contents of the upper mantle are lower than these reservoirs implying an enrichment of the source region in Zn prior to melting. These nonigneous enrichments have implications not only for arc magma genesis but also for continental crust formation and crust-mantle evolution. The mobility of Pb, As, Sb, and B in hot, reducing, acidic hydrothermal fluids may be greatly enhanced relative to the large-ion lithophile elements (LILEs; including U) as a result of HS −, H 2S, OH −, or other types of complexing. In the case of Pb, continued transport of Pb from subducted slabs into arc magma source regions throughout Earth history coupled with a U fluxing of the mantle a the end of the Archean may account for the depletion of Pb in the upper mantle, the low U/Pb of most arc volcanics and continental crustal rocks, and provide an explanation for the Pb- Paradox (Hofmann et al., 1986; McCulloch, 1993; Miller et al., 1994). Recycled slabs will then retain high U/Pb ratios upon entering the deep mantle and may eventually become incorporated into the source regions of many OIBs; some with HIMU (high 238U/ 204Pb) signatures.

Distribution of local anesthetics in lipid membranes

Absorption of local anesthetics into lipid membranes and adsorption onto their surfaces were studied as a function of the pH of aqueous bulk solutions by measuring lipid vesicle electrophoretic mobility, the partition of the anesthetics between the aqueous and membrane phases by the use of fluorescence and radioactive tracer methods, and the effect of the anesthetics on interfacial tension of lipid monolayers formed at the oil/aqueous interface. At a pH much lower than the p K a value of the local anesthetic, the charged form of the local anesthetic was only adsorbed onto the membrane surface, as determined from vesicle electrophoretic mobility, radioisotope tracer and the monolayer surface tension studies. Surface partition coefficients of the charged form of the local anesthetics on phosphatidylcholine and phosphatidylserine membranes were obtained from the data of electrophoretic mobilities for lipid vesicles. The surface partition coefficients of various local anesthetics paralleled those of the bulk partition coefficients. As the pH of the solutions increased, the adsorbed amount of the charged form of the anesthetic at the membrane interface decreased, while the absorption of the uncharged form of the local anesthetic into the membrane increased. The total amount of local anesthetic adsorbed per unit area of the membrane generally increased as the pH of the solution increased. This was also observed from the measurements of the fluorescence of local anesthetics adsorbed into the membranes. At lower pH than that corresponding to the p K a value of the local anesthetic, the amount of anesthetic adsorbed depended greatly upon the membrane surface charge. At a higher pH than its p K a, it did not depend appreciably on the surface charge density of the membrane but did depend on the bulk partition coefficients between the aqueous and oil phases.

Increase of [formula omitted] in residual liquids of high-Rb/Sr magmas that evolve by fractional crystallization

Equations are developed which describe the increase with time of the 87Sr86Sr and 87Rb86Sr ratios and Sr concentration in the residual liquid during a magmatic fractional crystallization process.Although at present the changes in the initial 87Sr86Sr ratio can be disregarded in most cases, for magmas with high Rb/Sr ratios and/or sufficiently long crystallization time-spans these changes may become important.The equations derived may be used to calculate Rb and Sr bulk partition coefficients when applied to the evolution of high-Rb/Sr magmas by fractional crystallization.

UThRa systematics in Kilauea and Mauna Loa basalts, Hawaii

We describe 226Ra 230Th 238U (dis)equilibria and RbBaThULa concentration relationships in historical and prehistoric lavas from Kilauea and Mauna Loa. ( 226 Ra 230 Th ), ( 230 Th 232 Th ), Th/U, Ba/Rb and Ba/La ratios [but not (Rb,Ba,La)/(Th,U) ratios] are essentially identical in both volcanoes, whereas the absolute concentrations (after correction for olivine crystallization) differ by up to a factor of 2, in response to varying melt fractions. This shows that bulk partition coefficients of these elements are significantly smaller than melt fractions. Very small or absent 230Th 238U disequilibrium implies very small or negligible magmatic fractionation between Th and U. 226Ra 230Th disequilibria are significantly larger (∼ 20% excess 226Ra on average) but are also independent of melt fraction. The combination of significant RaTh fractionation together with small or absent ThU fractionation provides constraints on recently proposed models to explain U-series disequilibria during partial melting and melt extraction. Instantaneous melt extraction models are rejected: (a) because they are inconsistent with experimentally determined partition coefficients; and (b) more generally because they would require significant covariation of ( 226 Ra 230 Th ) with melt fraction. On the other hand, dynamic melting models involving slow fractional melting or melt infiltration within the garnet stability region, followed by rapid movement through the lithosphere, are consistent with the results and yield melt porosities between 10 −3 and 10 −2 for plume upwelling velocities of 1 m yr −1. In addition, we tentatively proposed alternative models for creating the Ra excesses in the magma. One such process involves the mobilization of Ra within the volcanic edifice, subsequent advection toward and redeposition within the roof region of the magma chamber, and finally incorporation into the magma itself. Another mechanism for incorporating excess Ra in the magma might be transport of very small amounts of carbonate fluids or carbonatite melts (containing very large excesses of 226Ra) into partially molten regions in the mantle. Given the currently available data and state of knowledge about magma extraction processes, there is no obvious preference for either the purely magmatic models or those involving “extraneous” fluids in the mantle or within the volcanic edifice.

Geochemical effects of dynamic melting beneath ridges: Reconciling major and trace element variations in Kolbeinsey (and global) mid‐ocean ridge basalt

Zero‐age basalts dredged from the Kolbeinsey Ridge directly north of Iceland are mafic quartz tholeiites (MgO 6–10 wt. %), strongly depleted in incompatible elements. Fractionation‐corrected Na2O contents (“Nag”) are amongst the lowest found on the global ridge system, implying that the degree of partial melting at Kolbeinsey is amongst the highest for all mid‐ocean ridge basalt (MORB). In contrast, the basalts show large ranges of incompatible‐element ratios (e.g., K2O/TiO2 of 0.01 to 0.12 and Nd/Sm of 2.1 to 2.9) not related to variations in radiogenic isotope ratios; this suggests recent enrichment/depletion events associated with small‐degree partial melting as their cause, rather than long‐lived source heterogeneity. Tholeiitic MORB from many regions globally show similar or more extreme variations in K2O/TiO2. Dynamic melting of an adiabatically upwelling source can reconcile these conflicting indications of the degree of melting. Through dynamic melting, the incompatible elements are partially separated into different melt fractions based on their bulk partition coefficients, more incompatible elements being concentrated in deeper, smaller‐degree partial melts. The final erupted magma is a mix of melts from all depths in the melting column. The concentration of highly incompatible elements in the mix will be very sensitive to the physical processes allowing the deep melts to separate and migrate to the site of mixing, and small fluctuations in the efficiency of the separation process can account for the large range of trace element ratios seen at Kolbeinsey. The major element chemistry of the erupted mix (and Na8) is much more robust, depending mainly on the integrated total amount of melting. The large variations of incompatible element ratios seen at Kolbeinsey, and in MORB in general, therefore give no information about the total degree of melting occurring beneath the ridge, nor do they require a heterogeneous source.

Melting rates beneath Hawaii: Evidence from uranium series isotopes in recent lavas

238U, 232Th, 230Th and 226Ra abundances have been measured in six samples of recent Hawaiian basalt by high precision mass spectrometry, in an attempt to compare the melting process in plumes and at spreading ridges. The data reveal a very small range in ( 230Th/ 238U) activity ratio up to a maximum value of 1.02 ± 0.01, and ( 226Ra/ 230Th) activity ratios which lie between 1.10 ± 0.015 and 1.19 ± 0.02. U Th and Ra Th abundances are linearly correlated demonstrating that the disequilibria predate crystallisation and differentiation. Using recently published estimates for the bulk partition coefficients of U and Th, the results are consistent with melting rates > 10 −3 kg m −3 a −1 at porosities < 10 −3 for dynamic fractional melting in the garnet stability field.

Variability of trace elements in basaltic suites

This paper discusses the distribution of trace element concentrations in cogenetic suites of lavas, based on the study of trace element histograms. For each histogram, the variability is calculated ( V = s/m, where m is the mean concentration and s its standard deviation). The study of two basaltic series, each specific of a single igneous process (fractional crystallization and partial melting), shows that trace element variabilities are directly related to bulk partition coefficients. Considering different trace element behaviour models, quantitative relationships are established between the variability and the descriptive parameters for the processes (bulk partition coefficients, mean degree of fractionation and related standard deviation). An important feature is that the variability is independent of the initial concentrations in the source. This method is applied for comparison to several basalt suites on which case studies of magmatic processes have been conducted. It is also applied to study the behaviour of trace elements in different geodynamical contexts (hotspot, mid-ocean ridge and arc volcanism). Finally, a set of numerical values of bulk partition coefficients is proposed that accounts for N-MORB genesis.

The generation of uranium series disequilibria by partial melting of spinel peridotite: constraints from partitioning studies

Alkaline earth, Pb and actinide partition coefficients have been measured for olivine-, orthopyroxene- and clinopyroxene-melt pairs in experimental charges using secondary ion mass spectrometry. The bulk partition coefficients for spinel peridotites in equilibrium with basaltic melts are 1.1 × 10 −4 and 1.7 × 10 −4 for U and Th, respectively; both are dominated by the presence of clinopyroxene. These new data demonstrate that the bulk partition coefficient for uranium is lower than that for thorium in spinel peridotites, even if hydrous or accessory phases are stable at the solidus. U is similarly incompatible compared to Th for spinel peridotite in equilibrium with carbonate melts. These results demonstrate that the partial melting of a spinel peridotite at chemical equilibrium cannot account for the ( 230Th/ 238U) ratios in excess of unity observed in MORB. The generation of the observed ( 230Th) excesses by disequilibrium partial melting of spinel peridotite requires that the diffusivity of Th is greater than that of U, which is improbable, as the ionic radius of Th is greater than that of U. Similarly, ( 230Th) excesses can be produced on melt percolation only if the bulk partition coefficient of Th is smaller than that of U, and cannot therefore be generated in the spinel stability field. ( 230Th/ 238U) ratios greater than unity might be produced by slow, near fractional partial melting in the garnet stability field, if D U * gt/L is large compared to D Th * gt/L and D Th * cpx/L.

Terminology for trace-element partitioning

A self-consistent terminology for partitioning data is presented. Ratios of the concentration of a component in two phases are termed partition coefficients and given the symbol D. Ratios of partition coefficients are termed exchange coefficients and given the symbol K D . The prefix “bulk” implies that these coefficients are weighted according to the proportions of coexisting phases. Bulk partition and bulk exchange coefficients are denoted by D ̄ and K ̄ D , respectively.

The behaviour of platinum-group elements during partial melting, crystal fractionation, and sulphide segregation: An example from the Cape Smith Fold Belt, northern Quebec

The behaviour of the Pt-group elements (PGE) during both normal igneous processes and during the formation of PGE deposits is poorly understood. This is in part because of the limited data set available for nonmineralized rocks. Accordingly, PGE concentrations have been determined in komatiitic basalts associated with Ni-Cu sulphide deposits rich in PGE from the Proterozoic Cape Smith Fold Belt of northern Quebec. The lavas have been divided into olivine-phyric, pyroxene-phyric, and plagioclase-phyric on the basis of the phenocrysts present. Olivine-phyric lavas have Mg#s greater than 0.66 and are thought to be primary partial melts. The bulk partition coefficients of the PGE during partial melting calculated from the average of olivine-phyric basalts are 6, 2, 0.6, and 0.2 for Ir, Rh, Pt, and Pd, respectively, which indicates a decrease in compatibility of the PGE in this order. The pyroxene and plagioclase-phyric lavas are thought to have formed from the olivine-phyric lavas by crystal fractionation of olivine and chromite in the pyroxene-phyric lavas and olivine, chromite, and pyroxene in the plagioclase-phyric lavas. Ir shows a strong positive correlation with Mg#, Cr, and Ni, and this is attributed to Ir having partitioned into olivine or chromite. If olivine, chromite, and clinopyroxene were the only phases to have crystallized, then Pd should have behaved as an incompatible element; however, it does not correlate with the lithophile incompatible elements. It does, however, correlate with Rh and Pt. This is interpreted to suggest that Rh, Pt, and Pd were controlled by sulphide segregation. The removal of a small amount of sulphide along with the olivine and chromite during crystal fractionation could have resulted in the mildly compatible behavior exhibited by Rh, Pt, and Pd. The composition of the sulphides in the sulphide deposits may be modelled by assuming that the silicate magma from which they segregated was similar in composition to the spinifex-textured komatiites containing 15% MgO.

MORBCAL: a program for calculating the compositions of primary basaltic melts produced by decompression-induced melting below mid-ocean ridges

A widely accepted model proposes that mid-ocean ridge basalts (MORB) are produced by decompression-induced partial melting of oceanic upper mantle which adiabatically upwells in response to plate separation (McKenzie, 1984; Klein and Langmuir, 1987; McKenzie and Bickle, 1988). In this model, rising mantle begins to melt as it intersects the solidus at a depth determined by its temperature and melting characteristics. Continued upwelling involves continuous melting, matrix compaction, and melt segregation. To quantitatively evaluate this type of melting, Niu and Batiza (1991) developed an empirical scheme based on the peridotite melting experiments of Jaques and Green (1980), Falloon and Green (1987, 1988), and Falloon and others (1988). We used the experimental data to calculate the bulk partition coefficients (Kd) for the major elements. In addition, we determined pressure and temperature (extent of partial melting) dependance of these Kds and used these to calculate the compositions of pooled polybaric melts created by incremental melting. Finally, we developed a method for calculating the melting parameters (initial and final melting pressures and temperatures, plus total extent of melting) for actual MORB melt compositions. This method assumes that primary polybaric pooled melts fractionate only at low pressure and employs the chemical back-tracking method of Klein and Langmuir (1987). The purpose of this Short Note is to present the MORBCAL program. This program is an easy-to-use tool for readers interested in mid-ocean ridge processes and igneous petrogenesis. For details of the melting model, readers are referred to Niu and Batiza (1991). In the rest of the note, we briefly discuss MORBCAL and its availability.

Melt inclusions and crystal-liquid separation in rhyolitic magma of the Bishop Tuff

Melt inclusions in quartz phenocrysts from a single clast of pumice near the base of the plinian pumice fall of the Bishop Tuff were studied to test ideas concerning separation of melt and crystals in silicic magmas. Ten analyzed inclusions from the pumice clast are of high silica rhyolite composition with very low contents of the highly compatible elements Ba, Sr, and Eu, consistent with extensive fractionation. The concentrations of U, La, Ce, Mg, and Ca of these ten melt inclusions vary considerably as determined by ion microprobe. Petrologic considerations indicate that uranium is an incompatible element with a maximum bulk partition coefficient D of about 0.2 and that the evolution of the uranium content of the melt was controlled by crystallization of the magma. A minimum of 33 wt% perfect fractional crystallization is required to explain the observed range in uranium. However, only 17 wt% crystals occurred in the pumice clast. The greater calculated fraction of crystals requires significant separation of crystals and melt before the eruption of the plinian pumice fall in spite of the fact that crystal mixing (settling, etc.) did not occur in the Bishop magma.

Modelling the petrogenesis of high Rb/Sr silicic magmas

Rhyolites can be highly evolved with Sr contents as low as 0.1 ppm and Rb Sr > 2,000 . In contrast, granite batholiths are commonly comprised of rocks with Rb Sr < 10 and only rarely > 100. Mass-balance modelling of source compositions, differentiation and contamination using the trace-element geochemistry of granites are therefore commonly in error because of the failure to account for evolved differentiates that may have been erupted from the system. Rhyolitic magmas with very low Sr concentrations (⩽1 ppm) cannot be explained by any partial melting models involving typical crustal source compositions. The only plausible mechanism for the production of such rhyolites is Rayleigh fractional crystallization involving substantial volumes of cumulates. A variety of methods for modelling the differentiation of magmas with extremely high Rb/Sr is discussed. In each case it is concluded that the bulk partition coefficients for Sr have to be large. In the simplest models, the bulk D Sr of the most evolved types is modelled as > 50. Evidence from phenocryst/glass/whole-rock concentrations supports high Sr partition coefficients in feldspars from high silica rhyolites. However, the low modal abundance of plagioclase commonly observed in such rocks is difficult to reconcile with such simple fractionation models of the observed trace-element trends. In certain cases, this may be because the apparent trace-element trend defined by the suite of cognetic rhyolites is the product of different batches of magma with separate differentiation histories accumulating in the magma chamber roof zone.

An inversion approach to assimilation and fractional crystallisation processes

A numerical inversion program has been developed to investigate isotope and trace element variations in suites of related rocks in terms of assimilation and fractional crystallisation processes. Major element compositions constrain the degree of fractionation, and the program may be used to calculate the isotope and trace element composition of either the crustal contaminant, or of the parental magmas once the contaminant is known. The ratio of the rate of assimilation to the rate of fractional crystallisation, and the bulk partition coefficients, can be varied with changes in magma composition. The approach is illustrated with a suite of calc-alkaline rocks from the Cerro Galan centre in NW Argentina, and a suite of continental flood basalts from the southern Parana in Brazil. Both exhibit striking increases in 87Sr/86Sr with increasing SiO2 consistent with progressive contamination during differentiation within the continental crust. In detail, both suites contain a subset of samples which have undergone relatively little contamination, and a subset characterised by relatively high SiO2 and 87Sr/86Sr reflecting larger crustal contributions. Individual samples within the more contaminated subset can be modelled in terms of assimilation with fractional crystallisation (AFC) processes, although no clear progressively contaminated liquid lines of descent are preserved. At both Cerro Galan and the southern Parana, the associated dacites and rhyodacites appear to have similar compositions to the crustal contaminants. Thus the inversion program has been used to investigate the amount of contamination, the relationship between the amount of contamination and the amount of crystals removed by fractional crystallisation, and the isotope and trace element contents of the parental magmas prior to AFC. The latter are estimated to have had 87Sr/86Sr=0.7055 and 143Nd/144Nd=0.51256 at Cerro Galan, and 0.7090 and 0.51226 respectively in the southern Parana.

Th isotope and U-series studies of subduction-related volcanic rocks

The activities of 6–7 radionuclides measured by alpha spectrometry are reported for 35 samples of recent volcanic rocks from 24 volcanoes in 9 arcs. These include continental margins (Cascades, Alaska, Costa Rica), island arcs (the Sunda, Banda, and Sangihe arcs of Indonesia, Marianas, Japan, Aleutians, Antilles, Tonga), and a backarc basin (Lau Basin). Enrichment of 238U over 230Th is more common (over 2 5 of the samples) and greater (up to 59%) in these subduction-related volcanics than in those from other tectonic environments. 230Th 232Th ratios also extend to higher values, both in absolute numbers and relative to other isotope ratios. Enrichment of 210Po and 226Ra over 230Th is widespread and is more common and greater in island arcs than continental margins. The level of Po or Ra enrichment (50–300% in island arcs, 10–50% at continental margins) is similar to that in ocean island and ridge basalts and decreases with differentiation. The differences in Th-U systematics between subduction-related and other volcanic rocks are attributed to variations in the process of melt extraction, changes in bulk partition coefficients within the mantle wedge, or preferential addition of U from subducted lithosphere. Ra enrichments are attributed to partial melting processes which are similar to those at ocean ridges. Smaller excesses at continental margins are attributed either to slower ascent or to differences in the process of melt formation beneath continents.

U‐Th‐Pb partitioning behavior during partial melting in the upper mantle: Implications for the origin of high Mu Components and the “Pb Paradox”

The partitioning behavior of U, Th, and Pb in the upper mantle has been investigated through the analysis of ultramafic nodules found in lava flows at two localities in the southwestern United States. Secondary (i.e., pedogenic) components were distinguished from primary (igneous) components on the basis of Nd and Sr isotope ratios. Most of the primary components have Nd, Pb, and Sr isotope ratios within the ranges observed in oceanic basalts. The 238U/204Pb (mu) ratios of residual (i.e., type IA) clinopyroxene separates analyzed in this study are generally high (60–120) and show an excellent positive correlation with 147Sm/144Nd ratios. Because the latter are consistently greater than the chondritic ratio and because clinopyroxene contains most of the U in the nodules, we suggest this mineral is a “high‐mu” component in the upper mantle. Calculated “whole rock” U contents, based on analyses of purified separates of the major silicate minerals, are consistent with recent estimates for mid‐ocean ridge basalt sources. Clinopyroxene/melt partition coefficients, based on the assumption that the lavas bearing the nodules are the melts with which the nodule minerals last equilibrated, decrease in the order , consistent with available experimental data. Sulfide is inferred to be a major Pb‐bearing phase in the nodules. Bulk partition coefficients for Pb, Th, and U during melting are inferred to depend primarily on the abundance of clinopyroxene and sulfide in the residual mantle. On the basis of the model presented, the “Pb paradox” is explained as a direct consequence of melt depletion from the upper mantle.

Phase equilibria constraints on liquid lines of descent generated by paired assimilation and fractional crystallization: Trace elements and Sr and Nd isotopes

Utilizing a phase equilibria based trace element isotope model for low pressure igneous differentiation, it is now possible to update the methods presently being used to evaluate the effect of assimilation and fractional crystallization (AFC) processes. The results of the calculations presented here indicate that bulk partition coefficients (D) for Sr and Nd are strongly dependent on the chemistry of the assimilant and the rate of mixing. Virtually all silicic assimilants will increase bulk D for network‐modifying cations because of the positive correlation of most trace element partition coefficients with reciprocal temperature, and the amount of network forming components in the melt. In addition to the effect of temperature and melt composition, the addition of a magma of different composition from the magma chamber will generally cause a change in the fractionating mineral proportions. For example, a peraluminous assimilant will increase the proportion of plagioclase, and a peralkaline assimilant will increase the proportion of augite. This will in turn have an effect on bulk D. For most multiply saturated mafic to intermediate systems differentiating at low pressure, bulk DSr is &gt;1, and bulk DNd is &lt;1. Thus the effect of a peraluminous assimilant is to drive bulk DSr away from 1 and a peralkaline assimilant drives bulk DNd toward 1. Therefore a peralkaline assimilant will generally drive the liquid path away from a simple mixing curve (or D = 1 AFC curve) and a peralkaline assimilant will drive the liquid path toward a simple mixing curve. Compared to the results derived in this study, existing models using fixed bulk Ds calculated from closed system conditions will generate results that underestimate the assimilation rate, 87/86Sr and bulk Sr content for a peraluminous assimilant, and over‐estimate the assimilation rate, 143/144Nd and Nd content of the assimilant for a peralkaline assimilant.

Chemical differentiation of the Earth: the relationship between mantle, continental crust, and oceanic crust

The average chemical compositions of the continental crust and the oceanic crust (represented by MORB), normalized to primitive mantle values and plotted as functions of the apparent bulk partition coefficient of each element, form surprisingly simple, complementary concentration patterns. In the continental crust, the maximum concentrations are on the order of 50 to 100 times the primitive-mantle values, and these are attained by the most highly incompatible elements Cs, Rb, Ba, and Th. In the average oceanic crust, the maximum concentrations are only about 10 times the primitive mantle values, and they are attained by the moderately incompatible elements Na, Ti, Zr, Hf, Y and the intermediate to heavy REE. This relationship is explained by a simple, two-stage model of extracting first continental and then oceanic crust from the initially primitive mantle. This model reproduces the characteristic concentration maximum in MORB. It yields quantitative constraints about the effective aggregate melt fractions extracted during both stages. These amount to about 1.5% for the continental crust and about 8–10% for the oceanic crust. The comparatively low degrees of melting inferred for average MORB are consistent with the correlation of Na 2O concentration with depth of extrusion [1], and with the normalized concentrations of Ca, Sc, and Al (⋍ 3) in MORB, which are much lower than those of Zr, Hf, and the HREE (⋍ 10). Ca, Al and Sc are compatible with clinopyroxene and are preferentially retained in the residual mantle by this mineral. This is possible only if the aggregate melt fraction is low enough for the clinopyroxene not to be consumed. A sequence of increasing compatibility of lithophile elements may be defined in two independent ways: (1) the order of decreasing normalized concentrations in the continental crust; or (2) by concentration correlations in oceanic basalts. The results are surprisingly similar except for Nb, Ta, and Pb, which yield inconsistent bulk partition coefficients as well as anomalous concentrations and standard deviations. The anomalies can be explained if Nb and Ta have relatively large partition coefficients during continental crust production and smaller coefficients during oceanic crust production. In contrast, Pb has a very small coefficient during continental crust production and a larger coefficient during oceanic crust production. This is the reason why these elements are useful in geochemical discrimination diagrams for distinguishing MORB and OIB on the one hand from island arc and most intracontinental volcanics on the other. The results are consistent with the crust-mantle differentiation model proposed previously [2]. Nb and Ta are preferentially retained and enriched in the residual mantle during formation of continental crust. After separation of the bulk of the continental crust, the residual portion of the mantle was rehomogenized, and the present-day internal heterogeneities between MORB and OIB sources were generated subsequently by processes involving only oceanic crust and mantle. During this second stage, Nb and Ta are highly incompatible, and their abundances are anomalously high in both OIB and MORB. The anomalous behavior of Pb causes the so-called “lead paradox”, namely the elevated U/Pb and Th/Pb ratios (inferred from Pb isotopes) in the present-day, depleted mantle, even though U and Th are more incompatible than Pb in oceanic basalts. This is explained if Pb is in fact more incompatible than U and Th during formation of the continental crust, and less incompatible than U and Th during formation of oceanic crust.

Infrared spectroscopic measurements of CO 2 and H 2O in Juan de Fuca Ridge basaltic glasses

Dissolved H 2O and CO 2 contents in basaltic glasses from the Juan de Fuca Ridge and neighboring seamounts were determined by infrared spectroscopy. CO 2 contents range from about 45 to 360 ppm by weight, with carbonate ion complexes the only detectable form of dissolved carbon. Samples erupted at a given depth exhibit a large range in dissolved CO 2 contents that we interpret to be the result of variable amounts of degassing. The lowest CO 2 contents at each depth are in reasonable agreement with the experimentally determined CO 2 solubility curve for basalt at low pressures. All glasses with CO 2 values higher than the experimentally determined solubility at the eruption depth are oversaturated because of incomplete degassing. The highest CO 2 contents are spatially associated with the local topographic highs for each ridge segment. Lavas from relatively deep areas may have had greater opportunity to degas during ascent from a magma chamber or during lateral flow in dikes or seafloor lava flows. The highest observed CO 2 concentrations are from the axial seamount and lead to an estimate of a minimum depth to the magma chamber of 2.7 km beneath the ridge axis. H 2O contents vary from 0.07 to 0.48 wt.%, with hydroxyl groups the only detectable form of dissolved water. Water contents correlate positively with FeO */MgO and the highest water contents are found in the incompatible element-enriched Endeavour segment lavas. Variations in ratios of water to other incompatible elements suggest that water has a bulk partition coefficient similar to La during partial melting ( D ∼ 0.01).