Rare earth element systematics of hydrous liquids from partial melting of basaltic eclogite a re-evaluation

The origin of orogenic andesitic magmas is tested by calculations of REE fractionation in hydrous melts derived from partial melting of subducted ocean basalt in eclogite facies. New data on the subsolidus phase proportions of basaltic eclogite, the enrichment of LREE in altered ocean basalts, and experimentally determined REE partition coefficients (K_D's) between garnet and melt have been included in trace element fractionation equations. Non-modal melting of phases combined with variation inK_D's during melting is a unique feature of these calculations.Variation ofK_D, melting proportions, initial proportion of subsolidus phases, degree of melting, and initial REE concentrations yield a wide range of input parameters that produce REE profiles in partial melts of basaltic eclogite matching REE profiles of some orogenic andesites. The positive correlation of REE concentration with silica content for many andesitic suites can be accounted for by non-modal melting if quartz (or a similar phase with low REEK_D values) melts at a high melting proportion and garnet melts at a low melting proportion during the first stages of fusion. However, no mineralogic fractionation scheme can account for REE/silica systematics if REEK_D values are linearly decreasing with increasing melting. Earlier workers who have used similar calculations to discredit the eclogite fractionation model have set overly strict, and sometimes incorrect, constraints concerning the range in REEK_D values for garnet, the subsolidus proportions of phases in basaltic eclogite, and the relative concentrations of REE in subducted ocean crust undergoing partial melting.     

Lithium tracer-diffusion in an alkali-basaltic melt — An ion-microprobe determination

An ion-microprobe-based technique has been used to measure lithium tracer-diffusion coefficients (D_Li) in an alkali-basaltic melt at 1300, 1350 and 1400°C. The results can be expressed in the form:D_Li=7.5 ×10^?2exp(?27,600/RT)cm²S^?1The results show significantly faster diffusion rates than those previously recorded for other monovalent, divalent and trivalent cations in a tholeiitic melt. Consequently, diffusive transport of ions acting over a given time in a basaltic melt can produce a wider range of transport distance values than hitherto supposed. Hence, it is concluded that great care should be exercised when applying diffusion data to petrological problems.     

Pressure dependence of nickel partitioning between forsterite and aluminous silicate melts

Nickel partitioning between forsterite and aluminosilicate melt of fixed bulk composition has been determined at 1300°C to 20 kbar pressure. The value of the forsterite-liquid nickel partition coefficient is lowered from >20 at pressures equal to or less than 15 kbar to <10 at pressures above 15 kbar.Published data indicate that melts on the join Na₂O-Al₂O₃-SiO₂ become depolymerized in the pressure range 10–20 kbar as a result of Al shifting from four-coordination at low pressure to higher coordination as the pressure is increased. This coordination shift results in a decreasing number of bridging oxygens in the melt. It is suggested that the activity coefficient of nickel decreases with this decrease in the number of bridging oxygens. As a result, the nickel partition coefficient for olivine and liquid is lowered.Magma genesis in the upper mantle occurs in the pressure range where the suggested change in aluminum coordination occurs in silicate melts. It is suggested, therefore, that data on nickel partitioning obtained at low pressure are not applicable to calculation of the nickel distribution between crystals and melts during partial melting in the upper mantle. Application of high-pressure experimental data determined here for Al-rich melts to the partial melting process indicates that the melts would contain about twice as much nickel as indicated by the data for the low-pressure experiments. If, as suggested here, the polymerization with pressure is related to the Al content of the melt, the difference in the crystal-liquid partition coefficient for nickel at low and high pressure is reduced with decreasing Al content of the melt. Consequently, the change ofD_Ni^{ol-andesite melt} is greater than that ofD_Ni^{ol-basalt melt}, for example.     

Zircon saturation revisited: temperature and composition effects in a variety of crustal magma types

Hydrothermal experiments in the temperature range 750–1020°C have defined the saturation behavior of zircon in crustal anatectic melts as a function of both temperature and composition. The results provide a model of zircon solubility given by: In D_Zr^zircon/melt= ?3.80?[0.85(M?1)]+12900/T where D_Zr^zircon/melt is the concentration ratio of Zr in the stoichiometric zircon to that in the melt, T is the absolute temperature, and M is the cation ratio (Na + K + 2Ca)/(Al · Si). This solubility model is based principally upon experiments at 860°, 930°, and 1020°C, but has also been confirmed at temperatures up to 1500°C for M = 1.3. The lowest temperature experiments (750° and 800°C) yielded relatively imprecise, low solubilities, but the measured values (with assigned errors) are nevertheless in agreement with the predictions of the model.For M = 1.3 (a normal peraluminous granite), these results predict zircon solubilities ranging from ～ 100 ppm dissolved Zr at 750°C to 1330 ppm at 1020°C. Thus, in view of the substantial range of bulk Zr concentrations observed in crustal granitoids (～ 50–350 ppm), it is clear that anatectic magmas can show contrasting behavior toward zircon in the source rock. Those melts containing insufficient Zr for saturation in zircon during melting can have achieved that condition only by consuming all zircon in the source. On the other hand, melts with higher Zr contents (appropriate to saturation in zircon) must be regarded as incapable of dissolving additional zircon, whether it be located in the residual rocks or as crystals entrained in the departing melt fraction. This latter possibility is particularly interesting, inasmuch as the inability of a melt to consume zircon means that critical geochemical “indicators” contained in the undissolved zircon (e.g. heavy rare earths, Hf, U, Th, and radiogenic Pb) can equilibrate with the contacting melt only by solid-state diffusion, which may be slow relative to the time scale of the melting event.     

Correlation between δ~(18)O,Sr/Ca and Mg/Ca of coral Acropora and seawater temperature from coral culture experiments

To be used as proxies of seawater surface temperature(SST), the δ 18Oc values and Sr/Ca and Mg/Ca ratios of scleractinian coral skeletons must be verified by coral culture experiments in the laboratory. This paper describes a coral culture experiment that was conducted at several seawater temperatures T(21–28°C) using a tandem aquarium system and the new method for depositing coral skeletons grown under controlled conditions. The δ 18Oc values and the Sr/Ca and Mg/Ca ratios of the cultured coral were measured. We concluded that the δ 18Oc values and Sr/Ca and Mg/Ca ratios of the cultured coral are clearly correlated with T. The linear regression curve is δ18Oc(‰)=δ0.1427δT(°C)δ0.1495(n=18, r=0.955, p0.0001), and the slope of δ0.1427‰/°C is at the low end of the range of published values(δ0.13–δ0.29‰/°C). The Sr/Ca ratio decreases with increasing T, whereas the Mg/Ca ratio increases with increasing T, indicating a negative correlation between Sr/Ca and Mg/Ca. Their linear regression curves are Sr/Ca(mmol/mol)=δ0.04156δT+10.59(n=15, r=0.789, p0.005) and Mg/Ca(mmol/mol)= 0.04974δT+2.339(n=17, r=0.457, p0.05), respectively, which demonstrate that when Mg/Ca and Sr/Ca are increased by one unit, T increases by 5.19°C and decreases by 15.62°C, respectively. These variations are significantly lower than published values.     

Ion microprobe studies of water in silicate melts: Temperature-dependent water diffusion in obsidian

The temperature dependence of water diffusivity in rhyolite melts over the range 650–950°C and [P_T(H₂O] = 700 bars is evaluated from water concentration-distance profiles measured in glass with an ion microprobe. Diffusivities are exponentially dependent on concentration over this temperature range and vary from about 10^?8 cm²/s at 650°C to about 10^?7 cm²/s at 950°C at 2 wt.% water. Water solubility also varies with temperature at a rate of ?0.14 wt. per 100°C increase. The avtivation energy (E_a) appears to be constant at 19 ± 1kal/mole for 1, 2,and 3 wt.% H₂O. Comparison of these data with results for cation diffusion indicates that this value is a minimum E_a for diffusion of any species in a rhyolite melt.Compensation plots of log₁₀D₀ (the frequency factor) versus E_a indicate that hydrous rhyolite melts follow the same trend as anhydrous basalts. D₀ increases for H₂O and Ca²⁺ [1] as E_a decreases. This suggests that these molecules may diffuse by different mechanisms than do monovalent cations, and that hydration of the melt affects diffusion of Ca²⁺ and H₂O differently than it does monovalent cation diffusion. The results imply that dramatic increases in cation diffusivities by hydration [1] may occur with additions of less than 1 wt.% H₂O.     

A geochemical study of island-arc and back-arc tholeiites from the Scotia Sea

⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd ratios, REE and selected minor and trace elements are presented and compared for present-day volcanic rocks in the Scotia Sea.Tholeiitic basalts from the South Sandwich Islands show widely ranging contents of some lithophile elements, e.g. K₂O (0.09–0.55%) and Rb (1.55–14.2 ppm), but fairly constant Na₂O and Sr. Total REE contents range from about 4–20 times chondritic abundances with significant light-REE depletion and both positive and negative Eu anomalies. The variations in minor and trace element abundances are consistent with low-pressure fractional crystallization of plagioclase and clinopyroxene but only minor amounts of olivine. The⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd ratios of the parental magmas are thought be 0.7038–0.7039 and 0.51301–0.51314 respectively, and indicate derivation of at least some⁸⁷Sr from subducted ocean crust.The back-arc tholeiites in the Scotia Sea have lower⁸⁷Sr/⁸⁶Sr ratios (0.7028–0.7033), similar¹⁴³Nd/¹⁴⁴Nd ratios (0.51305) and are variably light-REE-enriched(Ce_N/Yb_N= 1.0–1.6). Total REE contents are comparable to those of the South Sandwich Islands tholeiites.     

Geochemistry, K–Ar geochronology and Sr–Nd–Pb isotope compositions of pitchstone in Gohado, southwestern Okcheon Belt, South Korea

Abstract Geochemical and Sr–Nd–Pb isotope characteristics, as well as K–Ar geochronology of a massive pitchstone (volcanic glass) stock erupted into Late Cretaceous lapilli tuff and rhyolite in the Gohado area, southwestern Okcheon Belt, South Korea, are reported. The pitchstones are highly evolved with SiO₂ contents ranging from ～72 to 73 wt%, K₂O/Na₂O ratios of 1.04–1.23 and low MgO/FeO^t values (0.17–0.20). The pitchstones are weakly peraluminous and the ASI (molar Al₂O₃/Na₂O + K₂O + CaO) values are significantly lower than 1.1. The pitchstones also display a general calc‐alkaline nature with significant alkali contents. The rare earth elements (REE) compositions show moderately fractionated nature with (La/Yb)_N ranging from 11 to 16. Chondrite normalized REE patterns show relative enrichment of light REE over heavy REE and moderate Eu anomaly (Eu/Eu* ratio varies from 0.53 to 0.57). A distinct negative Nb anomaly is observed for all pitchstones on a primitive mantle normalized trace element diagram, typical of subduction‐related magmatism and crustal‐derived granites. All these features are characteristic of I‐type granites derived from a continental arc. The pitchstones have Zr contents of 98.5–103.5 ppm with zircon thermometry yielding temperatures of 749–755°C (mean 752°C). The K–Ar analyses of representative pitchstone samples yielded ages of 58.7 ± 2.3 and 62.4 ± 2.1 Ma with a mean age of 61 Ma. The rocks show nearly uniform initial ⁸⁷Sr/⁸⁶Sr isotopic ratios of 0.7104–0.7106 and identical ¹⁴³Nd/¹⁴⁴Nd initial ratio of 0.5120. The rocks display negative εNd (61 Ma) values of ?12. The depleted mantle model ages (T_DM) range from 1.54 Ga to 1.57 Ga. The Pb isotope ratios are ²⁰⁶Pb/²⁰⁴Pb = 18.522–18.552, ²⁰⁷Pb/²⁰⁴Pb = 15.642–15.680 and ²⁰⁸Pb/²⁰⁴Pb = 38.794–38.923. These ratios suggest that the Gohado pitchstones were formed in a continental arc environment by partial melting of a 1.54 Ga to 1.57 Ga parental sources of lower crustal rocks probably of mafic or intermediate compositions.     

Experimental reproduction of textures of chondrules

The textures of chondrules have been reproduced by crystallizing melts of three different compositions at 1 atm with cooling rates ranging from 400 to 20°C/min under 10^?9 to 10^?12 atmP_O2. A porphyritic olivine texture has been formed from a melt of olivine-rich composition (SiO₂ = 45 wt.%), a barred-olivine texture from melt of intermediate composition (SiO₂ = 47 wt.%), and radial-olivine texture from melt of pyroxene-rich composition (SiO₂ = 57 wt.%). The cooling rate for producing barred olivine is most restricted; the rate ranges from 120 to 50°C/min. Other textures can be formed with wider ranges of cooling rate. The results of the experiments indicate that some of the major types of textures of chondrules can be formed with cooling rate of about 100°C/min. With this cooling rate, the texture varies depending on the composition of melt.     

Apatite and phosphorus in mantle source regions: An experimental study of apatite/melt equilibria at pressures to 25 kbar

The solubility of fluorapatite in a wide variety of basic magmatic liquids was experimentally determined over a range of upper mantle P-T conditions (8–25 kbar, 1275–1350°C). Fluorapatite is stable over the entire range of conditions investigated, but its solubility in melts is variable, depending negatively on SiO₂ content of the melt and positively upon temperature, with relatively little sensitivity to pressure above 8 kbar. At upper mantle pressures and a temperature of 1250°C, molten basalt (50% SiO₂) will dissolve 3–4 wt.% P₂O₅ before saturation in apatite is reached. For a magma 100°C cooler or containing 10 wt.% more SiO₂, apatite saturation occurs at less than 2 wt.% dissolved P₂O₅. The observed high solubility of apatite in basic magmas at their normal near-liquidus temperatures virtually precludes the occurrence of residual apatite in mantle source regions. If relatively low-temperature melting conditions prevail (e.g., 1100°C), as might be possible in H₂O-bearing regions of the upper mantle, apatite could remain in the residue, but only in amounts too small to have significant effects on the rare earth patterns of the liquids.Because of the high solubility of apatite in basic magmas, phosphorus can be confidently treated as an incompatible element in peridotite melting models. Such models, in combination with observed characteristics of basic lavas, indicate that the upper mantle contains ～200 ppm of phosphorus, much less than the chondritic abundance of ～900 ppm.     

Co2+ as a chemical analogue for Fe2+ in high-temperature experiments in basaltic systems

High-temperature experiments on ferromagnesian compositions have been hampered by the rapid absorption of up to 95% of the original iron by platinum and 40% by silver-palladium capsules. Molybdenum or iron capsule materials can decrease or alleviate iron loss, but restrict oxygen fugacities to values near the iron-wustite buffer. Because Co²⁺ is stable at f_O2 =HM and because the solubility of Co in platinum in this range of f_O2 is ～0.05% at temperatures to 1350°C, its use as an analogue for Fe²⁺ is possible. In addition, experiments simulating various Fe²⁺ ratios can be easily performed by choosing appropriate Co²⁺/Fe³⁺ ratios. The cobalt phases produced possess brilliant and distinctive colors which are valuable aids in optical identification of minute phases. The cobalt analogue hypothesis was tested with atmospheric pressure experiments in air on the cobalt analogue of the 1921 Kilauea basalt at three simulated Fe²⁺/Fe³⁺ ratios. The results were compared with those of R.E.T. Hill (1969) for the natural 1921 basalt. The phase relations were the same, with the cobalt system stability fields systematically shifted by about +50°C. Microprobe analysis of olivines and the coexisting glasses indicate that the distribution of Co²⁺ between olivine and melt is independent of temperature and liquid composition. Although the analogue liquid composition differs from the equilibrium composition of the natural system, it may be corrected be employing distribution coefficients (K_D = 0.61 for the Co system; K_D = 0.33 for the Fe system) to closely approximate what the natural system would yield if iron loss did not occur.     

Ion microprobe studies of water in silicate melts. Concentration-dependent water diffusion in obsidian

The ion microprobe at Johnson Space Center has been calibrated for in situ water determinations on a 10-μm scale over the range 0.2 wt.% H₂O to 1.8, 6.8, and 3.7 wt.%, for basaltic, albitic, and rhyolitic glasses, respectively. The basalt glass calibration curve differs substantially from those of albite and rhyolite glasses, indicating a need to carefully match composition and/or melt structure between H₂O standards and unknowns.A value for the diffusivity of water as a function of concentration and time has been calculated from water diffusion profiles measured in rhyolite glasses prepared at 850°C and 700 barsP_t(H₂O) [1]. Transient diffusion into a semi-infinite medium is described by the equation:?(φ/2)?¸/?φ=?(D_w?¸/?φ)/?φ #x003B8;=1, φ=0, θ→ 0, θ→∞, wherex =distance from the cylinder edge,t =time,C₀ =initial concentration,C_s =concentration at the edge,C =concentration at x,θ = C ? C₀/C_s ? C₀,φ = x/t^1/2, andD_w =diffusivity of water. An iterative technique has been used to calculate solutions to the diffusion equation as a function ofD_w [2]. Comparison of these solutions with the ion probe data indicate that, for0.2wt.% ≤ C ≤ 3.7wt.%H₂O,D_w can be described by an exponential function of θ, of the formD_w = D₀exp(bθ), withD₀ (i.e.,D_w at 0.2%) = (0.8?2.2) × 10^?8 cm²/s and2 ≤ b ≤ 4.     

Nickel partitioning between olivine and silicate melt

Partitioning of Ni between olivine and silicate melt has been determined for compositions in the system Fo-Ab-An (1 atm) for temperatures ranging from 1250°C to 1450°C. Nickel concentrations were determined by electron microprobe; concentration levels in the liquids ranged from 0.1% to 0.5%. Platinum capsules or Pt wire loops were used as containers. Equilibrium was evaluated from kinetic considerations and by variation of run parameters; it was documented in one case by a bracketed reversal. No evidence was found for a dependence of the partition coefficient D (Ni in olivine/Ni in liquid) on Ni concentration. D is strongly dependent on melt composition, varying linearly with (1/MgO) at constant temperature. The intrinsic temperature dependence of D is small; the apparent temperature dependence reported in previous studies is largely related to the variation of melt composition with temperature. Our D values determined in the simple system Fo-An-Ab agree well with those reported by Leeman for natural (Fe-bearing) basalt systems. Overall variation of D in our system (and in natural basalts) can be expressed by the regression: D = (124/MgO) ? 0.9Our data are used to evaluate published Ni-MgO relationships in natural basalt series from Kilauea, Crozet, Cape Verde and Baffin Bay. A combination of olivine accumulation and fractional crystallization processes are sufficient to model these series. Using our data, unique “parental” liquids can be specified for each of these series; the MgO content of these liquids varies from 6% to 13%. Basalts with MgO contents greater than these “parental” liquids must be accumulative. The linear Ni-MgO trends, high absolute Ni concentrations, and large spread of Ni contents for the high-MgO basalts argue convincingly against their being “primary” liquids. Models such as those of O'Hara [6,13] and Clarke [24], based on the assertion of primary high-MgO liquids, must therefore be re-evaluated.Because of the high Si/O ratio and low MgO content of island arc andesites, the Ni partition coefficient D may be quite high. Therefore, the relatively low Ni content of such andesites may not be an argument against their derivation as direct partial melts of the mantle.     

Author index volume 40

Tracer diffusion coefficients for Li in glasses of albite, orthoclase, and obsidian composition have been determined by a method involving deposition of a thin source on polished glass wafers, anneal under controlled temperature conditions (300–900°C), and ion-microprobe determination of the concentration profile. All results conform to an Arrhenius-type relationship,D = D₀exp(?Q/RT), whereQ is 23, 17, and 22 kcal mol^?1;D₀ is 0.2, 0.003, and 0.03 cm²s^?1 for albite and orthoclase glasses, and obsidian respectively. Lithium is thus a fast diffusing ion and behaves similarly to sodium in the same glasses. A mechanism involving jumps of the diffusing ions through oxygen hexagonal rings is suggested by consideration of ionic radii ratio of alkali (H, Li, Na, K, Rb, and Cs) ions to the oxygen anions.     

Petrogenesis of basalts from the project FAMOUS area: experimental study from 0 to 15 kbars

Tholeiitic basalt glasses from the FAMOUS area of the Mid-Atlantic Ridge are among the most primitive basaltic liquids reported from the ocean basins. One of the more primitive of these[Mg/(Mg+Fe²⁺) = 0.68;Ni= 232ppm;TiO₂ = 0.61] glasses (572-1-1) was selected for an experimental investigation. This study found olivine to be the liquidus phase from 1 atm to 10.5 kbar where it is replaced by clinopyroxene. The sequence of appearance of phases at 1 atm pressure is olivine (1268°C), plagioclase (1235°C) and clinopyroxene (1135°C). The sample is multiply saturated at 10.5 kbar with olivine (Fo₈₈), clinopyroxene (Wo₃₂En₆₀Fs₉), and orthopyroxene (Wo₅En₈₃Fs₁₂). From the 1-atm data we have measured (FeO/MgO) olivine/(FeO*/MgO) liquid (K′_D) for olivine-melt pairs equilibrated at 12 temperatures in the range 1268–1205°C.K′_D varies from 0.30 at 1205°C to 0.27 at 1268°C. Analysis of high-pressure olivine melt pairs indicates a systematic increase inK′_D with pressure.Evaluation of the 1-atm experiments reveals that fractionation of olivine followed by olivine + plagioclase can generate much of the variation in major element chemistry observed in the FAMOUS basalt glasses. However, it cannot account for the entire spectrum of glass compositions — particularly with respect to TiO₂ and Na₂O. The variations in these components are such as to require different primary liquids.Comparison of clinopyroxene microphenocrysts/xenocrysts found in oceanic tholeiites with experimental clinopyroxenes reveal that the majority of those in the tholeiites may have crystallized from the magma at pressures greater than ～ 10 kbar and are not accidental xenocrysts. Clinopyroxene fractionation at high pressures may be a viable mechanism for fractionating basaltic magmas.The major and minor element mineral/meltK′_d's from our experiments have been used to model the source region residual mineralogy for given percentages of partial melting. These data suggest that ～20% partial melting of a lherzolite source containing 0–10% clinopyroxene can generate the major and minor element concentrations in the parental magmas of the Project FAMOUS basalt glasses.     

Isotopic and REE studies of lunar basalt 12038: implications for petrogenesis of aluminous mare basalts

We report Sr, Nd, and Sm isotopic studies of lunar basalt 12038, one of the so-called aluminous mare basalts. A precise internal Rb-Sr isochron yields a crystallization age of 3.35±0.09 AE and initial⁸⁷Sr/⁸⁶Sr=0.69922?2 (2σ error limits, 1AE=10⁹ years, λ(⁸⁷Rb)=0.0139AE^?1). An internal Sm-Nd isochron yields an age of 3.28±0.23AE and initial¹⁴³Nd/¹⁴⁴Nd=0.50764?28. Present-day¹⁴³Nd/¹⁴⁴Nd is less than the “chondritic” value, i.e. ?(Nd, 0)=?2.3±0.4 where ?(Nd) is the deviation of¹⁴³Nd/¹⁴⁴Nd from chondritic evolution, expressed as parts in 10⁴. At the time of crystallization ?(Nd, 3.2AE)=1.5±0.6.We have successfully modeled the evolution of the Sr and Nd isotopic compositions and the REE abundances within the framework of our earlier model for Apollo 12 olivine-pigeonite and ilmenite basalts. The isotopic and trace element features of 12038 can be modeled as produced by partial melting of a cumulate mantle source which crystallized from a lunar magma ocean with a chondrite-normalized REE pattern of constant negative slope. Chondrite-normalized La/Yb=2.2 for this hypothetical magma ocean pattern. A plot of I(Sr) versus ?(Nd) for the Apollo 12 basalts clearly shows the influence of varying proportions of olivine, clinopyroxene, orthopyroxene, and plagioclase in the basalt source regions. A small percentage of plagioclase (～5%) in the 12038 source apparently is responsible for low I(Sr) and ?(Nd) in this basalt. Aluminous mare basalts from Mare Crisium (Luna 24) and by inference Mare Fecunditatis (Luna 16) occupy locations on the I(Sr)-?(Nd) plot similar to that of 12038, implying that some basalts from three widely separated lunar regions came from plagioclase-bearing source regions. A summary of model calculations for mare basalts shows a record of lunar mantle solidification during the period when REE abundances in the lunar magma ocean increased from ～20× chondritic to >100× chondritic. Although there is a general trend from olivine to clinopyroxene-dominated source regions with progressive magma ocean evolution, significant mineralogical heterogeneities in mantle composition apparently formed at any given stage of evolution, as evidenced in particular by the three Apollo 12 magma types.     

Mineralogy and petrology of sample 67075 and the origin of lunar anorthosites

Plagioclase in cataclastic anorthosite 67075 occurs as angular matrix grains and as recrystallized clasts of micro-anorthosite. Olivines are Fe-rich and fall into two compositional groupings. Large grains of pyroxene show exceptionally well-developed exsolution lamellae analogous to those observed in pyroxenes from layered complexes. The low-Ca component in both pigeonites and augites shows varying degrees of inversion to orthopyroxene. The lattices of host and lamellae may deviate slightly (up to 2°) from the ideal orientation. Very slow cooling from magmatic temperatures is required to produce the coarse exsolution textures and inversion features. Augite macrocrystals are distinctly subcalcic indicating crystallization at temperatures around1100 ± 50°C while host-lamellae pairs and small grains in lithic clasts and matrix indicate reequilibration on a micron scale to temperatures less than 800°C. Pyroxene compositions tend to cluster into two groups both of which are among the most Fe-rich reported for highland pyroxenes. Ti and Al contents of pyroxenes are very low and Ti, Cr, and Mn follow well-established magmatic differentiation trends. The high Cr content may reflect low?O₂ conditions and/or early crystallization of olivine and plagioclase.The⁸⁷Sr/⁸⁶Sr ratios in lunar anorthosites are the lowest reported for any lunar rock. It is likely that anorthosites formed as cumulates during the major differentiation episode which occurred prior to～4.3AE. Recrystallization features are common and³⁹Ar/⁴⁰Ar ages cluster around 4.0 AE. This may be the result of the intense bombardment prior to 4.0 AE which caused repeated cycles of in-situ fracturing and granulation followed by recrystallization. The low siderophile element content and the inferred slow cooling indicate a plutonic source region (10km) not frequently plumbed by impact events. The Fe-rich silicates indicate crystallization from a melt at an advanced stage of fractionation. However, the low REE abundances are not consistent with late-stage crystallization. Plagioclase apparently crystallized relatively early and was concentrated by flotation and/or convection currents while the mafic minerals crystallized from a fractionated trapped liquid. The chemical, isotopic, and mineralogical data place stringent constraints on the nature of genetically related rocks and the relationship of anorthosites to other members of the ANT suite does not appear to be one ofsimple fractionation. The data presented in this paper are consistent with the Taylor-Jake?model of lunar evolution.     

An assessment of local and regional isotopic equilibrium in the mantle

The assumption of local equilibrium during partial melting is fundamental to the interpretation of isotope and trace element data for mantle-derived rocks. If disequilibrium melting is significant, the scale of the chemical and isotopic heterogeneity in the mantle indicated by the data could be as small as the grain size of the mantle rock, and the isotope data themselves are then of doubtful value to the understanding of mantle processes. To assess the scale of isotopic heterogeneity in a partially molten asthenosphere we review the Sr isotopic data of volcanic rocks from oceanic regions and the available experimental data on diffusion kinetics in minerals and melts similar to those existing in the mantle. Although diffusion data are scarce and afflicted with uncertainties, most of the diffusion coefficients for cations in mantle minerals at temperatures of 1000–1200°C appear to be greater than 10^?13 cm² s^?1. Sr diffusion in liquid basalt is more rapid, with diffusion coefficients of D = 10^?7 to 10^?6cm²s^?1 near 1300°C. Simple model calculations show that, with these D values, a fluid-free mantle can maintain a state of disequilibrium on a centimeter scale for periods of 10⁸ to 10⁹ years. The state of disequilibrium found in many mantle-derived xenoliths is thus easily explained. A partially molten mantle, on the other hand, will tend to equilibrate locally in less than 10⁵ to 10⁶ years. The analytical data on natural rocks likewise indicate that the inhomogeneities are both old (>FX1.5 b.y.) and regional in character and that the consistent isotopic difference between ocean island and ocean floor volcanics cannot be explained by small-scale heterogeneity of the source rock.     

CO2- and LREE-rich mantle below eastern Australia: a REE and isotopic study of alkaline magmas and apatite-rich mantle xenoliths from the Southern Highlands Province,Australia

Alkaline magmatism in the Southern Highlands Province, New South Wales, Australia is associated with continental rifting. Near-primary liquids have a wide range in Nd and Sr isotope composition that indicates gross isotopic and chemical heterogeneities in a mantle source region depleted in light rare earth elements (LREE) for much of Earth's history. The large-ion lithophile element and LREE-enriched nature of the primary lavas ((Ce)_N = 95–182 and (Yb)_N = 8.5–13.3) is consistent with an enriched mantle source region. This elemental enrichment may be accomplished by veining of the subcontinental mantle with volatile-rich phases like amphibole, apatite and carbonate which provide the volatile flux necessary to trigger anatexis.Degassing of mantle CO₂ has led to migration of LREE-enriched fluids and local transformation of the lherzolitic mantle to pyroxenite veined by apatite ± kaersutite ± mica ± diopside. The mantle veining event may be related to upwelling of silica-undersaturated incompatible element-enriched magmas similar to the host magma of the Kiama xenoliths. In a relatively short period of time (100 m.y.), the Sr and Nd isotopes in essentially LREE-depleted mantle have evolved in response to low Sm/Nd and low Rb/Sr ratios, and now define a near-vertical vector on a isotope-isotope plot. From this rather unique signature we can infer that CO₂- and LREE-rich, Rb-poor mantle is a potentially suitable mantle source region for the genesis of alkali-potassic volcanic rocks characterized by a narrow range in⁸⁷Sr/⁸⁶Sr ratio and a wide range in¹⁴³Nd/¹⁴⁴Nd ratio (e.g. Leucite Hills).     

Isotope and trace element evidence for late-stage intra-crustal melting in the High Andes

Purico-Chascon is an acid igneous complex less than 1.5 Ma old rising to 5800 m in the North Chilean Andes, and consisting of andesite-dacite cones and dacite domes on an ignimbrite shield. The rocks are subdivided into two groups: those from Chascon appear to exhibit evidence for magma mixing with more basic material now preserved as xenoliths, whereas among those at Purico no xenoliths have been found.⁸⁷Sr/⁸⁶Sr=0.7095?0.7081 at Purico, 0.7079?0.7069 at Chascon, and 0.7061-0.7057 in the xenoliths from the Chascon lavas:¹⁴³Nd/¹⁴⁴Nd=0.51222?0.51236 overall. The Purico lavas are characterised by higher SiO₂, Rb/Sr,⁸⁷Sr/⁸⁶Sr, and REE abundances, and lower Sr/Nd, Sr/Ba and¹⁴³Nd/¹⁴⁴Nd than most Andean igneous suites. There is no indication ofselective crustal contamination of Sr, or any systematic change in isotope ratios during differentiation. Nonetheless the trend of, for example, high Sr/Nd and Sr contents in rocks with low⁸⁷Sr/⁸⁶Sr (0.704, Ecuador) to low Sr/Nd and Sr and high SiO₂ in rocks with⁸⁷Sr/⁸⁶Sr=0.7081?0.7095 at Purico is interpreted as a shift from subduction zone related magmatism to one with greater crustal affinity. The formation of the least evolved Purico lavas (～60%SiO₂) is discussed in terms of bulk assimilation of crustal material, mixing between crustal- and mantle-derived magmas, and partial melting of pre-existing crust. Although such models are still extremely primitive, the simplest explanation of the observed chemical variations is that the Purico rocks evolved from parental magmas derived by crustal anatexies. Thermal considerations suggest that such late-stage crustal anatexis is a predictable response to crustal thickening which in the Andes is thought to have taken place during the Cenozoic.