A combined Sm-Nd, Rb-Sr, and U-Pb isotopic study of Mg-suite norite 78238: Further evidence for early differentiation of the Moon

Lunar Mg-suite norite 78238 was dated using the Sm-Nd, Rb-Sr, and U-Pb isotopic systems in order to constrain the age of lunar magma ocean solidification and the beginning of Mg-suite magmatism, as well as to provide a direct comparison between the three isotopic systems. The Sm-Nd isotopic system yields a crystallization age for 78238 of 4334 ± 37 Ma and an initial value of −0.27 ± 0.74. The age-initial (T-I) systematics of a variety of KREEP-rich samples, including 78238 and other Mg-suite rocks, KREEP basalts, and olivine cumulate NWA 773, suggest that lunar differentiation was completed by 4492 ± 61 Ma assuming a Chondritic Uniform Reservoir bulk composition for the Moon. The Rb-Sr isotopic systematics of 78238 were disturbed by post-crystallization processes. Nevertheless, selected data points yield two Rb-Sr isochrons. One is concordant with the Sm-Nd crystallization age, 4366 ± 53 Ma. The other is 4003 ± 95 Ma and is concordant with an Ar-Ar age for 78236. The ²⁰⁷Pb-²⁰⁶Pb age of 4333 ± 59 Ma is concordant with the Sm-Nd age. The U-Pb isotopic systematics of 78238 yield linear arrays equivalent to younger ages than the Pb-Pb system, and may reflect fractionation of U and Pb during sample handling. Despite the disturbed nature of the U-Pb systems, a time-averaged μ (²³⁸U/²⁰⁴Pb) value of the source can be estimated at 27 ± 30 from the Pb-Pb isotopic systematics. Because KREEP-rich samples are likely to be derived from source regions with the highest U/Pb ratios, the relatively low μ value calculated for the 78238 source suggests the bulk Moon does not have an exceedingly high μ value.     

The age of Dar al Gani 476 and the differentiation history of the martian meteorites inferred from their radiogenic isotopic systematics

Samarium-neodymium isotopic analysis of the martian meteorite Dar al Gani 476 yields a crystallization age of 474 ± 11 Ma and an initial ε_Nd¹⁴³ value of +36.6 ± 0.8. Although the Rb-Sr isotopic system has been disturbed by terrestrial weathering, and therefore yields no age information, an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.701249 ± 33 has been estimated using the Rb-Sr isotopic composition of the maskelynite mineral fraction and the Sm-Nd age. The Sr and Nd isotopic systematics of Dar al Gani 476, like those of the basaltic shergottite QUE94201, are consistent with derivation from a source region that was strongly depleted in incompatible elements early in the history of the solar system. Nevertheless, Dar al Gani 476 is derived from a source region that has a slightly greater incompatible enrichment than the QUE94201 source region. This is not consistent with the fact that the parental magma of Dar al Gani 476 is significantly more mafic than the parental magma of QUE94201, and underscores a decoupling between the major element and trace element-isotopic systematics observed in the martian meteorite suite.Combining the ε_Nd¹⁴²-ε_Nd¹⁴³ isotopic systematics of the martian meteorites yields a model age for planetary differentiation of 4.513_+0.033^−0.027 Ga. Using this age, the parent/daughter ratios of martian mantle sources are calculated assuming a two-stage evolutionary history. The calculated sources have very large ranges of parent/daughter ratios (⁸⁷Rb/⁸⁶Sr = 0.037-0.374; ¹⁴⁷Sm/¹⁴⁴Nd = 0.182-0.285; ¹⁷⁶Lu/¹⁷⁷Hf = 0.028-0.048). These ranges exceed the ranges estimated for terrestrial basalt source regions, but are very similar to those estimated for the sources of lunar mare basalts. In fact, the range of parent/daughter ratios calculated for the martian meteorite sources can be produced by mixing between end-members with compositions similar to lunar mare basalt sources. Two of the sources have compositions that are similar to olivine and pyroxene-rich mafic cumulates with variable proportions of a Rb-enriched phase, such as amphibole, whereas the third source has the composition of liquid trapped in the cumulate pile (i.e. similar to KREEP) after ∼99% crystallization. Correlation between the proportion of trapped liquid in the meteorite source regions and estimates of f_O2, suggest that the KREEP-like component may be hydrous. The success of these models in reproducing the martian meteorite source compositions suggests that the variations in trace element and isotopic compositions observed in the martian meteorites primarily reflect melting of the crystallization products of an ancient magma ocean, and that assimilation of evolved crust by mantle derived magmas is not required. Furthermore, the decoupling of major element and trace element-isotopic systematics in the martian meteorite suite may reflect the fact that trace element and isotopic systematics are inherited from the magma source regions, whereas the major element abundances are limited by eutectic melting processes at the time of magma formation. Differences in major element abundances of parental magma, therefore, result primarily from fractional crystallization after leaving their source regions.     

Experimental study of polybaric REE partitioning between olivine, pyroxene and melt of the Yamato 980459 composition: Insights into the petrogenesis of depleted shergottites

A synthetic composition representing the Yamato 980459 martian basalt (shergottite) has been used to carry out phase relation, and rare earth element (REE) olivine and pyroxene partitioning experiments. Yamato 980459 is a sample of primitive basalt derived from a reduced end-member among martian mantle sources. Experiments carried out between 1-2 GPa and 1350-1650 °C simulate the estimated pressure-temperature conditions of basaltic melt generation in the martian mantle. Olivine-melt and orthopyroxene-melt partition coefficients for La, Nd, Sm, Eu, Gd and Yb (D_REE values) were determined by LA-ICPMS, and are similar to the published values for terrestrial basaltic systems. We have not detected significant variation in D-values with pressure over the range investigated, and by comparison with previous studies carried out at lower pressure.We apply the experimentally obtained olivine-melt and orthopyroxene-melt D_REE values to fractional crystallization and partial melting models to develop a three-stage geochemical model for the evolution of martian meteorites. In our model we propose two ancient (∼4.535 Ga) sources: the Nakhlite Source, located in the shallow mantle, and the Deep Mantle Source, located close to the martian core-mantle boundary. These two sources evolved distinctly on the ε¹⁴³Nd evolution curve due to their different Sm/Nd ratios. By partially melting the Nakhlite Source at ∼1.3 Ga, we are able to produce a slightly depleted residue (Nakhlite Residue). The Nakhlite Residue is left undisturbed until ∼500 Ma, at which point the depleted Deep Mantle Source is brought up by a plume mechanism carrying with it high heat flow, melts and isotopic signatures of the deep mantle (e.g., ε¹⁸²W, ε¹⁴²Nd, etc.). The plume-derived Deep Mantle Source combines with the Nakhlite Residue producing a mixture that becomes a mantle source (herein referred to as “the Y98 source”) for Yamato 980459 and the other depleted shergottites with the characteristic range of Sm/Nd ratios of these meteorites. The same hot plume provides a heat source for the formation of enriched and intermediate shergottites. Our model reproduces the REE patterns of nakhlites and depleted shergottites and can explain high ε¹⁴³Nd in depleted shergottites. Furthermore, the model results can be used to interpret whole rock Rb-Sr and Sm-Nd ages of shergottites.     

Determination of Sr and Ba partition coefficients between apatite from fish (Sparus aurata) and seawater: The influence of temperature

The Sr/Ca and Ba/Ca ratios in inorganic apatite are strongly dependent on the temperature of the aqueous medium during precipitation. If valid in biogenic apatite, these thermometers would offer the advantage of being more resistant to diagenesis than those calibrated on biogenic calcite and aragonite. We have reared seabreams (Sparus aurata) in tanks with controlled conditions during experiments lasting for more than 2 years at 13, 17, 23 and 27 °C, in order to determine the variations in Sr and Ba partitioning relative to Ca (D^Sr and D^Ba, respectively) between seawater and fish apatitic hard tissues (i.e. teeth and bones), as a function of temperature. The sensitivity of the Sr and Ba thermometers (i.e. ∂D^Sr/∂T and ∂D^Ba/∂T, respectively), are similar in bone (/∂T = 0.0036 ± 0.0003 and /∂T = 0.0134 ± 0.0026, respectively) and enamel (/∂T = 0.0037 ± 0.0005 and /∂T = 0.0107 ± 0.0026, respectively). The positive values of ∂D^Sr/∂T and ∂D^Ba/∂T in bone and enamel indicate that D^Sr and D^Ba increase with increasing temperature, a pattern opposite to that observed for inorganic apatite. This distinct thermodependent trace element partitioning between inorganic and organic apatite and water highlights the contradictory effects of the crystal-chemical and biological controls on the partitioning of Ca, Sr and Ba in vertebrate organisms. Taking into account the diet Sr/Ca and Ba/Ca values, it is shown that the bone Ba/Ca signature of fish can be explained by Ca-biopurification and inorganic apatite precipitation, whereas both of these processes fail to predict the bone Sr/Ca values. Therefore, the metabolism of Ca as a function of temperature still needs to be fully understood. However, the biogenic Sr thermometer is used to calculate an average seawater temperature of 30.6 °C using the Sr/Ca compositions of fossil shark teeth at the Cretaceous/Tertiary boundary, and a typical seawater Sr/Ca ratio of 0.02. Finally, while the present work should be completed with data obtained in natural contexts, it is clear that Sr/Ca and Ba/Ca ratios in fossil biogenic apatite already constitute attractive thermometers for marine paleoenvironments.     

Petrogenesis of the magmatic complex at Mount Ascutney,Vermont, USA

The Ascutney Mountain complex of eastern Vermont, USA, is a composite epizonal pluton of genetically related gabbro to granite intrusives. Nd isotopic data are reported for mafic rocks, granites, and nearby country rock. The parental mafic magma producing the complex 122 m.y. ago had ⁸⁷Sr/⁸⁶Sr=0.7039, ¹⁴³Nd/¹⁴⁴Nd=0.512678 ( _Nd=+3.8) and ¹⁸O=6.1, indicating a mantle source with time-integrated lithophile element depletion. Uniform initial radiogenic isotope ratios for granites, which are undistinguishable from those for the most primitive gabbro, suggest that the granite magma evolved from the mafic magma without crustal contamination and that the increase in ¹⁸O, to about 7.8, is the result of fractional crystallization. Mafic rocks show a large range in initial ¹⁴³Nd/¹⁴⁴Nd ratio, from about 0.51267 to 0.51236 ( _Nd= +3.7 to –2.5), which is correlated with elevated ⁸⁷Sr/⁸⁶Sr ratios and ¹⁸O. These data substantiate the production of mafic lithologies by fractional crystallization of the parental magma accompanied by assimilation of up to about 50% crust. The local country rocks include gneiss and schist and assimilation involved representatives of both rock types. The isotopic and chemical relationships preclude derivation from a single batch of magma undergoing contamination and indicate that a large magma body at depth evolved largely by fractionation with batches of melt issued from this chamber being variably contaminated at higher levels or at the level of emplacement.The Precambrian gneisses of the Chester dome and overlying lower Paleozoic schists have essentially identical Nd isotope systematics which suggest a crustal formation age of about 1.6. b.y. The parental sediments for the schists were apparently derived from a protolith similar to the gneissic basement without appreciable Sm/Nd fractionation.     

The early differentiation history of Mars from W-Nd isotope systematics in the SNC meteorites

We report here the results of an investigation of W and Nd isotopes in the SNC (Shergottite-Nakhlite-Chassignite (martian)) meteorites. We have determined that ε¹⁸²W values in the nakhlites are uniform within analytical uncertainties and have an average value of ∼3. Also, while ε¹⁸²W values in the shergottites have a limited range (from 0.3-0.7), their ε¹⁴²Nd values vary considerably (from −0.2-0.9). There appears to be no correlation between ε¹⁸²W and ε¹⁴²Nd in the nakhlites and shergottites. These results shed new light on early differentiation processes on Mars, particularly on the timing and nature of fractionation in silicate reservoirs. Assuming a two-stage model, the metallic core is estimated to have formed at ∼12 Myr after the beginning of the solar system. Major silicate differentiation established the nakhlite source reservoir before ∼4542 Ma and the shergottite source reservoirs at 4525 Ma. These ages imply that, within the uncertainties afforded by the ¹⁸²Hf-¹⁸²W and ¹⁴⁶Sm-¹⁴²Nd chronometers, the silicate differentiation events that established the source reservoirs of the nakhlites and shergottites may have occurred contemporaneously, possibly during crystallization of a global magma ocean. The distinct ¹⁸²W-¹⁴²Nd isotope systematics in the nakhlites and the shergottites imply the presence of at least three isotopically distinct silicate reservoirs on Mars, two of which are depleted in incompatible lithophile elements relative to chondrites, and the third is enriched. The two depleted silicate reservoirs most likely reside in the Martian mantle, while the enriched reservoir could be either in the crust or the mantle. Therefore, the ¹⁸²W-¹⁴²Nd isotope systematics indicate that the nakhlites and the shergottites originated from distinct source reservoirs and cannot be petrogenetically related. A further implication is that the source reservoirs of the nakhlites and shergottites on Mars have been isolated since their establishment before ∼4.5 Ga. Therefore, there has been no giant impact or efficient global mantle convection to thoroughly homogenize the Martian mantle following the establishment of the SNC source reservoirs.     

Ammonium loss and nitrogen isotopic fractionation in biotite as a function of metamorphic grade in metapelites from western Maine, USA

Ammonium fixed in micas of metamorphic rocks is a sensitive indicator both of organic-inorganic interactions during diagenesis as well as of the devolatilization history and fluid/rock interaction during metamorphism. In this study, a collection of geochemically well-characterized biotite separates from a series of graphite-bearing Paleozoic greenschist- to upper amphibolite-facies metapelites, western Maine, USA, were analyzed for ammonium nitrogen () contents and isotopic composition (δ¹⁵N_NH4) using the HF-digestion distillation technique followed by the EA-IRMS technique. Biotite separates, sampled from 9 individual metamorphic zones, contain 3000 to 100 ppm with a wide range in δ¹⁵N from +1.6‰ to +9.1‰. Average contents in biotite show a distinct decrease from about 2750 ppm for the lowest metamorphic grade (∼500 °C) down to 218 ppm for the highest metamorphic grade (∼685 °C). Decreasing abundances in are inversely correlated in a linear fashion with increasing K⁺ in biotite as a function of metamorphic grade and are interpreted as a devolatilization effect. Despite expected increasing δ¹⁵N_NH4 values in biotite with nitrogen loss, a significant decrease from the Garnet Zones to the Staurolite Zones was found, followed by an increase to the Sillimanite Zones. This pattern for δ¹⁵N_NH4 values in biotite inversely correlates with Mg/(Mg + Fe) ratios in biotite and is discussed in the framework of isotopic fractionation due to different exchange processes between or , reflecting devolatilization history and redox conditions during metamorphism.     

Molybdenum, tungsten and manganese partitioning in the system pyrrhotite-Fe-S-O melt-rhyolite melt: Impact of sulfide segregation on arc magma evolution

Experiments were performed to determine the partitioning of molybdenum, tungsten and manganese among a rhyolitic melt (melt), pyrrhotite (po), and an immiscible Fe-S-O melt (Fe-S-O). Sulfide phases such as these may be isolated from a silicate melt along with other crystallizing phases during the evolution of arc magma, and partition coefficients are required to model the effect of this process on molybdenum and tungsten budgets.We developed an experimental design to take advantage of properties of the phases under study. Careful control of temperature allowed pyrrhotite and magnetite to be stable along with an Fe-S-O melt, and this phase assemblage allowed the composition of run-product pyrrhotite to be used to calculate both fS₂ and fO₂ for the experiments. At run temperature, (1042 ± 2 °C), a rhyolitic melt can be formed at low pressure, under nominally dry conditions, which removed the need for confining pressure as well as externally imposed fugacities. The silica-saturated melt allowed the charges to be contained in sealed evacuated silica tubes without danger of reaction, and with closed system behavior for molybdenum and tungsten.Experiments were run for durations up to 2000 min. Molybdenite (mb) and wolframite (wo) were added to the experiments as sources for molybdenum and tungsten, respectively. Manganese was added to the system as both a component of the starting rhyolitic pumice, and of Mn-bearing wolframite. Oxygen fugacity in these experiments was fixed at the Ni-NiO oxygen fugacity buffer. Sulfur fugacity was 10⁻¹ bar. Run products were analyzed by EPMA and LA-ICP-MS. Analysis of the run products yielded ( standard deviation of the mean): , , , and . The partition coefficients for manganese in this system are and .Simple Rayleigh fractionation modeling suggests that oxidized felsic melts produced through fractional crystallization may have lost as much as 14% of their initial molybdenum, but only 2% of their initial tungsten, through the removal of an Fe-S-O melt along with crystalline phases. Modeling consistent with conditions of oxygen and sulfur fugacity influenced by assimilation of sulfide (with low concentrations of molybdenum and tungsten) from, for example, sedimentary rock, results in evolved magmas significantly depleted in molybdenum, but only moderately depleted in tungsten. The molybdenum:tungsten ratio can vary by two orders of magnitude. These systematics may help to explain some of the variability in metal ratios of intrusion-related hydrothermal ore deposits.     

Carbon solubility in core melts in a shallow magma ocean environment and distribution of carbon between the Earth’s core and the mantle

The solubility of carbon in Fe and Fe-5.2 wt.% Ni melts, saturated with graphite, determined by electron microprobe analysis of quenched metal melts was 5.8 ± 0.1 wt.% at 2000 °C, 6.7 ± 0.2 wt.% at 2200 °C, and 7.4 ± 0.2 wt.% at 2410 °C at 2 GPa, conditions relevant for core/mantle differentiation in a shallow magma ocean. These solubilities are slightly lower than low-pressure literature values and significantly beneath calculated values for even higher pressures [e.g., Wood B. J. (1993) Carbon in the core. Earth Planet. Sci. Lett.117, 593-607]. The trend of C solubility versus temperature for Fe-5.2 wt.% Ni melt, within analytical uncertainties, is similar to or slightly lower (∼0.2-0.4 wt.%) than that of pure Fe. Carbon content of core melts and residual mantle silicates derived from equilibrium batch or fractional segregation of core liquids and their comparison with our solubility data and carbon content estimate of the present day mantle, respectively, constrain the partition coefficient of carbon between silicate and metallic melts, in a magma ocean. For the entire range of possible bulk Earth carbon content from chondritic to subchondritic values, of 10⁻⁴ to 1 is derived. But for ∼1000 ppm bulk Earth carbon, is between 10⁻² and 1. Using the complete range of possible for a magma ocean at ∼2200 °C, we predict maximum carbon content of the Earth’s core to be ∼6-7 wt.% and a preferred value of 0.25 ± 0.15 wt.% for a bulk Earth carbon concentration of ∼1000 ppm.     

Geochemical evolution of historical lavas from Askja Volcano, Iceland: Implications for mechanisms and timescales of magmatic differentiation

The mechanisms and the timescales of magmatic evolution were investigated for historical lavas from the Askja central volcano in the Dyngjufjöll volcanic massif, Iceland, using major and trace element and Sr, Nd, and Pb isotopic data, as well as ²³⁸U-²³⁰Th-²²⁶Ra systematics. Lavas from the volcano show marked compositional variation from magnesian basalt through ferrobasalt to rhyolite. In the magnesian basalt-ferrobasalt suite (5-10 wt% MgO), consisting of lavas older than 1875 A.D., ⁸⁷Sr/⁸⁶Sr increases systematically with increasing SiO₂ content; this suite is suggested to have evolved in a magma chamber located at ∼600 MPa through assimilation and fractional crystallization. On the other hand, in the ferrobasalt-rhyolite suite (1-5 wt% MgO), including 1875 A.D. basalt and rhyolite and 20th century lavas, ⁸⁷Sr/⁸⁶Sr tends to decrease slightly with increasing SiO₂ content. It is suggested that a relatively large magma chamber occupied by ferrobasalt magma was present at ∼100 MPa beneath the Öskjuvatn caldera, and that icelandite and rhyolite magmas were produced by extraction of the less and more evolved interstitial melt, respectively, from the mushy boundary layer along the margin of the ferrobasalt magma chamber, followed by accumulation of the melt to form separate magma bodies. Ferrobasalt and icelandite lavas in the ferrobasalt-rhyolite suite have a significant radioactive disequilibrium in terms of (²²⁶Ra/²³⁰Th), and its systematic decrease with magmatic evolution is considered to reflect aging, along with assimilation and fractional crystallization processes. Using a mass-balance model in which simultaneous fractional crystallization, crustal assimilation, and radioactive decay are taken into account, the timescale for the generation of icelandite magma from ferrobasalt was constrained to be <∼3 kyr which is largely dependent on Ra crystal-melt partition coefficients we used.     

Evidence for shock heating and constraints on Martian surface temperatures revealed by Ar/Ar thermochronometry of Martian meteorites

The thermal histories of Martian meteorite are important for the interpretation of petrologic, geochemical, geochronological, and paleomagnetic constraints that they provide on the evolution of Mars. In this paper, we quantify ⁴⁰Ar/³⁹Ar ages and Ar diffusion kinetics of Martian meteorites Allan Hills (ALH) 84001, Nakhla, and Miller Range (MIL) 03346. We constrain the thermal history of each meteorite and discuss the resulting implications for their petrology, paleomagnetism, and geochronology. Maskelynite in ALH 84001 yields a ⁴⁰Ar/³⁹Ar isochron age of 4163 ± 35 Ma, which is indistinguishable from recent Pb-Pb (Bouvier et al., 2009a) and Lu-Hf ages (Lapen et al., 2010). The high precision of this result arises from clear resolution of a reproducible trapped ⁴⁰Ar/³⁶Ar component in maskelynite in ALH 84001 (⁴⁰Ar/³⁶Ar = 632 ± 90). The maskelynite ⁴⁰Ar/³⁹Ar age predates the Late Heavy Bombardment and likely represents the time at which the original natural remanent magnetization (NRM) component observed in ALH 84001 was acquired. Nakhla and MIL 03346 yield ⁴⁰Ar/³⁹Ar isochron ages of 1332 ± 24 and 1339 ± 8 Ma, respectively, which we interpret to date crystallization. Multi-phase, multi-domain diffusion models constrained by the observed Ar diffusion kinetics and ⁴⁰Ar/³⁹Ar age spectra suggest that localized regions within both ALH 84001 and Nakhla were intensely heated for brief durations during shock events at 1158 ± 110 and 913 ± 9 Ma, respectively. These ages may date the marginal melting of pyroxene in each rock, mobilization of carbonates and maskelynite in ALH 84001, and NRM overprints observed in ALH 84001. The inferred peak temperatures of the shock heating events (>1400 °C) are sufficient to mobilize Ar, Sr, and Pb in constituent minerals, which may explain some of the dispersion observed in ⁴⁰Ar/³⁹Ar, Rb-Sr, and U-Th-Pb data toward ages younger than ∼4.1 Ga. The data also place conservative upper bounds on the long-duration residence temperatures of the ALH 84001 and Nakhla protolith to be  °C and  °C over the last ∼4.16 Ga and ∼1.35 Ga, respectively. MIL 03346 has apparently not experienced significant shock-heating since it crystallized, consistent with the fact that various chronometers yield concordant ages.     

Origin of Sr,Nd and Pb isotopic systematics in high-Sr basalts from central Arizona

Alkalic and tholeiitic basalts were erupted in the central Arizona Transition Zone during Miocene-Pliocene time before and after regional faulting. The alkalic lava types differ from the subalkaline lavas in Sr, Nd and Pb isotopic ratios and trace element ratios and, despite close temporal and spatial relationships, the two types appear to be from discrete mantle sources. Pre-faulting lava types include: potassic trachybasalts (⁸⁷Sr/⁸⁶Sr = 0.7052 to 0.7055, _Nd= –9.2 to –10.7); alkali olivine basalts (⁸⁷Sr/ ⁸⁶Sr = 0.7049 to 0.7054, _Nd= –2 to 0.2); basanite and hawaiites (⁸⁷Sr/⁸⁶Sr = 0.7049 to 0.7053, _Nd= –3.5 to –7.8); and quartz tholeiites (⁸⁷Sr/⁸⁶Sr = 0.7047, _Nd= –1.4 to –2.6). Post-faulting lavas have lower ⁸⁷Sr/⁸⁶Sr (<0.7045) and _Nd from –3.2 to 2.3. Pb isotopic data for both preand post-faulting lavas form coherent clusters by magma type with values higher than those associated with MORB but within the range of values found for crustal rocks and sulfide ores in Arizona and New Mexico. Pb isotopic systematics appear to be dominated by crustal contamination. Effects of assimilation and fractional crystallization are inadequate to produce the Sr isotopic variations unless very large amounts of assimilation occurred relative to fractionation. It is impossible to produce the Nd isotopic variations unless ancient very unradiogenic material exists beneath the region. Moreover the assumption that the alkalic lavas are cogenetic requires high degrees of fractionation inconsistent with major- and trace-element data. Metasomatism of the subcontinental lithosphere above a subduction zone by a slab-derived fluid enriched in Sr, Ba, P and K could have produced the isotopic and elemental patterns. The degree of metasomatism apparently decreased upward, with the alkalic lavas sampling more modified regions of the mantle than the tholeiitic lavas. Such metasomatism may have been a regional event associated with crustal formation at about 1.6 Ga. Disruption and weakening of the subcontinental lithosphere in the Transition Zone of the Colorado Plateau by volcanism probably made deformation possible.     

Nd and Sr isotopes in ultrapotassic volcanic rocks from the Leucite Hills,Wyoming

Nd and Sr isotopic compositions and Rb, Sr, Sm and Nd concentrations are reported for madupites, wyomingites and orendites from the Pleistocene volcanic field of the Leucite Hills, Wyoming. All Leucite Hills rocks have negative εNd signatures, indicating derivation or contribution from an old light rare earth element (LREE) enriched source. In this respect they are similar to all occurrences of high potassium magmas so far investigated. But Sr isotopic variations are comparatively small and ⁸⁷Sr/ ⁸⁶Sr ratios are unusually low for high-K magmas (0.7053–0.7061, one sample excluded). These values suggest that the light REE enrichment of the source was not accompanied by a strong increase in Rb/Sr. Wyomingites and orendites are isotopically indistinguishable which is consistent with chemical and petrographic evidence for their derivation from a common magma series depending on emplacement conditions. Basic to ultrabasic madupites and more silicic wyomingites/orendites are distinct in their Nd isotopic variations (madupites: εNd= ?10.5 to ?12.3; wyomingites/orendites: εNd= ?13.7 to ?17.0) despite similar Sm/Nd ratios and complete overlap in ⁸⁷Sr/⁸⁶Sr. Selective or bulk assimilation of crustal material is unlikely to have significantly affected the Nd and Sr isotopic compositions of the magmas. The measured isotopic ratios are considered to reflect source values. The distinct isotopic characteristics of madupite and wyomingite/orendite magmas preclude their derivation by fractional crystallization, from a common primary magma, by liquid immiscibility or by partial melting of a homogeneous source. Two isotopically distinct, LREE enriched and slightly heterogeneous sources are required. Heterogeneities were most pronounced between magma sources from each volcanic centre (butte or mesa). The relationship between the madupite and wyomingite/orendite sources and their evolution is discussed on the basis of two simple alternative sets of models:

1. a two-stage evolution model with an old enrichment event (a metasomatic event?) perhaps taking place during the stabilization of the Wyoming Craton 3.2 to 2.5 Gyr ago but not later than 1.2 Gyr ago or
2. a mixing model involving mixing between one endmember with εNd near zero and another end-member with a strong negative εNd signature.

     

Complexation of Cu in Hydrothermal NaCl Brines: Ab initio molecular dynamics and energetics

Chloride complexation of Cu⁺ controls the solubility of copper(I) oxide and sulfide ore minerals in hydrothermal and diagenetic fluids. Solubility measurements and optical spectra of high temperature CuCl solutions have been interpreted as indicating the formation of CuCl, , and complexes. However, no other monovalent cation forms tri- and tetrachloro complexes. EXAFS spectra of high temperature Cu-Cl solutions, moreover, appear to show only CuCl and complexes at T > 100 °C. To reconcile these results, I investigated the nature and stability of Cu-Cl complexes using ab initio cluster calculations and ab initio (Car-Parrinello) molecular dynamics simulations for CuCl-NaCl-H₂O systems at 25 to 450 °C. Ab initio molecular dynamic simulations of 1 m CuCl in a 4 m Cl solution give a stable complex at 25 °C over 4 ps but show that the third Cl⁻ is weakly bound. When the temperature is increased along the liquid-vapour saturation curve to 125 °C, the complex dissociates into and Cl⁻; only forms at 325 °C and 1 kbar. Even in a 15.6 m Cl brine at 450 °C, only the complex forms over a 4 ps simulation run.Cluster calculations with a static dielectric continuum solvation field (COSMO) were used in an attempt directly estimate free energies of complex formation in aqueous solution. Consistent with the MD simulations, the complex is slightly stable at 25 °C but decreases in stability with decreasing dielectric constant (ε). The complex is predicted to be unstable at 25 °C and becomes increasingly unstable with decreasing dielectric constant. In hydrothermal fluids (ε < 30) both the and complexes are unstable to dissociation into and Cl⁻.The results obtained here are at odds with recent equations of state that predict and complexes are the predominant species in hydrothermal brines. In contrast, I predict that only complexes will be significant at T > 125 °C, even in NaCl-saturated brines. The high-temperature (T > 125 °C) optical spectra of CuCl solutions and solubility measurements of Cu minerals in Cl-brines need to be reinterpreted in terms of only the CuCl and complexes.     

An experimental study of Cobalt (II) complexation in Cl and H2S-bearing hydrothermal solutions

The speciation of cobalt (II) in Cl⁻ and H₂S-bearing solutions was investigated spectrophotometrically at temperatures of 200, 250, and 300 °C and a pressure of 100 bars, and by measuring the solubility of cobaltpentlandite at temperatures of 120-300 °C and variable pressures of H₂S. From the results of these experiments, it is evident that CoHS⁺ and predominate in the solutions except at 150 °C, for which the dominant chloride complex is CoCl₃⁻. The logarithms of the stability constant for CoHS⁺ show moderate variation with temperature, decreasing from 6.24 at 120 °C to 5.84 at 200 °C, and increasing to 6.52 at 300 °C. Formation constants for chloride species increase smoothly with temperature and at 300°C their logarithms reach 8.33 for , 6.44 for CoCl₃⁻, 4.94 to 5.36 for , and 2.42 for CoCl⁺. Calculations based on the composition of a model hydrothermal fluid (Ksp-Mu-Qz, KCl = 0.25 m, NaCl = 0.75 m, ΣS = 0.3 m) suggest that at temperatures ?200 °C, cobalt occurs dominantly as CoHS⁺, whereas at higher temperatures the dominant species is .     

Constraints on the petrogenesis of Martian meteorites from the Rb-Sr and Sm-Nd isotopic systematics of the lherzolitic shergottites ALH77005 and LEW88516

Detailed Rb-Sr and Sm-Nd isotopic analyses have been completed on the lherzolitic shergottites ALH77005 and LEW88516. ALH77005 yields a Rb-Sr age of 185 ± 11 Ma and a Sm-Nd age of 173 ± 6 Ma, whereas the Rb-Sr and Sm-Nd ages of LEW88516 are 183 ± 10 and 166 ± 16 Ma, respectively. The initial Sr isotopic composition of ALH77005 is 0.71026 ± 4, and the initial ε_Nd value is +11.1 ± 0.2. These values are distinct from those of LEW88516, which has an initial Sr isotopic composition of 0.71052 ± 4 and an initial ε_Nd value of +8.2 ± 0.6. Several of the mineral and whole rock leachates lie off the Rb-Sr and Sm-Nd isochrons, indicating that the isotopic systematics of the meteorites have been disturbed. The Sm-Nd isotopic compositions of the leachates appear to be mixtures of primary igneous phosphates and an alteration component with a low ¹⁴³Nd/¹⁴⁴Nd ratio that was probably added to the meteorites on Mars. Tie lines between leachate-residue pairs from LEW88516 mineral fractions and whole rocks have nearly identical slopes that correspond to Rb-Sr ages of 90 ± 1 Ma. This age may record a major shock event that fractionated Rb/Sr from lattice sites located on mineral grain boundaries. On the other hand, the leachates could contain secondary alteration products, and the parallel slopes of the tie lines could be coincidental.Nearly identical mineral modes, compositions, and ages suggest that these meteorites are very closely related. Nevertheless, their initial Sr and Nd isotopic compositions differ outside analytical uncertainty, requiring derivation from unique sources. Assimilation-fractional-crystallization models indicate that these two lherzolitic meteorites can only be related to a common parental magma, if the assimilant has a Sr/Nd ratio near 1 and a radiogenic Sr isotopic composition. Further constraints placed on the evolved component by the geochemical and isotopic systematics of the shergottite meteorite suite suggest that it (a) formed at ∼4.5 Ga, (b) has a high La/Yb ratio, (c) is an oxidant, and (d) is basaltic in composition or is strongly enriched in incompatible elements. The composition and isotopic systematics of the evolved component are unlike any evolved lunar or terrestrial igneous rocks. Its unusual geochemical and isotopic characteristics could reflect hydrous alteration of an evolved Martian crustal component or hydrous metasomatism within the Martian mantle.     

Trace element systematics and Sm-Nd and Lu-Hf ages of Larkman Nunatak 06319: Closed-system fractional crystallization of an enriched shergottite magma

Combined ¹⁴⁷Sm-¹⁴³Nd and ¹⁷⁶Lu-¹⁷⁶Hf chronology of the martian meteorite Larkman Nunatak (LAR) 06319 indicates an igneous crystallization age of 193 ± 20 Ma (2σ weighted mean). The individual ¹⁴⁷Sm-¹⁴³Nd and ¹⁷⁶Lu-¹⁷⁶Hf internal isochron ages are 183 ± 12 Ma and 197 ± 29 Ma, respectively, and are concordant with two previously determined ¹⁴⁷Sm-¹⁴³Nd and ⁸⁷Rb-⁸⁷Sr internal isochron ages of 190 ± 26 Ma and 207 ± 14 Ma, respectively (Shih et al., 2009). With respect to the ¹⁴⁷Sm-¹⁴³Nd isotope systematics, maskelynite lies above the isochron defined by primary igneous phases and is therefore not in isotopic equilibrium with the other phases in the rock. Non-isochronous maskelynite is interpreted to result from shock-induced reaction between plagioclase and partial melts of pyroxene and phosphate during transformation to maskelynite, which resulted in it having unsupported ¹⁴³Nd relative to its measured ¹⁴⁷Sm/¹⁴⁴Nd ratio. The rare earth element (REE) and high field strength element (HFSE) compositions of major constituent minerals can be modeled as the result of progressive crystallization of a single magma with no addition of secondary components. The concordant ages, combined with igneous textures, mineralogy, and trace element systematics indicate that the weighted average of the radiometric ages records the true crystallization age of this rock. The young igneous age for LAR 06319 and other shergottites are in conflict with models that advocate for circa 4.1 Ga crystallization ages of shergottites from Pb isotope compositions, however, they are consistent with updated crater counting statistics indicating that young volcanic activity on Mars is more widespread than previously realized (Neukum et al., 2010).     

Petrogenesis of the Laguna del Maule volcanic complex,Chile (36° S)

From 33°–42° S in central-south Chile, there are numerous volcanoes which form part of the Andean magmatic arc caused by subduction of the Nazca plate beneath western South America. The <0.3 m.y. old Laguna del Maule volcanic complex at 36° S is in a transition region between volcanoes at 33°–34° S formed dominantly of hornblende-bearing andesite and volcanoes south of 37° S dominantly composed of basalt and basaltic andesite. The Laguna del Maule complex ranges in composition from basalt (0.3 m.y.) to rhyolite (post-glacial). Although there is abundant evidence for magma mixing, basalt and rhyolite have similar Sr and Nd isotopic ratios, thereby requiring that the mixing members had the same isotopic ratios (⁸⁷Sr/ ⁸⁶Sr 0.70419 and ¹⁴³Nd/¹⁴⁴Nd 0.51274). In contrast, dacitic dikes and a volcanic neck which also have evidence for magma mixing are isotopically distinct. Major and trace element abundances are consistent with a genetic relationship between the basalt and rhyolite, either by low-pressure, plagioclase-dominated, fractional crystallization or by partial melting of a plagioclase-rich assemblage. There is no evidence that the rhyolites contain more of a crustal component than the associated basic volcanics.