Bulk Rock Scale Research Articles

Zirconium stable isotope fractionation during intra-crustal magmatic differentiation in an active continental arc

Zirconium (Zr) stable isotope variations occur among co-existing Zr-rich accessory phases as well as at the bulk-rock scale, but the petrologic mechanism(s) responsible for Zr isotope fractionation during magmatic differentiation remain unclear. Juvenile magma generation and intra-crustal differentiation in convergent continental margins may play a crucial role in developing Zr isotope variations, and the Northern Volcanic Zone of the Andes is an ideal setting to test this hypothesis. To investigate the influence of these processes on Zr stable isotope compositions, we report δ94/90ZrNIST of whole rock samples from: 1) juvenile arc basalts from the Quaternary Granatifera Tuff, Colombia; 2) lower crust-derived garnet pyroxenites (i.e., arclogites), hornblendites, and gabbroic cumulates found in the same unit; and 3) felsic volcanic products from the Doña Juana Volcanic Complex, a dacitic composite volcano in close proximity to and partially covering the Granatifera Tuff. The basalts have δ94/90ZrNIST values ranging from −0.025 ± 0.018 ‰ to +0.003 ± 0.015 ‰ (n = 8), within the range of mid-ocean ridge basalts. The dacites have δ94/90ZrNIST values ranging from +0.008 ± 0.013 ‰ to +0.043 ± 0.015 ‰ (n = 14), slightly positive relative to the Granatifera and mid-ocean ridge basalts. In contrast, the (ultra)mafic cumulates have highly variable, predominantly positive δ94/90ZrNIST values, ranging from −0.134 ± 0.012 ‰ to +0.428 ± 0.012 ‰ (n = 15). Individual grains and mineral fractions of major rock-forming phases, including garnet (n = 21), amphibole (n = 9), and clinopyroxene (n = 18), were analyzed from 8 (ultra)mafic cumulates. The mineral fractions record highly variable Zr isotopic compositions, with inter-mineral fractionation (Δ94/90Zrgarnet-amphibole) up to 2.067 ‰. Recent ab initio calculations of Zr–O bond force constants in rock-forming phases predict limited inter-mineral Zr isotope fractionation in high-temperature environments, suggesting that the large fractionations we observe are not the product of vibrational equilibrium processes. Instead, we propose a scenario in which large Zr isotopic fractionations develop kinetically, induced by sub-solidus Zr diffusion between coexisting phases via changes in Zr distribution coefficients that arise from changes in temperature. Altogether, Zr isotope variability in this calc-alkaline continental arc setting exhibits no correlation with indices of magmatic differentiation (e.g., Mg#, SiO2), and is not a simple function of fractional crystallization. Furthermore, the garnet clinopyroxenite cumulates studied here represent density-unstable lower arc crust material; consequently, material with isotopically variable δ94/90Zr can be recycled into the mantle as a consequence of lower crustal foundering.

Alteration of basaltic glass within the Surtsey hydrothermal system, Iceland – Implication to oceanic crust seawater interaction

Low-temperature hydrothermal alteration of basaltic glass on the seafloor has important implications on the chemical evolution of the oceanic crust and seawater composition. However, mass fluxes resulting from seawater-glass interaction in this type of environment remain poorly understood. This study presents new results on element mobilities for the palagonitization of basaltic glass and bulk rock hydrothermal alteration at Surtsey volcano, Iceland over a time period of ~50-years. Assessments of element mobilities were based on 1) immobile trace element isocon mass balance for palagonitization, 2) the assumption of large scale TiO2 immobility during bulk rock alteration and 3) chemical analyses of pore fluids and authigenic minerals.Element mobilities of glass alteration vary between initial palagonitization and subsequent palagonite maturation: Initially, palagonitization of sideromelane leaches SiO2, Al2O3, MgO, CaO, Na2O, K2O from the glass. Following this, the maturation of the palagonitized glass causes re-uptake of all of the abovementioned elements except for CaO, which shows either no change or slight uptake, and Na2O, which continues to be lost from the palagonitized glass. Among major elements TiO2 and FeO can be considered immobile during palagonitization, but not during the subsequent aging process of palagonitized glass. Among trace elements Hf, Ta, Zr, Nb, Y and REE were found to be immobile at all stages of the alteration. On the bulk rock scale, comprising both glass alteration and the formation of authigenic phases, early-stage alteration at Surtsey releases SiO2, Al, Mg and Ca to the fluid, whereas late-stage alteration poses a sink of SiO2, Al, Mg, Ca and Na. Net element budgets during early- and late-stage alteration indicate that chemical exchange between basaltic tuffs and seawater derived fluids at Surtsey acts as a long-term source of dissolved Ca, Al and SiO2 for seawater as well as a long-term sink for seawater Mg and Na.

Stable isotope geochemistry of silicon in granitoid zircon

Zircon is often used to study granites (sensu lato) and continental crust because it is very resistant and can be analyzed for various isotope systems that provide time and source information about their parental melts. Granites with different petrological histories have distinct bulk-rock silicon isotope compositions but it is unclear if these differences are also detectable in zircon because of superimposed fractionation effects (e.g., related to temperature, silica content, magmatic processes). The present study explores the Si isotope signatures of zircon from various granite types to constrain their isotope fractionation behavior and uses them as igneous petrogenetic tools, and possibly as granite source discriminators when zircon is found in detrital sediments. Our results show that although Si isotope compositions in zircon can be modified by secondary (post-crystallization) processes such as alteration/weathering and metamorphism, they are primarily controlled by zircon-melt isotope fractionation, which depends on both zircon crystallization temperature and magma silica content. Once these fractionation effects are understood and filtered out, a pattern emerges between Si isotope signatures of zircons from different granite types that is consistent with theoretical and experimental results as well as with known Si isotope differences at the bulk-rock scale. Silicon isotope ratios in zircon can track magma evolution (e.g., temperature and SiO2 changes) and, hence, reveal complex processes that involved magma mingling, fractional crystallization, and/or multiple sources. This study, therefore, illustrates that Si isotopes in zircon can be used to investigate magma evolution and represents a useful complement to existing techniques in granite studies involving zircon (e.g., U-Th-Pb, Lu-Hf and O isotopes) provided that it is not used as a stand-alone technique.

The combined Zr and Hf isotope inventory of bulk rock and sequentially leached chondrite samples

The stepwise acid digestion of primitive chondritic meteorites allows the identification of nucleosynthetic isotope anomalies that are otherwise hidden on the bulk rock scale. Here, we present for the first time combined isotope data for acid leachates, residues, and bulk rock aliquots of several primitive chondrites for the geo- and cosmochemically similar elements Zr and Hf. Our analyses reveal significant Zr and Hf isotope anomalies that (i) are complementary between acid leachates and residues and (ii) well-correlated with each other. The observed Zr and Hf anomalies strongly suggest variable contributions of common s-process carrier phases to the different leachates and residues. Ratios of r- (and p-process) Zr and Hf isotopes appear to be uniform in leachates and residues. In contrast to the well-correlated anomalies found in our leaching experiments, nucleosynthetic Zr and Hf isotope signatures seem to be decoupled on the bulk rock scale. This contrast may result from the heterogeneous distribution of neutron-rich Zr material devoid Hf, or alternatively be caused by the presence of anomalous CAI material which overprinted s-process deficits that were initially correlated.In contrast to a previous study, we find no direct evidence for the presence of a third isotopically distinct nucleosynthetic Zr component.

The effects of high-temperature fractional crystallization on calcium isotopic composition

We report Ca isotope fractionation among a co-genetic suite of samples from the Mauritanian Guelb el Azib ultramafic-mafic-anorthosite complex (GAC), which represents the fractional crystallization sequence of an igneous magma chamber. We investigate how the composition of an evolving crystallizing liquid and the resultant mineralogy of co-crystallizing assemblages affects inter-phase Ca isotope fractionation. Because the GAC is an Archean metamorphosed complex, we first investigate the resistance of the Ca isotope signature to secondary hydrothermal alteration and to metamorphism. While we cannot conclude here that Ca isotopes are undisturbed at the mineral scale, we show that they are preserved at the bulk rock scale. This is adequate for our study where several samples are almost monomineralic at the bulk scale level. The δ44/40Ca (relative to NIST standard SRM 915a) of GAC layers range from −1.53 to 1.61‰, with the earliest-formed ultramafic cumulate layer being isotopically heaviest and the later, more felsic layers being isotopically lighter. For the first time, we show that plagioclase is much more enriched in light Ca isotopes compared to olivine and (Ca-) pyroxene. Monte Carlo simulations suggest that Ca isotopes are fractionated among co-existing silicate minerals during fractional crystallization, possibly in relation with the residual liquid composition. In qualitative agreement with computational models based on first principles lattice dynamics, we observe that Ca isotope fractionation is mineralogically controlled and importantly, that the degree of fractionation can vary according to the CaO composition of the residual liquid. While previous studies have aimed to understand partial melting as a source of Ca isotope fractionation, our results suggest that fractional crystallization is also a source of Ca isotope variability among co-genetic samples.

Redox induced sulfur-selenium isotope decoupling recorded in pyrite

This study presents the first combined S and Se isotope investigation of sulfide suited to explore differences in fractionation between these two redox sensitive isotope systematics as recorded in the same mineral. A case study of Cretaceous Navajún pyrite from the Mesozoic Cameros Basin, Spain, with known petrogenesis and geological context shows systematic decoupling at the microscale: Variable S isotope values within the analyzed pyrite coincide with rather constant Se isotope values and vice versa. These signatures were not generated during pyrite growth but record previous redox induced fractionations in fluids that each contributed both elements from two sources. It is likely that both S and Se isotope fractionation occurred during strong reduction from one fully oxidized source whereas only S but no Se isotope fractionation occurred during minor reduction following sulfide dissolution via H2O from another source. Subsequent mixing of these two H2S-H2Se fluids at different elemental S-Se ratios during incorporation into the pyrite can then explain the S-Se isotope variations in the investigated specimen. These inferences are in accordance with a larger range in the redox potential Eh of Se relative to S, resulting in coupled or decoupled Se and S isotope fractionation depending on the oxygen fugacity during the reduction process. If extended to other sulfides of diverse origin, for a given pH, combined Se and S isotope studies may allow to investigate the magnitude of redox variations and place more robust constraints on minimum and maximum oxygen concentrations in the source. We therefore suggest combined S-Se isotope analyses in sulfide as a new powerful proxy for studying Earth’s redox evolution beyond the bulk rock scale.

Variable distribution of s-process Hf and W isotope carriers in chondritic meteorites – Evidence from 174Hf and 180W

The stepwise acid digestion of primitive chondritic meteorites allows the identification of nucleosynthetic isotope anomalies that are otherwise hidden on the bulk rock scale. Here, we present combined Hf and W isotope data for acid leachates, residues, and bulk rock aliquots of several primitive chondrites that include highly precise analyses of the heavy p-process isotopes 174Hf and 180W. Including data for these two p-process isotopes enables, for the first time, the clear-cut discrimination between s- and r-process contributions to the Hf and W isotope inventory. Our analyses reveal Hf and W isotopic homogeneity at the bulk rock scale, but significant Hf and W isotope anomalies that are complementary between acid leachates and residues. Since both r- to p-process isotope ratios are invariant in leachates and residues, the observed anomalies can unambiguously be tied to variable contributions of carrier phases enriched in s-process nuclides, as previously inferred for, i.e., Mo and Ru in leaching experiments. Hafnium and W isotope anomalies co-vary in leachate and residue fractions from CM chondrites, whereas CO and CV chondrites are characterized by distinctly larger Hf isotope anomalies compared to W. This observation is most likely explained by more efficient homogenization of s-process W carrier(s) or, alternatively, by local redistribution of anomalous W into secondary less resistant phases during parent body and/or nebular processing. This implies the presence of different s-nuclide carrier phases for Hf and W. Several carriers of s-process-material appear to have been selectively dissolved by our leaching protocol, while contributions from r- and p-process Hf and W carrier phases appear invariant, possibly due to the generally more labile nature of their carrier phases during solar nebula and/or parent body processing.

Silicon isotopic compositions of altered oceanic crust: Implications for Si isotope heterogeneity in the mantle

This study reports Si isotope compositions of the altered oceanic crust recovered from IODP Site 1256, East Pacific Rise to investigate the behavior of Si isotopes during low-temperature seawater and/or high-temperature hydrothermal alteration processes in the oceanic crust. These samples, including basalts and gabbros, were altered by seawater and hydrothermal fluids at various temperatures and water/rock ratios. The δ30Si of these samples range from −0.38‰ to −0.27‰, with an average of δ30Si=−0.32±0.06‰ (2SD, N=50), regardless of the depth and extent of alteration. The δ30Si values of the altered oceanic crust are in general agreement with the previous estimate of the global fresh MORBs (−0.27±0.06‰, 2SD; Savage et al., 2014), suggesting that either low-temperature seawater alteration or high-temperature hydrothermal alteration does not cause resolvable Si isotopic fractionation at the bulk-rock scale in the altered oceanic crust.Because the altered oceanic crust could be partially melted during subduction or in the convective mantle, we estimated the Si isotope fractionation between the melt and residue minerals of the altered oceanic crust. Our calculations show that Si isotopes can be fractionated between the melt and residues enriched in garnet and clinopyroxene. Although the bulk altered oceanic crust has a mantle-like Si isotopic signature, the residual solids of the recycled altered oceanic crust could be enriched in light Si isotopes and the melt could be enriched in heavy Si isotopes. This indicates that melting and recycling of the subducted oceanic crust can produce heterogeneous Si isotope compositions in the convective mantle, which could contribute to the source of ocean island basalts with low δ30Si.

Zn and Pb mobility during metamorphism of sedimentary rocks and potential implications for some base metal deposits

Comprehension of the genesis of Pb-Zn ore systems is currently limited by a poor understanding of where these metals are sourced from. Our study of metal mobility during regional metamorphism in the Mt. Lofty Ranges, South Australia, demonstrates that in staurolite-absent siliciclastic metasedimentary rocks, biotite contains >80 % of the bulk rock Zn, as well as a considerable proportion of the total Pb. Fluid flow through these metasedimentary rocks led to a continuous depletion of Pb and Zn on a mineral and bulk rock scale during prograde regional metamorphism. We calculate that ∼80 % of the bulk rock Zn and ∼50 % of the bulk rock Pb were mobilised, mainly through reactions involving biotite. These reactions led to a calculated Pb and Zn “loss” of ∼2.7 and 27 Mt, respectively, in the high-grade metamorphic zone. Halogen contents of apatite and biotite and bulk rock Zn isotope data provide evidence that Cl-rich metamorphic fluids were important for metal transport. Hence, fluid flow accompanying prograde metamorphism of typical sedimentary rocks can mobilise base metals to the degree required to potentially supply significant Pb-Zn ore systems.

Magnesium isotopic compositions of altered oceanic basalts and gabbros from IODP site 1256 at the East Pacific Rise

To investigate the behaviour of Mg isotopes during alteration of oceanic crust and constrain the Mg isotopic compositions of the altered oceanic crust (AOC), high-precision Mg isotope analyses have been conducted on forty-four altered basalts and gabbros recovered from IODP site 1256, which represent the carbonate-barren AOC formed at the East Pacific Rise (EPR). These samples were altered by interaction of seawater-derived fluids with oceanic crust at different temperatures and water/rock ratios. With the exception of one sample that has a slightly heavier Mg isotopic composition (δ26Mg=0.01±0.08‰), all the other samples have relatively homogenous and mantle-like Mg isotopic compositions, with δ26Mg ranging from −0.36 to −0.14‰ (an average value of −0.25±0.11‰, 2SD, n=43). This suggests that limited Mg isotope fractionation occurred during alteration of oceanic crust at the EPR at bulk rock scale, irrespective of highly variable alteration temperatures and variable water/rock ratios. Thus, our study suggests that the offset of δ26Mg values between seawater and global runoff dominantly results from the formation of marine dolomite as a sink for Mg. The mantle-like Mg isotopic composition further indicates that recycling of carbonate-barren AOC would not result in Mg isotope heterogeneity of the mantle at global scale. Consequently, the light Mg isotopic compositions of the mantle at local scale must result from incorporation of recycled Mg isotopically light carbonates.

Evaluation of the applicability of acid leaching for the 238U–230Th internal isochron method

The 238U–230Th internal isochron method has great potential for determining eruption ages of young volcanic materials if the separated groundmass phases have a sufficiently wide range of U/Th ratios. We examined the fractionation behavior of U and Th in aged (i.e., >0.5Ma) basaltic samples by 6M hydrochloric acid leaching to evaluate the applicability of acid-leaching treatment for the 238U–230Th internal isochron method. Acid leaching of aged basaltic rocks in 238U–234U–230Th secular equilibrium at the bulk-rock scale results in 230Th–238U and 234U–238U radioactive disequilibria for both leachates and residues. These radioactive disequilibria can be explained by redistribution of 234Th (parent of 234U) and 230Th between acid-soluble and acid-resistant phases due to α-recoil. The number of 230Th atoms redistributed by α-recoil can be calculated by using a mass conservation equation for 234U atoms and by the relative amount of recoiled 230Th and 234Th, the latter proportional to the kinetic energy of the recoiled nuclide. When the fraction of daughter nuclide 234U remaining in either the residue or leachate, after α-recoil redistribution of 238U, is large enough (>95%), the corrected (230Th/238U) values of leachate and residue show radioactive equilibria. This result demonstrates that preferential fractionation between U and Th does not occur during acid leaching for basaltic samples if there is no selective etching of the α-recoil track. This study implies that acid-leaching can be used in conjunction with the 238U–230Th internal isochron method for dating young volcanic rocks by evaluating the degree of the α-recoil redistribution of 234U.

On the iron isotope homogeneity level of the continental crust

Assessing the level of homogeneity of different planetary reservoirs is an essential step to establish a new isotopic tracer. In recent studies, we concluded that most igneous rocks from the continental crust were homogeneous to < ± 0.05‰ in δ 57Fe [Poitrasson, F., Halliday, A.N., Lee, D.C., Levasseur, S., Teutsch, N., 2004. Iron isotope differences between Earth, Moon, Mars and Vesta as possible records of contrasted accretion mechanisms. Earth Planet. Sci. Lett., 223, 253-266.; Poitrasson, F., Freydier, R., 2005. Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS. Chem. Geol., 222, 132-147.]. This conclusion was previously reached by Beard et al. [Beard, B.L., Johnson, C.M., Skulan, J.L., Nealson, K.H., Cox, L., Sun, H., 2003. Application of Fe isotopes to tracing the geochemical and biological cycling of Fe. Chem. Geol., 195, 87-117.], though defined by a broader range (± 0.08‰). The main purpose of the Beard and Johnson [Beard, B.L., Johnson, C.M., 2006. Comment on “Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS” by F. Poitrasson and R. Freydier, Chemical Geology, volume 222, pages 132-147. Chem. Geol., this issue.] comment is to emphasize again this conclusion reached by the two groups: the degree of the iron isotope variability observed in most bulk rocks from the continental crust is small. However, controversy lies with high silica magmatic rocks (> ∼ 71 wt.% SiO 2) that we concluded were significantly heavier than less silicic rocks from the crust on the basis of Fe isotopic analyses of seven bulk rock samples from different geological settings. Beard and Johnson [Beard, B.L., Johnson, C.M., 2006. Comment on “Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS” by F. Poitrasson and R. Freydier, Chemical Geology, volume 222, pages 132-147. Chem. Geol., this issue.] challenge this finding on the basis of previously “published” data. It is shown here that this controversy stems from the confusion between whole-rock and mineral iron isotope analyses, since this isotopic tracer, like many others, is more heterogeneous at the mineral scale than the bulk rock scale. Thus, minerals cannot be used in place of bulk rock Fe isotope compositions. The controversy also arises from different levels of analytical uncertainties. If only the best Fe isotope probes of planetary reservoirs — bulk rocks — are considered, then the conclusion that igneous rocks with SiO 2 > 71 wt.% are significantly heavier than the mean δ 57Fe of the continental crust [Poitrasson, F., Freydier, R., 2005. Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS. Chem. Geol., 222, 132-147.] remains valid. Nonetheless, given that granites represent only a minor fraction of the continental crust, this finding does not change the bulk igneous Fe isotope composition estimate previously defined for the Earth.

Methods for the microsampling and high-precision analysis of strontium and rubidium isotopes at single crystal scale for petrological and geochronological applications

Micromilling of single crystals to yield microgram-sized solid samples for subsequent isotopic analysis can yield important petrogenetic information from crystals (especially feldspars) in magmatic rocks. Exceptional detail can be gained because the textural context of the sample area can be fully evaluated before milling. Such information is otherwise unknown or lost when performing analysis at the bulk rock scale. Here we present a comprehensive methodology for the precise analysis of ng-levels of Rb and Sr purified from microgram-sized solid samples extracted from minerals in thin sections. Physical sampling techniques are based around a computer numerical control (CNC) milling machine (Micromill™), newly designed specifically for the sampling of complex accretionary and growth structures in crystalline materials. Chemical procedures for the isolation of Sr and Rb for TIMS and MC-ICPMS analysis, respectively, are presented, and the performance of these analytical techniques is assessed in the context of the small sample sizes produced by micromilling. Despite being time consuming, mechanical sampling, conventional dissolution and chemical separation followed by analysis by TIMS remain the best methods for both highly accurate and precise determinations of Sr isotopic compositions in most geological materials over a wide range of Sr concentrations, Rb/Sr ratios and matrix types. Using these techniques, it is possible to achieve long-term 2 S.D. external precisions of 50 ppm for load sizes as small as 3 ng Sr. We demonstrate the validity of these techniques using two examples. The first is from a < 50 ka single feldspar crystal from Parinacota volcano (Chile) to show that 87Sr/ 86Sr isotope ranges as small as 0.00006 (2 × 2 S.D. external) can be resolved under conditions where radiogenic-Sr ingrowth can be neglected. The second is from the 28.4 Ma Fish Canyon Tuff (Colorado) to demonstrate the utility of isotope dilution measurements of Rb and Sr contents to calculate 87Rb/ 86Sr and thus perform age corrections to the 87Sr/ 86Sr ratios in order to establish 87Sr/ 86Sr i variations between single crystals or zones. We demonstrate that biotite crystals in the Fish Canyon Tuff show Sr-isotopic variations well beyond analytical errors and hence that the crystals involved were not in isotopic equilibrium and cannot be used to establish an isochron age. On the other hand, the precision of our isotope dilution measurements may be used to obtain Rb–Sr geochronological information at the single crystal scale, provided that homogeneity in 87Sr/ 86Sr at the time of crystallisation can be demonstrated.

Rhenium‐187—osmium‐187 in iron meteorites and the strange origin of the Kodaïkanal meteorite

Abstract— A set of iron meteorites was investigated for Re‐Os isotopes and provides a well‐defined isochron age of 4.624 ± 0.017 Ba and an initial 187Os/188Os ratio of 0.095636 ± 0.00009. Actual ages may be a few percent lower due to uncertainty on the 187Re decay constant. Within the small sample number presented here, there is no evidence for age differences between classes. A more detailed study of the meteorite Kodaïkanal shows that the metal reservoir of this meteorite formed approximately at the same time as the other iron meteorites, in contrast with the silicate inclusions which display a formation age of 3.67 Ba with all other chronometers. A collisional origin is in agreement with the data on this meteorite. Major secondary events do not affect the Re‐Os system at the bulk rock scale in metal as it is the sole significant reservoir of these two elements. A Re‐Os and Rb‐Sr investigation of meteoritic troilite exhibits disturbed chronometric systems, which we attribute to the terrestrial history of the meteorite.