Inter-mineral Isotope Fractionation Research Articles

Chromium isotope variations in garnet-facies mantle rocks and their minerals: Implications for Cr isotope behavior in high-temperature processes

Previous work on Cr isotope composition of the lithospheric mantle focused on shallow spinel-facies peridotites and invoked the effects of partial melting and metasomatism. However, little is known on the Cr isotope composition and behavior in the deeper and more voluminous garnet-facies mantle, notably during generation and passage of magmas and fluids. Here, we present Cr isotope data for 26 garnet and ten spinel whole-rock peridotite xenoliths (mainly harzburgites) from the Udachnaya kimberlite on the Siberian craton and minerals from five of them, as well as for minerals of 14 eclogites and four garnet clinopyroxenites from the Dabie orogen in eastern China. Garnet (Gt) and clinopyroxene (Cpx) are major Cr hosts in the mantle. Our ionic modeling suggests the following order of 53Cr/52Cr values in coexisting minerals: garnet ≈ spinel > Cpx > olivine. Cr isotopes are equally partitioned between garnet and Cpx in four peridotites, while garnet is enriched in heavy Cr in one peridotite and all the eclogites (Δ53CrGt-Cpx 0.01‰ to 0.27‰, av. 0.10‰), but depleted in heavy Cr in the pyroxenites (Δ53CrGt-Cpx −0.03‰ to −0.16‰, av. −0.10‰). The Δ53CrGt-Cpx variations are attributed to effects of chemical composition, equilibration temperature and oxygen fugacity on equilibrium Cr isotope fractionation. Further, the anomalous Δ53CrGt-Cpx values in the Dabie pyroxenites and one peridotite may indicate disequilibrium inter-mineral isotope fractionation via kinetic diffusion during phase transitions as a result of pressure-temperature (P-T) changes and metasomatism.The whole-rock δ53Cr range in the garnet peridotites (−0.27 to 0.13‰; average −0.08 ± 0.18‰, 2 SD, n = 25, excluding an outlier with −0.86‰) overlaps bulk silicate Earth estimates whereas the spinel peridotites tend to have higher δ53Cr (−0.12 to 0.16‰; av. 0.02 ± 0.20‰, 2 SD, n = 10). While these rocks are residues of high degrees (30–40%) of melt extraction, their range contrasts with limited equilibrium Cr isotope fractionation we calculated for melting residues of both spinel and garnet peridotites (Δ53Crresidue-source < 0.03‰). The δ53Cr values in the spinel peridotites show correlations with modal abundances of Cpx, garnet and spinel, as well as major and trace element composition. We posit that the Cr isotope variations in these peridotites are somehow linked to solid–melt and inter-mineral diffusive exchanges during P-T changes, deformation and ingress of metasomatic media previously documented in the Udachnaya peridotites. The average δ53Cr value (−0.09 ± 0.16‰, 2 SD, n = 31) for garnet peridotites (excluding U9) in this and previous studies is similar to those previously obtained for (mainly off-craton) spinel peridotites and komatiites, and appears to be representative of the Cr isotope composition for bulk mantle. Numerical modeling for mantle magmas derived at different depths and by different melting degrees of garnet and spinel peridotites and eclogites yields a narrow δ53Cr range, possibly affected by oxygen fugacity.

The factors controlling equilibrium inter-mineral Ca isotope fractionation: Insights from first-principles calculations

Equilibrium isotope fractionation factors are crucial to quantitatively interpreting Ca isotope data of natural samples. Recent studies have revealed significant equilibrium Ca isotope fractionation between minerals, but the controlling factors remain poorly understood. Using density functional theory, this study calculates the reduced partition function ratios (RPFRs) among amphiboles (richterite and tremolite), sorosilicates (akermanite and gehlenite), K-bearing carbonates (butschliite), Na-bearing diopside (jadeite), and other Ca-bearing minerals (fluorapatite, anhydrite, CaTiO3 perovskite, and fluorite) at 0 GPa. The RPFRs of diopside and anorthite over a pressure range from 0 to 5 GPa are calculated to investigate the pressure effect. The effect of force constant, bond length, coordination number, anion type, solid solution composition, and pressure on RPFRs are discussed by compiling the literature data. The RPFRs show good correlations with the force constant and bond length but have no clear correlation with the coordination number. This suggests that the bond length is more reliable than the coordination number for roughly predicting the signs and magnitudes of inter-mineral isotope fractionation. Na-bearing clinopyroxene (e.g., jadeite) has similar RPFR with Na-free clinopyroxene, suggesting that the jadeite effect should be insignificant in natural samples. The RPFR of anorthite is lower than that of diopside at low pressure, and the two minerals both show a positive correlation between RPFR and pressure. Notably, the heavy Ca isotope enrichment between these two minerals can be reversed at high pressure (>3 GPa at 1000 K) because anorthite’s RPFR changes more sharply with increasing pressure than that of diopside. However, such a reverse may not occur in Earth's modern crust due to the stability of anorthite at lower pressures. Combining the theoretical predictions of amphibole and plagioclase and natural sample observation on granitoids, we infer that the RPFR of granitic magma may be lower than that of basaltic magma.

Ca isotopic compositions of zoned granitoid intrusion: Implications for the emplacement and evolution of magma bodies

To investigate the mechanism of magma emplacement of the intruded zoned pluton, we report high-precision Ca isotopic data of 23 granitoids (including granodiorite and quartz diorite), 5 mafic enclaves, and 7 hornblende and plagioclase mineral pairs from the Early Cretaceous Fangshan pluton, Beijing, China. Δ44/40CaHbl-Pl of rocks span from 0.02 ± 0.15‰ to 0.23 ± 0.12‰, which likely reflects equilibrium inter-mineral isotope fractionation. The granodiorites exhibit ∼0.30‰ variation in Ca isotopic compositions, with δ44/40Ca ranging from 0.47 ± 0.08‰ to 0.77 ± 0.09‰. The diorites and the enclaves yield δ44/40Ca of 0.51 ± 0.03‰ to 0.64 ± 0.05‰, and 0.54 ± 0.06‰ to 0.66 ± 0.11‰, respectively, which are lower than that of the upper continental crust (0.72 ± 0.10‰). Ca isotopic variation cannot be ascribed to K decay due to their low εCa values (< ±1.0). Theoretical study showed that Hbl fractional crystallization could counteract Ca isotope fractionation effect of their co-existing Pl crystallization. Source heterogeneity cannot account for Ca isotopic variation because δ44/40Ca does not correlate to (87Sr/86Sr)i. Instead, the Ca isotopic variation is likely controlled by partial melting and residual mineralogy. Specifically, δ44/40Ca of the granodiorites are negatively correlated with (Dy/Yb)N, reflecting various magma pools derived from different crustal depths with clinopyroxene and garnet residua. Modeling calculations suggest that the low Ca isotopic signature can be caused by ∼10% partial melting of sources with various proportions of garnet and clinopyroxene (from 5% garnet + 95% clinopyroxene to 40% garnet + 60% clinopyroxene). The diorites intruded earlier than the granodiorites, which can be slightly contaminated by surrounding basement rock. Ca isotopic compositions of enclaves are similar to those of their host rocks. Considering the spatial structure of the Fangshan pluton, it should be assembled by at least four magma pulses derived from various crustal depths through ballooning. Our results show that Ca isotopes can be used as a potential tool to trace magma source depth and decipher the process of magma emplacement of intrusive pluton, which is important to understand the continental crustal growth.

Genesis of the Xinfang Gold Deposit, Liaodong Peninsula: Insights from Fluid Inclusions and S-Sr Isotopic Constraints

The North China Craton (NCC) has been continuously reactivated since the Mesozoic and this decratonization is responsible for its economically important gold mineralization in the Mesozoic. The Early Cretaceous (110–130 Ma) gold mineralization in the NCC has been well-studied due to its significance, but little attention has been given to other episodes of gold mineralization related to polyphased reactivation of the NCC. The Xinfang mesozonal gold deposit (143 Ma) in the Liaodong Peninsula is related to the one of the episodes of the Yanshanian orogeny. The orebodies of the Xinfang gold deposit were controlled by the low angle transpressive fault systems and hosted by the Neoarchean monzogranitic gneiss. Fluid inclusion microthermometry reveals that the mineralizing temperatures range from 220 to 280 °C, with salinities from 6 wt.% NaCl eqv. to 15 wt.% NaCl eqv., pressures from 199 to 321 Ma. The S isotopic characteristics of sulfides not only record a heterogeneous source including magmatic or gneissic sulfur but also record inter-mineral isotope fractionation. The initial 87Sr/86Sr values of pyrite (0.713 480–0.729 031) indicate a radiogenic crustal origin for the sources. The metamorphic dehydration of the underlying basement resulted in the genesis of the Xinfang gold deposit. We summarize three episodes of gold mineralization in the Liaodong Peninsula related to continuous reactivation of the NCC, which indicates a great exploration potential of this area.

Calcium isotope fractionation during crustal melting and magma differentiation: Granitoid and mineral-pair perspectives

Abstract In order to investigate Ca isotope fractionation during crustal melting and magma differentiation, we report high-precision Ca isotopic data of 17 well-characterized granitoids from the Dabie orogen, China, and 22 mineral separates from two representative granitoids and nine local eclogites and garnet peridotites. Hornblende and plagioclase pairs have nearly constant Δ44/42Cahbl–plg, ranging from +0.09 ± 0.04 (in 2SE if not specified) to +0.12 ± 0.03, which likely reflects equilibrium inter-mineral isotope fractionation at 660–760 °C according to the plagioclase–hornblende geothermometry. Δ44/42Cagrt–cpx of eclogites and garnet peridotites range from +0.06 ± 0.03 to +0.38 ± 0.04, consistent with a stronger Ca O bond in garnet than clinopyroxene. Agrt–cpx ( A grt - c p x = Δ 44 / 42 Ca grt - c p x × T 2 / 10 6 , T in Kelvin), representative of inter-mineral isotope fractionation corrected for temperature effect, decreases with increasing jadeite proportion in clinopyroxene, indicating a compositional control on equilibrium inter-mineral isotope fractionation. Eight I-type non-adakitic granitoids with CaO contents ranging from 1.59 wt.% to 6.96 wt.% have homogeneous Ca isotopic compositions with an average δ44/42Ca of +0.37 ± 0.03 (2SD, N = 8), identical to the mean value of the local basement (+0.36 ± 0.06, 2SD), indicating insignificant Ca isotope fractionation during crustal melting and magma differentiation at low pressures (e.g., without a role of garnet). According to the negligible isotope fractionation between hornblende and plagioclase ( Δ 44 / 42 Ca hbl - p l g = ( + 0.10 ± 0.02 ) × 10 6 / T 2 ) at the liquidus temperatures, the Ca isotope fractionations between melt and hornblende, plagioclase are very small. Nine low-Mg adakitic granitoids have δ44/42Ca ranging from +0.24 ± 0.02 to +0.38 ± 0.02. Given their low chemical index of alteration (

High-temperature inter-mineral Cr isotope fractionation: A comparison of ionic model predictions and experimental investigations of mantle xenoliths from the North China Craton

Recent works have proposed that chromium (Cr) isotopes could be fractionated during peridotite partial melting and basaltic magma crystallization. However, until now, inter-mineral Cr isotope fractionation behavior for major mantle minerals has been poorly constrained. To investigate the mechanism and magnitude of equilibrium inter-mineral Cr isotope fractionation and to explore its implications for planetary mantle and crust lithology evolutions, we performed a systematic study of equilibrium Cr isotope fractionation between major mantle minerals by coupling theoretical ionic modeling with laboratory isotope analyses of natural samples and Cr valence determinations by X-ray absorption near edge spectroscopy (XANES). The ionic model considers both charges and coordination environments of Cr species in mantle minerals, which have proven to be critical factors affecting inter-mineral isotope fractionation. The ionic modeling results predict a general order of spinel (Spl) > pyroxene (Py) ≥ olivine (Ol) in 53Cr/52Cr. Systematic Cr isotope analyses of coexisting mantle minerals of seventeen peridotite xenoliths from Beiyan in the North China craton were performed. Chromium isotope results for different mineral pairs of lherzolites (e.g., Δ53CrSpl–Ol of 0.11‰ to 0.16‰, Δ53CrSpl–Py of 0.04‰ to 0.11‰ and Δ53CrPy–Ol of 0.05‰ to 0.10‰ at 870 °C to 970 °C), document measurable and systematic inter-mineral Cr isotope fractionation, in excellent agreement with the modeling results predicted with XANES-determined Cr2+/ΣCr values of the mineral separates. This fractionation order could essentially account for Cr isotope behaviors during partial melting and magmatic crystallization observed in terrestrial peridotites and lunar basalts.In contrast, we found that metasomatisms could influence Cr isotope compositions of minerals from Beiyan metasomatized clinopyroxene- (Cpx-) rich lherzolites and wehrlites by means of mineral–melt interaction and/or kinetic diffusion, leading to disequilibrium inter-mineral Cr isotope fractionation.

Effect of Ca content on equilibrium Ca isotope fractionation between orthopyroxene and clinopyroxene

Concentration effect on equilibrium inter-mineral isotope fractionation is ubiquitous in solid solution systems, but it is not clear in which concentration range such effect is prominent. Using first-principles calculations, we examine the effect of Ca and Fe contents in orthopyroxene (opx) on its average CaO bond length and the equilibrium Ca isotope fractionation factor (103lnα) between opx and clinopyroxene (cpx). Our results reveal that the average CaO bond length in opx is much smaller than that in cpx and it does not change with variable Ca content x (x and y are mole ratios in CaxFeyMg1−x−ySiO3 thereafter here) when x≤1/48. Incorporation of Fe (y≥1/32) into opx with a fixed Ca content can only slightly increase the average CaO bond length. 103lnαopx-cpx of 44Ca/40Ca is linearly correlated with the average CaO bond length in opx, suggesting that 103lnαopx-cpx of 44Ca/40Ca is controlled by opx CaO bond strength. Our calculations indicate that the Ca concentration effect on 103lnαopx-cpx is significant when x in opx ranges from 2/16 to 1/48, while Fe in natural opx only causes a slight decrease in 103lnαopx-cpx.Our results provide insights into Ca isotope fractionation in high-temperature geochemical processes. Given that Ca content x in opx from natural peridotites is usually lower than 1/32 and Fe content y is generally ∼10mol%, Ca and Fe concentration effects on 103lnαopx-cpx in natural samples are negligible. Rather, 103lnαopx-cpx is mainly controlled by temperature. 103lnαopx-cpx of 44Ca/40Ca decreases from 0.50‰ to 0.26‰ when temperature increases from 1000K to 1400K if the Fe effect is taken into account. Therefore, if Ca isotope fractionation between opx and cpx (Δ44/40Caopx-cpx) in natural peridotites is greater than 0.50‰ or lower than 0.26‰, it may indicate disequilibrium of Ca isotopes. Finally, the large 103lnαopx-cpx relative to our current analytical precision suggests that Δ44/40Caopx-cpx can be used as an independent thermometer with a precision comparable to elemental thermometers. Because most naturally occurring minerals are solid solutions with variable chemical compositions, this study presents a guideline to explore the concentration effect on equilibrium isotope fractionation among minerals.

Calcium isotopic composition of mantle xenoliths and minerals from Eastern China

This study presents calcium isotope data for co-existing clinopyroxenes (cpx), orthopyroxenes (opx), and olivine (ol) in mantle xenoliths to investigate Ca isotopic fractionation in the upper mantle. δ44/40Ca (δ44/40Ca (‰)=(44Ca/40Ca)SAMPLE/(44Ca/40Ca)SRM915a−1) in opx varies from 0.95±0.05‰ to 1.82±0.01‰ and cpx from 0.71±0.06‰ to 1.03±0.12‰ (2se). δ44/40Ca in ol (P-15) is 1.16±0.08‰, identical to δ44/40Ca of the co-existing opx (1.12±0.09‰, 2se). The Δ44/40Caopx−cpx (Δ44/40Caopx−cpx=δ44/40Caopx–δ44/40Cacpx) shows a large variation ranging from −0.01‰ to 1.11‰ and it dramatically increases with decreasing of Ca/Mg (atomic ratio) in opx. These observations may reflect the effect of opx composition on the inter-mineral equilibrium fractionation of Ca isotopes, consistent with the theoretical prediction by first-principles theory calculations (Feng et al., 2014). Furthermore, Δ44/40Caopx−cpx decreases when temperature slightly increases from 1196 to 1267K. However, the magnitude of such inter-mineral isotopic fractionation (1.12‰) is not consistent with the value calculated by the well-known correlation between inter-mineral isotope fractionation factors and 1/T2 (Urey, 1947). Instead, it may reflect the temperature control on crystal chemistry of opx (i.e., Ca content), which further affects Δ44/40Caopx−cpx. The calculated δ44/40Ca of bulk peridotites and pyroxenites range from 0.76±0.06‰ to 1.04±0.12‰ (2se). Notably, δ44/40Ca of bulk peridotites are positively correlated with CaO and negatively with MgO content. Such correlations can be explained by mixing between a fertile mantle end-member and a depleted one with low δ44/40Ca, indicating that Ca isotopes could be a useful tool in studying mantle evolution.

Large magnesium isotope fractionation in peridotite xenoliths from eastern North China craton: Product of melt–rock interaction

To investigate the effects of melt–rock interaction on Mg isotope fractionation and mantle Mg isotopic heterogeneity, we report high-precision Mg isotopic data of olivine (Ol), orthopyroxene (Opx), clinopyroxene (Cpx) and spinel (Spl) for 25 peridotite xenoliths from the eastern North China craton. These xenoliths range from lherzolites, Cpx-rich lherzolites to wehrlites, and are variably metasomatised. The lherzolites have Ol with Fo contents from 89 to 90 and have a homogeneous Mg isotopic composition (δ26Mg=−0.26 to −0.20), similar to the typical mantle value. By contrast, Cpx-rich lherzolites and wehrlites have lower Ol with Fo contents (78–88) and exhibit larger Mg isotopic variations, with δ26Mg ranging from −0.39 to +0.09. The δ26Mg values of minerals in these xenoliths are also variable and range from −0.45 to −0.03 in Ol, from −0.26 to −0.01 in Opx, from −0.34 to +0.22 in Cpx and from −0.16 to +0.25 in Spl. Inter-mineral fractionations between coexisting pyroxene and Ol in lherzolites and most isotopically light wehrlites (Δ26MgOpx–Ol=−0.04 to +0.09‰; Δ26MgCpx–Ol= +0.02 to +0.25‰) vary as a function of temperature and are consistent with equilibrium inter-mineral isotope fractionations. By contrast, large disequilibrium Mg isotope fractionation occurs between coexisting pyroxene and Ol in the majority of Cpx-rich lherzolites (Δ26MgOpx–Ol=+0.16 to +0.32‰; Δ26MgCpx–Ol=+0.04 to +0.34‰). Both types of isotope fractionations also occur between Spl and Ol, with Spl being consistently heavier than Ol (Δ26MgSpl–Ol=+0.15 to +0.55‰). Overall, the isotopically equilibrated but light wehrlites may result from mantle metasomatism by isotopically light melt, whereas large disequilibrium isotope fractionations in Cpx-rich peridotites likely reflect kinetic isotope fractionation during melt–peridotite interaction. Both processes result in the heterogeneous Mg isotopic composition of the lithospheric mantle beneath the eastern North China craton.

Petrology and oxygen-isotope geochemistry of the Yamba Lake kimberlite rocks, N.W.T.

The Torrie, Sputnik, and Eddie kimberlite rocks, located near Yamba Lake, central Slave province, N.W.T., are volcaniclastic, macrocrystic, heterolithic, olivine-rich tuff, and olivine-rich tuff breccia. Torrie and Sputnik kimberlite rocks contain pyroxene and garnet xenocrysts and megacrysts with major-element compositions consistent with derivation mostly from disaggregated garnet lherzolite, with subordinate contributions from eclogite, spinel lherzolite, garnet harzburgite, and websterite. The presence of primary groundmass phlogopite and compositionally evolved spinel, and the absence of mantle xenocrysts, xenoliths, and megacrystic ilmenite distinguish the Eddie kimberlite pipe from the other two kimberlite pipes. Large variations in δ18O of garnet and clinopyroxene in xenocrysts and xenoliths (+3.98 to +6.36‰), nonequilibrium intermineral isotopic fractionation, and major-element heterogeneity are interpreted as resulting from infiltration of fluids or melts produced by dehydration or melting of subducted oceanic crust into overlying peridotite. Although the timing is unconstrained for the xenocysts, the xenolith must have experienced this metasomatic interaction shortly before entrainment in the kimberlite. Variable δ18O values for magnesian ilmenite are also interpreted to result indirectly from such metasomatic activity in the mantle as well. The Torrie and Sputnik kimberlite rocks have low concentrations of diamond indicator minerals consistent with their low-diamond grades. These kimberlite rocks did not sample a significant amount of garnet harzburgite, the rock type commonly associated with high-diamond grades in other kimberlite rocks. Furthermore, metasomatism just prior to kimberlite eruption may have caused the resorption of any diamond present.