Diffusion-driven extreme Mg and Fe isotope fractionation in Panzhihua ilmenite: Implications for the origin of mafic intrusion

doi:10.1016/j.gca.2019.10.004

Geochimica et Cosmochimica Acta

Volume 278, 1 June 2020, Pages 361-375

https://doi.org/10.1016/j.gca.2019.10.004 Get rights and content

Abstract

To investigate the petrogenesis of Fe-Ti oxide ore deposits, we report Mg and Fe isotopic compositions for coexisting olivine, clinopyroxene and ilmenite in Fe-Ti oxide ores, magnetite-bearing gabbros and gabbros from the Panzhihua Fe-Ti-oxide-bearing layered mafic intrusion, Southwest China. Olivine and clinopyroxene have δ²⁶Mg values ranging from −0.47 to −0.32‰ and −0.40 to −0.18‰, and δ⁵⁷Fe values from −0.21 to +0.23‰ and −0.16 to +0.20‰, respectively. Most of these mineral pairs display disequilibrium inter-mineral fractionation as they fall off the theoretically predicted equilibrium fractionation lines. Ilmenites from oxide ores and magnetite-bearing gabbros (Group 1) have mantle-like or lower δ²⁶Mg values of −0.80 to −0.13‰ and δ⁵⁷Fe values of −0.33 to −0.23‰, and those from gabbros (Group 2) display slightly to extremely higher δ²⁶Mg values of +0.48 to +23.10‰ and much lower δ⁵⁷Fe values of −0.59 to −0.33‰. The δ²⁶Mg negatively correlates with δ⁵⁷Fe in all ilmenites, which cannot be explained by simple extensive fractional crystallization or Soret diffusion. Instead, the negative correlations between δ²⁶Mg and δ⁵⁷Fe and between MgO and FeO in ilmenites result from Mg-Fe inter-diffusion among ilmenite, silicate minerals and high-Ti basaltic melts. The extremely large isotopic variations were produced by the large Mg activity gradient between different types of ilmenites and melts, which was enhanced by interstitial liquid immiscibility process. Our study therefore demonstrates that combined Mg-Fe isotopes can be used to trace the genesis of Fe-Ti-oxide-bearing ore.

Introduction

Understanding the origins of Fe-Ti oxide ore deposit is of primary importance for exploration, efficient mining operations and ore processing (Charlier et al., 2015). The economic concentrations of Fe and Ti are mainly carried by ilmenite and magnetite, which are commonly emplaced in the mafic layered intrusions that contain olivine, plagioclase, clinopyroxene and apatite (Klemm et al., 1985, Hunter and Sparks, 1987, Toplis and Carroll, 1996, Zhou et al., 2002, Zhou et al., 2005, Cawthorn and Ashwal, 2009, Namur et al., 2010, Song et al., 2013). However, the petrogenesis of the giant Fe-Ti oxide ore is still controversial and several hypotheses have been proposed to explain its origin, including accumulation of Fe-Ti oxides in the late magmatic fractionation processes (Klemm et al., 1985, Song et al., 2013), an Fe-rich immiscible liquid segregated from mafic magma (Reynolds, 1985, Von Gruenewaldt, 1993, Zhou et al., 2005), or with magma addition and/or mixing (Robinson et al., 2003).

Magnesium and Fe isotopes can provide new constraints on the origins of Fe-Ti oxide ores because (1) fractional crystallization and partial melting do not significantly fractionate Mg isotopes but can cause large Fe isotopic variation (Teng et al., 2007, Teng et al., 2008, Teng et al., 2013); (2) combined Mg-Fe isotopic studies provide evidence to distinguish Soret diffusion from chemical diffusion in minerals (Richter et al., 2008, Richter et al., 2009a, Huang et al., 2010, Teng et al., 2011, Sio et al., 2013, Oeser et al., 2015, Sio and Dauphas, 2017); and (3) liquid immiscibility could lead to large Mg and Fe isotope fractionation with Si-rich phase enriched in heavy isotopes relative to the Fe-rich phase due to its high degree of polymerization (Zhu et al., 2015). In view of this, a few studies have combined Mg and Fe isotopes to explore the petrogenesis of Fe-Ti oxide ores although no consensus has been reached. For example, Liu et al. (2014b) suggested that the disequilibrium Fe isotopic fractionation among coexisting olivine, clinopyroxene and titanomagnetite from Fe-Ti oxide ores in Baima (Southwest China) was caused by changes in oxygen fugacity during phase segregation. By contrast, Chen et al., 2014, Chen et al., 2018) suggested that isotopic disequilibrium from the Baima intrusion resulted from chemical diffusion under sub-solidus conditions. These different interpretations lead to different implications on ore-forming processes. Hence, more systematic studies are required to better understand the mechanisms responsible for the large Mg and Fe isotopic variations in ilmenite and coexisting silicates in the mafic intrusion.

The Panzhihua layered mafic intrusion, located at the central part of the Emeishan large igneous province, Southwest China (Fig. 1), hosts a world-class Fe-Ti ore deposit, which is considered to be genetically related to the Emeishan mantle plume (e.g., Pang et al., 2008a). Here, we analyzed Mg and Fe isotopic compositions of olivine, clinopyroxene and ilmenite from a set of samples ranging from Fe-Ti-rich ore deposit layer to Fe-Ti-poor layers including oxide ores, magnetite-gabbros (Mt-gabbro), ilmenite-gabbros (ilm-gabbro) and gabbros from the Panzhihua layered mafic intrusion. Our results suggest that large Mg and Fe isotopic variations exist in ilmenites and coexisting silicates, which was mainly caused by chemical diffusion during the interstitial liquid immiscibility process.

Section snippets

Geological background and samples

The Emeishan large igneous province (ELIP) is located in Southwest China, between the Tibetan Plateau and Yangtze block (Fig. 1a), and consists of flood basalts, minor picrite and a variety of mafic-ultramafic to felsic intrusive rocks with an area of more than 2.5 × 10⁵ km² (Fig. 1b), which are interpreted to be the product of a mantle plume at the end of the Guadalupian (∼260 Ma) (Chung and Jahn, 1995, Xu et al., 2001, Zhou et al., 2002). It is subdivided into a central part and an outer

Major elements in the minerals

The chemical compositions of olivine, clinopyroxene and ilmenite were determined with a JEOL JXA8100 electron microprobe at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). The analytical conditions included an accelerating voltage of 15 kV, a beam current of 12 nA, a beam size of 5 μm and 10–30 s counting time. Natural minerals and oxides were used as standards to monitor the whole procedure. The precisions for major (>1.0 wt.%) and minor (<1.0 wt.%) elements are

Results

The chemical compositions of olivine, clinopyroxene and ilmenite are reported in Table S1. The Mg and Fe isotopic compositions of standard reference materials, four olivines, thirteen clinopyroxenes and fifteen ilmenites are presented in Table 1.

Discussion

Our results indicate large disequilibrium isotope fractionation among these coexisting minerals because most of these mineral pairs fall off the equilibrium Mg and Fe isotope fractionation lines at magmatic temperature of ∼950–1200 °C (Fig. 5, Fig. 6). This disequilibrium isotopic fractionation is also evidenced from the large intra-mineral Mg isotope fractionation in Group 2 ilmenites. For example, δ²⁶Mg values in ilmenite grains vary from +4.34 to +6.70‰ in sample PZH12-16 and from +20.03 to

Conclusions

The main conclusions drawn from the high-precision Mg and Fe isotopic analyses of olivine, clinopyroxene and ilmenite from the Panzhihua layered mafic intrusion are:

(1)
The δ²⁶Mg and δ⁵⁷Fe values range from −0.47 to −0.32‰ and −0.21 to +0.23‰ in olivine, from −0.40 to −0.18‰ and −0.16 to +0.20‰ in clinopyroxene, and from −0.80 to +23.10‰ and −0.59 to −0.23‰ in ilmenites. The δ²⁶Mg of +23.10‰ is the highest value reported to date.
(2)
The negative correlation between δ²⁶Mg and δ⁵⁷Fe in ilmenites, as well

Acknowledgements

FZT is grateful to Roberta L. Rudnick for her great mentorship throughout his PhD studies and her continuous encouragement and inspiration since then. We thank Prof. Zhaochong Zhang for help in field sampling and appreciate the discussion with Dr. Lie-Meng Chen on an early version. Constructive comments from anonymous reviewers and Guest Editor Dr. Sonja Aulbach are thanked, which have significantly improved the manuscript. This work was supported by the Strategic Priority Research Program of

References (68)

Y.H. Cao et al.
Iron isotope systematics of the Panzhihua mafic layered intrusion associated with giant Fe-Ti oxide deposit in the Emeishan large igneous province, SW China
J. Geophys. Res.
(2019)
R.G. Cawthorn et al.
Origin of anorthosite and magnetitite layers in the bushveld complex, constrained by major element compositions of plagioclase
J. Petrol.
(2009)
B. Charlier et al.
Experiments on liquid immiscibility along tholeiitic liquid lines of descent
Contrib. Mineral. Petrol.
(2012)
B. Charlier et al.
Fe-Ti-V-P ore deposits associated with proterozoic massif-type anorthosites and related rocks
Earth Sci. Rev.
(2015)
L.-M. Chen et al.
Magnesium isotopic evidence for chemical disequilibrium among cumulus minerals in layered mafic intrusion
Earth Planet. Sci. Lett.
(2018)
L.-M. Chen et al.
Controls on trace-element partitioning among co-crystallizing minerals: Evidence from the Panzhihua layered intrusion, SW China
Am. Mineral.
(2017)
L.-M. Chen et al.
Iron isotope fractionation during crystallization and sub-solidus re-equilibration: Constraints from the Baima mafic layered intrusion, SW China
Chem. Geol.
(2014)
S.L. Chung et al.
Plume-lithosphere interaction in generation of the Emeishan flood basalts at the Permian-Triassic boundary
Geology
(1995)
P.R. Craddock et al.
Iron isotopic compositions of geological reference materials and chondrites
Geostand. Geoanal. Res.
(2011)
P.R. Craddock et al.
Abyssal peridotites reveal the near-chondritic Fe isotopic composition of the Earth
Earth Planet. Sci. Lett.
(2013)

J.T. Crank

The Mathematics of Diffusion

(1975)

N. Dauphas et al.

Routine isotopic analysis of iron by HR-MC-ICPMS: How precise and how accurate?

Chem. Geol.

(2009)

N. Dauphas et al.

Magnesium and iron isotopes in 2.7 Ga Alexo komatiites: Mantle signatures, no evidence for Soret diffusion, and identification of diffusive transport in zoned olivine

Geochim. Cosmochim. Acta

(2010)

N. Dauphas et al.

Iron isotope systematics

Rev. Mineral. Geochem.

(2017)

Y.-S. He et al.

High-precision iron isotope analysis of geological reference materials by high-resolution MC-ICP-MS

Geostand. Geoanal. Res.

(2015)

M.B. Holness et al.

Silicate liquid immiscibility within the crystal mush: late-stage magmatic microstructures in the skaergaard intrusion, east greenland

J. Petrol.

(2011)

H. Huppert et al.

Double-diffusive convection

J. Fluid Mech.

(1981)

F. Huang et al.

Isotope fractionation in silicate melts by thermal diffusion

Nature

(2010)

R.H. Hunter et al.

The Differentiation of the Skaergaard Intrusion

Contrib. Mineral. Petrol.

(1987)

S. Ke et al.

Mg, Sr, and O isotope geochemistry of syenites from northwest Xinjiang, China: Tracing carbonate recycling during Tethyan oceanic subduction

Chem. Geol.

(2016)

D.D. Klemm et al.

The geochemistry of titanomagnetite in magnetite layers and their host rocks of the eastern bushveld complex

Econ. Geol.

(1985)

P.-P. Liu et al.

Immiscible Fe- and Si-rich silicate melts in plagioclase from the Baima mafic intrusion (SW China): Implications for the origin of bi-modal igneous suites in large igneous provinces

J. Asian Earth Sci.

(2016)

P.-P. Liu et al.

Open magma chamber processes in the formation of the Permian Baima mafic–ultramafic layered intrusion, SW China

Lithos

(2014)

P.-P. Liu et al.

Disequilibrium iron isotopic fractionation during the high-temperature magmatic differentiation of the Baima Fe-Ti oxide-bearing mafic intrusion, SW China

Earth Planet. Sci. Lett.

(2014)

O. Namur et al.

Crystallization sequence and magma chamber processes in the ferrobasaltic sept iles layered intrusion, Canada

J. Petrol.

(2010)

M. Oeser et al.

Processes and time scales of magmatic evolution as revealed by Fe–Mg chemical and isotopic zoning in natural olivines

Geochim. Cosmochim. Acta

(2015)

K.N. Pang et al.

Mineral compositional constraints on petrogenesis and oxide ore genesis of the late Permian Panzhihua layered gabbroic intrusion, SW China

Lithos

(2009)

K.N. Pang et al.

Abundant Fe-Ti oxide inclusions in olivine from the Panzhihua and Hongge layered intrusions, SW China: evidence for early saturation of Fe-Ti oxides in ferrobasaltic magma

Contrib. Mineral. Petrol.

(2008)

K.N. Pang et al.

Origin of Fe-Ti oxide ores in mafic intrusions: Evidence from the Panzhihua intrusion, SW China

J. Petrol.

(2008)

V.B. Polyakov et al.

The use of Mossbauer spectroscopy in stable isotope geochemistry

Geochim. Cosmochim. Acta

(2000)

I.M. Reynolds

The nature and origin of titaniferous magnetite-rich layers in the upper zone of the bushveld complex - a review and synthesis

Econ. Geol.

(1985)

F.M. Richter et al.

Isotope fractionation by diffusion in molten oxides

Geochim. Cosmochim. Acta

(1999)

F.M. Richter et al.

Isotopic fractionation of the major elements of molten basalt by chemical and thermal diffusion

Geochim. Cosmochim. Acta

(2009)

F.M. Richter et al.

Non-traditional fractionation of non-traditional isotopes: Evaporation, chemical diffusion and Soret diffusion

Chem. Geol.

(2009)

Cited by (28)

Magnesium isotope behavior during titanomagnetite fractionation in basaltic lavas
2024, Chemical Geology
Knowledge of the behavior of magnesium (Mg) isotopes during magmatic differentiation is a prerequisite for applying Mg isotopes as a tracer of crustal recycling and mantle heterogeneity. Crystal fractionation of mafic silicate minerals leads to limited Mg isotope fractionation; hence, Mg isotopic variations in basalts largely result from contributions of subducted crustal materials into their mantle sources. However, recent studies have revealed that the segregation of iron oxides, such as chromite during early-stage basaltic differentiation and titanomagnetite and ilmenite during late-stage differentiation, can potentially modify Mg isotopic compositions of mantle-derived lavas. Titanomagnetite is much more commonly observed in terrestrial basaltic lavas than chromite and ilmenite, and can also form during early-stage basaltic differentiation. To constrain the effects of titanomagnetite crystallization on Mg isotopic systematics of basaltic magma, we report Mg isotopic data for a suite of well-characterized alkaline basaltic lavas (MgO wt% = 4.99 to 6.52) from Gerba Guracha, western Ethiopian Plateau, which have undergone extensive titanomagnetite fractionation. Their δ²⁶Mg values range from −0.29‰ to −0.17‰ and do not correlate with indicators of titanomagnetite fractionation such as TiO₂ and V. These observations suggest that the segregation of titanomagnetite produced limited Mg isotope fractionation, possibly because of its composition-dependent, highly variable Δ²⁶Mg_{Ti-Mgt−melt} (Δ²⁶Mg_{Ti-Mgt−melt} = δ²⁶Mg_Ti-Mgt − δ²⁶Mg_melt) values. Considering that most iron oxides occur as complex solid solutions that are controlled by the oxygen fugacity of magmas, the effects of oxide segregation on Mg isotopic systematics of basalts require further evaluation.
Layered intrusions: Fundamentals, novel observations and concepts, and controversial issues
2024, Earth-Science Reviews
Layered intrusions are fossilized natural laboratories that historically have constrained many fundamental principles of igneous petrology. Layered intrusions are typically stratiform, usually sill-like bodies of cumulate rocks, at least a few hundred metres to as much as 10 km thick, characterized by the presence of a variety of different types of layering over a range of length scales. They are the solid record of crystallization, differentiation and solidification processes of mainly basaltic magmas. The importance of layered intrusions also lies in hosting a significant proportion of the world's known reserves and resources of important critical metals: particularly, the majority of the global resource of platinum-group elements (PGE), chromium (Cr) and vanadium (V) and also very large resources of nickel (Ni), copper (Cu) and cobalt (Co). This paper summarizes the progress that has been made in the study of layered intrusions during the last three decades. The progress is marked by a number of novel observations from layered intrusions. Among them are: (1) draping of igneous layering over a few-km-high sloping step in the chamber floor; (2) development of igneous layering on the overturned to undercutting portions of a chamber floor; (3) magmatic karstification of the floor cumulates, (4) existence of three-dimensional framework of crystals in (oxide) cumulates; (5) systematic variations in dihedral angles between touching grains, and other microtextural features; (6) Cr-rich structures at the base of magnetitite layers; (7) co-existence of melt inclusions of contrasting composition in minerals; (8) thermal and chemical histories recorded by plagioclase; (9) textural and chemical features of minerals revealed by X-ray microscopy, (10) intrusion-scale to mineral-scale isotopic heterogeneity; (11) out-of-sequence zircon ages; and (12) skeletal/dendritic growth of minerals revealed by minor element zonation. The progress is also evident from development of several new concepts and refinement of some established ones. These include: (1) time and length scales in layered intrusion processes, (2) catastrophically fast growth of magma chambers, (3) out-of-sequence emplacement in layered intrusions, (4) large-scale slumping and mineral sorting in layered intrusions, (5) production of monomineralic cumulates from single phase-saturated melts, (6) origin of non-cotectic cumulate by in situ growth, (7) the arrival of new phases on the liquidus, (8) inward propagation of solidification fronts, (9) mushy and hard chamber floor, (10) absence of roof sequences due to their disruption, (11) basal reversals and chilled margins, (12) adcumulus growth theory, (13) compositionally stratified magma chambers, (14) melt-sediment interactions during magma chamber growth, (15) lateral reactive infiltration in a crystal mush, (16) reactions involving conjugate immiscible liquids in crystal mushes, and (17) constraints on subsolidus processes from non-traditional Fe-Mg-Cr stable isotopes. Finally, we show that the major controversies regarding layered intrusions currently revolve around whether: (a) the microstructure of igneous rocks are primary or secondary and (b) compaction in layered intrusions is pervasive or non-existent (c) large, long-lived and entirely-molten magma chambers exist or not. The review shows that layered intrusions provide ground-truth information on the processes of magma crystallization, differentiation, and solidification in crustal chambers as well as on mechanisms of ore-forming elements concentration into economically viable mineral deposits. We propose a few lines for future research that may potentially raise igneous petrology to a new level of understanding of the processes that govern the evolution of terrestrial magmatic systems.
Zirconium stable isotope fractionation during intra-crustal magmatic differentiation in an active continental arc
2024, Geochimica et Cosmochimica Acta
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 δ^94/90Zr_NIST 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 δ^94/90Zr_NIST 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 δ^94/90Zr_NIST 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 δ^94/90Zr_NIST 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 (Δ^94/90Zr_{garnet-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 δ^94/90Zr can be recycled into the mantle as a consequence of lower crustal foundering.
Site-specific isotope effect: Insights from equilibrium magnesium isotope fractionation in mantle minerals
2023, Geochimica et Cosmochimica Acta
The equilibrium Mg isotope fractionation factors (lnα) derived from both theoretical calculation and experimental studies are not consistent with the values from natural sample observation. Different cation sites, which have identical coordinate numbers but different bond strengths, contribute equally to lnα in theoretical calculations. These non-equivalent sites might complicate the inter-mineral equilibrium isotope fractionations. Here, we investigate reduced partition function ratios (10³lnβ) of non-equivalent Mg cation sites (e.g., M1, M2, and M3) in forsterite, orthoenstatite, clinoenstatite, tremolite, omphacite, pyrope and spinel to explore the site-specific effect on the intra- and inter-mineral equilibrium isotope fractionation. Based on all-electron first-principles calculations, 10³lnβ in different cation sites decreases in the following sequence of spinel_M1 > tremolite_M3 > orthoenstatite_M1 > forsterite_M1 > omphacite_M1 > tremolite_M1 > tremolite_M2 > clinoenstatite_M1 > forsterite_M2 > orthoenstatite_M2 > clinoenstatite_M2 > pyrope_M1. These findings suggest that cation sites with larger polyhedron volumes prefer light Mg isotopes to those with smaller polyhedron volumes inside minerals, and the different contribution of intra-mineral cation sites could result in significant variations of equilibrium Mg isotope fractionation between minerals. The range of lnα between minerals induced by specific sites is defined as the equilibrium isotope fractionation domain. The site-corrected lnα can provide better evaluation of equilibrium fractionation factor of Mg isotopes between spinel and olivine and clarify the equilibrium state between mantle minerals. In addition, we establish spinel-olivine and omphacite-pyrope Mg isotope geothermometers based on the site-specific effect. This study suggests that the site-specific effect could be prevalent in solid materials and thus important for other isotope systems.
Diffusion-driven Zn and Mg isotope fractionation in magmatic Fe-Ti-Cr oxides and implications for timescales of magmatic processes
2023, Geochimica et Cosmochimica Acta
Diffusion-driven isotopic fractionation during crystal growth and crystal-melt interaction has important implications for identifying diffusive processes and estimating the timescales of magmatic processes. Especially, Fe-Ti-Cr oxides are common crystalline phases in both relatively unfractionated and highly evolved basaltic magmas and may record the evolution history of the host magmas. High-precision Zn and Mg isotopic compositions for a set of Fe-Ti-Cr oxides and their host lavas in three types of Cenozoic basalts (sodic, transitional and potassic) from northeast China are reported in this study. These oxide crystals exhibit a wide range of chemical compositions (e.g., FeO_T = 20.6–49.1 wt%, TiO₂ = 1.38–8.70 wt%, Cr₂O₃ = 16.2–43.1 wt%), reflecting substantial chemical disequilibrium with their host magmas. A large range of δ⁶⁶Zn_JMC-Lyon from −1.12‰ to 0.24‰ and δ²⁶Mg_DSM-3 from 0.04‰ to 0.80‰ is observed in oxides (n = 17) from all three lava types. Compared with the host basalts, the oxides are enriched in lighter Zn with △⁶⁶Zn_oxide-melt (δ⁶⁶Zn_oxide–δ⁶⁶Zn_melt) from −1.43‰ to −0.19‰ and heavier Mg with △²⁶Mg_oxide-melt (δ²⁶Mg_oxide–δ²⁶Mg_melt) from 0.43‰ to 1.13‰. Comparison with existing theoretical work indicates that these isotopic offsets, even if compositional effects of oxides are considered, are too large to be in equilibrium. There are negative correlations between tetrahedral-site Fe²⁺ + Zn²⁺ and Mg²⁺ (R² = 0.97) and between δ⁶⁶Zn and δ²⁶Mg (R² = 0.66) for the oxides, which are best attributed to isotopic fractionation induced by Zn–Mg inter-diffusion, supported by the core-to-rim increase of Zn contents and decline of Mg contents in zoned oxides. Our results thus suggest that inter-diffusion between Mg and Zn can occur during crystal growth and reaction with host magmas, which is commonly observed between Mg and Fe, and this process is accompanied by strong Zn and Mg isotope fractionations. Such fractionations can be utilized to estimate the timescales for the formation of zoned minerals. Numerical simulation yields a short (∼35 days) diffusion interval for the observed Zn and Mg isotopic variations in the investigated oxide crystals, which indicates rapid cooling of the host lavas. Our results also indicate that fractional crystallization of oxides would lead to slight δ⁶⁶Zn increase and δ²⁶Mg decrease in the residual melts. Such fractionation should be considered while characterizing the Zn and Mg isotopic compositions of evolved magmas with low MgO and Zn contents.
Petrogenesis and dynamic significance of Miocene-Holocene alkali basalts in the southeastern Tibetan Plateau
2023, Lithos
A series of Cenozoic post-collisional mantle-derived magmas developed in the Southeastern Tibetan Plateau (the SE Plateau), which can be utilized to trace source components and mantle metasomatism, and further constrain the mantle evolution. In this paper, we present a systematic study on petrography, whole-rock major and trace element, Sr-Nd-Pb-Mg isotopes of Miocene-Holocene alkali basalts at the southeastern margin of the SE Plateau. These Miocene-Holocene alkali basalts were the products of the northward extension of the Cenozoic mantle-derived magmatism in Southeast Asia (SE Asia), since they are comparable in terms of geographical distributions, petrographic characteristics, geochronology patterns, and geochemical compositions. The alkali basalts in this study are mainly basanite and trachybasalt, characterized by enriched LILEs, depleted Sr-Nd-Pb isotopes, positive anomaly in HFSEs, and significantly lighter Mg isotopes (−0.51‰ to −0.32‰) relative to primitive mantle. Integrated Sr-Nd-Mg isotopes and seismic tomography suggest a source regions from the mantle transition zone that has been enriched by significant carbonate sediments.
The decoupling of high Sr contents and depleted SrNd isotopes in the alkali basalts indicate that the mantle carbonation is subjected to recent subduction events and is possibly related to the subduction of the Neo-Tethyan seafloor. The calculated melting temperature of these basalts are generally high (1487–1583 °C), which is consistent with that of Cenozoic basalts in SE Asia (1470–1480 °C) and Hainan island (1480–1530 °C), showing a high-temperature gradient similar to that of mantle plume-related magmatism. Consequently, the generation of Miocene-Holocene alkali basalts is plausibly associated with the Hainan plume.

View all citing articles on Scopus

View full text

Diffusion-driven extreme Mg and Fe isotope fractionation in Panzhihua ilmenite: Implications for the origin of mafic intrusion

Abstract

Introduction

Section snippets

Geological background and samples

Major elements in the minerals

Results

Discussion

Conclusions

Acknowledgements

Iron isotope systematics of the Panzhihua mafic layered intrusion associated with giant Fe-Ti oxide deposit in the Emeishan large igneous province, SW China

J. Geophys. Res.

Origin of anorthosite and magnetitite layers in the bushveld complex, constrained by major element compositions of plagioclase

J. Petrol.

Experiments on liquid immiscibility along tholeiitic liquid lines of descent

Contrib. Mineral. Petrol.

Fe-Ti-V-P ore deposits associated with proterozoic massif-type anorthosites and related rocks

Earth Sci. Rev.

Magnesium isotopic evidence for chemical disequilibrium among cumulus minerals in layered mafic intrusion

Earth Planet. Sci. Lett.

Controls on trace-element partitioning among co-crystallizing minerals: Evidence from the Panzhihua layered intrusion, SW China

Am. Mineral.

Iron isotope fractionation during crystallization and sub-solidus re-equilibration: Constraints from the Baima mafic layered intrusion, SW China

Chem. Geol.

Plume-lithosphere interaction in generation of the Emeishan flood basalts at the Permian-Triassic boundary

Geology

Iron isotopic compositions of geological reference materials and chondrites

Geostand. Geoanal. Res.

Abyssal peridotites reveal the near-chondritic Fe isotopic composition of the Earth

Earth Planet. Sci. Lett.

The Mathematics of Diffusion

Routine isotopic analysis of iron by HR-MC-ICPMS: How precise and how accurate?

Chem. Geol.

Magnesium and iron isotopes in 2.7 Ga Alexo komatiites: Mantle signatures, no evidence for Soret diffusion, and identification of diffusive transport in zoned olivine

Geochim. Cosmochim. Acta

Iron isotope systematics

Rev. Mineral. Geochem.

High-precision iron isotope analysis of geological reference materials by high-resolution MC-ICP-MS

Geostand. Geoanal. Res.

Silicate liquid immiscibility within the crystal mush: late-stage magmatic microstructures in the skaergaard intrusion, east greenland

J. Petrol.

Double-diffusive convection

J. Fluid Mech.

Isotope fractionation in silicate melts by thermal diffusion

Nature

The Differentiation of the Skaergaard Intrusion

Contrib. Mineral. Petrol.

Mg, Sr, and O isotope geochemistry of syenites from northwest Xinjiang, China: Tracing carbonate recycling during Tethyan oceanic subduction

Chem. Geol.

The geochemistry of titanomagnetite in magnetite layers and their host rocks of the eastern bushveld complex

Econ. Geol.

Immiscible Fe- and Si-rich silicate melts in plagioclase from the Baima mafic intrusion (SW China): Implications for the origin of bi-modal igneous suites in large igneous provinces

J. Asian Earth Sci.

Open magma chamber processes in the formation of the Permian Baima mafic–ultramafic layered intrusion, SW China

Lithos

Disequilibrium iron isotopic fractionation during the high-temperature magmatic differentiation of the Baima Fe-Ti oxide-bearing mafic intrusion, SW China

Earth Planet. Sci. Lett.

Crystallization sequence and magma chamber processes in the ferrobasaltic sept iles layered intrusion, Canada

J. Petrol.

Processes and time scales of magmatic evolution as revealed by Fe–Mg chemical and isotopic zoning in natural olivines

Geochim. Cosmochim. Acta

Mineral compositional constraints on petrogenesis and oxide ore genesis of the late Permian Panzhihua layered gabbroic intrusion, SW China

Lithos

Abundant Fe-Ti oxide inclusions in olivine from the Panzhihua and Hongge layered intrusions, SW China: evidence for early saturation of Fe-Ti oxides in ferrobasaltic magma

Contrib. Mineral. Petrol.

Origin of Fe-Ti oxide ores in mafic intrusions: Evidence from the Panzhihua intrusion, SW China

J. Petrol.

The use of Mossbauer spectroscopy in stable isotope geochemistry

Geochim. Cosmochim. Acta

The nature and origin of titaniferous magnetite-rich layers in the upper zone of the bushveld complex - a review and synthesis

Econ. Geol.

Isotope fractionation by diffusion in molten oxides

Geochim. Cosmochim. Acta

Isotopic fractionation of the major elements of molten basalt by chemical and thermal diffusion

Geochim. Cosmochim. Acta

Non-traditional fractionation of non-traditional isotopes: Evaporation, chemical diffusion and Soret diffusion

Chem. Geol.