The distribution of fluid mobile and other incompatible trace elements in orthopyroxene from mantle wedge peridotites

doi:10.1016/j.chemgeo.2017.03.017

Chemical Geology

Volume 457, 10 May 2017, Pages 118-130

https://doi.org/10.1016/j.chemgeo.2017.03.017 Get rights and content

Abstract

Orthopyroxene is especially suited to decode and testify to the behavior of highly immobile elements during hydrous mantle melting. Laser ablation ICP-MS analyses from orthopyroxene hosted within peridotite from the Coast Range ophiolite (CRO) demonstrates that Group A peridotites (lherzolites) have similar compositions to mid-ocean-ridge abyssal peridotite, whereas other peridotites (Groups B and C; harzburgites) retain depleted signatures, but display ‘spoon-shaped’ enrichments for the light-REE. These patterns are consistent with variable degrees of partial melting of MORB-source asthenosphere initiated within the garnet stability field (< 10%) and continuing into the spinel stability field (< 15%). A few samples may have been subjected to subsequent melt/rock interaction. The supra-subduction zone (SSZ) environment represented by the CRO is illustrated by enriched fluid mobile elements (Li, Be, B, Pb) in all samples - up to 200 × depleted-MORB mantle (DMM). New applications of trace-element addition calculations [Shervais J. and Jean M.M. (2012) Inside the subduction factory: Modeling fluid mobile element enrichment in the mantle wedge above a subduction zone. GCA 95, 270–285] modified for orthopyroxene reveals that tens to hundreds of ppm were added to the DMM-source region. Our purpose is to demonstrate that orthopyroxene, in the absence of clinopyroxene, can be a constructive (and perhaps better) indicator of tectonic environment and magmatic processes that occurred within the North American Cordillera mantle wedge. Through this investigation we have captured all three stages of Coast Range ophiolite petrogenesis: starting with initial SSZ-coupled forearc spreading dominated by decompression melting, to a mature subduction zone with fluid-assisted partial melting, and the transition between the two.

Introduction

Ophiolites have long been used as natural laboratories for studying processes related to mid-ocean-ridge spreading and the generation of oceanic basalts. Much of this research has mainly focused on clinopyroxene trace-element chemistry from residual mid-ocean ridge mantle peridotites (e.g., Johnson et al., 1990, Johnson and Dick, 1992, Hellebrand et al., 2002) and ophiolites (e.g., Rampone et al., 1996, Batanova et al., 1998, Suhr et al., 1998, Saccani and Photiades, 2004, Sano and Kimura, 2007). Such studies estimated the degree of partial melting, as well as conditions of melt formation, porosity conditions, melt migration, fluid phase enrichment, and subsequent interactions with melts derived from deeper in the mantle tectosphere; however, all of these studies relied on the chemical analysis of clinopyroxene. Orthopyroxene, on the other hand, has been relegated to partitioning experiments or if measured for trace-elements, no further modeling or interpretation is offered, with preference given to clinopyroxene and/or spinel.

It is a common assumption that clinopyroxene carries most of the whole-rock's incompatible trace-elements in lherzolites, but clinopyroxene is rare to absent for clinopyroxene-poor or clinopyroxene-free peridotites, e.g., harzburgites and opx-bearing dunites; therefore, orthopyroxene is potentially our only source of information for geochemical models and geodynamic reconstructions. Harzburgites that form at subduction zones represent the depleted, refractory residues derived through extraction of relatively large melt fractions (> 20%) under hydrous melting conditions (e.g., Ishii et al., 1992, Parkinson and Pearce, 1998, Pearce, 2003, Jean et al., 2010), although melt/rock reactions have also been proposed (Kelemen et al., 1992). The fore-arc relationship has been confirmed by spoon-shaped rare earth element (REE) patterns of many refractory harzburgites and the light REE-depleted patterns of associated crustal sequence basaltic rocks (Suen et al., 1979, Pallister and Knight, 1981, Gruau et al., 1998). The predominantly harzburgitic nature of most ophiolite peridotites affords us the opportunity to examine this perception that orthopyroxene cannot be used in order to distinguish tectonic environments or mantle processes. This will provide a more complete evaluation of the ongoing processes within the mantle wedge.

In this contribution, we present new data on the trace-element chemistry of orthopyroxene from mantle peridotites of the Coast Range Ophiolite of California. In particular, we document enrichments in fluid mobile elements (FME) that characterize the mantle wedge during melt extraction. A modified algorithm, to carry out the same FME addition calculations modeled by Shervais and Jean (2012), allows for direct application to orthopyroxene, in order to calculate the addition of incompatible and fluid mobile trace elements to the mantle wedge during melting, which can be used in clinopyroxene-free harzburgites. In addition, we examine FME partitioning between clinopyroxene and orthopyroxene. We demonstrate that in clinopyroxene-poor harzburgites, orthopyroxene is the dominant phase that incorporates FME and other trace elements. This partitioning is linked to major-element chemistry and equilibration temperature(s). The results for which expands the use of trace element models to suprasubduction zone peridotites that contain little or no clinopyroxene,

Section snippets

Geologic background

The middle Jurassic Coast Range ophiolite, California, represents a short-lived episode of oceanic crust formation that affected much of western North America. Detailed mapping in the northern Coast Ranges suggests that the ophiolite formed over a proto-Franciscan subduction zone, which may have nucleated on a pre-existing fracture zone (Choi et al., 2008b, Shervais et al., 2011, Shervais and Choi, 2012). Peridotites with abyssal-type chemical signatures have been interpreted to represent a

Previous work

Choi et al. (2008b) examined orthopyroxene from CRO spinel lherzolites and demonstrated relatively high concentrations of Al with some enrichment in Ti, V, and Na, compared to orthopyroxenes from spinel harzburgites and orthopyroxenite dikes. Trace-elements also display the difference between orthopyroxene from spinel lherzolite compared to spinel harzburgites and orthopyroxenite (Jean et al., 2010). Chondrite-normalized REE patterns for spinel lherzolites, overall, have steep slopes and low

Petrography

Spinel lherzolites and clinopyroxene-rich harzburgites from the CRO are identified by ~ 3–8% modal clinopyroxene, and are characterized by either protogranular textures with smooth, curved grain boundaries, rare 120° triple grain interfacial angles, and a range of grain sizes and shapes (i.e., xenoblastic granular), or porphyroclastic textures, with large highly strained porphyroclasts of olivine and orthopyroxene surrounded by a groundmass of smaller strain free neoblasts with common 120°

Analytical methods

Twenty-one samples, with a total of 142 orthopyroxene grains, were analyzed for 28 elements, including REE (La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, and Lu), transition metals (TM: Sc, V), high field strength elements (HFSE: Nb, Sr, Ti, Zr, and Hf), and fluid mobile elements (FME: B, Pb, Li, Rb, Be, Ba, and Th). The NIST 612 glass standard was used as the external calibration standard, with CaO (wt%) determined by election microprobe analysis as the internal standard. Data reduction

Results

Orthopyroxene occurs in three dominant rocks types, which reflect their major and trace-element characteristics; e.g., spinel lherzolite (Group A) and spinel harzburgite (Groups B and C). Group A spinel lherzolites have spinel Cr#s [100*Cr/(Cr + Al)] from 10 to 38, Group B harzburgites have spinel Cr#s of 38–55, and Group C harzburgites have Cr#s > 55 (Choi et al., 2008b). All of these groups are refractory residues of melt extraction (Choi et al., 2008b, Jean et al., 2010). Orthopyroxenites are

Orthopyroxene based melt extraction models

Coast Range ophiolite pyroxenes were previously modeled using non-modal fractional melting (Jean et al., 2010). These models indicated that TM (V, Cr, Sc), HFSE (Zr, Ti, Hf, Sr), and REE + Y are relatively immobile and largely reflect magmatic processes. Starting with the DMM-source composition of Salters and Stracke (2004), orthopyroxene from Group A lherzolites (Black Diamond Ridge) were modeled with ~ 1.0% dry melting in the spinel (Sp) field and with 3.0% garnet (Gt) field melting transformed

Summary

This study synthesizes major- and trace-element data of orthopyroxene from 21 peridotitic rocks from the Coast Range ophiolite. The applications of this study are better served when harzburgite-rich suites can be used as a proxy, in the absence of cpx-rich lithologies. Our work provides further evidence for the three stages of Coast Range ophiolite petrogenesis: starting with intial SSZ–coupled forearc spreading dominated by decompression melting, to a mature subduction zone with fluid–assisted

Acknowledgments

We acknowledge the analytical help and data processing performed at Notre Dame University courtesy of Prof. Clive Neal and Dr. Anthony Simonetti. Discussions with Eric Hellebrand increased our knowledge about how the mantle melts and its geochemical response. This project was supported by NSF Grant EAR0440255. Don Dingwell provided editorial handling of this submission, but we especially thank Alessio Sanfilippo and Chenguang Sun for their careful and insightful reviews.

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Petrogenesis of Gerf Neoproterozoic carbonatized peridotites (Egypt): Evidence of convergent margin metasomatism of depleted sub-arc mantle
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Gerf Neoproterozoic ophiolitic rocks in the Southern Eastern Desert of Egypt represent the largest ophiolite nappe in the Arabian-Nubian Shield and have been preserved as part of the N-S striking Allaqi-Heiani suture zone. Landsat-8 OLI/TIRS, ASTER and Sentinel-1B data successfully discriminate the Gerf ophiolitic section and the structural framework of the study area. This study has applied spectral transform approaches, consisting of principal component analysis (PCA), band ratio (BR) and minimum noise fraction (MNF) for lithological and structural mapping. NW-SE and N-S structural trends are dominant and control the distribution of talc‑carbonates and ophicarbonates. The mantle section comprises dominantly harzburgites with subordinate dunites and is rarely cut by dike-like bodies of clinopyroxenites; these peridotites have been partially to completely converted to serpentinites and related rocks. The primary Gerf peridotites are low in TiO₂ (0.01 wt%), Al₂O₃ (0.50 wt%), and CaO (0.44 wt%) content on average, but are rich in Ni (up to 2758 ppm) and Cr (2906 ppm) relative to primitive mantle, suggesting their highly refractory residual nature after high degrees of partial melting, similar to forearc peridotites. This is confirmed by chemistry of their relic primary minerals, namely olivine (Fo: 91–93.7; NiO: 0.28–0.43 wt%), chromian spinel (Cr# 0.75 on average), orthopyroxene (Mg#: 0.92–0.93) and clinopyroxene (Mg#: 0.89–0.90). Gerf serpentinized peridotites also show ranges of oxygen fugacity (Δlog ƒO₂, FMQ + 0.2 – FMQ + 1.4) and equilibrium temperature (770–900 °C), consistent with those of forearc peridotites. Bulk-rock analyses of ultramafic rocks and in-situ analyses of their pyroxenes reveal enrichment of fluid mobile elements (FME: e.g., B, Cs, Pb, Sr) relative to high-field strength elements (e.g., Nb, Zr, Ti, Ta), indicating intense metasomatism of Gerf peridotites by slab-derived fluids. The Gerf peridotites have been subjected to intense CO₂ input from the subducted slab to form carbonate-rich rocks beneath the arc-forearc region. Carbonates (mainly magnesite) replace serpentine minerals and their formation synchronizes with the transition from lizardite to antigorite at temperatures, ~250° to ~350 °C. The CO₂-rich fluids increase the Au content because of alteration and break down of Au-bearing sulphides, and this process formed Au mineralization in carbonate-rich rocks. The maximum amount of CO₂ expulsion from the subducted slab increases with increasing mantle depth, and structural trends as ophicarbonates are abundant in thick parts of the ophiolite sequence in the Gerf area that was highly dissected by NW-SE, N-S and E-W striking faults. This confirms that structures control on distribution of the ophiocarbonate rocks in Gerf ophiolite. Calculated parental melts in equilibrium with Gerf peridotite spinels have boninitic affinities, suggesting their generation during the forearc stage, but parental melts of Gerf clinopyroxenite veins resemble N-MORB-like melts, indicating melt metasomatism of sub-arc mantle by impregnated mafic melts during early subduction initiation. Clinopyroxenite veins crystallized from MOR-like melts as a result of interaction between these melts and depleted peridotites in the sub-arc mantle.
Fingerprinting stealth metasomatism in ophiolitic peridotites
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Citation Excerpt :
These characteristics of clinopyroxene clearly demonstrate a hydrous melt/fluid metasomatic origin rather than formation by partial melting (Fig. 6a, b). Orthopyroxene can also be easily modified in mantle peridotites (Jean and Shervais, 2017). In general, partial melting results in shrinking grain size of orthopyroxene and increases of MgO and Cr2O3 contents and decrease of Al2O3 in orthopyroxene, and thus leads to positive correlations between MgO and Cr# values as defined from abyssal harzburgites (Fig. 4b; Warren, 2016).
Mantle wedge peridotites are characterized by extraordinarily depleted compositions and their genesis is still controversial. In this study, we present microstructural observations and mineral chemical analyses of mantle harzburgites in the Lycian ophiolite of SW Turkey that reveal their formation in a suprasubduction zone mantle wedge. The textures of clinopyroxene in the harzburgites, particularly chromite-clinopyroxene symplectites and grains containing mineral inclusions, do not support a partial melting origin. Zoning textures and the presence of additional phases within the orthopyroxene porphyroclasts indicate that they underwent melt/fluid infiltration. Olivine crystals and interstitial grains are enclosed in orthopyroxene porphyroclasts, implying that the orthopyroxene grains crystallized by consuming olivine. Chromite grains in the harzburgites not only have zoned, but also contain a variety of mineral inclusions. Amphibole grains in some of the harzburgites also have compositions similar to those in the chromitites. Therefore, all mineral phases in the Lycian harzburgites have well-preserved metasomatic signatures that overprinted their partial melting features. The enrichment of fluid-mobile elements, such as Cs, Rb, U, and Pb in clinopyroxene and amphibole, coupled with their high Mg# values (92.8–95.6 and 93.7–96.4, respectively), suggest formation by Cr- and Mg-rich, hydrous metasomatic melts/fluids. The consistent compositions of clinopyroxene and chromite in the harzburgites and chromitites, together with the decreasing intensity of alteration and crystallization temperature of amphibole in the chromitite pods through dunites to harzburgites, imply that the hydrous melts/fluids were partially released from chromite surfaces during chromitite formation and/or from dehydration of the metamorphic sole, which likely attributed to metasomatism at different depths. Moreover, chromite-hosted inclusions in the harzburgites may reflect the reaction before solidification of an inclusion-rich zone possibly signifying a rapid increase of chromite-oversaturated melt. Positive correlations between Cr and Al in pyroxenes of ophiolites are widely preserved in metasomatic mantle peridotites from different tectonic settings and thus they provide a robust chemical proxy for the metasomatism.
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We examine the thermal history of spinel lherzolite xenoliths to test models suggesting recent replacement of mantle lithosphere beneath the Canadian Cordillera. The xenoliths are hosted in basanite lavas and bedded pyroclastic deposits with an ArAr age of 465 ± 22 ka at Mt. Timothy, British Columbia, Canada. The xenolith suite is mostly fertile spinel peridotite (Fo_89.5_–_91.5, bulk Al₂O₃ 2–5.6 wt%) showing enrichment in highly incompatible elements. Geothermometry on coexisting ortho- and clinopyroxene using Ca-Mg-Fe (T_BKN) and REE (T_REE) exchange yield similar temperature distributions (950–1100 °C) indicating a sampling between 32 and 55 km depth along regional geotherm. The differences in T_REE and T_BKN (ΔT_REE-BKN) observed in the samples are statistically insignificant – with a maximum of 80 °C, similar to other subcontinental mantle xenoliths, and far less than observed for rapidly cooled mantle exhumed in oceanic settings (> 150 °C). The maximum ΔT_REE-BKN values can be used to place limiting values on the cooling rate of the lithosphere, as functions of the grain size of ortho- and clinopyroxene. The observed ΔT_REE-BKN requires a cooling rate < 10⁻⁵ °C /Myr for the xenoliths, far less than that expected for foundering or replacement of lithosphere in the past 50 Myr from asthenosphere (>10 °C /myr). The slow cooling rates are more consistent with a view that the mantle lithosphere of the Canadian Cordillera is ancient, has been maintained to be thin and hot for at least the past 30 Myr, and not cooled from asthenosphere in the past ~50 Myr. Extension of our result to other xenolith suites throughout the Canadian Cordillera can further test this conclusion.
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2021, Lithos
Citation Excerpt :
Orthopyroxenes in the New Caledonia harzburgites show different trace element composition from those in abyssal peridotites, i.e., they have higher (Ce/Yb)N ratios than those in abyssal peridotites at given (Sm/Yb)N ratios (Fig. 13). Such characteristics have also been previously reported for orthopyroxenes in mantle peridotites of the Coast Range ophiolites (Jean et al., 2010; Jean and Shervais, 2017). This is consistent with the fore-arc origin of the Coast Range ophiolites (Shervais, 2001).
The forearc mantle has generally experienced high degrees of partial melting and pervasive melt-rock reactions. Large ultramafic massifs of the New Caledonia ophiolite (Peridotite Nappe) provide opportunities for studying processes within the forearc mantle. Samples of harzburgite, dunite and websterite from three massifs (i.e., Me Maoya, Ouassë Bay, Massif du Sud) have been selected in this study. The peridotites have low contents of Al₂O₃ (0.10–0.78 wt%) and CaO (0.21–0.58 wt%), but high olivine Fo (90.8–92.5) and spinel Cr# (0.55–0.74) values. This indicates that they have experienced high degrees of partial melting, resulting in the complete consumption of primary clinopyroxene. Trace elements of orthopyroxenes have been measured to constrain the evolution of the New Caledonia harzburgites. Results show that orthopyroxenes have low contents of heavy rare earth element (HREE); their Yb contents are 0.04–0.39 times of chondrites, which can be modeled by ~25–30% of fractional melting of a depleted MORB mantle (DMM) source. The orthopyroxenes also show variable enrichment of light and middle rare earth elements (LREEs and MREEs) and large ion lithophile elements (LILEs), implying late-stage metasomatism. The websterites have high CaO (9.02–9.75 wt%) but low Al₂O₃ (1.59–1.60 wt%) contents. They have depleted Sr-Nd-Hf isotope compositions similar to that of New Caledonia CE-boninites and plot within the MORB field. Both websterites and dunites have ¹⁸⁷Os/¹⁸⁸Os ratios (0.12348–0.13067) comparable to that of abyssal peridotites and the New Caledonia harzburgites (0.1203–0.1534). Geochemical compositions and modeling of whole-rock and minerals of harzburgites, dunites and websterites suggest that the New Caledonia peridotites have experienced at least two-stage partial melting and reactions with various migrating melts. Moreover, we also demonstrated that trace element of orthopyroxenes can be applied to discriminate ultra-refractory harzburgites from different tectonic settings.
Sr, Nd, Pb and trace element systematics of the New Caledonia harzburgites: Tracking source depletion and contamination processes in a SSZ setting
2020, Geoscience Frontiers
Citation Excerpt :
At a first approximation we consider that interaction with slab-related hydrous fluids during melting may explain the FME enrichments. In order to constrain FME fluxes in the mantle wedge, we have applied the algorithm presented by Shervais and Jean (2012) and modified by Jean and Shervais (2017) for four selected samples (KPT1, KPT3, KPT5, YA1). This algorithm allows to model the fluid enrichment process and determine the amount of subduction component added to the mantle wedge peridotite for each melt increment, starting from orthopyroxene trace element composition.
The New Caledonia ophiolite (Peridotite Nappe) consists primarily of harzburgites, locally overlain by mafic-ultramafic cumulates, and minor spinel and plagioclase lherzolites. In this study, a comprehensive geochemical data set (major and trace element, Sr-Nd-Pb isotopes) has been obtained on a new set of fresh harzburgites in order to track the processes recorded by this mantle section and its evolution.
The studied harzburgites are low-strain tectonites showing porphyroclastic textures, locally grading into protomylonitic textures. They exhibit a refractory nature, as attested by the notable absence of primary clinopyroxene, very high Fo content of olivine (91–93 mol.%), high Mg# of orthopyroxene (0.91–0.93) and high Cr# of spinel (0.44–0.71). The harzburgites are characterised by remarkably low REE concentrations (<0.1 chondritic values) and display "U-shaped" profiles, with steeply sloping HREE (Dy_N/Yb_N = 0.07–0.16) and fractionated LREE-MREE segments (La_N/Sm_N = 2.1–8.3), in the range of modern fore-arc peridotites. Geochemical modelling shows that the HREE composition of the harzburgites can be reproduced by multi-stage melting including a first phase of melt depletion in dry conditions (15% fractional melting), followed by hydrous melting in a subduction zone setting (up to 15%–18%). However, melting models fail to explain the enrichments observed for some FME (i.e. Ba, Sr, Pb), LREE-MREE and Zr–Hf. These enrichments, coupled with the frequent occurrence of thin, undeformed films of Al₂O₃, and CaO-poor orthopyroxene (Al₂O₃ = 0.88–1.53 wt.%, CaO = 0.31–0.56 wt.%) and clinopyroxene with low Na₂O (0.03–0.16 wt.%), Al₂O₃ (0.66–1.35 wt.%) and TiO₂ (0.04–0.10 wt.%) contents, point to FME addition during fluid-assisted melting followed by late stage metasomatism most likely operated by subduction-related melts with a depleted trace element signature.
Nd isotopic ratios range from unradiogenic to radiogenic (−0.80≤ε_{Nd_i}≤+13.32) and negatively correlate with Sr isotopes (0.70257≤⁸⁷Sr/⁸⁶Sr ≤ 0.70770). Pb isotopes cover a wide range, trending from DMM toward enriched, sediment-like, compositions. We interpret the geochemical signature displayed by the New Caledonia harzburgites as reflecting the evolution of a highly depleted fore-arc mantle wedge variably modified by different fluid and melt inputs during Eocene subduction.
Harzburgite found in the Hegenshan ophiolite, southeastern Central Asian Orogenic Belt: Petrogenesis and geological implications
2019, Gondwana Research
This paper reports detailed studies on harzburgite and serpentinite in the Hegenshan ophiolitic mélange. Harzburgite consists mainly of olivine and orthopyroxene with trace amounts of clinopyroxene and chromian spinel. Clinopyroxene occurs as isolated crystals or in the intergrowth of chromian spinel–clinopyroxene–orthopyroxene. Harzburgite is moderately to highly depleted, displaying high Fo contents in olivine (90.8–92.2), moderate Al₂O₃ contents in orthopyroxene (1.59–2.79 wt%), low heavy REE abundances in clinopyroxene, and moderate Cr# values of spinel (0.50–0.62). The modal proportions of olivine and orthopyroxene pseudomorph grains imply that the parent of the Hegenshan serpentinite should be harzburgite. Whole-rock compositions of the harzburgite and serpentinite samples are characterized by depletions in Al₂O₃ and CaO and enrichments in light REE, Sr, and U. Geochemical modeling suggests that the Hegenshan harzburgite represents residues after 17–18% partial melting of the primitive mantle. The melt in equilibrium with clinopyroxene is more depleted than typical forearc basalt and boninite. Various pyroxene thermobarometers yield equilibrated temperatures of 945–1067 °C and pressures of 4.8–8.0 kbar for the Hegenshan harzburgite. The oxygen barometer yields results of +0.4 to +1.7 log units above the fayalite–magnetite–quartz buffer for the Hegenshan harzburgite. These petrological and geochemical characteristics, as well as the estimated P–T–fO₂ conditions support a back-arc setting for the Hegenshan ophiolitic mélange.

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Published by Elsevier B.V.