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Geochemical and Sr-Nd-Pb isotope investigation of the New Caledonia harzburgite: unravelling the evolution of a sub-arc mantle source
No full textThe New Caledonia Ophiolite hosts one of the largest obducted mantle sections worldwide, offering a unique opportunity to investigate key mantle processes. The mantle section is dominated by a harzburgite-dunite sequence but it also includes minor spl and pl lherzolites. Geochemical data indicate that the harzburgites suffered multiple melting episodes followed by localized interaction with fluids in a supra-subduction zone setting, while the lherzolites are akin to abyssal-type peridotites (Secchiari et al., 2016).
In order to constrain how these processes affected the behaviour of highly siderophile (HSE: PGE=Os-Ir-Ru-Rh-Pt-Pd+Au-Re) and chalcophile elements (S-Se-Te), a set of fully characterised peridotites (major, trace element, Sr-Nd-Pb isotopes) has been studied.
The lherzolites are slightly serpentinized and display chondritic to slightly suprachondritic 187Os/188Osi (0.1273-0.1329 at 53 Ma). The gently sloping HSE patterns with increasing depletion towards Au are similar to other oceanic or continental lherzolites. These features were inherited from sulphide melt-silicate partitioning during partial melting, melt infiltration and mixing of different generation of sulphides. S contents (202-1268 ppm) were likely increased by serpentinization, whereas Se/Te are similar to other lherzolites.
The harzburgites can be grouped in two sub-types. Type-A (+9.3≤Ndi≤+13.3) have subchondritic 187Os/188Osi (0.1203-0.1254), low Os (0.55-1.51 ppb) and very low Re/Os. Their HSE patterns display strong fractionations, enriched Os-Ir-Ru segments and Pt-Au positive spikes. S-Se-Te are often below the detection limit. These patterns can be ascribed to high melting degrees, leading to sulphide exhaustion and PGE alloys stabilization.
Type-B harzburgites (-0.8≤Ndi≤+4.0) show chondritic to suprachondritic measured 187Os/188Os (0.1273-0.1524), notably low Os-Ir contents (0.003-0.277 ppb) and highly variable 187Re/188Os (2-30). The “melt-like” HSE patterns exhibit strongly fractionated Os-Ir-Ru (OsN/RuN=0.02-0.46), negative Pt anomalies and positive Au spikes. S-Se-Te are close to or below the detection limit. We interpret these compositions as reflecting localized modification of type-A harzburgites by subduction-related fluids and/or hydrous melts, leading to partial destabilization of Os-Ir rich alloys due to high fO2.
Our work suggests that some of the features shown by arc lavas (e.g., positive Pt spikes) may mirror the geochemical signature of the sub-arc mantle.
REFERENCES
Secchiari, A., Montanini, A., Bosch, D., Macera, P., Cluzel, D. (2016): Melt extraction and enrichment processes in the New Caledonia lherzolites: Evidence from geochemical and Sr–Nd isotope data. Lithos, 260, 28-4
Origin and fate of refractory domains in the oceanic mantle - Secchiari A., Godard M., Montanini A.
No full textOrigin and fate of refractory domains in the oceanic mantle - Secchiari A., Godard M., Montanini A.</p
Highly siderophile and chalcophile element behaviour in abyssal-type and supra-subduction zone mantle: constraints from the New Caledonia ophiolite
No full textExposures of arc-related mantle sections preserved in ophiolites constitute a natural laboratory for investigating subduction zone processes. The New Caledonia ophiolite consists primarily of harzburgites, locally overlain by mafic-ultramafic cumulates, and minor spinel and plagioclase lherzolites (Secchiari et al., 2016). Recent studies have shown that the harzburgites and the associated cumulates represent a crust-mantle transect formed in a nascent arc environment (Marchesi et al., 2009; Pirard et al., 2013; Secchiari et al., submitted). However, scarce geochemical data are available for the harzburgites, so that the processes recorded by this mantle section are still poorly constrained.
In order to unravel the evolution of the New Caledonia harzburgite, a comprehensive geochemical dataset (major and trace element, Sr-Nd-Pb isotopes) has been obtained on a new set of fresh samples unaffected by serpentinization.
The studied harzburgites are low-strain tectonites showing porphyroclastic textures, locally grading into promylonitic textures. They exhibit a refractory nature, as attested by the notable absence of primary clinopyroxene, very high Fo content of olivine (90.9-92.9 mol.%), high Mg# of orthopyroxene (89.8-94.2) and Cr# of spinel (39-71). Thin films of undeformed clinopyroxene with very low Na2O (0.03-0.13 wt.%) and TiO2 (0.04-0.10 wt.%) contents have been also recognized in association with Al2O3- (0.88-1.53 wt.%) and CaO- (0.37-0.97 wt.%) poor secondary orthopyroxene.
The harzbugites display U-shaped REE profiles (GdN/YbN=0.06-0.48) with remarkably low REE concentrations (< 0.1 chondritic values), in the range of modern subduction zone peridotites. Geochemical models shows that the HREE composition of the harzburgites can be reproduced with high degrees (up to ~25%) of fractional melting of a DMM source in the spinel stability field. Extended trace element diagrams highlights depleted compositions coupled with strong positive anomalies for Pb and FME (i.e. Cs, Ba, Sr).
Nd isotopic ratios range from poorly to slightly radiogenic (-0.8≤Ndi≤+13.3) 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 harzburgite as reflecting a highly depleted sub-arc mantle source variably modified by fluid input and/or hydrous melts percolation during Eocene subduction.
REFERENCES
Marchesi, C., Garrido, C.J., Godard, M., Belley, F., Ferré, E. (2009): Migration and accumulation of ultra-depleted subduction-related melts in the Massif du Sud ophiolite (New Caledonia). Chem Geol, 266, 3-4, 171-186
Pirard, C., Hermann, J., O’ Neill H. (2013): Petrology and Geochemistry of the Crust–Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific. J Petrol, 54, 9, 1759–1792
Secchiari, A., Montanini, A., Bosch, D., Macera, P., Cluzel, D. (2016): Melt extraction and enrichment processes in the New Caledonia lherzolites: Evidence from geochemical and Sr–Nd isotope data. Lithos, 260, 28-43
Secchiari A., Montanini, A., Bosch, D., Macera, P., Cluzel, D. (2018): The contrasting geochemical message from the New Caledonia gabbronorites: insights on depletion and contamination processes of the sub-arc mantle in a nascent arc setting. Submitt. to Contrib Mineral Pet
Hydrous mafic-ultramafic intrusives at the roots of a proto-arc: implications for crust building and mantle source heterogeneity in young forearc regions
No full textThe New Caledonia ophiolite represents a rare example of proto-arc section originated during subduction infancy. The
sequence is dominated by refractory harzburgites overlain by ultramafic (dunites and wehrlites) and mafic (gabbronorites)
lithologies.
In this contribution, we report the first occurrence of amphibole-bearing intrusives in the New Caledonia forearc sequence.
This study deals with a petrological and geochemical investigation of a pyroxenite intrusion cut by mafic–ultramafic dikes.
The intrusion consists of medium grain websterites, composed of orthopyroxene (30–75 vol %), clinopyroxene (20–50 vol
%) and amphibole (2–30 vol %), which occurs as interstitial or poikilitic phase. Whole rocks display moderate Mg# (71–82)
and concave downward REE patterns, bearing depleted to flat LREE (
LaN/NdN = 0.5–1) and flat HREE segments (
DyN/
LuN = 0.8–1.1). Bulk rocks mirror clinopyroxene at higher absolute values. Fluid mobile element (FME) enrichments, coupled
to Zr–Hf depletion, are observed for both clinopyroxene and bulk rock. Mineral major element variations and textural
relationships indicate that the investigated lithotypes derived from hydrous magmas, which underwent extensive fractional
crystallisation and post-cumulus processes. Geochemical modelling shows that the parental melts in equilibrium with the
pyroxenites share remarkable similarities with the New Caledonia CE-boninite. However, they significantly differ from the
equilibrium melts previously reported for the other intrusive rocks of the sequence. As a whole, our new results highlight a
greater compositional variability for the liquids ascending into the Moho transition zone and lower crust. This may be also
related to the involvement of a highly heterogeneous mantle source during subduction initiation
GEOCHEMICAL VARIABILITY OF MANTLE ROCKS DRILLED IN AN OCEAN-CONTINENT TRANSITION: NEW INSIGHTS FROM THE IBERIAN MARGIN PERIDOTITES (ODP LEG 149 AND 173)
No full textMantle rocks exposed along ocean-continent transitions represent an exceptional archive for investigating processes occurring during lithospheric extension and break-up, providing important clues on the chemical heterogeneities in the Earth’s mantle.
In this contribution, we revisit the Iberian margin peridotites through a combined petrological and geochemical investigation performed on a new set of drilled samples (ODP Holes 899B, 1068A, 1070A).
Hole 899B peridotites are relatively fresh tectonic harzburgites, displaying variable clinopyroxene amounts (≈ 3-8 vol.%) and mineral chemisty (0.898≤Mg#[Ol]≤0.915; Al2O3[Opx]= 2.95-3.46 wt.%; 0.198≤Cr#[Spl]≤0.483). Holes 1068A and 1070A peridotites are characterized by higher serpentinisation degrees and wider compositional ranges (Hole 1068A: Al2O3[Cpx]= 3.77-6.79 wt.%; 0.253≤Cr#[Spl]≤0.376; Hole 1070A: Al2O3[Opx]= 2.65-3.70 wt.%; 0.378≤Cr#[Spl]≤0.452).
In-situ trace element analyses revealed a wide spectrum of geochemical signatures. Primary clinopyroxene was found only in Holes 1068A and 899B. The clinopyroxene yields abyssal-type patterns, with gently (LaN/SmN= 0.22-0.27) to steeply plunging (LaN/SmN= 0.02-0.03) LREE for Holes 1068A and 899B, respectively. Orthopyroxene of Hole 899B has low REE concentrations (HREEN≤ 1) and spoon-shaped patterns, excepted for one plagioclase-bearing sample, which exhibits a straight pattern (LaN/YbN =0.001-0.02) and negative Eu anomalies.
Hump-shaped REE patterns, yielding variable LREE-MREE fractionation (LaN/SmN= 0.003- 0.16) and flat HREE, were found in Hole 1070A. These patterns are atypical for mantle peridotites and cannot be explained by melt depletion.
Our new data shed light on the occurrence of different mantle domains exposed along the ocean-continent transition of the Iberian margin. These features cannot be accounted by margin
extension and oceanization, as samples with the most oceanic affinity (Holes 1068A and 899B) are preserved close to the Iberia continental margin, while hump-shaped patterns appear common offshore (Hole 1070A). We interpret these contrasting signatures as related to a complex interplay of depletion and melt-rock interaction, partly inherited from a previous Wilson cycle
Temperatures and cooling rates recorded by the New Caledonia ophiolite: implications for cooling mechanisms in young forearc sequences.
Full text linkTo unveil how forearc lithosphere cools and re-equilibrates, we carried out a comprehensive geothermometric investigation of the New Caledonia ophiolite, which represents a rare example of proto-arc section generated during subduction infancy. A large dataset, including more than eighty samples (peridotites and mafic-ultramafic intrusives), was considered. Closure temperatures calculated for the lherzolites using slow (TREE-Y) and fast diffusing (TCa-in-Opx, TBKN, TCa-in-Ol, TOl-Sp) geothermometers provide some of the highest values ever documented for ophiolitic peridotites, akin to modern sub-oceanic mantle. Cooling rates deduced from TREE-Y and TBKN yield values of ≈ 10-3 °C/y, similar to those obtained with TCa-in-Ol. These features are consistent with a post-melting history of emplacement, possibly along a transform fault, and thermal re-equilibration via conduction. Cpx-free harzburgites register a high-T evolution, followed by quenching and obduction. The relatively high TCa-in-Ol, TOl-Sp and cooling rates computed from TCa-in-Ol (≈ 10-3 °C/y) are atypical for this geodynamic setting, mirroring the development of an ephemeral subduction system, uplift and emplacement of the Peridotite Nappe.
Temperature profiles across the crust-mantle transect point to high closure temperatures, with limited variations with depth. These results are indicative of injection and crystallization of non-cogenetic magma batches in the forearc lithosphere, followed by thermal re-equilibration at rates of ≈ 10-4-10-3 °C/y. Our study shows that the thermal conditions recorded by forearc sequences are intimately related to specific areal processes and previous lithospheric evolution. Thus, detailed sampling and exhaustive knowledge of the geological background are critical to unravel the cooling mechanisms in this geodynamic setting
Mantle heterogeneity generated by melt depletion and melt-rock interaction: the West Iberian margin peridotites (ODP Legs 149 and 173).
No full textUnderstanding the nature and evolution of mantle sequences exhumed along modern magma-poor ocean-continent transition zones (OCTs) is crucial for deciphering the complex interplay of tectonic and magmatic processes during rifting, lithospheric breakup, and the genesis of oceanic lithosphere. However, the existing data pertaining to these specific tectonic settings remain limited.
In this study, we reassessed peridotites from the West Iberian margin (WIM) by performing a petrological and geochemical investigation of variably serpentinized mantle exposures sampled in the Iberia Abyssal Plain (ODP Holes 899B, 1068A and 1070A). The investigated samples exhibit a diverse spectrum of mineral modes, textures and chemistry. Hole 899B spinel harzburgites are coarse-grained granular peridotites (cpx content ~ 2-3 vol.%), displaying evidence of melt-rock interaction, evidenced by olivine-forming, pyroxene-dissolving microstructures. Mineral chemical variations, along with in-situ trace element analysis, reveal that these peridotites experienced moderate amounts of melt extraction, occurring partly within the garnet stability field, followed by reactive percolation of LREE-enriched melts.
Microtextural analysis combined with mineral major element compositions, including high Cr# and TiO2 in spinel, as well as Na2O- and Al2O3-enriched clinopyroxene, supports a derivation from melt impregnation processes for plagioclase peridotites of Holes 899B and 1068A. Geochemical modelling based on clinopyroxene REE compositions indicates that these peridotites originated through melt-rock reactions driven by chemically heterogeneous melts, ranging from depleted to MORB-like compositions.
Among the examined samples, Hole 1070A spinel harzburgites (cpx ~ 2-5 vol.%) exhibit the most distinctive features. The low modal clinopyroxene abundances, preserving a positive correlation between Cr#, Al2O3 and Yb, indicate that Hole 1070A peridotites represent a suite of residual mantle rocks recording variable degrees of melt extraction. However, the unusual Cr-Na enrichments, coupled to hump-shaped clinopyroxene REE patterns, cannot be solely ascribed to melt depletion. The counterintuitive features of these samples can be explained by a process of open-system melting occurring in the spinel stability field associated to contemporaneous percolation of an enriched melt.
Calculated equilibrium temperatures for the WIM peridotites are within the range of fossil OCTs (TCa-in-Opx= 921-1029 °C). They record a history of melt-rock interaction at high temperature (TREE-Y= 1098-1244 °C), followed by thermal re-equilibration at relatively low rates (TOl-Sp= 770-813 °C).
Our data provide new compelling evidence for a sub-continental lithospheric origin of the peridotite samples from ODP Holes 899B and 1068A. We show that the petrological and geochemical signature of the WIM peridotites reflect a polyphase history, which was achieved through the superimposition of rifting-related processes to melt extraction events, related to, or older than, the last Wilson cycle
Heterogeneous mantle domains in a modern OCT: new insights from the West Iberian margin (ODP Legs 149 and 173).
No full textHydrous mafic-ultramafic intrusives in a nascent arc (Massif du Sud New Caledonia ophiolite).
Full text linkindicate enrichments for FME (i.e. Rb, Ba, U) and Th, coupled to Zr-Hf depletion. Strong Sr positive
spikes are only observed for the crosscutting dikes, while the pyroxenite body yields Sr negative
anomalies.
Geochemical modelling shows that the putative liquids in equilibrium with the websterites have
intermediate Mg# (57–63) and incompatible trace element patterns sharing remarkable
similarities with the New Caledonia CE-boninites [5]. However, they significantly differ from the
equilibrium melts reported for the other intrusive rocks of the sequence [1, 4], suggesting greater
compositional variability for the liquids ascending into the Moho transition zone and lower crust.
Our results support the notion that petrological and geochemical heterogeneity of magmatic products may be distinctive features of subduction infancy
Multi-stage evolution of peridotites of New Caledonia: preliminary results.
No full textThe New Caledonia Ophiolitic Nappe (Lower Cretaceous- Upper Eocene) is one of the largest and best exposed ophiolitic complex in the world. It is largely dominated by harzburgite tectonites but it also includes lherzolites and mafic and ultramafic cumulates.
Although the mantle rocks have been studied for almost 30 years, their history still remain controversial, partly because of the scarcity of geochemical data and the total lack of isotopic data. Recent studies (Marchesi et al., 2009; Ulrich et al., 2010) proved that these mantle rocks have experienced a complex evolution including different phases of melting, melt-rock interaction and re-melting that led to an overall ultra-depleted composition.
Here, we present the preliminary results of a petrological and geochemical study on a new set of peridotite samples from New Caledonia. Harzburgites, consisting of variable proportions of olivine (from 70-75% up to > 80 vol %), orthopyroxene (from 12%-15% to 25 vol %) and Cr-rich spinel (≈1% or less), are highly refractory rocks, as attested by the absence of primary clinopyroxene, very high Fo content in olivine (90.7-92.9 mol%), high Mg# in orthopyroxene ([Mg/(Mg+Fe)]= 91.0-92.7) and Cr# in spinel ([100 • Cr/(Cr+Al)]= 40-71). In contrast, lherzolites (clinopyroxene = 5-10 vol %), display a fairly fertile nature, with lower Fo in olivine (88.5-91.8 mol%) and Mg# in orthopyroxene (89.0-91.3), low Cr# in spinel (0.132-0.167) and relatively high Na2O (up to 0.80 wt%) and Al2O3 (3.1-6.7 wt%) contents in clinopyroxene. Secondary, interstitial and undeformed clino- and orthopyroxenes have also been observed in harzburgites. These phases testify melt percolation after partial melting and re-equilibration at lithospheric conditions. Their chemical compositions, i.e. low Al2O3 and CaO contents in orthopyroxene and very low or negligibile Na2O and TiO2 in clinopyroxene, may suggest a metasomatic origin by SiO2-rich fluids and/or depleted melts in a subduction-related setting.
Mineral compositional variations (e.g. Mg# (Ol) vs Cr# (Spl) and Cr# (Spl) vs Mg# (Spl), show that most investigated harzburgites plot in the field of SSZ (forearc) peridotites, whereas some cpx-poor lherzolites are more akin to abyssal peridotites, or, even, to passive margin peridotites for the most fertile types.
The peridotites are low strain tectonites with porphyroclastic textures partially overprinted by mosaic equigranular textures, probably recording an asthenospheric HT origin followed by sub-solidus re-equilibration. Geothermometric estimates provide temperatures of 930–1145°C and 870-1080°C for the porphyroclastic assemblages of harzburgites and lherzolites, respectively; lower temperatures are recorded for the spinel facies recrystallization (≈ 830°C–980°C for both lithotypes).
These preliminary results are consistent with a multi-stage history of melting, deformation, recrystallization and melt-rock interaction. Geochemical and radiogenic isotope analyses (in progress) are expected to decipher the depletion vs. refertilization evolution of the different peridotite types and put constrain on their geodynamic significance
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