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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
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
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
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
Geochemical and Sr-Nd-Pb isotope investigation of the New Caledonia peridotite nappe: unravelling the history of a poorly known mantle section.
No full textThe New Caledonia mantle section: tracking source depletion and contamination processes in a suprasubduction setting.
No full textThe Bogota pyroxenites (New Caledonia): new insights on mantle heterogeneity in young subduction systems
No full textSupra-subduction mantle pyroxenites in an infant subduction system: the New Caledonia ophiolite record.
No full textSubduction-related ultradepleted melts in a nascent arc: geochemical and isotopic evidence from the intrusive sequence of the New Caledonia ophiolite.
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