Geochemical and Sr‐Nd Pb isotope investigation of the New Caledonia peridotite nappe: unravelling the history of a poorly known mantle section"
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Data di Pubblicazione:
2019
Citazione:
Geochemical and Sr‐Nd Pb isotope investigation of the New Caledonia peridotite nappe: unravelling the history of a poorly known mantle section" / A. Secchiari, A. Montanini, D. Bosch, P. Macera, D. Cluzel. ((Intervento presentato al convegno New Caledonia Peridotite Amphibious Drilling Workshop ICDP/IODP tenutosi a Montpellier nel 2019.
Abstract:
The New Caledonia ophiolite hosts one of the largest and best preserved mantle sections in the world,
offering a unique insight into upper mantle processes. Mantle lithologies are dominated by harzburgites, with
minor lherzolites, and are locally capped by a mafic-ultramafic intrusive sequence. Although the New
Caledonia ophiolite has been the subject of several petrological and geochemical investigations starting from
the 1980s (e.g. Prinzhofer et al., 1980; Dupuy et al., 1981), its ultra-depleted nature prevented an adequate
geochemical characterisation for long time. However, a renewed phase of interest has recently led to the
publication of new works, thereby improving considerably our knowledge on the Peridotite Nappe (Marchesi
et al., 2009; Ulrich et al., 2010; Pirard et al., 2013; Secchiari et al., 2016, 2018, submit.).
This contribution results from a 5 year lasting collaboration among Montpellier, Parma and New Caledonia
Universities. Here we present a comprehensive petrological and geochemical dataset obtained on fresh or
little serpentinized peridotites. Spinel lherzolites are slightly depleted rocks, as attested by the presence of 7-
8 vol.% of clinopyroxene, moderate Fo content of olivine (88.5-90.0 mol.%) and low Cr# of spinel (13-17).
The harzburgites exhibit a highly refractory character, testified by the notable absence of primary
clinopyroxene, high Fo content of olivine (90.9-92.9 mol.%), high Mg# of orthopyroxene (89.8-94.2) and high
Cr# of spinel (44-71). Mineral compositional variations and REE geochemistry indicate abyssal-type and
supra-subduction zone affinity for lherzolites and harzburgites, respectively. Melting models show that the
lherzolites underwent 8-9% degrees of fractional melting of a DMM source, starting in the garnet field. By
contrast, the harzburgites record exceedingly high melting degrees (i.e. 15% degree of dry melting and up to
18% degree of hydrous melting). On the other hand, concomitant enrichments in FME, L-MREE and Zr-Hf
were likely inherited during interaction with slab-derived silicate-bearing fluids, as supported by the frequent
occurrence of secondary interstitial Al2O3, CaO and Cr2O3-poor orthopyroxene and Na2O, Al2O3, TiO2-poor
clinopyroxene. Nd isotopes are in the range of the DMM for the lherzolites (+6.98≤εNdi≤+10.97). For the
harzburgites, heterogeneous Nd isotopic ratios (-0.80≤εNdi≤+13.32) coupled with Pb isotopes, trending from
DMM toward sediment-like compositions, support a derivation from a DMM reservoir variably modified by
subduction fluids. The geochemical features of the lherzolites suggest an origin in a MOR setting, i.e. in a
marginal basin formed before Eocene subduction. Conversely, the geochemical signature shown by the
harzburgites reflects the evolution of a highly depleted fore-arc mantle wedge contaminated by fluid inputs in
the subduction zone. Based on our data, a possible genetic link among the peridotites remains difficult to
establish.
Tipologia IRIS:
14 - Intervento a convegno non pubblicato
Elenco autori:
A. Secchiari, A. Montanini, D. Bosch, P. Macera, D. Cluzel
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