Investigating peridotite samples exposed along ocean-continent transitions is crucial to shed light on processes occurring in the upper mantle during lithospheric breakup, unravelling, at the same time, source heterogeneity and inheritance. More than three decades ago, mantle rocks were recovered along the West Iberian margin during several ODP cruises (Leg 103, 149, and 173; see Abe, 2001; Chazot et al., 2005). Despite being one of the few sites worldwide where mantle has been accessed through scientific drilling, Iberian peridotites remain poorly characterized. Here, we revisit Iberian peridotites through a combined petrological and geochemical study performed on a new set of mantle samples from ODP Holes 899B, 1068A, and 1070A. Hole 899B peridotites are relatively fresh, clinopyroxene-rich (≈ 5-8 vol.%) tectonite harzburgites. They mostly consist of deformed olivine (≈ 80- 85 vol.%), showing variable Fo contents (89.1-92.1), and low amount (≈ 10-12 vol.%) of exsolved, Mg-rich orthopyroxene. Dark brown spinel (< 1 vol.%, Cr#=0.198-0.483; TiO2=0.10-0.72) mainly occurs as elongated grains, sometimes rimmed by altered plagioclase. In contrast to Hole 899B, peridotites from Holes 1068A and 1070A are highly serpentinized. Samples from Hole 1068A have chromian spinel (Cr#=0.253-0.376) mantled by a fine-grain white rim, possibly representing a pseudomorph after plagioclase. Clinopyroxene relics display highly variable Al2O3 (3.77-7.25 wt.%) and TiO2 contents (0.38-0.75 wt.%), in contrast to the lower and more homogenous values of Hole 1070A samples (3.87-4.70 wt.% and 0.14-0.20 wt.% for Al2O3 and TiO2, respectively). High Mg# (0.915-0.920) olivine and orthopyroxene with low- to moderate Al2O3 contents (2.52-3.70 wt.%) are only preserved in Hole 1070A. In situ trace element investigation revealed convex-upward patterns for Holes 899B and 1068A clinopyroxene, in the range of modern abyssal peridotites. Clinopyroxene shows LREE-depleted segments (LaN/SmN = 0.01-0.27) and flat HREE for YbN ≈ 10, coupled to Eu negative anomalies. Orthopyroxene of Hole 899B yields low REE concentrations (HREEN ≤ 1) and spoon-shaped patterns, except for the plagioclasebearing sample, which exhibits a straight pattern (LaN/YbN = 0.001-0.02) and negative Eu anomalies. Humpshaped REE patterns with steep LREE slopes (LaN/SmN = 0.003-0.01) and flat HREE segments are observed for Hole 1070A orthopyroxene. Our preliminary results indicate that the mantle exhumed along the ocean-continent transition of the Iberian margin experienced variable degrees of depletion and melt/rock interaction.

Mantle rocks exhumed along an ocean-continent transition: revisiting the Iberian margin peridotites (ODP Leg 149 and 173) / A. Secchiari, M. Godard, A. Montanini, A. Luguet. ((Intervento presentato al convegno CONGRESSO SGI-SIMP 2022 Geosciences for a sustainable future tenutosi a Torino nel 2022.

Mantle rocks exhumed along an ocean-continent transition: revisiting the Iberian margin peridotites (ODP Leg 149 and 173)

A. Secchiari
Primo
;
2022

Abstract

Investigating peridotite samples exposed along ocean-continent transitions is crucial to shed light on processes occurring in the upper mantle during lithospheric breakup, unravelling, at the same time, source heterogeneity and inheritance. More than three decades ago, mantle rocks were recovered along the West Iberian margin during several ODP cruises (Leg 103, 149, and 173; see Abe, 2001; Chazot et al., 2005). Despite being one of the few sites worldwide where mantle has been accessed through scientific drilling, Iberian peridotites remain poorly characterized. Here, we revisit Iberian peridotites through a combined petrological and geochemical study performed on a new set of mantle samples from ODP Holes 899B, 1068A, and 1070A. Hole 899B peridotites are relatively fresh, clinopyroxene-rich (≈ 5-8 vol.%) tectonite harzburgites. They mostly consist of deformed olivine (≈ 80- 85 vol.%), showing variable Fo contents (89.1-92.1), and low amount (≈ 10-12 vol.%) of exsolved, Mg-rich orthopyroxene. Dark brown spinel (< 1 vol.%, Cr#=0.198-0.483; TiO2=0.10-0.72) mainly occurs as elongated grains, sometimes rimmed by altered plagioclase. In contrast to Hole 899B, peridotites from Holes 1068A and 1070A are highly serpentinized. Samples from Hole 1068A have chromian spinel (Cr#=0.253-0.376) mantled by a fine-grain white rim, possibly representing a pseudomorph after plagioclase. Clinopyroxene relics display highly variable Al2O3 (3.77-7.25 wt.%) and TiO2 contents (0.38-0.75 wt.%), in contrast to the lower and more homogenous values of Hole 1070A samples (3.87-4.70 wt.% and 0.14-0.20 wt.% for Al2O3 and TiO2, respectively). High Mg# (0.915-0.920) olivine and orthopyroxene with low- to moderate Al2O3 contents (2.52-3.70 wt.%) are only preserved in Hole 1070A. In situ trace element investigation revealed convex-upward patterns for Holes 899B and 1068A clinopyroxene, in the range of modern abyssal peridotites. Clinopyroxene shows LREE-depleted segments (LaN/SmN = 0.01-0.27) and flat HREE for YbN ≈ 10, coupled to Eu negative anomalies. Orthopyroxene of Hole 899B yields low REE concentrations (HREEN ≤ 1) and spoon-shaped patterns, except for the plagioclasebearing sample, which exhibits a straight pattern (LaN/YbN = 0.001-0.02) and negative Eu anomalies. Humpshaped REE patterns with steep LREE slopes (LaN/SmN = 0.003-0.01) and flat HREE segments are observed for Hole 1070A orthopyroxene. Our preliminary results indicate that the mantle exhumed along the ocean-continent transition of the Iberian margin experienced variable degrees of depletion and melt/rock interaction.
Settore GEO/07 - Petrologia e Petrografia
https://www.socgeol.it/files/download/pubblicazioni/Abstract Book/Abstract Congresso SGI-SIMP 2022_def.pdf
Mantle rocks exhumed along an ocean-continent transition: revisiting the Iberian margin peridotites (ODP Leg 149 and 173) / A. Secchiari, M. Godard, A. Montanini, A. Luguet. ((Intervento presentato al convegno CONGRESSO SGI-SIMP 2022 Geosciences for a sustainable future tenutosi a Torino nel 2022.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2434/946372
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