New insights on highly siderophile and chalcophile element behaviour in abyssal-type and supra-subduction zone mantle sections of the New Caledonia ophiolite
Abstract
Data di Pubblicazione:
2018
Citazione:
New insights on highly siderophile and chalcophile element behaviour in abyssal-type and supra-subduction zone mantle sections of the New Caledonia ophiolite / A. Secchiari, H. Becker, P. Gleissner, C. Li, A. Montanini, D. Bosch. - In: GEOPHYSICAL RESEARCH ABSTRACTS. - ISSN 1607-7962. - 20:(2018), pp. EGU2018-13877.1-EGU2018-13877.1. ((Intervento presentato al convegno EGU General Assembly tenutosi a Wien nel 2018.
Abstract:
The New Caledonia Ophiolite hosts one of the largest obducted mantle sections worldwide. The mantle section is
mostly composed of harzburgite and dunite, however, minor serpentinized spinel and plagioclase lherzolites also
occur in the northern part of the island. The major and trace element and Sr-Nd-Pb isotopic composition of the
harzburgites can be reconciled with a complex history of multiple melting episodes followed by localized overprint
by fluids and/or hydrous melts in a supra-subduction zone setting, while the lherzolites are similar to abyssal-type
peridotites (Secchiari, 2016).
As the harzburgites were not altered by low-temperature processes and apparently reflect the transition from ‘normal’
oceanic lithospheric mantle to supra-subduction zone mantle, this rock association offers the unique opportunity
to study the behaviour of highly siderophile (HSE: Os, Ir, Ru, Rh, Pt, Pd, Au, Re) and chalcophile elements
(S, Se, Te) in mantle from these different geodynamic settings.
The lherzolites are homogeneous with chondritic to slightly suprachondritic initial 187Os/188Osi (0.1273-0.1329).
They display gently sloping primitive mantle-normalized HSE patterns with increasing depletion towards Re and
Au and Se-Te concentrations, similar to variably depleted upper mantle peridotites. Sulfur contents (202-1268
ppm) were likely increased by serpentinization and seawater-related alteration. The HSE and S, Se, Te abundances
of the lherzolites can be explained by sulphide melt-silicate partitioning during partial melting, melt infiltration
and mixing of different generation of sulphides.
Harzburgites can be grouped in two different sub-types. Type A harzburgites (+9.3 epsilonNdi +13.3) show
subchondritic 187Os/188Osi (0.1203-0.1254), coupled with low Os concentrations (0.55-1.51 ppb) and low Re/Os.
These harzburgites display fractionated, Os-Ir-Ru–enriched patterns, with increasing depletion towards Au and Re
and a pronounced positive spike of Pt. S, Se and Te are often at, or below, the detection limit. These features can
be ascribed to sulphide exhaustion after high melting degrees and Os-Ir-Ru and Pt-Ir alloy stabilization.
Type B harzburgites (+0.8 epsilonNdi +4.0) show chondritic to suprachondritic measured 187Os/188Os
(0.1273-0.1524), very low Os and Ir concentrations (0.003-0.277 ppb) and suprachondritic and highly variable
187Re/188Os (2-30). HSE have “melt-like” patterns with strongly fractionated Os-Ir-Ru (OsN/RuN=0.02-0.46)
coupled with strongly negative Pt anomalies and positive Au spikes. S, Se, Te are close to or below the detection
limit. Such element patterns have never been identified in mantle peridotites before. We interpret these features
as resulting from localized modification of type-A harzburgites after interaction with subduction-related oxidizing
fluids or S-undersatured hydrous melts, which caused the breakdown and dissolution of sulphides and alloys,
preferentially removing Os, Ir, Pt and elements hosted by mantle sulphides.
The present work indicates that some of the features displayed by arc lavas such as positive Pt spikes may be
complementary to the composition of sub-arc mantle sources. (Vol. 20)
Tipologia IRIS:
01 - Articolo su periodico
Elenco autori:
A. Secchiari, H. Becker, P. Gleissner, C. Li, A. Montanini, D. Bosch
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