He and Ne isotopes in oceanic crust: implications for noble gas recycling in the mantle

The Podong Permian ultramafic intrusion is only one ultramafic intrusion with massif Ni-Cu sulfide mineralization in the Pobei layered mafic-ultramafic complex, western China. It is obviously different in sulfide mineralization from the nearby coeval Poyi ultramafic intrusion with the largest disseminated Ni-Cu sulfide mineralization and mantle plume contribution (Zhang et al., 2017). The type and addition mechanism of the confirmed crustal contaminations and possible mantle plume involved in the intrusion formation require evidences from carbon and noble gas isotopic compositions. In the present study, we have measured C, He, Ne, and Ar isotopic compositions of volatiles from magmatic minerals in the Podong ultramafic intrusion. The results show that olivine, pyroxene, and plagioclase minerals in the Podong intrusion have variable δ13C of CO2 (-24.5‰ to -3.2‰). The CH4, C2H6, C3H8, and C4H10 hydrocarbon gases show normal or partial reversal distribution patterns of carbon isotope with carbon number and light δ13C1 value of CH4, indicating the hydrocarbon gases of biogenic origin. The δ13C of CO2 and CH4 suggested the magmatic volatile of the mantle mixed with the volatiles of thermogenic and crustal origins. Carbon and noble gas isotopes indicated that the Podong intrusion could have a different petrogenesis from the Poyi ultramafic intrusion. Two types of contaminated crustal materials can be identified as crustal fluids from subducted altered oceanic crust (AOC) in the lithospheric mantle source and a part of the siliceous crust. The carbon isotopes for different minerals show that magma spent some time crystallizing in a magma chamber during which assimilation of crustal material occurred. Subduction-devolatilization of altered oceanic crust could be the best mechanism that transported large proportion of ASF (air-saturated fluid) and crustal components into the mantle source. The mantle plume existing beneath the Poyi intrusion could provide less contribution of real materials of silicate and fluid components.

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Recycling of oceanic crust and sediments: the noble gas subduction barrier

Earth and Planetary Science Letters ◽

10.1016/0012-821x(88)90170-7 ◽

1988 ◽

Vol 89 (2) ◽

pp. 173-183 ◽

Cited By ~ 194

Author(s):

Thomas Staudacher ◽

Claude J. Allègre

Keyword(s):

Oceanic Crust ◽

Noble Gas

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Fluid kinematics, fluid residence times, and rock degassing in oceanic crust determined from noble gas contents of deep sea drilling project pore waters

Journal of Geophysical Research Atmospheres ◽

10.1029/jb092ib12p12491 ◽

1987 ◽

Vol 92 (B12) ◽

pp. 12491 ◽

Cited By ~ 11

Author(s):

Ross O. Barnes ◽

W. Brian Clarke

Keyword(s):

Oceanic Crust ◽

Deep Sea ◽

Noble Gas ◽

Residence Times ◽

Pore Waters ◽

Drilling Project

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First assessment of the noble gas and CO2 isotopic composition of fluid inclusions hosted in mantle xenoliths from El Hierro (Canary Islands)

10.5194/egusphere-egu21-12817 ◽

2021 ◽

Author(s):

Andres Sandoval Velasquez ◽

Andrea Luca Rizzo ◽

Alessandro Aiuppa ◽

Maria Luce Frezzotti ◽

Samantha Remigi ◽

...

Keyword(s):

Isotopic Composition ◽

Oceanic Crust ◽

Canary Islands ◽

Fluid Inclusions ◽

Upper Mantle ◽

Noble Gas ◽

Mantle Metasomatism ◽

Mantle Xenoliths ◽

Local Source ◽

El Hierro

Studying the isotopic composition of fluids trapped in mantle xenoliths opens avenues to understanding the origin and cycling of volatiles in the Earth&#8217;s upper mantle. Here, we present the first isotopic results for noble gases and CO2 in fluid inclusions (FI) trapped in mantle xenoliths from El Hierro the youngest island of the Canarian archipelago. Our results are based on 6 mantle xenolith samples (3 Spinel-lherzolites and 3 Spinel-harzburgites) collected from the El Julan cliff valley (Oglialoro et al., 2017), from which we hand-picked crystals of Ol, Opx, and Cpx. Isotopic determinations were performed at the INGV (Sezione di Palermo) noble gas and stable isotopes laboratories, following the preparation methods and analytical procedures described in Rizzo et al. (2018 and references therein).The Ne-Ar isotopic compositions reveal the presence of an atmospheric component in the FI. Most of the samples exhibit 4He/20Ne ratios > 60, 20Ne/22Ne ratios between 9.84 and 10.49, 21Ne/22Ne ratios from 0.0295 to 0.0330, and 40Ar/36Ar > 800, suggesting mixing between MORB-like mantle fluids and an air-derived component. We argue this latter may (at least in part) derive from upper mantle recycling of atmospheric fluids via paleo-subduction event(s). Excluding samples possibly affected by diffusive fractionation processes, the average Rc/Ra ratio (3He/4He ratio corrected for atmospheric contamination) measured in El Hierro xenoliths is ~7.45 + 0.26 Ra, within the MORB range (8 + 1 Ra; Graham, 2002). The He homogeneous signature of these xenoliths agrees well with the 3He/4He compositions previously reported in lava phenocrysts and cumulates (Day and Hilton, 2011) and is slightly below the maximum ratios measured in groundwater samples during the 2012 volcanic unrest (~8.2 Ra; Padron et al., 2013). All these pieces of evidence argue against a primordial source involved in the local lithospheric mantle. Putting these data in the context of previous literature results for FI and surface gases in the Canary Islands (La Palma, La Gomera, Tenerife, Gran Canaria, and Lanzarote), we identify an eastward 3He/4He decreasing trend that parallels a corresponding increase of the oceanic crust thickness. In addition to the mantle heterogeneity, we propose that part of the 3He/4He east-to-west variation along the archipelago is caused by the variable thickness of the oceanic crust (and hence, different interactions with 4He-rich crustal fluids during emplacement).The FI &#948;13C(CO2) isotopic composition ranges from -2.38 to -1.23&#8240; in pyroxenes and -0.2 to +2.0&#8240; in olivine. These unusually positive &#948;13C compositions support the existence of a recycled crustal carbon component in the local source mantle, likely pointing to mantle metasomatism (Oglialoro et al., 2017) from fluids carrying carbon from subducted sediments and/or altered oceanic crust (AOC).

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