| 1. |
- Kemp, Anthony I.S., et al.
(författare)
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A linked evolution for granite-greenstone terranes of the Pilbara Craton from Nd and Hf isotopes, with implications for Archean continental growth
- 2023
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Ingår i: Earth and Planetary Science Letters. - : Elsevier BV. - 0012-821X .- 1385-013X. ; 601, s. 117895-117895
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Tidskriftsartikel (refereegranskat)abstract
- In felsic igneous rocks, the parent and daughter elements in the widely used Sm–Nd and Lu–Hf isotope tracer systems are mainly hosted in accessory phases. Recrystallisation and/or breakdown of these minerals during metamorphism, deformation and weathering potentially compromises the chemical and isotopic composition of the respective whole rocks, impeding the utility of such information for deducing the timing, rates and processes of crust-mantle differentiation in the early Earth. The different abilities of zircon and REE-rich minerals to withstand metamorphism have been suggested as a reason for the decoupling of the Lu–Hf and Sm–Nd isotope systems observed in a number of ancient gneiss terranes. The controls on element mobility and subsequent isotopic disturbance during recrystallisation and breakdown of LREE-rich accessory minerals are, however incompletely understood. Here, we use petrography, element mapping, and microanalysis of accessory minerals, in tandem with whole rock Sm–Nd data, to assess the reliability of the Sm–Nd system in the 3.59–3.58 Ga Mount Webber Gabbros, the oldest rocks in the Pilbara Craton (Western Australia). We show that despite multiple thermal events, which reset the mineral Sm–Nd systematics, and decomposition of the REE-rich mineral allanite, the Mount Webber rocks retained the Sm–Nd isotope signatures of their magmatic protoliths at the whole-rock scale. We show that the allanite breakdown occurred during modern, near-surface weathering processes at low temperature, such that the REE were sequestered into secondary minerals rather than escaping in higher temperature metamorphic fluids. The whole rock Sm–Nd, and zircon O–Hf signatures, together with new 142Nd isotope data, suggest derivation of the Mount Webber rocks from undifferentiated mantle sources that preserve no evidence for Hadean silicate Earth differentiation. This study highlights the benefits of a combined analytical approach using both in-situ and whole-rock isotope analyses to obtain a more complete record of the source and thermal evolution of ancient, highly metamorphosed igneous rocks.
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| 2. |
- Petersson, Andreas, et al.
(författare)
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Formation of early Archean Granite-Greenstone Terranes from a globally chondritic mantle : Insights from igneous rocks of the Pilbara Craton, Western Australia
- 2020
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Ingår i: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 551
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Tidskriftsartikel (refereegranskat)abstract
- The continental crust grows via juvenile additions from the mantle. However, the timing of initial continent stabilisation and the rate of subsequent continental growth during the first billion years of Earth history is widely debated, in part due to uncertainty over the composition of the mantle source of new crust. Well-preserved Archean granite-greenstone terranes, as present within the Pilbara Craton (Western Australia), provide insights into the sources of felsic magmas and the processes of continental growth and evolution in the distant geological past at the regional scale. Here, we present zircon U-Pb, O and Hf isotope data from ancient gneissic and granitic rocks of the Pilbara Craton, to decipher magma sources and the timing and processes of craton growth. There is no evidence for depleted mantle compositions, and the simplest interpretation is that the crust of the Pilbara Craton was generated from mantle with a chondritic Hf isotope composition. Our results indicate crustal addition at ~3.59 Ga, represented by emplacement of gabbroic to anorthositic rocks. We suggest that the formation of these igneous rocks, and the foundering of the complementary residues, triggered extensive melting of hot, upwelling mantle, leading to the subsequent accumulation of the >12 km thick greenstone belt eruptive sequences from 3.53 Ga, with emplacement of coeval felsic magmas at depth. This process shaped the initial crustal configuration of the proto-craton, which subsequently underwent gravitationally driven overturn and reworking to generate stable, cratonic continental crust with the distinctive dome and keel architecture. The zircon Hf and O isotope signatures of the Pilbara igneous rocks from ~3.59–3.4 Ga do not support remelting of an ancient (> 3.8 Ga) basement, and reinforce the overwhelmingly chondritic to near-chondritic zircon Hf isotope composition of Eoarchean meta-igneous rocks from a number of different Archean cratons. A corollary of this remarkable global consistency is that a significant volume of the mantle maintained a chondritic composition for the Lu-Hf system from the formation of the Earth into the Paleoarchean (up to 3.6–3.5 Ga), as would be the case if stabilised volumes of felsic continental crust prior to 3.5 Ga were relatively small. One implication is that the common assumption of a linear evolution of depleted mantle from 4.5 Ga to the present day is inappropriate for determining the timing and volume of continental crust extraction in the Archean. The nearly identical early evolution of the Pilbara and Kaapvaal cratons suggests a common process to generate Archean granite-greenstone terranes that does not require extensive reworking of ancient crust, but rather involves juvenile crustal addition above persistent zones of upwelling, chondritic mantle.
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| 3. |
- Petersson, Andreas, et al.
(författare)
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Response of Sm–Nd isotope systematics to complex thermal histories: A case study from 3.58 Ga gneisses of the Pilbara Craton
- 2023
-
Ingår i: Earth and Planetary Science Letters. - 0012-821X .- 1385-013X. ; 620, s. 118346-118346
-
Tidskriftsartikel (refereegranskat)abstract
- In felsic igneous rocks, the parent and daughter elements in the widely used Sm–Nd and Lu–Hf isotope tracer systems are mainly hosted in accessory phases. Recrystallisation and/or breakdown of these minerals during metamorphism, deformation and weathering potentially compromises the chemical and isotopic composition of the respective whole rocks, impeding the utility of such information for deducing the timing, rates and processes of crust-mantle differentiation in the early Earth. The different abilities of zircon and REE-rich minerals to withstand metamorphism have been suggested as a reason for the decoupling of the Lu–Hf and Sm–Nd isotope systems observed in a number of ancient gneiss terranes. The controls on element mobility and subsequent isotopic disturbance during recrystallisation and breakdown of LREE-rich accessory minerals are, however incompletely understood. Here, we use petrography, element mapping, and microanalysis of accessory minerals, in tandem with whole rock Sm–Nd data, to assess the reliability of the Sm–Nd system in the 3.59–3.58 Ga Mount Webber Gabbros, the oldest rocks in the Pilbara Craton (Western Australia). We show that despite multiple thermal events, which reset the mineral Sm–Nd systematics, and decomposition of the REE-rich mineral allanite, the Mount Webber rocks retained the Sm–Nd isotope signatures of their magmatic protoliths at the whole-rock scale. We show that the allanite breakdown occurred during modern, near-surface weathering processes at low temperature, such that the REE were sequestered into secondary minerals rather than escaping in higher temperature metamorphic fluids. The whole rock Sm–Nd, and zircon O–Hf signatures, together with new 142Nd isotope data, suggest derivation of the Mount Webber rocks from undifferentiated mantle sources that preserve no evidence for Hadean silicate Earth differentiation. This study highlights the benefits of a combined analytical approach using both in-situ and whole-rock isotope analyses to obtain a more complete record of the source and thermal evolution of ancient, highly metamorphosed igneous rocks.
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