Maghemite soil nodules reveal the impact of fire on mineralogical and geochemical differentiation at the Earth's surface

Stefan C. Löhr, David T. Murphy, Luke D. Nothdurft, Robert Bolhar, Sandra Piazolo, Coralie Siegel

Research output: Contribution to journalArticleResearchpeer-review

Abstract

Fires occur frequently over large parts of the Earth's surface. They potentially exert a significant influence on the mineralogical and geochemical characteristics of an environment that is otherwise considered to be dominated by low temperature processes. We test this hypothesis by comparing the mineralogy and geochemistry of (i) magnetic, iron-rich soil nodules, (ii) non-magnetic iron soil nodules and (iii) a published dataset of surficial sediments from eastern Australia.

Maghemite-rich nodules are present in soils from around the world. It has been argued that they are thermal alteration products of non-magnetic precursors, but this remains controversial. We use detailed petrographic and mineralogical analyses to demonstrate that maghemite occurs as part of a high temperature mineral assemblage including hematite and χ-alumina, within a magnetic nodule microfabric indicative of fire-induced dehydroxylation and sintering of non-magnetic precursors at temperatures of up to 600 °C. The genetic link between magnetic and non-magnetic nodules means that their comparison offers insights into the geochemical impact of fire.

Our results show that magnetic nodules are depleted in Si, Y, Zr and HREE but enriched in Fe and Cr relative to non-magnetic nodules that occur in close spatial proximity. Magnetic nodules also show variable but distinctly low Y/Ho (21.4 ± 0.4) and Zr/Hf (29.3 ± 0.8) as well as anomalously low La relative to the other LREE. In situ laser ablation analyses show that this is largely due to the presence of χ-alumina that is depleted in HREEs and has extremely low Y/Ho (mainly <20), as well as the low Zr/Hf of χ-alumina and the maghemite-hematite matrix, with no involvement from zircon.

We propose a multi-stage process of formation where fire transforms non-magnetic nodule precursors into proto-magnetic nodules. This is associated with thermal transformation of clays as well as Fe and Al oxyhydroxides, followed by isochemical segregation into a sintered core with low Si, Y/Ho, Zr/Hf and La/Gd and a reciprocal cortex. Preferential loss of the weathering-sensitive cortex, which is rarely preserved on the magnetic nodules, then results in geochemical differentiation of magnetic nodules relative to their non-magnetic precursors. We propose that the elevated Zr/Hf and Y/Ho ratios previously reported for Australian fluvial sediments reflect, at least in part, the long history of palaeo-fires in the catchments of these rivers, with preferential removal, transport and sedimentation of the readily weathered, high Y/Ho and Zr/Hf cortex material that is a product of thermal alteration of Fe nodules. In addition, since magnetic Fe nodules are demonstrably related to fire, they may represent a promising, directly dateable record of severe fires, which can complement the sedimentary charcoal record.

LanguageEnglish
Pages25-41
Number of pages17
JournalGeochimica et Cosmochimica Acta
Volume200
DOIs
Publication statusPublished - 2017

Fingerprint

maghemite
Fires
Earth (planet)
Soils
Aluminum Oxide
aluminum oxide
soil
thermal alteration
hematite
Iron
Surficial sediments
dehydroxylation
iron
Geochemistry
Mineralogy
Charcoal
surficial sediment
Laser ablation
Weathering
ferric oxide

Keywords

  • Fire
  • High temperature mineral assemblage
  • Maghemite
  • χ-Alumina
  • Rare earth elements
  • Soil iron nodules
  • Thermal alteration

Cite this

Löhr, Stefan C. ; Murphy, David T. ; Nothdurft, Luke D. ; Bolhar, Robert ; Piazolo, Sandra ; Siegel, Coralie. / Maghemite soil nodules reveal the impact of fire on mineralogical and geochemical differentiation at the Earth's surface. In: Geochimica et Cosmochimica Acta. 2017 ; Vol. 200. pp. 25-41.
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abstract = "Fires occur frequently over large parts of the Earth's surface. They potentially exert a significant influence on the mineralogical and geochemical characteristics of an environment that is otherwise considered to be dominated by low temperature processes. We test this hypothesis by comparing the mineralogy and geochemistry of (i) magnetic, iron-rich soil nodules, (ii) non-magnetic iron soil nodules and (iii) a published dataset of surficial sediments from eastern Australia. Maghemite-rich nodules are present in soils from around the world. It has been argued that they are thermal alteration products of non-magnetic precursors, but this remains controversial. We use detailed petrographic and mineralogical analyses to demonstrate that maghemite occurs as part of a high temperature mineral assemblage including hematite and χ-alumina, within a magnetic nodule microfabric indicative of fire-induced dehydroxylation and sintering of non-magnetic precursors at temperatures of up to 600 °C. The genetic link between magnetic and non-magnetic nodules means that their comparison offers insights into the geochemical impact of fire. Our results show that magnetic nodules are depleted in Si, Y, Zr and HREE but enriched in Fe and Cr relative to non-magnetic nodules that occur in close spatial proximity. Magnetic nodules also show variable but distinctly low Y/Ho (21.4 ± 0.4) and Zr/Hf (29.3 ± 0.8) as well as anomalously low La relative to the other LREE. In situ laser ablation analyses show that this is largely due to the presence of χ-alumina that is depleted in HREEs and has extremely low Y/Ho (mainly <20), as well as the low Zr/Hf of χ-alumina and the maghemite-hematite matrix, with no involvement from zircon. We propose a multi-stage process of formation where fire transforms non-magnetic nodule precursors into proto-magnetic nodules. This is associated with thermal transformation of clays as well as Fe and Al oxyhydroxides, followed by isochemical segregation into a sintered core with low Si, Y/Ho, Zr/Hf and La/Gd and a reciprocal cortex. Preferential loss of the weathering-sensitive cortex, which is rarely preserved on the magnetic nodules, then results in geochemical differentiation of magnetic nodules relative to their non-magnetic precursors. We propose that the elevated Zr/Hf and Y/Ho ratios previously reported for Australian fluvial sediments reflect, at least in part, the long history of palaeo-fires in the catchments of these rivers, with preferential removal, transport and sedimentation of the readily weathered, high Y/Ho and Zr/Hf cortex material that is a product of thermal alteration of Fe nodules. In addition, since magnetic Fe nodules are demonstrably related to fire, they may represent a promising, directly dateable record of severe fires, which can complement the sedimentary charcoal record.",
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Maghemite soil nodules reveal the impact of fire on mineralogical and geochemical differentiation at the Earth's surface. / Löhr, Stefan C.; Murphy, David T.; Nothdurft, Luke D.; Bolhar, Robert; Piazolo, Sandra; Siegel, Coralie.

In: Geochimica et Cosmochimica Acta, Vol. 200, 2017, p. 25-41.

Research output: Contribution to journalArticleResearchpeer-review

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T1 - Maghemite soil nodules reveal the impact of fire on mineralogical and geochemical differentiation at the Earth's surface

AU - Löhr, Stefan C.

AU - Murphy, David T.

AU - Nothdurft, Luke D.

AU - Bolhar, Robert

AU - Piazolo, Sandra

AU - Siegel, Coralie

PY - 2017

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N2 - Fires occur frequently over large parts of the Earth's surface. They potentially exert a significant influence on the mineralogical and geochemical characteristics of an environment that is otherwise considered to be dominated by low temperature processes. We test this hypothesis by comparing the mineralogy and geochemistry of (i) magnetic, iron-rich soil nodules, (ii) non-magnetic iron soil nodules and (iii) a published dataset of surficial sediments from eastern Australia. Maghemite-rich nodules are present in soils from around the world. It has been argued that they are thermal alteration products of non-magnetic precursors, but this remains controversial. We use detailed petrographic and mineralogical analyses to demonstrate that maghemite occurs as part of a high temperature mineral assemblage including hematite and χ-alumina, within a magnetic nodule microfabric indicative of fire-induced dehydroxylation and sintering of non-magnetic precursors at temperatures of up to 600 °C. The genetic link between magnetic and non-magnetic nodules means that their comparison offers insights into the geochemical impact of fire. Our results show that magnetic nodules are depleted in Si, Y, Zr and HREE but enriched in Fe and Cr relative to non-magnetic nodules that occur in close spatial proximity. Magnetic nodules also show variable but distinctly low Y/Ho (21.4 ± 0.4) and Zr/Hf (29.3 ± 0.8) as well as anomalously low La relative to the other LREE. In situ laser ablation analyses show that this is largely due to the presence of χ-alumina that is depleted in HREEs and has extremely low Y/Ho (mainly <20), as well as the low Zr/Hf of χ-alumina and the maghemite-hematite matrix, with no involvement from zircon. We propose a multi-stage process of formation where fire transforms non-magnetic nodule precursors into proto-magnetic nodules. This is associated with thermal transformation of clays as well as Fe and Al oxyhydroxides, followed by isochemical segregation into a sintered core with low Si, Y/Ho, Zr/Hf and La/Gd and a reciprocal cortex. Preferential loss of the weathering-sensitive cortex, which is rarely preserved on the magnetic nodules, then results in geochemical differentiation of magnetic nodules relative to their non-magnetic precursors. We propose that the elevated Zr/Hf and Y/Ho ratios previously reported for Australian fluvial sediments reflect, at least in part, the long history of palaeo-fires in the catchments of these rivers, with preferential removal, transport and sedimentation of the readily weathered, high Y/Ho and Zr/Hf cortex material that is a product of thermal alteration of Fe nodules. In addition, since magnetic Fe nodules are demonstrably related to fire, they may represent a promising, directly dateable record of severe fires, which can complement the sedimentary charcoal record.

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KW - Maghemite

KW - χ-Alumina

KW - Rare earth elements

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