Why natural monazite never becomes amorphous: Experimental evidence for alpha self-healing

Seydoux‐Guillaume, Anne‐Magali; Deschanels, Xavier; Baumier, Cédric; Neumeier, Stefan; Weber, William J.; Peuget, S.

doi:10.2138/am-2018-6447

Cited by 62 publications

(49 citation statements)

References 36 publications

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“…The second similarity is that there is no spatial relationship between age and position of the analytical spot within the monazite grain, whether in the core or rim. These results suggest that monazite grains underwent several periods of coupled dissolution‐precipitation that lead to the presence of unsupported Pb and disturbance of U‐Pb systems in the newly grown sections of the grain (Seydoux‐Guillaume et al, ). Thus, while individual ages are meaningless, the collective age range of monazite reflects the duration of the Pampean thermal anomaly.…”

Section: Discussionmentioning

confidence: 96%

Prolonged Movement on a > 10‐km‐Wide Thrust During Early Paleozoic Orogens in the Gondwana Margin of NW Argentina

et al. 2019

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The >10‐km thick Paleozoic Guacha Corral shear zone in Central Argentina is arguably one of the thickest thrust zones anywhere. It played a key role during the two early Paleozoic orogenies of western Gondwana and currently separates two terranes: one dominated by events related to the >510‐Ma Pampean Orogeny and the other by the <500‐Ma events of the Famatinian Orogeny. The combination of structures and geochronometry suggests that the shear zone was active for over 100 Ma. Thrusting started during migmatization at 520 Ma as recorded by U‐Pb ages in zircon and monazite and was demonstrably active also at 480 and 470 Ma. The age coincidence between Ar plateau ages of muscovite and biotite at 420 Ma in the upper part of the shear zone, and at 405 Ma in the lower part suggests two intense cooling pulses. These are ascribed to thrusting, rapid exhumation, and cooling. Its anomalous width is likely a result of this prolonged kinematic history related to its role in transferring shortening to the Pampean forearc possibly as a basal detachment and later by acting as the main boundary between the Pampean and Famatinian tectonometamorphic terranes. The two low‐temperature shearing events inferred from the Ar ages suggest that deformation continued during the period between the end of the Famatinian cycle at ~440 Ma and the start of the Chanic or Achalian Orogeny and its magmatism after 400 Ma.

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Section: Discussionmentioning

confidence: 96%

Prolonged Movement on a > 10‐km‐Wide Thrust During Early Paleozoic Orogens in the Gondwana Margin of NW Argentina

et al. 2019

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“…If such aqueous processes are responsible for zero-age Pb loss in zircon, the absence of such Pb loss in monazite and sphene remains notable. While we may consider assigning this discrepancy to factors such as the remarkable resistance of monazite to radiation damage (Seydoux-Guillaume et al, 2018), even fully crystalline monazite and sphene appear susceptible to aqueous recrystallization (Harlov et al, 2010;Gysi et al, 2018). Consequently, we consider the speculative possibility that partial resetting of zircon and baddeleyite during aqueous recrystallization may instead reflect the extreme incompatibility of Pb in the zircon (and baddeleyite) crystal lattice under natural conditions (Watson et al, 1997).…”

Section: Chemical Abrasion and U-pb Geochronologymentioning

confidence: 99%

Stepwise chemical abrasion–isotope dilution–thermal ionization mass spectrometry with trace element analysis of microfractured Hadean zircon

Keller

Boehnke²,

Schoene

et al. 2019

Geochronology

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Abstract. The Hadean Jack Hills zircons represent the oldest known terrestrial material, providing a unique and truly direct record of Hadean Earth history. This zircon population has been extensively studied via high-spatial-resolution high-throughput in situ isotopic and elemental analysis techniques, such as secondary ionization mass spectrometry (SIMS), but not by comparatively destructive, high-temporal-precision (<0.05 % two-sigma) thermal ionization mass spectrometry (TIMS). In order to better understand the lead loss and alteration history of terrestrial Hadean zircons, we conduct stepwise chemical abrasion–isotope dilution–thermal ionization mass spectrometry with trace element analysis (CA-ID-TIMS-TEA) on manually microfractured Hadean Jack Hills zircon fragments previously dated by SIMS. We conducted three successive HF leaching steps on each individual zircon fragment, followed by column chromatography to isolate U–Pb and trace element fractions. Following isotopic and elemental analysis, the result is an independent age and trace element composition for each leachate of each zircon fragment. We observe ∼50 Myr of age heterogeneity in concordant residues from a single zircon grain, along with a protracted history of post-Hadean Pb loss with at least two modes circa ∼0 and 2–4 Ga. Meanwhile, stepwise leachate trace element chemistry reveals enrichments of light rare earth elements, uranium, thorium, and radiogenic lead in early leached domains relative to the zircon residue. In addition to confirming the efficacy of the LREE-I alteration index and providing new insight into the mechanism of chemical abrasion, the interpretation and reconciliation of these results suggest that Pb loss is largely driven by low-temperature aqueous recrystallization and that regional thermal events may act to halt – not initiate – Pb loss from metamict domains in the Hadean Jack Hills zircons.

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“…If such aqueous processes are responsible for zero-age Pb loss in zircon, the absence of such Pb loss in monazite and sphene remains notable. While we may consider assigning this discrepancy to factors such as the remarkable resistance of monazite to radiation damage [85], even fully crystalline monazite and sphene appear susceptible to aqueous recrystallization [86,87]. Consequently, we consider the speculative possibility that partial resetting of zircon and baddeleyite during aqueous recrystallization may instead reflect the extreme incompatibility of Pb in the zircon (and baddeleyite) crystal lattice under natural conditions [88].…”

Section: Chemical Abrasion and U-pb Geochronologymentioning

confidence: 99%

“…If correct, such a relatively uniformitarian Hadean would appear plausibly consistent with independent evidence for subductiondriven flux melting since at least 3. 85 Ga [32]. Nonetheless, a large proportion of the Archean geological community would strongly dispute such views [33,34], and controversy regarding the nature and origin of Earth's earliest crust is likely to persist.…”

Section: Introductionmentioning

confidence: 99%

Stepwise chemical abrasion–isotope dilution–thermal ionization mass spectrometry with trace element analysis of microfractured Hadean zircon

Keller¹,

Boehnke²,

Schoene³

et al. 2019

Preprint

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The Hadean Jack Hills zircons represent the oldest known terrestrial material, providing a unique and truly direct record of Hadean Earth history. This zircon population has been extensively studied via high spatial resolution, high throughput in situ isotopic and elemental analysis techniques such as secondary ionization mass spectrometry (SIMS), but not by comparatively destructive, high-temporal-precision (< 0.05 % two-sigma) thermal ionization mass spectrometry (TIMS). In order to better understand the lead loss and alteration history of terrestrial Hadean zircons, we conduct stepwise chemical abrasion isotope dilution thermal ionization mass spectrometry with trace element analysis (CA-ID-TIMS-TEA) on manually microfractured Hadean Jack Hills zircon fragments previously dated by SIMS. We conducted three successive HF leaching steps on each individual zircon fragment, followed by column chromatography to isolate U-Pb and trace element fractions. Following isotopic and elemental analysis, the result is an independent age and trace element composition for each leachate of each zircon fragment. We observe ~50 Myr of age heterogeneity in concordant residues from a single zircon grain, along with a protracted history of post-Hadean Pb-loss with at least two modes circa ~0 and 2–4 Ga. Meanwhile, step-wise leachate trace element chemistry reveals enrichments of light rare earth elements, uranium, thorium, and radiogenic lead in early leached domains relative to the zircon residue. In addition to confirming the efficacy of the LREE-I alteration index and providing new insight into the mechanism of chemical abrasion, the interpretation and reconciliation of these results suggests that Pb-loss is largely driven by low-temperature aqueous recrystallization, and that regional thermal events may act to halt – not initiate – Pb-loss from metamict domains in the Hadean Jack Hills zircons.

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Why natural monazite never becomes amorphous: Experimental evidence for alpha self-healing

Cited by 62 publications

References 36 publications

Prolonged Movement on a > 10‐km‐Wide Thrust During Early Paleozoic Orogens in the Gondwana Margin of NW Argentina

Prolonged Movement on a > 10‐km‐Wide Thrust During Early Paleozoic Orogens in the Gondwana Margin of NW Argentina

Stepwise chemical abrasion–isotope dilution–thermal ionization mass spectrometry with trace element analysis of microfractured Hadean zircon

Stepwise chemical abrasion–isotope dilution–thermal ionization mass spectrometry with trace element analysis of microfractured Hadean zircon

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