U-Th/He systematics of fluid-rich ‘fibrous’ diamonds – Evidence for pre- and syn-kimberlite eruption ages

Timmerman, Suzette; Yeow, Hanling; Honda, Masahiko; Howell, D.; Jaques, A. L.; Krebs, Mandy Y.; Woodland, Sarah; Pearson, D. Graham; Ávila, Janaína N.; Ireland, Trevor

doi:10.1016/j.chemgeo.2019.04.001

Cited by 15 publications

(24 citation statements)

References 88 publications

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“…The highly aggregated nitrogen in diamondites is consistent with long mantle residence times (Mikhail et al, 2019), which implies ages far greater than 91 Ma. This leaves open the likelihood that 4 He produced by alpha decay of U and Th in the fluid, or recoil of 4 He into the fluids, has decreased the 3 He/ 4 He (Timmermann et al, 2019a). A strong relationship between 3 He/ 4 He and 3 He concentration is apparent from Figure 2a and reflects radiogenic 4 He production since the fluids were trapped in the diamond.…”

Section: Helium Isotopesmentioning

confidence: 99%

“…Diamonds can trap fluid during their growth, either along the diamond fibres, between interlocking polycrystalline grains, and surrounding diamond-hosted mineral inclusions (Navon et al, 1988;Jacob et al, 2014). As well as providing the only direct samples of metasomatic fluids from the mantle (Weiss et al, 2015), the trapped fluids enable the application of noble gas isotope tracers to resolve the origin of diamond-forming fluids (Burgess et al, 1998;Gautheron et al, 2005;Broadley et al, 2018;Timmermann et al, 2018Timmermann et al, , 2019a). Here we combine C-N isotope and nitrogen abundance data from southern African diamondites with He isotope data from micro-inclusions in the same sample to investigate the origin of carbonaceous fluids in the mantle.…”

Section: Introductionmentioning

confidence: 99%

“…The mixing lines plotted inFigure 2bare hyperbolic as [ 4 He] mantle /[ 4 He] crust is assumed to be 10. Comparative data are fromBurgess et al (1998), Gautheron et al (2005 andTimmermann et al (2018Timmermann et al ( , 2019a.Letter Geochem. Persp.…”

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confidence: 99%

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A secretive mechanical exchange between mantle and crustal volatiles revealed by helium isotopes in 13C-depleted diamonds

Mikhail¹,

Crosby²,

Stuart³

et al. 2019

Geochem. Persp. Let.

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Fluid inclusions trapped in fast-growing diamonds provide a unique opportunity to examine the origin of diamonds, and the conditions under which they formed. Eclogitic to websteritic diamondites from southern Africa show 13 C-depletion and 15 N-enrichment relative to mantle values (δ 13 C =-4.3 to-22.2 ‰ and δ 15 N =-4.9 to +23.2 ‰). In contrast the 3 He/ 4 He of the trapped fluids have a strong mantle signature, one sample has the highest value so far recorded for African diamonds (8.5 ± 0.4 R a). We find no evidence for deep mantle He in these diamondites, or indeed in any diamonds from southern Africa. A correlation between 3 He/ 4 He ratios and 3 He concentration suggests that the low 3 He/ 4 He are largely the result of ingrowth of radiogenic 4 He in the trapped fluids since diamond formation. The He-C-N isotope systematics can be best described by mixing between fluid released from subducted altered oceanic crust and mantle volatiles. The high 3 He/ 4 He of low δ 13 C diamondites reflects the high 3 He concentration in the mantle fluids relative to the slab-derived fluids. The presence of post-crystallisation 4 He in the fluids means that all 3 He/ 4 He are minima, which in turn implies that the slab-derived carbon has a sedimentary organic origin. In short, although carbon and nitrogen stable isotope data show strong evidence for crustal sources for diamond-formation, helium isotopes reveal an unambiguous mantle component hidden within a strongly 13 C-depleted system.

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Section: Helium Isotopesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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A secretive mechanical exchange between mantle and crustal volatiles revealed by helium isotopes in 13C-depleted diamonds

Mikhail¹,

Crosby²,

Stuart³

et al. 2019

Geochem. Persp. Let.

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“…Recently, a different approach has been suggested for constraining the timing of HDF-bearing diamond formation and C-O-H metasomatism by combining He isotope analyses with U-Th-He abundance measurements 40,41 . Like (U-Th)/He chronometry of other minerals 42,43 , this approach is based on the accumulation of 4 He atoms produced by α-decay of U and Th, while at the same time it considers all possible sources contributing to the budget of He in a diamond.…”

mentioning

confidence: 99%

“…For example, it accounts for the presence of significant initial 4 He in many C-O-H fluid-bearing diamonds. This concept has been investigated in a study of alluvial HDF-bearing diamonds 41 , which showed a positive correlation between 4 He and U-Th concentrations, reflecting the low diffusivity of He in diamond at mantle temperatures [44][45][46][47] . However, that study did not take into account the possibility of some diffusive loss of He, which is critical for a meaningful interpretation of (U-Th)/He data and for constraining crystallization ages 43 .…”

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confidence: 99%

Helium in diamonds unravels over a billion years of craton metasomatism

et al. 2021

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Chemical events involving deep carbon- and water-rich fluids impact the continental lithosphere over its history. Diamonds are a by-product of such episodic fluid infiltrations, and entrapment of these fluids as microinclusions in lithospheric diamonds provide unique opportunities to investigate their nature. However, until now, direct constraints on the timing of such events have not been available. Here we report three alteration events in the southwest Kaapvaal lithosphere using U-Th-He geochronology of fluid-bearing diamonds, and constrain the upper limit of He diffusivity (to D ≈ 1.8 × 10−19 cm2 s−1), thus providing a means to directly place both upper and lower age limits on these alteration episodes. The youngest, during the Cretaceous, involved highly saline fluids, indicating a relationship with late-Mesozoic kimberlite eruptions. Remnants of two preceding events, by a Paleozoic silicic fluid and a Proterozoic carbonatitic fluid, are also encapsulated in Kaapvaal diamonds and are likely coeval with major surface tectonic events (e.g. the Damara and Namaqua–Natal orogenies).

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Diamond-forming media through time – Trace element and noble gas systematics of diamonds formed over 3 billion years of Earth’s history

Timmerman

Krebs

Pearson

et al. 2019

Geochimica et Cosmochimica Acta

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U-Th/He systematics of fluid-rich ‘fibrous’ diamonds – Evidence for pre- and syn-kimberlite eruption ages

Cited by 15 publications

References 88 publications

A secretive mechanical exchange between mantle and crustal volatiles revealed by helium isotopes in 13C-depleted diamonds

A secretive mechanical exchange between mantle and crustal volatiles revealed by helium isotopes in 13C-depleted diamonds

Helium in diamonds unravels over a billion years of craton metasomatism

Diamond-forming media through time – Trace element and noble gas systematics of diamonds formed over 3 billion years of Earth’s history

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