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[SGC38-P02] The origin of negative 182W anomalies in geochemically-depleted kimberlites
Keywords:kimberlite, 182W, ocean island basalt, core-mantle interaction
Kimberlites are volcanic rocks enriched in volatile species, especially H2O and CO2. They have peculiar high–field strength element fractionations that have also been attributed to deep mantle sources [1]. Recent studies of compiled 143Nd-176Hf isotope data for kimberlites globally suggest that kimberlites are derived from an ancient component in the lowermost mantle, where it could have been shielded from mantle convection throughout most of Earth evolution [2, 3]. This ancient component features slightly depleted incompatible trace elements based on suprachondritic Nd-Hf isotope compositions. The inferred component is indistinguishable from the Prevalent Mantle (PREMA) component of modern ocean island basalts (OIBs; [4]). Further, Nakanishi et al. [5] found small negative μ182W anomalies (where μ182W is the part per million deviation in 182W/184W relative to laboratory standards presumed to be representative of the bulk silicate Earth) in a small suite of kimberlites that formed over a wide range of ages. That study concluded that the negative μ182W values could reflect any of the processes set forth by prior studies to explain negative anomalies in mantle derived rocks including some form of core-mantle interaction, long-term preservation of an early magma ocean fractionate, or long-term preservation of a mantle reservoir characterized by excess late accreted material.
This study seeks to re-evaluate the origin of the ancient reservoir contributing material with negative μ182W to the mantle sources of some kimberlites by further constraining the W isotope composition of the anomalous component. Because Nd-Hf isotopic compositions are affected by crustal and lithospheric mantle contamination, which would also strongly attenuate anomalous μ182W values, kimberlite samples with the highest initial 143Nd/144Nd and 176Hf/177Hf ratios (i.e. those more closely resembling PREMA) may provide the most accurate constraint on the 182W composition of the reservoir. We report new 182W data for 18 kimberlites from seven regions (southern Africa, Canada, Western Australia, India, Brazil, Greenland, and Siberia), with eruption ages ranging from 2060 to 52 Ma and variable Nd-Hf isotope compositions ranging from supra- to sub-chondritic.
Kimberlites analyzed in this study are characterized by negative to near zero μ182W values, ranging from −8.0 to +3.0. Combined with data from Nakanishi et al. [5], we found that the kimberlite samples converge to an anomalous endmember composition with a μ182W value of ~ −8 at the highest initial 143Nd/144Nd and 176Hf/177Hf ratios. These results suggest that the kimberlite mantle source reservoir experienced a partial melting event resulting in formation of a residue with supra-chondritic Sm/Nd and Lu/Hf ratios such as may have resulted from the processes related to a basal magma ocean crystallization. This mantle domain would then have been infused with core-derived W, most likely by isotopic exchange, resulting in a negative 182W value. A similar process has also been suggested for some modern OIBs, supporting an interpretation that some kimberlites and modern OIBs contain contributions from the same mantle component most likely located at the core-mantle boundary.
[1] Pearson et al. (2019) Elements 15, 387–392. [2] Woodhead et al. (2019) Nature 573, 578–581.
[3] Giuliani et al. (2021) PNAS 118, e2015211118. [4] Zindler et al. (1986) Annu Rev Earth Planet Sci 14, 493–571. [5] Nakanishi et al. (2021) PNAS 118, e2020680118.
This study seeks to re-evaluate the origin of the ancient reservoir contributing material with negative μ182W to the mantle sources of some kimberlites by further constraining the W isotope composition of the anomalous component. Because Nd-Hf isotopic compositions are affected by crustal and lithospheric mantle contamination, which would also strongly attenuate anomalous μ182W values, kimberlite samples with the highest initial 143Nd/144Nd and 176Hf/177Hf ratios (i.e. those more closely resembling PREMA) may provide the most accurate constraint on the 182W composition of the reservoir. We report new 182W data for 18 kimberlites from seven regions (southern Africa, Canada, Western Australia, India, Brazil, Greenland, and Siberia), with eruption ages ranging from 2060 to 52 Ma and variable Nd-Hf isotope compositions ranging from supra- to sub-chondritic.
Kimberlites analyzed in this study are characterized by negative to near zero μ182W values, ranging from −8.0 to +3.0. Combined with data from Nakanishi et al. [5], we found that the kimberlite samples converge to an anomalous endmember composition with a μ182W value of ~ −8 at the highest initial 143Nd/144Nd and 176Hf/177Hf ratios. These results suggest that the kimberlite mantle source reservoir experienced a partial melting event resulting in formation of a residue with supra-chondritic Sm/Nd and Lu/Hf ratios such as may have resulted from the processes related to a basal magma ocean crystallization. This mantle domain would then have been infused with core-derived W, most likely by isotopic exchange, resulting in a negative 182W value. A similar process has also been suggested for some modern OIBs, supporting an interpretation that some kimberlites and modern OIBs contain contributions from the same mantle component most likely located at the core-mantle boundary.
[1] Pearson et al. (2019) Elements 15, 387–392. [2] Woodhead et al. (2019) Nature 573, 578–581.
[3] Giuliani et al. (2021) PNAS 118, e2015211118. [4] Zindler et al. (1986) Annu Rev Earth Planet Sci 14, 493–571. [5] Nakanishi et al. (2021) PNAS 118, e2020680118.