Abstract:
The now-extinct
short-lived 182Hf-182W radiometric isotope system with a
half life of 8.9 Myrs has been widely used to provide significant
geochronological constraints on planetary processes, such as core-mantle
segregation and early crust-mantle differentiation [1]. Evolving from a chondriticreservoir
(u182W = ~-190 calculated as the deviation of 182W/184W
from modern Earth’s mantle multiplied by 1 million), the modern Earth was long
believed to consist of two components –a bulk silicate earth with a homogeneous 182W isotopic composition(u182W = 0) and by inference a metalliccore with u182W = ~-220. Recently, high-precision (<±5 ppm 2σ SD on 182W/184W) measurements of
W isotopic ratios [2-5] have been able to discriminate subtle 182W
isotopic anomalies in terrestrial rocks, triggeringconsiderable exploration of
early Earth processes and their influence on modern Earth. In theory, the older
rock samples and the deeper mantle-derived rocks are more likely to preserve
the information of early-formed 182W isotopic variation within the silicate
Earth.
Previous studies
have revealed that Hadean to Eoarchean supracrustal rocks are often
characterized by positive u182W ranging from +10 to +15 [2, 6-15], while negative values
are also reported [9]. The positive 182W anomalies could reflect the
heritage of a pre-late accretionary mantle [2,8,13,14] or an early depleted,high
Hf/W parental mantle reservoir (while 182Hf was still extant) [6,7,10].
The negative values could record the influence of the mantle contaminated by
more proportional meteoritic materials during localized
late accretion or an early, enriched low Hf/W parentalmantle reservoir [9].
Given ~40% of the W budget of the silicate Earth resides in the continental
crust, the accessible continental crust over time can provide the long-term
evolution of W isotopic compositionfor the uppermost portion of the convecting
upper mantle where the crust was derived. Archean upper continental crust
sampled by diamictites is characterized by generally negative u182W of -13 that progressively diminished to zero in diamictites
through the Paleoproterozoic to Phanerozoic [16], most likely reflecting the
efficient homogenization of W isotopic composition in the upper mantle by plate
tectonics and crustal growth/rejuvenation. Provided by the W mobility in the
crust-mantle reservoirs [7,17], another way to inspect the W isotopic composition
is to analyze tungsten ore deposits (scheelite or wolframite) formed in ancient
to modern continents. Recently, Mundl et al. [18] reported negative 182W
anomalies for the deep plume-derived basalts, the deep sources of which may not
be well homogenized by subduction or it could reflect the primordial signature
in the deep mantle or the interaction between the lowermost mantle and metalliccore.
Surprisingly, Rizo et al. [19] found substantially largeru182W values from +24 to +48 for Phanerozoic flood basalts from
the Ontong Javal Plateau (120Ma) and the Baffin Bay (60Ma). However, Kruijer
and Kleine [20] proposed that the 182W excesses for the OJP sample reported
by Rizo et al. [19] and perhaps also the Baffin Bay samples were caused by the effect
of nuclear field shift leading tothe deficit of 183W, which is used
in the NTIMS analyses via a double normalization. Another critical issue is
that significant mass dependent fractionation of W isotopes (e.g., δ182W/184W up to >0.2‰) can occur in the natural samples [21]. Whether such
large amounts of mass dependent fractionation of W isotopes in the rocks can be
well corrected deserves further investigation before concluding subtle
variations of the W isotopic ratios on the terrestrial rocks. Collectively, the
interpretation of the highly precise and accurate 182W isotopic
anomalies of the terrestrial rocks should be assisted by data of 146Sm-142Nd, 190Pt-187Re-186,187Os, and highly siderophile
element abundances. In addition, how the mobility of W in the mantle affects the
magnitute of W anomaly demands detailed work coupled with elements with similar
incompatibilities.
Acknowledgements:This work has been supported by the National Natural
Science Foundationof China (No. 41822301) and China “1000 Youth Talents
Program”.
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