Peridotite xenoliths from the Polynesian Austral and Samoa hotspots: Implications for the destruction of ancient ¹⁸⁷Os and ¹⁴²Nd isotopic domains and the preservation of Hadean ¹²⁹Xe in the modern convecting mantle

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• 作者：M.G. Jackson^a ; ^{jackson@geol.ucsb.edu" class="auth_mail" title="E-mail the corresponding author} ; S.B. Shirey^b ; E.H. Hauri^b ; M.D. Kurz^c ; H. Rizo^d
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：15 July 2016
• 年：2016
• 卷：185
• 期：Complete
• 页码：21-43
• 全文大小：1414 K

文摘

The Re–Os systematics in 13 peridotite xenoliths hosted in young (<0.39 myr) rejuvenated lavas from the Samoan island of Savai’i and 8 peridotite xenoliths from 6 to 10 myr old lavas from the Austral island of Tubuai have been examined to evaluate the history of the oceanic mantle in this region. Modal mineralogy, trace element compositions and ¹⁸⁷Os/¹⁸⁸Os ratios suggest that these peridotites are not cognate or residual to mantle plumes but rather samples of Pacific oceanic lithosphere created at the ridge. Savai’i and Tubuai islands lie along a flow line in the Pacific plate, and provide two snapshots (separated by over 40 Ma in time) of Pacific mantle that originated in the same region of the East Pacific rise. Tubuai xenoliths exhibit ¹⁸⁷Os/¹⁸⁸Os from 0.1163 to 0.1304, and Savai’i (Samoa) xenoliths span a smaller range from 0.1173 to 0.1284. The ¹⁸⁷Os/¹⁸⁸Os ratios measured in Tubuai xenoliths are lower than (and show no overlap with) basalts from Tubuai. The ¹⁸⁷Os/¹⁸⁸Os of the Savai’i xenoliths overlap the isotopic compositions of lavas from the island of Savai’i, but also extend to lower ¹⁸⁷Os/¹⁸⁸Os than the lavas. ³He/⁴He measurements of a subset of the xenoliths range from 2.5 to 6.4 Ra for Tubuai and 10.8 to 12.4 Ra for Savai’i.

Like abyssal peridotites and xenoliths from oceanic hotspots that sample the convecting mantle, Os isotopes from the Savai’i and Tubuai xenolith suites are relatively unradiogenic, but do not preserve a record of depleted early-formed (Hadean and Archean) mantle domains expected from earlier cycles of ridge-related depletion, continent extraction, or subcontinental lithospheric mantle erosion. The lack of preservation of early-formed, geochemically-depleted Os-isotopic and ¹⁴²Nd/¹⁴⁴Nd domains in the modern convecting mantle contrasts with the preservation of early-formed (early-Hadean) ¹²⁹Xe/¹³⁰Xe isotopic heterogeneities in the convecting mantle. This can be explained if the initial isotopic signatures in Re–Os and Sm–Nd systems are erased by recycling because the parent and daughter elements are retained in subducting slabs and more efficiently returned to the mantle during subduction than Xe. In this way, early-formed Os and Nd-isotopic heterogeneities could have been overprinted with, and diluted by, younger isotopic signatures. In contrast, preservation of early-formed heterogeneities in the modern convecting mantle is possible for other elements, such as Xe, that are not as efficiently recycled back into the mantle, owing to greater fluid mobility that concentrates such elements in the near-surface. Differing recycling efficiencies for Os, Nd and Xe lead to wide differences in the preservation of Hadean isotopic signatures of these elements in the modern convecting mantle. In general, incompatible elements that are fluid mobile (e.g., Xe) concentrate in surface reservoirs and are more likely to preserve Hadean geochemical signatures in the convecting mantle than compatible elements (e.g., Os) and fluid immobile incompatible elements (e.g., Nd).