Noble gas state of the Precambrian mantle
Final Report Abstract
Within this project old rocks of mantle provenance were investigated for their noble gas inventory. The aim was searching for isotopic signatures of the Precambrian mantle and their potential change in time. For this purpose, samples differing in age from two Indian carbonatite complexes and Archean peridotites from SW Greenland were investigated. Because of difficulties in collecting the carbonatite samples due to the onset of the Corona pandemic only a reduced sample suite was available for this study. The petrology of both carbonatite suites was established by optical and electron microscopy. Three magnesio-carbonatite samples from Newania (age ca. 2 Ga) reveal the highly unusual mineral association of dolomite and magnesite with minor primary siderite that so far was not investigated in much detail before. The applicant determined REE-patterns on single minerals and the oxygen and carbon isotope composition by SIMS techniques, and could prove a clear mantle provenance of the Newania carbonatite. The occurrence and petrology of magnesite is in favor of a carbonatite magma that formed in more than 130 km depth by partial melting of a carbonated peridotite. Due to the fine-grained nature of these rocks time-consuming separation of minerals for noble gas analyses was post-poned and couldn’t be performed within the project term anymore. The ca. 800 Ma old Sevattur cabonatite consists of both dolomite and calcite carbonatite, and in this sample suite magmatic fractionation processes are visible. Neon isotopes in the two analyzed carbonate separates (dol, cc) clearly point to the presence of a (local) crustal fluid incorporated in the carbonatite. No mantle noble gases were detected so far. Finally, metabasite whole rock splits and mineral separates from the oldest known mantle peridotite in SW Greenland (3.8 Ga) were investigated for its noble gas compositions. All samples indicate high contributions of radiogenic isotopes, in particular 4He*, of either crustal origin or in situ production from K, U and Th parent nuclides. In spite of this imprint two separates also showed a clear mantle signal in its neon isotope composition with maximum 20Ne/22Ne-ratios up to 10.98±0.25 and 21 10.88±0.08 (1σ). The lowest air-corrected Ne/22Ne-ratio (at 20 Ne/22Ne=12.5) is 0.08. As one still expects contribution of a nucleogenic 21Ne* component by co-production with 4He*, this value is too high and the real air-corrected 21Ne/22Ne-ratio is very likely lower than for the modern convecting mantle (=0.06). This is predicted for an ancient upper mantle domain that should have suffered a less severe extent of degassing compared with its modern counterpart. No mantle 3He could be detected, which precludes application of time-evolution models of 3He/4He in the mantle and calculation of mantle degassing rates in course of Earth’s history.
