A global picture on correlating primordial mantle signatures in Ocean Island basalts
Abstract
The existence of high 3He/4He ratios in plume related rocks, such as ocean island basalts (OIB) has been used as evidence for the presence of a primordial source reservoir in Earth's mantle. Additionally, short-lived radiogenic isotope systems, such as Hf-W (182Hf -> 182W, t½ = 8.9 Ma) further strengthen the argument for contributions to plumes from a primordial reservoir that must have formed within the first 60 Ma of solar system history.
We have conducted a global study of 182W/184W ratios in >70 OIB from 15 different hotspots that are characterized by 3He/4He ratios ranging from MORB-like (8 R/RA) to 41 R/RA (the measured 3He/4He ratio normalized to the atmospheric ratio). The samples show µ182W values (the deviation of a sample's 182W/184W from that of laboratory standards in parts per million) varying from 0 to as low as -23 (±4.5 ppm;2SD). The variations in W isotopic composition are not correlated with major or trace element concentrations, nor with long-lived radiogenic isotope systems. By contrast, 182W/184W ratios are negatively correlated with 3He/4He, where samples with "normal" µ182W values have MORB-like 3He/4He ratios, and samples with negative µ182W values are accompanied by 3He/4He ratios higher than 8 R/RA. While the data of most hotspots follow a He-W trend previously defined by OIB from Samoa and Hawaii, some are characterized by shallower (e.g., Iceland) or steeper (e.g., Galapagos) trends. The He-W trends with different slopes could suggest separate source reservoirs with variable 182W/184W and/or 3He/(U+Th) ratios, progressing towards different source compositions with time. Alternatively, distinctly different slopes of certain OIB trends could result from decoupled mixing of He and W, where the initial He-W composition of the primordial source is identical for all OIB sources. However, differences in He and W concentrations of other mantle plume components, such as ambient mantle or (ancient) recycled crust, may result in variable mixing trends. The global existence and correlation of primordial signatures in OIB either suggests the presence of a single large primordial domain that was/is tapped by several different mantle plume systems, or the preservation of various smaller reservoirs that formed in the earliest stages of Earth's history and survived mantle mixing processes to the present.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V31D0147M
- Keywords:
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- 1009 Geochemical modeling;
- GEOCHEMISTRYDE: 1025 Composition of the mantle;
- GEOCHEMISTRYDE: 1040 Radiogenic isotope geochemistry;
- GEOCHEMISTRYDE: 1041 Stable isotope geochemistry;
- GEOCHEMISTRY