Accretionary Origin of Volatiles
Understanding how the terrestrial planets obtained their volatiles (e.g., N, C, O, the noble gases and water) is central to understanding why some planets (Earth) have the ability to support life, while others (Mars, Venus, to our current knowledge) do not. The noble gases (He, Ne, Ar, Kr, and Xe) are geochemically unique – as inert gases they act as valuable tracers of planetary processing. The halogens (Cl, Br and I), while not ‘true’ volatiles in the sense of N, C, and O, behave highly incompatibly during melting and are hydrophylic making them valuable tracers of geochemical processing.
The halogens, particularly Br and I, are present in ultra-low abundance in most terrestrial and extraterrestrial materials making them difficult to measure by most conventional electron or ion microprobe techniques. At the University of Manchester, we have the ability to measure these elements through Neutron-Irradiation Noble Gas Mass Spectrometry, or NI-NGMS. This technique utilizes the noble gases as proxies for the halogens through neutron-irradiation of samples. In this way, we use both the noble gases and the halogens as tracers of geochemical processing in terrestrial and extraterrestrial materials to decipher how the Earth and all other bodies in the solar system obtained, retained and distributed its volatile elements through time.
Through investigating the halogen compositions of the precursors to planets, i.e., materials that accreted to form the planets, best preserved in primitive chondritic meteorites, we can better understand the evolution of volatile budget on our own planet. Additionally, investigating other bodies, such as the Moon and Mars and differentiated meteorites, will provide insight into the halogen response to planetary processes, such as melting, differentiation and crust formation. Currently we are pursuing several threads of research into extraterrestrial halogen reservoirs that, in conjunction with the “Halogens in MORBS and OIBs” and “Halogen Phase Partitioning” projects will help us to construct a clearer picture of the origin, distribution and overall behavior of halogens and, hence, volatiles in the Earth and other planetary bodies through time.
Volatile Recycling and Evolution
Noble gases have played a central role in developing conceptual models describing the Earth’s mantle convection. While early models favoured a mantle that was convectively layered at the 670 km phase change (e.g. Allègre et al. 1983), seismic imaging and models (e.g. Van der Hilst 1997; Van Keken and Ballentine 1999) show that layered mantle convection is unlikely.Part of this discrepancy lies in the fact that the source of primitive volatiles in the mantle and how they have been preserved in the mantle are yet to be satisfactorily resolved. An equally important question is whether the volatiles in the form of noble gases, halogens and water can survive subduction and whether they can be carried into the deep mantle to significantly influence the volatile concentration and therefore the physical and isotopic character of both the convecting mantle and ocean island systems.
Recently, Ballentine and Holland (2006) suggested for the first time that the heavy noble gases signature of the mantle was seawater-derived, implying thus a recycling of these gases back into the mantle during subduction. To better understand and confirm this process, further investigation using other tracers and investigation of a variety of systems needs to be carried out.Halogens (Cl, Br, I) provide a powerful tracer of seawater and marine sedimentary involvement with distinct I/Cl and Br/Cl ratios between the mantle, seawater and marine sediments. However, due to analytical limitations most previous studies have been restricted to studying Cl.
At the University of Manchester, the mass spectrometers of our noble gas geochemistry group (see facilities) allow the measurements of noble gas at a very high precision, for both the fluid phases (using crushing devices) and the solid phases (using heating devices). Halogens can also be obtained by analyzing the noble gas released from irradiated samples.
The first work combining halogens and noble gases in mantle’s samples has been done in our noble gas geochemistry group and found a seawater-derived signature in exhumed high pressure peridotite (Sumino et al. 2010), in support of a recycling of these elements into the mantle (Holland and Ballentine 2006).To pursue this research, the current projects involve the measurements of noble gases and halogens in other key samples to find potential seawater components, in order to better understand the process of volatile recycling:
- Mid-Ocean Ridge Basaltic (MORB) glasses (Dr Lorraine Ruzie, also see NOBLE project)
- The altered oceanic crust i.e. altered Mid-Ocean Ridge Basalts (MORB), gabbros, sediments, coming from ODP/IODP sites (Dr Deborah Chavrit)
- Exhumed high pressure ultramafic rocks and oceanic crust (Dr Deborah Chavrit)
- Back-arc basin samples (Dr Deborah Chavrit)
- Ocean Island Basalt (OIB) samples;
– As a global approach, e.g. from Tristan Da Cunha island, which contain a component of ancient recycled subducted oceanic crust, Canaries and Azores (Lisa Abbott)
– As a local and temporal approach, e.g. the Hawaiian Emperor Chain basalts (Michael Broadley), Iceland hot spot (Bridget Weston)
Other parallel studies are in process, to better understand the mechanisms of how the fluids might be subducted into the mantle itself. This is being studied by looking at the interaction between fluids and ultra-high pressure rocks hosted in exhumed continental crust (Alexandra Quas-Cohen).
The Mantle Today
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