The NOBLE project
The Origin, Accretion and Differentiation of Extreme Volatiles in Terrestrial Planets
Identifying the mechanisms by which the terrestrial planets acquired, retained and redistributed extreme volatiles and highly incompatible elements remains a fundamental challenge in the Earth and planetary sciences.
The halogens, Cl, Br and I, are highly incompatible, volatile and represent a powerful potential tracer of these processes. Although Cl is readily analysed, the concentrations of Br and I within most samples of interest are below the detection limit of conventional techniques and the halogens as a tracer set has been almost completely ignored.
Neutron irradiation of samples converts halogens to noble gas isotopes that can be measured by conventional or laser resonance mass spectrometry. Pioneered by Manchester, this innovation in analytical technique development now provides detection limits that far exceed any other approach, is independent of matrix effects, and links the halogen results to naturally occurring noble gases; a key tracer set that Manchester has a lot of experience in interpreting.
Halogen Phase Partitioning
These partial melts rise to the surface and crystallize as Mid-Ocean-Ridge Basalts (MORB).
The source for melts, which crystallizes at the surface as Ocean Island Basalts (OIB), is the deep mantle. Their origin are mantle plumes, which are characterized by upwelling solid diapiric pods at the 660-km boundary. Decompression of the solid material during upwelling leads to the generation of partial melts, which reach the surface in intra-plate hotspots like Hawaii. Consequently, MORBs and OIBs are crystallized partial melts of the earth mantle, which can easily be sampled, as they are present at the earth surface.
Halogens are, as large anions, excluded from most mineral structures, which means that they will preferably partition into the melt. So far, estimated halogen concentrations of the MORB-source mantle vary by orders of magnitude while those of Bromine and Iodine in the OIB-source mantle are non-existent (Pyle and Mather 2009; Aiuppa et al. 2009)
The green rectangle in Figure 2 (modified after Green and Ringwood 1967) shows the mineralogy of a typical mantle pyrolite, consisting of olivine, orthopyroxene (opx), clynopyroxene (cpx), plagioclase (Plg) and some minor phases (mP). The size of the blocks within the rectangle is proportional to the respective mineral ratio.
Partial melting of this mantle pyrolite will result in generation of magma (red), coexisting with olivine and orthopyroxene. This melt will rise to the earth surface and crystallize as a basalt, consisting of olivine, orthopyroxene, clynopyroxene, plagioclase and some minor phases.
We want to simulate experimentally partial melting of the earth mantle, which will give us the opportunity to determine the partitioning behaviour of halogens between a basaltic melt and olivine respectively orthopyroxene.
The red circle represents the pressure temperature range, which we plan to use for our experiments. A melt (L) is coexisting with olivine (Ol) and orthopyroxene (Opx).
Experiments are performed in a conventional Boyd and England Type Piston Cylinder Apparatus. Figure 4 shows a schematic sketch of the inner part.
The sample is located in the centre within a platinum capsule (length: 1 cm). Al2O3-powder and a Talc-Pyrex assembly serve as pressure medium. The pressure is generated by a cylindrical piston, which condenses the sample by pushing from the bottom. Heat is produced by a graphite furnace and controlled via a W-Re thermocouple. Pressures range between 10 – 25 kbar, temperatures between 1500 – 1720°C.
Figure 5 shows a BSE-image of an experiment that was performed at 10 kbar, first heated to 1720°C and slowly cooled with a rate of 10°C/min to 1500°C. After 5 h the sample was quenched and afterwards polished for further analysis.
Forsterite grains with a diameter of about 20 up to 150 µm are embedded in a melt with almost MORB-composition, giving us the opportunity to investigate halogen partitioning between forsterite (olivine) and melt at conditions relevant for mantle melting (10 kbar, 1500°C).
Determination of the halogen contents within crystals and melt will be performed by Microprobe, Ionprobe (SIMS) and for very low concentrations via neutron-irradiation coupled with conventional noble gas mass spectrometry (Ni-NGMS).
Halogens in Meteorites
Halogens in MORBs and OIBs
Because of their incompatibility, relatively high concentrations and distinct elemental compositions in surface reservoirs, the heavy halogens (Cl, Br, I) represent key tracers of volatile transport processes in the Earth. However, as pointed out recently by Aiuppa et al. 2009, their concentrations and distributions of halogens in the mantle remain deep, dark and mysterious. The difficulty to conduct high precision simultaneous data of I with Cl and Br on material of interest (e.g. basalts glass-samples) explains the scarcity of data in the literature (Schilling et al. 1980; Deruelle et al 1992; Jambon et al. 1995). For example in MORB samples, Cl, Br and I may be as low as 1ppm, 10 ppb and 1.0 ppb respectively.The Manchester Isotope Geochemistry and Cosmochemistry group has pioneered an innovative halogen analytical technique involving neutron irradiation of samples to convert halogens to noble gases. Neutron irradiation converts Cl, Br and I into isotopes of Ar, Kr and Xe, respectively (Johnson, 2000; Burgess, 2002). The production of irradiation-derived noble gas isotopes are orders of magnitude higher than their natural abundances and readily measured using conventional noble gas mass spectrometry (NI-NGMS). Current NI-NGMS detection limits are 1×10-12g for I and up to three times better for Cl. In one milligram of sample, detection limits are at 1ppb. These limits will be pushed using multi-collector mass spectrometers (Argus, Helix) offering a factor 10 improvement in precision and detection limits. This provides detection limits unmatched by any other technique.
For the first time, we are in a position to accurately determine the halogen content and variance in different mantle samples/reservoirs and because of the technique being used, link these to the noble gases. Our aim is now to conduct analyses on MORB and OIB glass-samples in order to reconstruct their respective mantle-source halogen compositions.