Mantle Apatite Revisited: Major Reservoir for U and Th in the Mantle and Reflector of Mantle Fluid Sources

Suzanne Y. O'Reilly1, W. L. Griffin1,2, P. Morgan1,3, D. A. Ionov1 and M. D.Norman1: 1GEMOC National Key Centre, School of Earth Sciences, Macquarie University, Sydney, NSW 2109, Australia (sue.oreilly@mq.edu.au), 2CSIRO Div. of Exploration and Mining, Box 136, North Ryde, NSW 2113, Australia, 3Department of Geology, University of Northern Arizona, Flagstaff, Arizona, USA.

Laser-ablation microprobe ICPMS analyses of apatites from mantle-derived xenoliths from eastern Australia, France (Massif Central), Germany (Eifel) and Alaska have confirmed two distinct populations of apatites reflecting high-pressure precipitation from fluids of different origins. One compositional type (apatite A) occurs as veins of apatite+Cr-diopside and apatite+ amphibole± mica and as dispersed polygonal grains in microstructural equilibrium in metasomatised mantle wall-rock (Cr-diopside or Type I) lherzolites. Most of these are from the Bullenmerri/Gnotuk locality in western Victoria, but also include two examples from Alaska. The apatite is commonly associated with abundant fluid inclusions (dominantly CO2 with up to 10% H2O) that may be up to 10mm across and constitute up to 3% by volume of the mantle rocks (eg Andersen et al., 1984). Carbonates coat the walls of these fluid cavities as well as crystals of amphibole, pyroxene and carbonate that project into the cavities.

The other compositional type (apatite B) is associated with Al-augite (Type II) rock types and also may occur as veins (and re-equilibrated polygonal grains) in lherzolites that can be related to infiltration of parental type II magmatic fluids. The apatite-bearing Type II rocks represent fractions of magma frozen within the mantle and most of these appear to have MARID-type affinities (Dawson and Smith, 1977) although they occur in mantle with a Phanerozoic rather than Archean or Proterozoic tectonothermal age. Most of the Type II xenoliths analysed are from the Kiama locality, NSW Australia (eg Wass et al., 1979) and include amphibole-rich clinopyroxenites, cpx-apatite-spinel rocks and apatite-spinel rocks. All contain coexisting carbonate in textural equilibrium, and are interpreted as mantle crystallisation products of kimberlitic/carbonatitic magmas. Xenoliths from Massif Central and Eifel resemble the cpx-apatite-spinel rocks from Kiama.

Apatite A in mantle lherzolites contains significant structural CO2 and is a carbonate-bearing hydroxychlorapatite with relatively high Cl (1-3%) and Br (5-40 ppm), and low F (0-0.25 %): this appears to be a unique composition specific to this rock type. The Kiama and other B apatites are hydroxy-apatites with relatively low Br (1-3 ppm) and Cl (<0.01-0.6%) and moderate F (0.08-2%). Other significant differences for A and B apatites respectively include: La/NdCN 3 vs 2; Sr, 0.5-2.2% vs 0.2-0.8%; Ba, 100-400 ppm vs < 60 ppm (and not correlated with Sr); U, 30-140 vs 1-4; Th, 90-640 vs 4-12; Th/U, 3-5 (3.5 avge) vs 2.9; Pb, 2-70 vs <1; Sr/Y 42-67 vs 25-32.

Average abundances (where significant) of U ppm, Th ppm, K20 wt% in common mantle minerals from Bullenmerri/Gnotuk lherzolites are: apatite ~ 60, 200, 0%; amphibole, ~0.5, 1.5, 1%; mica, <0.5, <0.5, 10%; metasomatised cpx, ~0.2, 1, 0.01%. Apatite is thus the major reservoir for heat-producing elements in the mantle if present in abundances >0.05%. Calculations of heat production show that 0.1% apatite, 1% amphibole, and mica and 10% metasomatised cpx would account for .033, .004, .01 and .015 µW/m3 respectively, compared with a standard geophysical model mantle heat production of .015 µW/m3. For a lithosphere 100 km thick, a mantle with this mode would account for a heat flow of about 6 mW/m2 which is about 30% of the total heat flow normally attributed to the mantle (for conductive heat loss). This heat flow model is dominated by the 0.1% apatite; the 10% metasomatised cpx accounts for ~ 1.2 mW/m2.

Apatite may be overlooked in many mantle xenolith suites and may be ubiquitous globally in Phanerozoic mantle that has metasomatic imprints. In addition to its importance for the mantle heat budget, the presence of apatite in the lithospheric mantle can affect REE, Sr, Ba, U, Th and halogen contents of basaltic melts that traverse apatite-bearing metasomatised lithosphere, either by disequilibrium partitioning or by apatite melting. The distinct trace element signatures of different apatite types also reflects different source origins and nature of metasomatic fluids in the mantle.

References:

Andersen, T., O'Reilly, S.Y. and Griffin, W.L., 1984. The trapped fluid phase in upper mantle xenoliths from Victoria, Australia: implications for mantle metasomatism. Contrib. Miner. Petrol., 88, 72-85.

Dawson, J.B. and Smith, J.V., 1977. The MARID (mica-amphibole-rutile-ilmenite-diopside) suite of xenoliths in kimberlite. Geochim. Cosmochim. Acta, 41, 309-323.

Wass, S.Y., Henderson, P. and Elliott, C., 1980. Chemical heterogeneity and metasomatism in the upper mantle - evidence from rare earth and other elements in apatite-rich xenoliths in basaltic rocks from eastern Australia. Philosophical Transactions of the Royal Society of London, A297, 333-346.

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