Contrasting noble gas compositions of peridotitic and eclogitic monocrystalline diamonds from the Argyle lamproite, Western Australia

Abstract

He-Ne-Ar compositions were determined in diamonds from the Argyle lamproite, Western Australia, to assess whether subducted material affects the noble gas budget and composition of stable old sub-continental lithospheric mantle (SCLM). Twenty diamonds (both peridotitic and eclogitic) were characterized for their carbon isotopic compositions and N abundance and aggregation from which 10 eclogitic growth zones and 5 peridotitic growth zones were analysed for their He-Ne-Ar compositions. The eclogitic diamonds have δ13C values of −4.7 to −16.6 ‰. indicating a subduction signature, whereas the peridotitic diamonds have mantle-like compositions of −4.0 to −7.8 ‰. Mantle residence temperatures based on N-in-diamond thermometry showed that the eclogitic diamonds were mainly formed at 1260-1270 C or above 1300 C near the base of the lithosphere, whereas the peridotitic diamonds generally formed at lower temperatures (mostly 1135-1230 C). A noble gas subduction signature is present to various extents in the eclogitic diamonds and is inferred from a hyperbolic mixing relationship between R/Ra and 4He and δ13C values concentrations with a predominance of low R/Ra values (<0.5; R/Ra = 3He/4Hesample/3He/4Heair). In addition, low 40Ar/4He and 40Ar/36Ar ratios, high nucleogenic 21Ne/4He and low 3He/22Ne ratios are characteristic of subducted material and were found in the eclogitic diamonds. The peridotitic diamonds show generally higher R/Ra values (median 1.1 +- 1.1) and lower 4He/40Ar ratios compared to eclogitic diamonds (median 0.1 +- 0.8 R/Ra; with 7/10 samples having an average of 0.13 +- 0.14 R/Ra). The studied peridotitic diamond growth zones showed a negative correlation between R/Ra and 4He concentrations over 2 orders of magnitude and limited variation in 3He, that can be largely explained by radiogenic 4He ingrowth. At low 4He concentrations the R/Ra value is around 2.8 for both paragenesis of diamonds and is significantly lower than present-day SCLM values, suggesting (1) a more radiogenic helium isotope composition beneath the Halls Creek Orogen than those for typical SCLM from other cratons and/or (2) that the peridotitic diamonds are formed from fluids that also had a subduction input. The high mantle residence temperature and low R/Ra value in the core and low temperature and higher R/Ra value in the rim of a single peridotitic diamond indicate multiple growth events and that part of the lherzolitic diamond population may be genetically related to the eclogitic diamonds. Combining the diamond mantle residence temperatures with noble gas compositions shows that noble gas subduction signatures are present at the base of the lithosphere below 180 km depth beneath Argyle and that fluid migration and interaction with the SCLM occurred over scales of at least 15 km, between 180 and 165 km depth.