Juvenile helium in ancient rocks: II. U-He, K-Ar, Sm-Nd, and Rb-Sr systematics in the Monche Pluton. [formula omitted] ratios frozen in uranium-free ultramafic rocks

Abstract

The important geodynamic parameter, the 3He 4He ratio in rocks and fluids of the continental crust, is generally decreasing from the mantle values (≅10 −5) to the radiogenic ratio (≅10 −8) on the time scale of about 1 Ga or less. However, the ratios, observed in some ancient rocks and minerals, are much higher than the radiogenic value due to a preferential retention of trapped He, when compared with radiogenic helium and / or a low U 3He ratio in a sample. The distribution of He, Ar, Nd, and Sr isotopes, K, Rb, Sm, and U in ultrabasic rocks, in rock-forming minerals, in ores from the 2.49 Ga Monche Pluton, and in basic rocks of the Main Range (the Kola Peninsula) enables us to distinguish sources of the rocks and trapped fluids and outline some peculiarities of petrogenetic and fluid processes. The initial values of ϵ Nd ( T) = −0.9 ± 0.5, 87Sr 86Sr (T) = 0.7021 ± 0.0002 , for the 2.49 Ga Monche Pluton are rather similar to these for other layered intrusions of the Baltic Shield. They differ considerably from the model values for the depleted 2.5 Ga old mantle. Probably the melts originated in the specific enriched subcontinental mantle. [ 4He] concentrations of ( 10–480)∗ 10 −6 cm 3/ g ) and [U] concentrations of 0.01–0.001 ppm in rocks of the Pluton are translated into atomic ratio of 4He U up to ≅500 and U-He “age” up to ≅10 Ga (!). These rocks contain dominantly trapped He. Despite the fact that [U] in gabbros from the Main Range is about two orders of magnitude higher, this conclusion appears to be valid because of better retention of trapped He as compared with radiogenic one. The 3He 4He ratios, (0.16–0.74)∗ 10 −6, exceed considerably the radiogenic value but are much lower than those in the mantle due to a contribution of crustal He. Isotopic and geochemical evidence implies that a melt from which the ultrabasic rocks were formed originated from the mantle; thus crust-mantle fluid occurred in the mantle melt. Geological observations and the relationship between 3He 4He and Mg (Fe + Mg) values show that the high-magnesium divines formed during melt ascent i.e., under higher temperatures and, probably, at slightly greater depth as compared with those parameters for the magma chamber. The observed and rather different sources of 3 He and 4He as well as the con siderable constancy of their ratio in different minerals, separated from both the ultramafic rocks and gabbros, implies: 1. (1) an intensive process of mixing between mantle and crustal components: a melt convection in the chamber may have occurred; 2. (2) the two types of rocks could originate by crystallization differentiation of one and the same melt. Moreover, similar ratios of 3He 4He ≅ 0.2 ∗ 10 −6 are observed in the enclosing gneisses of the Kola-Belomorian series. Practically all 3He and 4He are concentrated in secondary amphiboles; hence the fluid which stimulated the metamorphic process was probably released from the ultramafite-bearing melt. The source of trapped 4He and 40Ar ∗ (corrected for air) appears to be the continental crust, but their ratio varies over a wide range, in contrast to approximately constant 3He 4He ratio. Degassing of the melt is probably responsible for extremely high 4He Ar ∗ values in some minerals.