Low mantle plume component in 370 Ma old Kola ultrabasic-alkaline-carbonatite complexes...
I. N. Tolstikhin et al.

4. Results

4.1. Helium

4.1.1. Abundances of He isotopes in whole-rock samples.

fig10 fig11 Whole-rock (mineral) concentrations of helium isotopes and 4He/ 3He ratios vary within a great range sim104 (Table 2, Figure 11). Generally ultrabasic rocks show somewhat higher concentrations of 3He and lower 4He/ 3He ratios; carbonatites contain widely variable abundances of He isotopes; intermediate concentrations and ratios are typical of alkaline rocks. The lowest abundances of 3He, down to 5times10-13cm3 STP g-1, and 4He, down to 2times10-7 cm3 STP g-1, are typical of carbonatites from the Khibiny and some rocks from dyke Complexes. Some rocks from these Complexes also show quite high 4He/3He ratios, similar to those in radiogenic crustal helium, sim108 [Mamyrin and Tolstikhin, 1984].

In contrast, ultrabasic rocks, carbonatites and related minerals from the Seblyavr and the Kovdor show high concentrations of 3He, from 5times10-9 to 5times10-10 cm3 STP g-1, along with the low 4He/3He ratios. Rocks and minerals from the Seblyavr appear to retain He isotopes better than any other and contain helium with 4He/3He ~1000 times below the radiogenic ratio. Two olivinites, from the Seblyavr and the Lesnaya Varaka (Table 2) contain He with as low 4He/3He as 6.4times104 and 6.8times104, respectively, which are substantially lower than the mean value in mid-oceanic ridge basalts, (8.9 pm 0.9)times104, implying a contribution of plume-related materials to Kola UACC. However, nuclear reactions could also produce high abundances of 3He and low 4He/3He ratios in certain environments, e.g., in Li-bearing rocks and minerals, and to identify sources of helium the measured concentrations should be compared with those expected for radiogenic in-situ produced helium.

4.1.2. Helium isotope inventories: Measured and expected from in-situ production.

The concentrations of radiogenic 4He* are calculated from measured U and Th concentrations (Table 2), and the age of the massifs, 370 pm 10 Ma [Kramm et al., 1993; Kramm and Kogarko, 1994, see Section 2).

Estimates of the in-situ produced 3He* appears to be a more complicated and less reliable than 4He*. The relevant nuclear reaction is 6Li(nt, a ) 3H rightarrow b- rightarrow 3He* with l3H) = 12.2 yr where nt defines thermal neutron and a is a -particle. The production of 3He* per one 4He* atom depends on abundance of major and some trace elements in a rock, peculiarities of U and Th distribution and Th/U ratio. Among trace elements Li is the most important but also Gd, Be, B could influence the flux of thermal neutrons.

Morrison and Pine [1955] were first who invented method for estimation 3He*/ 4He* production ratio. Gorshkov et al. [1966] illustrated a good agreement, within 20%, between measured and calculated thermal neutron fluxes in natural rocks. Gerling et al. [1976] and Mamyrin and Tolstikhin [1984] presented measured and calculated 3He/4He ratios for different rocks and inferred that the measured ratios are mainly controlled by Li and U+Th distribution among minerals and He losses from these minerals. Therefore a direct comparison of measured and calculated ratios can not be used to estimate the accuracy of calculations. Tolstikhin et al. [1996] illustrated a similarity (also within 20 %) between the calculated production ratio of 3He*/ 4He*=7.2times10-8 for Permian shists (the Molasses basin, Northern Switzerland) and the measured ratio, 9.4times10-8, in adjacent aquifer having stagnant waters with quite high helium concentrations, 4.5times10-3 cm3 STP per g H2O. From this brief review we consider that the accuracy of calculated 3He* concentrations are most probably within 50%.

fig12 Figure 12 comprises measured (m) over calculated (c) abundances of helium isotopes in ~40 rocks of different compositions and localities. All samples show 4Hem / 4Hec*le 1 implying that an additional source for 4He is not required. This is in a great contrast to extremely high 3Hem / 3Hec* ratios in these very samples, up to 105. A great excess of 3He in UACC samples can not be attributed to spallogenic production of this nuclide simply because more than 50% of the samples were collected from prospecting boreholes and quarries (Figures 2 - 10).

The comparison of measured and calculated abundances gives unambiguous evidence on contribution of mantle 3He-bearing fluid to a majority of ultrabasic, alkaline rocks and carbonatites. The crushing and step-wise heating experiments allow fluid-related helium to be separated, at least partially, from in-situ produced He*.

4.1.3. He isotope abundances in fluid inclusions.

fig13 Generally a substantial portion of 3He is related to vesicles and readily extracted by milling (the Average and Median are 0.42 and 0.37, respectively, Figure 13), whereas 4He appears to be better fixed within crystalline lattices (Av = 0.13, Me= 0.075).

fig14 Helium isotope abundances in fluid-related He vary in a wide range (Figure 14) similar to that observed for the whole-rock data (see Figure 11). However in a number of samples from different massifs 4He/3He ratios are substantially below the MORB value indicating a contribution of high- 3He plume-related fluid. Olivinite SV-1 from the Seblyavr massif and magnetic fraction SV-2 separated from this rock both show the lowest 4He/3He =(3.02 pm 0.01)times104 whereas 3He concentration in the magnetic fraction is by a factor of 4.5 exceeds that in the parent rock (Table 3). These relationships, also seen in Figure 14, indicate independence of the lowest ratios 4He/3He from helium concentrations exceeded some threshold, 3Hege10-10 cm3 STP g-1.

According to Figure 14, the 4He/3He range is getting narrower with increasing 3He. However samples with as high 3He as sim10-9 cm3 STP g-1 still show 4He/3He varying within a factor of ~20. The parent-daughter relationships allows to understand whether initial 4He/3He ratios varied substantially in magmatic He trapped 370 Ma ago or a post-magmatic contribution of in-situ produced 4He* is a reason of this spread.

4.1.4. Relationships between helium isotopes, parent element concentrations and age.

Enhanced concentrations of U, the major generator of radiogenic He, are typical of ultrabasic rocks from Kola UACC, U le 20 ppm, by a factor of 100 exceeding usual concentrations [Taylor and McLennan, 1985]. The Average U concentrations and Medians (both in ppm) are decreasing from ultrabasic rocks of UACC, 3.2 and 1.2 (13 samples) through carbonatites 2.40 and 1 (23 samples) to rocks of the dyke Complex 1.8 and 0.6 (29 samples). Th concentrations are quite high in some carbonatites and rocks from dykes, up to 70 ppm, which affect the average Th/U ratios observed in these rocks, 11 and 12, respectively. The average Th/U in ultrabasic rocks, 5.6, is also above the mean crustal value, 3.9 [Kramers and Tolstikhin, 1998].

fig15 A comparison of the combination U + 0.24Th proportional to 4He production [Zartman et al., 1961], and the whole-rock helium isotope abundances definitely shows an important role of the contribution of radiogenic 4He*: the higher the ratio of (U+0.24Th)/3He the higher the 4He/3He ratio (Figure 15). Assuming the closed system evolution for UACC, i.e., no gain/loss of species of interest since the formation age, 370 Ma ago, the data points would have situated on the evolution line having slope of 4He/(U+0.24Th) = 45 [cc/g] (solid line in Figure 15). Indeed the data-points are approaching the evolution line or lay below indicating an open system behaviour, i.e., helium loss. In regards to U-Th-He systematics, such a behaviour is quite typical for both igneous and sedimentary rocks ( Mamyrin and Tolstikhin, 1984; Tolstikhin et al., 1996]. Importantly, several data-points having low whole-rock 4He/3He ratios almost approach the evolution line, implying a narrow interval for the initial 4He/3He ratio in helium trapped by these rocks.

fig16 To estimate the initial ratio several samples with low 4He/3He and (U+0.24Th)/3He ratios are presented in a linear co-ordinate plot (Figure 16). The regression line indicates the initial 4He/3He = 30,000 which is exactly the same as the lowest measured ratio in helium released by milling (sample SB-1, Table 3). Slope of the regression is lower than that of the reference evolution line, indicating a moderate He loss.

Summarising, relationships between helium isotopes and parent radioactive elements (Figures 15 and 16) reveal the identical initial ratio of 4He/3He approx 30,000 in trapped helium and its highly variable concentrations. This initial ratio is intermediate between the mean MORB value, 89,000 [Tolstikhin and Marty, 1998] and the lowest value observed in Loihi basalt glasses 20,000 [Honda et al., 1993]. Subsequent variable contribution of radiogenic in-situ produced He* ensure a wide spread of present-day He isotope abundances in UACC, depending on relative abundance of the parent elements, (U+0.24Th)/3He, and portion of He * retained in a sample (see also Section 5.2).

4.2. Neon and He-Ne Relationships

fig17 The conventional three-isotope plot (Figure 17) shows a good correlation between 21Ne/22Ne and 20Ne/22Ne ratios in Ne extracted by milling. 20Ne/22Ne ratios vary from 10.5 to 12.1 (Table 4) implying mixing between the atmospheric, 9.8, and the solar, 13.7, end-members [Honda et al, 1991, 1993]. The slope of regression line, SR(Kola) = 190 pm 40, is slightly below that observed for Loihi samples, SR(Loihi) = 250 pm 25 but well above the MORB regression, SR(MORB) = 90 pm 4, corroborating conclusion on a plume-related fluid component inferred from study of He isotope abundances.

Tolstikhin et al. [1985) reported an extremely radiogenic Ne in 5 samples of Khibiny carbonatites having the average ratios 20Ne/22Ne = 5.3 and 21Ne/22Ne=0.1; because of high fluorine concentrations in Kola UACC, contribution of radiogenic 22Ne* appears to be important. Other UAC Complexes show less radiogenic signature with Ne compositions following approximately along the MORB trend (Figure 17).

Subtraction of the in-situ produced Ne isotopes using U-He-Ne systematics allow the plume-related compositions to be revealed in the less radiogenic samples from the Kovdor and other UACC. Measured isotope composition of neon (for example, point M in Figure 17, corresponding to sample KV-28, Table 4) reflects proportion of mixing of atmospheric, solar and radiogenic components. The first two end-members are well known and their adequate mixture is defined hereafter as the initial composition (SA in Figure 17). A plausible candidate for radiogenic "end-member'' appears to be neon from Khibiny carbonatites; the corresponding data-point is situated to the right off Figure 17; direction to this point is shown by R. The end members, i.e., initial (SA) and radiogenic R compositions, as well as measured M, and plume-related P compositions resulting from end-member mixing must all lay on one and the same line in the co-ordinate used. Data-points R and M, which co-ordinates are known from measurements, determine this line SAPMR. Shift of a data-point from the initial composition (SA) to the right along SAPMR is proportional to the addition of radiogenic Ne *, other things being equal. Proportion


allow the shift from initial to plume related composition to be quantified. This proportion relies on the constant production ratio 4He*/21Ne*approx (1.5 pm 0.5)times107 and known ( 4He/3He)plume = 30,000 (Sections 4.1.3 and 4.1.4), ( 4He/3He) prim = 3,000 [e.g., Anders and Grevesse, 1989], ( 4He/3He)meas = 230,000 (KV-28, Table 2) and gives the composition of plume-related Ne in this sample shown as P. While P deviates to the left from Kola UACC array, other 4 corrected compositions are well within this array corroborating occurrence of plume-related Loihi-like Ne even in samples containing a substantial radiogenic component.

4.3. Argon and Lighter Noble Gases

4.3.1. Parent-daughter and Ne-Ar relationships and 40Ar/36Ar ratio in trapped fluid 40Ar/36Ar ratio appears to be important characterisation of the trapped fluid particularly taking into account a poor knowledge of this ratio in a plume source [Allegre et al., 1986; Ozima and Zahnle, 1993; O'Nions and Tolstikhin, 1994, 1996; Porcelli and Wasserburg, 1995; Tolstikhin and Marty, 1998]. In contrast to He, three sources of Ar are significant for samples from Kola UACC: atmogenic Ar, 40Ar* produced in-situ, and trapped Ar. 40Ar/36Ar ratios in Ar extracted from the samples by milling vary within an order of magnitude, from air value 296 to ~3,000, implying a substantial contribution of atmospheric Ar in initially homogeneous (Section 4.1.4) trapped fluid.

fig18 Within the conventional 40Ar/36Ar versus K/36Ar plot the data points mainly cluster around the reference 370 Ma isochrone crossing atmospheric initial 40Ar/36Ar ratio (Figure 18). However several Kovdor samples deviate to the top off the isochrone, indicating an elevated initial 40Ar/36Ar ~4,000 in the trapped fluid.

fig19 20Ne/22Ne versus 40Ar/36Ar correlation (Figure 19) allows an independent estimate for the initial 40Ar/36Ar ratio [Marty et al., 1998]. This correlation resulting from mixing of atmospheric and mantle species definitely shows that the mantle end-member must have 40Ar/36Ar > 3,000. Generally mixing trajectory is a curve in these coordinates and extrapolation of this curve to solar 20Ne/22Ne requires special assumptions about the mantle end-member(s). If mixing of material from 3 reservoirs (Section 5.4) would have occurred, the extrapolation is not allowed. Assuming two end-member mixing, i.e., the atmosphere and the plume source, Marty et al. [1999] obtained the limiting 40Ar/36Ar plume sim 5,000 - 6,000.

fig20 4.3.2. 4He/40Ar* ratio in trapped fluid. Two post-magmatic processes appears to evolve 4He/40Ar * ratios in a trapped fluid. The first is addition of in-situ produced nuclides, particularly tacking into account that the mean K/U ratio observed in UAC samples (with the exception of the Dyke Complex, see Table 3) is by a factor of approx 5 less than the canonical mantle K/U = 12700 [Jochum et al., 1983]. The second is a preferential helium loss. Indeed, the whole-rock 4He/40Ar * versus (U+0.24Th)/K plot demonstrate that the two processes account for the observed distribution of data-points (Figure 20).

To avoid a substantial contribution of in-situ-produced nuclides, 23 samples with 4He/3He mill le 4He/ 3HeMORB were selected giving the average 4He/40Ar* = 6.4 pm 5.3 and Me = 5.0. Among those several carbonatites show insignificant fractionation between 4He and 21Ne * (See Figure 24) and 4He/40Ar* ratios in this sub-set vary within a narrow range averaging at 3.1 pm 1.1 which is in a good agreement with the model estimates of mantle ratios, 2.5 pm 1.5 for the upper and approx 3 for the lower mantle reservoirs [Tolstikhin and Marty, 1998].

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