Whole-rock (mineral) concentrations of helium isotopes and
4He/
3He ratios
vary within a great range
104 (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
5
10-13cm3 STP g-1, and
4He, down to
2
10-7 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,
108 [Mamyrin and Tolstikhin,
1984].
In contrast, ultrabasic rocks, carbonatites and related minerals from the
Seblyavr and the Kovdor show high concentrations of
3He, from
510-9
to
5
10-10 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
6.4
104 and
6.8
104, respectively, which are substantially
lower than the mean value
in mid-oceanic ridge basalts,
(8.9
0.9)
104,
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.
The concentrations of radiogenic
4He* are calculated from measured U and Th
concentrations (Table 2), and the age of the massifs, 370
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
b-
3He* with
l ( 3H) = 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*=7.210-8
for Permian shists (the Molasses
basin, Northern Switzerland) and the
measured ratio,
9.4
10-8, in adjacent aquifer having
stagnant waters with quite high
helium concentrations,
4.5
10-3 cm3 STP per g H2O.
From this brief review we
consider that the accuracy of calculated
3He* concentrations are most probably within
50%.
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*
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*.
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).
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
0.01)
104
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,
3He
10-10 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
10-9 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.
Enhanced concentrations of U, the major generator of radiogenic He, are
typical of ultrabasic rocks from Kola UACC, U
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].
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.
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
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).
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
40, is slightly below that observed for Loihi
samples, SR(Loihi) =
250
25 but well above the MORB regression, SR(MORB)
= 90
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* (1.5
0.5)
107
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.
4.3.1. 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.
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.
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
5,000 - 6,000.
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
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 4He/
3HeMORB were selected giving
the average
4He/40Ar* = 6.4
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
1.1 which is in a good agreement with the model
estimates of mantle ratios, 2.5
1.5 for the upper and
3 for the lower mantle
reservoirs [Tolstikhin and Marty, 1998].