14.3 Beryllium-10
Cosmic rays interact directly with nitrogen and
oxygen atoms in the atmosphere, causing spallation (fragmentation) into the
light atoms Li, Be and B. Amongst these, one of the nuclides produced is the
unstable isotope 10Be. Cosmogenic 10Be can also be
generated in the surface layer of exposed rocks by in situ production. However, this subject will be dealt with under
section 14.6.
10Be
decays by pure $ emission to 10B. It was first observed in naturally
occurring material by radioactive counting (
Analytically,
10Be determination by AMS is similar to 14C, but involves
some additional complications. Because Be does not form stable negative ions,
the BeO! species
must be used, upon which the isobaric interference of 10BO is a
serious problem. This is overcome by passing the ion beam through an absorber
gas (in front of the detector) whose pressure is adjusted to completely stop 10B
transmission. The high ion velocity of the 10Be beam generated by
AMS allows this species to pass through to the detector, which consists of a
gas ionisation counter in front of a surface barrier detector which finally
absorbs the ion beam. The first detector measures energy loss () E)
of the ions as they collide with gas molecules in the chamber (this property of
the ions is inversely proportional to their atomic number). The second detector
measures residual energy. Using this bivariate discriminant, 10Be
ions can be resolved from all other signals (Fig. 14.23) to yield a very low
background. 10Be contents of samples are measured relative to added 9Be
spike, and normalised for machine mass discrimination by frequent standard
analysis.
Fig. 14.23. Plot of )E against E for a typical geological sample, to
show resolution of 10Be from other species. Dot size indicates the
number of counts in each bin (smallest = 1 count). After Brown et al. (1982).
14.3.1 10Be in the atmosphere
10Be enters the hydrological cycle by attachment
to aerosols, from which it is scavenged by precipitation. Consequently, it has
a very short (ca. 1 week to 2 yr) residence time in the atmosphere and, unlike 14C,
is not homogenised within the atmosphere prior to its fallout.
It
was originally intended (Raisbeck et al.,
1979) that 10Be analysis of rainwater would allow accurate
constraints to be placed on the global average 10Be production rate.
However, two factors complicate a determination of the 10Be flux in
rainwater. One is the tendency for comparatively Be-rich soil particulates to
be caught up in the atmosphere and cause secondary contamination of rain
(Stensland et al., 1983). Once this
effect is removed, individual depositional events still turn out to have very
variable 10Be contents (Brown et
al., 1989).
One
way of gauging the effect of soil re-suspension on 10Be abundances
is to compare them with 7Be data. The latter species has relatively
similar atmospheric production rates to 10Be, but much lower levels
in soils due to its very short (53 day) half-life. It is measured by ( counting. An alternative way of
assessing the effects of soil re-suspension is to compare continental and
oceanic 10Be deposition rates. Brown et al. (1989) used both of these approaches in an analysis of 10Be
precipitation in
Fig. 14.24. Histograms of 10Be and 7Be
concentration respectively in rainfall from
The
variability of 10Be contents in individual rain showers makes it
difficult to determine accurate annual fluxes for mid-latitudes. A summary of
these data as a function of latitude (Fig. 14.25) shows the variability of
these estimates (Brown et al., 1992).
However, at tropical latitudes the estimates of annual 10Be flux are
more reproducible. The latter are in good agreement with a global 10Be
flux estimate of 106 atom/cm2/yr, based on cosmic-ray
intensity as a function of latitude (Lal and Peters, 1967). Therefore, at the
present time, the curve in Fig. 14.25 represents the best available estimate of
the atmospheric 10Be flux.
Fig. 14.25. Summary of empirical estimates of
annual 10Be flux in rainfall, as a function of latitude. These are
compared with a theoretical production curve based on cosmic-ray intensity.
After Brown et al. (1992).
14.3.2 10Be in soil profiles
Beryllium is partitioned very strongly from
rain water onto the surface of soil particles such as clay minerals. If we
assume that 10Be adsorption is perfect, and that a given soil
section is developed by weathering of rock or rock debris without the addition
or removal of sediment during the weathering process, the soil section should
contain a complete inventory of all deposited 10Be that has not yet
decayed. This process offers the opportunity of dating a soil profile by
measuring the total accumulation of 10Be in the section, but it is
apparent that there are a large number of assumptions.
The
10Be inventory of a soil profile in
q
N
= )
(1 ! e!8 t) [14.3]
8
where q
is the input flux from rainfall and t
is the accumulation time. If the profile is infinitely old, relative to the 1.5
Myr half-life of 10Be, it will reach saturation, whereupon input
flux from rainfall is balanced by the rate of decay (8). Equation [14.3] then simplifies
to N = q/8. Given an annual deposition flux of 1.3 H 106 atom/cm2
at this latitude, the saturation inventory will be 3 H 1012 atom/cm2,
which is three times the observed inventory. The discrepancy can be explained
by loss of 10Be-enriched soil from the top of the profile by
erosion, and its replacement at the bottom of the profile by weathering of
bedrock with no 10Be. Solving equation [14.3] for t (using the observed inventory) yields
the residence time of 10Be in the profile, equal to 800 kyr.
Very
different 10Be behaviour is demonstrated by a soil profile from the
Fig. 14.26. Plots of 10Be activity
as a function of depth in two soil profiles. a) Virginia Piedmont, after Pavich
et al. (1985); b)
Differences
in 10Be retention between the two cases described above can be
understood in the light of laboratory experiments on beryllium partition
between soil and water (You et al.,
1989). These studies showed that beryllium retention on soil particles is
strongly pH dependent, with distribution
coefficients of ca. 105 in neutral conditions (pH 7), but less than
100 at pH 2. Hence the more acidic conditions in tropical soils are less
favourable for beryllium retention.
In
alkaline soils, beryllium mobility within the soil profile may be very limited,
and in these conditions 10Be may be used as a stratigraphic tool. An
example is provided by a 10Be study of Chinese loess, in which
carbonate-rich conditions yield a pH value of 8 (Chengde et al., 1992). The profile was dated magnetically back to 800 kyr,
and represents the products of wind-borne deposition through varying climatic
conditions. Chengde et al. tuned
their profile to the Quaternary climatic record provided by sea-floor 18O
variations. They concluded that during arid periods, rapid loess deposition was
accompanied by high fluxes of 10Be, adsorbed onto wind-blown
particles. These sections were interspersed with wetter periods with lower 10Be
depositional fluxes.
14.3.3 10Be in snow and ice
Atmospheric 10Be accumulates in snow
and ice, but its half-life is too long to date such deposits. However, it can
be used as a tracer of climatic changes and to understand the processes
modulating cosmogenic 14C production in the atmosphere. The first
detailed study of this type was made by Raisbeck et al. (1981a) on a 906 m-long ice core from the Dome C station,
eastern
Subsequent
studies of
Studies
of the solar modulation of 10Be production were extended to a 1000
year period in a more recent study from the South Pole (Bard et al., 1997). This section was
precisely dated using 20 impurity layers which were correlated with known
volcanic eruptions. In this section, the peak at the Maunder Minimum (1700 AD)
is seen very sharply, but similar peaks are also visible at other periods,
particularly around 1050 AD (Fig. 14.27). Measurement of 10Be in
polar ice is particularly effective to study solar modulation of cosmogenic
production because the shielding effect of the Earth’s magnetic field is
reduced at the poles, enhancing the sensitivity of cosmogenic production to the
effects of the solar wind. Furthermore, these local production effects are
fully transmitted into the ice core record because 10Be is not well
homogenised in the atmospheric system. In contrast, the radiocarbon signal from
tree rings shows a strongly damped signal due to the buffering of atmospheric
radiocarbon abundances by exchange with the oceans. However, Bard et al. used a box model to unravel these
effects in the carbon system and calculate a synthetic record of ‘undamped’
radiocarbon production. As expected, this was well correlated with the 10Be
record (Fig. 14.27).
Fig. 14.27. Plot of 10Be variation in
the South Pole ice core during a 1000 yr period from ca. 1900 to 900 compared
with age-corrected 14C levels in tree rings. After Bard et al. (1997).
In
contrast to the successful use of ice core material to study solar modulation of cosmogenic nuclide
production, attempts to use ice core records of 10Be to chart geomagnetic modulation of cosmogenic
production were much less successful. For example, a reconnaissance study of
the long Dome C core (Raisbeck et al.,
1981a) revealed a strong correlation of 10Be with *18O, which was attributed to the climatic effect
of the last ice age. No significant correlation was observed with geomagnetic
field strength, which reached a minimum value 6000 years ago and is argued to
control long-term 14C variations. Yiou et al. (1985) suggested a partial solution to this problem by
attributing the 10Be maximum during the last glaciation to lower
precipitation at that time. This would sweep the same amount of 10Be
out of the atmosphere, but concentrate it in a lower volume of ice, causing the
10Be record to be compressed.
The
long-term Antarctic Dome C data were again matched by results from
Fig. 14.28. Plot of 10Be and *18O variations in the
It
is now realised that the dramatic variations in 10Be abundance over
the last glacial cycle are almost entirely a result of variable ice
accumulation rates. Indeed, the method has more recently been used to chart
paleo-accumulation rates in central
14.3.4 10Be in the oceans
Marine sediments were some of the first
materials to be successfully analysed for 10Be, since they have
concentrations measurable by $ counting. The objective was to use 10Be as a dating tool for
oceanic sediments. However, early studies, which simply compared 10Be
abundances at various depths with theoretical cosmogenic production rates, were
unreliable. A more rigorous study was made by Tanaka and Inoue (1979) on
paleomagnetically dated sediment cores from the
Fig. 14.29. Compilation plot of 10Be
activity (normalised relative to the sediment surface) against burial age
(depth) for five cores from the North Pacific. After Tanaka and Inoue (1979).
An
important application of 10Be as a dating tool has been in studies
of ferromanganese crusts. These deposits represent an ideal archive for
particle-reactive species in the ocean system, such as Nd and Pb (section 4.5).
Because the crusts grow into free space they are resistant to contamination by
clastic sediment particles and can accurately record long-term seawater isotope
variations, provided their growth rates can be accurately measured. For the
past 500 kyr, the dating of crusts can be performed using U-series isotopes
(section 12.3.2), but beyond the range of this method, 10Be is the
best technique. This was first applied by Turekian et al. (1979) and is now used as a standard technique in
oceanographic paleo tracer analysis (section 4.5.3).
Early
studies of the behaviour of stable 9Be in the oceans suggested that
it was one of the class of elements which very quickly precipitates from
seawater (Merrill et al., 1960). This
implies that fine particulates are the principal carrier of 10Be.
This model was tested by Tanaka and Inoue (1979) by plotting the 10Be
precipitation rate against sedimentation rate at different sites in the Pacific
(Fig. 14.30). The good positive correlation displayed by most of the data
suggests that the particulate model is valid. Therefore, the net 10Be
deposition flux at any given locality is dependent on the sedimentation rate
rather than the cosmogenic flux. 10Be deposition rates are actually
seen to vary by a factor of three above and below the theoretically predicted
flux from the atmosphere. These variations in the 10Be depositional
flux at different localities were attributed by Tanaka and Inoue to the lateral
transport (advection) of fine particulates by ocean currents.
Fig. 14.30. Plot of 10Be
precipitation rate against particulate sedimentation rate for the different
A
detailed understanding of the behaviour of 10Be in the aqueous
system requires a consideration of the oceanic residence time. Merrill et al. (1960) estimated the residence
time of beryllium using equation [14.4] (Goldberg and Arrhenius, 1958):
total oceanic inventory
residence time = ))))))))))))))) [14.4]
total rate of introduction
This equation holds for a steady-state
(equilibrium) system, which is approximated if the flux is constant for three
residence times. For 10Be the equation can conveniently be
calculated per unit area:
total
water column budget / unit area
residence time = ))))))))))))))))) [14.5]
supply
flux / unit area
Merrill et
al. determined a residence time for 9Be attached to particulate
matter of only 150 yr, but they estimated a longer residence time of 570 yr for
the dissolved beryllium budget.
The
first estimate of the soluble 10Be budget of the oceans was made by
Yokoyama et al. (1978) based on 10Be/9Be
ratios in manganese nodules. By assuming that these incorporated dissolved
beryllium directly from seawater, and using published 9Be abundances
in the oceans, they calculated the dissolved 10Be budget as 2 H 109 atoms/g. Almost
identical concentrations were determined by Raisbeck et al. (1980) in the first direct 10Be determinations on
deep ocean waters, but their estimated residence time (630 yr) differed
markedly from that of Yokoyama et al.
(1600 yr) due to the use of different cosmogenic flux estimates. Raisbeck et al. used their own estimate of the
cosmogenic 10Be flux, based on one year’s rain from a single
locality in
Fig. 14.31. Concentrations of dissolved 10Be
(atom/g) and 9Be (pico Mole) plotted as a function of water depth in
the open ocean of the east Pacific. After Kusakabe et al. (1987).
The
behaviour of 10Be in the near-shore environment is very different
from the open ocean, as proposed in
Similar
excess 10Be sedimentation rates have been seen in freshwater lakes
(Raisbeck et al., 1981b). In this
case, soil erosion in the drainage basin which supplied the lake caused the
introduction of beryllium-rich sediment. Lundberg et al. (1983) proposed that the excess 10Be was
introduced dominantly on organic matter rather than silicate particles,
consistent with the nutrient-like behaviour of 10Be proposed for the
oceanic system.
Comparisons
between the Be isotope ratio of modern ocean masses were first made by Ku et al. (1990). These revealed a
consistent global pattern of increasing 10Be/9Be ratio
along the ocean conveyer belt, with values ranging from 0.6 H 10!7 in NADW, through 1 H 10!7 in the Antarctic,
to values as high as 1.6 H 10!7 in deep Pacific water (Ku et al.,
1990). This trend is shown in terms of increasing 10Be
content with water age in Fig. 14.32. The positive trend is due to the low particulate
sedimentation rate from NADW, which allows be 10Be to be advected to
the Pacific, and implies a beryllium residence time similar to the circulation
age of the ocean water masses. The precise residence time is difficult to
constrain because of the different sources of the two isotopes: 10Be
from global rain and 9Be from river water. However, the curve shown
in Fig. 14.32 was modelled assuming a residence time of 600 yr (von Blankenburg
et al. (1996).
Fig. 14.32. Plot of 10Be concentration against
radiocarbon age in oceanic deep water to show the build-up of 10Be
along the Ocean Conveyer Belt. ( ! ) = NADW; ( " ) = Pacific deep water. After von
Blankenburg et al. (1996).
Beryllium isotope
ratios in the 10-7 range are less extreme than most cosmogenic
isotope ratios, offering the possibility of analysis by conventional (non
accelerator) mass spectrometry. Such a method was developed by Belshaw et al. (1995) and applied by von
Blankenburg et al. (1996) to the
beryllium isotope analysis of ocean floor ferro-manganese crusts. Uranium
isotope ratios were used to date the surface layer of the crust in order to
apply an age correction in cases where the original growth surface had been
removed by abrasion (section 12.3.2). Crusts of various ages (0 – 300 kyr) gave very consistent initial
Be isotope ratios within each ocean basin, consistent with the beryllium
composition of overlying deep ocean water. This suggests that the oceans have
generally maintained the same circulation pattern over the past 300 kyr.
However, more detailed comparisons of glacial and interglacial 10Be/9Be
ratios are needed to test for any interruptions to the conveyer belt during
glacial maxima.
14.3.5 Comparison of 10Be with other tracers
Ku et al.
(1995) made the first comparison of 26Al and 10Be
abundances in ocean water. They found 26Al/27Al ratios in
surface water which were consistent with the expected 26Al flux from
atmospheric production (by spallation of argon). Therefore, contributions of in situ cosmogenic 26Al from
cosmic dust or wind-blown continental dust do not appear to be significant in
sea water. However, the 26Al/10Be ratio measured in
surface water was an order of magnitude lower than the atmospheric production
ratio. This was therefore attributed to the much shorter ocean residence time
of 26Al.
Wang
et al. (1996) compared 26Al,
10Be and 230Th records in ocean floor sediments. The
authigenic (seawater-derived) fractions of these nuclides were removed from
core samples by leaching with NaOH. 10Be/9Be ratios in
the leachates were in good agreement with the composition of overlying (North
Pacific) deep water, whereas 10Be/9Be ratios in the bulk
sediment were 50% lower, due to input from detrital continental beryllium. The 26Al
budget in the sediment column was also in good agreement with the atmospheric
production flux and with 26Al in ocean water (Ku, et al., 1995).
Comparison
of authigenic 26Al, 10Be and excess initial 230Th
revealed 26Al/230Th ratios consistent with the estimated
production ratio, but excess abundances of 10Be relative to both the
other nuclides (Fig. 14.33). Hence, it was concluded that 26Al and 230Th
have similar (very short) residence times in ocean water, whereas the longer residence
time of 10Be allows advection of 10Be into the North
Pacific, where it is scavenged by high biogenic production. Similar effects are
seen in the 231Pa/230Th system (section 12.3.6), but the
effect on 10Be/26Al is larger, due to the longer ocean residence
of 10Be. On the other hand, ratios of 231Pa/230Th
are easier to measure, so both methods are likely to be very useful in
paleo-oceanography.
Fig. 14.33. Relative abundances of
radionuclides in a sediment core from the North Pacific, showing ratios close
to the production ratio for 26Al–230Th, but out of equilibrium with 10Be.
After Wang et al. (1996).
Despite the
complexities which may be introduced into 10Be sediment profiles by
advection, marine sediments offer a better prospect than snow deposits for
studying variations in paleo 10Be production. To overcome the
problem of advection, Lao et al.
(1992) compared 10Be abundances with the U-series nuclides 230Th
and 231Pa. These nuclides have similar ocean chemistry to beryllium,
but are produced at a constant rate from uranium in solution. Lao et al. compared 10Be
production at the present day and 20 kyr ago (corresponding to the last glacial
maximum when the 14C flux was 140% of its present value). They
normalised both 10Be and 231Pa fluxes against 230Th.
However, based on a comparison of ocean residence times (section 12.3.6), we
can best normalise climatic effects on 10Be deposition by comparing
the 10Be/231Pa ratio at the present day and 20 kyr ago.
After excluding one site with abnormal chemistry, 17 sites from the Pacific
gave an average 10Be flux enhancement of 144% during the last
glacial maximum, in excellent agreement with 14C.
Comparisons between the
geomagnetic field strength and 10Be deposition have also been made by
observing their variation over time in a single site. However, the existence of
a direct (inverse) relationship between these variables depends on the
neutralisation of climatic effects. In the absence of U-series data for
normalisation, this may only happen by chance. Thus, Robinson et al. (1995) observed a good inverse
relationship between 10Be and magnetic intensity in an 80 kyr old
sediment core from the central
To
avoid this type of problem, Frank et al.
(1997) used only 230Th-dated cores to compile a global average of 10Be
inventories for the past 200 kyr. This was based on 19 long cores which covered
most of this age range, supplemented by 18 shorter cores covering the last 25
kyr. The global stack of 10Be abundance data (Fig. 14.34a) was then
inverted to determine relative geomagnetic field intensity (Fig. 14.34b), which
was found to correlate very well with a globally stacked paleointensity record
(Fig. 14.34c).
Fig. 14.34. Comparison between a global stack
of 10Be paleointensity data (a, b) with a similar stack of magnetic
paleointensity (c), demonstrating the paleomagnetic modulation of global
cosmogenic isotope production. After Frank et
al. (1997).
Kok
(1999) expressed concern that both the 10Be and the paleointensity
records might still not be completely free of climatically induced variations
in sedimentation rate, despite the use of the 230Th dating method to
calibrate the sections. This concern was prompted by the observation that the
magnetic paleointensity record show a significant correspondence with the
SPECMAP record of oxygen isotope variations (Fig. 14.35, a,b). However, two
alternative explanations have since been offered for this correspondence. The
first suggestion (Yamazaki and Oda, 2002) is that the geomagnetic field
intensity is itself modulated by the
Earth’s orbital eccentricity (which is thought to modulate climatic cycles). In
that case, we would expect to see a correlation between variations in climate
and the geomagnetic field. However, close examination of Fig. 14.35 shows that
the 10Be-derived magnetic paleointensity stack (Fig. 14.35a) shows a
stronger correspondence with the SPECMAP *18O record
than the directly measured magnetic record. This suggests that it is the 10Be
record, rather than the magnetic record, that contains a component with a
climatic linkage.
Fig. 14.35. Comparison of paleointensity record
with the SPECMAP *18O record (dashed line). a) magnetic
paleointensity derived from the inversion of 10Be data; b) directly
measured magnetic paleointensity; c) normalised residual 10Be
modulation, attributed to solar field effects. After Kok (1999) and Sharma
(2002).
Sharma
(2002) examined this problem in more detail by extracting a residual 10Be
modulation effect, after the subtraction of the geomagnetic modulation. This
quantity was then normalised to its present day value, and is plotted in Fig.
14.35c along with the SPECMAP record. The controversial aspect of Sharma’s
analysis was that he attributed the residual modulation effect to solar
magnetic activity, which he suggested was also responsible for the 100 kyr
glacial climate cycles during this period. This would then provide a ‘rival’
explanation for glacial cycles, relative to the widely accepted Milankovich
model of ‘orbital tuning’ (section 12.4.2). The great success of the
‘astronomical timescale’ (section 10.4.2), obtained by orbital tuning of the oxygen isotope record, makes it hard
to disbelieve the fundamental importance of the Milankovich model. However,
there may be room for more than one influence on glacial cycles. Therefore, as
one reviewer commented, Sharma’s model “could just be right”. Unfortunately, it
will take time to accumulate the high quality 10Be and paleomagnetic
records need to test this question further. In the meantime, it should be
remembered that the Earth’s geomagnetic field is the principle modulating influence on cosmogenic isotope production.
14.3.6 10Be in magmatic systems
The most important application of 10Be as a petrogenetic
tracer is in studies of the relationship between sediment subduction and island
arc volcanism. In a reconnaissance study, Brown et al. (1982) demonstrated 10Be concentrations in island
arc volcanics (2.7 H 106 ) 6.9 H 106
atom/g) which were generally much higher than the levels seen in a control
group of continental and oceanic flood basalts. Brown et al. argued against high-level contamination of the analysed
volcanics on the grounds that the short half-life of 10Be renders it
extinct in all but surficial deposits, while 10Be levels in
rain-water are too low to cause the observed enrichments. In contrast, it has
long been known (e.g. Arnold, 1956) that pelagic sediments have very large 10Be
contents, in excess of 109 atoms/g. Brown et al. therefore attributed their data to the involvement of
subducted ocean floor sediment in the genesis of island arc magmatism.
Subsequent studies
(e.g. Tera et al., 1986) confirmed
the general observation of high 10Be in island arc volcanics and low
10Be in the non-arc control group (Fig. 14.36). Detailed studies
were also undertaken to assess the effects of weathering on the 10Be
contents of lavas. Analyses of material collected during or immediately after
eruption were shown to contain the same range of 10Be contents as
historical lavas. Contamination of non-arc samples by radiogenic 10Be
was only observed in severely altered samples. In situ cosmogenic production of 10Be in lavas at depths
of 1 cm was also excluded, based on the low 10Be abundances in 16
Myr-old
Fig. 14.36. Histograms of 10Be
abundance in volcanic rocks. a) Non-arc control group and low-10Be
arcs; b) high-10Be arcs. Symbols: A = sample from ‘active’ volcano;
H = historic flow; F = fresh sample collected during or immediately after
eruption. After Tera et al. (1986).
A surprising result of
the detailed study of Tera et al. was
that several arcs have 10Be levels as low as the maximum of 1 H 106 atoms/g in the control group.
These included all samples from the Mariana,
1) adequate 10Be
inventory in trench sediments;
2) subduction rather than
accretion of uppermost Be-enriched sediments;
3) incorporation of sediment in
the magma source area;
4) transport time from
sedimentation to magma source area < 10 Myr.
Failure of any one of these criteria could preclude a positive 10Be
signal. However, Tera et al. did not observe simple correlations between 10Be
and geophysical parameters such as age of the subducting plate.
In an attempt to
harmonise their data from different arcs, Tera et al. plotted 10Be against a model parameter involving
sedimentation rate, sediment thickness, plate velocity, and distance from
trench to magma source. This quantity is specified as:
0o s | 8 l |
Predicted
10Be abundance = ))) exp - | ))) | [14.6]
8 h | v |
where 0o = 10Be abundance in the sediment, s = Plio)Pleistocene sedimentation rate, h = sediment thickness, l = the arc )trench gap, and
v = plate velocity. Since the
volcanic front is always located about 100 km above the seismic plane, the arc )trench gap is inversely proportional to the dip
of the Benioff zone. Using this model, the contrast between the 10Be-rich
Central American data and other arcs is explained by the high sedimentation
rate and steep subduction angle of the former. However, both the Japanese and
Aleutian arcs have a single 10Be-rich data point (shown by the
dashed ranges in Fig. 14.37) which does not fit the model.
Fig. 14.37. Plot of 10Be data for seven arcs against a model
parameter for the efficiency of 10Be supply to arc magma sources. 10Be
signals are modelled for different bulk percentage sediment contributions to
magmas. After Tera et al. (1986).
A further step in rationalising
the 10Be systematics of arc volcanics was achieved by considering
the data relative to non-cosmogenic (9Be) abundances (Monaghan et al., 1988; Morris and Tera, 1989).
Within the different minerals of a single rock sample, 10Be is
normally strongly correlated with total Be content (Fig. 14.38), implying that
radiogenic and non-radiogenic Be were mixed before magmatic differentiation
occurred. This further strengthens the arguments against surficial
contamination of the lavas by 10Be, and also argues against crustal 10Be
assimilation by magmas. Finally, the enriched Be contents of the groundmass,
relative to phenocrysts, show that Be behaves as an incompatible element during
magmatic differentiation.
Fig. 14.38. Plot of 10Be against total Be content for
separated mineral phases, plus whole-rock (WR) and groundmass (GM) from two
young lavas. Samples are from the
In contrast to Be
mineral systematics, most whole-rock samples analysed by Morris and Tera did
not show a strong correlation between 10Be and total Be. However,
they did show a good correlation between 10Be/9Be ratios
and absolute 10Be abundances (Fig. 14.39). These findings suggest
that most of the rocks analysed, which were basalts, did not have their 10Be
contents perturbed by magmatic differentiation. However, some andesites lie
significantly to the right of the main trend, including the Japanese and
Aleutian samples with abnormally high 10Be contents in Fig. 14.37
(Tera et al., 1986).
Fig. 14.39. Plot of 10Be/9Be against 10Be
abundance for basalts ( ! ) and
evolved rocks ( " )
from different arc and non-arc environments. After Morris and Tera (1989).
Further constraints on
the timing of subduction-related processes were obtained by combining 10Be/9Be
and 238U/230Th data. Sigmarsson et al. (1990) observed that these ratios were correlated in the
Southern Volcanic Zone of the
A further step in
understanding subduction-zone processes was achieved by comparing 10Be/9Be
and boron/beryllium ratios in arc lavas (Morris et al., 1990). Several arcs displayed a strong positive correlation
between these two variables (Fig. 14.40), despite the fact that the two
beryllium isotopes and boron have different distributions in the subducted
slab. 10Be is concentrated in the uppermost sediment layers and
diminishes rapidly downwards, 9Be is distributed throughout the
sediment column, whereas B is principally concentrated in the hydrothermally
altered basaltic crust. Hence, there is no a
priori reason why these three species should display coherent behaviour in
arc volcanics. The fact that they do behave coherently in widely separated
volcanoes along the length of an arc suggested to Morris et al. a very thorough homogenisation mechanism for Be and B during
the process of subduction-related magma genesis. While such a process could
occur in the solid state, it is easiest to conceive of the mixing of fluids
driven off from different parts of the subducted slab. The convergence of all
of the correlation lines at the origin points to complete stripping out of all
boron from the subduction zone, with no long-term residence of this element in
the mantle.
Fig. 14.40. Plot of 10Be/9Be
against elemental B/Be ratio, showing correlation lines for arc volcanics
relative to possible mixing end-members. Numbered ticks denote calculated
percentages of slab-derived fluids necessary to generate the observed arrays by
contamination of the mantle source. After Morris et al. (1990).
The observed
correlation between 10Be/9Be ratio and elemental B/Be
ratio suggests that the latter may represent a useful proxy for the former.
This is important because it widens the applicability of beryllium data.
Firstly, the elemental B/Be ratio can be used as a tracer of the slab component
in arcs with low subduction rates, where 10Be is extinct by the time
of eruption. Secondly, elemental ratios can be measured with less sophisticated
analytical equipment such as ICP)MS. These advantages were demonstrated by Edwards et al. (1993b) in a study of basaltic lavas from the Indonesian
arc. Edwards et al. were able to
combine B/Be ratios with other radiogenic isotope systems in order to uniquely
specify the Pb, Sr and Nd isotope signatures of the slab-derived component,
which was modelled by a 80%)20%
mixture of basaltic crust and Indian Ocean sediment. This signature was also
distinguishable from enriched and depleted reservoirs in the mantle wedge. The
use of elemental B/Be data made these deductions possible despite the fact that
10Be abundances were at baseline, showing this nuclide to be extinct
in the analysed lavas.
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