Geochimica
et Cosmochimica
0016-7037/92/65.00
Acla Vol. 56, pp. 2955-2961
+ .oO
Copyright0 1992 Pergamon Press Ltd. Printedin U.S.A.
LETTER
Iridium content of basaltic tuffs and enclosing black shales
of the Balder Formation, North Sea
W. CRAWFORDELLIOTT,’ JAMESL. ARONSON,’ and HUGH T. MILLARDJR.*
‘Department of Geological Sciences, Case Western Reserve University, Cleveland, OH 44106, USA
*Branch of Isotope Geology, United States Geological Survey, MS 424, Box 25046, Federal Center, Denver, CO 80225-0046, USA
(Received February 5, 1992; accepted in revisedform May 29, 1992)
Abstract-The
anomalous levels of Ir and the presence of shocked metamorphosed quartz deposited at
the Cretaceous/Tertiary
(K/T) boundary worldwide is strong evidence that a meteorite impact took
place during the K/T boundary interval. However, because of observed high Ir contents at Kilauea vents,
it is still a major point of contention that the Ir anomaly could have been produced by flood basaltic
volcanism. This might especially be true at Stevns Klint, Denmark, where the K/T boundary marl
contains pyroclastic labradorite and Mg-smectite thought to have been produced by basaltic volcanism.
However, up to now, no study has determined whether or not a depositional Ir anomaly has formed in
association with a known major basaltic eruption. Herein, we report the concentrations of Ir, Pt, Au,
and Ag in basaltic tuffs and enclosing marine black shales of the widespread Paleocene-Eocene Balder
Formation. The tuffs in the Balder Formation represent explosive basaltic volcanism associated with the
major volcano/tectonic activity of the opening of the northern North Atlantic Ocean. As such, they are
the kind of eruption that could have possibly created a global K/T boundary-type Ir anomaly. Our results
show that the tuffs and the shales on a per-weight basis both contain concentrations of Ir (0.1-0.25 ppb)
that are higher than the Ir levels recently measured from terrestrial rocks including the Deccan Trap and
Columbia River flood basal@ but are comparable to Hawaiian and Reunion Island basalts. Because of
its thickness, the absolute amount of Ir expelled during the eruption of the main tuff sequence of the
Balder Tuff is sizable. Yet for such an eruption to have produced a global Ir anomaly would mandate it
having been one of an extremely high volatile content and it would have to have been erupted over a
very short interval of time. Furthermore, such a high proportion of the volatilized Ir would have to have
been injected into the stratosphere so that only small enrichments of Ir were formed in the proximal tuffs
on a per gram basis.
INTRODLJCHON
took place at the close of the Cretaceous period, 65 million
Years ago.
In spite of the many studies proposing that the Iris derived
from meteorite impact, some of the studies cogently argue
that the Ir may have been generated by the voluminous basaltic volcanism taking place on a worldwide scale during K/
T boundary time (for review, MCCARTNEY and LOPER,
1989). ZOLLERet al. ( 1983), OLMEZ et al. ( 1986), and RNNEGAN et al. ( 1989) have found that aerosols and particulates
associated with basaltic eruptions occurring at Kilauea contained significant enrichments of Ir relative to the basalt flows
themselves. ZOLLERet al. ( 1983 ) associated the high Ir content of these aerosols with high F contents of the Kilauea
vent gases and the low boiling point of compounds with Ir
and F. They, along with TOUTAINand MEYER( 1989), suggest
that Ir is released by basaltic hot-spot volcanoes having deep
mantle sources. Moreover, basaltic volcanism is proposed to
have been a significant cause in the mass extinctions at the
K/T boundary (e.g., HALLAM, 1987; OFFICER et al., 1987)
and potentially at the Permo-Triassic boundary as well
(RENNE and BASU, 1991).
The occurrences of multiple Ir anomalies in Cretaceous
and Tertiary rocks near the K/T boundary in several locations
THE ABUNDANCEAND occurrence of noble metals in sedimentary rocks (e.g., shales and limestones) and in volcanic
rocks (rhyolites and basalts) are not known very well relative
to gabbroic and ultramafic rocks. Nor were there many reasons to study the occurrences of these metals in sedimentary
and volcanic rocks until 1980, when ALVAREZet al. ( 1980)
discovered that Ir was present in anomalous concentrations
(i.e., > 1 ppb) at the Cretaceous/Tertiary
(K/T) boundary.
The Ir anomaly at the K/T boundary has been found in
greater than seventy-five localities worldwide (ALVAREZ,
1986) in both non-marine and marine sedimentary rocks.
At Stevns Klint, Denmark, GANAPATHY( 1980) showed the
Pt group metals are present at the K/T boundary in approximately chondrite-normalized
abundances, and Ir is present
there over 103-fold at the K/T boundary relative to the enclosing Cretaceous and Tertiary rocks (e.g., see KASTNER et
al., 1984; ROCCHIA et al., 1984). The source of the Ir and
the other Pt group metals are arguably meteoritic and, together with the presence of shock-metamorphosed quartz with
multiple planes of deformation lamellae (BOHORet al., 1984,
1987), compose strong evidence that a meteorite impact
2955
2956
W. C. Elliott, J. L. Aronson, and H. T. Millard Jr.
(i.e., Bavaria, Italy, South Atlantic Ocean) have also supported this rival hypothesis ( CROCKETet al., 1988; HLJF~~AN
et al., 1990; GRAUP et al., 1989). Moreover, KOEBERL( 1989)
found volcanic dust bands preserved in ablating glacial ice
in Antarctica containing up to 7.5 ppb Ir. The Ir levels correlated positively with chalcophile elements (i.e., Ar, Sb, Se)
in these volcanic dust bands, and elsewhere these chalcophile
elements have been associated with volcanism (OFFICER and
DRAKE, 1985 ). FELITSYNand VAGANOV(1988) found iridium content in Holocene ashes from Kamchatka increased
with distance from the source volcano, and iridium was most
abundant in the silt-sized fractions. In Scandinavia, we have
argued previously that labradorite and the Mg-smectite within
the K/T boundary layer at Stevns Klint, Denmark, are both
derived from pyroclastic basaltic volcanism, the former as a
primary phenocryst, the latter by alteration of basaltic glass
ash (ELLIOTT et al., 1989). The observations of enhanced
volatilization of Ir compounds during basaltic eruptions by
ZOLLER et al. ( 1983), OLMEZ et al. ( 1986), and TOUTAIN
and MEYER ( 1989), together with our evidence of basaltic
volcanism at the K/T boundary transition in Denmark, permit basaltic volcanism as a possible source of all or part of
the anomalous Ir seen at the K/T boundary in northwestern
Europe, and possibly elsewhere as well.
However, it is difficult to generalize from isolated modemday vent Ir analyses of aerosols (OLMEZ et al., 1986) and
vent condensates (TOUTAIN and MEYER, 1989) as to the
potential of a basaltic eruption to form a widespread Ir
anomaly in sedimentary rocks. In the research reported here,
we examine the best possible case we could readily find in
the rock record where such an Ir anomaly could have formed
from basaltic volcanism. In designing our research, we reasoned that should Ir and the other noble metals be preferentially enriched in volatiles emitted during basaltic volcanism, a special type of basaltic eruption would be required
to create a K/T boundary-type Ir anomaly of global proportions. Namely a significant fraction of the Ir in a very
large volume of basaltic magma would have to have been ( 1)
volatilized out of this huge reservoir and (2) globally distributed. The only way we can envision the latter is by insertion
of this large fraction of Ir into the stratosphere which we in
turn believe would require a major explosive eruption. This
is not commonly associated with the eruption of highly fluid
basaltic magma. Thus, this line of reasoning led us to investigate here the thick and widespread pyroclastic basaltic tuffs
of the Balder Formation, the largest pyroclastic basaltic episode of which we are aware.
The eruption of the Balder Formation tuffs was associated
with one of Earth’s major flood basalt sequences, the northern
North Atlantic B&o-Arctic Province. Such major explosive
eruptions would be caused by excessive build-up of volatiles
(perhaps exacerbated by water flooding the magma chamber)
and would result in the anomalously large emission of these
volatiles, including any volatile compounds of Ir and F. It is
conceivable that a magma chamber large enough to erupt a
flood basalt sequence might be capped by a large pocket of
Ir-enriched volatiles. During explosive eruption this Ir would
be co-erupted with the glassy ash as a separate volatized phase
as indicated by ZQLLERet al. ( 1983), TOUTAIN and MEYER
(1989), and OLMEZ et al. ( 1986). We also reasoned that
such a major basaltic explosive eruption that could inject Irenriched volatiles into the stratosphere also would be characterized by a major halo of enhanced Ir deposition proximally around the eruption site. Thus, even though the proximal regional areas received a large flux of basaltic tephra
diluting the co-erupted volatile Ir, the resulting deposit would
still be anomalously enriched in Ir on a per weight basis. Not
only would the resulting glassy basaltic ash tuffs be chemically
reactive, but so also would be the associated volatiles which
probably condensed on the ash surfaces. During and after
deposition, these metals could have migrated from the tuffs
during diagenesis. Thus, to test the contention of basaltic
volcanism having contributed markedly to the Ir llux at the
K/T boundary, we analyzed the Ir, Pt, Au, and Ag contents
not only in the relevant basaltic tuffs but also in the enclosing
strata of the Balder Formation.
THE BALDER FORMATION
Basaltic tuffs are rare in the rock record. However, two
early Tertiary basaltic tuffs occur in two different stratigraphic
associations in northwestern Europe: ( 1) the MO-clay of the
Fur Formation in northern Jutland, Denmark (NIELSENand
HEILMANN-CLAUSEN,1988; PEDERSENand SURLYK, 1983;
PEDERSENet al., 1975; and KNOX and MORTON, 1988), and
(2) the Balder Formation, North Sea ( MALM et al., 1984).
The MO-clay basaltic tuffs are highly weathered and enclosed
within diatomaceous earth. The Balder Formation basaltic
tuffs are found in the subsurface, enclosed in marine shales.
The Balder Formation is a prominent stratigraphic North
Sea marker due to its thickness and its distinct seismic and
well-log characteristics. Though penetrated by numerous oil
wells throughout the North Sea, the Balder Formation has
been cored in part in only two locations, wells 30 / 2- 1 and
25/ 10-l (Fig. 1). We utilized the core of STATOIL’s well
30/2-I where the tuff grain size and the number and thickness
of tuffs is greater and presumeably closer to the source, and
the enclosing shales are carbonaceous ( MALM et al., 1984).
The percentages of C,,, of the shales studied herein range
from 1.38- 1.80% (unpubl. data from authors) on a wholerock basis. We also chose to study core 30 /2- 1 because if the
noble metals diffused from the tuffs during diagenesis, then,
if not in the tuff themselves, they would have the best chance
of being preserved in the adjacent organic-rich black shales
( SCHMITZet al., 1988) or secondarily incorporated into the
clay minerals (ELLIOTT et al., 1989). For an opposing view,
espousing that the noble metals may be present in the insoluble residue as refractory particles rather than adsorbed on
organic material, see GILMOUR and ANDERS( 1989 ).
It is not well appreciated that the amounts of basalt expelled
during the latest Paleocene and earliest Eocene in the northern
North Atlantic region are comparable to the amounts expelled
in the formation of the Deccan Traps during the K/T
boundary transition. Most of the volume of these basalts are
in the sub-sea parts of the Greenland and Norway continental
shelves as well as the Rockall and Faero plateaus. The northern North Atlantic flood basalt activity rivaled that of the
Deccan Traps in volume, perhaps only about 30% smaller
(ROBERTSet al., 1984). Furthermore, like the Deccan Traps
(JAEGERet al., 1989), indications are the bulk of the northern
North Atlantic flood basalts erupted in a narrow span of time,
2951
Ir content of pyroclastic rocks in relation to K/T boundary
:
E&i
Ke
Predominantly
El
m
Bosolt
Flows
Bosolt
Sills or Flows
Known
Area of Pyroclostic
Trace
of Midoceon
Ridge
Ash Fall
FIG. 1. Pre-spreading distribution of Paleocene and Eocene basaltic flows, sills, and known areas of pyroclastic
basaltic ash falls presented by ROBERTS
et al. ( 1984). The basaltic ashes shown in the North Sea are those of the Balder
Formation. a nrominent
seismic marker. cored in wells 30/2-l and 25/ IO-1. The fine dots outline the continental
.
shelves and plateaus of the northern North Atlantic Ocean.
certainly less than a few million years and maybe less than
1 million years (ROBERTS et al., 1984). Thus, the Balder
Formation tuffs are likely to represent basaltic volcanism from
the deep mantle of the type most likely capable of transporting
high amounts of Ir to the surface.
Furthermore, ZOLLER et al. ( 1983) note the importance
of high F content as a criterion of the type of basaltic volcanism likely to produce high Ir levels. We do not know if
the upper Paleocene northern North Atlantic flood basalt
volcanism was of a type unusually rich in F emissions. However, Hekla, an active Icelandic volcano, is a modem representative of the same hot spot and resulting spreading system, and it is among the most prominent F-emitting basaltic
volcanoes that have been surveyed (ZOLLER et al., 1983).
The Balder Formation tuffs are associated with very extensive
deepmantle flood basalt eruptions that may have been rich
in F. Thus,. the analysis of noble metals in basaltic tuffs in
the Balder Formation, and, as importantly, the enclosing
shales rich in C,, deposited under anoxic conditions ( MALM
et al., 1984), is a reasonable test to see if significant Ir enrichments form in tuffs and enclosing rocks resulting from
explosive basaltic volcanism comparable in amount to that
occurring at the K/T boundary.
The core and its stratigraphic context have been described
completely by MALM et al. ( 1984). On the basis of biostratigraphy, MALMet al. ( 1984) correlate the main tuff sequence
seen in core 30/2- 1 with ( 1) the middle of the main North
Sea pyroclastic phase of JACQUE and THOUVENIN( 1975),
(2) subphase 2b of the Paleogene North Sea pyroclastic stratigraphy dated at about 53 m.y. ago (KNOX and MORTON,
1988). NIELSENand HEILMANN-CLAUSEN(1988) indicate
the Balder Formation is correlative with the Fur Formation
and that the main phase of the volcanism took place during
the Paleocene-Eocene transition. The 16-meter-long core be-
2958
W. C. Elliott, J. L. Aronson, and H. T. Millard Jr.
tween 1952-1968 meters deep is from the 47-meter-thick
main tuff zone of the Balder Formation. MALM et al. ( 1984)
estimate the main tuff zone has approximately 500 tuffs, all
basaltic, ranging in thickness from 0.0 1 to 10’s of centimeters.
The tuffs comprise 44% of the sediment volume. These tuffs
are now chlorite/smectite bentonites containing plagioclase
phenocrysts ( MALM et al., 1984; unpubl. studies by author).
were encapsulated, along with the flux monitors wires (2 cm lengths
of aluminum wire containing 980 ppm Co and 5.9 1 ppm Au), and
irradiated for 36 h in the central thimble of the United States Geological Survey TRIGA reactor (GSTR, thermal flux = 2.0 X 10 I3n/
cm*/sec, epithermal flux = 1.2 X lO’*n/cm*/sec). The capsule was
rotated during irradiation.
Following irradiation and four days of cooling, the quartz ampoules
were opened and the sample powders poured into zirconium crucibles
containing carriers for Ir, Au, and Ag (50 pg Ir, 25 pg Au, and 10
pg Ag). The mixture was fused with 5 g of sodium peroxide for 5
min and cooled. 1 g of lead oxide powder and 3 g of borax glass were
added, the mixture heated carefully to expel1 water, and then fused.
2 g of potassium cyanide powder were added in small portions over
a period of 15 min to reduce the lead to the metallic state. The reduced
lead collected the noble metals, then formed a drop. The crucible
was removed from the flame and the melt swirled as it cooled. The
lead drop was made to roll out of the melt just as the melt solidified
and the lead drop was poured into another zirconium crucible, where
it immediately froze. The resulting lead bead was flattened to form
a disk about 0.050 cm thick.
The lead disks, monitors in aluminum foil, and the weighed aluminum flux monitor wires were taped to aluminum planchets and
counted at 0.5 cm (6.0 cm for the aluminum wires) from an intrinsic
Ge gamma detector according to the following schedule and conditions given in Table I. The count rates are corrected for decay, pulse
METHODS
Samples of the Balder Formation tuffs and enclosing shales were
collected at 1962.64-1962.68 m (base of the thickest tutI’) and at
1968.0- 1968.1 m (the lowest cored tuff of the Balder Formation
unit). Basaltic tuffs and enclosing shales were collected at 1959.231959.27 m and at 1961.0-1961.3 m (Fig. 2). These tuffs and shales
and the ~0.1 p fraction of the K/T boundary at Nye IUov were
analyzed for Ag, Au, Ir, and Pt as described in the following text.
The Nye Klev sample served as a low-b standard.
The radiochemical procedure used to determine the noble metals
was adapted from that described by MILLARD( 1987). Powdered
samples were sealed in quartz ampoules. Monitors ( 1.348 pg Ag,
0.2111 pg Et, 0.001833 ng Ir, and 0.000973 pg Au) were prepared
by pipetting aliquots of standard solutions onto aluminum foil, evap
orating to dryness, and folding the foil. The samples and monitors
_--__-ElSample8
(m-1
1,-.23
AQ @pb)
0
1w
200
Ir @pB)
200
0
.l .2 .3 .4 .5
Au (I@)
0
2
4
6
310
___- G
_-----_-
c4
____--++++++++
++++++++
++++++++
c3
1959.25 ++++++++
++++
++++++++
++++++++
++++++++
++++ _______
---__-_
1,959.27 -:-:-I-I
c2
i5
-
1,961.00~
I-T-I---_- w
__-_
____
_______
----.
-_-___-_-____
__-_
_--_--l,gs1.05.
-1-I-I-I
__----_--ii
---++++++++ r2
++++++++ ++++++++
++++++++
++++++++
++++++++
:;:g:+
++++++++
++++++++
++++++++
1,961.10~++++++++,
++++++++
++++++++
:&:+
+++++++;++++++++
++++++++
Dl
:+;+z+;+
w-
FIG. 2. Distribution of Ag, Ir, and Au in the Balder Formation tuff-shale pairs at 1959.23-1959.27 m and at 1961.001961.13 m.
Ir content of pyroclastic rocks in relation to K/T boundary
TABLE1. Counting parameters
Gamma Energies
Nuclide
(keV)
Samples (decay times = 7 and 20 days, counting times = 4000 to
10000 set) and Monitors (decay times 7 and 20 days, counting
times = 1008 set):
Au (Au-198, t1,2= 2.70 days)
Pt (Au-199, tl,z = 3.14 days)
Ir (Ir-192, tllz = 74.2 days)
Ag (Ag-110 m, tl,2 = 252 days)
411.8
158.4
308.4, 316.5, 468.1
657.7, 937.5
Al Wires (decay time = 7 days, counting time = 200 set)
Au (Au-198, tllz = 2.70 days)
Co (Co-60, tl12= 5.26 years)
411.8
1332.5
Carrier Yields (decay time = 7 to 10 days, counting time = 200 to
4000 set):
Au (Au-198, t,,2 = 2.70 days)
Ir (Ir-192, tllz = 74.2 days)
Aa (Aa-110 m. t,,, = 252 davs)
Note: m-denotes
411.8
316.5
657.7. 937.5
metastable phase.
pileup, and the contribution to ‘99Aufrom Au (only i9’Au from Pt
is desired).
The rotation of the capsule during irradiation was found to eliminate the effect of the horizontal flux gradient and thus only the vertical
flux gradient requires further evaluation. The aluminum flux wires
were used to determine this correction. The count rates for the wires
were corrected for decay, pulse pileup, and counts per minute per
microgram (cpm/rcp) computed for Au and Co. These values were
then used to compute the thermal and epithermal neutron fluxes for
the lowest layer of samples in the capsule and the fluxes at the other
layers relative to the lowest layer.
Carrier yields for lr, Au, and Ag were determined by reirradiation
of the lead disks along with suitable reirradiation carrier monitors
prepared from the carrier solutions. The reirradiation lasted 5 min
and was performed in the rotating specimen rack of the GSTR. The
lead disks and monitors were taped to aluminum planchets and
counted at 0.5 cm from an intrinsic Ge gamma detector according
to the schedule in Table 1.The count rates were corrected for decay,
pulse pileup, and residual activity for the original irradiation and
carrier yields for Ir, Au, and Ag computed for each sample.
2959
The cpm/pg of Ir, Au, Et, and Ag were computed for each monitor;
these were corrected for the vertical flux gradient, and weighted averages of the corrected cpm/pg were calculated. The concentrations
of Ir, Au, Et, and Ag in each sample were computed using the averaged
monitor cpm/pg values and these were then corrected for the vertical
flux gradient and the carrier yields. Weighted averages for the concentration of each element in each sample were computed for the
various peaks for each nuclide and the several countings of each
sample.
RESULTS
The concentrations of Ag, Ir, Et, and Au in the tuffs and
enclosing shales are summarized in Table 2, and the variations
of Ag, Ir, and Au in shale-tuff pairs are shown in Fig. 2. As
listed in Table 2, the concentrations of Ir in both tuffs and
shales range from 0.102-0.257 ppb. These concentrations
are an order of magnitude lower than the Ir levels we and
others have measured from a K/T boundary with low-Ir levels
(e.g., Nye Klev, 1.85 ppb on a whole rock basis), and these
concentrations are more than two orders of magnitude lower
than the levels we and others have measured at K/T boundaries with high-b levels like Stevns Klint (e.g., 5.3-61 ppb
Ir, Table 2 ) . The Ir levels for the Balder Formation tuffs and
shales listed in Table 2 would not be considered to be anomalous. They are comparable to the Ir levels in basalts measured
from Kilauea, Hawaii (analysis by C. Orth in OLMEZ et al.,
1986) and from Reunion Island ( TOUTAIN and MEYER,
1989). However, the Ir concentrations would be considered
enriched relative to the levels of Ir measured in volcanic ashes
across the K/T boundary in Montana, USA, where the concentrations were co.04 ppb Ir (B. Schmitz, pers. commun.,
1992) in basalts and intertrap sediments from Deccan Traps
(ROCCHIA et al., 1988) and in basalts from the Columbia
River ( GANAPATHY, 1980).
The concentrations of Ag and Au are more variable ranging
from 52.5-28 1 ppb (Ag) and 0.82-8.43 ppb (Au). These
levels are within an order of magnitude of those from the K/
T boundary at Nye Klov, and they are an order of magnitude
lower than the concentrations of Au and Ag at Stevns Klint
(ELLIOTT et al., 1989). Moreover, Balder Formations shales
(samples Cl-C2, C4-C5, D3D4) contain more Ag and Au
than the basaltic tuffs (Fig. 2).
TABLE2. Concentrations of platinum group metals and silver in the Balder Formation
Sample
ID
A 0)
B (t)
Cl (sh)
C2 (sh)
C3 (t)
C4 (sh)
c5 (shj
Dl (t)
D2 (t)
D3 (sh)
D4 (sh)
Depth (m)
Ag* r CV*
Au*, CV
1962.64-1962.71
1968.0-1968.1
1959.23-1959.27
1959.23-l 959.27
1959.23-1959.27
1959.23-1959.27
1959.23-1959.27
1961.0-1961.13
1961.0-1961.13
1961.0-1961.13
1961.0-1961.13
108.4 + 2%
52.4 f 1.5%
199.8 ? 1.2%
212.7 + 1.0%
106.8 -+ 1.3%
179.9 + 1.6%
214.5 + 0.8%
75.3 f 1.4%
100.8 + 1.1%
179.6 * 0.1%
281.6 + 3.3%
4.23 + 6.9%
1.53 f 8.5%
8.43 + 8.2%
7.06 f 7.6%
2.07 f 7.5%
7.59 1- 7.5%
5.81 f 7.6%
0.82 + 5.8%
1.46 f 8.2%
4.26 & 9.7%
5.15 + 7.2%
257
0.69
1.86
4.1
49
35-3830
0.83
1.6-101
2.01
5.3-61
CO.1
<4-<250
Nye Klev (K/T boundary)’
Nye KIov (K/T boundary, ~0.1 pm)
this study
Stevns Klint (K/T boundary)’
Notes: (t)-tuff;
(sh)-shale,
*-Concentration
in ppb, CV-Coefficient
of variation, ‘-Data
If, cv
0.134
0.175
0.221
0.130
0.249
0.257
0.183
0.102
0.128
0.139
0.105
+
*
+
-t
f
f
+
f
+
+
f
3.8%
2.0%
4.0%
3.7%
4.2%
1.9%
1.9%
6.2%
2.3%
3.2%
12.4%
from Elliott et al. (1989).
Et*
co.5
co.2
co.9
~0.8
co.2
~0.8
~0.6
<o. 1
co.2
<OS
~0.6
2960
W. C. Elliott, J. L. Aronson, and H. T. Millard Jr.
INTERPRETATION
AND DISCUSSION
Iridium is present in the tuffs and the enclosing shales in
the Balder Formation at concentrations well below that seen
at the K/T boundary in Denmark, especially that at Stevns
Klint, a K/T boundary rich in C,,. Silver and gold are present
in the tuffs and enclosing shales at levels comparable to the
low end of the range of concentrations of these metals seen
at the K/T boundary in Denmark (Table 1) .
According to our reasoning, the Balder Formation at well
30/2- 1 was a good location to look for an enhanced Ir anomaly on a per weight basis. However, no anomaly was found
in either the tuffs or the shales. The 47-meter-thick main tuff
sequence of well 30/2-l and associated black shales of the
Balder Formation, which are probably themselves largely
tuffaceous, actually do contain approximately 1800 ng Ir/
cm*. If 5% of that Ir is a volatilized component that rained
out globally from stratospheric injection and the eruptive
sequence represents a small amount of time, then a global Ir
anomaly could have formed on the scale of the K/T boundary
Ir anomaly. Not knowing the Ir concentration in the parent
magma, it is possible that the subanomalous concentrations
(0.1-0.257 ppb Ir) seen in the tuffs and shales at well 30/21 are enrichments and indeed do represent such a proximal
halo.
It is interesting to note that the black shales enclosing these
tuffs are as enriched in Ir as the tuffs. They have slightly
higher levels of Au and Ag relative to the tuffs. Thus, Ir enrichment is not a function of it being in either shale or basaltic
tuff. Both lithologies are argillaceous. It seems likely that the
shales represent an admixture of reworked tuff possibly diluted with epiclastic clay. The simplest explanation for the
comparable content of Ir in the shales is that the shales are
dominantly tuffaceous or that Ir has been mobilized during
diagenesis and migrated from the tuffs to the shales.
Lastly, we note that the Ir levels of the Deccan Trap basalts
and intertrap sediments are markedly low ( ROCHIA et al.,
1988) relative to the Ir levels of the Reunion Island (TOUTAIN
and MEYER, 1989), Kilauea basalts (OLMEZ et al., 1986),
and Balder Formation basalts and enclosing shales listed in
Table 2. OLMEZ et al. ( 1986) suggest Ir may have outgassed
from the Deccan Traps magma to form the global Ir anomaly
at the K/T boundary. Our results too indicate that a sizeable
amount of Ir was released during the eruption of the Balder
Formation basal& and that subanomalous Ir levels are present in proximal basaltic tuffs and the enclosing shales. The
absence of even a subanomalous level of Ir in the Deccan
Traps basalts is curious. Either all the measurable Ir was volatilized (e.g., OLMEZet al., 1986) or the Deccan Traps basalts
were possibly formed from an h-poor magma.
CONCLUSIONS
The Paleocene/Eocene Balder Formation of the North Sea
represents explosive basaltic activity that emitted sizeable
amounts of iridium. The basaltic tuffs and enclosing shales
of the Balder Formation in well 30 / 2- 1 on a per-weight basis
have only subanomalous amounts of Ir (0.1-0.25 ppb Ir),
and the Ir concentrations in the shales are not markedly different from the Ir concentrations in the tuffs. Yet because of
the great thickness of the tuff, and their relatively high con-
tents of Ir, a large absolute amount of Ir was deposited over
the time span of their eruption ( - 1800 ng/cm2) but greatly
diluted by the continuous deposition of tuff and shale. We
hypothesized that the proximal areas adjacent to a major
explosive eruption of basaltic magma would not only receive
large amounts of tephra but also large amounts of the coerupted h-rich condensed vapor, aerosols, and particulates.
Therefore, we expected, but did not find, a major per-weight
Ir anomaly at a proximal location for these major eruptions
accompanying the opening of the northern North Atlantic
Ocean. Thus, the test which we regarded as the best case was
negative.
These results do not entirely eliminate basaltic volcanism
as the cause of a global Ir anomaly at the K/T boundary.
Conceivably, an eruption could segregate the h-rich cap of
volatiles from the tephra by having shot the volatile phase
strongly and directly upward into the stratosphere while the
bulk of the magmatic tephra fell proximally to the vent. Our
intuition dictates against the likelihood of such an eruption.
Acknowledgments-We thank the staff of the USGS TRIGA reactor
for performing the irradiation. We also thank S. N. Ehrenberg and
K. A. Gibbons of STATOIL for allowing us to examine and collect
core samples from well 30/2-l. Linda J. Abel and Karen S. Wehn
drafted the figures, and P. A. Dove typed the tables. We also thank
B. Schmitz, C. 0. Officer, and an anonymous reviewer for helpful
comments. This studv was funded bv award #EAR 89-16473 from
the National Science Foundation to W. C. Elliott and J. L. Aronson.
Case Western Reserve University Geology Department Contribution
#185.
Editorial handling: G. Faure
REFERENCES
ALVAREZW. ( 1986) Toward a theory of impact crises. Eos 67,649658.
ALVAREZL. W., ALVAREZW. L., ASAROF., and MICHELH. ( 1980)
Extraterrestrial cause for the Cretaceous/Tertiary extinction. Science 208, 1095-l 108.
BOHOR B. F., FOORD E. E., MODRESKIP. J., and TRIPLEHORN
D. M. ( 1984) Mineralogical evidence for an impact event at the
Cretaceous/Tertiary boundary. Science 224, 867-869.
BOHORB. F., MODRESKIP. J., and FOORD E. E. ( 1987) Shocked
quartz in the Cretaceous/Tertiary boundary clays: Evidence for a
global distribution. Science 236, 705-709.
CROCKETJ. H.. OFFXER C. B.. WEZELF. C., and JOHNSONG. D.
( 1988) Distribution of noble metals across the Cretaceous/Tertiary
boundary at Gubbio, Italy: Iridium variation as a constraint on
the duration and nature of Cretaceous/Tertiary boundary events.
Geology 16,77-80.
ELLIOTTW. C., ARONSONJ. L., MILLARDH. T., and GIERLOWSKIKORDESCHE. ( 1989) The origin of the clay minerals at the Cretaceous/Tertiary boundary in Denmark. GSA Bull. 101,702-7 10.
FELITSYNS. B., VAGANOVP. A. (1988) Iridium in the ash of Kamchatkan volcanoes. Intl. Sci. Rev. 30, 1288- 129 1.
FINNEGAND. L., ZOLLERW. H., and MILLERT. M. ( 1989) Iridium
emissions from Hawaiian volcanoes (abstract). Global Catastrophes in Earth History; LPI Contrib. 673, 48. Lunar Planetary Institute.
FITCH F. J. and HURFORDA. J. ( 1973) Fission track dating of the
Tardee Rhvolite. Co. Antrim Proc. Geol. Assoc. 88, 267-284.
GANAPATHY~. ( 1980) A major meteorite impact on the earth 65
million years ago. Science 209,92 l-923.
GILMOURI. and ANDERSE. ( 1989) Cretaceous/Tertiary boundary
event: Evidence for a short time scale. Geochim. Cosmochim. Acta
s3,503-5 1I.
GRAUPG., SPETTELB., HERM D., and WEIDICHK. F. ( 1989) Mineralogy and phase chemistry of an b-enriched pie-K/T layer from
Ir content of pyroclastic rocks in relation to K/T boundary
the Lattengebirge, Bavarian Alps, and significance for the KTB
problem. Earth Planet. Sri. Lett. 95,271-290.
HALLAMA. ( 1987) End-Cretaceous Mass Extinction Event: Argument for Terrestrial Causation. Science 238, 1237- 1242.
HUFFMANA. R., CROCKETJ. H., CARTERN. L., BORELLAP. E.,
and OFFICERC. B. ( 1990) Chemistry and mineralogy across the
Cretaceous/Tertiary boundary at DSDP Site 527, Walvis Ridge,
South Atlantic Ocean in Global Catastrophes. In Earth History:
An InterdisciplinaryConference on Impacts, Volcanismand Mass
Mortality(ed. V. L. SHARFTONand P. D. WARD); GSA Special
Paper 247, 3 19-334.
JACQUE M. and THOUVENINJ. ( 1975) Lower Tertiary Tuffs and
volcanic activity in the North Sea: Petroleum and Continental
Shelf of Northwest Europe. Appl. Sci., 455-465.
JAEGERJ., CURTILLOTV., and TAPPONNIER( 1989) Pateontological
view of the ages of the Deccan Traps, the Cretaceous/Tertiary
Boundary, and the India Asia collision. Geology 17, 3 16-3 19.
KASTNERM., ASAROF., MICHELH., ALVAREZW., and ALVAREZ
L. ( 1984) The precursor of the Cretaceous/Tertiary boundary clay
at Stevns Klint, and DSDP Hole 465A. Science 226, 137-143.
KNOXR. W. O’B. and MORTONA. C. ( 1988) The record of earlv
Tertiary N Atlantic volcanism in sediment in the North Sea Basin.
In Early Tertiary Volcanismand the Opening of the NE Atlantic
(ed. A. C. MORTON and L. M. TROTMAN), GSA Special Publ.
No. 39, 407-419.
KOEBERLC. ( 1989) Iridium enrichment in volcanic dust from blue
ice fields, Antarctica, and possible relevance to the K/T boundary
event. Earth Planet. Sci. Lett. 92, 3 17-322.
MALM 0. A., CHRISTENSEN0. B., FURNESH., ~VLIE R., RUSELATTENH., and LORANGEOSTBYK. ( 1984) The Lower Tertiary
Balder Formation: An organogenic and tuffaceous deposit in the
North Sea region. In Petroleum Geologyof the North Sea Margin
(ed. GRAHAMand L. M. TROTMAN);pp: 149-170.
MCCARTNEYK. and LOPERD. E. ( 1989 ) Emeraence of a rival oaradigm to account for the Cretaceous/Tertiary Event. J. Geol. E&c.
37,36-48.
MILLARDH. T., JR. ( 1987) Neutron Activation determination of
iridium, gold, platinum, and silver in geologic samples. J. Radiochem. Nucl. Chem. 113, 125-132.
NIELSEN0. B. and HEILMANN-CLAUSEN
C. ( 1988) Palaeogene vol-
2961
canism: The sedimentary record in Denmark. In Tertiary Voicanism and the Opening of the NE Atlantic(ed. A. C. MORTONand
L. M. TROTMAN);GSA Special Publ. No. 39, 395-405.
OFFICERC. B. and DRAKE C. L. ( 1985) Terminal Cretaceous Environmental Events. Science 227, 116 I- 1167.
OFFICERC. B., HALLAMA., DRAKEC. L., and DEVINEJ. D. ( 1987)
Late Cretaceous and paroxysmal Cretaceous/Tertiary extinctions.
Nature326, 143-149.
OLMEZI., FINNEGAND. L., and ZOLLERH. H. ( 1986) Iridium emissions for Kiluea volcano. J. Geophys.Res. 91, 7 18l-7191.
PEDERSENA. K. and SURLYKF. ( 1983) The Fur Formation, a late
Paleocene ash-bearing diatomite from northern Denmark. Bull.
Geol. Sot. Denmark 32,43-65.
PEDERSENA. K.. ENGELLJ.. and RONS& J. G. ( 1975) Earlv Tertiarv
in the Skagarrak, new chemical evidence from ash layers in the
ma-clay of northern Denmark. Lithos 8,255-268.
RENNE P. R. and BASU A. R. ( 199 1) Rapid Eruption of Siberian
Traps Flood basalts at the Permo-Triassic Boundary. Science 253,
176-179.
ROBERTSD. G., MORTONA. C., and BACKMANJ. ( 1984) Late Paleocene-Eocene volcanic events in the northern north Atlantic
ocean. Initial Reports of DSDP Project81,9 13-923.
ROCCHIAR., RENARDM., BOCLETD., and BONTEP. ( 1984) ESSAI
d’evolution de la duree de la transition Cretace/Tertiare par l’evolution de l’anomalie en iridium; implication dans la recherch de
la caum de la ctise biologique. Bull. Sot. Geoi. France 7, 11931202.
ROCCHIAR., BOCLETD., COURTILLOTV., and JAEGERJ. ( 1988) A
search for iridium in the Dcccan Traps and inter traps. Geophys.
Res. Lett. 15, 812-815.
SCHMITZB., ANDERSSONP., and DAHL J. ( 1988) Iridium, sulfur
isotopes and rare earth elements in the Cretaceous-Tertiary
boundary clay at Stevns Klint, Denmark. Geochim. Cosmochim.
Acta 52,229-232.
TOUTAINJ. P. and MEYERG. ( 1989) Iridium-bearing sublimates at
a hot-spot volcano (Piton de la Foumaise, Indian Ocean). Geophys.
Res. Lett. 16, 1391-1394.
ZOLLERW. H., PARINGTONJ. R., and PHELANKOTRAJ. M. ( 1983)
Iridium enrichment in airborne particles from Kiluea Volcano.
Science222, 1118-1121.