The Cause
of Anomalous Potassium-Argon “Ages” for Recent Andesite Flows at Mt. Ngauruhoe,
New Zealand, and the Implications for Potassium-Argon “Dating”
Andrew A. Snelling, PhD,
Answers in Genesis,
PO Box 6302, Acacia Ridge, DC, Qld, 4110, Australia.*
*current address:
Answers in Genesis, PO Box 510, Hebron, Kentucky, 41048, USA.
Presented at the Fourth International Conference
on Creationism, Pittsburgh, Pennsylvania, August 3–8, 1998. Published in: Proceedings
of the Fourth International Conference on Creationism, R. E. Walsh (editor), pp. 503–525.
© 1998 Creation Science Fellowship, Inc., Pittsburgh,
Pennsylvania, USA. All Rights Reserved.
Abstract
New Zealand’s newest and most active volcano, Mt. Ngauruhoe in
the Taupo Volcanic Zone, produced andesite flows in 1949 and 1954, and avalanche
deposits in 1975. Potassium-argon “dating” of five of these flows and deposits yielded
K-Ar model “ages” from <0.27Ma to 3.5±0.2 Ma. “Dates” could not be reproduced,
even from splits of the same samples from the same flow, the explanation being variations
in excess 40Ar* content. A survey of anomalous K-Ar “dates” indicates they are common,
particularly in basalts, xenoliths, and xenocrysts such as diamonds that are regarded
as coming from the upper mantle. In fact, it is now well established that there
are large quantities of excess 40Ar* in the mantle, which in part represent primordial
argon not produced by in situ radioactive
decay of 40K and not yet outgassed. And there are mantle-crust domains between,
and within, which argon circulates during global tectonic processes, magma genesis,
and mixing of crustal materials. This has significant implications for the validity
of K-Ar and 40Ar/39Ar “dating.”
Keywords
Andesite, 1949–1975 Flows, Mt. Ngauruhoe, New Zealand, Potassium-Argon
Dating, Anomalous Model “Ages,” Excess 40Ar*, Excess 40Ar* in Rocks and Minerals,
Upper Mantle, Geochemical Reservoirs, Mantle-Crust Domains, Crustal Mixing, Magma
Genesis
Introduction
Mt. Ngauruhoe is an andesite stratovolcano of 2291m elevation,
rising above the Tongariro volcanic massif within the Tongariro Volcanic Center
of the Taupo Volcanic Zone, North Island, New Zealand (Figure 1) (Graham & Hackett,
1987; Williams, 1994). Though not as well publicized as its neighbor, Mt, Ruapehu
(about 12km to the south), Ngauruhoe is an imposing, almost perfect cone that rises
more than 1000m above the surrounding landscape. Eruptions from a central 400m diameter
crater have constructed the steep (33°), outer slopes of the cone (Hackett &
Houghton, 1987; Williams).
Geologic Setting
The Taupo Volcanic Zone, a volcanic arc and marginal basin of
the Taupo-Hikurangi arc-trench (subduction) system (Cole & Lewis, 1981), is
a southward extension on the Tonga-Kermadec arc into the continental crustal environment
of New Zealand’s North Island. It has been interpreted as oblique subduction of
the Pacific plate beneath the Australian plate. The zone extends approximately 300km
north- northeast across the North Island from Ohakune to White Island (Figure 1)
and is up to 50km wide in the central part, narrowing northwards and southwards.
This volcano-tectonic depression (Taupo-Rotorua depression (Grindley, 1960) comprises
four rhyolitic centers (Rotorua, Okataina, Maroa, and Taupo), plus the calc-alkaline
Tongariro Volcanic Center, part of a young (<0.25Ma) andesite-dacite volcanic
arc with no associated rhyolitic volcanism extending along the eastern side of the
zone (Graham & Hackett, 1987).
The Tongariro Volcanic Center extends for 65km south-southwest from Lake Taupo at the
southern end of the Taupo Volcanic Zone (Figure 1) and consists of four large predominantly
andesite volcanoes Kakaramea, Pihanga, Tongariro, and Ruapehu (Figure 2); two smaller
eroded centers at Maungakatote and Hauhungatahi; a satellite cone and associated
flows at Pukeonake and four craters at Ohakune (Figure 2) (Cole, Graham, Hackett,
& Houghton, 1986; Hackett & Houghton, 1987).
Most vents lie close to the axis of a large graben in which
Quaternary volcanic rocks overlie a basement of Mesozoic greywacke and Tertiary
sediments (Gregg, 1960; Nairn & Wood, 1987). North-northeasttrending normal
faults with throws up to 30 m cut the volcanoes within the graben. Nearly all
vents active within the last 10 ka lie on a gentle arc which extends 25 km
north-northeast from the Rangataua vent on the southern slopes of Ruapehu
through Ruapehu summit and north flank vents, Tama Lakes, Ngauruhoe, Red Crater,
Blue Lake, and Te Mari craters. None of the young vents lie on the mapped
faults, which mostly downthrow towards the axis of the graben. The vent lineation
lies above this axis, which is considered to mark a major basement fracture
(Gregg, 1960; Grindley, 1965; Nairn & Wood) that allows the intrusion of
andesite dikes. The Tongariro volcanics unconformably overlie late Miocene
marine siltstones beneath Hauhungatahi, and a minimum age for the onset of
volcanism is measured by the influx of andesite pebbles in early Pleistocene
conglomerates of the Wanganui Basin to the south (Cole et al., 1986; Grindley,
1965). The oldest dated lavas from the Tongariro massif are hornblende andesites
exposed at Tama Lakes between Ngauruhoe and Ruapehu, at 0.26±0.003 Ma; from
Ruapehu, 0.23±0.006 Ma; and from Kakoramea, 0.22±0.001 Ma (potassium-argon
dates) (Stipp, 1968).
Tongariro itself is a large volcanic massif that consists of
at least twelve composite cones, the youngest and most active of which is
Ngauruhoe. A broad division has been made into older (>20 ka) and younger
(<20 ka) lavas (Cole et al., 1986; Topping, 1973). There is a
north-northeast alignment of the younger vents of Tongariro, particularly
evident between Te Mari and Ngauruhoe.
Ngauruhoe
Ngauruhoe is the newest cone of the Tongariro massif and has
been active for at least 2.5 ka (Grindley, 1965; Nairn & Wood, 1987;
Williams, 1994). It has been one of the most active volcanoes in New Zealand, with
more than seventy eruptive episodes since 1839, when the first steam eruption
was recorded by European settlers (Gregg, 1960; Nairn & Wood; Williams).
Prior to European colonisation the Maoris witnessed many eruptions from the
mountain (Gregg, 1960). The first lava eruption seen by European settlers
occurred between April and August 1870, with two or three flows witnessed
spilling down the north-western flanks of the volcano on July 7 (Gregg, 1960;
Nairn & Wood). Following that event there have been pyroclastic (ash)
eruptions every few years (Nairn & Wood), with major explosive activity in April–May
1948.
The next lava extrusion was in February 1949, beginning suddenly
with ejection of incandescent blocks, and a series of hot block and ash flows down
the north-western slopes on February 9 (Gregg, 1960; Nairn & Wood, 1987). The
southern sub-crater filled with lava, which by late on February 10 had flowed over
the lowest part of the rim and down the north- west slopes of the cone. By February
12 the flow had ceased moving, subsequent mapping placing its volume at about 575,000m3 (Figure 3)
(Battey, 1949; Gregg, 1960). Further explosive pyroclastic (ash) eruptions followed,
reaching a maximum about February 1921. The eruptions ended on March 3.
The eruption from May 13, 1954 to March 10, 1955 began with explosive
ejection of ash and blocks, although red-hot lava had been seen in the crater five
months previously (Gregg, 1960; Nairn & Wood, 1987). The eruption was remarkable
for the estimated large volume of almost 8 millionm3 of lava that then flowed from
the crater from June through September 1954, and was claimed to be the largest flow
of lava observed in New Zealand (that is, by the European settlers) (Gregg, 1960;
Williams, 1994). The lava was actually expelled from the crater in a series of seventeen
distinct flows on the following dates (Gregg, 1956; 1960):
June 4, 30
July 8, 9, 10, 11, 13, 14, 23, 28, 29, 30
August 15 (?), 18
September 16, 18, 26
Figure 3 shows the distribution of those 1954 lava flows that
are still able to be distinguished on the north-western and western slopes of
Ngauruhoe.
All flows were of aa lava (as was the February 1949 flow),
typified by rough, jagged, clinkery surfaces made up of blocks of congealed
lava. The lava flows were relatively viscous, some being observed at close quarters
slowly advancing at a rate of about 20 cm per minute (Gregg, 1956, 1960;
Williams, 1994). The August 18 flow was more than 18 m thick and still warm
almost a year after being erupted. Intermittent explosive eruptions and
spectacular lava fountaining during June and July 1954 built a
spatter-and-cinder cone around the south sub-crater, modifying the western
summit of the mountain. Activity decreased for two months after the last of the
lava flows on September 26, but increased again during December 1954 and
January 1955 with lava fountaining and many highly explosive pyroclastic (ash)
eruptions. The last ash explosion was reported on March 10, 1955, but red-hot
lava remained in the crater until June 1955 (Gregg, 1956, 1960).
After the 1954–1955 eruption, Ngauruhoe steamed semi-continuously,
with numerous small eruptions of ash derived from comminuted vent debris. Incandescent
ejecta were seen in January 1973, and ash erupted in December 1973 contained juvenile
glassy andesite shards (Nairn & Wood, 1987). Cannon-like, highly explosive eruptions
in January and March 1974, the largest since 1954–1955, threw out large quantities
of ash and incandescent blocks, one of which was reported as weighing 3000 tonnes
and thrown 100m (Nairn & Wood; Williams, 1994). Pyroclastic avalanches flowed
from the base of large convecting eruption columns, down the west and north slopes
of the cone, and the crater became considerably shallower (Nairn, Hewson, Latter,
& Wood, 1976; Nairn & Wood).
A series of similar but more violent explosions occurred on February
19,
1975, accompanied
by clearly visible atmospheric shock waves and condensation clouds (Nairn, 1976;
Nairn & Self, 1978; Nairn & Wood, 1987; Williams, 1994). Ash and blocks
up to 30m across were ejected and scattered within a radius of 3km from the summit.
The series of nine, cannon-like, individual eruptions followed a 1.5 hour period
of voluminous gas-streaming emission, which formed a convecting eruption plume between
11km and 13km high (Nairn & Self; Nairn
& Wood; Williams, 1994). The explosions took place at 20–60
minute intervals for more than five hours. Numerous pyroclastic avalanches were also
generated by fallback from the continuous eruption column, the avalanches consisting
of a turbulent mixture of ash, bombs, and larger blocks which rolled swiftly down
Ngauruhoe’s sides at about 60km per hour (Nairn & Wood; Williams). The deposits
from these avalanches and the later explosions accumulated as sheets of debris in
the valley at the base of the cone, but did not extend beyond 2km from the summit.
It is estimated
that a minimum bulk volume of 3.4 millionm3 of pyroclastic material
was erupted in the seven-hour eruption sequence on that day (Nairn & Self).
Figure 3 shows the location of these avalanche deposits.
There have been no eruptions since February 1975. A plume of
steam or gas is still often seen above the summit of the volcano, as powerful fumaroles
in the bottom of the crater discharge hot gases. However, the temperature of these
fumaroles in the crater floor has steadily cooled significantly since 1979, suggesting
that the main vent is becoming blocked.
Sample Collection
Fieldworkandcollectionofsampleswasundertaken in January 1996.
The Ngauruhoe area was accessed from State
Highway 47 via Mangateopopo Road. From the parking area at the end of the
road, the Mangateopopo Valley walking trail
was followed to the base of the Ngauruhoe cone, from
where the darker-colored recent lava flows were clearly visible and each one easily
identified on the north-western slopes against the lighter-colored older portions
of the cone (Figure 3).
Eleven 2–3kg samples were collected two each from the February
11, 1949, June 4, 1954, and July
14, 1954 lava flows and from the February 19, 1975 avalanche deposits,
and three from the June 30, 1954 lava flows. The sample locations are marked on Figure
3. Care was taken to ensure correct identification of each lava
flow and that the samples collected were representative of each flow and any variations
in textures and phenocrysts in the lavas.
Laboratory Work
All samples were sent first for sectioning one thin section from
each sample for petrographic analysis. A set of representative pieces from each
sample (approximately 100g) was then despatched to the AMDEL Laboratory in Adelaide,
South Australia, for whole-rock major, minor and trace element analyses. A second
representative set (50–100g from each sample) was sent progressively to Geochron
Laboratories in Cambridge (Boston), Massachusetts, for whole-rock potassium-argon
(K-Ar) dating first a split from one sample from each flow, then a split from the
second sample from each flow after the first set of results was received, and finally,
the split from the third sample from the June 30, 1954 flow.
At the AMDEL Laboratory each sample was crushed and pulverized.
Whole-rock analyses were undertaken by total fusion of each powdered sample and
then digesting them before ICP-OES for major and minor elements, and ICP-MS for
trace and rare earth elements. Fe was analyzed for amongst the major elements by
ICP-OES as Fe2 O3 and reported
accordingly, but separate analyses for Fe as FeO
were also undertaken via wet chemistry methods. The detection
limit for all major element oxides was 0.01%. For minor and trace elements the detection
limits varied between 0.5 and 20ppm, and for rare earth elements between 0.5 and
1 ppm.
The potassium and argon analyses were undertaken atGeochronLaboratoriesunderthedirectionofRichard
Reesman, the K-Ar laboratory manager. No specific location or expected age information
was supplied to the laboratory. However, the samples were described as andesites
that probably contained “low argon” and therefore could be young, so as to ensure
the laboratory took extra care with the analytical work.
|
Petrography and Chemistry
Clark (1960) reported that most of the flows from Ngauruhoe are
labradorite-pyroxene andesite with phenocrysts of plagioclase (labradorite), hypersthene
and rare augite in a hyalopilitic (needle-like microlites set in a glassy mesostasis)
groundmass containing abundant magnetite. However, all lava, lapilli, and incandescent
blocks that have been analyzed from eruptions this century also contain olivine;
chemically they may be classed as low-silica (or basaltic) andesites (using the
classification scheme of Gill, 1981). The published analyses in Table 1 show only
trivial changes in composition between 1928 and 1975. In fact, the 1954 and 1974
andesites are so similar that Nairn et al. (1976) suggested that a solid plug of
1954-andesite was heated to incandescence and partially remobilized on top of a
rising magma column in 1974. This plug was disrupted and blown from the vent as
ejecta ranging in texture from solid blocks, through expanded scoria to spatter
bombs.
Table 2 lists the whole-rock major element analyses of the eleven
samples collected in this study. Comparison of the data for each flow with the corresponding
data in Table 1 indicates that in their bulk chemistries all the samples analyzed
(and thus all the flows) are virtually identical to one another, the trivial differences
being attributable to the statistics of analytical errors, sampling and natural
variations. Thus it is not unreasonable to conclude that these basaltic andesites
are cogenetic, coming from the same magma and magma chamber, even as they have been
observed to flow from the same volcano.
Nevertheless, Nairn et al. (1976) suggested that even though
the 1949 and 1954 lavas were both olivine-bearing andesite, the chemical analyses
(Table 1) showed the 1954 lava to be slightly more basic than the 1949 lava, with
slightly higher MgO, CaO and total iron oxides, but lower SiO2 and alkalis.
However, these trends are not duplicated with any statistical significance by
the analytical results of this study (Table 2). At least they found that their
analyses of the 1974 lava blocks and bombs were identical within the limits of
error with the 1954 lava (Table 1), which was also substantiated in this study
with respect to the 1975 avalanche material and the 1954 lava (Table 2).
Clark (1960) and Cole (1978) recognized five lava types in
the Tongariro Volcanic Center based on the modal proportions of the phenocryst
minerals. Graham (1985) modified this scheme to six types based on a combination
of mineralogy and chemistry, but given their uniform bulk chemistry and
petrology, these Ngauruhoe lava flows group together as plagioclasepyroxene andesite
within Graham’s “Type 1.” Cole et al. (1986) have described Type 1 lavas as
volumetrically dominant within the Tongariro Volcanic Center and as exhibiting
coherent chemical trends with increasing silica content. They are relatively Fe-rich
and follow a typical calc alkaline trend on the AFM diagram. Adapting the
terminology of Gill (1981), the Ngauruhoe lavas are described as basic
andesites (53–58 wt% SiO2) (Cole et al.). Their designation as
plagioclase-pyroxene andesites is based on the predominant phenocrysts present,
with plagioclase greater than or equal to pyroxene. Two modal analyses are
listed in Table 3 which very closely resemble the samples collected for this
study.
All samples of the five lava flows examined in this study
exhibited a porphyritic texture, with phenocrysts (up to 3 mm across)
consistently amounting to 35–40% by volume. The phenocryst assemblage is
dominated (2:1) by plagioclase, but orthopyroxene and augite (clinopyroxene)
are always major components, while olivine and magnetite are only present in
trace amounts. This POAM phenocryst assemblage is a typical anhydrous
mineralogy (Cole et al., 1986). The groundmass consists of microlites of
plagioclase, orthopyroxene, and clinopyroxene, and is crowded with minute
granules of magnetite and/or Fe-Ti oxides. Small amounts (9–10%) of brown transparent
(acid-residuum) glass are also present, and the overall texture is generally
pilotaxitic.
Steiner (1958) stressed that xenoliths are a common
constituent of the 1954 Ngauruhoe lava, but also noted that Battey (1949)
reported the 1949 Ngauruhoe lava was rich in xenoliths. All samples in this
study contained xenoliths, including those from the 1975 avalanche material.
However, many of these aggregates are more accurately described as
glomerocrysts and mafic (gabbro, websterite) nodules (Graham et al., 1995).
They are 3–5 mm across, generally have hypidiomorphic-granular textures, and
consist of plagioclase, orthopyroxene, and clinopyroxene in varying
proportions, and very occasionally olivine. The true xenoliths are often rounded
and invariably consist of fine quartzose material. Steiner also described much
larger xenoliths of quartzo-feldspathic composition and relic gneissic structure.
The plagioclase phenocrysts have been reported as ranging in
composition from An89 to An40 (andesine to bytownite), but in Ngauruhoe lavas
are usually labradorite (An68-55). They are subhedral and commonly exhibit
complex oscillatory zoning with an overall trend from calcic cores to sodic rims
(Cole, 1978; Cole et al. 1983, 1986). Thin outer rims are usually
compositionally similar to groundmass microlites. Twinning and hourglass
structures are common.
Orthopyroxene predominates (>2:1) over clinopyroxene.
Subhedral-euhedral orthopyroxene is typically pleochroic and sometimes zoned. Compositions
range from Ca4 Mg74 Fe22 to Ca3 Mg47 Fe50 (Cole et al. 1983, 1986), but
representative bulk and partial analyses of Ngauruhoe orthopyroxenes (Cole,
1978; Ewart, 1971; Graham & Hackett, 1987) indicate a hypersthene
composition predominates, which is confirmed by optical determinations (Clark, 1960;
Cole). Euhdral-subhedral clinopyroxene is typically twinned and zoned, but
compositions show a restricted range of Ca43 Mg47 Fe10 to about Ca35 Mg40 Fe25,
all of which is augite (Cole et al., 1986; Graham et al., 1995).
The olivine present is strongly magnesian, analyses indicating
some compositional zoning from Fo88 to Fo78. The magnetite present in the
groundmass is titanomagnetite, judging from the amount of TiO2 present in
whole-rock analyses (Tables 1 and 2), but some ilmenite is likely to occur
sporadically in association with it (Cole et al., 1986; Graham et al., 1995).
K-Ar
Results
All analytical results received from Geochron Laboratories are
listed in Table 4, grouped in chronological order according to the historic date
of each flow. The 40Ar* quantity refers to the amount of radiogenic 40Ar measured
in each sample. All other quantities are self-explanatory, some of them being calculated
from the analytical results supplied by the laboratory.
The “age” of each sample is calculated from the analytical results using the general model-age equation (Faure, 1986; Dickin, 1995):—
where:
t = the “age”
λ = the decay constant of the parent isotope
Dt = the number of daughter atoms in the rock presently
Do = the number of daughter atoms initially in the rock
Pt = the number of parent atoms presently in the rock
To date a rock, Dt and Pt are measured, and
Equation 1 can then be used if an assumption about the original quantity of
daughter atoms (Do) is made.
Applied specifically to K-Ar dating, Equation 1 thus becomes:-
where:
t = the “age” in Ma (millions of years)
5.543 × 10-10 = the current estimate for the decay
constant of 40K
0.1048 = the estimated
fraction of 40K decays producing 40Ar
40Ar*/40K = the
calculated mole ratio of radiogenic
40Ar to 40K in the sample
It should be noted that to make Equation 2 equivalent to Equation
1, 40Ar* is assumed to be equal to (D −D ), which thus means the 40Ar* measurement
has included within it an assumption concerning the initial quantity of 40Ar in
the rock, namely, no radiogenic argon is supposed to have existed when the rock
formed (that is, Do = 0). Thus Equation 2 yields a “model age” assuming
zero radiogenic argon in the rock when it formed.
The model ages listed in Table 4 range from <0.27Ma to
3.5±0.2Ma. However, it should be noted that the samples, one from each flow, that
yielded model ages of <0.27Ma and <0.29Ma (that is, below the detection limits
of the equipment for 40Ar*) were all processed at the K-Ar laboratory in the same
batch, suggesting the possibility of a systematic problem with the analytical procedure
and equipment (in particular, the gas extraction “line”). When this question was
raised with the laboratory manager, Richard Reesman, he kindly rechecked his equipment
and then re-ran several of the samples, producing similar results and thus ruling
out a systematic laboratory “error.”
However, an independent blind check was then made, by submitting
to the K-Ar laboratory duplicate splits from two samples already analyzed, to establish
if results really were reproducible. The samples chosen were the A and B samples
of the June 30, 1954 flow, because their first splits had produced the lowest and
highest model ages, <0.27Ma and 3.5±0.2Ma respectively. The results of these
additional analyses are shown in Table 4 as A#2 and B#2, and yielded model ages
of 1.3±0.3Ma and 0.8±0.2Ma respectively. Clearly, reproducibility was not obtained,
but this is not surprising given the analytical uncertainties at such low to negligible
levels of 40Ar*, which are at the detection limits of the laboratory's equipment
(Reesman, 1997, 1998).
Discussion
In spite of the wide variations in model “ages” obtained between
and within these recent lava flows, and of the difficulties obtaining analytical reproducibility,
it is apparent that the cause of the anomalous K-Ar model “ages” is excess argon
in the lavas, that is, non-zero concentrations of radiogenic argon (40Ar*). This
of course is contrary to the assumption of zero radiogenic
argon in Equation
2 for calculating the model “ages.” When analyzed the oldest of the lavas was less
than 50 years old, so there has been insufficient time since cooling for measurable
quantities of 40Ar* to have accumulated within the lavas due to the slow radioactive
decay of 40K. Thus the measurable 40Ar* can’t be from in situ radioactive decay
since cooling, and therefore must have been present in the molten lavas when extruded
from Mt. Ngauruhoe.
No Radiogenic Argon Assumption Violated by Many Anomalous “Ages”
The assumption of no radiogenic argon (40Ar*) when
the rocks formed is usually stated as self-evident. For example,
Geyh & Schleicher (1990, p.56) state:
What is special about the K-Ar method is that the daughter nuclide
is a noble gas, which is not normally incorporated into minerals and is not bound
in the mineral in which it is found.
Similarly, Dalrymple & Lanphere (1969, p.46) state: a silicate
melt will not usually retain the 40Ar that is produced, and thus the potassium-argon
clock is not “set” until the mineral solidifies and cools sufficiently to allow the
40Ar to accumulate in the mineral lattice.
Dalrymple (1991, p.91) has recently put the argument more strongly:
The K-Ar method is the only decay scheme that can be used with
little or no concern for the initial presence of the daughter isotope. This is because
40Ar is an inert gas that does not combine chemically with any other element and
so escapes easily from rocks when they are heated. Thus, while a rock is molten
the 40Ar formed by decay of 40K escapes from the liquid.
However, these dogmatic statements by Dalrymple are inconsistent
with even his own work on historic lava flows (Dalrymple, 1969), some of which he
found had non-zero concentrations of 40Ar* in violation of this key assumption of
the K-Ar dating method. He does go on to admit that “Some cases of initial 40Ar
remaining in rocks have been documented but they are uncommon” (Dalrymple, 1991),
but then refers to his study of 26 historic, subaerial lava flows (Dalrymple, 1969).
Five (almost 20%) of those flows contained “excess argon,” but Dalrymple still then
says “that ‘excess’ argon is rare in these rocks!” The flows and their “ages” were
(Dalrymple, 1969):
|
Hualalai basalt, Hawaii (A.D.1800–1801)
|
1.6±0.16Ma
|
|
1.41±0.08Ma
|
|
|
Mt. Etna basalt, Sicility (122B.C.
|
0.25±0.08Ma
|
|
Mt. Etna basalt, Sicility (A.D.1792)
|
0.35±0.14Ma
|
|
Mt. Lassen plagioclase, California (A.D.1915)
|
0.11±0.03Ma
|
|
Sunset Crater basalt, Arizona (A.D.1064–1065)
|
0.27±0.09Ma
|
|
0.25±0.15Ma
|
Far from being rare, there are numerous examples reported in
the literature of excess 40Ar* in recent or young volcanic rocks producing excessively
old whole-rock K-Ar “ages” as shown in Table 5.
Other studies have also reported measurements of excess 40Ar*
in lavas. Fisher (1970) investigated submarine basalt from a Pacific seamount and
found “the largest amounts of excess 4He and 40Ar ever recorded” (at that time).
McDougall (1971) not only found “extraneous radiogenic argon present
in three of the groups of basalt flows” on the young volcanic island of Réunion in
the Indian Ocean,
but “extraneous argon” was also “detected in alkali feldspar and amphibole in hyperbyssal
drusy syenites that are exposed in the eroded core of Piton des Neiges volcano.”
Significant quantities of excess 40Ar* have also been recorded in submarine basalts,
basaltic glasses and olivine phenocrysts from the currently active Hawaiian volcanoes,
Loihi Seamount and Kilauea, as well as on the flanks of Mauna Loa and Hualalai volcanoes,
also part of the main island of Hawaii (Honda et al., 1993; Valbracht et al., 1996),
and in samples from the Mid-Atlantic Ridge, East Pacific Rise, Red Sea, Galapagos
Islands, McDonald Seamount and Manus Basin (Marty & Humbert, 1997; Staudacher
et al., 1989). Patterson, Honda, & McDougall (1990) claimed that some of the
initial Loihi analytical results were due to atmospheric contamination of the magma
either during intrusion or eruption, but subsequent work (Honda et al., 1993; Valbracht
et al., 1996b) has confirmed that the excess 40Ar* is not from atmospheric contamination
at all.
Table 5. Examples of excess
40Ar* in recent
or young volcanic rocks producing excessively
old whole-rock K-Ar “ages.”
|
Akka Water Fall flow, Hawaii (Pleistocene)
|
32.3±7.2 Ma (Krummenacher, 1970)
|
|
Kilauea Iki basalt, Hawaii (A. D. 1959)
|
8.5±6.8 Ma
(Krummenacher, 1970)
|
|
Mt. Stromboli, Italy, volcanic bomb (September 23, 1963)
|
2.4±2 Ma
(Krummenacher, 1970)
|
|
Mt. Etna basalt, Sicily (May 1964)
|
0.7±0.01 Ma
(Krummenacher, 1970)
|
|
Medicine Lake Highlands obsidian, Glass Mountains, California
(<500 years old)
|
12.6±4.5 Ma (Krummenacher, 1970)
|
|
Hualalai basalt, Hawaii (A. D. 1800– 1801)
|
22.8±16.5 Ma
(Krummenacher, 1970)
|
|
Rangitoto basalt, Auckland, New
|
0.15±0.47 Ma
|
|
(McDougall,
|
|
|
Zealand (<800 years old)
|
Polach, & Stipp,
|
|
1969)
|
|
|
Alkali basalt plug, Benue, Nigeria (<30 Ma)
|
95 Ma (Fisher, 1971)
|
|
Olivine basalt, Nathan Hills, Victoria Land, Antarctica (<0.3
Ma)
|
18.0±0.7 Ma (Armstrong, 1978)
|
|
Anorthoclase in volcanic bomb, Mt.
|
0.64±0.03 Ma
|
|
(Esser, McIntosh,
|
|
|
Erebus, Antarctica (1984)
|
Heizler, & Kyle,
|
|
1997)
|
|
|
Kilauea basalt, Hawaii (<200 years old)
|
21±8 Ma (Noble & Naughton, 1968)
|
|
Kilauea basalt, Hawaii (<1000 years
|
42.9±4.2 Ma
|
|
(Dalrymple &
|
|
|
Moore, 1968)
|
|
|
old)
|
30.3±3.3 Ma
|
|
(Dalrymple &
|
|
|
Moore, 1968)
|
|
|
East Pacific Rise basalt (<1 Ma)
|
690±7 Ma
(Funkhouser Fisher, & Bonatti, 1968)
|
|
Seamount basalt, near East Pacific Rise (<2.5 Ma)
|
580±10 Ma
(Funkhouser, Barnes,& Naughton, 1966) 700±150 Ma
(Fisher, 1972)
|
|
East Pacific Rise basalt (<0.6 Ma)
|
24.2±1.0 Ma
(Dymond, 1970)
|
Excess 40Ar* Occluded in Minerals
Austin (1996) has investigated the 1986 dacite lava flow from
the post-October 26, 1980 lava dome within the Mount St. Helens crater, and has
established that the 10-year-old dacite yields a whole-rock K-Ar model “age” of
0.35±0.05Ma due to excess 40Ar* in the rock. He then produced concentrates of the
constituent minerals, which yielded anomalous K-Ar model “ages” of 0.34±0.06Ma (plagioclase),
0.9±0.2Ma (hornblende), 1.7±0.3Ma (pyroxene), and 2.8±0.6Ma (pyroxene ultra-concentrate).
While these mineral concentrates were not ultra-pure, given the fine-grained glass
in the groundmass and some Fe-Ti oxides, it is nonetheless evident that the excess
40Ar* responsible for the anomalous K-Ar “ages” is retained within the
different constituent minerals in different amounts. Furthermore, the whole-rock
“age” is very similar to the “age” of the plagioclase concentrate because plagioclase
is the dominant constituent of the dacite.
That the excess 40Ar* can be occluded in the minerals within
lava flows, rather than between the mineral grains, has been established
by others also. Laughlin et al. (1994) found that the olivine, pyroxene, and plagioclase
in Quaternary basalts of the Zuni-Bandera volcanic field of New
Mexico contained very significant quantities of excess 40Ar*, as did the olivine and clinopyroxene
phenocrysts in Quaternary flows from New Zealand
volcanoes (Patterson, Honda, & McDougall, 1994). Similarly, Poths, Healy, &
Laughlin (1993) separated olivine and clinopyroxene phenocrysts from young basalts
from New Mexico and Nevada and then measured “ubiquitous excess argon” in them.
Damon, Laughlin,
& Precious (1967) have reported several instances of phenocrysts
with K/Ar “ages” 17 million years greater than that of the whole rocks, and one
K/Ar “date” on olivine phenocrysts of greater than 110Ma in a recent (<13,000
year old) basalt. Damon et al. thus suggested that large phenocrysts in volcanic
rocks contain the excess 40Ar* because their size prevents them from completely
degassing before the flows cool, but Dalrymple (1969) concluded that there does not
appear to be any correlation of excess 40Ar* with large phenocrysts or with any
other petrological or petrographic parameter.
Most investigators have come to the obvious conclusion that the
excess 40Ar* had to have been present in the molten lavas when extruded, which then
did not completely degas as they cooled, the excess 40Ar* becoming “trapped” in
the constituent minerals, and in some instances, the rock fabrics themselves. Laboratory
experiments have tested the solubility of argon in synthetic basalt melts and their
constituent minerals near 1300°C at one atmosphere pressure in a gas stream containing
argon (Broadhurst, Drake, Hagee, & Benatowicz, 1990, 1992). When quenched, synthetic
olivine in the resultant material was found to contain 0.34ppm 40Ar*. Broadhurst
et al. (1990) commented that “The solubility of Ar in the minerals is surprisingly
high,” and concluded that the argon is held primarily in lattice vacancy defects
within the minerals.
In a different experiment, Karpinskaya, Ostrovsky,
& Shanin (1961) heated muscovite to 740°–860°C underhighargonpressures(2800–5000
atmospheres) for periods of 3 to 10.5 hours. The muscovite absorbed significant quantities
of argon, producing K/Ar “ages” of up to 5 billion years, and the absorbed argon
appeared like ordinary radiogenic argon (40Ar*). Karpinskaya (1967) subsequently
synthesized muscovite from a colloidal gel under similar argon pressures and temperatures,
the resultant muscovite retaining up to 0.5wt% argon at 640°C and a vapor pressure
of 4000 atmospheres. This is approximately 2,500 times as much argon as is found
in natural muscovite. These experiments show that under certain conditions argon
can be incorporated into minerals and rocks that are supposed to exclude argon when
they crystallize.
Applications to the Mt. Ngauruhoe Andesite Flows
Therefore, the analytical results from the very recent (1949–1975)
andesite flows at Mt. Ngauruhoe, New Zealand, that yield anomalous K-Ar model “ages”
because of excess 40Ar*, are neither unique nor an artifact of poor analytical equipment
or technique. This realization that the presence of the excess 40Ar* in these rocks
is both real and measurable, and has not been derived from radioactive decay of
40K in situ, leads to the obvious questions as to whether there is any pattern in
the occurrences of excess 40Ar*, and from whence came this excess 40Ar*?
It is clear that the excess 40Ar* was in the lavas when they
flowed from the Mt. Ngauruhoe volcano and were trapped in the andesite as it cooled.
That there were gases in the lavas is readily evident from the copious “frozen”
bubble holes now in the rock, implying that much of the gas content escaped as the
lavas flowed and cooled. When choosing samples, care was taken to select pieces from
each flow that were different from one another (for example, copious “frozen” gas
bubble holes compared with virtually no such holes). It is hardly surprising, therefore,
that the 40Ar* measurements on four of the five flows were consistentwithsuchdifferences
thesamplesfromeach flow which had very few or virtually no “frozen” gas bubble holes
yielded excess 40Ar* and thus anomalous K-Ar model “ages,” whereas the other samples
from each of these flows which contained copious “frozen” gas bubble holes failed
to yield detectable 40Ar* (<0.27Ma and <0.29Ma in Table 4).
The exception was the June 30, 1954 flow—not only was this expected
relationship between excess 40Ar* and lack of “frozen” gas bubble holes not duplicated,
but analyses on duplicate splits off the same samples yielded widely divergent results
(<0.27Ma versus 1.3±0.3Ma and 3.5±0.2Ma versus 0.8±0.2Ma, see Table 4). Thus
the presence (or absence) of excess 40Ar* must also depend on which portion of a
rock sample is being analyzed, which in turn implies dependence on the mineral constituents
present, including the glass in the groundmass. As already noted, Austin (1996)
found widely different amounts of excess 40Ar* in the mineral separates concentrated
from Mount St. Helens 1986 dacite, while numerous other studies (Laughlin et al., 1994; Patterson et al., 1994; Poths et
al., 1993; Valbracht et al., 1996b) have located excess 40Ar* in phenocrysts.
Cooling Rates, Pressures, and Potassium Alteration
Another factor is the rate of cooling of lavas. Dalrymple &
Moore (1968) found that the 1cm thick glassy rim of a pillow in a Kilauea submarine
basalt had greater than forty times more excess 40Ar* than the basalt interior just
10cm below. The glassy pillow rim is, of course, produced by rapid quenching of
the hot basalt lava immediately it contacts the cold ocean water, so the excess
40Ar* in the lava is rapidly trapped and retained. Dymond (1970) obtained similar
results on four deep-sea basalt pillows from near the axis of the East Pacific Rise.
Dalrymple & Moore (1968) also found that the excess 40Ar* contents of the glassy
rims of basalt pillows increased systematically with water depth, leading them to
conclude that the amount of excess 40Ar* is a direct function of both the hydrostatic
pressure and the rate of cooling. In a parallel study, Noble & Naughton (1968)
reported K-Ar “ages” from zero to 22Ma with increasing sample depth for submarine
basalts probably less than 200 years old, also from the active Kilauea volcano.
Seidemann (1977) has reported yet another intriguing relationship.
He analyzed deep-sea basalt samples obtained from DSDP drillholes in the floor of
the Pacific Ocean basin and found K-Ar “ages” increased with increasing K contents
of the basalts, a relationship he noted also appeared in similar data published
by DSDP staff (Seidemann, Figure 1).
In basalt pillows the K content increases from the margin to a maximum at an intermediate
distance
