Sunday, July 8, 2018


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.
2
 
Because the sample pieces were submitted as whole rocks, the K-Ar laboratory undertook the crushing and pulverizing preparatory work. The concentrations of K O (weight %) were then measured by the flame photometry method (Dalrymple & Lanphere, 1969), the reported values being the averages of two readings for each sample. The 40K concentrations (ppm) were calculated from the terrestrial isotopic abundance using the measured concentrations of K O. The concentrations in ppm of 40Ar*, the supposed “radiogenic” 40Ar, were derived using the conventional formula from isotope dilution measurements on a mass spectrometer by correcting for the presence of atmospheric argon whose isotopic composition is known (Dalrymple & Lanphere). The reported concentrations of 40Ar* are the averages of two values for each sample. The ratios 40Ar*/Total Ar and 40Ar/36Ar are also derived from measurements on the mass spectrometer and are also the averages of two values for each sample.

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

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