one little thing, magmic rocks are
not completely degassed below 1200� C
which is to say, that small melt inclusions
are not degassed until temperatures in excess
of 1200� C are reached, [R. P. Esser
et.al.]
meaning, completely melted does
-not- mean completely degassed.
degassing is itself -temperature- dependant.
first, think of water,
water is outgassed stepwise as
the applied pressure reaches
the external pressure.
at 32� C it's liquid, but still
has considerable gas content,
as you raise the temperature, the applied pressure
nears the atmospheric pressure and gases are released
but even -at- the boiling point, some small amount
of gas remains in the water in an equilibrium
described by some constant K.
even neglecting this small amount -at- the boiling point,
we cannot neglect the amounts -below- the boiling point.
and so you can see, that below the boiling point
of magmic rocks, some gases are likely to be present
and well below such a temperature,
complete degassing cannot possibly occur.
and that is what Esser sees and this correlates
well with the consideration that ash from volcanoes
commonly have initial argon well above zero.
this because the ash eruptions are at a
lower temperature and well below that
of boiling magma.
not and never completely degassed.
trouble is, the hottest volcanoes like those in
hawaii reach temperatures of -less- than 1200� C
and so, one may conclude that there is never
a complete degassing even in the hottest volcanoes.
therefore, initial argon = zero is always unwarranted.
also, ash bursts are caused because the magma
is cooler already, allowing gas pressure
beneath to build up and blow the ash
out explosively.
so these sources never completely degassed pre-eruption.
it's considered common for ash deposits
to be heavy in argon 'contamination'
===
http://www.tulane.edu/~sanelson/geol204/volcan&magma.htm
Explosive Eruptions
Explosive eruptions are favored by high gas content and
high viscosity (andesitic to rhyolitic magmas).
Explosive bursting of bubbles will fragment the magma into
clots of liquid that will cool as they fall through the air.
These solid particles become pyroclasts (meaning - hot fragments)
and tephra or volcanic ash, which refer to sand-
sized or smaller fragments.
Temperature of Magmas
Temperature of magmas is difficult to measure (due to the danger
involved),
but laboratory measurement and limited field observation indicate that
the eruption temperature of various magmas is as follows:
Basaltic magma - 1000 to 1200�C
Andesitic magma - 800 to 1000�C
Rhyolitic magma - 650 to 800�C.
===
http://adsabs.harvard.edu/abs/1997GeCoA..61.3789E
Title:
Excess argon in melt inclusions in zero-age anorthoclase
feldspar from Mt. Erebus, Antarctica, as revealed by
the 40 Ar/ 39 Ar method *1
Authors: Esser, R. P.; McIntosh, W. C.; Heizler,
M. T.; Kyle, P. R.
Publication: Geochimica et Cosmochimica Acta, vol. 61,
Issue 18, pp.3789-3801 (GeCoA Homepage)
Publication Date: 09/1997
Origin: ELSEVIER
DOI: 10.1016/S0016-7037(97)00287-1
Bibliographic Code: 1997GeCoA..61.3789E
Abstract
Historically erupted (1984) anorthoclase phenocrysts from Mt. Erebus
yield K/Ar and 40 Ar/ 40 Ar apparent ages as old as 700 ka indicating
the presence of excess argon. 40 Ar/ 39 Ar furnace step heating results
from anorthoclase reveal a positive correlation between the Cl/K ratio
and apparent age. Because chlorine (up to 1700 ppm) is present in melt
inclusions but not in the anorthoclase crystal lattice, this correlation
suggests that the excess argon is associated with melt inclusions
trapped within the anorthoclase during rapid crystal growth.
Confirmation
of the source of excess argon comes from step-heating experiments on
multiple anorthoclase aliquots separated from two phenocrysts and one
glass aliquot prepared from the matrix of a volcanic bomb. The
anorthoclase
phenocrysts were crushed and HF etched to yield aliquots containing 30%,
10%, and 1 % melt inclusions. The step-heated anorthoclase with 30% and
10% melt inclusions yielded the highest Cl/K ratios and apparent
integrated
ages of 640 � 30 ka and 179 � 16 ka, respectively. The anorthoclase with
I% melt inclusions yielded significantly lower Cl/K ratios and apparent
integrated ages of 48 � 8 ka and 50 � 30 ka. The step-heated volcanic
glass yielded the least variable Cl/K ratios and a total gas age of
101 � 16 ka. Argon released from the anorthoclase and the trapped melt
inclusions can be distinguished by differences in their degassing
behavior,
allowing geologically more reasonable ages to be obtained. Melt
inclusions
exposed on the exterior of anorthoclase grains principally degas during
furnace extraction at temperatures less than 1200�C. Inclusions entirely
within anorthoclase grains principally degas at temperatures greater
than 1200�C when incongruent melting of the anorthoclase allows melt
inclusion hosted excess argon to escape. Anorthoclase aliquots prepared
with less than 1% inclusions can be fitted with a plateau for heating
steps below 1200�C to yield ages as young as 8 � 2 ka, whereas steps
above 1200�C yield ages in excess of 100 ka. However, anorthoclase
aliquots containing 10-30% melt inclusions yield ages in excess
of 200 ka for heating steps below 1200�C. Minimizing the effects
of the excess argon from melt inclusions relies on sample preparation
and step-heating. Fine crushing and treatment with hydrofluoric acid
removes many of the larger melt inclusions. Small melt inclusions
which remain within the anorthoclase degas primarily at temperatures
above 1200�C. Temperatures below 1200�C yield the most accurate ages.
Attempts at post-analytically correcting for the chlorine-correlated
excess argon are hindered by the variations in 40 Ar E /Cl within
and between samples. Elevated 40 Ar E /Cl ratios in bubbles within
the melt inclusions, as deduced from in vacuo crushing experiments,
are the most likely cause for some or all of a sample's total
40 Ar E /Cl variation. In addition, relative solubilities of argon
and chlorine within phonolitic melts may be partly responsible
for variations in 40 Ar E /Cl.
http://www.agu.org/pubs/crossref/2001/2001GL013855.shtml
GEOPHYSICAL RESEARCH LETTERS, VOL. 28, NO. 22, PAGES 4279�4282, 2001
Excess argon in Mount St. Helens plagioclase
as a recorder of magmatic processes
Paul W. Layer
Geophysical Institute and Department of Geology and Geophysics,
University of Alaska, Fairbanks, Fairbanks, Alaska
James E. Gardner
Geophysical Institute and Department of Geology and Geophysics,
University of Alaska, Fairbanks, Fairbanks, Alaska
Abstract
Excess argon in plagioclase crystals from young (<4000 ybp)
tephra layers from Mount St. Helens, Washington, illustrates
the importance of argon isotopes in understanding magmatic
processes. 40Ar/39Ar step-heating identifies two distinct
argon reservoirs in the plagioclase; a high Cl/K, low Ca/K
reservoir with atmospheric 40Ar/36Ar and a low Cl/K, higher
Ca/K reservoir with variable 40Ar/36Ar. The first is
probably glass, whereas the second is �true� plagioclase.
Felsic dacite plagioclases have little or no excess argon,
but those from more mafic layers have significantly higher
40Ar/36Ar, indicating a non-atmospheric source. These variations
are seen within a layer that contains both mafic and felsic dacite,
and are inconsistent with either xenocrystic or restitic origin
for plagioclase. The magma chamber exhibits long-term open-system
degassing behavior punctuated by short-term fluctuations from
influx of high 40Ar/36Ar basalt. The relative timing between
mixing and eruption can affect the amount of excess argon
recorded in plagioclase. � 2001 American Geophysical Union
http://cat.inist.fr/?aModele=afficheN&cpsidt=10337098
Titre du document / Document title
EVIDENCE FOR DISTORTION OF TERTIARY K/AR AGES BY EXCESS ARGON :
EXAMPLE GIVEN BY THREE ALKALI OLIVINE BASALTS FROM
NORTHERN HESSE, GERMANY
http://cat.inist.fr/?aModele=afficheN&cpsidt=13846991
The K-Ar and Ar-Ar dating techniques occasionally produce
anomalously old ages attributed to excess argon, and such
data is often rejected as not offering any insight into
the age, thermal history or geochemistry of the rock.
However, improvements in the quantification of argon
geochemistry now provide a framework to model excess argon
in both open and closed systems. Solubility data for argon
in hydrous fluids, melts and emerging data for minerals can
be used to understand the behaviour of excess argon, and
provide valuable insights into the environment in which
the samples cooled to their argon retention or 'closure'
temperature. Treating excess argon as a trace element also
throws light on its behaviour in minerals above the
closure temperature, in deeply buried dry systems such
as eclogites, blueschists, granulites and even in the
lithospheric mantle. Extremely low partition coefficients
between K-feldspar and hydrous fluid phases predict lower
excess argon susceptibility than micas and this is
observed in fluid-poor systems. Variation of partition
coefficients can lead to excess argon in fluids being
introduced into minerals or removed from minerals as
grain boundary fluids change during flow through a rock.
However, excess argon can also be introduced or removed
from minerals by varying temperature, without the need
for fluid flow. High mineral/melt and mineral/fluid
partition coefficients are also the reason why excess argon
is often concentrated in inclusions within minerals.
Partition coefficients between minerals and hydrous fluids
as low as 10[-][6] lead fluid inclusions to dominate the
radiogenic argon budget, particularly in low potassium minerals.
Melt inclusions are less dominant but become critical
in dating younger samples.
http://www.dlinde.nl/schepping/Excess%20Argon%20at%20Mount%20St_%20Helens.htm
The conventional K-Ar dating method was applied to the 1986
dacite flow from the new lava dome at Mount St. Helens, Washington.
Porphyritic dacite which solidified on the surface of the lava dome
in 1986 gives a whole rock K-Ar 'age ' of 0.35 � 0.05 million years
(Ma).
Mineral concentrates from the dacite which formed in 1986 give K-Ar
'ages 'from 0.34 � 0.06 Ma (feldspar-glass concentrate) to 2.8 � 0.6 Ma
(pyroxene concentrate). These 'ages 'are, of course, preposterous.
The fundamental dating assumption ('no radiogenic argon was present
when the rock formed ') is questioned by these data. Instead, data
from this Mount St. Helens dacite argue that significant 'excess argon'
was present when the lava solidified in 1986. Phenocrysts of
orthopyroxene,
hornblende and plagioclase are interpreted to have occluded argon within
their mineral structures deep in the magma chamber and to have retained
this argon after emplacement and solidification of the dacite.
The amount of argon occluded is probably a function of the argon
pressure when mineral crystallization occurred at depth and/or
the tightness of the mineral structure. Orthopyroxene retains
the most argon, followed by hornblende, and finally, plagioclase.
The lava dome at Mount St. Helens dates very much older than its
true age because phenocryst minerals inherit argon from the magma.
The study of this Mount St. Helens dacite causes the more
fundamental question to be asked �how accurate are K-Ar 'ages'
from the many other phenocryst-containing lava flows world-wide?
this bit;
http://www.onafarawayday.com/Radiogenic/Ch10/Ch10-1.htm
also has a look at this 'excess argon' problem.
http://www.ees.nmt.edu/Geol/labs/Argon_Lab/SiteMap.html
http://geoinfo.nmt.edu/staff/esser/RE_Pub.html[dead link]
http://www.pathlights.com/ce_encyclopedia/06dat4.htm[dead link]
http://www.onafarawayday.com/Radiogenic/Ch10/Ch10-1.htm
===
Kelly & Wartho, Science, 28 July, 2000,
Rapid Kimberlite Ascent and the Significance
of Ar-Ar Ages in Xenolith Phlogopites, page 609
The ages yielded by large phlogopites from xenoliths are,
however, commonly older than the eruption, a phenomenon
that has been interpreted as the incorporation of excess
radiogenic Ar from a deep fluid source.
===
it's my contention that the "excess argon" problem,
throws the entire chronometry into doubtful question.
you never get complete degrassing and so,
you never get a true 'zero' to that 'ages' gathered in.
there are subterranean gases which follow along
right up into the magmic chamber
and it never escapes completely.
one can pick and choose through a given sample
to skew the outcome in any direction you like.
contemporary eruptions can read to be very ancient,
and if you pick through the rubble,
very "ancient" deposits can be made
to show a recent deposition.
and there's considerable overlap of possible dates
between upper and lower shelving of such igneous intrusions.
a higher shelf can have samplings which
show a more 'ancient' date that a lower shelf,
and vicey versey, and, various artifacts
ere not usually recovered from -within- these
igneous intrusions anyway, but in and amongst
the -mobile- phases which are found
between igneous[volcanic] intruuions.
and so, being a much more -mobile- phase,
artifacts may be washed in and out of them
with very little relationship to when such
igneous volcanic intrusions are deposited.
meaning, so you find some animal bones in the
hardened mud between two layers of volcanic sheet,
this still tells you very little about -when-
such animal bones were deposited in the -wet-
muddy blobs inasmuch as a little flooding can
soften these muddy deposits right up again
and more anuimal detritus
can find its way into teh newly softened mud which
is then rehardened during prolonged dry spells.
all without ever being able to conclusively establish
that any animal detritus was deposited
in mud above a lower shelf before an upper
shelf of igneous [volcanic] matter
was then, deposited.
meaning, the animal detritus can get in between
the shelves of volcanic matter at any time,
even _after_ -both- shelves are deposited.
and so, the timelines are ambiguous
and remain doubtful.
reasonable doubt...