JOURNAL OF PETROLOGY
VOLUME 42
NUMBER 11
PAGES 2169–2171
2001
A Note on the IUGS Reclassification of the
High-Mg and Picritic Volcanic Rocks
A. C. KERR1∗ AND N. T. ARNDT2
1
DEPARTMENT OF EARTH SCIENCES, CARDIFF UNIVERSITY, PO BOX 914, MAIN BUILDING, PARK PLACE,
CARDIFF CF10 3YE, UK
2
LGCA, UNIVERSITÉ DE GRENOBLE, BP 53, 1381 RUE DE LA PISCINE, 38031 ST MARTIN D’HERES, FRANCE
RECEIVED APRIL 30, 2001; REVISED TYPESCRIPT ACCEPTED MAY 23, 2001
The purpose of this short note is to offer some comments
and observations on the new IUGS classification of highMg rocks (Le Bas, 2000), particularly its use of the terms
‘komatiite’ and ‘picrite’ with no reference to texture or
the original usage of these terms. At the outset of this
discussion we would like to place on record that we are
in agreement with the need to redefine the nomenclature
of high-Mg rocks. There are, however, important issues
relating to both the use of the term ‘komatiite’ for all
extrusive volcanic rocks with >18 wt % MgO and the
definition and use of the term ‘picrite’, neither of which
are discussed by Le Bas (2000).
KOMATIITES—A DEFINITION
THE 1991 IUGS CLASSIFICATION
The new classification of high-MgO rocks stems in part
from the 1991 IUGS classification proposal (Le Bas &
Streckeisen, 1991), which defines picrites as possessing
>18 wt % MgO with between 1 and 2% total alkalis.
Komatiites, on the other hand, were defined as having
>18 wt % MgO with <1 wt % total alkalis. Although
this classification permitted slightly more latitude in the
use of the terms picrite and komatiite, the 2000 IUGS
classification still uses both these terms in a purely chemical sense and ignores the distinctive textural features of
picrites and komatiites.
TESTING THE NEW IUGS
CLASSIFICATION SYSTEM: LATE
CRETACEOUS KOMATIITES AND
PICRITES FROM GORGONA ISLAND,
COLOMBIA
Komatiites were first recognized by Viljoen & Viljoen
(1969) in the Barberton greenstone belt, South Africa.
The features that marked these rocks out as distinctive
were not only their high Mg content but also their large,
skeletal, platy, bladed or acicular grains of olivine, which
Viljoen & Viljoen (1969) called ‘crystalline quench texture’. Thus, komatiites from the type locality were first
distinguished on the basis of their chemistry and texture.
The term ‘spinifex texture’ was first introduced by Nesbitt
(1971).
The next serious attempt to define komatiite was by
Arndt & Nisbet (1982), who pointed out that a komatiite
is not simply an ultramafic lava, but it also possesses a
well-developed spinifex texture. A recent overview of
komatiite lava flows and spinifex textures has been given
by Arndt (1994) and the reader is referred to that work
for further information.
The only known post-Cambrian komatiites, from Gorgona Island, Colombia (Echeverrı́a, 1980; Kerr et al.,
1996a; Arndt et al., 1997) largely preserve their original
mineralogy and chemistry. In addition to spinifex-textured komatiites (Fig. 1a), picritic dykes and breccias
with polyhedral and some skeletal olivines, but no spinifex
textures (Fig. 1b), are also present. This suite of rocks
represents ideal samples on which to assess the applicability of the new classification.
Figure 2 shows all available analyses of Gorgona komatiites and picrites plotted in fig. 6 of Le Bas (2000).
The Gorgona data are taken from Echeverrı́a (1980),
∗Corresponding author. E-mail: kerra@cardiff.ac.uk.
 Oxford University Press 2001
JOURNAL OF PETROLOGY
VOLUME 42
NUMBER 11
NOVEMBER 2001
Fig. 1. Photomicrograph of (a) spinifex-textured, platy olivines in a Gorgona komatiite, (b) subhedral olivines in a Gorgona picrite dyke. Field
of view 4 mm in each photomicrograph.
Fig. 2. Plot of total alkalis vs MgO showing the classification fields
proposed for high-Mg rocks by the IUGS Subcommission (Le Bas,
2000). Plotted in the diagram are spinifex-textured komatiites and nonspinifex-textured picrites from Gorgona. Data sources: Echeverrı́a
(1980); Kerr et al. (1996a); Arndt et al. (1997).
Kerr et al. (1996a) and Arndt et al. (1997). This diagram
reveals that all non-spinifex-textured (Fig. 1b) picrites
from Gorgona now classify as komatiites, as they all
possess >18 wt % MgO. Furthermore, and perhaps more
worrying, 67% of available analyses of spinifex-textured
Gorgona komatiites are, according to the new classification system, renamed ‘picrites’. This is despite the
fact that they possess the widely recognized textural
characteristics of komatiites, namely spinifex-textured
olivines (often up to 5 cm long).
More confusion arises when one realizes that within
many individual differentiated komatiite flows, samples
from the non-spinifex cumulate zone classify as komatiites
under the new definition (>18 wt % MgO) whereas the
spinifex-textured portion of the flow classifies as a ‘picrite’.
In fact, many worldwide occurrences of high-MgO lavas
with polyhedral olivines now classify as ‘komatiites’, without possessing the main textural feature of Precambrian
and Gorgona komatiites—spinifex olivines. A case in
point are the thick high-Mg lava flows of the Curaçao
lava succession, which, like the Gorgona lavas, are a part
of the Caribbean oceanic plateau (Kerr et al., 1996b).
These lavas possess polyhedral, skeletal olivines, and
individual flows commonly display some degree of crystal
settling. Using the new classification, the olivine cumulates
in the lower parts of Curaçao flows classify as ‘komatiites’,
and those in the upper parts as ‘picrites’. Similarly, the
lower cumulates of olivine-rich flows in continental flood
volcanic sequences such as those of Baffin Bay, Karoo
and Noril’sk all become ‘komatiite’ if texture and petrography are ignored! We believe that all these cases
graphically illustrate the very real problems of a nomenclature system for igneous rocks that fails to take
account of texture.
PICRITES AND THE NEW IUGS
CLASSIFICATION
The problem with the use of the term ‘picrite’ stems
from the way the term is employed by different groups
of geologists. Traditional petrologists, trained to name a
rock using criteria that can be applied in the field, are
normally happy with a definition like that normally found
in petrological texts, which emphasizes an abundance of
olivine phenocrysts. The problem arises when other
groups use picrite to imply formation of the rock from a
highly magnesian ‘picritic’ liquid. The IUGS criteria—
MgO between 12 and 18% and 1–2% alkalis—include a
variety of rocks that are very different from the traditional
concept of picrite.
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KERR AND ARNDT
Let us consider four examples, all volcanic rocks, all
of which conform to the IUGS criteria:
(1) an olivine porphyritic lava formed by minor crystal
accumulation from a silicate liquid magma with 11%
MgO;
(2) an olivine porphyritic lava formed by partial crystallization of a liquid with 15% MgO;
(3) the upper pyroxene spinifex-textured part of a
differentiated komatiitic basaltic flow; such rocks commonly contain large acicular grains of augite or pigeonite,
but no olivine;
(4) a pyroxene–plagioclase cumulate in the lower part
of the same flow.
Only the first two are picrites in the petrological
sense—the last two do not even contain olivine! Examples
(2), (3) and (4) form through the crystallization of a highly
magnesian ‘picritic’ liquid, but not example (1).
The only real solution is to insist that the meaning
implied by the use of the term picrite be clearly spelt
out. For most purposes an objective definition that incorporates both textural and chemical criteria is probably
best. Even in the IUGS classification scheme (Le Bas
2000), texture is not completely ignored—it is used to
identify the rock as volcanic and not plutonic. For highly
magnesian lavas in which rocks with similar bulk composition can be produced by radically different processes,
texture must also be incorporated into any classification
scheme. The issue of whether a rock forms from highly
magnesian liquid, or not, is a subject of discussion and
interpretation and it should be considered separately, not
as part of a general classification scheme.
CONCLUDING REMARKS
In summary, there are several issues that need to be
addressed. First, the classification of two-thirds of Gorgona spinifex-textured komatiites as picrites obviously
calls into question the stringent use of a purely chemical
classification system for highly magnesian rocks. Second,
the unqualified use of the term ‘komatiite’ for all volcanic
rocks with >18 wt % MgO and <1 wt % TiO2 will
undoubtedly lead to confusion in the literature. Indeed,
the confusion has commenced with the non-spinifex
Gorgona picrites having already been classified as ‘komatiites’ (Thompson & Gibson, 2000). Such indiscriminate usage of the term komatiite may well result
in names such as ‘petrographic’ komatiite, ‘non-spinifextextured’ komatiite, or even (heaven forbid) ‘real’ komatiite or ‘pseudo-’ komatiite being used to help differentiate between picrites and spinifex-textured komatiites.
In regard to a resolution of these problems we would
strongly recommend that the term komatiite be reserved
solely for lavas with characteristic spinifex-textured olivines, or lavas that can be related directly, using field or
COMMENT
petrological criteria, to lavas with this texture. This would
resolve much of the ambiguity in nomenclature that
exists within the new IUGS classification scheme. We do
not recommend the introduction of a new name for lavas
with MgO >18 wt % that do not possess spinifex-textured
olivines. It is usually sufficient simply to describe such
rocks as ‘the basal olivine-rich parts of differentiated
flows’ or some such similar phrase.
We wish to stress that this contribution is not written
in a spirit of criticism, rather its purpose is to highlight
potential problems that may arise through the use of this
classification system and that other geologists, less familiar
with komatiites, and picrites, may not be aware of. It is
our belief that the unqualified use of the current IUGS
classification scheme will create more problems than it
solves. We trust that this note will stimulate much debate
and discussion on the nomenclature of high-MgO volcanic rocks in the wider petrological community. We
hope that it will allow the issues raised here to be
satisfactorily resolved, and a workable consensus to be
reached on the classification of these rocks.
REFERENCES
Arndt, N. T. (1994). Komatiites. In: Condie, K. C. (ed.) Archean Crustal
Evolution. Amsterdam: Elsevier, pp. 11–44.
Arndt, N. T. & Nisbet, E. G. (1982). What is a komatiite? In: Arndt,
N. T. & Nisbet, E. G. (eds) Komatiites. London: George Allen and
Unwin, pp. 19–27.
Arndt, N. T., Kerr, A. C. & Tarney, J. (1997). Dynamic melting in
plume heads: the formation of Gorgona komatiites and basalts. Earth
and Planetary Science Letters 146, 289–301.
Echeverrı́a, L. M. (1980). Tertiary or Mesozoic komatiites from Gorgona Island, Colombia: field relations and geochemistry. Contributions
to Mineralogy and Petrology 73, 253–266.
Kerr, A. C., Marriner, G. F., Arndt, N. T., Tarney, J., Nivia, A.,
Saunders, A. D. & Duncan, R. A. (1996a). The petrogenesis of
komatiites, picrites and basalts from the Isle of Gorgona, Colombia;
new field, petrographic and geochemical constraints. Lithos 37,
245–260.
Kerr, A. C., Tarney, J., Marriner, G. F., Klaver, G. Th., Saunders,
A. D. & Thirlwall, M. F. (1996b). The geochemistry and petrogenesis
of the late-Cretaceous picrites and basalts of Curaçao, Netherlands
Antilles: a remnant of an oceanic plateau. Contributions to Mineralogy
and Petrology 124, 29–43.
Le Bas, M. J. (2000). IUGS reclassification of the high-Mg and picritic
volcanic rocks. Journal of Petrology, 41, 1467–1470.
Le Bas, M. J. & Streckeisen, A. L. (1991). The IUGS systematics of
igneous rocks. Journal of the Geological Society, London 148, 825–833.
Nesbitt, R. W. (1971). Skeletal crystal forms in the ultramafic rocks of
the Yilgarn Block, Western Australia: evidence for an Archaean
ultramafic liquid. Geological Society of Australia Special Publication 3,
331–347.
Thompson, R. N. & Gibson, S.A. (2000). Transient high temperatures in
mantle plume heads inferred from magnesian olivines in Phanerozoic
picrites. Nature 407, 502–506.
Viljoen, M. J. & Viljoen, R. P. (1969). The geology and geochemistry
of the Lower Ultramafic Unit of the Onverwacht Group and a
proposed new class of igneous rocks. In: Upper Mantle Project. Geological
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