Archaean rocks provide a record of the early stages of planetary evolution. The interpretation is frustrated by the probable unrepresentative nature of the preserved crust and by the well known ambiguities of tectonic geological synthesis. Broad constraints can be placed on the tectonic processes in the early Earth from global scale modeling of thermal and chemical evolution of the Earth and its hydrosphere and atmosphere. The Archean record is the main test of such models. Available general model constraints are outlined based on the global tectonic setting within which Archaean crust evolved and on the direct evidence the Archaean record provides, particularly the thermal state of the early Earth

Bickle, M. J.

Nisbet, E. G.

English

NASA Technical Reports Server

GREENSTONE BELT TECTONICS - THERMAL CONSTRAINTS: M.J. Bickle (Dept
Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, U.K.) and
E.G. Nisbet (Dept Geological Sciences, University of Saskatchewan, S7N OWO,
Canada).
Archaean rocks provide a unique record of the early stages of evolution
of a planet. Their interpretation is frustrated by the probable unrepresenta-
tive nature of the preserved crust and by the well known ambiguities of tec-
tonic geological synthesis. Broad constraints can be placed on the tectonic
processes in the early earth from global scale modelling of thermal and chemi-
cal evolution of the earth and its hydrosphere and atmosphere. The Archaean
record is the main test of such models. It is the purpose of this contribu-
tion to outline what general model constraints are available on the global
tectonic setting within which Archaean crust evolved, and what direct evidence
the Archaean record provides on particularly the thermal state of the early
earth.
The distinct tectonic style of Archaean granite-greenstone terrains un-
doubtedly reflects secular variation in the earth's tectonic processes as a
result of chemical and thermal evolution. Since tectonic processes are a
direct manifestation of heat loss processes in the earth, changes in the
earth's thermal state are likely to be primarily responsible for changes in
tectonic style. However, the geological record of tectonic processes is also
influenced by the state of chemical evolution of the solid earth and its
hydrosphere and atmosphere. As discussed, below the basic volcanic dominated
nature of greenstone belts is probably as much a consequence of higher mantle
temperatures as any specific tectonic setting. Until proved otherwise we
must assume that 'greenstone belts' formed in as wide a range of tectonic
environments as modern sedimentary sequences. Care must be taken to distin-
guish features which are due to a specific tectonic environment from those
indicative of general tectonic processes in the Archaean earth.
Global Thermal Histories
Calculations of global thermal evolution are based on derivations of
relationships between internal temperature and heat loss. Given such a rela-
tionship and the present temperature and radiogenic heat producing element
distribution within the earth it is possible to calculate temperature distri-
butions in the past with the assumption that the heat loss processes (convec-
tion) varied only in rate throughout earth history. Most current models are
formulated to satisfy the cosmochemical constraint that present day radiogenic
heat production produces about half of the total heat loss and that the earth
was hot soon after accretion [e.g. 1]. The main area of uncertainty intrinsic
in the modelling is the treatment of convection in a fluid of temperature
sensitive and non-Newtonian viscosity. One set of models, the 'parameterised1
convection calculations, derives a relationship between internal temperature
and heat loss by computing heat loss as a function of viscosity for a series
of models run with internally constant but differing viscosities and assuming
some form for the viscosity temperature dependence. Implicit in such model-
ling is the assumption that convection in a variable viscosity fluid can be
approximated by a constant viscosity appropriate to a characteristic tempera-
ture within the system. However, as first demonstrated by McKenzie and Weiss
[2] the assumptions of parametrical convection calculations are not approp-
riate to convection in variable viscosity fluids. Christensen [3] points out
https://ntrs.nasa.gov/search.jsp?R=19860016430 2020-03-20T14:25:14+00:00Z
MODEL CONSTRAINTS
Bickle, M.J. and Nisbet, E.G.
2. All the models predict that higher internal temperatures result in thin-
ner, higher thermal gradient boundary layers (Plates)[l,3]. Further con-
straints must come from Archaean geology, which provides evidence on two cri-
tical parameters, upper mantle temperatures and continental lithospheric
thermal gradients.
1. Mantle Temperatures
The presence in Archaean greenstone belts of komatiitic lavas more mag-
nesian than any younger lava is one of the few distinctive features of the
Archaean and prime evidence that mantle temperatures were higher. To quantify
the difference we need to know (1) the eruption temperature of komatiites and
(2) the relationship between komatiite eruption temperatures and mantle
temperatures. The first question has provoked surprisingly little discussion
given its significance [e.g. 5,6]. Liquidus temperatures of komatiitic lavas
are proportional to MgO content but this may be increased by olivine accumula-
tion. Glassy, near phenocryst free lavas [7], and relict forsterite-rich
olivine compositions have been taken to indicate liquids at least as magnesian
as 27-30?o MgO [5] although this is disputed [6]. Alternatively excess H20 or
alkalis have been suggested as fluxes lowering liquidus temperatures [e.g.8].
The latter is potentially testable through the temperature dependence of Ni
olivine:liquid partition coefficients although such systematic tests have not
been made. Even so eruption temperatures of ~1500°C (25% MgO) to ~1600°C
(30?o MgO) are 100-200°C hotter than any more recent lava.
The relationship between komatiite temperature and mantle temperature is
more problematic. Adiabatically upwelling mantle cools along substantially
higher thermal gradients (higher dT/dP) above the solidus as a result of the
latent heat of melting (Fig. 2). If komatiites represent ~50?o melts at high
1100 1500 2000
100
200]
depth
km
GPa
8
Figure 2. Mantle liquidus and solidus and adiabatic ascent paths calculated
with the assumption that melt and solid do not segregate on ascent, after
McKenzie and Bickle [23].
PRECEDING PAGE BLANK KOT
4:
MODEL CONSTRAINTS
Bickle, M.J. and Nisbet, E.G.
level with an olivine residue then a 1600°C komatiite must be derived from E
mantle in excess of 2000°C from depths >300km where we are essentially igno-
rant of mantle solidus-liquidus temepratures. Such temepratures substantially
exceed the upper bound of mantle temperatures derived from global thermal
modelling. Alternatively it has been suggested that eutectic melts at high
pressure shift to komatiite compositions [9]. Available phase equilibrium
data suggests this might be in the region 50-100 kbar [Fig.3]. If so koma-
tiites might be derived from mantle temperatures of 1800°C-1900°C, a potential
temperature of 1700-1800°C, and 400-500°C hotter than present day average
mantle. If komatiites are derived from anomalously hot upwelling convective
instabilities the potential temperatures of such regions are 200°C-300°C
hotter than mantle in present day thermal plumes.
1200 2000
Figure 3. Phase relations for melting mantle-like compositions from
experiments on komatiites. Note intersection of garnet melting curve
with the pyroxene melting curve is hypothetical.
The chemistry of komatiites is not obviously reconcilable with their
being small degrees of eutectic melts. Incompatible element concentrations
are surprisingly uniform and are consistent with komatiites being ~50?o melts
of plausible mantle materials [10,11]. Small degrees of melt would be
expected to be substantially enriched in incompatible elements although parti-
tion coefficients at the pressures of komatiite genesis are unknown and sub-
stantial modifications to komatiite chemistry by wall rock interaction might
be expected during their ascent [12].
Komatiite genesis is therefore problematic. However, even the most con-
servative estimates of komatiite eruption temperatures (a 25% MgO 1500°C lava)
implies mantle potential temperatures ~200°C hotter than at present and a 30?o
MgO, 1600°C lava is inferred to imply mantle potential temperatures ~400°C
greater than today. One further complication is the possibility that at high
pressure the komatiite melt density exceeds that of solid mantle. If the in-
version in density is associated with a change in sign of the pressure
MODEL CONSTRAINTS
Bickle, M.J. and Nisbet, E.G.
derivative of the potential temperature on the melting curve existence of a
stable magma ocean at depth is probable [13]. The implications of such a
magma ocean for global thermal and chemical evolution are profound.
2. Crustal Thermal Gradients
Metamorphic pressures and temperatures record anomalous thermal condi-
tions in tectonically active crust. If sufficient is known about the tectonic
setting of the metamorphism it is possible to invert the perturbed thermal
conditions to infer steady state lithospheric thermal gradients [14]. Models
for such inversion are mostly based on the thermal time constant over litho-
spheric thicknesses being rather greater than that of tectonic events (~<50
Ma). Given the possibility of magmatic or fluid heat transfer, such models
tend to put upper bounds on lithospheric thermal gradients.
Archaean metamorphic conditions exhibit as wide a range of thermal gra-
dients as modern erogenic provinces. High thermal gradients may at least
locally be associated with magmatic advection of heat [15]. The lower thermal
gradient, higher P/T metamorphism has attracted most interest as it places
limits on the magnitude of lithospheric thermal gradients. The widespread
8-10 Kb, 700°C-900°C conditions recorded by gneiss terrains [16] imply back-
ground gradients little different from those in modern continental litho-
sphere. However, Morgan [17] suggests that these metamorphic conditions are
buffered by crustal melting and heat flow in these regions is underestimated.
Comparable high P/T metamorphism is known from upper-greenschist and amphi-
bolite facies Archaean terrains [15,18-20] although it is less well documen-
ted. This is inconsistent with high heat flow through the underlying crust
and not explicable as buffered by melting.
The inference from the metamorphic conditions of relatively low litho-
spheric thermal gradients has received substantial support from the observa-
tion of the formation and preservation of Archaean age diamonds [21]. These
imply lithospheric thicknesses of ~150-200 km and mantle heat flux as low as
20 mWrri .
The observation that greenstone belts may have formed or been preserved
in continental crust with relatively low thermal gradients has far-reaching
implications for Archaean tectonics. Study of the metamorphism and its tec-
tonic setting in greenstone belts would seem to be one rather neglected area
of greenstone tectonics.
Implications on Global Thermal Evolution
The evidence for a significantly hotter mantle implied by komatiites is
irreconcilable with the evidence for a thick cool continental lithosphere if
the lithosphere behaved as its modern counterpart. There is good evidence
from the depth-age relationships of oceanic lithosphere and sedimentary basin
evolution that Phanerozoic oceanic and continental lithosphere behaves as a
simple thermal boundary layer. To preserve a similar or greater thickness of
Archaean lithosphere requires some additional process to stabilise the con-
tinental lithosphere. Morgan [17] suggests that increasing the concentration
of radiogenic heat production might achieve this. It might but thermal gra-
dients over such enriched lithosphere would have to be at least as high as
those over correspondingly thin but unenriched lithosphere. An alternative
mechanism is that the stabilisation results from density changes on melting
[e.g. 22]. One consequence of a higher temperature mantle is that melting
would start at much greater depths (Fig. 2)(~1T5 km for a 1600°C mantle versus
g MODEL CONSTRAINTS
Bickle, M.J. and Nisbet, E.G.
~60 km for the present day ~1300°C mantle). The depleted zone is comparative-
ly less dense than unmelted mantle although whether the relatively small
changes are sufficient to stabilise the lithosphere against convective instab-
ilities is open to question. The mechanism of stabilisation of Archaean con-
tinental lithosphere and the formation and preservation of Archaean diamonds
is a key question. It has implications both for Archaean tectonic interpre-
tations as well as subsequent global evolution given the significance of the
continental lithosphere to continental tectonics.
There is one further significant tectonic implication of a hotter mantle.
The amount of melt produced by upwelling mantle is proportional to mantle
temperature [Fig. 4; 23]. With a 1600°C mantle any tectonic activity such as
50
i.0
30
20
10
Melt thickness
for infinite stretching
km
Solidus intersection temperature
UOO 1600 1800
1200 UOO 1600 °C
Potential temperature
Figure 4. Melt thickness as a function of mantle temperature for infinite
stretching (oceanic ridge case) after McKenzie & Bickle [23].
crustal extension which led to mantle upwelling would produce significant
magma. It seems probable that the basalt dominated nature of both Archaean
greenstone and late Archaean cratonic supracrustal sequences is a reflection
of mantle temperature and not necessarily of a special tectonic setting.
The extrusion of thick dense basaltic volcanics in supracrustal sequences
may be an important factor in the development of the characteristic tectonic
style of granite-greenstone terrains.
Archaean Tectonic Regimes
The prime assumption of all the global scale thermal models is that heat
loss processes changed only in rate. One hotly debated point is whether
plate tectonics or some alternative tectonic scheme operated during the
Archaean. For example, Richter [1] has suggested that once convecting mantle
penetrated the melt region below continental lithosphere the surface tectonic
regime would be dominated by vertical recycling rather than horizontal
MODEL CONSTRAINTS
Bickle, M.J. and Nisbet, E.G.
motions. This scheme does not explain the preservation of the early Archaean
crustal relicts for which some special survival mechanism must be proposed.
Perhaps the best evidence for major horizontal (plate) motions lies in the
linear tectonic belts characteristic of the larger Archaean terrains (Superior
Province, Yilgarn Block) and the evidence for large scale overthrust nappe
tectonics in the high-grade gneiss belts. Other geological evidence is open
to interpretation. For example, the significance of the calc-alkaline-like
granite suites, possible analogies between some greenstone belt mafic sequen-
ces and ophiolites and the tectonic state of greenstone belts (allochthonous
or autochthonous) are all disputed. One additional line of evidence does
strongly suggest division of the Archaean earth into continental and oceanic-
type regions. The heat loss through the Archaean continental regions inferred
from metamorphic thermal gradients is too low by an order of magnitude to be
representative of heat loss from the Archaean earth [24,25]. The extra heat
is plausibly lost through oceanic-like regions as is the case today. This
would involve substantial melting and recycling of volcanic crust.
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Greenstone belt tectonics: Thermal constraints

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