Download Meteorite Impacts as Triggers to Large Igneous Provinces

Meteorite Impacts
as Triggers to Large
Igneous Provinces
Shocked quartz showing
decorated planar
deformation features
from newly discovered
impact deposits in SW
China, of unknown age.
Partially crossed polars,
field of view approximately 1 mm. PHOTO
ADRIAN JONES
Adrian P. Jones1
A
meteorite impacting on the surface of the Earth produces not only
a crater but also, if the impactor is sufficiently large, high melt
volumes. Computer simulations suggest that, in addition to shockinduced melting produced by impact, additional decompression melting
of the hot target mantle beneath the crater can produce melt volumes
comparable to those found in large igneous provinces (LIPs). The coincidence
between the expected frequency of such impact events combined with the
similarity in magma volumes of LIPs suggests that large meteorite impacts
may be capable of triggering LIPs and mantle hotspots from a point source
which is subsequently buried. Can the impact model explain any LIP? What
are the distinctive macroscopic criteria predicted from an impact model, and
how may they be recognised or rejected in the geological record of the Earth?
500 Myr. Thus, terrestrial impact
events capable of generating global
ejecta layers should occur relatively frequently in the geological
record, and some LIPs may overlap
in age with an unrelated impact
event (White and Saunders 2005).
LIP volumes are typically ~106 km3,
but some are smaller, e.g. the
Columbia River basalt plateau
(~105 km3), and some are larger,
e.g. the Siberian Traps and the
Ontong Java Plateau (106–107 km3).
This paper summarises recent
modelling to test the impact forKEYWORDS: meteorite impact, decompression melting, mantle, hotspot
mation of the Ontong Java Plateau
(OJP), which due to its size is an
INTRODUCTION
extreme case. The case for cause and effect between a speThe idea that large meteorite impacts may trigger volcanic cific impact event and a LIP would be strengthened if the
activity (impact-induced volcanism) has been around for stratigraphy demonstrates that the impact event immediseveral decades. A meteorite impact was proposed to ately predated the LIP, and if the initial chemistry of the
explain the differentiated melts of the Sudbury Igneous LIP, where underlain by oceanic mantle, was ultramafic (e.g.
Complex, including the currently unfavoured idea that the high-magnesium basalt or picrite). Such an impact event
nickel-rich deposits are cosmogenic (Dietz 1972). Hot melt- preceding a LIP has been described from West Greenland
ing of impacted peridotite mantle was proposed by Green and is profiled below. The volcanic expression of impacts is
(1972) as an explanation for the very-high-temperature unknown, but key igneous features of a large differentiated
lavas called komatiites. Rogers (1982) proposed that large impact melt at Sudbury provide some clues.
oceanic basalt plateaus represent the relics of moderatesized meteorite impact craters [diameter (D) > 100 km] and IMPACTS AND MELTING
promoted Grieve’s (1980) idea that upwarping of the
Typically, the relationship between the body size of the
asthenosphere might be sufficient to initiate a long-lived
impactor and the diameter of the transient crater is 1:10.
thermal plume in the mantle. Melt productivity beneath
Rebound and gravity-assisted relaxation produce a final
impact craters may be significantly increased by decomcrater much shallower than the transient crater and with D
pression melting of the mantle and high ambient geotherapproximately twice that of the transient crater. A compilamal gradients.
tion of estimated melt volumes for terrestrial impact craters
Very large terrestrial impact craters (D > ~500 km) can cre- in crystalline rocks by Grieve and Cintala (1992) shows a
ate more melt than the volume of the impact crater, i.e. suf- simple logarithmic correlation with crater diameter (FIG. 1A).
ficient for LIPs (~106 km3, FIG. 1A), and simulations of the The conventional view is that melt volume scales with the
largest conceivable impact, the giant moon-forming event, kinetic energy (1/2mv2) of the impactor (Pierazzo et al.
predict reorganisation and substantial melting of the man- 1997) and that the volume of melt produced by a meteorite
tle and lithosphere on a global scale (Canup and Asphaug impact (for a crater with D ~100–200 km) is insufficient to
2001). Glikson (2001) estimates that 390 ± 36 craters (D > explain the amount of melt typical of a LIP (Ivanov and
100 km) and 45 ± 4 craters (D > 250 km) formed on Earth Melosh 2003). Until recently, most impact cratering models
in the last 3.8 Gyr. Even when adjusted for a reduction in simulated the effect of impacts into cold targets. Since hot
the number of hits with time, this translates to one large targets melt at lower shock pressures, modelling the true
impact (D > 250 km) every ~100–150 Myr during the last thermal structure of the target is vital for understanding the
effects of impacts on Earth. This is particularly true for large
impactors that penetrate through the Earth’s crust and into
1 Department of Geological Sciences
the mantle. Jones et al. (2002) used hydrodynamic comUniversity College London
puter modelling to demonstrate that the melting response
Gower Street
of the Earth’s peridotite mantle to decreasing pressure
London WC1E 6BT
United Kingdom
(decompression melting) beneath a large impact crater
[email protected]
might increase the total melt volume, depending on the
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PP.
277–281
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D ECEMBER 2005
A
B
A
(A) Theoretical correlation of impact melt volume versus
crater diameter for terrestrial impact craters (sloping line)
with locus of terrestrial impact melt below this line (after Grieve and
Cintala 1992). Also shown is the melt volume typical of a LIP (horizontal dashed line at 106 km3). (B) Hypothetical increase in impact melt volume above the theoretical sloping line, due to additional decompression
melting of lithospheric mantle for large crater diameters, not precisely
determined (see Jones et al. 2002, 2003).
FIGURE 1
target thermal profile and lithology. For impacts into the
thermally active Earth, we contend that above some threshold crater diameter (not yet determined), where decompression melting becomes significant, the volume of melt
produced might be considerably greater than that predicted
by the conventional relationship between melt volume and
crater diameter. Melt volumes of approximately 106 km3 –
comparable to LIP volumes – might be produced (FIG. 1B).
GEOLOGICAL EVIDENCE OF IMPACT
Amongst an array of geological impact signatures, two
important criteria are (1) the presence of shock metamorphic effects in mineral and rock inclusions in breccias and
melt rocks, and (2) evidence for a minor extraterrestrial geochemical component in these rocks (Koeberl 2002).
Shocked quartz is the foremost mineralogical criterion for
recognition of shock metamorphism in terrestrial materials;
it is petrographically distinctive and recognisable even as
rare fragments (FIG. 2A). However, shocked quartz can only
be produced in target rocks containing quartz, ruling out
parts of the Earth’s crust and lithosphere composed of
quartz-free rock like basalt (e.g. oceanic crust). Therefore
ejecta products of oceanic impacts will be free of shocked
quartz and must be identified by other mineralogical and
geochemical criteria. These include quenched melt droplet
spherules (FIG. 2B), Ni-spinel (FIG. 2C, D) and geochemical
anomalies for the siderophile elements, especially platinumgroup elements (e.g. iridium, osmium) and chromium. In
addition, there may be field evidence for unusual geological
activity, such as tsunami deposits.
B
C
IMPACT MODEL FOR THE ONTONG
JAVA PLATEAU
The mid-Cretaceous Ontong Java Plateau (OJP) is the largest
oceanic LIP. It is thought by many to have formed from a
deep mantle plume, although this is not universally
accepted. It may instead have been triggered by a meteorite
impact (Rogers 1982; Ingle and Coffin 2004) as examined
here. Jones et al. (2005a) modelled the first few hundred
seconds of a vertical impact between a hypervelocity meteorite projectile and a dry peridotite lithosphere target using
geotherms for young oceanic crust at the onset of the OJP.
At the scale of tens to hundreds of kilometres, the complexities of atmosphere and ocean are ignored. In the simulation, changing the target to hot oceanic lithosphere has
a dramatic effect and produces massive melting by both
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D
(A) Shocked quartz from Tertiary breccia, Antrim,
Northern Ireland. Field of view 2.5 mm, plane polarized
light (PPL). (B)–(D) From Tertiary spherule bed, Nuussuaq, West
Greenland (after Jones et al. 2005b). (B) Impact melt glass spherules,
field of view 2.5 mm (PPL). (C) Skeletal quench-textured Ni-spinel
occurring as radiating “christmas trees” (PPL). (D) Detail of (C). Backscattered electron image (width ~50 µm) showing Ni-spinel crystals,
which have an irregular core of nearly pure Ni metal.
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FIGURE 2
D ECEMBER 2005
0
50
100
300
200
100
0
kilometres
shock and decompression. The OJP was formed on young
oceanic crust close to a spreading ridge, and the precise age
of this crust determines the geothermal gradient, which
controls the amount of melt in our impact model. Simply
changing the age of the oceanic crust from 20 to 10 Ma produces about five times more melt, and for our 20 km
impactor, can produce ~106 km3 melt, equivalent to a LIP.
The largest event modelled is extreme, and involved a 30 km
diameter dunite projectile impacting at 20 km/s. Within
~10 minutes of impact, the melt was distributed predominantly as a giant sub-horizontal disc with a diameter in
excess of 600 km at ~150 km depth in the upper mantle,
although most of the melt was shallower than ~100 km
(FIG. 3). The total volume of melt produced is ~2.5 × 106 km3
and ranges from 100% melt (superheated liquid >500°C
above the solidus) within 100 km of ground zero, to nonequilibrium partial melts varying in amount with depth
and distance (FIG. 3). Some of this melt will quench, but
most will crystallise slowly, taking perhaps tens of thousands of years to solidify (Jones et al. 2005a). Massive reorganisation of the affected upper mantle, driven by largescale physical disturbances such as displaced crust,
juxtaposed hot and cold materials and mobile melts, is virtually inevitable (Price 2001). This has not been modelled,
but thermodynamic relaxation of heterogeneously melted
mantle could be redistributed through a much larger
mantle volume as a conventional partial melt, to a maximum of approximately ~7.5 × 106 km3 of basalt, assuming
20–30% partial melting.
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The map shows the outline of the Ontong Java Plateau. Its
size can be compared with the impact melt derived from
a hydrodynamic simulation of a large impact. The round “bulls eye” at
the same scale as the map represents the area of melt generated ~20
minutes after vertical impact of a 30 km meteorite into young oceanic
crust. The simulation assumes a young oceanic mantle geotherm, minimal dry melting and maximum credible projectile size (Jones et al.
2005a). The enlarged cross section beneath, with horizontal equal to
vertical scale (in kilometres), shows melt distribution in the mantle on
the left and deformation paths as disturbed layers on the right. Colours
on the left-hand side show the extent of melting: red, up to 100% melt
(corresponding to superheated conditions at temperature >500 degrees
above solidus); yellow 50% melt; blue >1% melt. In peridotite mantle
the colours red-yellow-blue correspond approximately with regions
where the products of melting are peridotite or komatiite, picrite and
basalt, respectively. The maximum depth of melting for this model is
approximately 150 km. Most of the melt is confined to a diameter of
300 km, but a thin surface layer and a deeper disc at ~50–70 km extend
to >600 km. The total melt volume generated both by shock and
decompression melting is ~2.5 × 106 km3. Massive reorganisation of the
affected upper mantle is expected, and could trigger further mantle
upwelling and additional melting.
FIGURE 3
This volume of impact melt exceeds that of most LIPs but is
slightly less than the total volume of the OJP (>30 × 106 km3).
Larger melt volumes will result from hotter mantle potential temperatures (>1500 degrees, e.g. Chazey and Neal
2004). Various other parameters could be changed – projectile dimensions, velocity, hydrous mantle, or anomalous
mantle composition. Part of the impact-induced melt
would be buoyant and erupt rapidly. This episode might be
followed by an extended secondary period of additional
melting during mantle upwelling, as envisioned by Grieve
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(1980) and described in more detail in the lunar mare
model of Elkins-Tanton et al. (2004). A primary feature of
the oceanic impact model is that voluminous ultramafic or
mafic melts are expected early in the igneous activity.
The geochemistry of the OJP and the absence of thermal
uplift have proved difficult to reconcile with deep mantle
plume models, but these features may be consistent with an
impact origin (see Ingle and Coffin 2004). The major and
trace element chemistry of the OJP lavas is notably uniform, with many incompatible element concentrations
comparable to those for shallow mid-ocean ridge basalt
(MORB), but with isotopic characteristics more like those of
ocean island basalts. A shallow-melting origin (~100 km), is
compatible with the impact model.
The subject of correlations between specific impact events,
LIPs and mass extinctions has been reviewed by Alvarez
(2003). Given the likely size of the impact required, a global
record is expected, but will it be recognisable? Oceanic
impacts away from the continental shelf will not yield
shocked quartz, and shocked ejecta of glasses and silicates
are highly susceptible to replacement by clays. Global fallout layers of fine glass-rich ash and dust could be similar to
conventional volcanic deposits. Volcanic clay layers and
«Fullers Earth» horizons are distinctive marker bands in
Barremian–Aptian geological sequences of northern
Europe, coinciding with the ~120 Ma age of the OJP event;
might one of these contain the distinctive geochemical signature of extraterrestrial components linked to a distal OJP
impact layer? However, no mass extinction correlates with
the date of formation of the OJP, although there was a
global oceanic anoxic event and a global negative Sr isotope
anomaly (Jones and Jenkyns 2001).
IMPACT EJECTA BENEATH A LIP?
The early Tertiary (~62 Ma) lavas forming Disko Island in
central West Greenland and the Nuussuaq Peninsula immediately to the north have been correlated with those on
Baffin Island and form the earliest western extremity of the
North Atlantic Tertiary Igneous Province, in which volcanic
activity continues today in Iceland. These highly magnesian lavas require high temperature melting of shallow
mantle (60–90 km) and may constitute a precursor to the
plume which became established under East Greenland
(Gill et al. 1995). A distinctive spherule bed horizon crops
out over a 3 m interval in shallow water sediments approximately 10 m beneath the local base of the flood lava pile
on the Nuussuaq Peninsula. The glassy spherule layers have
many of the hallmarks of impact ejecta: immiscible melt
textures, distinctive Ni-spinel, and high Ir, PGE and
siderophile element anomalies (Jones et al. 2005b). The
iron-rich silicate glass spherules (~3 wt% NiO, ~35 wt%
FeO) are circular in cross section and show evidence of surface dissolution, smectite replacement and calcite infilling
of vesicles, though many glasses are optically unaltered.
Their pronounced Fe–Ni correlation is dissimilar to volcanic
suites, but can be explained by mixing of a basaltic melt
and an iron–nickel source. Distinctive Ni-spinel grains
(~7–10 wt% NiO) possess very nickel-rich cores (FIG. 2D).
Rare glass spherules show compositional gradients towards
resorbed silicates (plagioclase, clinopyroxene); shocked plagioclase (maskelynite glass) has an anomalous texture comparable to that seen in impact-melted lunar breccias.
Although anomalously high copper and sulphur concentrations (up to ~1% in spherules) have led other researchers to
suggest terrestrial explanations, such as the possibility that
they are products of fire-fountaining of exotic or hot,
picritic Disko lavas (see Jones et al. 2005b for details), a
strong case can be made that they are impact deposits.
Delicate preservation features rule out substantial sedimenELEMENTS
tary reworking, and spherule sizes and bed thicknesses
imply a large source crater. The age of the spherule beds is
constrained by nannofossils and magnetostratigraphy to be
close to the age of initiation of the West Greenland flood
lavas (~62 Ma).
EXPRESSIONS OF IMPACT VOLCANISM?
Although there is no direct evidence that large volumes of
extrusive rock have been produced by impact, the Sudbury
structure shows that large volumes of subsurface magma
can be generated by impact. The Sudbury structure is a large
(D ~200 km, 1850 Ma), deformed and eroded impact crater,
whose central region was occupied by melt. An eight-year
multidisciplinary study by Stoffler et al. (1994) concluded
that the impact excavated deep into the crust, almost to the
mantle (~30 km), before collapse and rebound. The present
eccentric shape is due to subsequent tectonism. The melt
(> ~12,000 km3), possibly superheated, formed by impact
melting of crust within just a few minutes. The magma differentiated by gravity settling of crystals and immiscible
sulfides to produce hundreds of metres of noritic cumulates
(norite is a type of gabbro). Early formed pyroxene and sulfides were swept into basal depressions to form mineralised
norite, overlain by slowly cooled igneous-textured rocks
with differentiated compositions.
There is no record of volcanism at Sudbury but it may have
been spectacular. The high temperatures implied by coexisting immiscible melts and mafic magmas are comparable
to those of many large igneous intrusions, representing the
mid- to upper-crustal reservoirs feeding surface volcanism.
At Sudbury, the presence of pseudotachylites (veins of
shock-induced glassy rock), contact zone breccias and an
array of peripheral shock features is well established. Any
mantle melt component is thought to have been small, but
could have been delivered almost instantly via crust-spanning dykes with rapid post-crater closure (Price 2001).
Rapid closure of fractures may explain the absence of feeders in impact-induced melt bodies such as Sudbury. The
Sudbury nickel deposits are crudely concentrated around
the margins of the impact cavity and form the largest nickel
mining district ever mined. The source of the nickel could
be the impactor in terms of mass balance, although isotope
data suggest a crustal source for the accompanying sulphur.
In other impact craters, there is evidence for associated
downwards and outwards injection of magma, forming
dykes, breccias, and pseudotachylites, and for the establishment of vigorous hydrothermal systems. The Sudbury rootless (?) impact melt, the likelihood of superheat, and the
formation of immiscible sulphides are valuable lessons for
mainstream igneous petrology and global ore prospecting.
FURTHER WORK
Hypervelocity impact models predict that large oceanic
meteorite impacts can generate melts with volumes comparable to those of typical LIPs. Future modelling should
incorporate longer time scales in order to allow for mantle
flow. Impact-triggered mantle melts are expected to have
geochemical signatures like those of mantle plumes. Rapid
mixing of melts from sub-horizontal sub-crater reservoirs to
depths where pyrope garnet and/or diamond are stable is
possible (Jones et al 2003). Impact melting of the mantle
can generate peridotitic melts like komatiites and other
high-degree partial melts (Jones 2002). Reprocessing of
parts of the upper mantle via large bolide impacts is consistent with models of planetary accretion following the late
heavy bombardment and provides an alternative explanation for primitive geochemical signatures currently attributed to plumes entraining material from the core. For LIPs
with mafic to ultramafic initial volcanic products, and
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D ECEMBER 2005
especially for oceanic targets, impact volcanism is a testable
hypothesis. Continental LIPs (e.g. the Siberian Traps; Jones
et al. 2002) may also be the result of impacts and might be
expected to show an admixed component from melted
crust; such LIPs need to be modelled. Evidence of an initiating catastrophe, such as an ejecta layer at the base of a
LIP, may in principal be found, but in practice may be missing because of burial or lack of exposure, as in the case of
the OJP. The West Greenland spherule horizon appears to
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