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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 ELEMENTS, VOL. 1, PP. 277–281 277 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 ELEMENTS 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. 278 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. ELEMENTS 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 279 D ECEMBER 2005 (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 280 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 REFERENCES Alvarez W (2003) Comparing the evidence relevant to impact and flood basalt at times of major mass extinctions. Astrobiology 3: 153-161 Canup RM, Asphaug E (2001) Origin of the Moon in a giant impact near the end of the Earth’s formation. Nature 412: 708-712 Chazey WI, Neal CR (2004) Large igneous province magma petrogenesis from source to surface: platinum-group element evidence from Ontong Java Plateau basalts recovered during ODP legs 130 and 192. In: Fitton JG, Mahoney JJ, Wallace PJ, Saunders AD (eds) Origin and Evolution of the Ontong Java Plateau. Geological Society of London Special Publication 229, pp 219-238 Dietz RS (1972) Sudbury astrobleme, splash emplaced sub-layer and possible cosmogenic ores. In: Guy-Bray JV (ed) New Developments in Sudbury Geology. Geological Association of Canada Special Paper 10, pp 29-40 Elkins-Tanton LT, Hager BH, Grove TL (2004) Magmatic effects of the lunar late heavy bombardment. Earth and Planetary Science Letters 222: 17-27 satisfy the impact–LIP criteria of superposition and mafic LIP startup geochemistry and is accessible to future scrutiny. ACKNOWLEDGMENTS I thank colleagues at UCL for many discussions, and particularly David Price and Paul DeCarli for mainstream partnership central to the theme of this review. Ian Parsons and Stephanie Ingle are thanked for providing most thoughtful reviews. . Jones AP, Price GD, Price NJ, DeCarli PS, Clegg RA (2002) Impact induced melting and the development of large igneous provinces. Earth and Planetary Science Letters 202: 551-561 Jones CE, Jenkyns HC (2001) Seawater strontium isotopes, oceanic anoxic events, and seafloor hydrothermal activity in the Jurassic and Cretaceous. American Journal of Science 301: 112-149 Jones AP, Price GD, De Carli PS, Price NJ, Clegg RA (2003) Impact decompression melting: a possible trigger for impact induced volcanism and mantle hotspots? In: Koeberl C, Martinez-Ruiz F (eds) Impact markers in the Stratigraphic Record. Springer, Berlin, pp 91-120 Koeberl C (2002) Mineralogical and geochemical aspects of impact craters. Mineralogical Magazine 66: 745-768 Jones AP, Wunemann K, Price GD (2005a) Modeling impact volcanism as a possible origin for the Ontong Java Plateau. In: Foulger GR, Natland JH, Presnall DC, Anderson DL (eds) Plates, Plumes and Paradigms. Geological Society of America Special Paper 388: 711-720 Jones AP, Kearsley AT, Friend CRL, Robin E, Beard A, Tamura A, Trickett S, Claeys P (2005b) Are there signs of a large Palaeocene impact, preserved around Disko bay, W. Greenland?: Nuussuaq spherule beds origin by impact instead of volcanic eruption? In: Kenkman K, Hörz F, Deutsch A (eds) Large Meteorite Impacts III. Geological Society of America Special Paper 384: pp 281-298 Gill RCO, Holm PM, Nielsen TFD (1995) Was a short-lived Baffin Bay plume active prior to initiation of the present Icelandic plume? Clues from the high-Mg picrites of West Greenland. Lithos 34: 27-39 Pierazzo E, Vickery AM, Melosh HJ (1997) A reevaluation of impact melt production, Icarus 127: 408-423 Price NJ (2001) Major Impacts and Plate Tectonics. Routledge, London, 416 pp Rogers GC (1982) Oceanic plateaus as meteorite impact signatures. Nature 299: 341-342 Stöffler D, Deutsch A, Avermann M, Bischoff L, Brockmeyer P, Buhl D, Lakomy R, Müller-Mohr V (1994) The formation of the Sudbury structure, Canada: toward a unified impact model. In: Dressler BO, Grieve RAF, Sharpton VL (eds) Large Meteorite Impacts and Planetary Evolution. Geological Society of America Special Paper 293: 303-318 White RV, Saunders AD (2005) Volcanism, impact and mass extinctions: incredible or credible coincidences? Lithos 79: 299-316 . Glikson AY (2001) The astronomical connection of terrestrial evolution: crustal effects of post-3.8 Ga mega-impact clusters and evidence for major 3.2 ± 0.1 Ga bombardment of the Earth–Moon system. Journal of Geodynamics 32: 205-229 Green DH (1972) Archaean greenstone belts may include terrestrial equivalents of lunar maria? Earth and Planetary Science Letters 15: 263-270 Grieve RAF (1980) Impact bombardment and its role in proto-continental growth on the early earth. Precambrian Research 10: 217-247 Grieve RAF, Cintala MJ (1992) An analysis of differential impact melt-crater scaling and implications for the terrestrial impact record. Meteoritics 27: 526-538 Ingle S, Coffin MF (2004) Impact origin for the greater Ontong Java Plateau? Earth and Planetary Science Letters 218: 123-134 Ivanov BA, Melosh HJ (2003) Impacts do not initiate volcanic eruptions: Eruptions close to the crater. Geology 31: 869-872 Jones AP (2002) Komatiites: new information on the type locality (Barberton), and some new ideas. Geology Today 18: 23-25 ELEMENTS 281 D ECEMBER 2005