Ivanov and Melosh (2003) define constraints on the likelihood of asteroid impact-triggered volcanic activity. I agree with the authors that to date there is no evidence for an impact-triggered origin of oceanic and continental large igneous provinces, possibly with the exception of the Cretaceous-Tertiary (K-T) boundary Decan basalts (Alt et al., 1988), and that impact reactivation of hot spot loci (Abbott and Isley, 2002) must represent a rare coincidence in Earth history. However, here I point out (1) crustal and petrologic factors that greatly increase the probability of impact-triggered volcanism in geothermally active regions of oceanic basins, and (2) Archean to early Proterozoic field and geochemical evidence of large impact events with likely volcanic consequences.

For a flux of impact craters (Ds > 20 km, where Ds = outer structural diameter) on the order of 4.3–6.3 × 10−15 km−2 yr−1 (Shoemaker and Shoemaker, 1996) on a post–Late Heavy Bombardment (LHB, post–3.8 Ga) Earth occupied by >80% time-integrated oceanic crust (McCulloch and Bennett, 1996), with a cumulative asteroid and crater size/frequency distribution NDD 10−1.8 (ND = number of craters of diameter D), some 360 craters with D >100 km and some 40 craters with D >300 km would form in oceanic basins post-LHB (Glikson, 2001). With a present-day high-geotherm (30 K km−1) oceanic crust occupying ~10% of oceanic crust (Ivanov and Melosh, 2003), assuming higher Archean geothermal gradients and smaller-scale convection cells and plate dimensions (Lambert, 1983), thin (<5 km) oceanic crustal spreading regions overlying shallow asthenosphere (<50 km) can be expected to have occupied >25% of the Archean oceanic basins. In this estimate some ~90 craters with Ds ~ 100 km and ~10 craters with Ds ~ 300 km impacted thermally active oceanic crust post-LHB.

For a 300 km impact structure, using morphometric estimates after Grieve and Pilkington (1996), the stratigraphic uplift SU = 0.086Ds1.03 is ~30 km. Alternatively, assuming a Ds/Dt ratio of ~2 (Dt = diameter of transient crater), the transient crater depth, dt = 0.28Dt1.02, would be ~45 km. Under geothermal gradients of ~30 K km−1 near-solidus asthenosphere would occur at depths of ~40–50 km. The close agreement between the excavation and near-solidus parameters for craters Ds ~ 300 km suggests that impacts on this scale would result in intersection of the peridotite solidus by impact-rebounded asthenosphere, with consequent partial melting. The assumption that catastrophic mantle melting took place during the Archean is supported by the occurrence of peridotitic komatiites (>30% MgO) (Green, 1972, 1981).

Ivanov and Melosh (2003, p. 872) state: “The role of large-scale impacts in triggering volcanism has been small, if not negligible, for the past 3–3.3 b.y.” The identification in Archean impact fallout units of high iridium fluxes and large spherule radii (<2 mm) (3.26–3.24 Ga impact fallout deposits in the eastern Transvaal—Lowe et al., 2003 and Kyte et al., 2003; 2.63 Ga and 2.47–2.50 impact fallout units in the Hamersley Basin, Western Australia—Glikson and Vickers, 2003), coupled with the mafic geochemistry and absence of planar deformation features–bearing quartz in these units (Simonson et al., 1998), suggest very large impact events in the contemporaneous oceanic basins. The ca. 3.26–3.24 Ga impact cluster in the Barberton greenstone belt (Lowe et al., 2003) broadly correlates with ca. 3.2 Ga peak impact events on the Moon deduced from Ar-Ar ages of lunar impact spherules (Culler et al., 2000) and with ca. 3.2 Ga Rb-Sr and Ar-Ar ages of volcanics in some of the lunar maria (Glikson, 2001). That some of the Archean impact fallout deposits are accompanied by volcanic tuffs and are succeeded by banded iron formations hints at contemporaneous volcanic and hydrothermal activity.

Finally, I note the title of the article refers to “eruptions close to the crater.” Further studies are required to test whether impact-generation and propagation of deep crustal fractures in distal crustal sectors may have taken place. I suggest that the jury is still out regarding the question of the role of large asteroid impacts (Ds >> 100 km) in potentially triggering and/or enhancing mantle fusion events in thermally active oceanic crustal regions (Stothers and Rampino, 1990).

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