Supplementary File. The mineral melt partition coefficient for Tm in olivine should be and not as stated by Gibson & Geist (2010).

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1 DR Supplementary File A. REE inversion modelling We have used a modified version of the rare-earth element (REE) inversion procedure of McKenzie & O Nions (1991) to calculate the depth of the base of the rigid lithosphere during the eruption of the Deccan flood-basalts. Further details of the REE inversion technique used here, including updated partition coefficients for garnet, clinopyroxene, orthopyroxene and olivine1 are given by Gibson & Geist (010). This work showed that REE inversion models using the average depleted MORB mantle composition of Workman & Hart (005) or Salters & Stracke (004) with the primitive mantle composition of McDonough & Sun (1995) generated melt columns that were systematically deeper by ~ km than models which used the depleted and primitive mantle compositions of McKenzie & O Nions (1991) Supplementary Figure 1. Results of REE inversion modelling of Deccan flood basalts using the rare-earth-element inversion programme (INVMEL) of McKenzie & O Nions (1991). Plots a) - f) show the mean of the observed data (closed circles) versus modelled results (red continuous line) of the inversion modelling for all of the trace elements in the various Deccan formations. Error bars give standard deviations of element abundances, which are normalised to the depleted mantle source of McKenzie & O Nions (1991). Misfits between the observed and calculated concentrations of Rb, K and Pb may be due to hydrothermal alteration. Plots g), h) and i) show cumulative melt 1 The mineral melt partition coefficient for Tm in olivine should be and not as stated by Gibson & Geist (010). 1

2 fraction versus depth curves in the convecting mantle. Dashed lines are theoretical melt distributions predicted for isentropic decompression melting. In our study, we assume that the depth to the top of the top of the melt column approximates to the base of the lithosphere. Data are from this work and Vanderkluysen et al. (011) The most crucial elements for estimating the depth of melting in the convecting mantle are the middle and heavy REEs, owing to the depth control on garnet stability. An over-estimate of the light/middle REE ratio, perhaps due to lithospheric contamination, causes the REE inversion models to predict a smaller melt fraction. In all of our models the garnet-spinel reaction zone was set at a depth of 80 to 100 km corresponding to a mantle potential temperature of 1500 o C (Klemme and O Neill, 000). B. Whole-rock geochemical data for Deccan basalts Only a limited number of high-precision ICP-MS analyses have been published for Deccan basalts. To supplement these we have analysed samples from Narmada Rift, which represent the basal part of the Deccan CFB succession, and some lavas from the Ambenali Formation that outcrop on the main escarpment at Mahabaleshwar. These samples were collected during fieldwork in India with K.V. Subbarao, D. McKenzie, M. Anand and M. Widdowson. Samples of Deccan basalts were analysed for major elements and some trace elements (Ni, Cr) by XRF in the Dept of Earth Sciences at the Open University, UK by J Watson and for other trace elements by ICP-MS in the Dept of Earth Sciences at the University of Cambridge, UK by JA Day. Full details of these analytical techniques, together with details of analytical precision, can be found in Gibson et al. (010). Supplementary Table 1: Analyses of Deccan lavas used in REE inversion models Formation Ambenali Fm Ambenali Fm Narmada Narmada Sample no 97SG1 97SG 98SG14 98SG15 Grid ref (WGS84) N17 o 56.8' E7 o 5.8' N17 o 56.55' E7 o.06' N1 o 50.94' E7 o 41.0' N1 o 50.94' E7 o 41.0' SiO TiO Al O Fe O t MnO MgO CaO Na O K O P O Total LOI Ba Co Cr Cs Cu Hf Nb Ni Pb Rb Sc Sr Ta Th U V Y Zn Zr Ga La Ce Pr

3 Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu *Fe O t represents total Fe. ** ICP-MS analyses undertaken at the University of Durham on 98SG14 & 98SG15 were first published by Gibson (00). These samples were recently re-analysed by ICP-MS at the University of Cambridge, UK under the same running conditions as 97SG1 and 97SG.

4 Appendix: Mathematical model for permeability and flow response within the Deccan plume head to dynamic stresses Similar to the response of hydrological systems to the dynamic stresses of earthquakes, we consider the possible response of a CFB magmatic system to seismic stresses in terms of characteristic timescales for flow response and recovery. For a long duration response lasting many eruptive events, there must be a change in the effective permeability of some part of the system rather than a change in the hydraulic head driving flow, which would generate a response too short lived to account for a significant volume release. As described in the main text, field evidence suggests that Deccan eruptions around the time of Chixculub were emanating from a relatively localized vent system from un oriented dikes that contrast with earlier strongly aligned Deccan dike complexes. This suggests that the flux of mantle melt rising through the crustal plumbing system were large enough that tectonic influence on dike orientation was suppressed. Constraints are fewer on the extent of deeper plumbing, but mantle plume models suggest a laterally extensive zone of 5% melt available in the upper mantle to fuel the plumbing system. The melting region extends up to km radius in classical plume head models (Farnetani and Richards, 1994). We characterize the deep plumbing system by two effective permeabilities (regime 5 in the main text, and Figure A1). κ 1 is the permeability of the partially molten plume head where flow is dominated by flow along grain boundaries. κ is the effective permeability of the system at a level where channelization of migrating melt occurs, localizing a lateral recharge region to a more confined vertical transport system with a higher permeability. Melt migration through this system to crustal depths (where transport is accomplished through magma chambers and dikes more rapidly) is governed in our model by the equations for porous flow between regions with differing permeabilities, forced with constant hydraulic head h 0 h µβ eff t = κ h z + κ 1 wd (h 0 h). (A1) Here h is the hydraulic head in the vertical channelized region of melt transport, varying only in the z direction. µ is the viscosity of melt and β eff = β s + φβ l is the effective compressibility of the two phase system with solid compressibility β s, liquid compressibility β l and porosity φ. w is the width (in x direction, Figure A1) of the vertical melt channels, D the width of the partially molten recharge zone with fixed head h 0 at x = D. We assume no flow at the bottom boundary h/ z = 0 at z = L and no head at the top boundary h = 0 at z = 0. This is a crude model for a catchment area with more complex geometry as sketched by regime 5 in the main text, but captures the basic partitioning of transport efficacy in the system. We study the response of this model to perturbation that suddenly increases the vertical permeability κ at t = 0 from κ,i to κ,f. The discharge Q from this porous flow region to higher levels subsequently increases from an initial to a new steady state over some flow timescale τ f. We consider however that this permeability increase is transient, that is, there is a structural relaxation of permeability to pre impact values that occurs over a compaction timescale τ p. Thus the discharge Q decreases back to pre impact values eventually. It is the competition between these two processes 1

5 that determines the overall magnitude and duration of increased melt influx to the crust and hence increased surface eruption rate. Discharge can be found from Darcy s law, with ρ l the melt density and g gravity as Q = ρ lgκ w µ evaluated at z = 0. We utilize an analytic solution to equation (A1) (Carslaw and Jaeger, 1959; Manga and Rowland, 009): ( ) 1/ τv (1 cosh ( τ v /τ h )) Q(t) = q τ h sinh ( + (A) τ v /τ h ) [ ] R(τ v /τ h ) 8 q (n 1) π + 4R(τ v /τ h ) (τ v /τ h ) (n 1) π exp [ (1/τ h + (n 1) π /τ v )t]. + 4(τ v /τ h ) n=1 Equation (A) exhibits adjustment of discharge from pre- to post- impact permeability structure governed by four parameters h z, (A) q = ρ lgw κ,f µl Scaled magnitude of discharge (A4) R = κ,f κ,i Ratio of final to initial vertical permeabilities (A5) τ h = µβ eff Dw κ 1 Timescale for lateral flow (A6) τ v = µβ eff L κ,f Timescale for vertical flow (A7) with the flow adjustment timescale well approximated by τ f (1/τ h + π /τ v ) 1. Because the effective permeability of a channelized flow system is considerably larger (e.g., Hewitt and Fowler, 009), the smaller vertical timescale for flow τ v dominates the response. Thus τ f τ v. Competing with this increased discharge is the structural adjustment of flow pathways to pre impact conditions. For a two phase viscous system a natural timescale for such relaxation is the compaction timescale δ c /v, where δ c = κφµ s /µ is the compaction length (e.g., Hewitt and Fowler, 009; McKenzie, 1984; Spiegelman, 199) with matrix viscosity µ s and v the flow velocity. δ c ranges from 100 m to 10 km for the range of likely parameters in our problem. Although the compaction length sets the scale over which pressure gradients are felt in a two phase viscous system, consideration of the volume of melt extracted suggests a catchment dimension for the crustal plumbing system that exceeds a compaction length by at least an order of magnitude. We thus consider the possibility that kinematics of plume impingement on the lithosphere may provide alternative constraints on melt extraction and also consider a scale length for permeability recovery similar to the lateral dimension of the melting zone (δ r 100 km). If we assume a typical buoyancy driven scaling for melt flow v = κ ρg/φµ with ρ 00 kg/m the density difference between melt at matrix driving flow, we have τ p = δ c v = 1 φ µ s µ ρg κ. (A8) Alternatively if the lateral dimension of the melting zone sets relaxation, we have τ p = δ r v = δ rφµ κ ρg. (A9)

6 Here κ is the vertical permeability that relaxes after the impact. A simple model for the structural relaxation of permeability to pre impact values is (Manga and Rowland, 009) κ (t) = κ,i + (κ,f κ,i )e t/τp, (A10) and we use equation (A) for flow adjustment along with this relation to explore the range of response times of the deep plumbing system. These estimates provide a quantitative basis for the arguments posed in the main text regarding possible response of Deccan eruption rates to an impactor that adjusts the deep permeability. We obtain the background head h 0 that recharges the channelized melt system through the steady state solution h 0 = Q 0 κ,i Dw sinh L κ1 κ,i Dw κ,i w κ 1 1 cosh L, (A11) κ 1 κ,i Dw using Q 0 = 100 km /yr. To remain consistent with our assumption of decoupled flow and structural relaxation, we focus on two limits: one in which the flow response timescale τ f is much smaller than the structural relaxation timescale τ p, and one in which τ p is smaller. These two limits are illustrated in Figure A, along with the bounds on relaxation dimension discussed above. Requiring a melt region capable of producing observed flow volumes, we take illustrative parameter values as follows: L = 100 km (vertical extent of channelized flow region), D = 100 km recharge length scale (melt catchment region dimension), and w = 100 m (sum total cross sectional dimension of multiple melt channels that comprise the vertical transport system). For a long duration flow response as given by red/blue curves in Figure A, we take representative κ 1 = m, κ,i = 10 1 m, with a two order of magnitude increase in permeability occurring as a result of the impact κ,f = 10κ,i. µ = 0.1 Pas (appropriate for ultramafic melts), µ s = 10 1 Pas, φ = 0.0, ρ l = 000 kg/m, ρ = 00 kg/m, β eff = Pa 1. For a short duration response of small amplitude given by orange/green curves in Figure A, we take κ,i = 10 1 m, a single order of magnitude increase in permeability, and µ s = 19 Pas. Dotted curves are for a permeability relaxation length scale of δ c, while solid curve are form δ p

7 z = 0 Moho level chambers Channelized melt zone, width w, height L, permeability κ Porous flow in plume head, lengthscale D, permeability κ1, constant head assumed at x = D { w z = L, no vertical flow z x Figure A1: Schematic diagram of the model magmatic plumbing system. A laterally extensive and hydraulically connected reservoir of plume head melt feeds through porous flow a network of channels with higher effective permeability that transport melt vertically to Moho level magma chambers where melt stalls beneath lower density crust. Both regimes of plume drainage are part of regime 5 in the main text. 4

8 τ < τ p f τ > τ p f Flow adjustment to increased permeability Permeability relaxation to pre-impact conditions 800 Flux from plume head (km /yr) Compaction length limit Melt region lateral dimension limit Time since impact (years) Figure A: Model results from equations (A) (red and orange curves) and (A10) (blue and green curves). Separation in terms of thick and dotted lines illustrates the predicted flow response: discharge from the deep plume system follows the flow response until permeability shuts off, at which point flow decreases following the permeability readjustment. Note log 10 scale on the x axis in both panels. Red and blue curves illustrate an example of system response with τ p > τ f. The descending dotted and solid blue curves bound the structural relation time: dotted curve is for compaction dominated response, while solid is for relaxation set by the lateral dimension of melting. In this case there is prolonged period of increased flow at roughly an order of magnitude greater volumetric flux. τ p = years when δ c is used, τ p = years when δ f is used, while τ f =.4 10 years. Orange and green curves illustrate a case where τ p < τ f. Here permeability rapidly recovers to pre impact values and there is little long lived flow response. τ p = years when δ c is used, τ p = years when δ f is used, while τ f =.4 10 years. 5

9 Carslaw, H., and Jaeger, J., 1959, Conduction of heat in solids: Oxford University Press, nd edition. Farnetani, C., and Richards, M., 1994, Numerical investigations of the mantle plume initiation model for flood basalt events: Journal of Geophysical Research, v. 99, p Gibson, S. A., 000, Ferropicrites: geochemical evidence for Fe-rich streaks in upwelling mantle plumes: Earth and Planetary Science Letters, v. 174, p Gibson, S.A., 00, Major element heterogeneity in Archean to Recent mantle plume starting-heads: Earth and Planetary Science Letters, v. 195, no. 1-, p , doi: /S001-81X(01) Gibson, S.A., and Geist, D.J., 010, Geochemical and geophysical mapping of lithospheric thickness variations beneath Galápagos: Earth and Planetary Science Letters, v. 00, p , doi: /j.epsl Hewitt, I., and Fowler, A., 009, Melt channelization in ascending mantle: Journal of Geophysical Research, v. 114, doi:10.109/008jb Klemme, S., and O Neill, H. S. C., 000, The near-solidus transition from garnet lherzolite to spinel lherzolite: Contributions to Mineralogy and Petrology, v. 18, p Manga, M., and Rowland, J., 009, Response of Alum Rock springs to the October 0, 007 Alum Rock earthquake and implications for the origin of increased discharge after earthquakes: Geofluids, v. 9, p Manga, M., and Bonini, M., 01, Large historical eruptions at subaerial mud volcanoes, Italy: Natural Hazards and Earth System Science, v. 1, p Manga, M., Beresnev, I., Brodsky, E.E., Elkhoury, J.E., Elsworth, D., Ingegritsen, S.E., Mays, D.C., and Wang, C.-Y., 01, Changes in permeability caused by transient stresses: Field observations, experiments, and mechanisms: Reviews of Geophysics, v. 50, doi: /1/011RG0008. McDonough, W.F. and Sun, S.S., 1995, The Composition of the Earth: Chemical Geology, v. 10, p. -5. McKenzie, D., 1984, The generation and compaction of partially molten rock: Journal of Petrology, v. 5, p McKenzie, D., and O Nions, R. K., 1991, Partial melt distributions from inversion of rare earth element concentrations: Journal of Petrology, v., p Salters, V.J.M., and Stracke, A., 004, Composition of the depleted mantle: Geochemistry Geophysics Geosystems, v. 5, /00GC000597, doi: /00GC Spiegelman, M., 199, Flow in deformable porous mdeia. part 1: Simple analysis: Journal of Fluid Mechanics, v. 47. p Vanderkluysen, L., Mahoney, J.J., Hooper, P.R., Sheth, H.C., and Ray, R., 011, The feeder system of the Deccan Traps (India): Insights from dike geochemistry: Journal of Petrology, v. 5, p. 15-4, doi: /petrology/egq08. Workman, R.K., and Hart, S.R., 005, Major and trace element composition of the depleted MORB mantle (DMM): Earth and Planetary Science Letters, v. 1, no. 1-, p. 5 7, doi: /j.epsl

EMMR25 Mineralogy: Ol + opx + chlorite + cpx + amphibole + serpentine + opaque

EMMR25 Mineralogy: Ol + opx + chlorite + cpx + amphibole + serpentine + opaque GSA Data Repository 2017365 Marshall et al., 2017, The role of serpentinite derived fluids in metasomatism of the Colorado Plateau (USA) lithospheric mantle: Geology, https://doi.org/10.1130/g39444.1 Appendix