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SUPPLEMENTARY INFORMATION DOI: 10.1038/NGEO2392 Information Continental Supplementary crust generated in oceanic arcs Esteban Gazel, Jorden Hayes, Kaj Hoernle, Peter Kelemen, Erik Everson, W. Steven Holbrook, Folkmar Hauff, Paul van den Bogaard, Eric A. Vance, Shuyu Chu, Andrew J. Calvert, Michael J. Carr, Gene M. Yogodziski 1. The Central America Land Bridge: From arc to continent The Central American Land Bridge (CALB) includes Costa Rica and Panama (Fig. S1). Its formation started ~15 Ma1 and culminated ~3 Ma2. Current models suggest that the land bridge resulted mainly from the interaction of this intra-‐ oceanic arc with Galapagos Hotspot tracks3, 4. The formation of the land bridge enabled the exchange of fauna between the Americas and played an important role in changing oceanic circulation and global climate2, 5-‐9. Tectonic reconstructions and regional studies suggest that the CALB developed in the western-‐ most edge of the Caribbean Plateau, an area without old, evolved continental crust10-‐12. Along-‐arc variations in trace elements and Pb, Sr, Nd and O isotopes in Central America are similar among silicic deposits and mafic lavas13-‐15. This is evidence that silicic magmas are genetically related to basaltic magmas produced in the mantle wedge and that no interaction with old, evolved continental crust occurred14. In general, O-‐isotope Figure S1. Tectonic setting of the CALB15, 79-‐81. Note that the isotopic compositions of the erupted lavas in the CALB match the isotopic variations show no evidence of domains of the subducting Galapagos Hotspot tracks. Costa Rican lavas hydrothermal alteration, supporting require the input of the subducting Seamount Province (blue areas) the conclusion that assimilation of and southern Costa Rican and Panamanian magmas require the 3, 4 subducting Cocos/Coiba Ridge (green areas) . CCR: central Costa altered volcanic rocks was not a Rica, CNS: Cocos-‐Nazca Spreading Center, EPR: East Pacific Rise, NAP: major process in the arc14. N. American Plate. Along the volcanic front in Central America only Guatemalan lavas clearly show effects of crustal contamination13 (Fig. S2). Thus, interaction with pre-‐existing continental crust may be ruled out as a possible source of CALB magmas. Our new data (Table S1) combined with recent data from Panama16 NATURE GEOSCIENCE | www.nature.com/naturegeoscience 1 1 yield the most complete geochemical reconstruction of the evolution of a subduction system (Costa Rica from 30 Ma; Panama from 70 Ma) (Fig. 1). We discovered that the Galapagos signature appeared in the geochemical record in at least two pulses (Fig. 1). After the most recent pulse (<12 Ma) there was a dramatic change in the geochemistry of magmas in southern Central America, evolving from a depleted primitive arc composition to one closer to continental crust. Recent seismic velocity models from active-‐source surveys in Costa Rica also suggest that this island arc has the average P-‐wave velocities that are closest to continental crust of any non-‐ continental arc, worldwide (Fig. 3). The record of evolution from depleted arc composition towards a young continental arc makes this region a natural laboratory to test continental evolution models. Figure S2. Pb-‐Nd isotopes for magmas of the active Central America 2. Global comparison of volcanic front. Mixing lines connect the required end-‐members, intra-‐oceanic island arcs: Where depleted mantle (MORB), subducting Seamount Province (SP) and Cocos/Coiba Ridge (CCR)4. The Galapagos signature decreases along the juvenile continental crust is volcanic front from central Costa Rica/Panama towards Nicaragua. Note being formed today? that only some samples from Guatemala are affected by pre-‐existing continental crust. Understanding whether present conditions can form juvenile continental crust is fundamental to reconstructing the evolution of Earth. We produced a global comparison of major (wt%) and trace element (ppm) compositions of modern intra-‐oceanic arcs, limited to intra-‐oceanic arcs and the CALB that lacks older continental crust basement. Geochemical data for the CALB is from RU_CAGeochem (http://dx.doi.org/10.1594/IEDA/100263), Gazel et al.4, 15, Wegner et al.16 and this study (Tables S1 and S2). The Aleutian average was computed from compilations of Kelemen et al.17, Singer et al.18, and personal communications with Gene Yogodzinki, Kaj Hoernle and Maxim Portnyagin. Additional data from Panama and other intra-‐ oceanic arcs (Aleutians, Lesser Antilles, Izu-‐Bonin, Marianas, South Scotia, Tonga and Vanuatu) used in this study are from the Georoc Database (http://georoc.mpch-‐ mainz.gwdg.de). The compilation, including original references, is available upon request from the corresponding author (E. Gazel). Continental crust is andesitic in composition and enriched in elements incompatible in the mantle (e.g., K2O, LREE). These incompatible elements are fractionated by melting and crystallization processes, possibly at different stages19, 20. While silicic magmas (SiO2 >60 wt%) are common in all arc systems we evaluated, only the CALB, W. Aleutians, and the Lesser Antilles show a normal distribution with a significant portion (more than half) of the data in the range of estimated, bulk continental crust (Fig. S4). The rest of the arcs have bimodal distributions dominated by basaltic compositions with a minor population of silicic melts. K2O shows an overall log-‐normal distribution with the exception of CALB, Iwo-‐Jima, the E. and W. Aleutians and Vanuatu, which clearly reach continental crust K2O values. 2 CALB (Costa Rica and Panama) 100 150 C. Crust N=259 M=0.545 SD=0.641 SE=0.0399 N=244 M=53.5 SD=6.24 SE=0.399 60 100 N=412 M=57.2 SD=4.31 SE=0.212 40 <10 Ma >12 Ma Frequency Frequency 80 C. Crust N=414 M=2.19 SD=0.853 SE=0.0419 50 20 0 0 45 50 55 60 65 70 75 0 1 2 SiO 2 3 4 5 K2O Western Aleutians (Adak and West) N=284 M=57.3 SD=4.96 SE=0.294 80 N=285 M=1.39 SD=0.466 SE=0.0276 120 C. Crust C. Crust 100 60 Frequency Frequency 80 40 60 40 20 20 0 0 45 50 55 60 65 70 75 0 1 2 SiO2 3 4 5 K 2O Eastern Aleutians N=1090 M=1.26 SD=0.688 SE=0.0208 N=1087 M=55.1 SD=5.40 SE=0.164 250 300 C. Crust 250 Frequency Frequency 200 150 100 200 150 100 50 0 C. Crust 50 45 50 55 60 SiO 2 65 70 75 0 0 1 2 3 K2O 4 5 Figure S3: SiO2 (wt%) and K2O (wt%) comparison of the CALB and modern intra-‐oceanic arcs with continental Figure S2. crust. The gray bar represents the range of average continental crust reported in Rudnick and Gao20 (Table 9). 3 L. Antilles N=1565 M=56.9E01 SD=5.46 SE=0.138 300 N=1513 M=0.926 SD=0.578 SE=0.0148 600 C. Crust C. Crust 500 Frequency Frequency 200 100 400 300 200 100 0 0 45 50 55 60 65 70 75 0 2 SiO 2 Vanuatu 4 5 N=656 M=1.67 SD=1.15 SE=0.0449 150 C. Crust C. Crust 100 Frequency 100 50 0 50 0 45 50 55 60 65 70 75 0 1 2 SiO 2 N=316 M=54.4 SD=4.52 SE=0.255 80 4 5 N=321 M=0.869 SD=0.473 SE=0.0264 100 C. Crust C. Crust 80 Frequency 60 40 20 0 3 K2O Marianas Frequency 3 K2O N=664 M=53.5 SD=5.63 SE=0.219 150 Frequency 1 60 40 20 45 50 55 60 SiO 2 65 70 0 0 1 2 3 K2O 4 5 Figure S3 (continued): SiO2 (wt%) and K2O (wt%) comparison of CALB and modern intra-‐oceanic arcs with continental crust. The gray bar represents the range of average continental crust reported in Rudnick and Gao20 (Table 9). 4 Izu-Bonin N=1391 M=0.614 SD=0.666 SE=0.0179 N=1424 M=54.5E01 SD=6.19 SE=0.164 800 C. Crust 400 C. Crust North of Iwo-Jima 300 Frequency Frequency 600 200 400 200 100 0 Iwo-Jima (high La/Yb) 45 50 55 60 65 70 0 75 0 1 2 SiO 2 Tonga 4 5 N=819 M=0.483E SD=0.342 SE=0.0119 N=805 M=55.9E01 SD=6.48 SE=0.228 200 3 K2O C. Crust C. Crust 400 300 Frequency Frequency 150 100 50 200 100 0 0 45 50 55 60 65 70 75 0 1 2 SiO 2 South Scotia 4 5 K2O N=352 M=0.381E SD=0.279E SE=0.0149 N=353 M=53.8 SD=4.00 SE=0.213E-01 120 3 200 C. Crust C. Crust 100 150 Frequency Frequency 80 60 100 40 50 20 0 0 45 50 55 60 SiO 2 65 70 75 0 1 2 3 K2O 4 5 Figure S3 (continued): SiO2 (wt%) and K2O (wt%) comparison Figure S2. of the CALB and modern intra-‐oceanic arcs with continental crust. The gray bar represents the range of average continental crust reported in Rudnick and Gao20 (Table 9). 5 3. Production of young continental crust in intra-‐oceanic arcs: Melting of the enriched oceanic crust Fractional crystallization can produce silicic melts25, but unless primary magmas start with enriched compositions, fractional crystallization itself won’t produce the incompatible-‐ element composition of continental crust14, 17. During the CALB geochemical evolution, it is likely that partial melting of subducting lavas from Galapagos Hotspot tracks resulted in the output of young, felsic continental crust4. Thus, we suggest that the process required to produce these “young continents” initiates with partial melting of incompatible-‐element enriched (relative to MORB) subducting oceanic crust and reaction of those melts with the mantle wedge. Once this process produces the “enriched starting material,” the evolution of primary magmas by shallow processes like fractional crystallization, assimilation and anatexis of the lower crust26, 27 will add to the process of “brewing” young continental crust. Melting of incompatible-‐element enriched intraplate volcanics (Galapagos-‐modified subducting crust in the CALB)4, 15 may also supply the 5-‐20% intraplate component inferred to be present in continental crust20. The presence of a component formed by partial melting of subducting sediment and/or basalt in arc magmas worldwide is now widely accepted17, 23, 28, 30, 31. Modern thermal models indicate that temperatures above the aqueous fluid saturated solidus are common along the top of most subducting plates worldwide, including that beneath Central America29, 32, 33, 34. However, only in the W. Aleutians and the CALB is it clear that isotopically juvenile lavas with “continental” major and trace element compositions are produced at present time3, 4, 17. In other intra-‐oceanic arcs, there are either (a) isotopic data indicating a substantial component derived from subducting, continental sediments – raising the possibility that trace element similar to continental crust are inherited from this component – and/or (b) trace element abundance are substantially depleted in highly incompatible elements relative to continental crust. Although partial melting of the subducting slab may play a significant role in the generation of continental crust, this process would be facilitated if the slab was enriched in incompatible-‐elements by intraplate processes. This will explain why the estimates of incompatible trace elements in continental crust are more enriched than those found in intra-‐ oceanic volcanic arcs19, 20, 35. An intraplate input could occur when the eruptive products of a plume (e.g., ridges and seamount tracks) get recycled in a subduction system4. Partial melting of subducting, hotspot lavas can “re-‐fertilize” the arc mantle wedge, and subsequent melting of this metasomatized mantle may produce geochemically enriched lavas in an arc setting4 (Fig. S4). For example, Bryant et al.36 reported Pb, Sr, and Nd isotopes and trace element evidence for the interaction between melts from the subducting Carnegie Ridge (Galapagos track) and the mantle wedge in the northern Andean Volcanic Zone in Ecuador. In the northern Marianas, the subduction of the Wake and Magellan seamounts37 also correlates with an enriched incompatible element signature of the eruptive lavas38-‐40. Similar results have been reported in central Vanuatu, by the interaction of this subduction system with the D’ Entrecasteaux Ridge and associated seamounts41. Gazel et al.15 reported migration of primitive andesites/adakites at 35 mm/y from northwest to southeast along the CALB, tracking the eastward movement of the triple junction where the Panama Fracture Zone intersects the Middle America Trench. The primitive 6 Rock/Pyrolite Mantle Rock/Pyrolite Mantle Rock/Pyrolite Mantle andesite/adakite compositions are consistent with magmas produced by high-‐pressure partial melting of a mafic protolith (e.g., steep REE, high La/Yb and Sr/Y) 15, 42-‐44. According to Bindeman et al.45 the upper 10000 Average CR <6 Ma Costa Rica <6 Ma mantle-‐like oxygen isotopes require mixing of slab ABS model 1000 melts from the upper low-‐temperature and lower high-‐temperature altered parts of the subducting 100 crust, but the oxygen isotope data can also be explained by reaction with the mantle wedge17. 10 Seismic data46 suggest that the presence of a major 1 slab window beneath southern Costa Rica and Panama is present in the area above where adakites .1 are common (green shaded area in Fig. S1). The 10000 Average CR 12-8 Ma Costa Rica 12-8 Ma numerous hotspot tracks and fracture zones on the ABS model 1000 subducting Cocos and Nazca plates47 could make the subducting slab below this part of the arc relatively 100 easy to tear. Gazel et al.15 suggested that the latest collision of the Galapagos Hotspot tracks with the 10 arc that started~15-‐10 Ma4, 48, 49 clogged the subduction zone and triggered detachment of a 1 major portion of the subducting slab below CALB. .1 The detached slab was then replaced by hot 10000 Average CR >12 Ma Costa Rica >12 Ma asthenosphere, which explains elevated mantle ABS model temperatures 1400-‐1450 °C in the Costa Rica-‐ 1000 (altered MORB+sed.) Panama mantle wedge15. 100 The W. Aleutians are in a similar tectonic scenario, and primitive andesites/adakites with a 10 clear slab signature are widespread along this part of the arc17, 29, 42, 50-‐52. The thickness of the arc crust 1 is 30 km, with a seismic and geochemical signature Pyrolite Mantle (McDonough and Sun, 1995) .1 close to continental crust17, 53. Although the Ba U Ta La Pb Sr Zr Eu Dy Yb Rb Th Nb K Ce Pr Nd Sm Ti Y Lu geochemical evolution in the W. Aleutians is not as Figure S4. Geochemical evolution in Costa Rica as well constrained as in the CALB, the available a result of the interaction with Galapagos tracks. 40Ar/39Ar data suggest that arc magmatism began in The three panes show the mean composition of the Middle Eocene (circa 46 Ma)54 and references therein. samples with SiO2 <55 wt%. Range of all data used for the average in the gray area. Data The age of the adakites in the island of Adak is sources are listed in section S2. Partial melting Middle Miocene54 but the appearance of similar and metasomatism modeled with ABS72 with the rocks in the Bowers Ridge is as early as Oligocene-‐ sources and parameters described in Gazel et al.4 55 and mantle wedge conditions from Gazel et al.15. Early Miocene (32-‐22 Ma) . Model results suggest that the samples <12 Ma Previous models suggested that although a (close to upper continental crust values) require component produced by partial melting of an enriched, subducted component from the Galapagos tracks and sediment, while the samples subducting oceanic crust is ubiquitous throughout >12 Ma (close to lower continental crust values) the arc, the presence of slab melts is most evident in can be reproduced with a subduction component the W. Aleutians due to lower extents of mantle composed of altered MORB slab and sediments. S. melting17 and/or the presence of a slab tear-‐ Prov: Seamount Province, Sed: Sediments. window during the Oligocene-‐Miocene that allowed hot asthenosphere to flow into the W. Aleutians and triggered slab melting56, 57. It is possible that to some extent, the W. Aleutians also interacted with ocean floor related to the Emperor 7 Ridge (Hawaiian Track). Although isotopically indistinguishable from depleted Pacific MORB, the incompatible-‐element compositions of the Emperor Ridge are more enriched than MORB58, 59, providing a similar scenario to the Cocos Ridge and related seamounts (Galapagos tracks) interaction with southern Central America. Considering that the Emperor Ridge resulted from the NE motion of the Pacific Plate above the Hawaiian Plume, that mid-‐ Cretaceous (120-‐93 Ma) segments of the Emperor Chain could be accreted onto Kamchatka60 and the older sections of the Emperor Ridge (when the motion of the Pacific Plate was towards the north) are now subducted, it is possible that the interaction between Hawaiian tracks (or ocean floor affected by the Hawaiian Plume) and different parts of the W. Aleutians can go as far back as the Oligocene. This could explain the presence of an enriched, continental-‐like signature in the W. Aleutians. On the other hand, (a) the compositions of these lavas have been successfully modeled as the outcome of reaction between small degree partial melts of subducting MORB and the overlying mantle wedge 17, 42, 51 and (b) the eastern Aleutians – far from the subducting Emperor seamounts – also show trace element characteristics very similar to bulk continental crust. Thus, the role of a subducting intraplate component (if any) needs further evaluation, including sampling the actual subducting crust along the Aleutians. Another global example of the effect of subducting enriched intraplate magmas and the effect in the volcanic output with continental crust affinity is the Iwo-‐Jima segment of the Izu-‐ Bonin Arc. In this segment there is significant geochemical evidence (enrichments of incompatible elements, radiogenic Pb and unradiogenic Nd isotopes, etc.) of the effect of the subducting Ogasawara Plateau and associated seamounts in the volcanic output 61. Figure S3 shows that the volcanic rocks from the Iwo-‐Jima segment reach high SiO2 and K2O values as expected in continental crust. 4. Subduction of plume-‐enriched oceanic crust and implications for global production of continental crust As mentioned in the last section, geochemical and petrological evidence indicate that in order to produce continental crust, partial melting of basalt in subduction system must be deep (>1 GPa), followed by the interaction of those melts with mantle peridodite. Constant recycling of basalt in subduction zones will result in a steady-‐state production of continental crust; nevertheless, the record of crustal production is characterized by periods of high-‐ productivity in the Archean ~ 3.8-‐2.7 Ga, followed by other peaks at ~1.8, ~1.1 and ~0.5 Ga 62, 63, 64, 65. In a recent work, Martin et al.66 concluded that in order to produce continental crust (especially tonalite-‐trondhjemite-‐granodiorite TTG suites), recycling of enriched oceanic crust that resulted from mantle plume activity is necessary. Their model agrees with the interpretation presented here for the production of juvenile continental crust in the CALB, where contrary to Ecuador36 the arc developed and evolved on oceanic crust14, 15. This model may also explain the record of cycles of production of continental crust (rather than a steady-‐ state production). It is possible that those cycles are controlled by deep-‐Earth peaks in mantle plume activity and the subsequent recycling of oceanic plateaus or tracks. Figure S5 shows a comparison with preserved oceanic crust from the Archean and Paleoproterozoic (same age range as 3.8, 2.7 and 1.8 Ga crustal production episodes) from the Superior Province. The composition of the subducting Cocos Ridge is very similar to those older oceanic rocks (Fig. S5). The Seamount Province, on the other hand, is more enriched in 8 large-‐ion lithophile elements (e.g., Rb-‐K) and other incompatible elements. Nevertheless, the subducting crust offshore of the CALB has the required composition to be recycled into continental crust by subduction processes as it is more enriched than MORB, satisfying the composition necessary to produce continental crust during subduction recycling as suggested by Martin et al. 66 and this study. The Paleoproterozoic (~1.8 Ga) samples from the Superior Province plotted in Fig. S5 are interpreted as an ancient oceanic plateau produced by mantle plume activity67. There is also record of incompatible-‐element enriched oceanic crust in the Archean (Fig. S5); however, an important question is whether the Archean oceanic crust was produced by plume interaction or the ambient mantle was not as depleted as today, thus producing basalts that is more enriched than modern MORB. Nevertheless, the presence of incompatible element enriched oceanic crust compared to modern MORB (Fig. S5) in the Archean and Paleoproterozoic is supporting evidence for the need of an enriched basaltic component necessary to produce continental crust 68, 66. Galapagos Tracks Seamount Province Cocos Ridge Reference values Superior Province Archean 1 OIB Th/Yb 100 Rock/Pyrolite Mantle 10 Galapagos Tracks Seamount Province Cocos Ridge Reference values MORB 10 EMORB .1 NMORB Superior Province Archean 1 Rb Ba Th U Nb Ta K La Ce Pb Pr Sr .01 .1 1 Nb/Yb Galapagos Tracks Seamount Province Cocos Ridge Reference values Superior Province Paleoproterozoic ~1.8 Ga 100 1 OIB Th/Yb 10 10 10 Galapagos Tracks Seamount Province Cocos Ridge Reference values MORB 100 Rock/Pyrolite Mantle Nd Sm Ti Y Lu P Zr Eu Dy Yb EMORB .1 NMORB Superior Province Paleoproterozoic ~1.8 Ga 1 Rb Ba Th U Nb Ta K La Ce Pb Pr Sr Nd Sm Ti Y Lu P Zr Eu Dy Yb .01 .1 1 Nb/Yb 10 100 Figure S5. Comparison of subducting Galapagos tracks offshore of the CALB with Archean and Paleoproterozoic oceanic crust from the Superior Province, Canada. Data from Seamount and Cocos Ridge from Hoernle et al.80 and Werner et al.82, MORB value from Gale et al.83. Additional data from Minifie et al.68 and the Georoc Database (http://georoc.mpch-‐ mainz.gwdg.de/georoc). Diagrams on the right panel are after Pearce 84 to compare ratios of the same samples plotted in the multi-‐element diagrams on the right. Superior Province data were selected with no Nb anomalies to represent the composition of oceanic and intraplate samples. 9 5. Data and Methods 5A. Geochemical data and analytical methods New major and trace element data from Panama were collected at GEOMAR and the University of Kiel. Samples were first crushed into small pieces, then washed in de-‐ionized water and carefully hand-‐picked under a binocular microscope. Major elements and some trace elements (e.g., Cr, Ni, Zr, Sr) of whole rock samples were determined on fused beads using a Philips X'Unique PW1480 X-‐ray fluorescence spectrometer (XRF) equipped with a Rh-‐ tube at IFM-‐GEOMAR. H2O and CO2 were analyzed in an infrared photometer (Rosemount CSA 5003). Additional trace elements (e.g., Rb, Ba, Y, Nb, Ta, Hf, U, Th, Pb and all REE) were determined by ICP-‐MS on a VG Plasmaquad PQ1-‐ICP-‐MS at the Institute of Geosciences (University of Kiel) after the methods of Garbe-‐Schönberg69. Sr-‐Nd-‐Pb analyses were carried out on whole rock powders and chips that were leached in hot (130°C, 1h) 6N HCl for Sr and 2N HCl (70°C, 1h) for Pb prior to digestion. Sr and Nd were analyzed on a TRITON thermal ionization mass spectrometer (TIMS) and Pb on a MAT 262 RPQ2+ TIMS at GEOMAR. Within run mass bias correction uses 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219. NBS987 gave 87Sr/86Sr = 0.710258 ±0.000008 (N=51) and La Jolla 143Nd/144Nd = 0.511846 ±0.000005 (N=49). NBS981 (n=70) yielded 206Pb/204Pb = 16.900 ±0.007, 207Pb/204Pb = 15.438 ±0.009 and 208Pb/204Pb = 36.528 ±0.030 and are corrected to the values given in Todt et al.70. Accordingly, a mass bias correction of 0.111‰ / amu is applied to the sample data. Replicate analyses of M155a and CP98 are within the external 2 sigma errors of the standards stated above. Total chemistry blanks were <100 pg and thus considered negligible. Step-‐heating 40Ar/39Ar dates were collected in key samples. Mineral separates, rock matrix samples and irradiation monitor TCR-‐2 (sanidine from Taylor Creek Rhyolite; Age = 27.87 Ma)71 were irradiated in position E6 of the FRG-‐1 nuclear reactor at the GKSS Research Center, Geesthacht, using a Cd shielding. The reactor constants and J-‐value monitor data are at the end of Table S2. Step-‐heating 40Ar/39Ar analyses were carried out with a 20 W argon-‐ion laser in a MAP 216 mass spectrometer at IFM-‐GEOMAR. Analysis of system blanks were measured prior to each sample and after each fifth sample heating step, typically comprising 10%, 1%, and 2% of the measured 36Ar, 39Ar, and 40Ar isotopes, respectively. The data reported included more than 50% of 39Ar in each plateau (Table S2). 10 5B. Geophysical data and methods We present combined results from two wide-‐angle active-‐source seismic experiments conducted across the Central Cordillera in Costa Rica. A 2005 onshore survey had twenty buried explosive sources recorded by Reftek portable seismometers at ~200 meter spacing72. In 2008, a double-‐sided onshore-‐offshore survey was conducted using the airgun source from the R/V Marcus G. Langseth with onshore seismometers spaced at ~2 km intervals. The explosion and onshore-‐offshore wide-‐angle refraction data were of high quality. The explosion data had all first break picks grouped into a single phase and the onshore-‐offshore data were interpreted to have three distinct phases: a lower-‐crustal refraction P2, Moho reflection PMP, and upper-‐mantle refraction Pn72, 73. Travel time uncertainties were assigned by visual phase-‐by-‐ phase inspection of individual Figure S6: Ray coverage of the velocity model presented in Figure phases and range from 80-‐150 3c. For clarity, every tenth ray is plotted. The location of the 1-‐D velocity model used in Fig. 3B is shown in blue. ms. The travel time picks were modeled using RAYINVR and damped least squares inversion74. The velocity model was created using a layer-‐stripping approach, modeling each layer from top to bottom. Boundary and velocity nodes were placed to maximize ray coverage (Figure S6) and thus, sensitivity of the velocity nodes within each layer. The presented velocity model has a RMS error of 134 ms, reduced Chi-‐squared of 1.2, and a total of 23,047 rays traced. Average velocities for reported table S4 were obtained from different arcs from their original 2_D velocity models (compiled in ref. 75). This calculation was done every 0.2-‐0.5 km where the velocity models cross the arc, and then averaging (over distance) these velocity functions to obtain a single representative value. The V_2D were determined by averaging V(x,z) from depth=10 km to the depth where the velocity reached 7.6 km/s, which is taken as indicating the uppermost mantle. The standard deviation of this average and range of values are also reported in Table S4. 11 5C. Statistical methods To determine a CI that integrates all the available data of 36 major and trace-‐elements for intra-‐oceanic arcs (data sources in Section 2) and appropriately take into account the variability of these data, we used the statistical program R to perform the following algorithm76: 1. Calculate the average of each of the 36 elements for all the estimates of continental crust (Table S3.). 2. For each arc, randomly sample one value from the data sources in section 2 for every element, getting a total of 36 sample data points. This statistical technique is also known as bootstrapping the sample77. 3. Calculate the CI for this sample, using Eq. 1 for the 36 sampled values from each arc (data sources in Section 2) and the 36 expected continental crust average values from Step 1 (Table S3). 4. Repeat steps 2 and 3 100,000 times to produce 100,000 CI values for each arc (data sources in Section 2). The histogram plotted in Fig. S7 of these 100,000 CI values is the Monte Carlo approximation to the bootstrap distribution78. 5. From the 100,000 CI values, we found the mean, mode of the distribution and the 68% highest posterior density (HPD) interval of the distribution for each arc using the “emp.hd” (empirical HPD) 78 function in R. By using this bootstrap method of resampling from all the actual values previously measured, we not only get a reasonable CI, but also obtain a distribution for each CI, which gives more information about the behavior of each CI. The distribution of the continental crust is approximately normally distributed around a mean of ~18 and has a standard deviation of 3.2. For the distributions of the 15 arcs/arc segments, we can clearly see that all the indexes are distributed differently from the continental crust, with different means, modes, distributional shapes, and larger standard deviations. From all arc/arc segments analyzed the data from Costa Rica <10 Ma with a mean of 45.58 and a standard deviation of 9.68 is the closest to the integrated CI computed for the continental crust estimates reported in Table S3. For most of the 15 arcs/arc segments (Fig. S7, Table S3), the distributions are fairly symmetric, meaning that the mode is close to the average. The true index may vary around the mode, and we are 68% confident that the value will be within the 68% highest posterior HPD interval (Fig. S8, Table S3). The CI for the Tonga-‐Kermadec Arc (Fig. S8) has a left heavy tail, which indicates that the index is more likely to be greater than the average value of the bootstrapped samples. The index distribution of the Marianas Arc (active) (Fig. S8) are right heavy tailed, indicating that the index is more likely to be less than their averages. Nevertheless, when plotting CI from mean vs. mode values resulted in very similar correlations (r2 of 0.87 and 0.85, respectively, see Table S3C). We selected the CI-‐mean for Fig. 5, as it produced a slightly better correlation with arc Vp data but both indexes (mean and mode) are reported in Table S3C. The index for the Iwo-‐Jima segment of the Izu-‐Bonin Arc has a bimodal distribution (Fig. S8). To separate the two populations of the simulations for Iwo-‐ Jima, 57% were estimated from the first normal distribution with mean CI=59.70±0.60, while 43% came from the second normal distribution with mean CI=97.50±5.30. 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