Evolution of Geochemical Variations Along the Central American Volcanic Front

Article Volume 00, Number 00 0 MONTH 2013 doi: 10.1002/ggge.20259 ISSN: 1525-2027

Evolution of geochemical variations along the Central American volcanic front

Ian Saginor Division of Natural Sciences and Mathematics, Keystone College, One College Green, La Plume, Pennsylvania 18440, USA ([email protected]) Esteban Gazel Department of Geosciences, Virginia Tech, Blacksburg, Virginia, USA Claire Condie Natural Science Department, Middlesex County College, Edison, New Jersey, USA Michael J. Carr Department of Earth and Planetary Science, Rutgers University, New Brunswick, New Jersey, USA

[1] New geochemical analyses of volcanic rocks in El Salvador add to existing data from Nicaragua and Costa Rica to create a comprehensive set of geochemical data for Central American volcanics. These data coupled with previously published 40Ar/39Ar ages covering the past 30 Ma show that Costa Rica and Nicaragua had similar U/Th and Ba/La values until 10 Ma when the region developed the distinctive along arc variations that made this margin famous. U/Th values increased in Nicaragua since the Miocene, while remaining unchanged along the rest of the volcanic front. This coincides temporally with the Carbonate Crash, which caused a transition in Cocos plate sediments from low-U carbonates to high- U, organic rich hemipelagic muds. Increases in uranium are not observed in Costa Rica because its lower slab dip produces a more diffuse zone of partial melting and because of the contribution from Galapagos- derived tracks dilutes this signal. Ba/La has been used as a geochemical proxy for contributions from the subducting slab; however, our analyses indicate that the Ba concentrations do not vary signiﬁcantly along strike either in the subducting sediment or the volcanic front. Along-arc variation is controlled by changes in La, an indicator of the degree of partial melting or source enrichment. Trace element models of ﬁve segments of the volcanic front suggest that a subducting sediment component is more important to magmas produced in El Salvador and Nicaragua than in Costa Rica, where the geochemistry is controlled by recent (<10 Ma) recycling of Galapagos tracks.

Components: 11,913 words, 9 ﬁgures, 2 tables. Keywords: Central American volcanic front; subduction processes; geochemical evolution. Index Terms: 1031 Subduction zone processes: Geochemistry; 3060 Subduction zone processes: Marine Geology and Geophysics; 3613 Subduction zone processes: Mineralogy and Petrology; 8170 Subduction zone processes: Tectonophy- sics; 8413 Subduction zone processes: Volcanology. Received 28 May 2013; Revised 8 August 2013; Accepted 15 August 2013; Published 00 Month 2013.

Saginor, I., E. Gazel, C. Condie, and M. J. Carr (2013), Evolution of geochemical variations along the Central American volcanic front, Geochem. Geophys. Geosyst., 14, doi:10.1002/ggge.20259.

Figure 1. Map of the evolution of the Central American volcanic front. Triangles are active volcanoes. Inset on the lower left shows locations of core sites mentioned in this study. Location of Miocene to present volcanic front after Bundschuh and Alvarado [2007]. Gray shaded box is the Cocos Plate peak fault relief from Kelly [2003].

1. Introduction into seven right stepping lines that vary in length from 100 to 300 km [Carr, 1984]. 2 [ ] We report here new geochemical data on [4] Geochronological and geological data define Oligocene-Mid-Miocene as well as Early Quater- an approximate age for the volcanic front of Costa nary rocks from El Salvador, which allow us to Rica and Nicaragua of 600 and 330 ka, respec- extend the along the arc profiles in Ba/La and tively [Carr et al., 2007]. Based on this age con- U/Th for the youngest Oligocene-Mid-Miocene straint, the estimated extrusive volcanic flux arc. These new geochemical data (major and trace ranges from 1.3 1010 kg/m/Ma in western Nicar- elements) for volcanic rocks in El Salvador pro- agua to 2.4 1010 kg/m/Ma in central Costa Rica, vide us with the most complete data of Central but overlap within the calculated error [Carr et al., American volcanics published to date. Along with 40 39 2007]. The similar fluxes and similar Ba contents Ar/ Ar ages and geochemical modeling, these (Figure 2) along the margin suggest that regional data extend our knowledge over the past 30 my of Ba/La variations are not related to the total amount the evolution of this arc and help us better define of released slab component but the mechanism of the space-time variations in Ba/La and U/Th and fluid delivery into the mantle wedge. A focused understand their origin. fluid mechanism that produces high degree melts (low La/Yb) is preferred for Nicaragua; mean- 1.1 Geologic Setting while a diffuse mechanism that will produce relatively lower degree of partial melting (high La/ [3] The Central American volcanic front extends Yb) is more likely in Costa Rica and El Salvador 1,100 km from the border between Mexico and [Carr et al., 2007]. Guatemala to Costa Rica (Figure 1) and is gener- ated by the northeasterly subduction of the Cocos Plate underneath the Caribbean Plate at a rate that 1.2. Stratigraphy of the Subducting Cocos ranges from 6 cm/yr off southern Guatemala to 9 cm/yr off southern Costa Rica [DeMets, 2001]. Plate One of the obvious features in Central America is [5] The subducting Cocos Plate has a simple sedi- the physical segmentation of the volcanic front mentary stratigraphy consisting primarily of an

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Figure 2. Variation of Ba, La, Th, and U along the volcanic front. Colored data symbols are volcanic front data from Carr et al. [2013] plotted against distance along arc. Gray circles are Miocene samples from Plank et al. [2002], Gazel et al. [2009], and Saginor et al. [2011]. Gray crosses are modeled values at different degrees of partial melting from Table 2. Note the uniform concentrations of ﬂuid-mobile Ba all along the volcanic front but high concentrations of La, U, and Th in central Costa Rica. upper hemipelagic layer overlying a carbonate et al., 2002; Solomon et al., 2006]. However, sig- sedimentary layer [Patino et al., 2000]. It is gener- niﬁcant 10Be enrichment in Nicaraguan front vol- ally thought that these sediments are not accreted canoes may suggest incomplete subduction of the and that the complete subduction of the Cocos hemipelagic sediments toward the edges of the Plate allows comparison of the subducted input margin in Guatemala and Costa Rica [Leeman and volcanic output [Patino et al., 2000; Plank et al., 1994].

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files, the corrected age versus U concentration profiles (Figure 3b) agree within error. Although the elevated U concentrations appear to be related to both time and proximity to the Central Ameri- can Volcanic Front as volcanic ash could be a possible contributor to the elevated U concentrations, a more likely cause is elevated organic carbon concentration related to upwelling near the coast, coupled with organic carbon’s ability to precipi- tate U from the water column [Plank et al., 2002]. In contrast to U, both sites 844 (600 km offshore Nicaragua) and 845 (435 km offshore Guatemala) have similar Ba profiles as 1039 and 495 [Patino et al., 2000; Plank et al., 2002; Solomon et al., 2006]. The similar barium values in these four cores suggest that barium levels are relatively constant throughout sedimentary layers on the Cocos Plate and are not controlled solely by distance from the margin.

[7] In the carbonate section, most incompatible elements are in low concentration or vary little with depth (e.g., Sr, Ba). In the hemipelagic section, there are some strong gradients: Ba and Pb increase and U decreases with depth. Th, K, Rb, Cs, and Sr show little variation with depth. The mean values of Ba, La, Y, and Pb are only slightly higher in the hemipelagic section than in the carbonate section [Patino et al., 2000; Solomon et al., 2006]. However, the mean values of U, Cs, Th, K, and Rb are much higher in the hemipelagic section.

Figure 3. (a) U concentration in core samples vs. age [after Plank et al., 2002]. (b) U concentration in core samples versus 1.2. Along Arc Geochemical Variation adjusted age. Note the increase of U after 5 Ma. [8] Large geochemical variations occur in Central American front lavas [Figure 4, Carr et al., 1990; Morris et al., 1990; Leeman et al., 1994] that [6] Ocean Drilling Program (ODP) sites 844 and made this arc a focus site for the National Science 845 [Plank et al., 2002], Deep Sea Drilling Pro- Foundation (NSF) Margins Program. Based on gram (DSDP) Site 495 [Patino et al., 2000], and Na O content [Plank and Langmuir, 1993] and ODP Site 1039 [Solomon et al., 2006] provided 2 La/Yb [Carr et al., 1990], these authors suggested the sedimentological and chemical stratigraphy for that degree of melting is highest beneath Nicara- the subducting Cocos Plate (Figure 3, core loca- gua (where the highest Ba/La is also reported) and tions in Figure 1). The two sites closest to the that the extent of melting decreases toward the trench are Site 1039 off the Nicoya Peninsula of northwest and southeast. Similarly, the element Costa Rica and Site 495 off Guatemala. Despite ratios that were thought to trace slab input, Ba/La, being 600 km apart, these two sites have very sim- U/Th, and Ba/Th are highest in Nicaragua and ilar lithologies and barium distributions through- decrease toward the northwest and southeast (Fig- out both the carbonate and hemipelagic layers. ure 4). Uranium values of sites 495, 844, and 845 increase 18 sharply in the upper hemipelagic layer. The [9] Oxygen isotopes ( Oolivine) from phenocrysts increase occurs earliest at Site 495, which is clos- in basalts and basaltic-andesites from the Central est to the trench. If we calculate the time needed American volcanic front vary from a minimum of for sites 844 and 845 to reach the near trench posi- 4.6% centered in western Nicaragua to a maxi- tion of Site 495 and add that time to their age pro- mum of 5.7% in Guatemala [Eiler et al., 2005].

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Figure 4. Geochemical and geophysical variation along the volcanic front in Central America. Colored symbols are from the active front [Carr et al., 2013] and gray circles are Oligocene-Mid-Miocene samples [Plank et al., 2002; Gazel et al., 2009; Saginor et al., 2011]. Balsamo samples are represented by the gray circles within the Salvadoran segment. Colored symbols for geophysical measures refer to present day. Slab age and crustal thickness are from Carr et al. [1990]. Slab dip is from Syracuse and Abers [2006].

This variation correlates with major and trace ele- variations in 18O slab component contribution to ments, and Sr and Nd isotope values of the host the mantle. Models suggest that these variations lavas. Eiler et al. [2005] interpreted this trend as are produced by two end members, a water rich

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flux with low 18O produced by dewatering of ser- Central America using many of the units defined by pentine from the altered oceanic crust of the Cocos Wiesemann [1975]. An elongated Oligocene-Mid- Plate and a relatively water-poor sediment melt Miocene volcanic belt strikes N 70 W, roughly par- with high 18O. The first end member dominates allel to the Pacific Coast for 800 km through Guate- the slab component in the center of the arc, the mala, El Salvador, and into Honduras. In central second is more evident in the northwestern part of and eastern Honduras and northern Nicaragua, the the arc. In central Costa Rica, both fluxes are small Oligocene-Mid-Miocene belt widens significantly. or even insignificant [Eiler et al., 2005]. Reynolds [1980] divided the Oligocene-Mid- Miocene volcanic sequence into three lithostrati- [10] Leeman et al. [1994] found that B/La ratios graphic formations that roughly parallel the Pacific correlated with 10Be/9Be, which allowed the Coastline: the Chalatenango Formation (Middle to authors to estimate that the flux of subducted sedi- Upper Miocene) composed of rhyolitic tuffs and ment is at least twice as high in Nicaragua than in lavas, the Balsamo Formation (Upper Miocene to Costa Rica. They proposed that the higher flux of Pliocene) composed of andesitic lavas, tuffs, and sediments is due to subduction of older, cooler, lahars, and the Cuscatlan Formation (Pliocene) and steeper slab below Nicaragua. composed of rhyolitic tuffs and basaltic lavas. The [11] The slab contribution for volcanic front origi- only ages ever published for the Balsamo Formation nates from both the sedimentary layers and the were from Saginor et al. [2011] and they were both altered oceanic crust subducting along the Middle found to be approximately 1 Ma, significantly American Trench [Patino et al., 2000; Hoernle younger than suggested by Reynolds [1980]. et al., 2008; Gazel et al., 2009, 2012]. Patino et al. [14] The Chalatenango Formation occurs inland [2000] show that the sediment signal is a mixture from the presently active volcanic belt. The Bal- between the upper organic rich hemipelagic section samo Formation is coincident with or on the of the Cocos Plate (with low Ba/Th and high U/La) Pacific coastal side of the currently active volcanic and the lower pelagic carbonate section (with high belt. In eastern and central El Salvador, the Ba/Th and low U/La) of the Cocos Plate. Cuscatlan Formation overlies the Balsamo Forma- [12] TheincreaseinU/ThvaluesinNicaraguanvol- tion on the coastal side of the volcanic belt. In canic material since the Miocene has been attributed western El Salvador, the Cuscatlan Formation to changes in the subducting sediments following occurs on the northern side of the Oligocene-Mid- the Carbonate Crash [Lyle et al., 1995; Plank et al., Miocene volcanic belt, where it overlies the Chala- 2002] at 10–12 Ma. At that time, the Central Ameri- tenango Formation [Reynolds, 1980]. The charac- can isthmus began to close, which shut thermoha- teristics of the volcanics changed from dominantly line circulation from the Pacific to the Caribbean silicic tuffs to andesitic flows to basaltic flows dur- [Coates et al., 1992; Lyle et al., 1995; Farrell ing the Late Pliocene to Quaternary [Reynolds, et al., 1995; Montes et al., 2012]. This caused the 1980]. A secondary type of volcanism, composed carbonate compensation depth to rise and sediments of bimodal basalt-rhyolite suites called Cuscatlan to become enriched in organic carbon at the expense occurred since the Late Pliocene in the region of carbonate [Hoffmann et al., 1981]. The result is behind the volcanic front. The extensive flows and that Cocos Plate sediments consist mainly of a tuffs of the Chalatenango Formation in the Middle lower carbonate layer and an upper hemipelagic Miocene occurred inland of the present volcanic layer, with the latter unit showing enrichment in U front and along the central and northern parts of [Patino et al., 2000]. This increase in uranium con- the Oligocene-Mid-Miocene volcanic belt. These tent of subducted sediments is reflected in Nicara- extensive, predominantly silicic deposits represent guan lavas in the form of a substantial increase in a substantially more productive, broader, and lon- U/Th values between the Miocene and active vol- ger lasting volcanic episode. canic front [Patino et al., 2000; Plank et al., 2002]. [15] In Nicaragua, the Oligocene-Mid-Miocene Recently, Saginor et al. [2011] found that the transi- was dominated by basaltic and andesitic lavas of tion to high U/Th in the volcanic output of Nicara- the Coyol and Tamarindo formations, with the lat- gua after the Central American gateway closure was ter being the only Miocene Nicaraguan volcanism more gradual than previously thought. to the southwest of the active front [Plank et al., 2002; Saginor et al., 2011]. The Coyol is often 1.3. Study Area found associated with Miocene ignimbrites of the [13] Reynolds [1980] summarized the Oligocene- Matagalpa Formation [McBirney and Williams, Mid-Miocene volcanic stratigraphy of Northern 1965]. Costa Rica was similarly active during this

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time period with the exception of 10–7 Ma, when the volcanic front, we collected 20 samples the a gap in volcanism was associated with pluton units mapped as Balsamo Formation across El Sal- emplacement [Macmillan et al., 2004] and pulses vador. Sample locations documented with GPS are of slab detachment following the collision with reported in Table 1. Major element (wt%) data for Galapagos tracks [Gazel et al., 2012]. the Balsamo samples were obtained using by X-ray fluorescence (XRF) at Michigan State Uni- [16] It is now clear that the volcanic front has versity following procedures outlined in Hannah shifted through time [Plank et al., 2002; Carr et al. [2002]. Data are reported in Table 1. et al., 2007; Saginor et al., 2011]. Figure 1 shows the approximate past positions of the volcanic [20] Trace element data (ppm) were collected front for El Salvador, Costa Rica, and Nicaragua, from rock sample solutions, digested by concen- which appear to have shifted location over time. In trated HF-HNO3, and analyzed on a Finnigan general, the volcanic belt has migrated toward the MAT Element, high-resolution inductively trench, implying either a steeper slab dip over time coupled plasma mass spectrometer (HR-ICP-MS) or slab rollback; however, there are substantial (Table 1) at the Institute of Marine and Costal Sci- differences along the margin. In Nicaragua, the ences at Rutgers University. The analytical run modern front has been moving trenchward for at consisted of 20 rocks sample solutions, two diges- least 10 Ma [Plank et al., 2002] and based on the tion blanks, two USGS rock standards (BIR-1a, available geochronology data there is a gap in vol- BHVO-1), five duplicate samples, and three stand- canism between 7 and 3.6 Ma [Saginor et al., ard additions (consisting of nine samples). Stand- 2011]. In western Costa Rica, the strike of the vol- ard additions were distributed evenly through the canic front has rotated 30 degrees counterclock- run. Indium was added to all the samples during wise since about 12 Ma [Macmillan et al., 2004]. solution preparation for drift monitoring. Drift was reduced by normalizing the indium intensities of [17] Available geochronological data indicate that all the measurements in the run to the first mea- the currently active volcanic front started at surement in the run. Indium drift was about 20% approximately 600 ka in the same location as the during the run. The relative percent difference earlier front that was active from 2 to 1 Ma [Carr between the calculated and given reference values et al., 2007]. For the most part, the Oligocene- for BHVO-1 (USGS) were below 6% with the Mid-Miocene volcanic belt is located inland from exception of Cs (26%), Eu (11%), Ta (9%), W the Quaternary to modern (<2 Ma) volcanic front (15%), Pb (19%), Th (16%), and Y (10%). Pb and with the exception of Costa Rica where the mod- Th agree within 6% of the ICP-MS data reported ern front is underlain by volcanism of Early Mio- by Jenner et al. [1990]. Precision determined by cene to Pliocene age (Figure 1). duplicate analyses was better than 7% RDS. All [18] Over the last 20 Ma, the volcanic front of the details of the method (standard values used, Central America has migrated, usually in the blanks, etc.) can be found in Bolge et al. [2009]. trenchward direction, allowing the geochemical history to be sampled [Ehrenborg, 1996; Plank et al., 2002; Carr et al., 2007; Gazel et al., 2009; 3. Results Saginor et al., 2011]. Plank et al. [2002] provided the first geochemical reconstruction of Nicaraguan [21] The new samples from the Balsamo formation volcanism from the Miocene to present. Their Ba/ range from basalts to andesites, dominated by La profile for the Oligocene-Mid-Miocene arc is basaltic-andesites (Figure 5). In part, this reflects a essentially the same as that of the modern vol- sampling bias in favor of mafic lavas. The major canics, there is some variability through time (sec- element data from Balsamo Formation are within tion 4.3), while the U/Th profile along the the range of modern lavas from the volcanic front Oligocene-Mid-Miocene arc in Nicaragua is very in El Salvador and Nicaragua, belonging to the different from the profile derived from the pres- calc-alkaline series (Figure 5). The new trace ele- ently active volcanoes. ment data from the Balsamo formation in El Sal- vador support the results presented by Plank et al. [2002] and Saginor et al. [2011] for the 2. Data and Analytical Methods Oligocene-Mid-Miocene lavas of Nicaragua. Ba/ La values for the Balsamo samples are lower than [19] To address the lack of analyses of nonactive in NW Nicaragua and similar to the profile for the segments of the arc from the northern segment of modern volcanic front (Figure 4). The volcanic

7 Table 1. New XRF and ICP-MS Geochemical Data for El Salvadorian Balsamo Formation

Sample Bal-1 Bal-2 Bal-3A Bal-3B Bal-4 Bal-5 Bal-6 Bal-7 Bal-8 sample Bal-9A Bal-9B Bal-10 Bal-11 Bal-12 Bal-13 Bal-14 Bal-15 Bal-16 Sample Bal-17 Bal-18

Age Age Age SiO[2] 53.08 53.61 53.96 54.66 62.63 54.42 54.02 54.75 50.61 SiO[2] 49.05 49.37 56.9 51.07 52.02 53.49 52.24 63.21 54.38 SiO[2] 55.3 49.83 TiO[2] 0.82 0.92 0.82 0.82 0.67 0.83 0.8 0.85 0.72 TiO[2] 0.87 0.84 0.66 0.98 0.96 0.87 0.81 0.65 0.81 TiO[2] 0.72 1.02 Al[2]O[3] 17.54 19.07 16.25 16.27 17.2 18.26 17.76 17.89 17.19 Al[2]O[3] 20.66 20.86 17.37 18.46 19.61 19.32 18.27 15.26 18.51 Al[2]O[3] 18.25 19.55 Fe[2]O[3] 9.8 8.74 8.61 8.25 6.4 9.77 9.14 9.58 9.82 Fe[2]O[3] 10.33 10.07 7.81 10.47 9.96 9.21 9.56 5.14 8.48 Fe[2]O[3] 8.4 10.35 MnO 0.16 0.14 0.14 0.15 0.16 0.18 0.18 0.17 0.16 MnO 0.17 0.15 0.15 0.18 0.17 0.15 0.16 0.12 0.13 MnO 0.15 0.18 MgO 4.74 3 4.62 5.04 1.47 3.69 3.9 3.52 3.59 MgO 3.17 3 3.22 4.75 3.66 3.05 3.72 1.19 4.17 MgO 3.38 4.43 CaO 9.55 8.87 7.89 8.37 4.9 8.53 8.13 7.92 8.06 CaO 10.63 11.16 7.38 8.94 8.93 7.93 7.41 3.54 8.42 CaO 7.74 9.87 Na[2]O 2.8 3.39 3.33 3.29 4.53 3.37 3.3 3.49 3.16 Na[2]O 2.71 2.65 3.43 3.2 3.31 3.6 3.22 3.83 3.71 Na[2]O 3.3 3.02

K[2]O 1.23 1.51 1.89 1.89 1.73 0.82 1.32 0.88 0.64 K[2]O 0.62 0.61 1.27 0.78 0.75 1.21 1.22 3.8 0.77 K[2]O 1.32 0.63 AL ET SAGINOR P[2]O[5] 0.17 0.21 0.29 0.26 0.17 0.18 0.2 0.18 0.15 P[2]O[5] 0.16 0.15 0.14 0.22 0.18 0.18 0.18 0.24 0.16 P[2]O[5] 0.16 0.2 Li 9.12 9.14 11.77 9.65 13.91 8.39 9.27 7.39 8.57 Li 6.2 6.45 8.87 9.44 9.53 7.13 6.35 25.25 8.75 Li 6.13 5.4 V 254.77 215.19 188.24 180.35 89.58 218.49 229.39 186.86 183.55 V 230.5 263.44 169.7 255.56 250.68 145.6 224.05 44.88 225.68 V 167.23 243.34 Cr 12.77 5.04 165.57 149.2 0.52 7.82 16.76 1.01 2.4 Cr 0.48 2.42 4.98 20.6 6.49 0.88 2.64 2.28 25.53 Cr 5.35 7.53 Co 30.35 21.26 24.96 24.13 10.36 23.79 26.65 18.55 24.62 Co 20.22 23.63 19.14 33.63 29.41 17.99 24.72 7.76 23.66 Co 20.2 27.87

Ni 15.38 7.25 48.32 48.3 0.88 7.44 12.1 1.96 5.81 Ni 2.51 3.41 5.53 24.55 14.87 3.99 5.54 0.6 16.17 Ni 5.23 10.34 .:

Cu 91.16 124.18 87.99 56.85 5.83 103.24 65.05 51.69 93.16 Cu 104.92 174.52 87.79 138.49 108.76 101.85 58.64 19.56 127.43 Cu 67.56 67.11 VARIATIONS GEOCHEMICAL AMERICAN CENTRAL Zn 83.38 84.59 90.27 83.71 84.59 83.38 85.86 65.35 84.19 Zn 69.47 78.79 78.45 90.12 86.7 56.34 78.26 76.8 77.15 Zn 92.33 84.88 Rb 23.36 38.27 58.97 51.45 37.72 15.44 35.29 15.02 10.24 Rb 7.34 9.36 31.67 9.79 11.63 21.9 25.79 114.19 10.89 Rb 19.67 6.97 Sr 479.72 513.37 507.57 521.6 505.16 573.99 533.31 449.71 539.03 Sr 419.78 504.54 484.59 447.82 415.85 378.72 374.15 274.09 488.71 Sr 448.69 564 Y 20.44 23.98 26.59 25.51 24.71 19.14 36.66 18.51 17.29 Y 17.48 17.76 21.82 28.68 23.79 17.33 23.17 34.66 23.47 Y 17.15 16.71 Zr 113.32 129.07 138.4 136.11 132.34 67.37 102.04 66.01 53.49 Zr 46.31 52.78 84.88 83.86 86.74 71.68 105.43 252.63 66.59 Zr 87.39 70 Nb 2.62 3.02 3.31 3.23 3.19 1.81 2.75 1.72 1.43 Nb 1.13 1.29 2.01 2.38 2.35 1.77 2.7 6.27 1.64 Nb 2.12 2.15 Cs 0.43 0.78 1.45 0.77 0.64 0.4 1.05 0.42 0.49 Cs 0.12 0.13 0.87 0.31 0.25 0.44 0.79 4.23 0.46 Cs 0.85 0.28 Ba 540.99 607.52 712.79 678.1 1007.21 583.27 833.58 514.14 445.24 Ba 375.15 402.5 680.1 414.43 359.5 404.17 577.12 1089.84 387.67 Ba 434.52 348.38 La 9.46 10.33 14 13.54 13.11 6.98 15.14 8.73 5.78 La 4.93 5.19 9.29 9.79 6.6 6.6 10.13 22.2 7.2 La 6.78 6.35 Ce 22.8 24.88 31.66 32.12 27.13 16.74 25.87 15.93 14.11 Ce 11.2 12.69 19.79 19.25 16.74 15.61 19.89 50.45 13.17 Ce 16.21 15.8 Pr 3.55 3.91 5.16 4.98 4.41 2.73 5.04 3.06 2.33 Pr 1.95 2.15 2.96 3.58 2.75 2.52 3.74 7.18 2.41 Pr 2.48 2.55 Nd 14.85 16.42 21.51 20.92 18.03 11.96 21.11 13.12 10.28 Nd 8.85 9.72 12.18 15.71 12.28 10.84 15.94 27.69 10.79 Nd 10.61 11.37 Sm 3.9 4.34 5.63 5.44 4.49 3.31 5.37 3.37 2.85 Sm 2.59 2.85 3.12 4.28 3.62 2.98 4.28 6.68 2.91 Sm 2.91 3.14 Eu 1.03 1.19 1.47 1.49 1.37 1.07 1.55 1.03 0.91 Eu 0.82 0.91 1.04 1.24 1.09 0.89 1.22 1.46 0.95 Eu 0.94 1 Gd 3.89 4.42 5.47 5.37 4.47 3.41 5.76 3.48 2.99 Gd 2.88 3.11 3.33 4.76 4.03 3.16 4.51 6.53 3.53 Gd 3.14 3.34 Tb 0.61 0.69 0.82 0.8 0.69 0.53 0.89 0.53 0.47 Tb 0.46 0.5 0.51 0.74 0.65 0.5 0.71 1.02 0.54 Tb 0.5 0.52 Dy 3.73 4.21 4.79 4.51 4.22 3.33 5.46 3.21 3 Dy 2.91 3.16 3.19 4.66 4.24 3.11 4.37 6.11 3.37 Dy 3.06 3.14 Ho 0.77 0.87 0.96 0.91 0.88 0.69 1.17 0.66 0.63 Ho 0.61 0.66 0.69 0.98 0.9 0.65 0.9 1.28 0.73 Ho 0.64 0.65 Er 2.19 2.48 2.74 2.56 2.57 2 3.42 1.85 1.82 Er 1.75 1.9 2.01 2.81 2.6 1.87 2.6 3.75 2.1 Er 1.84 1.84 Tm 0.34 0.38 0.42 0.39 0.4 0.31 0.54 0.28 0.29 Tm 0.26 0.29 0.31 0.42 0.41 0.29 0.41 0.6 0.31 Tm 0.29 0.28 Yb 2.1 2.38 2.66 2.44 2.6 1.93 3.45 1.76 1.79 Yb 1.63 1.78 1.94 2.61 2.56 1.82 2.6 3.86 1.89 Yb 1.85 1.75 Lu 0.32 0.37 0.42 0.37 0.41 0.3 0.57 0.27 0.28 Lu 0.25 0.27 0.31 0.4 0.39 0.28 0.4 0.6 0.3 Lu 0.29 0.27 Hf 3.09 3.5 3.54 3.63 3.42 1.9 2.72 1.79 1.58 Hf 1.36 1.54 2.37 2.3 2.41 2.03 2.92 6.88 1.89 Hf 2.38 1.98 10.1002/ggge.20259 Ta 0.15 0.17 0.17 0.18 0.19 0.1 0.16 0.09 0.07 Ta 0.06 0.08 0.13 0.13 0.14 0.11 0.16 0.43 0.09 Ta 0.13 0.11 Pb 3.82 4.2 7.74 7.59 5.8 2.92 3.46 2.31 2.53 Pb 1.29 1.28 4.61 2.95 2.26 3.9 3.97 10.18 3.35 Pb 4.03 4.86 Th 1.93 2.22 2.03 2.13 2.18 0.84 1.75 0.77 0.42 Th 0.53 0.6 1.66 0.68 0.77 1.23 1.13 7.14 0.35 Th 0.9 1.03 U 0.92 1.02 1.01 1.08 1.05 0.44 0.88 0.41 0.25 U 0.39 0.44 0.75 0.38 0.4 0.56 0.57 2.93 0.17 U 0.37 0.41 Easting 288.194 296.705 296.705 315.114 321.397 325.39 330.822 Easting 400.638 400.638 378.724 378.389 231.205 235.336 211.067 Easting 222.816 212.772 Northing 1518.3 1518.024 1518.024 1510.303 1504.746 1504.268 1506.846 Northing 1461.25 1461.25 1460.157 1464.927 1538.577 1532.133 1518.507 Northing 1516.611 1515.563 Lat 13.727 13.437 13.72512 13.72512 13.65657 13.60673 13.60265 13.62628 13.215 Lat 13.21723 13.21723 13.22 13.20654 13.24966 13.90545 13.84762 13.72219 13.67 Lat 13.70621 13.69577 Lon 88.95868 88.9 88.87998 88.87998 88.7093 88.65088 88.61395 88.56391 88 Lon 87.91703 87.91703 88.1 88.11921 88.1225 89.48731 89.4485 89.67145 89.53 Lon 89.56272 89.6554 8 SAGINOR ET AL.: CENTRAL AMERICAN GEOCHEMICAL VARIATIONS 10.1002/ggge.20259

[24] The melting model used in this study was aggregated fractional melting [Shaw, 1970] described by the following equation:

1=D CL=C0 ¼ 1=F ½1 ðÞ1 F 0 : ð1Þ

where CL is the average concentration of the element in the melt, C0 is the initial concentration of the element in the source, F is the melt fraction, and D0 is the initial bulk partition coefficient. Equation (1) is derived from the mass balance equation C0 ¼ F CLþ(1 F)CS, and the bulk partition coefficient D ¼ CS/CL [Shaw, 1970]. CS is the concentration of the element in the solid phase. The partition coefficients used in our mod- Figure 5. Arc rock type classification from Peccerillo and eling (peridotite and eclogite sources) were from Taylor [1976]. Note that the new data from El Salvador Bal- samo Formation belonging to the calc-alkaline series overlaps the compilation of Kelemen et al. [2003]. The DM modern El Salvador and Nicaragua volcanic fronts. composition was inverted from sample SO-144-1 from Werner et al. [2003]. Modeled melts in eclogite facies from the subducting Seamount Province front segment of El Salvador is symmetrical to and Cocos/Coiba Ridge were based on the average the values of SW Nicaragua and northern Costa calculation of the data published by Hoernle et al. Rica. [2000] and Werner et al. [2003]. The subducting sediments were obtained from Patino et al. [2000]. 4. Discussion [25] Our models were produced by metasomatism 4.1. Trace Element Modeling Along the of depleted mantle (DM) by variable contributions of partial melts from the subducting slab. The Volcanic Front modeling followed an optimization approach until [22] Previous Central American trace element a match was obtained for the trace element compo- models were limited by the lack of estimates of sition of the volcanic front lavas. The Salvadorian the composition of magma sources (subducting source was modeled with the highest sediment sediments, mantle wedge composition, etc.). For component of the four segments (1%) and no Ga- the different arc models presented here, we used lapagos component, while central Costa Rica was the complete magma source compositions and modeled with the lowest sediment component model constraints summarized by Gazel et al. (0.1%) and the highest Galapagos component [2009]. Furthermore, Hoernle et al. [2008] and (3%). The metasomatized mantle (MM) was then Gazel et al. [2009, 2012] showed that a component melted (by aggregated fractional melting, equation from subducting Galapagos-derived tracks is nec- (1) over a range of partial melts chosen to bracket essary for modeling isotopic and trace element the range of compositions found within each of data in southern Central America. these four segments (Figure 6). Trace element concentrations in all the segments are in an overall [23] Trace element concentrations for the modern agreement with modeled values. To avoid fractio- volcanic front were modeled for five segments nation correction, we restricted our samples to along the volcanic front (Figure 6, model parame- basalts and basaltic andesites (SiO2 < 55 wt %). ters and results in Table 2): El Salvador, North- Therefore, the values we are reporting here should west Nicaragua, Southeast Nicaragua, Northwest not be considered compositions of primary mag- Costa Rica, and Central Costa Rica. The northern mas but as a test of the different components nec- segment (Guatemala) of the volcanic front was not essary to reproduce the volcanic front data. modeled because the volcanic material from that Modeled Ba/La and U/Th (Figure 6) values also region shows evidence of crustal contamination, reflect the overall decrease toward Costa Rica as most likely due to relatively thicker Paleozoic con- seen in the data, although model values for U/Th tinental crust basement [Feigenson and Carr, are slightly elevated in the Northwest Nicaraguan 1986]. segment. Nevertheless, the overall pattern was

9 SAGINOR ET AL.: CENTRAL AMERICAN GEOCHEMICAL VARIATIONS 10.1002/ggge.20259

Figure 6. Modeled trace element concentrations for El Salvador, NW Nicaragua, SE Nicaragua, NW Costa Rica, and central Costa Rica (black crosses and dashed lines). High-precision volcanic front data from Carr et al. [2013]. Details about modeling and modeled data in Table 2. reproduced and modeled values for Ba, La, U, and necessarily mean that more sediment is entering Th closely match the actual data (Figures 2 and 6). the subduction zone offshore Nicaragua. It is also possible that the sediment component makes up a [26] According to the modeling results, the sediment component in El Salvador and Nicaragua smaller percentage of the MM source in Costa plays a more important role than in the generation Rica because of the additional Galapagos compo- of Costa Rican magmas; however, this does not nent. This supports the conclusion of Carr et al.

10 Table 2. Trace-Element Modeling Results Following the Methodology Described in Gazel et al. [2009]

Metasomatized Distance Sample Mantle TiO[2] K[2]O P[2]O[5] Rb Sr Y Zr Nb Ba La Ce Pr Nd Sm Eu Dy Yb Lu Ta Pb Th U (km)

Central CR_5%F DMþ3%Galapagosþ 1.47 2.90 1.40 64.59 1154.95 27.50 189.72 23.48 1009.14 73.58 139.71 18.26 47.42 6.15 1.83 5.58 2.56 0.36 1.27 5.22 7.64 4.02 1050.00 0.1 Sed 1 Central CR_10%F DMþ3%Galapagosþ 1.21 1.45 0.79 32.30 618.90 23.83 115.11 11.78 504.57 36.85 71.34 9.89 28.02 4.18 1.31 4.45 2.21 0.32 0.64 2.64 3.82 2.01 1050.00 AIO TAL ET SAGINOR 0.1 Sed 1 Central CR_15%F DMþ3%Galapagosþ 1.01 0.97 0.54 21.53 414.32 20.71 79.37 7.85 336.38 24.57 47.57 6.63 19.15 3.04 0.98 3.61 1.91 0.28 0.42 1.76 2.55 1.34 1050.00 0.1 Sed 1 Central CR_20%F DMþ3%Galapagosþ 0.85 0.73 0.40 16.15 310.81 18.06 59.87 5.89 252.28 18.43 35.68 4.97 14.42 2.34 0.76 2.97 1.65 0.25 0.32 1.32 1.91 1.00 1050.00 0.1 Sed 1 NW CR_5% DMþ0.6%Galapgosþ 1.41 1.68 0.53 34.07 708.55 28.88 136.61 10.64 1620.53 32.56 61.13 8.29 24.90 4.66 1.52 5.39 2.71 0.39 0.61 4.98 3.52 2.44 910.00 .: 0.6 Sed.1 ETA MRCNGOHMCLVARIATIONS GEOCHEMICAL AMERICAN CENTRAL NW CR_10% DMþ0.6%Galapgosþ 1.16 0.84 0.30 17.03 380.54 24.60 82.36 5.34 810.27 16.31 31.24 4.50 14.74 3.17 1.08 4.26 2.29 0.34 0.31 2.52 1.76 1.22 910.00 0.6 Sed.1 NW CR_15% DMþ0.6%Galapgosþ 0.96 0.56 0.20 11.36 254.82 21.06 56.66 3.56 540.18 10.87 20.84 3.02 10.09 2.30 0.81 3.43 1.95 0.29 0.20 1.68 1.17 0.81 910.00 0.6 Sed.1 NW CR_20% DMþ0.6%Galapgosþ 0.81 0.42 0.15 8.52 191.16 18.14 42.72 2.67 405.13 8.15 15.63 2.26 7.59 1.77 0.63 2.82 1.67 0.25 0.15 1.26 0.88 0.61 910.00 0.6 Sed.1 SE Nicaragua_ DMþ0.4%Galapagosþ 1.39 1.00 0.36 21.20 528.07 28.83 122.92 6.01 1458.72 18.90 36.41 5.42 18.92 4.34 1.45 5.35 2.71 0.39 0.37 4.26 1.97 1.92 750.00 5%F 0.4 Sed.1 SE Nicaragua_ DMþ0.4%Galapagosþ 1.15 0.50 0.21 10.60 283.61 24.57 74.10 3.02 729.36 9.47 18.61 2.94 11.20 2.95 1.04 4.23 2.29 0.34 0.18 2.15 0.99 0.96 750.00 10%F 0.4 Sed.1 SE Nicaragua_ DMþ0.4%Galapagosþ 0.95 0.33 0.14 7.07 189.91 21.03 50.99 2.01 486.24 6.31 12.41 1.97 7.66 2.14 0.78 3.41 1.95 0.29 0.12 1.44 0.66 0.64 750.00 15% F 0.4 Sed.1 SE Nicaragua_ DMþ0.4%Galapagosþ 0.80 0.25 0.10 5.30 142.47 18.11 38.44 1.51 364.68 4.73 9.31 1.48 5.77 1.65 0.61 2.79 1.67 0.25 0.09 1.08 0.49 0.48 750.00 20%F 0.4 Sed.1 NW Nicaragua_ DMþ0.2%Galapagosþ 1.14 0.55 3.59 11.57 316.31 24.53 72.08 2.56 1047.64 8.69 16.16 2.66 10.57 2.90 1.03 4.22 2.29 0.33 0.16 2.76 0.96 1.18 650.00 10%F 0.6 Sed1 NW Nicaragua_ DMþ0.2%Galapagosþ 0.95 0.36 2.43 7.71 211.81 21.00 49.59 1.70 698.43 5.79 10.78 1.78 7.23 2.11 0.77 3.40 1.95 0.29 0.11 1.84 0.64 0.79 650.00 15%F 0.6 Sed1 NW Nicaragua_ DMþ0.2%Galapagosþ 0.80 0.27 1.82 5.78 158.90 18.09 37.39 1.28 523.82 4.35 8.08 1.34 5.44 1.62 0.60 2.79 1.67 0.25 0.08 1.38 0.48 0.59 650.00 20%F 0.6 Sed1 NW Nicaragua_ DMþ0.2%Galapagosþ 0.68 0.22 1.46 4.63 127.12 15.68 29.94 1.02 419.06 3.48 6.47 1.07 4.36 1.31 0.49 2.33 1.44 0.22 0.06 1.10 0.38 0.47 650.00 25%F 0.6 Sed1 El Salvador_5%F DMþ 1%Sed2 1.46 1.46 0.26 31.04 799.30 36.36 122.45 4.64 1143.94 18.83 30.73 5.06 19.19 4.70 1.63 6.25 3.42 0.50 0.32 3.49 2.30 0.82 450.00

El Salvador_10%F DMþ 1%Sed2 1.19 0.73 0.15 15.52 416.46 29.20 72.07 2.33 571.97 9.42 15.53 2.66 10.84 3.03 1.10 4.69 2.72 0.40 0.16 1.75 1.15 0.41 450.00 10.1002/ggge.20259 El Salvador_15%F DMþ 1%Sed2 0.97 0.49 0.10 10.35 278.04 23.79 49.21 1.55 381.31 6.28 10.35 1.78 7.32 2.14 0.80 3.63 2.20 0.33 0.11 1.17 0.77 0.27 450.00 El Salvador_20%F DMþ 1%Sed2 0.81 0.37 0.08 7.76 208.54 19.70 37.03 1.17 285.99 4.71 7.77 1.34 5.50 1.63 0.61 2.89 1.81 0.28 0.08 0.88 0.58 0.21 450.00 11 SAGINOR ET AL.: CENTRAL AMERICAN GEOCHEMICAL VARIATIONS 10.1002/ggge.20259

[2007] that the flux of incompatible elements to [1994] that 10Be/9Be is high in Nicaragua and low the volcanic front does not change significantly in Costa Rica because more sediment is subducted along strike. The model also shows that the sub- in Nicaragua, but this is only one reason why ducting Galapagos tracks component decreases Costa Rica has such a low sediment signal. The from Costa Rica to Nicaragua, which is expected other is that, as our current modeling shows (Fig- because the Galapagos hotspot tracks collide with ure 6), the influx of incompatible elements from the Galapagos hotspot tracks dilutes the signal the margin in central Costa Rica [see details in from whatever small amount of sediment is able to Hoernle et al., 2008; Gazel et al., 2009, 2012]. enter the subduction zone.

[30] La/Yb coupled with Pb-isotopes are good dis- 4.2. Geochemical Variations Along and criminators between source enrichment and degree Across the Volcanic Front of partial melting in Central America, as this par- ticular arc is not controlled by sediments [Feigen- [27] Central American volcanoes have a roughly symmetrical distribution of Ba/La, with a peak son et al., 2004; Hoernle et al., 2008; Gazel et al., centered in western Nicaragua and lower values 2009, 2012]. In the case of NW Nicaragua, La/Yb values are very low (<5) and the Pb isotopes are towards both ends of the volcanic front (Guate- 206 204 mala and central Costa Rica) (Figure 3). This ratio consistent with a DM mantle ( Pb/ Pb <18.6) has been used as a geochemical proxy for contri- [Feigenson et al., 2004; Hoernle et al., 2008; butions from the subducting slab, as Ba is a well- Gazel et al., 2009]. Thus, in NW Nicaragua the known fluid mobile element in subduction systems Ba/La variations can be explained by high melt [e.g., Pearce and Parkinson, 1993; Plank and fraction (controlled by flux melting) of a depleted Langmuir, 1993; Pearce and Peate, 1995]. The mantle source that is only enriched in fluid mobile high Ba content of Cocos Plate sediments is the elements (Ba, B, Sr, etc.) by subduction processes. cause of the high Ba/La in Central America rela- On the other hand, in southern Central America tive to other arcs [Plank and Langmuir, 1993]. (central Costa Rica segment) highly radiogenic Pb isotopes (206Pb/204Pb > 18.8) correlate with high [28] Carr et al. [1990] suggested that high Ba/La La/Yb (>10). Thus, an enriched component is nec- occurs in Nicaragua because the slab’s steep dip essary to explain the low Ba/La in this segment of causes a more focused flux of material from the the arc. subducting slab. Figure 2, which contains data from both Nicaragua and Costa Rica [Carr et al., [31] Different explanations have been proposed 2013], shows the along arc profile for Ba/La and for the higher degrees of partial melting in Nicara- also separates the two elements into their own gua. Carr et al. [1990] first suggested that a steep plots, so that the effect of each on along arc varia- slab dip in Nicaragua forced the slab-derived fluids into a smaller volume of mantle wedge that subse- tion of Ba/La can be better understood. This figure quently melted it to a higher degree of partial makes clear that Ba does not change significantly melting. However, determining slab dip beneath along strike. Instead, the variation is controlled by the volcanic front is difficult especially at margins variations in La (Figure 2), a proxy for degree of like Central America, where the dip increases with partial melting or source enrichment. Furthermore, depth. Syracuse and Abers [2006] compiled a data from ODP sites 844, 845, 1039, and 495 global set of dips at arcs by measuring the average reveal that the Ba profile in offshore sediments are dip between 50 and 250 km. Using those depth remarkably similar throughout the Cocos Plate contours, we estimated a dip beneath the volcanic [Patino et al., 2000]. Carr et al. [2007] determined front by fitting a second-order polynomial to the Ba fluxes from the volcanoes of Nicaragua and three contours closest to the volcano and evaluat- Costa Rica and found that there is no significant ing the gradient at the volcano. The result is a variation between these two segments of the vol- broad region of maximum dip across all of Nicara- canic front. gua and shallower dips to the NW and SE. The 10 9 [29] Be/ Be has been used as a proxy for the sub- peak of slab dip in Central America is actually a duction of the upper portion of the sediment col- relatively flat plateau encompassing all of Nicara- umn [Leeman et al., 1994] and a positive gua. In contrast, the geochemical data have gener- correlation has been observed between 10Be/9Be ally sharper peaks that all lie within this region of and Ba/La. This correlation is one of the reasons high and constant slab dip, which is expected if Ba/La was used as a tracer of the sediment signal slab dip was a controlling factor. Ba/La peaks [Carr et al., 2003]. We agree with Leeman et al. roughly at Cerro Negro (660 km in Figure 4), Sr

12 SAGINOR ET AL.: CENTRAL AMERICAN GEOCHEMICAL VARIATIONS 10.1002/ggge.20259

Figure 7. Variation of the slab dip and depth along Central America. (a) Location of the seismic profiles along the volcanic front, (b) Northern Nicaragua segment, (c) Northern Costa Rica segment, and (d) Central Costa Rica segment. Note the systematic decrease in slab dip and depth from north to south. Slab structure from Syracuse et al. [2008] and crustal thickness from Carr [1984] and Hayes et al. [2013]. isotopes peak at Masaya (740 km), U/Th peaks at regardless of variations in slab hydration. Never- Nejapa (720 km), and La/Yb has its minimum at theless, Ranero et al. [2003] discovered pervasive Cerro Negro. bending related faulting that reactivated ridge parallel faults now oriented nearly parallel to the [32] Figure 7 shows three across arc profiles of the subducting slab [Syracuse et al., 2008] together trench. These faults allow the downgoing Nicara- with the overlying Caribbean Plate that demon- guan slab to establish a steeper dip and may lead strates that the maximum possible height of the to increased serpentinization that can penetrate melting column varies from 35 km in central Costa into the upper mantle of the subducting Cocos Rica to 150 km in northern Nicaragua. Thus, fluid Plate. Alternatively, these cracks could simply act focusing coupled with the higher melting potential as more efficient pathways to deliver water from (extended melting column) in Nicaragua suggests the slab into the overlying mantle wedge. Since that the degree of partial melting would be higher this faulting is most pronounced in Nicaragua, it is

13 SAGINOR ET AL.: CENTRAL AMERICAN GEOCHEMICAL VARIATIONS 10.1002/ggge.20259

also possible that the Nicaraguan slab carries more influence of the Galapagos derived hotspot tracks water into the subduction zone than in Costa Rica. overwhelms the signal coming from the sediments Kelly [2003] placed the peak in fault relief at a dis- themselves [Gazel et al., 2009]. While in Nicara- tance along the arc between 670 and 720 km, gua high U/Th values are likely due to the higher using the same scale as Figure 4. Thus, the peak in U/Th values in the subducting sediments since the Carbonate Crash [Plank et al., 2002; Saginor fluid release should occur between volcanoes et al., 2011]. Momotombo and Nejapa or at the right step separating the western and eastern Nicaraguan segments. This is consistent with the location of the 4.3. Temporal Controls on Ba/La and U/ geochemical peaks and the substantial variation in Th in Central America the fault roughness height from 20 to 80 m is con- [36] In this section, we evaluate the variation of U/ sistent with the sharp peaks in the geochemical Th and Ba/La along volcanic front through time. ratios. Seismic refraction data suggests that the 40Ar/39Ar ages [Plank et al., 2002; Saginor et al., amount of water stored in the Cocos Plate may be 2011; Alvarado et al., 2012] in Costa Rica and 2.5 times higher offshore Nicaragua than the Nic- Nicaragua have been compiled to create a 30 my oya Peninsula [Van Avendonk et al., 2011]. record of geochemical evolution in the region (Figure 8). El Salvador does not have enough [33] The relatively high age of the subducting slab in NW Nicaragua can help explain the higher dated samples to include in this figure and two degrees of partial melting. The age of the slab cur- samples thought to be Oligocene-Mid-Miocene rently subducting underneath Nicaragua is approx- from the Balsamo Formation were instead found imately 3 my older than the crust entering to be from only around 1Ma [Saginor et al., 2011]. southern Costa Rica [Carr et al., 1990], which [37] Previous diagrams showing along arc varia- suggests it should be more highly serpentinized tions in Ba/La and U/Th grouped samples into ei- and carry additional fluids into the subduction ther ‘‘modern’’ or ‘‘Tertiary’’ volcanism [Carr zone [Rupke€ et al., 2004], although this effect is et al., 1990; Patino et al., 2000; Plank et al., likely to be minor. This is because the Nicaraguan 2002; Carr et al., 2007; Saginor et al., 2011]. Fig- slab has spent more time sitting on the ocean floor ure 8 provides details into the evolution of Central and also because it is cooler, which promotes American volcanism through time by including increased serpentinization. available ages for volcanism prior to the advent of the modern front. It is well documented that mod- [34] In summary, all these mechanisms suggest that subducting slab beneath Nicaragua should be ern Ba/La values are higher in Nicaragua than in releasing more fluids, and thus, increasing the Costa Rica; however, Figure 8 demonstrates that degree of partial melting within the mantle wedge. was not always the case. Instead, along arc Ba/La The measured water contents of melt inclusions was remarkably similar until regional variations along the volcanic front in Central America appear higher in Nicaragua, lower in Costa Rica at [Sadofsky et al., 2008] have a limited range and no approximately the time of the collision of the large peak in Nicaragua. It is likely that the water Coiba Ridge [12–8 Ma, Macmillan et al., 2004; contents of magmas are buffered to some extent as Gazel et al., 2009] and during the Carbonate high degrees of melting lower the water content in Crash. By looking at Ba and La independently the melt [Plank et al., 2013]. High degree of par- (Figure 8), we see that this change in Nicaragua tial melting explains the depletions in La, and thus was driven by an increase in Ba, while in Costa controlling the high Ba/La in Nicaragua since Ba Rica, the change was driven by an increase in La. does not change along the arc strike. [38] In Nicaragua, the volcanic front has migrated [35] Neither U nor Th concentrations vary signifi- trenchward since the Miocene [Plank et al., 2002, cantly along the arc with the exception of Costa Saginor et al., 2011], which means that it is now Rica, which is elevated in both (Figure 2). The closer to the major source of Ba, the subducting incoming sediments do not vary significantly sediments. This may explain the Ba increase in along strike in either element, which suggests that Nicaragua through time. Figure 8 also shows that high levels in Costa Rica are controlled by other La has not changed in Nicaragua since the Mio- factors. Radiogenic isotopes [Gazel et al., 2009] cene. Tamarindo samples from Nicaragua rein- together with trace-element models (Figure 6) sug- force the case that changes in Ba are driving local gest that Costa Rica is affected by a smaller per- changes in Ba/La. Several of these samples appear centage of sediment than Nicaragua and that the distinctly elevated (they are circled in Figure 8) in

14 SAGINOR ET AL.: CENTRAL AMERICAN GEOCHEMICAL VARIATIONS 10.1002/ggge.20259

Figure 8. Evolution for key trace elements (Ba, La, U, and Th) in Costa Rica (blue) and Nicaragua (red) in the last 30 Ma [data from Plank et al., 2002; Carr et al., 2007; Gazel et al., 2009; Saginor et al., 2011; Carr et al., 2013]. Note how the Ba/La and U/Th ratios, used to trace the sediment component, clearly changed after 10 Ma, separating the trends of Costa Rica and Nicaragua. Although there is an overall increase in Ba in Costa Rica over time, the separation from Nicaragua is found in La, U, and Th, all of which dramatically increased in Costa Rica in the last 10 Ma following the interaction of the arc with Galapagos tracks [Gazel et al., 2009]. both Ba/La and Ba when compared to other Nica- nism in the Nicaraguan Highlands; however, it raguan samples of similar ages. The Tamarindo was emplaced 80 km closer to the trench [Saginor Formation is coeval with Middle Miocene volca- et al., 2011]. Another possibility for the Ba

15 SAGINOR ET AL.: CENTRAL AMERICAN GEOCHEMICAL VARIATIONS 10.1002/ggge.20259

Figure 9. Across arc variation in Ba/La, Ba, and La. Conchagua, Conchaguita, Meanguera, and Cosiguina are part of the active volcanic front. El Tigre appears morphologically younger than Conchagua but has no historic activity. Zacate Grande is a mix of older andesites and young basalts, although there are no available ages. In the backarc region, the gradient in Ba/La continues [Patino et al., 1997]. Esteli and Tegucigalpa are lava ﬁelds of subalkaline basalts and basaltic andesites erupted in grabens well behind the volcanic front. Yojoa is a very young alkaline basalt complex adjacent to the strike-slip Caribbean-North American plate boundary. increase in both Nicaragua and Costa Rica is an La. The change in La occurs because the mantle increase in primary productivity due to upwelling source is enriched by metasomatic interaction with following the initial closure of the isthmus in Galapagos-derived tracks [Gazel et al., 2009, response to the collision of the Coiba and then the 2012]. Although melt fraction may also be lower, Cocos Ridge. Cocos Plate sediments do show a source enrichment is more consistent with the modest increase in Ba following this transition; change of La/Yb together with contemporaneous however, this change affected the entire region, changes to Galapagos-derived isotopic signature not just Nicaragua. Also, as previously discussed, [Hoernle et al., 2008; Gazel et al., 2009, 2012]. Ba/La levels are far more sensitive to changes in [40] The modest Ba enrichment in Costa Rica over the degree of partial melting and therefore changes time may be due to the high Ba concentrations in in La. the Galapagos tracks; however, since this increase [39] In Costa Rica, the decrease in Ba/La is con- is also seen in Nicaragua as well, this explanation trolled not by Ba (which increases with time along may be insufﬁcient. One consequence of the arc with Nicaragua), but rather by a sharp increase in changing shape over time is that the slab dip and

16 SAGINOR ET AL.: CENTRAL AMERICAN GEOCHEMICAL VARIATIONS 10.1002/ggge.20259 the arc-trench distance may well have changed as 5. Conclusions well. For the present arc, there is some evidence that Ba/La decreases as arc-trench distance [43] Variations in Ba/La along the arc and through increases. The clearest example occurs in the vol- time are primarily controlled by La, a proxy for canoes around the Gulf of Fonseca, which is the degree of partial melting or source enrichment. bounded by El Salvador, Honduras, and Nicaragua This is true for both Miocene and modern vol- (Figure 9). There is a right step in the volcanic canics. Changes in Ba/La in Central America front northwest of Cosiguina€ Volcano and there is through time can be controlled by changes in Ba an unusually wide distribution of composite cones. only if the degree of partial melting does not Cosiguina€ is 165 km from the trench and Zacate change (Ex: Nicaragua since the Miocene). Grande is 30 km further back. Yet across these 30 km, Ba/La decreases from about 100 to about 50. [44] Nicaragua has the highest degree of partial Most of the cross-arc variation in Ba/La occurs in melting along the Central American volcanic the first 30 km behind the volcanic front where the front. A number of factors can help explain the depth to the seismic zone increases from 116 km high melt fractions in Nicaragua. First, a steeper at Cosiguina€ to 200 km at Zacate Grande. Zacate slab dip under Nicaragua creates a higher melting Grande also marks the furthest landward extent of column, which focuses the fluid flux from the sub- the Wadati-Beniof Zone. The wide variation ducting slab. Second, the distribution of trench within individual volcanic centers makes it doubt- parallel faulting related to slab bending results in ful that cross-arc gradients in Ba/La can be reli- deep faults that appear to be allowing an added ably recognized across the short distance within a influx of water in Nicaragua and the water leads to volcanic center. a higher degree of partial melting. Third, variations in the age of the subducting slab can affect [41] Figure 9 shows that Ba/La increases toward the degree of serpentinization of subducting ocean the trench; however, this could be either due to crust and the upper mantle. increasing Ba flux from the subducted sediments or an increase in La, and therefore the degree of [45] Plank et al. [2002] identified a long-term high partial melting (assuming a constant mantle com- level (>70) of Ba/La along the Nicaraguan vol- position). When these elements are examined sep- canic arcs. New age data and geochemical data arately, it is clear that Ba does not change better define the origin of the very high Ba/La val- systematically across the arc, while La decreases ues found in Nicaragua. The first occurrence of steadily towards the trench. Ba/La >100 was about 15 Ma in the Tamarindo Formation at the very front of the then active vol- [42] U/Th values have also shifted through time. canic belt. Consistently high values of Ba/La Figure 8 shows that U/Th was similar along the arc began sometimes after 9 Ma and after the collision until the collision of the Coiba Ridge at 10 Ma, of Galapagos tracks and Carbonate Crash. Overall, when the ratio decreased in Costa Rica and the Ba concentrations of lavas appear to have increased in Nicaragua. U shows no along arc varia- increased gradually from the earliest samples at tion until 10 Ma when it increased in both coun- around 25 Ma to the present. tries,buttoagreaterextentinCostaRica.Thhas no along arc variation until 10 Ma when Th [46] U/Th values in Nicaragua experienced a sig- increased in Costa Rica, but remained the same in nificant increase since the Miocene due to Nicaragua. In Costa Rica, a modest increase in U increases in U-bearing sediment following the coupled with a greater increase in Th produced the Carbonate Crash. While U also increased in Costa temporal change in U/Th. The measured increase in Rica, U/Th remained low due to increases in Th U in the subducting sediments since the Carbonate derived from the subducting Galapagos tracks. Crash is a possible explanation; however, that does not explain why Costa Rican volcanoes currently have more U than Nicaraguan volcanoes. We sug- Acknowledgments gest that the increase in Costa Rican U since the Miocene is also influenced by the subduction of [47] This study was supported by NSF Tectonics Program grant EAR- 1221414 to Gazel, and through the NSF Margins trace-element enriched Galapagos tracks, which are Program, grants EAR0203388 and OCE 0505924 to Carr, absent in Nicaragua. The increase in U/Th in Nicar- Saginor, and Condie. We would like to thank Editor Cin-Ty agua is more straightforward and can be explained Lee and two anonymous reviewers, as well as C. Saginor and by the sediment transition from low U carbonates N. Sou whose comments and revisions greatly improved this to high U hemipelagics. manuscript.

17 SAGINOR ET AL.: CENTRAL AMERICAN GEOCHEMICAL VARIATIONS 10.1002/ggge.20259

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