Downloaded from geology.gsapubs.org on September 2, 2010 Crustal assimilation no match for slab fl uids beneath Volcán de Santa María, Guatemala

Brian R. Jicha1*, Katy E. Smith1, Brad S. Singer1, Brian L. Beard1, Clark M. Johnson1, and Nick W. Rogers2 1Department of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA 2Department of Earth and Environmental Science, Open University, Milton Keynes MK7 6AA, UK

ABSTRACT 200 Guatemala 238 230 40 39 New U- Th and Sr isotope data from Ar/ Ar-dated mafi c El Salvador/Honduras of the composite cone, A.D. 1902 dacitic tephra, and 1922–pres- 150 Costa Rica ent dacitic dome lavas from the Santa María–Santiaguito volcanic Santa Maria complex in northwestern Guatemala indicate that genesis of magma beneath the western end of the Central American volcanic arc 100

requires sources much different than previously envisioned. Although Ba/La U-series data from historical lavas along the Central American vol- canic arc, including Guatemala, show variable but small degrees of 50 either 230Th or 238U excess, 72–35 ka and basaltic andesites from the Santa María cone have 15%–26% 238U excesses, among the largest measured in . This implies that the mantle 0 wedge beneath this sector of the arc was signifi cantly modifi ed by 0.7048 slab-derived fl uids. A decrease in 87Sr/86Sr and (238U/230Th) ratios over the past 72 k.y. as basaltic andesite gave way to dacite is consistent with fractional crystallization coupled with progressive assimilation 0.7044 of crust that has relatively unradiogenic, mid-oceanic ridge – like Sr isotope composition. Overall, our U-series data along with 0.7040 published 238U/230Th isotope results from Central America demon- Sr/ Sr

87 86 strate that (1) slab-fl uid fl ux can be high throughout Central America, including regions of relatively thick crust, and (2) the middle to lower 0.7036 crust beneath northwestern Guatemala may not be dominated by ancient metamorphic and granitic rocks. 0.7032 INTRODUCTION 3.5 The geochemistry of arc lavas refl ects contributions from the sub- 3.0 ducted slab, sediments on the slab, mantle, and subvolcanic crust; how- ever, the mechanisms and extent by which these different components are 2.5 incorporated into arc magmas remain enigmatic. The Central American 2.0

arc is an ideal location to examine the role of subduction zone inputs and /Th) crustal contamination owing to prominent, well-documented, along-arc 1.5

238 232 variations in geochemistry and crustal thickness (Carr et al., 1990). A (U consensus has developed that variations in erupted lava compositions (Ba/ 1.0 La, 10Be/9Be, U/Th) along the Central American arc refl ect changes in the 0.5 amount of subducted material from the Cocos plate involved in magma genesis (Fig. 1; Carr et al., 1990, 2003; Leeman et al., 1994; Patino et al., 0.0 2000; Rüpke et al., 2002; Eiler et al., 2005). It is generally accepted that the 0 200 400 600 800 1000 1200 intensity of the slab signal peaks in west-central Nicaragua and decreases Distance along the volcanic front (km) toward both ends of the arc. The crust is also thinnest beneath central Nica- Figure 1. Ba/La, 87Sr/86Sr, and (238U/232Th) variations along Central ragua (25 km) and becomes thicker (>40 km) toward the edges of the arc in American volcanic front. Black circles—data from Santa María. Cen- northwestern Guatemala and central Costa Rica (Carr et al., 2003). tral American data from modifi ed version of Centam database (M. Carr, 238 232 238 230 2009, personal commun.). Open symbols represent ( U/ Th) data ac- The U- Th isotope system has been used widely, including in quired by mass spectrometric or alpha counting methods, whereas Central America, as a tracer of hydrous fl uids released from the subduct- (238U/232Th) for closed symbols was calculated based on U and Th ing oceanic crust and its overlying sediments (e.g., Reagan et al., 1994). contents in Centam database. U-series data are from McDermott and It has been used recently to elucidate intracrustal processes in continental Hawkesworth (1991), Allegre and Condomines (1976), Herrstrom et al. arcs (e.g., Jicha et al., 2009; Walker et al., 2007). This study combines (1995), Clark et al. (1998), Walker et al. (2007), Condomines and Sig- marsson (1993), Reagan et al. (1994), and Thomas et al. (2002). new U-series and Sr isotope data from a suite of lavas erupted over the past 72 k.y. at the Santa María–Santiaguito volcanic complex, Guatemala, to explore the long-term evolution of this explosive and compare SANTA MARÍA–SANTIAGUITO VOLCANIC COMPLEX the subduction zone input beneath this complex to other active Central Santa María–Santiaguito is one of 39 volcanic complexes that com- American arc volcanoes. pose the Central American volcanic arc (Carr et al., 2003). It is located near the western end of the arc in the Guatemalan highlands and is built *E-mail: [email protected]. upon ~50-km-thick crust. On 24 October 1902, the second largest eruption

© 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, September September 2010; 2010 v. 38; no. 9; p. 859–862; doi: 10.1130/G31062.1; 3 fi gures; Data Repository item 2010238. 859 Downloaded from geology.gsapubs.org on September 2, 2010 of the twentieth century commenced from a vent on the southwest fl ank of fl uids to the mantle wedge during magma genesis. Thus, U-series isotope the Santa María composite volcano (Williams and Self, 1983). In 1922, the data from Santa María lavas suggest that magmas were generated from Santiaguito dome complex began to grow inside the crater formed during a mantle wedge that was signifi cantly modifi ed by fl uids released from the 1902 eruption, and remains active, having erupted more than 1.2 km3 of the subducted slab. Eiler et al. (2005) analyzed oxygen isotope ratios of dacitic magma. Volcanism at Santa María–Santiaguito is bimodal in both olivine and plagioclase phenocrysts in mafi c Central American arc lavas composition and time, including roughly equal volumes of older basaltic- and proposed that the slab-derived component beneath Guatemala was andesitic composite cone lavas (~8 km3) and historic dacitic (~9 km3) lava a water-poor partial melt of subducting sediment. Forward modeling of and tephra (Rose, 1987; Escobar-Wolf et al., 2010). Recent 40Ar/39Ar and Cameron et al. (2003) also required a sediment melt component for volca- paleomagnetic results reveal that cone building occurred at Santa María noes in southeastern Guatemala. However, Walker et al. (2007) suggested in four phases from 103 to ca. 35 ka (Escobar-Wolf et al., 2010). In this that sediment melt is not feasible in Guatemala because the subducting study we focus on the evolution of the last three cone-building phases and sediment has very high (230Th/232Th) ratios and Guatemalan lavas have the historic tephra and lava. some of the lowest (230Th/232Th) ratios in Central America. We concur with Walker et al. (2007) because U will not be signifi cantly fractionated from LARGE FLUID FLUX IN GUATEMALA Th during sediment melting, and thus a sediment melt is unlikely to be the All 19 whole-rock samples from Santa María–Santiaguito are in dominant medium by which U is preferentially transported to the mantle 238U excess [(238U/230Th) > 1] (see the GSA Data Repository1 for meth- beneath Santa María–Santiaguito. ods), similar to the results of Walker et al. (2007), who analyzed two ca. 35 ka lavas from the Santa María composite cone. There are 12 mafi c ASSIMILATION OF OCEANIC CRUST BENEATH SANTA lavas from the Santa María cone, with the exception of a lava taken near MARÍA the summit, that have 238U excesses ranging from 15% to 26%, among Unlike Santa María basalts and basaltic andesites, which have large the highest measured in the Central American volcanic arc (Fig. 2). 238U excesses, pumice and scoria from the 1902 eruption and dacitic lavas Lavas from western Nicaragua, where the slab signal is inferred to be from Santiaguito have small 238U excesses (4%–10%) and 87Sr/86Sr ratios at a maximum, have elevated (230Th/232Th) ratios, but relatively small that are generally less radiogenic than the older, mafi c cone lavas (Fig. 3). 238U excesses (4%–10%) (Thomas et al., 2002). Only basalts from Cerro Carr et al. (2003) proposed that the increase in 87Sr/86Sr ratios in lavas from Negro in western Nicaragua, which have high water content in melt central to western Guatemala refl ects enhanced assimilation of radiogenic, inclusions (Roggensack et al., 1997), have 238U excesses (14%–21%) Paleozoic crust of the Chortis block, which underlies western Guatemala. comparable to those found in Santa María basalts and basaltic andesites In contrast, we propose that the decline in both (238U/230Th) and 87Sr/86Sr (Fig. 2). Moreover, several basalts from Poás and Arenal volcanoes in ratios with time at Santa María–Santiaguito implies that dacites were central Costa Rica also have large 238U excesses (18%–26%) (Herrstrom produced from basaltic andesitic magma that underwent assimilation- et al., 1995; Allegre and Condomines, 1976) (Fig. 2). fractional crystallization and incorporated ancient, low-87Sr/86Sr mafi c Signifi cant 238U excesses in continental arc lavas are relatively crust. Alternatively, the trend toward more silica-rich magma with lower uncommon; most lavas with (238U/230Th) >> 1 are found in island arcs. 87Sr/86Sr ratios and smaller 238U excesses might refl ect evolution of the Because experiments show that U is highly mobile in oxidizing aque- mantle wedge and parental melts over the past 72 k.y. Because covariation ous fl uids and Th is not (Brenan et al., 1995), the presence of large 238U of incompatible trace elements and their ratios are strikingly linear for the excesses in arc lavas has been attributed to the addition of slab-derived basaltic to dacitic compositions observed (Escobar-Wolf et al., 2010) and

30 50 0.70405 Santiaguito dome complex 1922-present dacite lavas 20 Cerro 45 mafic enclave Negro 0.70400 1902 eruption 1902 dacite pumice 10 40 238 1902 scoria

% U excess Santa Maria composite cone 0 35 35 ka lavas 0.70395 60-46 ka lavas

thickness (km) 10 30 72 ka lavas

Guatemala Sr/ Sr

87 86 Nicaragua Crustal 0.70390 20 Costa Rica 25

230 Santa Maria

%30 Th excess 20 0 200 400 600 800 1000 1200 0.70385 Distance along the volcanic front (km)

Figure 2. Percent 230Th and 238U excess versus distance along Cen- Th excess U excess tral American volcanic front. Heavy gray line represents variation in 0.70380 crustal thickness along strike of the arc. (Sources of published U-Th 0.9 1.0 1.1 1.2 1.3 238 230 isotope data as in Fig. 1.) Activity ratios determined by mass spec- (U/Th) trometry (in Reagan et al., 1994) are preferred over those measured by alpha counting. Figure 3. 87Sr/86Sr versus (238U/230Th) for Santa María–Santiaguito la- vas and tephras. Older mafi c stratocone lavas have highest 87Sr/86Sr ratios and largest U excesses, whereas 1902 dacite pumice and sco- 1GSA Data Repository item 2010238, U-Th methods, is available online ria and Santiaguito dacite dome lavas have lower 87Sr/86Sr ratios and at www.geosociety.org/pubs/ft2010.htm, or on request from editing@geosociety smaller 238U excesses. Youngest Santa María cone lava has lowest .org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. 87Sr/86Sr and (238U/230Th) ratios.

860 GEOLOGY, September 2010 Downloaded from geology.gsapubs.org on September 2, 2010 the range in (230Th/232Th) ratios is limited (Table DR1 in the Data Reposi- wedge, thereby leading to higher degrees of melting (e.g., lower La/Yb tory), we instead prefer a hypothesis that dacitic magmas are derived from ratios). Less-focused fl uids in other parts of the arc where dip is not as earlier erupted basaltic andesitic magma via fractional crystallization cou- steep will result in smaller degrees of melting. There are two potential pled with assimilation of crustal melts. The crustal melts that contributed pitfalls of this focused versus defocused slab fl uid–fl ux model. First, the to the generation of Santa María–Santiaguito dacites probably dampened correlation of slab signal with degree of melting in Central America is large 238U excesses that likely existed in the parent magmas. Potential crude. La/Yb ratios from lavas spanning ~600 km of the arc from crustal assimilants for Guatemalan lavas identifi ed by Walker et al. (1995) in south-central Guatemala to Zapatera in eastern Nicaragua show lim- from Chortis crust have 87Sr/86Sr ratios ranging from 0.70650 to 0.71427 ited variability (average La/Yb = 3.4 ± 1.7; 2 standard deviations) and no (Walker et al., 1995), and therefore are not suitable assimilants for Santia- correlation along strike (r2 = 0.07). Second, if 238U excesses truly refl ect guito dacites. Wholesale melting of young precursor basalts that stalled in U-Th fractionation due to fl uid transport of U+6 to the mantle wedge, our the crust, as suggested by Vogel et al. (2006), seems unlikely to have pro- new U-series data, along with published 238U/230Th isotope results, imply duced the dacites because gabbroic fragments ejected in the 1902 eruption that fl uid fl ux is highly variable along strike of the arc. This observation is and older mafi c lavas from edifi ces surrounding Santa María have 87Sr/86Sr consistent with water contents that range from 1 to 6 wt% in phenocryst- ratios higher than those of the dacites. hosted melt inclusions in Central American volcanic arc lavas (Sadofsky Hoernle et al. (2004) proposed that a belt of mafi c igneous rocks et al., 2008, their fi gure 6). Moreover, olivine-hosted melt inclusions in extending from Costa Rica to Guatemala has been accreted to the Pacifi c the A.D. 1723 tephra erupted from Irazú volcano in Costa Rica contain margin of Central America. Although these rocks have relatively unra- high water concentrations (>3 wt%), and require signifi cant recycling of diogenic 87Sr/86Sr ratios (0.70306–0.70319), major and trace element water from hydrous sources in the subducting slab despite the fact that the geochemistry refl ect an enriched intraplate origin (Hoernle et al., 2004). melt inclusions have low Ba/La ratios (~16) (Benjamin et al., 2007). This

Energy-constrained recharge-assimilation-fractional crystallization (EC- apparent disconnect between measured H2O contents in melt inclusions of RAFC) model calculations (i.e., Bohrson and Spera, 2007) indicate that historically erupted phenocrysts and trace element and isotope proxies for the chemical and isotopic compositions of Santiaguito dacites can be pro- slab fl uid in whole-rock samples poses a serious obstacle to understand- duced from Santa María basalt that undergoes fractional crystallization ing volatile transport in arcs. Further scrutiny of larger sets of combined coupled with assimilation of ~20% by mass of a partial melt of altered U-series and melt inclusion data obtained from the same eruptive material normal mid-oceanic ridge basalt (N-MORB)–like crust, and not ocean will be needed to resolve this conundrum. island basalt–like crust. Ophiolites that crop out along the south side of In contrast to models suggesting that lavas in western Nicaragua the Motagua fault in central Guatemala may compose a signifi cant por- have maximum sediment and fl uid signals, we propose that the elevated tion of the middle to lower crust beneath the active volcanic centers in Ba/La ratios in western Nicaraguan lavas only refl ect a more pronounced northwestern Guatemala (Beccaluva et al., 1995; Alvarado et al., 2003). sediment signature and are not related to fl uid delivery to the mantle. These rocks have a tholeiitic, N-MORB–like composition that could rep- This should not be surprising because Ba/La ratios correlate positively resent the potential crustal contaminant involved in Santa María–Santia- with 10Be/9Be (Plank and Langmuir, 1998; Patino et al., 2000; Reagan guito magma genesis. Assimilation of MORB-like crust is not unique to et al., 1994). A diminished role of slab-derived fl uids in Nicaragua chal- Guatemala; it has been inferred from the negative correlation between lenges interpretations garnered from most studies of Central American 87Sr/86Sr and U-series isotopes in basalts at Lopevi volcano in the Vanuatu arc magma genesis (e.g., Carr et al., 1990, 2007; Leeman et al., 1994; arc (Handley et al., 2008). Herrstrom et al., 1995; Clark et al., 1998; Patino et al., 2000; Rüpke et The uppermost lava fl ow in a package of ca. 35 ka lavas atop the al., 2002; Eiler et al., 2005; Sadofsky et al., 2008) and contradicts a recent Santa María cone has the smallest 238U excess (3%) and least radiogenic tomographic study (Rychert et al., 2008). 87Sr/86Sr ratio of any of the Santa María lavas (Fig. 3). We interpret this to suggest that crustal melting and assimilation were ongoing well in CONCLUSIONS advance of the 1902 eruption. Evidently, repeated injections of mantle- Basalts and basaltic andesites from Santa María have 238U excesses derived basalt during a >50 k.y. period (Escobar-Wolf et al., 2010) pro- among the largest measured in Central America, and are likely the result moted crustal melting and long-term incubation of a hybrid dacitic magma of signifi cant fl uid modifi cation of the mantle wedge during magma gen- that eventually erupted explosively in 1902. esis. We suggest that elevated Ba/La, U/Th, and 10Be/9Be ratios in west- ern Nicaraguan lavas simply refl ect a pronounced sediment signature, IMPLICATIONS FOR ALONG-ARC TRENDS IN CENTRAL most likely a partial melt of the sediment, not a large fl ux of slab-derived AMERICA fl uids. This contradicts previous interpretations that favor a maximum The slab signal in most arc lavas is likely composed of multiple com- fl uid signal in western Nicaragua. Production of dacite composing the ponents, including a partial melt of the sediment, fl uid released from the 1902 Plinian fall deposit and Santiaguito dome complex requires assimi- downgoing plate, and overlying sediment pile, all of which transfer ele- lation of a MORB-like crustal component, which differs from ancient ments to the mantle wedge. Sediments atop the subducting Cocos plate are crustal rocks assumed to predominate in this region of Guatemala. Were relatively uniform in composition along the strike of Central America and we to have concentrated our investigation on only historical eruptions are composed of pelagic carbonates capped by hemipelagic oozes (Patino of Santa Maria–Santiaguito, far different conclusions regarding magma et al., 2000). The sediment column is characterized by unusually high Ba genesis may have been reached, highlighting the fact that models based and U contents but relatively constant Ba/La and U/Th ratios (Plank and on along-arc geochemical observations will benefi t by considering the Langmuir, 1998; Patino et al., 2000). In western Nicaragua, Ba/La and magmatic history recorded by all products of long-lived volcanic cen- U/Th ratios in the lavas reach a maximum, and this is where the slab sig- ters, not just the most recent. nal is inferred to be the strongest, and it is commonly associated with slab-derived fl uids. The popular model of Carr et al. (1990) for produc- ACKNOWLEDGMENTS ing the elevated Ba/La and U/Th ratios in Nicaraguan lavas proposes that We thank Rudiger Escobar-Wolf and Bill Rose for guidance on fi eld and pet- rologic aspects of Santa Maria, and John Hora for help with the U-Th analyses. fl uid fl ux from the subducted slab is nearly uniform along the arc but fl uid We also thank Barry Cameron for his informal review. Kaj Hoernle, Jim Walker, addition to the mantle wedge varies with slab dip. In Nicaragua, where and an anonymous reviewer provided helpful comments that greatly improved this slab dip is steepest, fl uids are focused to a narrow window in the mantle manuscript. Supported by U.S. National Science Foundation grant EAR-0738007.

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