Dacite Petrogenesis on Mid-Ocean Ridges

JOURNAL OF PETROLOGY VOLUME 51 NUMBER 12 PAGES 2377^2410 2010 doi:10.1093/petrology/egq056

Dacite Petrogenesis on Mid-Ocean Ridges: Evidence for Oceanic Crustal Melting and Assimilation Downloaded from https://academic.oup.com/petrology/article/51/12/2377/1540213 by guest on 29 September 2021 V. D. WANLESS1,2*, M. R. PERFIT1, W. I. RIDLEY3y AND E. KLEIN4

1DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF FLORIDA, GAINESVILLE, FL 32611, USA 2DEPARTMENT OF GEOLOGY AND GEOPHYSICS, WOODS HOLE OCEANOGRAPHIC INSTITUTION, WOODS HOLE, MA 02540, USA 3US GEOLOGICAL SURVEY, DENVER, CO 80225, USA 4NICHOLAS SCHOOL OF THE ENVIRONMENT, DUKE UNIVERSITY, DURHAM, NC 27708, USA

RECEIVED NOVEMBER 23, 2009; ACCEPTED AUGUST 26, 2010

Whereas the majority of eruptions at oceanic spreading centers geochemical signature similar to a MOR dacite. This supports the produce lavas with relatively homogeneous mid-ocean ridge basalt hypothesis that crustal assimilation is an important process in the (MORB) compositions, the formation of tholeiitic andesites and formation of highly evolved MOR lavas and may be significant in dacites at mid-ocean ridges (MORs) is a petrological enigma. the generation of evolved MORB in general. Additionally, these pro- Eruptions of MOR high-silica lavas are typically associated with cesses are likely to be more common in regions of episodic magma ridge discontinuities and have produced regionally significant vol- supply and enhanced magma^crust interaction such as at the ends umes of lava. Andesites and dacites have been observed and sampled of ridge segments. at several locations along the global MOR system; these include propagating ridge tips at ridge^transform intersections on the Juan de Fuca Ridge and eastern Gala¤pagos spreading center, and at the KEY WORDS: assimilation; dacite; fractional crystallization; 98N overlapping spreading center on the East Pacific Rise. Despite mid-ocean ridge; MORB; partial melting the formation of these lavas at various ridges, MOR dacites show remarkably similar major element trends and incompatible trace element enrichments, suggesting that similar processes are controlling INTRODUCTION their chemistry. Although most geochemical variability in MOR bas- Fast to intermediate oceanic spreading centers typically alts is consistent with low-pressure fractional crystallization of vari- erupt geochemically diverse basaltic lavas (e.g. Klein, ous mantle-derived parental melts, our geochemical data for MOR 2005); however, a much more extensive range of lava com- dacitic glasses suggest that contamination from a seawater-altered positions, including ferrobasalts and FeTi basalts as well as component is important in their petrogenesis. MOR dacites are char- rarer high-silica andesites and dacites have been recovered acterized by elevated U,Th, Zr, and Hf, low Nb and Taconcentra- (Perfit et al., 1983; Langmuir et al., 1986; Natland & tions relative to rare earth elements (REE),and Al2O3,K2O, and MacDougall, 1986; Natland et al., 1986; Regelous et al., Cl concentrations that are higher than expected from low-pressure 1999; Smith et al., 2001). High-silica lavas have erupted on fractional crystallization alone. Petrological modeling of MOR several ridges and are commonly associated with specific dacites suggests that partial melting and assimilation are both inte- tectonic settings; these include propagating ridge tips gral to their petrogenesis. Extensive fractional crystallization of a (Christie & Sinton, 1981; Perfit & Fornari, 1983; Fornari MORB parent combined with partial melting and assimilation of et al., 1983), overlapping spreading centers (OSCs) amphibole-bearing altered crust produces a magma with a (Christie & Sinton, 1981; Perfit et al., 1983; Sinton et al.,

*Corresponding author.Telephone: (352) 392-2231. Fax: (352) 392-9294. E-mail: [email protected] ß The Author 2010. Published by Oxford University Press. All yPresent address: National Science Foundation, Ocean Drilling rights reserved. For Permissions, please e-mail: journals.permissions@ Program, Arlington,VA22230, USA oxfordjournals.org JOURNAL OF PETROLOGY VOLUME 51 NUMBER 12 DECEMBER 2010

1983; Ba zin et al., 2001), regions of ridge^hotspot inter- 2009). Other studies indicate that low degrees of dehydra- action (Chadwick et al., 2005; Haase et al., 2005), and at tion partial melting of altered basalt, similar in compos- 10830’N on the East Pacific Rise near the ridge^transform ition to dikes of the upper ocean crust, can produce intersection (Regelous et al., 1999). These wide variations dacitic melts (Beard & Lofgren, 1991). These experimental in composition are commonly attributed to low magma studies suggest that oceanic crust will begin to melt at tem- supply and/or cooler crust at ridge segment ends, or the peratures as low as 850^9008C and 510% melting of cold edge effect, which promote greater differentiation of the crust will yield dacitic or tonalitic melts (Beard & magmas prior to eruption (Christie & Sinton, 1981; Perfit Lofgren, 1991; Koepke et al., 2004; Kvassnes & Grove, et al., 1983; Sinton et al., 1983; Perfit & Chadwick, 1998; 2008). Kvassnes & Grove (2008) stated that mineral pairs Rubin & Sinton, 2007). (plagioclase^olivine and plagioclase^augite) similar to Downloaded from https://academic.oup.com/petrology/article/51/12/2377/1540213 by guest on 29 September 2021 The formation of highly evolved, silicic magmas in oceanic gabbros from the lower crust will melt quickly non-ridge settings (e.g. ocean islands, arc volcanoes, and and easily at temperatures similar to that of primitive continental interiors) has been attributed to various pro- MOR magmas (1220^13308C). All of these studies indicate cesses, including crystal fractionation, partial melting of that partial melting of ocean crust can produce high-silica overlying crust, and/or assimilation of crustal material melts at MORs, but the role that this process may play in into an evolving magma chamber (e.g. Bowen, 1928; De the formation of extrusive silicic lavas on the seafloor has Paolo, 1981; Hildreth, 1981). On mid-ocean ridges (MORs), not yet been assessed. many studies have documented the dominant role crystal The compositional variability observed in arc and fractionation plays in magma differentiation (e.g. Clague continental volcanic rocks is commonly ascribed to the & Bunch,1976; Bryan & Moore,1977; Byerly,1980) whereby associated processes of assimilation and fractional crystal- extensive crystallization of olivine (Ol), plagioclase (Plag), lization (AFC; e.g. Bowen, 1928; De Paolo, 1981). Similar pyroxene (Px) and Fe^Ti oxides leads to the generation of processes may also occur where thickened oceanic crust highly evolved melts enriched in SiO2 and depleted leads to magma^crust interaction; for instance, within in MgO, FeO and TiO2 (e.g. Juster et al., 1989). However, Icelandic volcanoes (e.g. Nicholson et al., 1991). On smaller although crystal fractionation is undoubtedly a primary scales, the combined effects of these processes have been process involved in the differentiation of most MORB observed in ophiolites, where sub-axial intrusive magmas magmas, it may not be the only mechanism involved in have been in contact with and have partially melted the the formation of high-silica MOR andesites and dacites. overlying sheeted dikes (Gillis & Coogan, 2002; Coogan, Partial melting (or anatexis) of basaltic crustal material 2003; Gillis, 2008; France et al., 2009). AFC processes have may produce evolved compositions, particularly in settings also been invoked to explain the high Cl concentrations where magma^rock interactions are likely, such as the top observed in some MORBs (Michael & Schilling, 1989; of an axial magma chamber (e.g. Coogan et al., 2003b; Michael & Cornell, 1998; le Roux et al., 2006). During this Gillis, 2008). It has been suggested as the origin for process, a magma undergoes crystal fractionation, and the high-silica lavas erupted on many ocean islands (e.g. resultant latent heat of crystallization provides the heat Iceland; O’Nions & Gronvold, 1973; Sigurdsson & Sparks, needed to partially melt the surrounding wall-rock. These 1981; Martin & Sigmarsson, 2007; Galapagos Islands; melts are then assimilated into, and homogenized with, McBirney, 1993; Socorro Island; Bohrson & Reid, 1997; the fractionating magma reservoir. AFC processes can Bohrson & Reid, 1998; Hawaii; Van Der Zander et al., produce a wide range of rock types depending on the ini- 2010). This process may also explain the formation of tial composition of the intruding magma, the degree of high-silica lavas in back-arc settings, most recently within crystal fractionation, the initial wall-rock composition, the Lau Spreading Center (e.g. Kent et al., 2002) and and the amount of melting and assimilation. Manus Basin (Sinton et al., 2003), although the influence High-silica compositions are found throughout the of a subduction zone in these tectonic settings makes ocean crust and are commonly observed as intrusive or magma genesis much more complicated. plutonic material. As mentioned above, plagiogranites are Evidence from ophiolites suggests that the top of the a ubiquitous component of the ocean crust and have been axial magma chamber at MORs is a dynamic boundary observed as small intrusions and veins in ophiolites (e.g. where magmas may interact with and melt different Pedersen & Malpas, 1984), drill cores from the ocean crust layers of crustal material, including both gabbros and (Casey, 1997; Dick et al., 2000; Wilson et al., 2006), and as sheeted dikes (Coogan et al., 2003b). Recent experimental xenoliths in Icelandic lavas (Sigurdsson, 1977). The origin evidence suggests that partial melting of hydrous gabbroic of these silicic rocks remains unclear but two main hypoth- rock similar to that in the lower ocean crust can form sili- eses are (1) partial melting of gabbroic crust (e.g. Koepke cic magmas (Koepke et al., 2004; Kvassnes & Grove, 2008) et al., 2004, 2007; Nunnery et al., 2008) and (2) extreme and may explain the presence of highly evolved plagiogra- crystal fractionation of tholeiitic basalt magmas nite veins in the ocean crust (Koepke et al., 2004; Brophy, (Coleman & Donato, 1979; Beccaluva et al., 1999; Niu

2378 WANLESS et al. DACITE PETROGENESIS ON MORs

et al., 2002). There are many examples of highly evolved the role the tectono-magmatic setting may play in their plutonic rocks from slower spreading centers (e.g. petrogenesis. Mid-Atlantic Ridge, Aumento, 1969; South West Indian Ridge, Dick et al., 2000), which may suggest that AFC or partial melting processes are occurring on much smaller GEOLOGICAL AND TECTONIC scales, deeper in the ocean crust. SETTING In this study we examine the geochemistry of high-silica 98N East Pacific Rise overlapping lavas from three MORs, the East Pacific Rise, Juan de spreading center Fuca Ridge, and Gala¤ pagos Spreading Center (Fig. 1), and The 98N OSC (Fig. 1a) is located on the East Pacific Rise show they have remarkably similar major and trace elem- between the Clipperton and Siqueiros transform faults. Downloaded from https://academic.oup.com/petrology/article/51/12/2377/1540213 by guest on 29 September 2021 ent compositions (Fig. 2), suggesting that similar sources It consists of two north^south-trending ridges that overlap and processes control their petrogenesis. More specifically, by 27 km and partly enclose a large overlap basin we examine the roles that crystal fractionation, partial (Sempere & Macdonald, 1986). The limbs are separated melting, and AFC may have played in the formation of by 8 km (Singh et al., 2006).The eastern limb is propagat- an exceptional suite of high-silica lavas from the 98N ing to the south into older crust at a rate of 42 km OSC on the East Pacific Rise, and evaluate if these results Myr^1 (Carbotte & Macdonald, 1992). apply generally to the formation of high-silica lavas on The 98N OSC has been the focus of several geophysical other MORs. We focus on the petrogenesis of dacites at studies (Detrick et al., 1987; Harding et al., 1993; Kent the 98N OSC because it is the most complete and geo- et al., 1993, 2000; Bazin et al., 2001; Dunn et al., 2001; Tong logically well-constrained dataset available; however, et al., 2002); including the first 3D multi-channel seismic descriptions of the geological settings of high-silica lavas survey of a MOR (Kent et al., 2000) and a 3D seismic re- in the other environments are important to ascertain fraction study (Dunn et al., 2001). These studies resulted in

Fig. 1. Bathymetric maps showing the tectonic setting of the MOR dacites discussed in the text (data from GeoMapApp; Carbotte et al., 2004). Boxes show the general locations of high-silica lavas on each ridge. Dacites are commonly associated with the ends of ridge segments, such as overlapping spreading centers (OSCs) and propagating ridge tips at ridge^transform intersections. (a) 98N OSC on the East Pacific Rise (EPR), (b) propagating ridge tip on the Juan de Fuca Ridge and Axial Seamount and (c) possible OSC near the propagating ridge tip on the Gala¤ pagos Spreading Center (GSC).

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Fig. 1. Continued.

the first 3D image of a subsurface magma chamber along a White et al., 2009). Several high-silica lavas were also re- MOR, which showed that a shallow melt lens lies beneath covered from this area during dredging operations in the both limbs of the OSC with an anomalously large melt late 1980s (Langmuir et al., 1986). The siliceous lavas are lens in the interlimb region, north of the overlap basin. primarily confined to the northern section of the This suggests that the region currently has an unusually neo-volcanic zone on the eastern, propagating limb, along high magma supply rate for a ridge segment end (Kent the eastern edge of the melt lens (Fig. 3). Morphologically, et al., 2000). the dacites form large single bulbous to elongate pillows High-silica andesites and dacites were recovered from that can each be several meters in diameter (Fig. 4). The the eastern limb during the MEDUSA2007 cruise pillows are highly striated and have a coarse bread-crust (AT15-17) using the ROV JasonII (Wanless et al., 2008; surface texture. Typically, the pillows are stacked into

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Fig. 1. Continued. mounds, which can be several meters high, or construc- High-resolution bathymetric maps show that there are tional domes. Dacites largely occur in two regions: as a numerous other constructional domes in the region but nearly linear pillow mound in the center of the east limb they have not yet been sampled, although we surmise that neo-volcanic zone and as large, elongate pillow lavas on they are also composed of high-silica lavas. Rare andesites the flanks of the axial graben (Fig. 3). Their axial and have also been recovered within the axis and along the near-axial positions, low sediment cover and unaltered bounding faults of the southern Cleft segment (Stakes nature suggest they are relatively young. et al., 2006). Seismic studies of the southern Juan de Fuca Ridge indi- Juan de Fuca Ridge propagating ridge cate the presence of an axial magma chamber beneath tip and Axial Seamount most of the Cleft segment (Canales et al., 2005). However, The Cleft segment is the southernmost segment of the Juan an axial magma chamber reflector is absent south of de Fuca Ridge (Fig. 1b). It terminates at 44827’N at a 44838’N where the high-silica lavas were recovered, sug- ridge^transform intersection, where it intersects and over- gesting the presence of small melt volumes resulting from laps the Blanco Transform Zone (Embley et al., 1991; weak melt supply to the ridge^transform intersection. Embley & Wilson, 1992; Smith et al., 1994). This intersec- Zircon thermochronology and U-series data indicate that tion is characterized by a series of curved ridges that over- the dacites erupted less than 30 kyr ago (Perfit et al., 2007). shoot the Blanco Transform Zone onto the older Pacific The axial segment of the Juan de Fuca Ridge is a plate (Stakes et al., 2006). second-order ridge segment that currently overlies the High-silica andesites and dacites form two small con- Cobb hotspot, which has produced a chain of seamounts structional domes on the Pacific plate, where the axial trending NW away from the ridge axis (Chadwick et al., ridge intersects and is believed to propagate past the 2005). The Juan de Fuca Ridge is migrating NW at a rate Blanco Transform Zone into the older ocean crust (6·3 of 3·1cm a^1 and has been situated above the Cobb hotspot Ma) that was created at the Gorda Ridge (Embley & for the last 0·2^0·7 Myr, creatinga large on-axis seamount, Wilson, 1992; Stakes et al., 2006). The domes are 20^30 known as Axial Seamount (Karsten & Delaney,1989). m high and 200^500 m in diameter and were sampled Axial Seamount is the largest feature on this segment of using a rock core and the ROV Tiburon during research the Juan de Fuca Ridge and has a large summit caldera cruises in 2000 and 2002 (Cotsonika, 2006). (Embley et al., 1999) underlain by a large seismically

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Fig. 2. Comparison of major and trace element compositions in MOR high-silica andesites and dacites from the East Pacific Rise (OSC Dacite), Juan de Fuca Ridge (JdF Dacite), Gala¤ pagos Spreading Center (GSC Dacite) and Axial Seamount (Axial Andesite). MOR dacitic lavas have similar major and trace element compositions, whereas andesites have more variable compositions that lie between dacites and highly evolved MORBs. (a) Mantle-normalized diagram showing similarities in trace element compositions between dacites from the three ridges and an andesite from Axial Seamount on the Juan de Fuca Ridge. Average composition for N-MORB from the 9817 0^108N segment of the East Pacific Rise (EPR) is shown for comparison. MOR dacites are characterized by low Nb and Ta and high U, Th, Zr and Hf relative to elements of similar incompatibilities. (b, c) Major element variation diagrams showing the range of compositions of the MOR dacites compared with East Pacific Rise MORB. Gray field shows the range of compositions from 41200 analyses of MORB glasses from the segment of the East Pacific Rise north of the 98N OSC (from PetDB, Perfit et al.,1994; Sims et al., 2002, 2003; our unpublished data).

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Fig. 3. Bathymetric map of the 98N OSC showing the locations of samples collected during the MEDUSA cruise in 2007. Samples are divided into rock types based on silica content (dacite 462 wt % SiO2; andesite 57^62 wt % SiO2; basaltic andesite 52^57 wt % SiO2; basalt 552 wt % SiO2 and FeTi basalt552 wt % SiO2 and412wt % FeO). Dacites are primarily found on-axis on the eastern limb of the OSC. The 50 m contour intervals are shown.

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Fig. 4. Photographs of MOR high-silica lavas from 98N OSC East Pacific Rise (a, b), Gala¤ pagos Spreading Center (c) andJuan de Fuca Ridge (d). Morphologically, the dacites typically form blocky angular flows and large elongate pillows with roughly corrugated or striated surfaces.

imaged axial magma chamber (West et al., 2001). It has two 1983; Perfit et al., 1983; Embley et al., 1988). The evolved prominent rift zones, extending to the north and south, lavas erupted within the axial valley and along the which create bathymetric highs. Extensive sampling of the axis-bounding faults of the Gala¤ pagos Spreading Center main edifice shows that it is composed of moderately 20 km east of the ridge^transform intersection with the evolved and slightly enriched MORB (Chadwick et al., Inca transform fault (Fig. 1c). Bathymetric data reveal two 2005). The rift zones have linear ridges that appear to ac- curved ridges surrounding a depression within this region, commodate extensive diking from the main caldera which has been interpreted as an old, small, extinct OSC system (Chadwick et al., 2005). Rare high-silica andesites or deviation from axial linearity (Embley et al., 1988; sampled by three rock cores (Chadwick et al., 2005) are Perfit et al., 1999) that has been rifted away from the located east of the northern rift zone and may be asso- neo-volcanic zone. Most of the evolved lavas (63% ciated with dike propagation from the main axial magma SiO2) at the Gala¤ pagos Spreading Center were found chamber into older ridge crust. off-axis along the bounding faults associated with the southern portion of the extinct OSC. Gala¤ pagos Spreading Centerçextinct OSC or propagating ridge tip? High-silica lavas were sampled at the eastern end of the PETROGRAPHY Gala¤ pagos Spreading Center at 858W. The area was ex- The 98N OSC dacites are glassy and predominantly aphy- tensively studied though dredging and Alvin exploration ric, with sparse microphenocrysts and very rare, small in the early 1980s (Fornari et al., 1983; Perfit & Fornari, phenocrysts of plagioclase and clinopyroxene. The small

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phenocrysts of plagioclase commonly have resorbed edges Small glass fragments (10^50 mm) were handpicked, and sieve textures. Several of the samples contain small avoiding microphenocrysts and alteration, cleaned, and basaltic xenoliths composed of subophitic plagioclase and dissolved for trace element and isotope analyses following clinopyroxene surrounded by dacitic or andesitic glass. methods described by Goss et al. (2010). Fourteen dacites Juan de Fuca Ridge dacites typically are microcrystal- from the 98N OSC were analyzed for trace element line to nearly aphyric and also contain rare basaltic xeno- concentrations by inductively coupled plasma mass spec- liths and xenocrysts. Unlike spatially associated MORB, trometry (ICP-MS) at medium resolution using a high- which have low vesicularity, the dacites are moderately precision Element2 system at the University of Florida vesicular with 10^15% (by volume) elongate vesicles (Table 1). Radiogenic isotope ratios (Pb, Sr, and Nd) were (millimeter-sized to 10cm long). More crystalline samples determined for 10 dacites by multi-collector ICP-MS using Downloaded from https://academic.oup.com/petrology/article/51/12/2377/1540213 by guest on 29 September 2021 contain microphenocrysts of ferroaugite^hedenbergite and a Nu-Plasma system at the University of Florida Center ferropigeonite^ferrosilite, and lesser amounts of sodic for Isotope Geoscience (Table 2). A detailed description of plagioclase and FeTi oxides set in a glassy matrix with sample preparation, dissolution procedures, standards, crystallites (20^30%) of plagioclase and pyroxene with and errors has been given by Kamenov et al. (2007) and rare fayalite, quartz and zircon crystals. Clinopyroxene Goss et al. (2010). External calibration was done to quantify microphenocrysts exhibit both normal and reverse zoning results using a combination of in-house basalts (ENDVç with rims that have the same composition as groundmass Endeavour and ALV 2392-9) and USGS (AGV-1, BIR-1, pyroxenes. Titanomagnetite is common in most samples, BHVO-1, BCR-2 and STM-1) rock standards (Kamenov whereas ilmenite is significantly less abundant (Cotsonika, et al., 2007; Goss et al., 2010). 2006). The petrology and mineral chemistry of andesites and GEOCHEMICAL RESULTS dacites from the Gala¤ pagos Spreading Center have been described in detail by Perfit & Fornari (1983). Many of Major elements the lavas are nearly aphyric and extremely glassy with Major element compositions of the 98N OSC high-silica moderate vesicularity (55 vol. %). In the more crystalline andesites and dacites are presented (along with the trace andesites rare phenocrysts and microphenocrysts of element abundances) in Table 1. The major element geo- augite^ferroaugite, pigeonite, FeTi oxides and intermediate chemistry of the 98N OSC lavas is similar to that of the composition plagioclase predominate. Apatite crystals Juan de Fuca Ridge and Gala¤ pagos Spreading Center are present but extremely rare. Similar to the Juan de lavas (Fig. 2). Here, only data from the 98N OSC are dis- Fuca high-silica lavas, calcic plagioclase and magnesian cussed in detail; however, it is important to note the simi- clinopyroxene xenocrysts are present in the Gala¤ pagos larity in major and trace element contents and elemental Spreading Center andesites. trends in high-silica lavas from all three ridges. New analyses of some of the high-silica samples previously analyzed and discussed by Perfit et al. (1983,1999) and rep- resentative samples from the Juan de Fuca Ridge are pre- ANALYTICAL METHODS sented as Supplementary Data (http://www.petrology Glass chips from the outer rims of 18 dacites collected at .oxfordjournals.org/). All high-silica samples from the the 98N OSC were analyzed using a JEOL 8900 Electron 98N OSC appear unweathered with minimal amounts of Microprobe for major element concentrations at the Fe^Mn oxide coating and are essentially aphyric. USGS in Denver, Colorado (Table 1). Eight to 10 points The 98N OSC tholeiitic andesites to dacite samples ex- were analyzed per sample and averages are reported in hibit increasing SiO2 with decreasing FeO, TiO2, and Table 1. USGS mineral standards were used to calibrate MgO (Figs 2 and 5), with the most differentiated dacites the microprobe and secondary normalizations were done having 67 wt % SiO2 and51wt % MgO.Al2O3 concen- to account for instrument drift using the JdF-D2 glass trations in the dacites (12·9^13·3 wt %), however, do not ‘standard’ (Reynolds, 1995), University of Florida in-house show a large decrease compared with the OSC basalts standard ALV 2392-9 from the East Pacific Rise (Fig. 5).The dacites have high incompatible major element (Smith et al., 2001) and USGS standard dacite glass GSC concentrations (K2O40·90 wt % and Na2O43·4wt%; (for more detail on methods see Smith et al., 2001). The Fig. 5) but low P2O5 concentrations (50·26 wt %; Fig. 5) beam diameter during glass analyses was 20 mm, and an compared with basalts. Chlorine concentrations in the accelerating voltage of 15keV and a beam current of 20 dacites range from 0·24 to 0·70 wt % compared with nA were used. High-precision chlorine and potassium con- 50·01 to 0·04 wt % in the OSC basalts (Fig. 6). centrations were also determined by microprobe on seven of the dacites using 200 s peak and 100 s background count- Tr ace e le me nts ing times. St 7820 Sodalite was used as the chlorine The 98N OSC dacites are enriched in incompatible trace standard. elements compared to 98N OSC basalts (Fig. 7); the latter

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Table 1: Major and trace element compositions of dacites from the 98N OSC, East Pacific Rise

Latitude (N): 9·148 9·145 9·145 9·143 9·147 9·146 9·128 9·145 9·137 Longitude (W): 104·198 104·207 104·207 104·207 104·203 104·204 104·211 104·206 104·211 Sample no.: 266-58 265-65 265-64 265-67 266-50 266-53 265-84 265-63 266-47

SiO2 63·063·864·064·164·364·364·464·464·5

TiO2 1·10 1·26 1·28 1·34 1·07 1·06 1·13 1·29 0·99

Al2O3 13·113·213·113·313·213·313·213·313·2 Downloaded from https://academic.oup.com/petrology/article/51/12/2377/1540213 by guest on 29 September 2021 FeO 8·43 8·14 8·27 8·49 8·08 8·06 8·18 8·22 7·74 MnO 0·16 0·15 0·16 0·16 0·14 0·14 0·15 0·15 0·14 MgO 1·75 1·34 1·60 1·49 1·27 1·12 1·23 1·29 1·02 CaO 4·34 4·21 4·45 4·41 3·78 3·73 3·92 4·21 3·53

Na2O3·63 3·84 3·46 3·93 4·23 4·16 3·41 3·71 4·94

K2O0·96 0·97 0·97 0·95 1·10 1·09 1·19 0·99 1·22

P2O5 0·26 0·22 0·20 0·23 0·24 0·25 0·22 0·21 0·23 Cl 0·24 0·65 Total 96·69 97·17 97·55 98·42 97·35 97·20 96·98 97·76 97·54 Trace elements (ppm) Li 32 34 32 30 29 30 Sc 17·419·918·214·815·217·2 V 122 140 121 61 102 121 Cr 12·912·312·43·81·79·8 Co 14·817·315·512·814·014·9 Ni 9·09·89·25·87·08·4 Cu 17·319·117·618·420·816·7 Zn 110·1124·1122·4 109·0100·4112·7 Ga 29·735·428·128·829·430·9 Rb 9·110·59·612·812·79·5 Sr 76 90 89 86 70 76 Y 132 148 142 157 133 132 Zr 735 842 622 856 968 745 Nb 13·014·813·015·514·613·2 Cs 0·11 0·12 0·12 0·14 0·13 0·10 Ba 50·557·152·964·659·849·8 La 23·25 26·31 23·43 28·53 27·27 23·53 Ce 67·48 76·53 67·97 82·17 77·81 68·11 Pr 10·06 11·37 10·05 11·87 11·21 10·16 Nd 46·452·845·953·750·947·3 Sm 14·09 15·37 14·62 16·25 14·32 13·77 Eu 3·00 3·35 3·12 3·18 2·80 2·99 Gd 16·83 18·13 17·43 19·00 16·58 16·37 Tb 3·19 3·43 3·36 3·65 3·09 3·10 Dy 21·08 22·81 22·85 24·21 20·31 20·53 Ho 4·58 4·88 4·92 5·19 4·33 4·40 Er 13·55 14·65 14·86 15·79 13·16 13·19 Tm 2·19 2·33 2·37 2·50 2·10 2·10 Yb 14·25 15·20 15·66 16·74 13·66 13·54 Lu 2·15 2·31 2·44 2·56 2·10 2·06 Hf 18·69 20·95 17·54 22·28 23·14 18·62 Ta 0·81 1·10 0·92 1·03 1·39 0·99 Pb 3·33 5·05 3·41 3·15 4·89 5·82 Th 1·74 1·82 1·64 2·29 2·28 1·65 U0·59 0·65 0·64 0·86 0·82 0·59

(continued)

2386 WANLESS et al. DACITE PETROGENESIS ON MORs

Table 1: Continued

Latitude (N): 9·128 9·133 9·116 9·164 9·140 9·155 9·128 9·116 9·154 Longitude (W): 104·210 104·216 104·198 104·205 104·211 104·213 104·208 104·198 104·215 Sample no.: 265-85 266-46 265-94 264-09 265-70 265-42 265-83 265-95 265-40

SiO2 65·065·065·265·866·366·567·567·5

TiO2 1·06 0·94 0·97 0·89 0·87 0·94 0·76 0·77

Al2O3 13·112·913·013·213·213·013·313·1 Downloaded from https://academic.oup.com/petrology/article/51/12/2377/1540213 by guest on 29 September 2021 FeO 7·99 7·17 7·90 7·03 7·17 7·92 6·68 6·47 MnO 0·16 0·15 0·14 0·13 0·14 0·16 0·13 0·12 MgO 1·18 1·41 1·13 1·06 0·80 0·89 0·67 0·94 CaO 3·78 3·71 3·54 3·48 3·23 3·50 2·98 3·01

Na2O3·67 4·76 4·29 4·24 4·08 3·99 3·88 4·43

K2O1·22 1·19 1·14 1·21 1·33 1·20 1·37 1·21

P2O5 0·20 0·17 0·23 0·20 0·19 0·21 0·16 0·15 Cl 0·64 0·58 0·70 0·51 0·67 Total 97·41 97·43 97·61 97·78 97·27 98·91 97·37 97·67 Trace elements (ppm) Li 31 31 27 34 32 32 31 31 Sc 14·513·111·912·313·811·010·512·5 V 73 63464558325251 Cr 4·61·53·83·73·43·43·04·2 Co 12·310·710·29·910·78·38·59·9 Ni 6·64·95·75·05·94·75·45·3 Cu 18·816·815·915·915·514·014·716·6 Zn 107·6104·688·8 106·4 119·0103·198·3 103·3 Ga 28·130·228·328·829·529·230·430·2 Rb 13·812·412·915·013·715·512·512·4 Sr 81 68 78 78 83 76 61 73 Y 154 146 151 160 160 159 145 146 Zr 872 1050 824 934 816 922 985 1013 Nb 15·316·114·915·916·415·616·516·6 Cs 0·15 0·13 0·13 0·17 0·16 0·17 0·13 0·13 Ba 68·160·065·772·770·076·462·259·7 La 29·07 29·10 28·98 30·89 29·03 30·69 29·16 29·47 Ce 82·46 83·93 83·89 88·15 82·95 87·16 83·65 85·00 Pr 11·78 12·15 11·97 12·49 11·98 12·32 12·02 12·44 Nd 52·155·252·755·053·554·054·656·6 Sm 16·01 15·74 15·92 16·67 16·69 16·46 15·64 16·89 Eu 3·01 3·02 2·95 3·09 3·39 3·05 2·89 3·32 Gd 18·51 18·17 18·54 19·47 19·69 18·90 17·66 19·68 Tb 3·55 3·39 3·55 3·69 3·76 3·64 3·35 3·67 Dy 23·80 22·36 23·80 25·06 25·37 24·51 22·13 23·89 Ho 5·12 4·77 5·12 5·38 5·47 5·27 4·74 5·20 Er 15·50 14·37 15·66 16·42 16·48 16·18 14·61 15·35 Tm 2·49 2·29 2·52 2·63 2·64 2·62 2·32 2·49 Yb 16·69 14·81 16·84 17·54 17·56 17·54 15·12 16·19 Lu 2·58 2·30 2·61 2·73 2·75 2·72 2·32 2·42 Hf 22·96 24·87 22·68 24·80 22·24 24·75 24·51 24·97 Ta 1·05 1·18 1·04 1·10 1·12 1·08 1·23 1·02 Pb 3·59 5·75 2·76 3·84 3·47 3·80 4·12 3·64 Th 2·43 2·35 2·59 2·68 2·36 2·80 2·36 2·49 U0·92 0·84 0·98 1·04 0·91 1·05 0·86 0·84

2387 JOURNAL OF PETROLOGY VOLUME 51 NUMBER 12 DECEMBER 2010

Table 2: Radiogenic isotopic compositions of lavas from 98N OSC, East Pacific Rise

208Pb/204Pb 2s 207Pb/204Pb 2s 206Pb/204Pb 2s 87Sr/86Sr 2s 143Nd/144Nd* 2seNd

East Limb basalts 264-04 37·6992 17 15·4758 8 18·2789 9 0·702496 11 0·513163 11 10·2 265-18 37·6766 19 15·4723 7 18·2749 8 0·702487 18 0·513190 13 10·8 265-35 37·6635 16 15·4704 6 18·2496 7 0·702472 12 0·513164 5 10·3 265-43 37·6608 15 15·4674 5 18·2492 6 0·702502 21 0·513154 9 10·1 Downloaded from https://academic.oup.com/petrology/article/51/12/2377/1540213 by guest on 29 September 2021 265-113 37·6949 17 15·4767 6 18·2750 7 0·702494 16 0·513172 5 10·4 266-01 37·6417 20 15·4693 7 18·2348 9 0·702456 15 0·513158 7 10·1 266-33 37·6831 22 15·4739 9 18·2771 9 0·702444 13 0·513160 5 10·2 265-05 37·6873 20 15·4776 6 18·2936 7 0·702430 12 0·513191 8 10·8 East Limb basaltic andesites 265-24 37·6834 17 15·4731 6 18·2644 6 0·702562 13 0·513187 5 10·7 265-56 37·6813 15 15·4736 6 18·2702 6 0·702528 12 0·513187 4 10·7 265-91 37·6827 14 15·4745 5 18·2659 5 0·702532 12 0·513179 4 10·6 265-103 37·6742 21 15·4721 8 18·2608 9 0·702450 12 0·513162 4 10·2 265-109 37·6729 18 15·4714 7 18·2594 7 0·702493 16 0·513145 7 9·9 265-125 37·6717 19 15·4712 7 18·2648 8 0·702458 45 0·513154 6 10·1 264-20 37·6899 15 15·4758 6 18·2694 6 0·702438 11 0·513179 4 10·6 265-49 37·6890 16 15·4745 6 18·2685 6 0·702438 12 0·513196 7 10·9 East Limb andesites 264-14 37·6725 21 15·4724 10 18·2624 11 0·702428 14 0·513153 5 10·0 265-25 37·6614 17 15·4688 7 18·2511 7 0·702544 18 0·513152 5 10·0 265-90 37·6754 15 15·4716 6 18·2617 7 0·702496 11 0·513159 6 10·2 265-100 37·6804 15 15·4735 6 18·2650 6 0·702489 12 0·513179 4 10·6 266-54 37·6877 17 15·4749 7 18·2717 8 0·702463 12 0·513193 5 10·8 East Limb dacites 264-09 37·6764 18 15·4721 7 18·2679 8 0·702536 19 0·513171 8 10·4 265-40 37·6737 21 15·4713 8 18·2655 9 0·702466 11 0·513149 8 10·0 265-42 37·6782 17 15·4719 7 18·2703 7 0·702458 15 0·513140 6 9·8 265-64 37·6822 25 15·4756 10 18·2670 11 0·702576 15 0·513185 7 10·7 265-70 37·6760 20 15·4707 8 18·2663 8 0·702476 24 0·513141 5 9·8 265-83 37·6814 15 15·4728 6 18·2712 7 0·702534 15 0·513147 7 9·9 265-84 37·6887 17 15·4761 7 18·2739 7 0·702507 11 0·513179 7 10·6 265-85 37·6767 21 15·4713 8 18·2674 8 0·702481 19 0·513148 5 9·9 265-95 37·6795 17 15·4746 7 18·2633 8 0·702465 19 0·513165 6 10·3 266-53 37·6780 13 15·4718 5 18·2676 5 0·702471 15 0·513175 10 10·5

The 2s error reflects in-run machine error (precision at the last significant figure). Long-term reproducibility estimates are: 87Sr/86Sr ¼0·00003, 143Nd/144Nd ¼0·000018, 206Pb/204Pb ¼0·0034 (205 ppm), 207Pb/204Pb ¼ 0·0028 (184 ppm), 208Pb/204Pb ¼0·0086 (234 ppm). *Unknowns were normalized to an 143Nd/144Nd value of 0·511215 0·000007 for JNdi-1, which was reported by Tanaka et al. (2000) relative to a La Jolla 143Nd/144Nd value of 0·511858 (Lugmair & Carlson, 1978). Major and trace element data for basalts, FeTi basalts, basaltic andesites and andesites have been presented by Wanless (2010). have compositions typical of normal, incompatible trace The dacites also contain high concentrations of Rb, Ba, U element-depleted mid-ocean ridge basalts (N-MORB) and Th, but relatively low Sr and Eu contents. Compared from the northern East Pacific Rise. For example, Zr and with East Pacific Rise N-MORB the dacites have relatively Hf concentrations in the dacites range from 622 to flat REE patterns (Fig. 2). On mantle-normalized trace 1050 ppm and from 18 to 25 ppm, respectively (Table 1). element diagrams the dacites have positive Zr, Hf, U, and

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Fig. 5. Major element variations vs MgO (wt %) in dacites from the 98N OSC on the East Pacific Rise (filled squares). Gray field represents all lavas collected in 2007 from the 98N OSC. Dacites are compared with three low-pressure fractional crystallization trends (calculated using MELTS; Ghiorso & Sack,1995) using parental compositions (FC-1, FC-2, FC-3) from OSC basalts (see text for modeling parameters and details and Table 3 for compositions). Not all dacite major element variations can be explained by fractional crystallization alone (e.g. Al2O3,K2O, and P2O5). Experimental compositions from partial melting of altered basalt (Beard & Lofgren,1991) are shown for comparison (B&L PMelts).

Th anomalies, and negative Nb and Ta anomalies (Fig. 2). (i.e. Ni and Cr) and Nb/La are positively correlated. Consequently, the dacites also have slightly lower Nb/La U/Nb, Nd/Y Th/Nb and Ce/Yb ratios are negatively corre- and higher Zr/Dy ratios compared with OSC basalts lated with MgO in the dacites. (Fig. 8), and Ce/Yb, Th/Nb and Nd/Y ratios increase from basalt to dacites (Fig. 8). Compatible trace elements are Isotopic data low in the dacites, with Ni concentrations ranging from The 98N OSC dacites have very limited ranges of Pb, Sr 9·8to4·9. ppm (Fig. 7) and Cr concentrations from 13 to and Nd isotopic compositions, which lie within the general 1ppm. Most incompatible trace elements exhibit negative field of East Pacific Rise MORB (Table 2; Fig. 9). 87Sr/86Sr correlations with MgO; however, compatible elements ratios range from 0·70246 to 0·70258, with an average of

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Fig. 6. Variation diagram showing Cl (wt %) vs MgO (wt %) for OSC lavas. Superimposed are three model liquid lines of descent (calculated using MELTS; Ghiorso & Sack, 1995), showing that the maximum amount of Cl enrichment owing to extensive fractional crystallization cannot produce the high Cl concentrations in the OSC dacites. The dashed rectangle represents the range of compositions that can be produced through 1^15% partial melting of altered basalt with 350 ppm Cl (star); 350 ppm Cl is the median value of Cl analyzed in sheeted dikes from ODP Hole 504B, with minimum and maximum values (49 and 650 ppm) shown with an error bar. Partition coefficients for Cl are from Gillis et al. (2003).The range of MgO values for partial melts was taken from experimental partial melts of less than 15% (Beard & Lofgren, 1991).

0·70250. These values are well within the range of differentiated major element concentrations, high concen- N-MORB East Pacific Rise lavas from 9 to 108N(Sims trations of incompatible elements, distinct trace element et al., 2002, 2003; Goss et al., 2010) and similar to patterns, and N-MORB-like isotopic signatures. The N-MORB lavas from the 98NOSC.143Nd/144Nd ratios are models must also be able to explain relatively high U, Th, also similar to East Pacific Rise N-MORB and range Zr and Hf, and low Nb and Ta as well as the flat REE pat- from 0·51314 0 ( eNd ¼ 9·8) to 0·513196 (eNd ¼10·9). Pb terns. Additionally, markedly high Cl, K (and high Cl/K), isotopes ratios for the 98N OSC dacites are indistinguish- Al2O3, and low P2O5 must be accounted for in successful able from those for 98N OSC basalts and other East petrogenetic schemes. Pacific Rise lavas, with average 206Pb/204Pb ¼18·268, 207Pb/204Pb ¼15 ·473, and 208Pb/204Pb ¼ 37·679. The Pb isotope compositions of the 98N dacites form a tight cluster Crystal fractionation in the center of the field defined by other 98Nlavas(Fig.9). Several petrological models, including MELTS thermodynamic modeling (Ghiorso & Sack, 1995), Rayleigh crystal fractionation, crystal^melt segregation, and in situ crystal- PETROGENETIC MODELS FOR lization are investigated here to determine if various pro- HIGH-SILICA LAVAS cesses of crystal fractionation can account for the major We now examine the results of various models of fractional and trace element compositions of MOR dacites. crystallization, partial melting and assimilation and compare the results with the geochemical data described above to evaluate their relevance to the formation of Rayleigh fractional crystallization MOR dacites. Specifically, we focus on physically reason- The program MELTS (Ghiorso & Sack, 1995) provides able models that are consistent with the highly a useful framework to evaluate if the major element

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Fig. 7. Trace element variations vs Zr (ppm) in 98N OSC lavas. Superimposed on the diagrams are calculated trends for fractional crystallization (model 1 and 2 using the Rayleigh fractionation equation), 1^15% partial melting (assuming batch melting), and AFC simulations (EC-AFC; Bohrson & Spera, 2001). (See text for model parameters.) Inflection points in model trend lines represent changes in crystallizing phases.

compositions of MOR dacites can be produced by crystal magma could partially crystallize to produce a dacite. fractionation. Petrological modeling was also carried out These included a slightly evolved MORB (265-113), a ferro- using the program PETROLOG (Danyushevsky, 2001) basalt (265-43), and a FeTi basalt (264-08). Pressures for with similar results for high-silica lavas, although the pro- each MELTS run were set at 1 kbar to simulate an approx- grams generate somewhat different results for intermediate imate minimum depth of crystallization in the shallow compositions. Additionally, the MELTS calculations are oceanic crust, the oxygen fugacity was set at the quartz^ consistent with results from Perfit et al. (1983), which were fayalite^magnetite (QFM) buffer, and the H2O concentra- calculated using least-squares best-fit equations. Several tions varied from 0·2to0·35 wt % depending on the 98N N-MORBs were used as starting parental melt com- parent melt composition. Both Perfit & Fornari (1983) and positions (Table 3) to determine if a moderately evolved Juster et al. (1989) determined that extensive crystallization

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Fig. 8. Normalized trace element ratio diagrams showing the range of dacite compositions at the 98N OSC. Fractional crystallization, partial melting and AFC trends are shown as in Fig. 4. Tick marks on trend lines for partial melts are in 1% increments, whereas fractional crystallization tick marks represent 10% intervals. Concentrations are mantle-normalized (Sun & McDonough, 1989). occurred at low pressures and oxygen fugacities slightly similar end-member compositions at low MgO and high above QFM. Liquid lines of descent were also calculated SiO2 as fractional crystallization proceeds. for higher pressures (up to 5 kbar) to simulate depths of Liquids with MgO and SiO2 contents similar though crystallization within the nascent layer 3 and the shallow not identical to 98N OSC dacites can be produced by mantle. However, the liquid lines of descent converge on 75^85% crystal fractionation of a ferrobasaltic parent.

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Fig. 9. Radiogenic isotope compositions of 98N OSC lavas. (a) Pb-isotope diagram showing that the 98N OSC dacites have 208Pb/204Pb and 206Pb/204Pb ratios similar to OSC N-MORB basalts and the northern East Pacific Rise N-MORB (gray field; data from Sims et al., 2002, 2003; Goss et al., 2010). (b) Nd and Sr isotope data show that 98N OSC lavas are similar to East Pacific Rise and OSC N-MORB. Black bar represents calculated Sr enrichment (0·0001) during assimilation of altered crust (see text for details). (See Table 2 for data.)

Results predict a crystallization sequence of Ol, followed total amount of crystallization varies slightly depending by Ol þ Plag, Ol þ Plag þ Cpx, Plag þ Cpx þ Sp (titano- on whether the starting composition was a moderately magnetite), and in some models, late-stage crystallization evolved basalt, ferrobasalt, or FeTi basalt (Fig. 5; maxi- of apatite. No orthopyroxene crystallization is predicted, mum of 85% crystallization). In contrast, calculated which is similar to some experimental results (Juster et al., abundances of K2O, P 2O5,Al2O3, and Cl do not match 1989); however, pigeonite is predicted by these experiments the dacitic end-member compositions using any of the par- and is observed in some MOR andesites and dacites. ents; modeled residual liquids have higher P O by factors The calculations suggest that temperatures of59808C are 2 5 of 5^10, lower K O by factors of 1·5^2·5, lower Al O by reached when residual liquids attain compositions similar 2 2 3 factors of 1·4^1·5 and lower Cl by factors of 10^12 (Figs 5 to dacites. The models are in agreement with anhydrous experimental results that indicate that residual dacitic and 6). Although MELTS does not predict apatite satura- liquids form after 87% crystallization at temperatures tion in andesitic liquids the decreasing P2O5 contents in of 10408C (Juster et al., 1989). some andesites and very low values in dacites strongly sug- For several major elements (TiO2, FeO, and SiO2), com- gest apatite crystallization. Juster et al. (1989) calculated positions similar those of to 98N OSC dacites are obtained that apatite saturation would occur at 0·7wt%P2O5 in through crystal fractionation of a MORB magma; the Gala¤ pagos Spreading Center andesites.

2393 JOURNAL OF PETROLOGY VOLUME 51 NUMBER 12 DECEMBER 2010

Table 3: Starting compositions for geochemical modeling Additional tests of Rayleigh fractionation include modeling the behavior of trace elements using as input parameters the degree of crystallization and mineral modal Sample: 264-08 (FC-3) 265-43 (FC-1) 265-113 (FC-2) proportions determined from the MELTS modeling. Basaltic partition coefficients are used in the trace element SiO 50·150·551·9 2 modeling up to 57% SiO 2, and andesitic partition coeffi- TiO 2·68 1·92 2·17 2 cients are used for 457% SiO 2 (Table 4). The Rayleigh Al O 12·713·913·4 (D ^1) 2 3 fractionation equation [Cl/Co ¼ F ] is used to simulate FeO 14·111·612·8 a continuously evolving magma chamber in which MnO 0·26 0·21 0·23 phenocrysts are immediately separated from the liquid Downloaded from https://academic.oup.com/petrology/article/51/12/2377/1540213 by guest on 29 September 2021 MgO 5·69 6·98 5·93 (Cl ¼ concentration of element in the liquid; CaO 9·58 11·14 9·48 Co ¼ concentration of the element in the parent; F ¼ %

Na2O3·27 2·86 3·28 melt; and D ¼ bulk partition coefficient). The starting

K2O0·21 0·13 0·22 composition for these calculations was a ferrobasalt from

P2O5 0·28 0·19 0·28 the 98NOSC(265-43). Cl 0·07 0·01 0·02 As shown in Fig. 7, the trace element concentrations Total 99·24 99·67 99·72 observed in the dacites cannot be reproduced using Trace elements (ppm) Rayleigh fractionation (Fig. 7) with the constraints Li 10 8 11 imposed by major element variations. This model repro- Sc 42·142·338·0 duces some dacite incompatible element compositions but V 450 347 323 does not reproduce the observed enrichments in most of Cr 18·0 108·632·1 the incompatible elements. Although the most incompati- Co 43·541·439·0 ble elements (Rb, Ba, Th, U) show the greatest difference between the observed and calculated compositions, even Ni 33·854·333·7 less incompatible elements (Nb, Zr, Y, Hf) require 490% Cu 60·458·850·8 crystal fractionation. For instance, maximum calculated Zn 116·193·7 103·2 Zr and Nb concentrations are 705 ppm and 13ppm, Ga 21·318·321·0 respectively, compared with an average of 870 and 15ppm Rb 2·11·12·2 in the dacites. In addition, the UN/NbN is predicted by Sr 126 120 111 modeling to be 51, whereas the measured dacite values Y58·943·861·0 are 41 and the modeling does not reproduce the high Zr 180 126 229 ZrN/DyN and CeN/YbN and low NbN/LaN ratios in the Nb 5·53·15·7 98N OSC lavas (Fig. 8). The middle to heavy REE Cs 0·03 0·01 0·03 (MREE to HREE) concentrations (i.e. Nd, Sm, Eu, Dy, Ba 17·18·415·6 Yb, and Lu) can be generated only by 490% crystal frac- La 6·81 4·54 7·63 tionation. Regardless, such extreme degrees of crystalliza- Ce 20·51 14·16 23·69 tion are inconsistent with major element model Pr 3·32 2·34 3·92 calculations. Nd 17·31 12·56 20·09 In summary, the calculated liquid lines of descent do not Sm 5·99 4·42 6·68 provide a good fit to the observed major and minor com- Eu 1·94 1·48 1·99 positions of the high-silica lavas, and trace element Gd 7·74 5·78 8·52 models parameterized from the MELTS calculations do Tb 1·45 1·08 1·58 not reproduce measured trace element abundances or Dy 9·69 7·22 10·13 trace element ratios. Thus, we conclude that extensive Ho 2·05 1·52 2·20 low-pressure crystal fractionation is unlikely to be the sole Er 5·94 4·39 6·26 mechanism to explain the genesis of the high-silica lavas Tm 0·91 0·67 0·96 at the 98NOSC. Yb 6·01 4·43 6·17 Lu 0·93 0·68 0·95 Hf 4·83 3·44 5·72 Crystal^melt segregation model Ta 0·37 0·21 0·36 Bachmann & Bergantz (2004) suggested that intermediate Tm 0·61 0·39 0·95 liquids (andesites and dacites) will separate from crystals Gd 0·34 0·18 0·40 (via filter pressing) when a magma has undergone 440^ Pb 0·13 0·08 0·15 50 vol. % crystallization. The segregated melt, which is

2394 WANLESS et al. DACITE PETROGENESIS ON MORs

Table 4: Partition coefficients used in Rayleigh fractional crystallization, partial melting and AFC calculations

Element Andesite partition coefficients Basalt partition coefficients

Olivine Cpx Plag Apatite Ilmenite Amphibole Olivine Cpx Plag Ilmenite

Rb 0·01 0·02 0·025 0·001 0·034 0·04 0·0003 0·0004 0·056 0·034 Ba 0·01 0·02 0·155 0·12 0·00034 0·1 0·00001 0·0003 1·45 0·00034 Th 0·01 0·01 0·19 1·28 0·00055 0·15 7·00E-06 0·0021 0·13 0·00055

U0·005* 0·0075* 0·34 1·40·0082 0·008 9·00E-06 0·001 0·051 0·0082 Downloaded from https://academic.oup.com/petrology/article/51/12/2377/1540213 by guest on 29 September 2021 Nb 0·00017 0·005 0·033 0·0011 20·28 0·00005 0·0089 0·045 2 Ta 0·00002 0·014 0·11 0·003 1·70·27 0·00012* 0·013 0·066 1·7 La 0·00006 0·062 0·082 11·40·000029 0·027 0·0002 0·054 0·13 0·000029 Ce 0·00006 0·116 0·072 12·90·000054 0·0293 7·00E-05 0·086 0·11 0·000054 Sr 0·00217 0·08 2·74·30·00027* 0·28 0·00004 0·091 1·40·00027* Nd 0·00015 0·33 0·045 32·80·00048 0·0325 0·0003 0·19 0·066 0·00048 Zr 0·00450 0·14 0·0009 0·042 0·29 0·26 0·001 0·26 0·048 0·29 Hf 0·00370 0·21 0·017* 0·014 0·38 0·43 0·0029 0·33 0·051 0·38 Sm 0·00044 0·41 0·033 16·10·00059 0·024 0·0009 0·27 0·054 0·00059 Eu 0·00056 0·57 0·55 25·5 0·009 0·0498 0·0005 0·43 0·65 0·009 Dy 0·00250 0·94 0·034 34·8 0·01 0·0136 0·0027 0·44 0·024 0·01 Y0·00380 0·90·01 7·10·0045 0·0196 0·0082 0·47 0·013 0·0045 Yb 0·05600 0·63 0·014 15·4 0·17 0·102 0·024 0·43 0·0079 0·17 Lu 0·03* 0·605 0·039 3·92 0·084 0·6 0·016 0·56 0·06 0·084 Primary Zanetti Klein et al., Dunn & Prowatke & Zack & Bottazzi Halliday Halliday Dunn & Zack & reference et al., 2004 2000 Sen, 1994 Klemme, 2006 Brumm, 1998 et al., 1999 et al., 1995 et al., 1995 Sen, 1994 Brumm, 1998 Secondary Gill, Gill, Rollinson, Fujimaki, Rollinson, Haase Rollinson, Rollinson, Rollinson, Rollinson, reference 1979 1979 1993 1986 1993 et al., 2005 1993 1993 1993 1993

*Values interpolated using nearest neighbors. Values for Lu are averages of Y and Yb. Italicized values indicate secondary reference. more evolved than the original melt, will once again crys- occurred after three segregation events or 87·5 wt % crystallize until it reaches 40^50% phenocrysts, when again tallization; however, as was noted during the MELTS cal- the new evolved melt will separate from the pheno- culations, several dacitic major and minor element crysts. Although this segregation model applies strictly concentrations (i.e. Al2O3,K2O and P2O5) could not be to systems of intermediate composition (Bachmann & reproduced. In addition, the calculated incompatible Bergantz, 2004), here we evaluate whether a basaltic trace element abundances were also lower than those magma can evolve geochemically, through a series of seg- observed in the MOR dacites (Fig. 10). regation events, to form compositions similar to the MOR dacites. To simulate these conditions a 98N OSC ferrobasalt In situ crystallization calculations was allowed to undergo equilibrium crystallization to an A different approach to magma crystallization is in situ andesitic composition, using MELTS thermodynamic cal- crystallization, where phenocrysts do not separate from culations (Ghiorso & Sack, 1995) and starting conditions the interstitial melt until a small remaining volume of described for Rayleigh fractionation models. At this ande- liquid is pressed from the crystallizing mush and mixed sitic composition, the liquid separates from the phenocrysts with the main body of melt (e.g. Langmuir, 1989; (Bachmann & Bergantz, 2004), creating a new parent Reynolds & Langmuir,1997; Pollock et al., 2005).This pro- melt. This parent composition becomes the new starting cess assumes that crystallization occurs along a tempera- concentration (an andesite) for the next run, which subse- ture gradient within a solidification front (or boundary quently crystallizes 50% by volume. This process was layer) and interstitial melt evolves independently from the repeated until MgO and SiO2 concentrations similar to main melt body. The mixing of interstitial melt back into those of the 98N OSC dacites were obtained (Fig. 10).This the main magma body gradually causes bulk increases

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Fig. 10. Elemental variation diagrams showing the 98N OSC dacites vs calculated liquid lines of descent produced during two alternative types of crystallization. Crosses show the evolution of a melt during in situ crystallization (Langmuir,1989). Circles show the liquid line of descent of a magma that undergoes 50% fractional crystallization and is then separated (via filter pressing) from the phenocrysts (melt-segregation model).The resulting magma undergoes two additional steps of 50% crystallization and melt segregation, following the model of Bachman & Bergantz (2004). Inflection points in models represent changes in crystallizing phases, particularly Fe-oxides. in incompatible element abundances and changes in trace remains (Reynolds & Langmuir, 1997). All of the residual element ratios (Langmuir, 1989). This process will cause liquid mixes back into the magma chamber during each an increase in highly incompatible elements compared iteration of boundary layer crystallization. As the magma with Rayleigh crystal fractionation, because these elements chamber continues to crystallize by this mechanism, the are continually returned to the residual magma body boundary layer moves inward leaving restite crystals (Langmuir,1989). behind and the volume of the magma body decreases. In situ crystallization was evaluated following Reynolds Theoretically, this process will continue to modify the & Langmuir (1997), with a starting composition of a 98N magma chamber liquid composition until an infinitesi- OSC ferrobasalt (265-43) using the same partition coeffi- mally small amount of melt is left. cients as for Rayleigh fractional crystallization. Crystalliz- After 85% in situ crystallization (Fig. 10) calculated ing phases include Ol, Plag, Cpx, and eventually, major and incompatible trace element concentrations Fe-oxides. In the modeled system, the boundary layer is do not match those measured in the MOR dacites. This always 5% of the liquid magma chamber volume and the process can only account for Zr concentrations in the melt boundary layer crystallizes until 35% interstitial liquid that are less than three times the original concentration,

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Fig. 11. Partial melting model showing a range of possible sources (gray lines) that could produce the 98N OSC dacites (bold line) from 1^15% partial melting. Sheeted dikes and the upper parts the gabbroic layer may be composed of a range of compositions depending on the composition of the starting material and degree of alteration. Possible source compositions were calculated using the batch melting equation and solving for the initial composition (Co). Three possible wall-rock compositions are superimposed on the calculated parental range. Altered basalt 1 (squares; Nakamura et al., 2007) provides the best match to the calculated source composition and is used as the source rock in subsequent partial melting and AFC models. reaching values of 320 ppm. Even495% in situ crystalliza- (Alt et al., 1986). To better evaluate the composition of the tion cannot reproduce the enriched trace element signa- wall-rock involved in the formation of dacites on the 98N tures of the MOR dacites. Although in situ crystallization OSC, we use the batch melting equation, Cl ¼ Co/[Dbulk does enrich the residual melt in incompatible elements (1 ^ F) þ F] and andesitic partition coefficients (Table 4) compared with Rayleigh crystal fractionation, the failure to solve for a range of possible wall-rock compositions to reproduce the observed enrichments in MOR dacites (Co) and then compare these results with compositions of suggests that the latter cannot result from this process fresh and altered MORB. Trace element patterns gener- either. ated from the calculations suggest that altered basalt provides a better fit than fresh MORB as a source (wall- Partial melting (anatexis) rock) composition for the 98N OSC dacites (Fig. 11). Experimental studies (Beard & Lofgren, 1991; Koepke Consequently, we model partial melting of an altered et al., 2004) suggest that 510% partial melting of altered MORB (Nakamura et al., 2007) to determine if this process oceanic crust can produce melt with major element could produce the geochemical characteristics observed in concentrations consistent with MOR dacite compositions. the 98N OSC l ava s ( Fi g. 11). Of particular note are the high SiO2,Al2O3 and K2O Altered basaltic wall-rock is melted using different and low FeO, TiO2, and P2O5 concentrations produced by modal mineralogies, one with amphibole (Haase et al., partial melting of amphibolite-facies or greenschist-facies 2005) and one without (Koepke et al., 2004). Calculated minerals because these are also the chemical characteris- trace element concentrations resulting from 1^15% partial tics of MOR dacites. Basaltic rocks are known to undergo melting of oceanic crust are shown in Figs 7, 8, and 12. partial or complete alteration and recrystallization at the Partial melting in the absence of amphibole (19% Ol, ridge axis as a result of pervasive high-temperature hydro- 30% Cpx, 50% Plag, 1% Ilm) can reproduce some, but thermal alteration (Alt et al., 1986; Gillis & Roberts, 1999). not all, of the trace element enrichments observed in the Such altered rocks are an attractive starting composition 98N OSC dacites (Fig. 12a). In particular, the HREE in for anatexis because their solidus temperatures are much the 98N OSC dacites are higher than the calculated abun- lower than those of fresh MORB. dances. The concentrations derived from partial melting Altered oceanic crust can have a wide range of trace ele- of altered crust with amphibole [20% Cpx, 25% Opx, ment concentrations, depending on the degree of alteration 49% Plag, 5% Amph, 1% Fe-oxide; based on modal

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Fig. 12. Mantle-normalized trace element diagrams showing the results of 1^15% partial melting (PM) of an altered basalt (see Fig. 11) using two modal mineralogies. Partial melts were calculated using the batch melting equation. (See text for further details.) (a) Partial melting results using a non-amphibole-bearing gabbro assemblage (19% Ol, 30% Cpx, 50% Plag,1% Ilm). (b) Partial melting results using an amphibole-bearing gabbro (20% Cpx, 25% Opx, 49% Plag, 5% Amph, 1% Fe-oxide; Haase et al., 2005). The amphibole-bearing gabbro provides the best fit to the 98N OSC dacites. proportions from Haase et al. (2005)] are much closer to dacites (Fig. 6). Although Cl partition coefficients are the concentrations of incompatible elements in the 98N poorly constrained, we use estimates to model Cl partition- OSC dacites, suggesting that amphibole is an important ing during melting (Gillis et al., 2003). Using the median component in the source rock (Fig. 12b). Cl concentration in altered basalts in Ocean Drilling The best-fit model relies on 510% partial melting Program (ODP) Hole 504B (350 ppm) as a starting com- of amphibole-bearing altered oceanic crust to produce position and modal proportions described above, we calcu- incompatible trace element compositions comparable with late a range of possible Cl enrichments for 1^15% melting those in the 98N OSC dacites. In particular, the important to be from 0·2wt% to41·0 wt %. This spans the range characteristics of this model are melts with positive Zr of Cl concentrations observed in MOR dacites (Fig. 6). and Hf anomalies, negative Nb and Ta anomalies on mantle-normalized diagrams (Fig. 12), relatively high U Assimilation^fractional crystallization and Th concentrations, and high UN/NbN and CeN/YbN The energy constrained assimilation^fractional crystalli- ratios (Fig. 8). zation (EC-AFC) formulation of Bohrson & Spera (2001) Melting of altered basalt can also produce elevated Cl is used to assess the role that these associated processes concentrations similar to those observed in the MOR play in the formation of MOR dacites. The amount of

2398 WANLESS et al. DACITE PETROGENESIS ON MORs

Table 5: AFC modeling parameters possible in the melt. Thus, crust with an initial temperature of 508C requires 485% magma crystallization to begin melting, but results in concentrations of 35 ppm La in Parameter Abbreviation Value Units the magma (Fig. 13), in agreement with the abundances observed in the dacites. Based on these competing pro- Magma liquidus temperature tlm 1200 8C cesses, the best-fit wall-rock temperature to generate the Magma temperature tmo 1200 8C 98N OSC dacites is between 650 and 7208C. Assimilant liquidus temperature tla 1100 8C The local solidus, as described by Bohrson & Spera Country rock temperature tao 720 8C (2001), is the solidus of the assimilant, in this case amphibo- Solidus temperature ts 875 8C litized basalt. Several experimental studies have examined –1 –1 Downloaded from https://academic.oup.com/petrology/article/51/12/2377/1540213 by guest on 29 September 2021 Magma specific heat cpm 1484 J kg K dehydration melting of altered oceanic crust and amphibo- –1 –1 Assimilant specific heat cpa 1388 J kg K lites (Hacker, 1990; Rapp et al., 1991; Wolf & Wyllie, 1994; –1 Crystal enthalpy Hcry 396000 J kg Johannes & Koepke, 2001) but few studies were performed –1 Fusion enthalpy Hfus 354000 J kg under conditions comparable with those expected at

Equilibration temperature Teq 900 8C MORs (Beard & Lofgren, 1991). These experiments determined that the solidus temperatures of altered basalts are between 8508C and 9008C (Beard & Lofgren, 1991). Gillis & Coogan (2002) discussed the effects of melting altered crust at the roof of an axial magma chamber and suggested crystallization required to produce enough heat to melt the a solidus temperature of 8758C. Ti-in-zircon thermometry surrounding crust is calculated and, in turn, this produces (905 348CataTiO2 activity of 0·32 0·02 estimated a specific mass of melt of a specific composition. Several from coexisting Fe^Ti oxides) on phenocrysts in the Juan physical parameters are required as inputs to EC-AFC cal- de Fuca Ridge dacites is broadly consistent with zircon culations (Table 5).These include the liquidus temperature saturation thermometry (average of 824 15 8C) and of the parental magma (12008C based on results of Fe^Ti oxide temperatures (8308C; A. Schmitt, personal MELTS modeling of OSC lavas), the temperature and soli- communication, 2009). Based on these combined results, dus of the wall-rock, and the temperature of equilibrium 8758C was used as the local solidus temperature for AFC between the wall-rock and the magma. The initial calculations. magma composition is assumed to be N-MORB and the The final thermal input parameter is the temperature of assimilant is amphibole-bearing altered oceanic crust equilibrium, which is defined as the final equilibrium tem- with the modal composition described above. perature of the magma and wall-rock. Generally, this tem- The wall-rock may span a range of temperatures (from perature should correspond to the temperature of the 8008Cto408C) depending on the age of the ocean crust erupted lava. Although the temperature of the erupted (Maclennan, 2008). Higher initial wall-rock temperatures 98N OSC dacites is uncertain, the temperatures of other allow crustal melting to begin earlier in the evolution of dacitic magmas have been estimated. The temperature of the magma reservoir because less additional heat is crystallization of a Gala¤ pagos Spreading Center andesitic required to raise the wall-rock above its solidus tempera- magma was calculated to be as low as 910^9408C based ture; that is, at the onset of assimilation, the amount of on coexisting titanomagnetite and ilmenite grains (Perfit fractional crystallization the magma has undergone is et al., 1983), and experimental partial melts resulting from lower. This lowers the overall amount of incompatible melting of oceanic gabbros showed that dacitic melts trace element enrichment in the resulting magma because formed at temperatures of 9008C (Koepke et al., 2004). the fractionating magma is less chemically evolved at the Consequently, we use 9008C as the input equilibrium tem- onset of assimilation and, despite the high trace element perature. The partition coefficients are the same as those concentrations in the anatectic melt, the overall enrich- used for partial melting and fractional crystallization ment is less than that of a highly fractionated magma models. Basaltic partition coefficients are used for the frac- (Fig. 13). For example, crust with an initial temperature tionating magma, whereas andesitic bulk Kd values were of 8008C will begin melting after 50^55% magma crystal- used for the assimilant in the absence of a comprehensive lization and the resultant magma has a maximum of dataset of dacite Kd values. 25 ppm La, which is too low to produce the dacitic com- Results of EC-AFC calculations suggest that combina- positions (Fig. 13). Antithetically, lowering the wall-rock tions of 73^85% crystal fractionation of a parental basaltic temperature decreases the total mass of wall-rock assimi- magma and assimilation of 5^20% by mass of partially lated, while increasing the amount of crystallization melted wall-rock produces melts that have trace element needed to initiate melting. Consequently, this causes an compositions consistent with those of the 98N OSC dacites. increase in the overall incompatible element concentration In the best-fit model, melting and assimilation begins

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Fig. 13. Diagram showing the calculated effects of varying wall-rock temperature on incompatible trace element compositions (La) during AFC. (a) Higher wall-rock temperatures cause earlier onset of assimilation compared with lower initial temperatures. Higher initial temperatures produce lower overall incompatible element abundances compared with lower wall-rock temperatures because the amount of fractional crystallization the magma has undergone is lower at the onset of assimilation. The average La concentration of the 98N OSC dacites is 28 ppm, which can be produced by assimilating partial melts of a wall-rock at starting temperatures of 650^7208C. (b) Plot showing the ratio of assimilation to crystallization (Ma*/Mc) for various temperatures of wall-rock. Lower ratios produce higher incompatible trace element concentrations in the melt.

after 68% crystallization and a further 5^17% crystalliza- La), an increase in Zr and Hf concentrations (relative to tion occurs as the wall-rock melt is assimilated. HREE), relatively flat mantle-normalized HREE patterns, EC-AFC trace element calculations suggest that many of and ratios of light REE (LREE) to HREE and MREE to the incompatible trace element concentrations and ratios HREE that are similar to those observed in MOR dacites observed in the 98N OSC dacites can be explained through (Fig. 14). For example, Zr concentrations in the AFC this combination of processes (Figs. 7, 8, 14). Of particular models are 852 ppm and Nb concentrations are 16ppm importance are negative Nb and Ta anomalies (relative to compared with average 98N OSC dacite concentrations of

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Fig. 14. Mantle-normalized trace element diagram showing results of the best-fit AFC model (fine black lines). This model requires a total of 73^85% fractional crystallization in combination with 5^20% assimilation of partially melted wall-rock to produce trace element compositions similar to the 98N OSC dacites (bold line). Fractional crystallization is the dominant process until 68% of the magma has crystallized. This is followed by 5^20% assimilation of partial melts in conjunction with an additional 5^17% crystallization.

870 and 15, respectively. Although the overall fit of the the importance of partial melting and assimilation of model data to the observed data is encouraging, the calcu- altered material in the formation of dacites on MORs. lated model values for Ba, Th, U and Hf are slightly under-enriched (Fig. 14). However, it should be pointed Geochemical evidence of partial melting out that some of the input parameters to these calculations, Will crustal anatexis create geochemical signatures similar such as the actual degree of alteration and hence composi- to those observed in MOR dacites? Based on elemental sys- tion of the crustal assimilant and the temperature of the tematics (e.g. Cl, U/Nb; Figs 6 and 7) and the partial melt- wall-rocks, are not well constrained and are very probably ing calculations presented, it appears that partial melts of not constant. altered oceanic crust may be involved in the generation of the MOR dacites. Partial melts of hydrothermally altered crust produce distinct signatures compared with those of DISCUSSION unaltered oceanic crust, as a result of changes in mineral- Petrogenesis of high-silica lavas ogy and bulk composition during hydrothermal circula- Extreme crystal fractionation, partial melting of crustal tion. Hydrothermal circulation in layer 2B or the top of material, and/or AFC processes have been proposed as layer 3 may cause alteration to greenschist or amphibolite explanations for the formation of highly silicic composi- assemblages, where Ca-rich plagioclase is replaced by tions in continental interiors, arc and ocean island settings, sodic plagioclase and pyroxenes develop rims or over- but only a few studies have focused on the formation of growths of amphibole (Alt et al., 1986; Coogan et al., high-silica lavas at MORs (Byerly & Melson, 1976; Perfit 2003b). The degree to which this occurs depends on tem- et al., 1983; Juster et al., 1989; Haase et al., 2005).The petro- perature, water/rock ratios and fluid chemistry. To explain genetic calculations presented above demonstrate that the geochemical signatures in the MOR dacites, our melt- crystal fractionation alone is not a viable mechanism for ing assemblage must include amphibole. the formation of high-silica MOR lavas, despite using a Melting of amphibole-bearing assemblages, a common range of starting compositions and several end-member component in altered layer 2B (Alt et al., 1986; Coogan, models (Figs 5, 7 and 8). Instead, the results emphasize 2003; Coogan et al., 2003b), can explain many of the

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major element concentrations of the MOR dacites, includ- of mineralogical effects. For example, the relatively high ing the anomalously high Al2O3 concentrations observed Zr (highly incompatible during melting) concentrations in the oceanic dacites (Fig. 5). Dehydration partial melting in the high-silica lavas are a consequence of partial melting experiments, where water exists only as hydrous phases of altered crustal material (Fig. 12). Additionally, the melt- within the rock, provide a better fit to MOR dacite com- ing and AFC models discussed above point to the impor- positions than hydrous partial melting results (Koepke tance of amphibole in the melting assemblage to explain et al., 2004). Dehydration melting experiments produce a the HREE (compare Fig. 12a and 12b). range of Al2O3 concentrations (Beard & Lofgren, 1991) that are similar to or higher than in MOR dacites (Fig. 5). The need for crystallization, assimilation and altered crust Comparatively high Na2O concentrations in the dacites in dacite petrogenesis Downloaded from https://academic.oup.com/petrology/article/51/12/2377/1540213 by guest on 29 September 2021 may result from the melting of albitic plagioclase. We propose that the partial melting and assimilation of Elevated Na2O concentrations are not observed in all oceanic crust plays a significant role in the formation the experimental results of Beard & Lofgren (1991) but high-silica MOR lavas; however, we stress that most of appear to be a function of the degree of albitization of the the heat required to melt the wall-rock is a consequence of starting material. Variable P2O5 concentrations are also extensive fractional crystallization. Coogan et al.(2003b) observed in the experimental melts, suggesting that P2O5 showed that the latent heat of crystallization from the for- contents are very low in the source rock or that apatite is mation of a 4 km thick gabbro sequence provides enough a residual phase in the melting residue. Similar conclusions energy to heat 1·3 km of overlying crust from 450 to can be applied to the MOR dacites, which have low phos- 1150 8C, which promotes partial melting. It is the assimila- phorus contents (Fig. 5); however, P2O5 low concentrations tion of these partial melts into a fractionally crystallizing are probably influenced by apatite crystallization during magma reservoir that produces the highly evolved melts AFC processes (see below). Additionally, low FeO and with enriched incompatible trace element signatures (e.g. TiO2 suggest that Fe-oxides are not a primary melting De Paolo, 1981; Be¤ dard et al., 2000). component in the source rock. This is consistent with our Major element compositions of MOR dacites often lie proposed source rock composed of olivine, plagioclase, between those of experimental partial melts of altered cpx, amphibole, Fe - ox ide s. basalt (Beard & Lofgren, 1991) and liquids produced by High Cl concentrations in MOR dacites also support the moderate to large extents of crystal fractionation (Fig. 5). role of partial melting of rocks altered by seawater-derived The major element compositions of magmas produced by fluids (Michael & Schilling, 1989; Michael & Cornell, AFC may, therefore, lie between those of partial melts of 1998; Coogan et al., 2003b; Gillis et al., 2003). Although Cl altered oceanic crust and fractionated basaltic magmas. behaves incompatibly during crystallization many MOR This is particularly apparent in Al2O3 and may explain lavas show over-enrichments compared with other ele- whyveryfewevolvedlavasatthe98N OSC (including fer- ments with similar compatibilities. For example, after robasalts and basaltic andesites) lie on the calculated 85% fractional crystallization of a MORB parent (with liquid lines of descent (Fig. 5). 0·01wt % Cl), there is less than a 10-fold enrichment in Results from EC-AFC calculations (Bohrson & Spera, Cl, resulting in concentrations of 0·07 wt %, compared 2001) confirm that assimilation of anatectic melts into a with an order of magnitude more (0·7wt% Cl)inthe residual fractionated magma can explain a wide range of dacites. Analyses of altered basalt from sheeted dikes in trace element concentrations in MOR dacites (Figs 7, 8 drill holes show that Cl concentrations span a range from and 14). The best-fit model for 98N OSC dacite composi- 49 to 650 ppm (Sparks, 1995). Partial melting (1^15%) of tions requires significant crystal fractionation (73^85 wt an amphibole-bearing wall-rock (with 350 ppm Cl) results %) of a ferrobasalt parental magma in combination with in anatectic melts with 0·3^0·9 wt % Clçcovering the 10^25% (by mass) anatectic melt, which provides the addi- range observed in dacites (Fig. 6). tional incompatible element enrichments observed. Our Hydrothermal alteration and metamorphism are known models also indicate that to explain the enrichments in to cause increased concentration of some trace elements, Rb, Ba, Th and U concentrations relative to other highly including U, Th, Rb, and Ba, as well as Cl (e.g. Alt & incompatible elements present in the MOR dacites assimi- Teagle, 2003). Observed positive anomalies of some highly lation of low-degree partial melts of hydrothermally incompatible elements (e.g. U and Th) relative to other altered oceanic basalt is required. It is important to note incompatible elements with similar distribution coefficients that the extreme Cl enrichments in MOR dacites require are consistent with partial melting of hydrothermally a seawater component that can be derived by AFC process altered ocean crust (Fig. 12). Partial melting may also and that Cl over-enrichments observed in many MORBs explain some of the anomalies in the high field strength have been explained by small amounts of assimilation of elements whose concentrations are not affected by altera- either hydrothermally altered ocean crust or Cl-rich tion or metamorphism but can be fractionated as a result brines stored in the crust (Michael & Schilling, 1989;

2402 WANLESS et al. DACITE PETROGENESIS ON MORs

Michael & Cornell, 1998; Perfit et al., 1999; Coogan et al., melt to anatectic melt of 3·5:1to produce the most radio- 2003a; le Roux et al., 2006). genic 87Sr/86Sr signatures observed in the MOR dacites Minor phases, such as FeTi oxides, zircon, and apatite, (0·70258). Therefore, this process has the potential to can affect both major and trace element concentrations in slightly affect the Sr isotope ratios in the dacitic magma, the MOR dacites and can potentially provide insight into but will not result in ratios as elevated those generated the roles of fractional crystallization, partial melting and directly from partial melting of altered oceanic crust assimilation. Relatively low Nb and Ta anomalies and low (0·7028). Additionally, slightly elevated Sr isotope ratios FeO and TiO2 concentrations may be a consequence of in high-silica lavas from the Gala¤ pagos Spreading Center both the removal of phenocrysts during late-stage crystal are consistent with AFC processes (Perfit et al., 1999). In fractionation and residual iron^titanium oxides in the par- comparison, Nd isotopes are unaffected during fractional tially melted wall-rock. Elevated Zr and Hf concentrations crystallization and are relatively immobile during hydro- Downloaded from https://academic.oup.com/petrology/article/51/12/2377/1540213 by guest on 29 September 2021 result from little to no zircon crystallization in the fractio- thermal alteration (Michard & Albare' de, 1986; Delacour nating magma and/or no residual zircon in the melting et al., 2008). Nd isotopes from the OSC dacites are similar assemblage. Apatite crystallization and/or low phosphorus to those of basalts from the region. contents in the source rock can account for the low P2O5 The 98N OSC dacites form a tight cluster in Pb isotopic concentrations observed in the MOR dacites. This is sup- composition compared with 98N OSC basalts (Fig. 9). Pb ported by experimental results, which suggest that apatite isotopes are not significantly affected by hydrothermal saturation should occur by 0·7wt % P2O5 (Juster et al., alteration provided sediments (which are not abundant in 1989). this environment) are not involved in the alteration process (e.g. Perfit et al., 1999). We suggest that the similarity Isotopic signature of assimilation in isotopic ratios in the dacites compared with the basalts Given that the 98N OSC dacites have radiogenic isotope represents an overall averaging of isotopic values from compositions similar to those in spatially related basalts basalts in the region as a result of melting and assimilating (Fig. 9) what effects might assimilation, particularly of a range of MORB compositions at the base of the sheeted altered crust, have on derivative melts? AFC processes dike layer. can change radiogenic isotopic ratios if the assimilant has Relatively low oxygen isotope ratios observed in MOR relatively high concentrations of the element in question dacites from the Gala¤ pagos Spreading Center (Perfit and significantly different isotopic ratios from the original et al., 1999) and the 98N OSC (Wanless et al., in prepara- magma reservoir (e.g. Taylor,1980; De Paolo, 1981). In gen- tion) also support these conclusions. Fresh MORBs will eral, however, the effect on isotopic compositions is less have mantle oxygen isotope values (5·5); however, sea- dramatic in AFC processes compared with partial melting water alteration (seawater d18O ¼ 0) will decrease this because AFC processes create mixtures of altered and ratio (Gillis et al., 2001), whereas fractional crystallization fresh material, whereas melting alone will retain the isoto- of Fe-oxides, and to a lesser extent olivine and pyroxene, pic signature of the altered crust. will cause an increase (Matsuhisa et al., 1973). Therefore, Assimilation of altered oceanic crust may increase Sr partial melting and assimilation of altered basalt should isotope ratios depending on the amount of assimilation produce melts with lower d18O values than predicted by and extents of fluid^rock interaction (e.g. Alt & Teagle, fractional crystallization calculations (Muehlenbach & 2003). Altered oceanic crust can have a range of Sr concen- Clayton, 1972). MOR dacites have oxygen isotope ratios trations (from less than to greater than typical MORB similar to MORB values (Perfit et al., 1999; Perfit et al., compositions) depending on the type or degree of altera- 2007; Wanless et al., in preparation), suggesting that fraction (Alt & Teagle, 2003). Assuming 75% fractional crys- tional crystallization alone cannot explain the formation tallization (which will not change the isotopic ratios) and of dacites on MORs. Taylor (1968) suggested that to a first assimilation of 10 mass % wall-rock, we can calculate the approximation, the effect of assimilation on oxygen iso- isotopic composition of the resulting melt using a ratio of topes can be determined using mass balance. Assuming 2·5:1 (fractionated melt to anatectic melt). Using reason- an evolved magma has a d18O value of 6·8 (largely owing able values for the isotopic ratios and Sr concentrations of to fractionation of silicates and iron oxides) and an assimi- altered sheeted dike lavas (0·7028; average of basal dikes lant has a d18O value of 3·5 (owing to seawater alteration), from Pito Deep; Hess Deep and Hole 504B; Barker et al., the resultant oxygen isotope ratio of the AFC magma 2008) and initial MORB parental magma (0·7025 and would be 6. This value is similar to those observed in 100 ppm, a typical altered East Pacific Rise concentration), the MOR dacites and is less than predicted by fractional mass-balance calculations indicate that assimilation of crystallization alone. altered crust will cause an increase of just 0·0001 in the 87Sr/86Sr of the final magma (Fig. 9). EC-AFC modeling AFC processes and tectonic setting of Sr isotopes produces similar results but requires less The remarkable geochemical similarity of dacites erupted crystal fractionation (73%) and a ratio of fractionated at the three MORs discussed here indicates that similar

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processes are controlling their petrogenesis (Fig. 2) and we information, we propose the following model for dacite propose that these processes are linked to tectono-mag- formation on MORs (Fig. 15). matic setting. Andesites and dacites have erupted on sev- (1) Basaltic magma is injected by down-rift lateral dike eral ridges, often in regions of propagation, such as propagation. Axial magma reservoirs are formed. propagating ridge tips and OSCs (Christie & Sinton, 1981; (2) Magma supply to the region is cut off or reduced, Perfit et al., 1983; Sinton et al., 1983, 1991). Similar high- allowing for extensive fractional crystallization of the silica lavas have also been found along the Pacific^ magma reservoir. Antarctic Rise (Haase et al., 2005), at Axial Seamount on (3) Extensive fractional crystallization and the conse- the Juan de Fuca Ridge (Chadwick et al., 2005) adjacent to quent release of latent heat of crystallization initially a large axial magma chamber, at a ridge^transform inter- heats, then partially melts the surrounding altered Downloaded from https://academic.oup.com/petrology/article/51/12/2377/1540213 by guest on 29 September 2021 section at the Juan de Fuca Ridge (Cotsonika, 2006), at an wall-rock, which might be layer 2b dikes or high- extinct OSC on the Galapagos Spreading Center (e.g. level altered gabbroic rocks. Perfit et al., 1983), and at the end of first- and second-order (4) Anatectic melts are assimilated into the fractionating ridge segments on the Northern East Pacific Rise magma body. High-silica AFC melts may vary in (8837’N and 10830’N; Langmuir et al., 1986). Collectively, composition depending on crustal temperature, most of these lavas erupted on intermediate to fast spread- extent of fractional crystallization, amount of anatec- ing ridges, in settings where magma reservoirs have the tic melt, and efficiency of assimilation. potential to undergo extensive fractional crystallization and interact with colder, and variably altered crust. This model may account for the formation of highly A key result of this study is that the geochemical signa- evolved magmas at OSCs, propagating ridge tips, ridge^ tures of MOR dacites require assimilation of partial melts transform intersections and along dikes associated with of hydrothermally altered crust into an extremely frac- the down-rift volcanism on Axial Seamount. This situation tionated magma (where the latent heat of crystallization may not be unique to traditional MORs as evidenced by provides the heat needed to melt the oceanic crust). The an analogous situation at Krafla volcano in Iceland, extensive amounts of fractional crystallization required where the imaged melt lens is thought to be primarily suggest episodic or sporadic magma supply to magma composed of iron-rich basalt but high-silica lavas are asso- reservoirs, which may not be characteristic of more ciated with the edges of the caldera rim, where increased ‘steady-state’ ridge environments. These requirements are magma^rock interactions may be likely (Nicholson met at the ends of ridge segments, where magma reservoirs et al., 1991). may have a sporadic magma supply. In these regions, magmas are considered to be fed intermittently to the Relationship of melt lens to dacites at 98N ridge tip through dike propagation from a more robust Dacitic lavas at 98N OSC erupted on-axis, over the eastern central region (Christie & Sinton, 1981). Between diking edge of a large, seismically imaged melt lens (Kent et al., events the ridge tip magma supply is cut off, allowing for 2000; Dunn et al., 2001). Despite the eruption of young, increased extents of crystal fractionation and interaction fresh high-silica lavas in the neo-volcanic zone, the under- of the melt with older, altered crust. This may increase the lying melt lens is not assumed to be dacitic in composition. likelihood of eruption of high-silica lavas through AFC The relatively evolved composition of the ferrobasalts processes. This is not to suggest that AFC processes do not currently overlying the main body of the wide axial occur in ‘steady-state’ ridge environments but that the magma chamber (Fig. 3) suggests that it has undergone a high-silica melts may not be preserved or erupted in these moderate amount of crystallization (to ferrobasalt). regions (see the section on ‘Effects of assimilation on typi- Mixing of the ferrobasaltic and dacitic magmas beneath cal MORB’). the east limb may explain the wide range of compositions erupting at the 98NOSC. Model for formation of MOR dacites The presence of a large, seismically imaged melt lens at Based on the similarity of composition of high-silica lavas 98N OSC does not contradict the episodic magma supply from three MORs, petrological modeling calculations requirement for dacite formation. Instead, it may allow applied to dacites at the 98N OSC, and published experi- and enhance AFC processes in the region. AFC modeling mental results, we suggest that MOR dacites form under suggests that extensive fractional crystallization is required specific conditions that include: (1) a tectono-magmatic set- to produce dacitic compositions, which suggests low or ting in which magma injection is episodic, allowing for episodic magma supply. Somewhat antithetically, the 98N extensive crystal fractionation; (2) the presence of altered region has an anomalously large axial magma chamber, crust, which facilitates the geochemical enrichments suggesting that the current melt lens may be only indir- observed in the MOR dacites. Based on these two require- ectly related to the dacites. Interaction of the eastern edge ments, the tectonic setting and available geophysical of the large melt lens with high-silica lavas may increase

2404 WANLESS et al. DACITE PETROGENESIS ON MORs Downloaded from https://academic.oup.com/petrology/article/51/12/2377/1540213 by guest on 29 September 2021

Fig. 15. Schematic illustration showing a possible scenario for dacite formation at MORs. (a) Injection of basaltic magma into a ridge segment end through dike propagation. (b) Magma supply rates diminish at segment end, abandoning pockets of magma and allowing for extensive fractional crystallization. The latent heat of crystallization begins to heat up and partially melt the hydrothermally altered wall-rock. (c) Partial melts of wall-rock are assimilated or mixed into the evolving magma chamber, resulting in dacitic magmas.

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the volume of dacites erupted at the OSC by acting as a trends and incompatible trace element enrichments, sug- mobilizer for the more viscous magmas. This is supported gesting similar processes controlled their petrogenesis. by geochemical evidence of small-scale local mixing of The formation of highly evolved lavas on MORs highly evolved compositions with the moderately evolved requires a combination of partial melting, assimilation basaltic melt lens beneath the east limb ridge axis, which and crystal fractionation. The highly enriched incompati- may account for the diversity of compositions erupted ble trace element signatures cannot be produced through at the OSC (Fig. 15). crystal fractionation alone and appear to require a melt component derived from partial melting of altered oceanic Effects of assimilation on typical crust. EC-AFC modeling suggests that significant amounts (475%) of crystallization of a MORB parent magma and

MORB compositions Downloaded from https://academic.oup.com/petrology/article/51/12/2377/1540213 by guest on 29 September 2021 The formation of dacite compositions on MORs requires modest amounts (5^20%) of assimilation of hydrother- assimilation of anatectic melts into residual fractionated mally altered oceanic crust can produce geochemical sig- magmas; however, AFC processes may also explain natures consistent with the dacite compositions. The AFC slightly elevated incompatible element abundances process explains the trace element abundances in high- observed in MORB lavas from some sections of MORs silica lavas and accounts for several major and minor ele- (Bryan & Moore,1977).The geochemical signatures of ana- ment concentrations (i.e. Al2O3,K2O and Cl). tectic melts may be subtle in less evolved magmas; how- An important constraint provided by AFC calculations is the temperature of the assimilant. Varying the wall-rock ever, elevated Cl, Al2O3, and K2O are common (Michael & Schilling, 1989; Michael & Cornell, 1998; Perfit et al., temperature can change the amount of crystallization or 1999; Coogan et al., 2003a; le Roux et al., 2006). At more assimilation that occurs and consequently the overall magmatically active ridge sections, the wall-rock may enrichment in incompatible trace element concentrations. have a higher initial temperature, which would allow for The formation of dacites at the 98NOSCrequiresthe melting and assimilation to begin earlier in the evolu- temperature of the surrounding crust to be 650^7208C, tion of the magma body and require less fractional crystal- which in turn requires the latent heat of crystallization lization to occur prior to melting (Fig. 13). For example, at from 468% fractional crystallization of ferrobasaltic 8008C assimilation begins after only 53% fractional crys- magma before melting can begin. Although this amount tallization compared with 68% crystallization for 7208C of crystallization is unlikely in regions of high or constant crust. Assimilation of anatectic melts into a magma reser- magma supply, the surrounding wall-rock in typical voir that has undergone less crystallization produces less ridge settings may be much warmer than at the ends of evolved compositions and therefore, cannot produce ridge segments, allowing for assimilation at much lower MOR dacites. It does, however, increase the incompatible amounts of crystallization. At wall-rock temperatures of element abundances in the melt phase and may explain 8008C, our calculations indicate that assimilation begins the commonly noted anomalous incompatible element after 53 wt % fractional crystallization. This suggests that although conditions are not appropriate for the petro- enrichments, low FeO and elevated Al2O3 concentrations in some ‘normal’ MORB lavas. genesis of dacites at typical ridge settings, assimilation of crustal material may be common but geochemically cryptic. CONCLUSIONS The formation of high-silica lavas on MORs appears The majority of eruptions at spreading centers produce to require a unique tectono-magmatic setting, where episo- basalts with relatively limited chemical variability; how- dic magma supply allows for extensive crystal fractiona- ever, high-silica lavas have been sampled at several ridges. tion, partial melting and assimilation. These conditions Eruptions of andesites and dacites are typically associated are met in regions of ridge propagation, such as OSCs with ridge discontinuities and produce significant volumes and propagating ridge tips, where diking allows for episo- of lava at a local scale. Limited amounts of these lavas dic injection of magma into older, altered ocean crust. have been sampled at the southern terminus of the Juan Here, magma undergoes extreme crystallization without de Fuca Ridge, along the eastern Gala¤ pagos spreading repeated replenishment, creating enough latent heat center, as well as at the axis at 8837’N and off-axis at of crystallization to melt and assimilate surrounding 10830’N on the East Pacific Rise. We have documented wall-rock. more voluminous eruptions of high-silica lavas including highly evolved dacite on the propagating eastern limb of the 98N overlapping spreading center (OSC) on the East FUNDING Pacific Rise. Collectively, the dacites from these various This work was supported by the RIDGE2000 program MOR environments appear to represent a common end- of the National Science Foundation (grant number member composition that shows similar major element OCE-0527075 to M.R.P. and OCE-0526120 to E.M.K.).

2406 WANLESS et al. DACITE PETROGENESIS ON MORs

SUPPLEMENTARY DATA Rica and geodynamic implications. Contributions to Mineralogy and Petrology 11, 1091^1107. Supplementary data for this paper are available at Journal Be¤ dard,J.H.,He¤ bert, R., Berclaz, A. & Varfalvy,V. (2000). Syntexis of Petrology online. and the genesis of lower oceanic crust. In: Dilek, Y., Moores, E. M., Elthon, D. & Nicolas, A. (eds) Ophiolites and Oceanic Crust: New Insights from Field Studies and the Ocean Drilling ACKNOWLEDGEMENTS Program. Geological Society of America, Special Papers 349, 105^119. Bohrson, W. A. & Reid, M. R. (1997). Genesis of silicic peralkaline We thank the Captain, officers and crew of the R.V. Atlantis volcanic rocks in an ocean island setting by crustal melting and for all their help during cruise AT15-17, the open-system processes: Socorro Island, Mexico. Journal of Petrology MEDUSA2007 Science Party (including S. White, K. Von 38,1137^1166.

Damm, D. Fornari, A. Soule, S. Carmichael, K. Sims, Bohrson, W. A. & Reid, M. R. (1998). Genesis of evolved ocean island Downloaded from https://academic.oup.com/petrology/article/51/12/2377/1540213 by guest on 29 September 2021 A. Zaino, A. Fundis, J. Mason, J. O’Brien, C. Waters, magmas by deep- and shallow-level basement recycling, Socorro F. Mansfield, K. Neely, J. Laliberte, E. Goehring and Island, Mexico: constraints from Th and other isotope signatures. L. Preston) for their diligence in collecting data and sam- Journal of Petrology 39, 995^1008. ples for this study. We thank the Jason II shipboard and Bohrson, W. A. & Spera, F. (2001). Energy-constrained open-system shore-based operations group for their assistance in collect- magmatic processes ii: application of energy constrained assimila- ing these data, and HMRG for processing all DSL-120 tion^fractional crystallization (EC-AFC) model to magmatic systems. Journal of Petrology 42, 1019^1041. side-scan and bathymetry data collected during this B owe n, N. L. (1928). The Evolution of Igneous Rocks.Princeton,NJ: cruise. Discussions with S. White and A. Goss are grate- Princeton University Press. fully acknowledged, and contributed to this research. Brophy, J. G. (2009). La^SiO2 and Yb^SiO2 systematics in mid-ocean Thanks go to G. Kamenov and the UF Center for Isotope ridge magmas: implications for the origin of oceanic plagiogranite. Geoscience for laboratory assistance. We thank Contributions to Mineralogy and Petrology 158,99^111. W. Bohrson for both the editorial handling of this manu- Bryan,W. B. & Moore, J. G. (1977). Compositional variations of young script and extensive reviews. L. Coogan, J. Maclennan, basalts in the Mid-Atlantic Ridge rift valley near 36849’N. and F. Meade are thanked for their thorough and insight- Geological Society of America Bulletin 88,556^570. ful reviews. Byerly, G. R. (1980). 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