¹DEPARTMENT OF GEOLOGY, ROYAL HOLLOWAY, UNIVERSITY OF LONDON, EGHAM, SURREY TW20 0EX, UK ²UNIVERSITY OF SOUTHAMPTON, SOUTHAMPTON OCEANOGRAPHY CENTRE, EMPRESS DOCK, EUROPEAN WAY, SOUTHAMPTON SO14 3ZH, UK ³SOUTHAMPTON OCEANOGRAPHY CENTRE, EMPRESS DOCK, EUROPEAN WAY, SOUTHAMPTON SO14 3ZH, UK

RECEIVED APRIL 7, 1997; REVISED TYPESCRIPT ACCEPTED JANUARY 14, 1998

Three hundred stratigraphically constrained samples from the Reykjanes Peninsula, SW Iceland, provide the basis for this study. This area is an elevated section of mid-ocean ridge influenced by the Iceland Plume. Selected chemical, Sr, Nd and laser-assisted fluorination oxygen isotope data are presented. The dataset is subdivided into groups based on criteria which are independent of degree of fractionation and petrography. Two of these groups, Depleted and Stapafell, include high-MgO aphyric samples with [delta]¹⁸O_olivine values in equilibrium with normal peridotite mantle. Depleted group samples have high ¹⁴³Nd/¹⁴⁴Nd, low Nb/Zr and low incompatible element abundances compared with the dataset as a whole, the reverse of the Stapafell group. The majority of the remaining samples have radiogenic isotope ratios, and incompatible element concentrations and ratios intermediate between the Depleted and Stapafell groups. Some samples, however, define a range in ⁸⁷Sr/⁸⁶Sr and [delta]¹⁸O_olivine at constant ¹⁴³Nd/¹⁴⁴Nd, and others possess positive Sr anomalies when normalized to primitive mantle values. We explore the possibility that these and other chemical characteristics have been produced by shallow crustal processes, including assimilation of xenocrysts, cumulates and hydrothermally modified crust. We conclude that although these processes are important, the major crustal process acting to modify characteristics indicative of mantle heterogeneity is magma mixing. Chemical variation previously thought to be a consequence of dynamic melting is more readily explained by magma mixing.

Keywords: assimilation; Iceland; oxygen isotopes; radiogenic isotopes; basalts

INTRODUCTION

Volcanic activity in Iceland takes place primarily through crust younger than 0·7 Ma along neovolcanic zones regarded as the present location of the Mid-Atlantic Ridge (Fig. 1). Although tholeiitic basalts are dominant in these regions of crustal generation, there is a higher incidence of acid and intermediate rocks than is usually associated with mid-ocean ridges (MOR). Silicic lavas are concentrated in central Iceland and decrease towards the northern and southwestern extremities of the rift zone. The remaining Icelandic crust away from the axial rift system, the marginal zone, is between 0·7 and 15 Ma old (Moorbath et al., 1968).

Figure 1. Sketch map of the Reykjanes Peninsula. The three most southwesterly geothermal fields are shown as shaded ellipses. Locations of samples for which data are presented in Table 1 are circled and labelled with the appropriate sample number. Inset: sketch map of Iceland. Shaded areas represent the main neovolcanic zones. Vatnajökull approximates to the centre of the Iceland Plume.

The ~3000 m elevation above normal mid-ocean ridges, and increased thickness (~15-35 km) of Icelandic crust compared with normal oceanic crust (e.g. Bott, 1988; Bjarnasson et al., 1993; White et al., 1996), regional broadband seismic data (Wolfe et al., 1997), and particular aspects of the lava chemistry are all cited as evidence for a mantle plume centred beneath central-east Iceland (Sigvaldason et al., 1974; Tryggvason et al., 1983). There is a long history of research on Iceland regarding aspects of the mantle plume (e.g. Jakobsson, 1972; Schilling, 1973; O'Nions et al., 1977; Jakobsson et al., 1978; Zindler et al., 1979; Imsland, 1983; Hemond et al., 1993). Studies based on the chemical and isotopic composition of the basalts have focused on the thermal, compositional and dynamic structure of the Iceland Plume, its contribution to the depleted upper-mantle reservoir over the past ~60 my, and its role in the evolution of the North Atlantic Basin.

The isotopic and chemical heterogeneity observed in axial basalts, however, need not necessarily be a true reflection of mantle sources and processes. Crustal processes such as assimilation and fractional crystallization are capable of extensive modification of mantle characteristics. Both are processes more usually associated with the evolved basalts and silicic rocks of central volcanoes; for example, Macdonald et al., (1987, 1990) and Furman et al., (1991, 1992) required varying contributions from crustal melting or assimilation and fractional crystallization in their studies of central volcanoes in Iceland. Similar processes have been proposed to account for the range of basalts erupted in the neovolcanic zones (e.g. Óskarsson et al., 1982, 1985; Steinthórsson et al., 1985). In addition, the elements Rb, Sr, U, K, Na, Ba and Pb may be variably mobilized in the different hydrothermal and metamorphic facies through which the crust passes as it subsides and moves away from the ridge axis. This could also give rise to substantial heterogeneity in Sr isotope ratios, especially in those areas where seawater is present in the hydrothermal systems. Óskarsson et al., (1985) proposed that variable degrees of metamorphism in association with lower-crustal melting would give rise to chemical stratification in the crust, more specifically, elevated large ion lithophile element concentrations in the upper layers. Periodic ridge jumps eastwards towards the plume centre could result in rifting and volcanism initially taking place in the older, thicker, chemically stratified crust. Óskarsson et al., (1985) concluded that assimilation of old, variably hydrothermally altered crust by melts from a depleted mantle source could produce the range of isotope and trace element ratios observed on Iceland today.

More recently, Hemond et al., (1993) proposed that a crustal component was responsible for some of the chemical `anomalies' in the lavas from the neovolcanic zones. They suggested that positive Sr, Ba and Rb anomalies (normalized to primitive mantle) in some primitive Icelandic rocks require addition of variably hydrothermally altered crustal material. The hydrothermal component in this contaminant was thought to be responsible for the elevated Sr, Ba and Rb, increased <sup>87</sup>Sr/<sup>86</sup>Sr ratios and low [delta]<sup>18</sup>O values of some Reykjanes Peninsula lavas. However, they concluded that `these processes are secondary and of lesser importance than variations in mantle source compositions'.

A crustal component has thus been invoked to explain a wide v