6. Trace and Rare Earth Element Chemistry of Volcanic Ashes from Sites 918 and 919: Implications for Icelandic Volcanism1

Home , Volcanic glass

Saunders, A.D., Larsen, H.C., and Wise, S.W., Jr. (Eds.), 1998 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 152

6. TRACE AND RARE EARTH ELEMENT CHEMISTRY OF VOLCANIC ASHES FROM SITES 918 AND 919: IMPLICATIONS FOR ICELANDIC VOLCANISM1

Peter D. Clift2 and J. Godfrey Fitton3

ABSTRACT

Miocene to Holocene sediments cored at Sites 918 and 919 in the Irminger Basin contain windblown ash layers derived from Icelandic volcanoes. They represent the most explosive volcanism of the rift system. In contrast, only those eruptions that produced lava flowing far from the rift zone are now exposed on Iceland due to subsidence close to the rift axis. Sediments older than 3.2 Ma contain little fresh glass, due to the hydrating effect of diagenetic fluids, although one basaltic layer dated at 10.5 Ma and three rhyolitic layers dated at 12.1−12.7 Ma were found to contain fresh glass. Major, trace, and rare earth element data have been collected from aphyric glass shards from these ash layers, using a combination of electron and ion microprobe technology. The ashes are dominantly basaltic and tholeiitic in character and are principally derived from the rift zone, with minor input from off-axis sources, probably the Snaefellsnes Peninsula. Their trace element characteristics are consistent with derivation from a mantle source similar to that below Iceland today, although the degree of this enrichment, as modeled by Zr/ Nb, can be seen to be variable over short periods of time and to have greater range than that found in exposed Icelandic lavas of the same age. The variations are not attributable to fractional crystallization. The ashes indicate that either different volcanoes were erupting compositionally variable lavas at any one time or that the composition varied rapidly with time, or both. These variations are homogenized by magma mixing prior to eruption in the large volume flows preserved in the Tertiary lava flows exposed in Iceland. Chemical variation seen in the ash layers may be due to variation in the degree of mantle melting, mantle heterogeneity, or derivation of melt from different depths within the melting column. Melting is thought to have commenced within a garnet-bearing mantle and continued in a spinel-bearing zone.

INTRODUCTION to address some of these problems mostly by examining the volcanic ash record recovered at Sites 918 and 919 in the Irminger Basin (Fig. Ocean Drilling Program (ODP) Leg 152 drilled a transect of holes 1). A few analyses were also made from the Paleogene at Site 914 on across the continent-ocean transition offshore East Greenland along the East Greenland Shelf. The Greenland Shelf sites were not exam- a latitude of approximately 63°N. One of the principal objectives of ined for Neogene ashes because this area was covered and strongly the cruise was to examine the nature of the regional volcanism and to eroded by glaciers during this time (Larsen, Saunders, Clift, et al., chart the evolution of the Icelandic mantle plume, which is thought 1994). In each case the ash material was analyzed using ion and elec- to be responsible for the enhanced topography and volcanism on tron microprobes and the chemistry examined to eliminate non-Ice- modern Iceland, compared with the rest of the Mid-Atlantic Ridge. landic volcanic centers as sources of the ash. During the breakup of the northeast Atlantic, the Iceland Plume may have contributed heat to the asthenosphere under the rift, so produc- ICELANDIC VOLCANISM ing the thick sequences of lavas imaged as the seaward-dipping re- flector series (SDRS). The recognition of a strong thermal input in the form of the thick lava sequences and absence of any chemically en- The nature of recent volcanism in Iceland has been extensively in- riched components in the lavas (Shipboard Scientific Party, 1994a; vestigated and compared with the older basalts exposed over east and 1994b) suggest that there may have been a marked detachment be- west Iceland and dating back into the middle Miocene (approximate- tween the thermal and chemical effects of the plume at the time of ly 16 Ma) in the northwest of the island (Hemond et al., 1993; Hard- continental breakup. A similar effect can be seen today, as the effects arson and Fitton, 1994). Except for the lone alkalic center at of the modern plume extend as far south as 62°N on the Reykjanes Snaefellsnes, modern volcanism on Iceland is concentrated in the Ridge in terms of trace and rare earth elements and radiogenic isotope eastern and western rift zones, representing the onshore continuation ratios (Hart et al., 1973; Schilling, 1973). However, elevated topog- of the Mid-Atlantic Ridge. Physically, the Pleistocene volcanoes dif- raphy on the spreading ridge extends as far south as 50°N (Klein and fer from the Tertiary basalts due to their eruption under the ice sheets Langmuir, 1987), as well as north along the Kolbeinsey Ridge (Mertz that then covered the island. Nonetheless, extensive evidence exists et al., 1991), indicating that the thermal effect of the plume is still for frequent explosive volcanism on Iceland (e.g., Sigurdsson and more widespread than the chemical. The goal of this paper is to in- Loebner, 1981; Schmincke et al., 1982), making it the major source vestigate the chemical character and evolution of the plume. Key of airfall ashes in the Northeast Atlantic. The volcanic rocks of the questions addressed were (1) to what extent is the modern volcanism neovolcanic rift zone differ from older Tertiary flows in terms of their in Iceland representative of past activity in terms of its chemistry and geochemical variability. Although the vast majority of the modern (2) has the chemistry of the plume undergone any systematic changes volcanic rocks are basalts, the trace element chemistry shows that the and if so, over what time scales do these changes occur. We attempt degree of enrichment of the lavas with respect to incompatible elements is variable. An incompatible element is one that is not easily incorporated into the crystal lattices of mantle minerals. Therefore, when a given piece of mantle melts, the liquid produced will have 1Saunders, A.D., Larsen, H.C., and Wise, S.W., Jr. (Eds.), 1998. Proc. ODP, Sci. more of any given incompatible element than of other more compat- Results,152: College Station, TX (Ocean Drilling Program). ible elements when compared to the original mantle source. In this 2 Department of Geology and Geophysics, Woods Hole Oceanographic Institution, paper we employ Zr/Nb to model the relative enrichment of magmas Woods Hole, MA 02543, U.S.A. [email protected] 3 Department of Geology and Geophysics, Grant Institute, University of Edinburgh, in high field strength elements, Zr being a more compatible high field West Mains Road, Edinburgh, EH9 3JW, United Kingdom. strength element than Nb. Both elements are readily measured and

67 P.D. CLIFT, J.G. FITTON

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500 Site 917 Site 916 63 30 N Site 915 500 1000 200 Site 914 1200

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Figure 1. Map of the East Greenland Margin and Irminger Basin showing the location of the Leg 152 drill sites (Larsen, Saunders, Clift, et al., 1994). the errors obtained at the concentrations encountered in Iceland lavas companied by massive outpourings of lavas (e.g., Surtsey; Walker are small, giving high confidence in the results. and Croasdale, 1972). Hardarson and Fitton (1994) demonstrated that the Tertiary lavas outside the neovolcanic zone have Zr/Nb values ranging from 6 to 13, whereas recent volcanism in the neovolcanic zone shows a much SEDIMENTOLOGY AND PETROGRAPHY wider spread, with values up to 34. The reason for the spread in values may be related to the nature of crust formation in Iceland, rather Figure 3 shows the location within the sedimentary column of than reflecting anomalous volcanism in the present day. The most ashes analyzed during this study. All the Neogene grains analyzed in widely accepted model for crustal formation is that of Pálmason this study were removed from primary airfall ash layers recovered at (1981; 1986), who proposed a spreading mechanism for the mid- Sites 918 and 919, whereas some of the Oligocene and Eocene grains ocean ridge on Iceland, featuring strong subsidence of material erupt- were reworked and are present as particles in shallow marine sand- ed in the rift zone (Fig. 2). As such, the rocks exposed at the surface stones. The Neogene ashes are typically interbedded with sequences over much of Iceland will only represent those flows that were able of clays and siliciclastic silts, interpreted as distal turbidites or the to travel far from the central rift zone. These flows will typically be glacially deposited material dumped from melting glacier fronts and the largest flows and may not be representative of the diversity of icebergs. This is especially true in the late Miocene–Holocene sec- magmatism at any one time. For the largest flows, small-scale chem- tion of Site 918 (0.0−543.5 meters below seafloor [mbsf]). There is ical heterogeneity will tend to be eliminated by mixing in large mag- no evidence to suggest major reworking of the ash layers as turbid- ma chambers. The variability in the modern rift zone would suggest ites, although disruption and particle dispersion by bioturbation is that the exposed Tertiary lavas only show the lower values of Zr/Nb. common. Ashes at Site 919, located closer to Iceland, are as much as To sample rocks with rarer high values of Zr/Nb, the melt needs to be 3 cm thick, whereas those layers at Site 918 are typically thinner and removed from the mantle and erupted with a minimum of mixing, a are often largely dispersed by burrowing. Where bioturbation was in- process that is unlikely in large magma chambers. tense, the presence of ash particles was only revealed by microscopic The ashes recovered at Sites 918 and 919 span an overlapping smear slide study because there was no perceptible banding in the time period with the lavas of onshore Iceland and allow this model of core itself. Nevertheless, it is reasonable to assume that these dissem- spreading and chemical variability to be tested. To have reached the inated particles are representative of the volcanism at the time of dep- Irminger Basin, the ash must have been blown to the southwest fol- osition of the background sediment. Bioturbation only distributes the lowing an explosive eruption. One obvious problem when attempting sediment particles locally, usually within 20 cm of the original layer, to see greater chemical variability in the ashes is that the largest erup- so that the overall stratigraphic progression is maintained. Thin ash tions could also have been the most explosive, in which case they will layers (Fig. 4) show minor disruption by bioturbation. Undisturbed be chemically identical to the outcropping Tertiary lavas. Although layers are sharp-based and show a grading in grain size and color up large volume eruptions are often explosive, studies of Icelandic activ- into hemipelagic sediments. Although slumping is known from the ity show that violent phreatomagmatic eruptions are not always ac- ocean islands of the Pacific (e.g., Hawaii; Garcia and Hull, 1994), a

68 TRACE AND RARE EARTH ELEMENT CHEMISTRY

Figure 2. Schematic diagram of Icelandic spreading model, showing the subsidence of lavas in the rift zone and the exposure of only the most distal of the older flows at the surface (Pálmason, 1981). Age ODP Lith. Units Age 0 ODP Lith. Units 0

100 Pleis. 20 200 U. Plio. 300 I 40 400 L. Plio. 500 60 600 U. Mioc. I 700 80

II Pleistocene 800 M. Mioc. Depth (mbsf) L. Mio. 100 900

1000 III 120 1100 U. Oligo. IV

V M-L Eoc. 1200 140 Pliocene ? 1300 Site 918 Site 919

Clay or claystone Gravel or conglomerate

Silt or siltstone Nannofossil ooze or chalk

Sand or sandstone Volcanic ash Figure 3. Schematic sedimentary logs for the sections Glacial diamicton Basaltic lavas cored at Sites 918 and 919. Arrows indicate level from which ash material was removed for analysis.

69 P.D. CLIFT, J.G. FITTON

cm turbidite origin for the layers recovered during Leg 152 can be reject- 125 ed, as no evidence exists to suggest erosion or redeposition by current activity (c.f. Bouma, 1962). The vast majority of the undisturbed ash layers are dark colored, suggestive of a basaltic chemistry. Smear-slide examination indicates that the ash is dominated by volcanic glass shards that are very angu- lar in form and usually show vesicles testifying to the volcanic origin. Up to 95% of grains within individual ash layers are glass, with the remainder made up by accessory minerals, principally feldspars and pyroxenes. The amount of fresh glass and the number of individual ash layers decreases very markedly below 290 mbsf at Site 918. Between 530 and 650 mbsf, the calcium content of the pore waters reaches a maximum and the magnesium content is at a minimum (Shipboard Scien- tific Party, 1994b). The chemistry within this zone is thought to be 130 due to the alteration of volcanic glass. Although this, and the presence of palagonitized glass within the sediment, may explain the pau- city of glass below 530 mbsf, it does not explain why there is so little present between 290 and 530 mbsf. There is not a clear explanation for such a drop off in material. Donn and Ninokovich (1980) suggest that there was a maximum in Icelandic volcanism during the Pliocene, as recorded at a number of drill sites throughout the northeast Atlantic. In contrast, Sigurdsson and Loebner (1981) used ashes recovered north of Iceland during Deep Sea Drilling Project (DSDP) Leg 38 to suggest a maximum in volcanism during the Pliocene– Pleistocene. Both of these studies were disputed by Bitschene et al., (1989), who used the better recovery from ODP Leg 104 cores on the Vøring Plateau to infer no maximum in volcanism. However, exactly how good a Pliocene–Pleistocene record this area provides is open to question because it was covered by pack ice. If Icelandic volcanism 135 really did peak during the Pliocene–Pleistocene, then other factors, such as climatic conditions, must have prevented the deposition of ash on the East Greenland Shelf, as the first appearance of abundant ash layers at Sites 919 and 918 is later than the 5.0 Ma date noted from northeast of Iceland (Sigurdsson and Loebner, 1981). Moberly and Campbell (1984) suggested that volcanism from the Hawaiian hot spot might be more intense during periods of normal geomagnetic polarity. Figure 5 shows the compiled data set for Sites 918 and 919 (discussed below) together with the geomagnetic reversal history of Cande and Kent (1992). It can be seen that although there is a corre- spondence between normal polarity and ashes from 0−1.5 Ma, this does not appear to be the case prior to that time. Even for the period 0−1.5 Ma, the small number of analyses makes this data set of ques- tionable statistical significance. The ashes at Sites 918 and 919 do not 140 support this model of episodic hot-spot magmatism. The prevailing wind systems may have an influence on the accumulation of volcanic ash in the Irminger Basin. It is possible that the winds after 3.2 Ma were more prone to blowing material toward the southwest than had previously been the case. It is noteworthy that this time only shortly predates the onset of intense North Atlantic glaciation (Shackleton et al., 1984), and was most probably a period of significant climatic change, as global cooling intensified. Interestingly, the position of the East Greenland drill sites is upwind of Iceland with respect to the dominant wind direction of the modern northeast Atlantic (Kennett, 1981). Stratospheric wind direc- tions in the northeast Atlantic blow towards the east-northeast. Major explosive eruption columns may reach over 40 km into the atmo- sphere (i.e., well within the normal stratospheric range of 15−50 km). 145 In Iceland during this century, eruptions from Hekla resulted in columns that reached 17 km high in 1971 and 25 km high in 1947. Thus the accumulation of much ash in this region would be unexpected, Figure 4. Representative example of distinct ash bed from Site 919. which may explain the relatively modest amount of material found even as far east as Site 919. In comparison with DSDP drill sites northeast of Iceland (Donn and Ninkovich, 1980; Sigurdsson and Loebner, 1981; Bitschene et al., 1989; Desprairies et al., 1989), the thickness of individual ash beds near East Greenland is small. The material that accumulated at the Leg 152 drill sites must therefore represent either the products of exceptionally violent Plinean erup-

70 TRACE AND RARE EARTH ELEMENT CHEMISTRY

A B 0 0

10 1

30 Age (Ma)

3 40

4 50

Figure 5. Variation in total silica content of glass grains 5 60 with time since (A) 5.0 Ma and (B) 60 Ma from Sites 75 70 65 60 55 50 45 40 75 70 65 60 55 50 45 40 918 (solid circles) and 919 (open circles). Geomagnetic SiO (%) SiO2 (%) 2 reversal history from Cande and Kent (1992). tions or eruptions that took place during periods of abnormal wind di- estimate of the age, and when plotted in sequence any age progression rection. In either case, the Leg 152 ash material may not be represen- in the volcanism will not be affected. tative of the full spectrum of Icelandic volcanism. Nevertheless, the ashes do provide valuable snapshots of the Icelandic activity, especially during the 0.0−3.2 Ma period. SAMPLE COLLECTION AND PREPARATION

Paleogene Volcaniclastics Samples were chosen from discrete volcaniclastic layers and were preferentially taken from the coarser grained bases of the layer where A few analyses were made from the older parts of the section at this was possible. Sediments were disaggregated by being mixed Site 918. Oligocene sands, interpreted as turbidites, yielded a number with water and placed in an ultrasonic bath for 24 hr. In the case of of glass fragments whose alteration was not too severe. In these sed- more indurated sediments, the sandstones and siltstones were treated iments, resedimentation has clearly occurred, which leaves their with dilute HCl and then NaOH for 24 hr each on a hotplate, agitating provenance using sedimentary data alone open to question. Their ba- with a mechanical stirrer where necessary. After this the sediments saltic character means that they could represent airfall ashes of Oli- were sieved through 125-µm, 63-µm, and 45-µm mesh and washed gocene age, derived from a proto-Iceland to the north. Alternatively, by a high-pressure water jet. A selection of grains from the largest their derivation from onshore Greenland remains a possibility. If so, size fraction (>125 µm) was then mounted in epoxy resin and pol- either the rift-related basalts of the seaward dipping sequences or late ished, using first 6-µm and then 1-µm diamond paste. The largest stage volcanism at Kangerlussuaq are possible sources. Although grain sizes were chosen due to the operational constraints of the ion eroded away today, basalts are presumed to have been exposed on- microprobe. To make a single analysis, the beam of the probe covers shore at 63°N during the Paleogene. The vast majority of the grains a spot approximately 30 µm across. Large grains were chosen to en- composing these sediments show clear evidence for derivation from hance the likelihood of being able to find such a spot for analysis. The the Greenland craton (Saito, this volume). A metamorphic source ter- slides were carbon coated before electron-probe analysis. rain is indicated by the presence of polycrystalline quartz, pyroxenes, amphiboles, and micas. A small number of volcanic glass fragments were extracted from ANALYTICAL PROCEDURES lower Eocene sediments directly overlying the weathered top of basalts at Site 918. These sediments are strongly dominated by volcanic The samples were analyzed using the Cameca SX50 electron mi- detritus and are of a shallow marine facies (Shipboard Scientific croprobe at Texas A&M University for Si, Al, Na, K, Ca, Mg, Fe, P, Party, 1994b). The sediments are reworked and, in view of their prox- Ti, Mn, and, in some cases, Cr. A beam current of 10 nA was em- imity to the basalts, were probably derived by erosion of the underly- ployed, operating at a voltage of 20 kV. A count time of 30 s was used ing basement, although additional input by airfall from volcanoes in for each element. the active rift, which at that time would have been close by and prob- Samples of basaltic and rhyolitic glass identified by the electron ably subaerial, remains a possibility. probe study were subsequently cleaned, coated in gold and analyzed using the Cameca ims 4f ion microprobe at the University of Edin- Dating of Ash Layers burgh, running Charles Evans and Associates software, for a suite of 20 trace and rare earth elements. The mafic glasses were chosen for The age of a single ash layer and its constituent grains is deter- trace element analysis to minimize the geochemical effect of frac- mined using a combination of the shipboard biostratigraphic and mag- tional crystallization and thus yield the greatest amount of informa- netostratigraphic data (Shipboard Scientific Party, 1994b; 1994c). tion on the petrogenetic history and source characteristics of the mag- These data provide a number of age-control points on the section and mas. Electron backscattered images of glass obtained on the electron yielded a stratigraphic age. A numerical age is then assigned from the microprobe often showed the presence of microphenocrysts within time scale of Cande and Kent (1992). Between control points, the age the glass. The glass grains that were analyzed by the ion microprobe at any one point is calculated assuming linear rates of sedimentation. were either aphyric or contained the minimum number of pheno- Although probably inaccurate in detail, this method provides a good crysts within the available sample set. There is no way of establishing

71 P.D. CLIFT, J.G. FITTON whether phenocryst phases were actually absent from the host mag- and three beds within an interval of 652−660 mbsf at Site 918. These ma of an aphyric glass fragment on eruption. beds contain only rhyolitic glasses. Of the Oligocene and Eocene Representative major element chemical analyses for the volcanic grains analyzed, much of the material proved to be basaltic but was glass grains probed are given in Table 1. Trace and rare earth element typically altered to smectite. However, the cores of a few grains, es- data are shown in Table 2. The reliability of the data is a key question. pecially those from calcite-cemented layers, provided high total per- Very few of the Neogene grains show any visible cloudiness attribut- centages of major elements, suggesting that alteration was not too ad- able to hydration. Alteration generally increases down section, a vanced and that some useful information could be gained by the con- common phenomenon in marine ashes. The high surface area to vol- sideration of water immobile elements. ume ratio of ash shards, especially vesicular ones, promotes the hy- The total silica contents of the freshest 0.0−3.2 Ma glasses are dis- dration of the glass and results in a relatively short life span for small played in Figure 5. The vast majority of the analyses show basaltic glass shards in the sedimentary column (Hay and Ijima, 1968). Alter- values, ranging from 47.0 to 52.5 wt% SiO2. The majority of grains − ation can be seen in the analyses as low analytical totals, although within this group show a more confined range of 48 51 wt% SiO2, these can also arise from an indigenous volatile content in the melt typical of Icelandic tholeiite. MgO is mostly high, typically >6 wt%, (Burnham and Jahns, 1962). In general, however, the probe data from ranging as high as 7.8 wt%, which suggests that the grains are not both sites show high totals, close to 100%, suggesting that alteration very evolved and thus appropriate for petrogenetic modeling. Figure has not affected these specimens to a significant degree. Lower totals, 7 shows a triangular alkali-iron-magnesium (AFM) diagram (Irvine around 93 wt%, are seen between 652.0 and 660.0 mbsf at Site 918. and Baragar, 1971). The analyzed grains clearly fall along the trend This is mostly due to the greater age and degree of hydration of these of a tholeiitic fractionation series. The major element ash data indi- specimens and to a lesser extent because the amount of water increas- cate that volcanism since 3.2 Ma has been dominantly basaltic and es with silica content during fractionation. tholeiitic. Rare silicic eruptions are noted at 1.0, 1.8, and 2.5 Ma, as An important additional source of error and uncertainty is the loss well as earlier at 12.1, 12.5, and 12.7 Ma. No temporal trend in the of sodium under the electron beam (Neilson and Sigurdsson, 1981). nature of the major element chemistry can be seen and effectively one The expected levels of total alkalies at around 8 wt% in the most sil- basaltic ash layer is indistinguishable from another. iceous glasses are not achieved because sodium declines sharply with silica content from basaltic andesite compositions onwards. Potassi- um values are scattered but are not so conspicuously subject to loss. TRACE AND RARE EARTH ELEMENT CHEMISTRY Sodium loss under the beam causes the apparent concentration of other elements to rise. As sodium loss is related to silica content and A group of 20 trace and rare earth elements was measured using is not subject to a constant loss factor and volatile content is also an ion microprobe. The full analyses, together with the corresponding broadly related to silica, the two effects counteract each other and major element analyses, are shown in Table 2. Table 3 shows a typi- measured abundances of most elements, except sodium, are probably cal analysis from Sample 152-918A-8H-2, 34 cm, with concentra- not far from their volatile-free levels. The data have therefore been tions and percentage error determined from the counting statistics. left uncorrected. Instead, analyses with abnormally low totals have This percentage varies with the total concentration of any given ele- been excluded. ment and its ionization potential. In general, however, the errors are It should be noted that the trace element data obtained on the ion low, the vast majority under 5 wt%. Notable exceptions are Sm and microprobe are not affected by effects analogous to sodium loss un- Lu, which approach 10%, whereas U, present in only tiny quantities, der the electron beam, although a correction would still be necessary records uncertainties of 31.6 wt%. The results from the ion probe to arrive at volatile-free values. This correction is likely to be very work are thus comparable, and in a number of instances superior to small, as Icelandic magmas are generally dry and thus even the acid other analytical techniques such as inductively coupled plasma mass glasses would not have been water saturated on eruption. spectrometry (ICP-MS) or X-ray fluorescence (XRF). In this paper we shall consider both the overall chemical character of the glasses Analytical Errors and individual elemental ratios. The degree of enrichment of incompatible elements over compatible elements is a key factor and for this The precision and reproducibility of the trace element measure- purpose we looked at two elemental groups, the high field strength el- ments are assumed to closely approximate to those given by the Pois- ements and the REE. son counting statistics for each element. This assumption is support- As mentioned above, the degree of alteration of the glasses ed by earlier work on the same probe (Hinton and Upton, 1991) and younger than 3.2 Ma is considered to be low due to the general clarity on other basaltic glass grains (Clift and Dixon, 1994). In particular, of the glass under microscopic examination and the high analytical the smoothness of the rare earth element (REE) plots obtained from totals derived from electron probe analysis. Hydration, however, has the glasses suggest that the errors are not significantly greater than affected older grains to a greater degree, resulting in totals less than the uncertainties in the counting statistics, even at low concentrations 90%. However, many trace and rare earth elements are considered to (<1.0 ppm). Chemically similar elements are seen to show the same be immobile in water (Pearce, 1983). Provided the hydration has not trends within the data set, regardless of whether the element is a rel- advanced too far, these elements may still be useful for petrogenetic atively abundant one (e.g., Zr) or present in low concentrations (e.g., purposes. Ho, Tb). Table 3 shows the count rate errors for a typical analysis The multi-element spider diagram is typically used to display the from Site 918. It is noteworthy that for some geochemically impor- overall incompatible element characteristics of a rock (Thompson et tant elements, such as Nb, the ion probe method is able to achieve al., 1983). In a spider diagram, the elements are ordered to give a good precision (2% error), as is shown by the correlation between Nb smooth curve for average mid-ocean ridge basalts (MORB; Sun, and Zr on Figure 6. Clift and Dixon (1994) estimated that background 1980), which in effect means an increasing incompatibility of the el- interference became a statistical problem in measuring Nb below 0.4 ements in lherzolite during incipient partial melting from right to ppm, well below the values being recorded here. The scatter on this left. Figures 8 and 9 show spider diagrams for glasses of different plot is primarily due to real variations in the Zr/Nb ratio. ages at Sites 918 and 919, respectively. In each case, the total concentration of the elements has been normalized against a primitive mantle standard (Sun and McDonough, 1989). The patterns shown MAJOR ELEMENT CHEMISTRY by all the Miocene–Pleistocene examples are typical of lavas derived from an undepleted mantle source. The flat or gently inclined pat- Fresh glass appears to be confined to Site 919 and Site 918 above terns have a relative enrichment in the most incompatible elements 290 mbsf. Below this level, glass is confined to a horizon at 619 mbsf (i.e., toward the left of the diagram), and contrast with natural

72 TRACE AND RARE EARTH ELEMENT CHEMISTRY Total (wt%) 5 O 2 P (wt%) 3 O 2 Cr (wt%) O 2 K (wt%) O 2 Na (wt%) CaO (wt%) MgO (wt%) MnO (wt%) FeO (wt%) 3 O 2 Al (wt%) 2 TiO (wt%) 2 SiO (wt%) Table 1. Representative major elemental compositions for individual glass grains as measured by electron probe. by electron major elemental compositions for individual glass grains as measured 1. Representative Table Age (Ma) Depth (mbsf) interval (cm) Core, sample, 2H-2, 342H-4, 242H-1, 432H-1, 432H-1, 1062H-1, 106 3.642H-1, 106 6.542H-1, 1062H-1, 106 0.04 9.63 0.08 9.63 10.263H-3, 142 10.263H-3, 142 10.26 0.14 49.4923H-3, 142 10.26 0.14 0.15 47.2034H-2, 48 10.26 0.155H-3, 103 0.155H-3, 103 48.572 3.056 0.15 15.42 49.087 49.062 2.470 0.15 15.42 49.0667H-6, 135 15.42 49.9507H-6, 135 13.551 3.067 0.16 22.78 49.8407H-6, 135 34.03 14.736 3.375 2.457 0.16 49.6057H-6, 135 34.03 2.653 0.16 14.281 2.526 13.023 0.26 49.442 2.806 0.556H-1, 122 64.35 12.886 13.327 9.645 48.469 2.591 0.55 64.35 12.914 48.697 64.35 13.497 13.9883H-3, 85 0.235 64.35 13.052 50.573 14.483 13.717 3.787 0.903H-3, 85 49.263 0.195 13.264 14.344 2.803 0.904H-3, 115 49.756 13.055 3.283 0.904H-3, 115 41.02 0.183 5.740 14.950 0.90 10.746 1.927 0.265 0.194 48.539 13.116 2.357 14.059 7.7958H-2, 34 0.243 48.298 2.233 93.85 12.850 1.0010H-3, 2 0.168 48.326 10.017 93.85 103.65 5.509 15.26810H-3, 2 0.310 13.239 50.048 103.65 13.971 4.741 5.903 12.267 3.247 12.616 0.227 13.808 5.364 13.289 3.470 1.315H-3, 99 67.722 5.820 3.316 1.31 1.47 2.636 13.890 9.6345H-3, 99 0.317 5.078 12.190 1.247 10.299 1.47 60.64 12.849 9.2695H-3, 99 0.207 2.176 6.205 12.325 80.82 12.916 9.6655H-3, 99 0.231 48.780 10.346 80.82 12.962 0.0425H-3, 99 48.968 49.023 0.400 0.244 14.518 2.677 6.592 14.674 9.7675H-3, 99 0.199 1.50 50.422 2.529 10.604 112.99 2.605 5.970 14.627 0.8496H-1, 43 1.60 0.208 112.99 2.612 5.745 15.358 2.803 1.60 18.638 2.682 112.99 10.837 2.431 0.778 11.145 0.007 6.031 0.4316H-1, 43 0.263 112.99 1.723 48.747 6.632 0.349 1.395 10.304 1.60 2.498 0.5786H-6, 87 0.215 0.030 112.99 49.721 6.721 10.233 1.60 13.080 0.4266H-6, 87 0.258 112.99 49.088 1.902 0.415 1.60 12.196 15.017 10.1686H-6, 87 0.206 118.93 2.091 0.288 11.296 1.60 0.009 14.463 5.057 0.423 3.0726H-6, 87 0.063 2.565 49.672 0.420 11.250 1.60 0.002 5.095 0.347 14.135 2.7676H-6, 87 2.538 49.721 1.60 0.023 118.93 5.100 14.126 2.379 0.024 0.011 48.372 9.220 1.69 99.703 126.87 2.284 7.748 11.204 0.504 2.341 12.866 48.555 0.369 31 9.48711H-2, 126.87 0.000 0.272 0.546 3.363 98.062 0.000 2.396 13.260 49.110 0.473 9.434 126.87 0.300 0.385 3.495 1.69 13.385 49.291 0.357 9.753 126.87 0.257 0.870 0.363 12.829 3.507 1.777H-1, 37 0.152 48.572 0.232 13.950 126.87 97.462 0.208 0.036 0.289 3.307 1.777H-1, 37 98.172 2.737 13.158 0.343 13.355 97.764 0.070 0.311 0.270 3.327 1.77 2.726 13.626 5.355 12.953 97.667 0.053 49.087 3.318 1.77 98.833 89.11 2.696 12.897 7.354 0.38313H-6, 35 2.201 48.893 8.822 3.067 1.77 0.033 0.233 12.879 7.253 0.345 15.085 98.292 13H-6, 35 0.038 48.994 0.557 98.841 0.201 12.981 0.380 15.61813H-6, 35 0.023 128.37 48.842 0.572 9.625 0.227 13.607 0.314 15.007 3.375 10.240 1.8013H-6, 35 128.37 48.787 0.573 13.728 100.273 13.023 8.244 0.155 14.364 2.92714H-3, 0 11.486 50.404 0.263 5.310 0.291 0.210 14.059 2.767 97.640 0.020 114.15 5.977 1.80 0.209 0.204 14.416 2.579 97.618 0.004 114.15 12.886 2.584 5.917 1.80 0.295 1.821 49.314 13.988 2.561 0.000 13.155 114.15 1.784 2.087 98.884 0.049 0.226 2.225 2.603 98.786 9.454 12.959 114.15 5.029 10.102 0.308 2.25 0.453 99.194 13.025 48.317 4.649 14.483 10.732 0.183 2.25 118.8 0.343 0.430 13.042 70.894 5.072 14.142 0.380 2.076 0.183 2.25 0.362 0.067 0.000 13.253 0.156 5.150 14.955 0.261 2.25 2.586 97.883 9.268 2.593 5.354 48.382 14.007 3.397 97.700 8.997 0.265 2.493 4.290 48.286 13.171 2.30 0.030 13.948 0.226 98.704 9.487 0.257 0.011 5.509 48.140 13.034 99.994 0.027 0.000 9.340 0.265 0.046 48.226 0.443 10.127 2.491 0.404 2.107 0.257 12.964 1.999 4.741 11.525 0.279 9.109 2.061 0.212 49.120 0.318 7.827 2.729 5.457 0.266 9.634 1.878 99.912 0.266 0.013 2.702 5.246 0.111 2.054 0.003 2.444 13.778 15.235 0.652 0.045 5.425 13.820 0.607 9.269 97.440 0.227 0.031 2.910 6.390 2.408 98.050 13.570 6.389 0.529 9.635 98.631 2.677 5.368 99.074 13.551 0.563 11.900 9.190 0.357 0.646 0.305 0.000 0.195 12.165 9.849 11.736 0.018 13.446 2.605 6.795 0.253 0.587 11.965 0.281 0.000 2.749 0.431 12.423 9.345 97.021 0.036 0.221 2.704 98.620 0.031 5.061 0.337 0.173 2.730 13.060 11.089 97.737 2.599 0.389 0.578 0.024 0.238 0.019 0.401 0.397 0.009 0.242 2.802 0.511 6.936 0.414 99.346 0.440 9.704 6.917 0.391 99.328 0.158 2.490 0.183 0.002 0.460 6.942 98.296 0.275 0.000 0.369 6.796 0.545 97.633 11.499 98.827 0.000 11.719 2.676 0.033 6.230 0.303 98.195 11.406 0.009 0.473 97.462 11.366 4.027 0.266 0.000 2.474 0.344 2.482 0.537 10.441 0.266 97.764 0.000 2.376 0.252 97.878 2.441 2.811 0.351 0.202 97.838 0.176 97.404 0.022 2.524 98.942 0.182 0.203 0.243 97.971 0.047 0.044 0.008 0.355 0.364 97.949 0.013 0.022 0.007 0.110 98.573 0.201 0.018 0.175 92.803 0.253 97.653 98.008 0.143 96.906 97.617 97.912 152-918A- 152-919B- 152-918A- 152-919A- 152-918A- 152-919B- 152-918A- 152-919B- 152-919B- 152-918A- 152-919B- 152-918A-

73 P.D. CLIFT, J.G. FITTON Total (wt%) 5 O 2 P (wt%) 3 O 2 Cr (wt%) O 2 K (wt%) O 2 Na (wt%) CaO (wt%) MgO (wt%) MnO (wt%) Table 1 (continued). FeO (wt%) 3 O 2 Al (wt%) 2 TiO (wt%) 2 SiO (wt%) Age (Ma) Depth (mbsf) interval (cm) Core, sample, 14H-3, 014H-3, 014H-3, 018H-CC, 1418H-CC, 1420H-3, 9 118.821H-2, 109 118.821H-2, 109 161.8 118.821H-2, 109 161.821H-2, 109 2.3023X-5, 14 2.30 174.39184.0923X-5, 14 2.50 2.30 184.0923X-5, 67 2.50 184.0923X-5, 67 48.877 184.0923X-5, 67 2.522.55 49.23023X-5, 67 71.092 2.55 205.94 47.02023X-5, 122 48.435 2.55 205.9423X-5, 122 2.55 2.493 206.47 49.337 49.89823X-5, 122 1.805 206.47 2.60 0.028 52.06123X-5, 122 3.086 206.47 2.60 3.824 50.841 206.47 13.265 2.63 207.02 52.274 14.22024X-3, 128 2.63 1.375 3.692 207.02 12.582 49.765 15.55924X-3, 128 2.63 207.02 3.565 13.033 48.86825X-4, 109 2.63 3.217 212.98 13.375 2.65 48.94025X-4, 109 3.492 11.184 15.24712.713 2.65 49.03425X-7, 4 11.899 1.486 12.550 2.65 3.247 212.98 13.374 49.09625X-7, 4 13.332 2.70 2.505 212.98 49.179 0.20525X-7, 4 49.165 12.661 2.242 10.31714.842 223.19 0.15825X-7, 4 47.875 2.355 13.914 223.19 12.904 0.16228X-2, 67 0.087 2.70 51.239 0.296 2.408 13.614 14.02433X-2, 59 2.70 48.916 2.367 14.916 5.898 226.64 13.64133X-2, 59 3.445 2.75 0.1880.267 7.280 226.64 13.78433X-2, 59 1.960 14.830 2.75 0.331 5.491 226.64 0.034 13.904 49.10133X-2, 59 2.767 12.437 4.564 0.204 226.64 13.708 10.557 48.256 2.051 13.599 2.80 246.27 13.141 0.330 11.934 50.024 13.225 7.9854.809 2.80 290.49 15.50136R-1, 17 0.234 43.104 13.123 3.625 2.80 290.49 13.165 8.862 0.64639R-4, 14 2.713 0.193 8.965 13.727 4.876 2.80 290.49 13.91040R-2, 36 14.605 2.528 2.90 3.178 0.204 49.337 3.812 290.49 12.05140R-2, 36 12.652 2.306 3.20 1.245 8.781 0.255 50.03140R-3, 114 13.657 4.990 3.20 1.339 7.708 0.160 50.434 13.379 3.679 2.33540R-3, 114 6.716 11.975 3.20 3.075 619.07 8.954 0.200 51.044 13.138 0.258 0.291 55.39840R-3, 114 6.680 3.20 3.221 652.54 8.247 13.548 2.118 0.224 0.283 48.09840R-3, 114 2.450 6.654 3.041 659.36 12.917 13.280 0.261 9.328 49.44651R-6, 67 2.993 6.932 3.162 10.50 659.36 1.148 10.912 5.409 14.903 661.64 0.202 49.201 1.476 2.783 6.426 3.018 12.10 10.848 13.320 5.862 0.000 0.122 12.358 661.64 49.116 2.731 12.50 0.408 10.756 13.399 7.835 0.050 3.17016R-1, 90 14.923 661.64 0.558 12.50 10.776 13.427 0.198 4.447 2.783 1.75116R-1, 90 12.70 661.64 0.793 45.012 0.051 2.480 0.000 10.625 13.264 0.217 6.96617R-1, 146 2.395 14.065 12.70 0.000 10.089 0.648 27.815 73.278 2.305 771.37 0.227 0.207 2.319 13.956 12.70 0.721 11.089 12.816 71.006 0.056 2.365 0.124 0.219 0.043 13.572 12.70 13.748 70.738 2.352 6.12288R-1, 42 8.710 0.512 72.999 0.048 13.908 0.402 226.7 0.276 0.636 11.669 13.361 0.000 2.388 5.63189R-4, 49 17.40 0.197 70.758 1.062 2.290 0.014 97.716 1.412 0.065 226.7 0.248 13.813 15.851 6.79289R-4, 49 236.96 0.239 70.901 2.343 0.000 98.574 11.577 0.303 0.098 0.274 12.83589R-4, 49 0.377 0.195 10.416 70.502 0.194 0.227 12.451 2.746 0.000 97.593 12.99189R-4, 49 0.155 93.699 32.10 0.508 0.224 10.443 2.329 0.011 98.104 0.221 1118.22 59.942 11.965 12.409 5.26889R-4, 49 0.263 32.90 32.10 0.396 0.397 0.000 11.360 0.055 10.823 0.252 1132.39 11.791 99.180 5.16491R-2, 10 0.279 0.246 0.540 98.430 0.028 0.221 1132.39 11.712 5.267 2.47593R-2, 64 0.279 14.356 11.505 0.051 98.096 0.703 0.240 39.70 0.189 1132.39 48.809 4.828 2.67293R-2, 64 0.231 0.208 11.909 0.038 98.880 0.270 1.253 44.04 9.466 0.226 1132.39 43.119 44.187 0.002 2.121 0.292 0.221 11.749 99.724 1.234 5.032 3.400 44.04 9.313 1132.39 0.007 0.221 11.898 6.501 3.56095R-3, 94 44.04 9.337 0.292 1148.1 0.223 98.833 1.501 0.279 0.000 98.630 53.759 5.88696R-1, 29 0.467 44.04 8.656 0.346 1167.94 10.906 0.000 3.614 0.208 10.567 0.101 98.983 54.750 2.695 6.59696R-1, 29 2.111 1.570 0.434 44.04 0.137 1167.94 3.377 0.968 9.495 0.146 98.951 59.404 2.72896R-3, 120 0.156 11.329 3.384 11.153 47.90 0.072 99.308 59.800 2.732 0.035100R-2, 27 48.30 10.345 0.061 0.489 0.336 1184.34 8.705 99.087 70.839 2.764 0.000 1.044 0.189 48.30 11.496 0.137 10.689 12.306 99.687 5.194 0.169 0.563 0.967 1185.39 59.861 0.007 0.101 99.888 0.029 2.700 0.068 0.768 0.563 1185.39 12.155 2.423 1189.3 0.117 53.499 0.093 0.760 0.559 0.299 50.60 98.184 61.268 2.665 0.076 11.964 1215.77 98.438 9.671 14.703 13.485 0.621 0.473 0.388 51.00 0.034 61.358 2.531 12.368 0.116 0.348 1.151 0.006 1.241 0.131 51.00 0.145 0.532 0.000 1.852 0.000 7.335 1.039 0.345 51.40 98.310 0.148 3.886 52.00 1.905 0.050 7.649 49.675 0.955 0.437 11.777 0.910 0.113 99.172 10.643 0.376 0.179 0.020 3.506 43.453 0.921 1.623 0.331 1.787 0.000 97.930 2.923 0.480 8.154 97.133 58.721 12.798 1.750 0.000 3.603 0.390 0.738 14.083 0.015 54.901 12.397 1.754 49.034 3.597 14.591 0.436 0.172 4.410 0.111 0.001 3.218 12.201 11.881 10.384 0.007 0.495 5.275 1.231 1.030 98.618 0.025 3.343 0.020 2.900 7.392 0.539 98.824 0.000 3.429 0.383 1.830 0.061 7.483 0.157 2.305 11.177 99.171 0.020 3.154 10.304 0.234 1.791 11.508 11.397 9.046 6.840 0.313 3.008 98.842 101.168 11.313 7.021 3.493 0.007 3.120 0.208 1.980 0.024 0.070 98.329 3.985 2.782 1.883 98.771 13.682 0.000 0.000 0.048 14.082 1.386 2.751 1.964 152-918A- 152-918D- 152-914B- 152-918D- 152-918D-

74 TRACE AND RARE EARTH ELEMENT CHEMISTRY Total (wt%) 5 O 2 P (wt%) 3 O 2 Cr (wt%) O 2 K (wt%) O 2 Na (wt%) CaO (wt%) MgO (wt%) MnO (wt%) Table 1 (continued). FeO (wt%) 3 O 2 Al (wt%) 2 TiO (wt%) 2 SiO (wt%) Age (Ma) Depth (mbsf) interval (cm) Core, sample, 100R-2, 27100R-3, 82100R-3, 82100R-3, 82100R-3, 82 1215.77100R-3, 82 1217.82 1217.82 52.00 1217.822H-2, 34 52.00 1217.822H-4, 24 52.00 1217.82 52.00 48.1962H-1, 43 52.00 53.8672H-1, 43 52.00 60.5182H-1, 106 58.4182H-1, 106 3.64 0.544 57.6672H-1, 106 6.54 1.222 56.7952H-1, 106 0.3912H-1, 106 1.001 0.04 9.63 17.835 0.671 0.08 9.63 10.26 15.942 0.4663H-3, 142 10.26 17.9363H-3, 142 10.26 15.665 17.826 0.14 49.4923H-3, 142 10.26 18.193 0.14 0.15 47.203 7.1694H-2, 48 10.26 17.696 0.15 9.5445H-3, 103 0.15 9.6035H-3, 103 0.000 48.572 3.056 0.15 15.42 8.381 49.087 49.062 2.470 0.15 15.42 0.018 8.161 49.0667H-6, 135 15.42 0.033 49.9507H-6, 135 0.077 13.551 2.366 3.067 0.16 22.78 49.8407H-6, 135 34.03 14.736 0.011 3.375 2.457 0.16 49.605 1.8067H-6, 135 34.03 0.000 2.653 0.16 2.363 14.281 2.526 13.023 2.278 0.26 0.547 49.442 2.806 0.556H-1, 122 64.35 12.886 13.327 2.158 9.645 48.469 2.591 0.55 64.35 0.203 12.914 2.075 48.697 64.35 0.196 13.497 13.9883H-3, 85 0.235 64.35 0.281 13.052 50.573 0.849 14.483 13.717 3.787 0.903H-3, 85 49.263 0.059 0.195 13.264 14.344 2.803 0.904H-3, 115 49.756 0.085 0.124 13.055 3.283 0.904H-3, 115 41.02 0.563 0.183 5.740 14.950 0.90 10.746 0.094 1.927 1.325 0.265 0.194 48.539 13.116 2.357 14.059 0.181 7.7958H-2, 34 0.243 48.298 2.233 1.612 93.85 12.850 0.033 1.0010H-3, 2 0.168 48.326 0.952 10.017 93.85 103.65 5.509 15.26810H-3, 2 0.310 13.239 50.048 1.907 103.65 13.971 0.039 4.741 5.903 12.267 3.247 12.616 0.227 1.420 13.808 5.364 13.289 3.470 1.31 0.000 1.4345H-3, 99 67.722 5.820 3.316 1.31 1.47 0.000 2.636 13.890 9.6345H-3, 99 0.317 5.078 12.190 1.247 10.299 1.47 60.64 12.849 0.055 0.113 9.2695H-3, 99 0.207 2.176 6.205 12.325 80.82 12.916 0.028 9.6655H-3, 99 0.231 48.780 0.000 10.346 80.82 12.962 0.000 0.0425H-3, 99 48.968 49.023 0.041 0.400 0.244 14.518 2.677 6.592 14.674 9.7675H-3, 99 1.50 0.199 89.640 50.422 2.529 0.020 10.604 112.99 2.605 5.970 14.627 0.8496H-1, 43 0.208 1.60 0.007 81.924 112.99 2.612 5.745 15.358 2.803 1.60 18.638 2.682 0.000 92.537 112.99 10.837 2.431 0.778 11.145 0.007 6.031 0.4316H-1, 43 0.263 89.399 112.99 1.723 48.747 6.632 0.349 1.395 10.304 1.60 2.498 0.5786H-6, 87 0.215 88.776 0.030 112.99 6.721 49.721 10.233 1.60 13.080 0.4266H-6, 87 0.258 86.784 112.99 49.088 1.902 0.415 1.60 12.196 15.017 10.1686H-6, 87 0.206 118.93 2.091 0.288 11.296 1.60 0.009 14.463 5.057 0.423 3.0726H-6, 87 0.063 2.565 49.672 0.420 11.250 1.60 0.002 5.095 0.347 14.135 2.7676H-6, 87 2.538 49.721 1.60 0.023 118.93 5.100 14.126 2.379 0.024 0.011 48.372 9.220 1.69 99.703 126.87 2.284 7.748 11.204 0.504 2.341 12.866 48.555 0.369 31 9.48711H-2, 126.87 0.000 0.272 0.546 3.363 98.062 0.000 2.396 13.260 49.110 0.473 9.434 126.87 0.300 0.385 3.495 1.69 13.385 49.291 0.357 9.753 126.87 0.257 0.870 0.363 12.829 3.507 1.777H-1, 37 0.152 48.572 0.232 13.950 126.87 97.462 0.208 0.036 0.289 3.307 1.777H-1, 37 98.172 2.737 13.158 0.343 13.355 97.764 0.070 0.311 0.270 3.327 1.77 2.726 13.626 5.355 12.953 97.667 0.053 49.087 3.318 1.77 98.833 89.11 2.696 12.897 7.354 0.383 2.201 48.893 8.822 3.067 1.77 0.033 0.233 12.879 7.253 0.345 15.085 98.292 0.038 48.994 0.557 98.841 0.201 12.981 0.380 15.618 0.023 128.37 48.842 0.572 9.625 0.227 13.607 0.314 15.007 3.375 10.240 1.80 128.37 48.787 0.573 13.728 100.273 13.023 8.244 0.155 14.364 2.927 11.486 50.404 0.263 5.310 0.291 0.210 14.059 2.767 97.640 0.020 5.977 1.80 0.209 0.204 14.416 2.579 97.618 0.004 12.886 2.584 5.917 1.80 0.295 1.821 49.314 13.988 2.561 0.000 13.155 1.784 2.087 98.884 0.049 0.226 2.225 2.603 98.786 9.454 12.959 5.029 10.102 0.308 0.453 99.194 13.025 48.317 4.649 14.483 10.732 0.183 0.343 0.430 13.042 70.894 5.072 14.142 0.380 2.076 0.183 0.362 0.067 0.000 13.253 0.156 5.150 14.955 0.261 2.586 97.883 9.268 2.593 5.354 14.007 3.397 97.700 8.997 0.265 2.493 4.290 13.171 0.030 13.948 0.226 98.704 9.487 0.257 0.011 5.509 13.034 99.994 0.027 0.000 9.340 0.265 0.046 0.443 10.127 2.491 0.404 0.257 12.964 1.999 4.741 11.525 0.279 9.109 0.212 0.318 7.827 2.729 5.457 0.266 9.634 99.912 0.266 0.013 2.702 5.246 0.111 0.003 2.444 15.235 0.652 0.045 5.425 0.607 9.269 97.440 0.227 0.031 2.910 6.390 98.050 6.389 0.529 9.635 98.631 2.677 5.368 99.074 0.563 9.190 0.357 0.646 0.305 0.000 0.195 9.849 11.736 0.018 2.605 6.795 0.253 0.587 0.281 0.000 2.749 0.431 9.345 97.021 0.036 2.704 98.620 0.031 5.061 0.337 2.730 11.089 97.737 2.599 0.389 0.578 0.024 0.019 0.401 0.397 0.009 2.802 0.511 0.414 99.346 0.440 9.704 0.391 99.328 2.490 0.183 0.002 0.460 98.296 0.275 0.000 0.369 0.545 97.633 98.827 0.000 2.676 0.033 0.303 98.195 0.009 0.473 97.462 4.027 0.266 0.000 0.344 0.537 0.266 97.764 0.000 0.252 97.878 2.811 0.351 97.838 97.404 0.022 98.942 0.182 97.971 0.047 0.355 97.949 0.007 98.573 92.803 152-918A- 152-919B- 152-918A- 152-919A- 152-918A- 152-919B- 152-918A- 152-919B- 152-919B- 152-918A- 152-919B-

75 P.D. CLIFT, J.G. FITTON Total (wt%) 5 O 2 P (wt%) 3 O 2 Cr (wt%) O 2 K (wt%) O 2 Na (wt%) CaO (wt%) MgO (wt%) MnO (wt%) Table 1 (continued). FeO (wt%) 3 O 2 Al (wt%) 2 TiO (wt%) 2 SiO (wt%) Age (Ma) Depth (mbsf) interval (cm) Core, sample, 13H-6, 3513H-6, 3513H-6, 3513H-6, 3514H-3, 014H-3, 0 114.1514H-3, 0 114.1514H-3, 0 114.1518H-CC, 14 114.15 2.2518H-CC, 14 2.25 118.820H-3, 9 2.25 118.821H-2, 109 2.25 118.821H-2, 109 161.8 48.382 118.821H-2, 109 161.8 48.286 2.3021H-2, 109 48.140 2.3023X-5, 14 48.226 2.30 174.39184.0923X-5, 14 2.107 2.50 2.30 184.0923X-5, 67 2.061 2.50 49.120 184.0923X-5, 67 1.878 48.877 184.0923X-5, 67 2.522.55 2.054 49.230 13.77823X-5, 67 71.092 2.55 205.94 47.020 13.82023X-5, 122 48.435 2.55 2.408 205.94 13.57023X-5, 122 2.55 2.493 206.47 13.551 11.900 49.33749.89823X-5, 122 1.805 206.47 2.60 12.165 0.028 52.06123X-5, 122 3.086 206.47 13.446 2.60 11.965 3.824 50.841 206.47 13.265 2.63 207.02 12.423 52.274 14.220 0.22124X-3, 128 2.63 1.375 3.692 207.02 12.582 49.765 15.559 0.17324X-3, 128 2.63 207.02 3.565 13.060 13.033 48.868 0.23825X-4, 109 2.63 3.217 212.98 13.375 2.65 48.940 0.24225X-4, 109 3.492 11.184 15.24712.713 2.65 6.936 49.03425X-7, 4 11.899 1.486 12.550 2.65 3.247 6.917 212.98 13.374 49.096 0.15825X-7, 4 13.332 2.70 2.505 6.942 212.98 49.179 0.20525X-7, 4 49.165 12.661 2.242 10.31714.842 6.796 223.19 0.158 11.49925X-7, 4 47.875 2.355 13.914 223.19 12.904 0.162 11.71928X-2, 67 0.087 2.70 51.239 0.296 2.408 13.614 6.230 14.024 11.40633X-2, 59 2.70 48.916 2.367 14.916 5.898 226.64 13.641 11.36633X-2, 59 3.445 2.75 0.1880.267 7.280 2.474 226.64 13.78433X-2, 59 1.960 14.830 2.75 0.331 5.491 2.482 226.64 0.034 13.904 10.441 49.10133X-2, 59 2.767 12.437 4.564 0.204 2.376 226.64 13.708 10.557 48.256 2.051 13.599 2.80 246.27 13.141 0.330 2.441 11.934 50.024 13.225 7.9854.809 2.80 290.49 15.501 0.20236R-1, 17 0.234 43.104 13.123 3.625 2.80 290.49 13.165 8.862 0.176 0.64639R-4, 14 2.524 2.713 0.193 8.965 13.727 4.876 2.80 290.49 13.910 0.20340R-2, 36 14.605 2.528 2.90 3.178 0.204 49.337 3.812 290.49 12.051 0.24340R-2, 36 12.652 2.306 3.20 1.245 8.781 0.255 50.031 0.04440R-3, 114 13.657 4.990 3.20 1.339 7.708 0.160 50.434 13.379 3.679 0.008 2.335 0.36440R-3, 114 6.716 11.975 3.20 3.075 619.07 8.954 0.200 51.044 13.138 0.013 0.258 0.291 55.39840R-3, 114 6.680 3.20 3.221 652.54 8.247 13.548 2.118 0.022 0.224 0.283 48.09840R-3, 114 2.450 6.654 3.041 659.36 0.110 12.917 13.280 0.261 9.328 49.44651R-6, 67 2.993 6.932 3.162 10.50 659.36 1.148 10.912 0.201 5.409 0.018 14.903 661.64 0.202 49.201 1.476 2.783 6.426 3.018 12.10 10.848 0.175 13.320 5.862 0.000 0.122 12.358 661.64 49.116 2.731 12.50 0.408 10.756 0.253 13.399 7.835 0.050 97.653 3.17016R-1, 90 14.923 661.64 0.558 12.50 10.776 13.427 0.198 4.447 2.783 98.008 1.75116R-1, 90 12.70 661.64 0.793 45.012 0.051 2.480 0.000 10.625 0.143 13.264 0.217 6.966 96.906 17R-1, 146 2.395 14.065 12.70 0.000 10.089 0.648 27.815 73.278 2.305 771.37 0.227 0.207 97.617 2.319 13.956 12.70 0.721 11.089 12.816 71.006 0.056 2.365 0.124 0.219 0.043 13.572 12.70 13.748 70.738 2.352 6.12288R-1, 42 8.710 0.512 72.999 0.048 97.912 13.908 0.402 226.7 0.276 0.636 11.669 13.361 0.000 2.388 5.63189R-4, 49 17.40 0.197 70.758 1.062 2.290 0.014 97.716 1.412 0.065 226.7 0.248 13.813 15.851 6.79289R-4, 49 236.96 0.239 70.901 2.343 0.000 98.574 11.577 0.303 0.098 0.274 12.83589R-4, 49 0.377 0.195 10.416 70.502 0.194 0.227 12.451 2.746 0.000 97.593 12.99189R-4, 49 0.155 93.699 32.10 0.508 0.224 10.443 2.329 0.011 98.104 0.221 1118.22 59.942 11.965 12.409 5.26889R-4, 49 0.263 32.90 32.10 0.396 0.397 0.000 11.360 0.055 10.823 0.252 1132.39 11.791 99.180 5.16491R-2, 10 0.279 0.246 0.540 98.430 0.028 0.221 1132.39 11.712 5.267 2.47593R-2, 64 0.279 14.356 11.505 0.051 98.096 0.703 0.240 39.70 0.189 1132.39 48.809 4.828 2.67293R-2, 64 0.231 0.208 11.909 0.038 98.880 0.270 1.253 44.04 9.466 0.226 1132.39 43.119 44.187 0.002 2.121 0.292 0.221 11.749 99.724 1.234 5.032 3.400 44.04 9.313 1132.39 0.007 0.221 11.898 6.501 3.560 44.04 9.337 0.292 1148.1 0.223 98.833 1.501 0.279 0.000 98.630 53.759 5.886 0.467 44.04 8.656 0.346 1167.94 10.906 0.000 3.614 0.208 10.567 0.101 98.983 54.750 2.695 6.596 2.111 1.570 0.434 44.04 0.137 1167.94 3.377 0.968 9.495 0.146 98.951 59.404 2.728 0.156 11.329 3.384 11.153 47.90 0.072 99.308 59.800 2.732 0.035 48.30 10.345 0.061 0.489 0.336 8.705 99.087 70.839 2.764 0.000 1.044 0.189 48.30 11.496 0.137 10.689 12.306 99.687 5.194 0.169 0.563 0.967 59.861 0.007 0.101 99.888 0.029 2.700 0.068 0.768 0.563 12.155 2.423 0.117 53.499 0.093 0.760 0.559 0.299 98.184 61.268 2.665 0.076 11.964 98.438 9.671 14.703 13.485 0.621 0.473 0.388 0.034 61.358 2.531 12.368 0.116 0.348 1.151 0.006 1.241 0.131 0.145 0.532 0.000 1.852 0.000 7.335 1.039 0.345 98.310 0.148 3.886 1.905 0.050 7.649 0.955 0.437 11.777 0.910 0.113 99.172 10.643 0.376 0.179 0.020 3.506 0.921 1.623 0.331 1.787 0.000 97.930 2.923 0.480 8.154 97.133 12.798 1.750 0.000 3.603 0.390 0.738 14.083 0.015 12.397 1.754 3.597 14.591 0.436 4.410 0.111 0.001 3.218 12.201 11.881 10.384 0.007 0.495 5.275 1.030 98 152-918A- 152-918A- 152-918D- 152-914B- 152-918D-

76 TRACE AND RARE EARTH ELEMENT CHEMISTRY U (ppm) Total (wt%) Lu (ppm) 5 O Tm 2 (ppm) P (wt%) Er (ppm) 3 O 2 Cr (wt%) Ho (ppm) O 2 Dy K (ppm) (wt%) Tb (ppm) O 2 Na (wt%) Gd (ppm) Eu CaO (ppm) (wt%) Sm (ppm) MgO (wt%) Nd (ppm) Pr MnO (ppm) (wt%) Ce (ppm) Table 1 (continued). FeO (wt%) La (ppm) 3 O 2 Ba (ppm) Al (wt%) Nb (ppm) 2 TiO (wt%) Y (ppm) 2 Sr SiO (wt%) (ppm) Table 2. Trace and rare earth element concentrations for individual glass grains measured by ion probe. earth element concentrations for individual glass grains measured and rare 2. Trace Table Rb (ppm) Age (Ma) Zr (ppm) Age (Ma) Depth (mbsf) Depth (mbsf) interval (cm) Core, sample, 95R-3, 9496R-1, 2996R-1, 2996R-3, 120100R-2, 27 1184.34100R-2, 27 1185.39100R-3, 82 1185.39100R-3, 82 1189.3 50.60100R-3, 82 1215.77 51.00100R-3, 82 1215.77 51.00100R-3, 82 1217.82 51.40 52.00 1217.82 49.675 52.00 1217.82 43.453 52.00 1217.82 58.721 52.00 1217.82 54.901 49.034 52.00 0.172 48.196 52.00 1.231 53.867 52.00 0.539 60.518 2.305 0.234 58.418 9.046 11.313 0.544 57.667 1.222 56.795 3.985 13.682 0.391 14.082 1.001 9.659 17.942 17.835 0.671 15.942 16.227 0.466 17.936 9.528 17.583 15.665 17.826 0.221 0.272 18.193 7.169 17.696 0.052 9.544 0.018 0.052 9.603 15.956 0.000 9.517 8.381 0.018 8.161 2.596 0.033 3.387 2.843 11.153 0.077 11.961 2.366 0.011 1.806 0.000 0.087 2.363 0.377 0.786 1.216 2.278 1.234 0.547 2.158 0.203 2.075 0.070 0.196 0.139 0.454 0.163 0.281 1.000 0.849 0.059 0.085 0.124 4.667 0.563 1.171 2.493 0.325 0.094 0.000 1.325 0.181 1.612 0.033 0.015 0.952 0.007 0.028 0.059 1.907 0.000 0.039 1.420 0.000 1.434 0.066 0.000 97.645 0.108 0.040 97.923 0.055 0.113 0.028 87.025 0.000 0.000 85.623 87.629 0.041 89.640 0.020 0.007 81.924 0.000 92.537 89.399 88.776 86.784 152-918D- 2H-2, 342H-2, 342H-2, 343H-2, 15 3.643H-3, 112 3.643H-3, 112 3.64 0.044H-2, 48 12.95 15.42 0.044H-2, 48 252 15.42 0.045H-3, 103 0.13 155 0.165H-3, 103 22.78 310 0.16 1998H-2, 34 22.78 7.03 200 34.03 13.908H-2, 34 0.26 125 34.03 40.408H-2, 34 0.26 185 0.55 430 6.21 13210H-3, 2 60.64 5.19 0.55 475 13510H-3, 2 60.64 2.75 141 186 3111H-2, 60.64 1.5 39.6 218 158 18.3 1.5313H-6, 35 80.82 1.5 248 33.5 3.6414H-3, 0 80.82 1.5 2.84 251 21.8 89.11 34.2 10614H-3, 0 35.8 114.15 1.6 30.3 4.51 266 10918H-CC, 14 83.2 1.6 20.0 161 101.0 141 118.8 1.8 296.020H-3, 9 17.9 2.25 161.8 180 6.40 153 19.3 118.8 32.6 549.021H-2, 109 6.46 218 11.7 19.2 33.2 14221H-2, 109 123 2.3 29.4 27.7 203 84.7 106.0 2.56 172.89184.09 2.523X-5, 14 11.9 2.3 51.2 29.1 194 3.65 184.09 73.1 47.923X-5, 14 12.3 162 4.91 62.1 123 11.6 16.3 2.522.55 18.9 3.51 1.36 11323X-5, 67 205.94 287 41.3 166 57.9 152 13.5 2.5523X-5, 67 205.94 10.8 6.93 43.1 177 58.9 105355 8.01 183 158 57.5 39.9 44.523X-5, 122 206.47 2.6 29.8 13.7 1.69 323 25.2 27.90 11.2 68.6 206.47 2.6 31.7 4.29 25.4 31.0 27.0 207.12 31.9 11.70 9.9 2.63 5.65 6.21 11.6 38.2 170 7.98 396 218 12.8 27.4 26.5 96.9 10.30 26.7 2.63 12.1 57.6 200 101.0 231 11.9 2.65 3.8 3.62 165 143 6.13 181 24.1 29.1 25.8 27.8 17.1 181 52.0 139 10.0 18.6 3.87 24.7 29.6 9.91 40.5 9.48 227 8.9 19.6 60.0 20.90 2.33 27.5 16.8 1.56 3.74 3.89 5.87 7.68 54.1 85.2 2.94 20.7 169 61.2 45.8 18.4 9.8 4.4 77.7 2.61 43.3 12.00 185 44.8 47.2 3.76 8.4 12.2 6.90 11.6 3.92 7.32 132 16.7 19.3 2.12 2.35 31.0 16.6 431.0 10.40 5.97 11.2 1.810 4.73 131 25.7 25.7 6.9 6.25 37.0 187 43.6 30.6 20.8 9.8 0.962 38.1 10.90 69.7 152.0 1.66 4.37 4.95 9.43 10.80 1.450 40.5 32.1 53.2 134.0 125.0 28.1 30.0 1.60 3.76 30.9 16.2 4.64 1.470 5.5 31.5 4.69 23.3 19.2 9.9 12.5 31.9 1.470 38.8 5.54 1.63 1.60 7.87 2.27 112 5.69 13.1 25.5 4.23 6.83 9.03 16.6 9.1 8.31 8.99 99.2 141.0 22.1 15.2 0.94 3.64 0.962 8.90 24.330.5 72.7 1.60 27.3 5.39 7.87 1.57 6.51 24.7 13.9 1.370 63.6 56.7 21.0 2.72 4.13 20.9 2.34 18.622.8 1.64 10.3 5.56 5.86 62.2 3.39 18.4 1.67 3.834.45 1.070 1.210 5.62 0.907 96.2 4.49 6.99 8.42 12.1 7.75 5.42 9.95 60.3 1.68 2.61 9.95 0.493 5.39 45.553.3 1.98 0.892 1.14 1.180 11.0 5.21 6.49 6.66 4.87 18.521.3 46.5 0.688 19.5 2.48 1.74 10.7 1.690 28.6 30.6 1.75 0.453 1.630 5.85 0.855 0.338 11.7 6.427.13 10.7 9.83 0.678 0.731 7.47 3.26 25 1.79 1.44 1.34 4.135.34 12.90 10.00 46 0.447 5.80 0.815 4.1 0.980 7.28 7.96 8.90 3.70 0.689 0.945 1.380 3.53 0.449 30.729.7 1.37 1.880 7.67 4.75 4.17 3.49 1.542.04 1.78 0.741 0.294 3.36 1.240 15.50 6.06 0.298 6.14 2.41 0.462 1.86 7.80 11.10 17.7 1.18 1.090 7.046.56 18.80 0.739 0.608 4.26 0.881 6.156.90 5.14 1.950 17.8 11.30 7.80 0.154 1.27 29.4 5.37 0.760 0.685 1.66 2.00 2.380 4.44 4.77 0.554 7.43 0.176 2.132.25 1.0401.320 10.70 0.675 2.010 4.02 1.51 0.856 15.10 0.091 0.102 3.85 6.14 0.626 0.814 11.60 4.81 6.26 1.44 0.556 1.45 9.18 7.267.34 10.90 2.03 0.688 1.53 0.483 0.114 4.48 2.87 1.640 2.57 0.745 0.819 4.78 0.280 1.490 6.42 4.13 1.411.30 2.14 5.14 0.362 0.574 0.515 6.7910.10 0.584 0.935 9.34 8.83 0.85 1.050 0.554 9.38 0.731 0.025 4.414.25 5.47 0.466 0.776 0.267 1.440 1.060 1.320 0.597 1.97 7.05 2.49 0.773 1.91 0.4870.651 0.805 0.144 1.250 8.83 6.77 0.177 0.661 0.382 5.17 0.5810.549 0.704 1.35 4.29 0.567 1.77 1.40 0.376 0.893 0.127 0.260 0.229 0.821 4.05 0.218 0.913 4.62 4.24 0.733 0.622 0.325 0.765 0.670 0.428 0.727 0.758 0.679 0.124 0.226 0.162 interval (cm) Core, section 152-918A-

77 P.D. CLIFT, J.G. FITTON U (ppm) Lu (ppm) Tm (ppm) Er (ppm) Ho (ppm) Dy (ppm) Tb (ppm) Gd (ppm) Eu (ppm) Sm (ppm) Nd (ppm) Pr (ppm) Ce (ppm) Table 2 (continued). La (ppm) Ba (ppm) Nb (ppm) Y (ppm) Sr (ppm) Rb (ppm) Zr (ppm) Age (Ma) Depth (mbsf) 23X-5, 12224X-3, 128 207.1224X-3, 128 212.9825X-4, 109 212.98 2.6525X-7, 4 223.19 2.725X-7, 4 132 2.725X-7, 4 226.64 2.75 12533X-2, 59 226.64 22033X-2, 59 2.20 226.64 2.8 78.633X-2, 59 290.49 2.8 3.17 290.49 204 2.8 4.29 561 290.49 3.2 1.3639R-4, 14 154 357 3.239R-4, 14 183 291 3.2 23.4 16.60 205 78.340R-2, 36 652.54 12.00 14540R-2, 36 652.54 25.2 13 174 255 12.340R-2, 36 659.36 42.9 26.5 5.69 13 173 7.3040R-2, 36 659.36 11.4 13.5 6.0740R-3, 114 659.36 170 56.1 19.2 13.5 50.9 194 121 8.61 5.940R-3, 114 659.36 660.86 13.5 45.0 151 458 44.740R-3, 114 514 10.7 660.86 13.5 235 95.0 36.688R-1, 42 512 13.5 43.0 73.10 31.9 660.86 34.0 27.196R-1, 29 481 13.5 31.80 42.40 8.4 1118.22 33.0 176.0 26.4100R-2, 27 545 17.3 13.5 23.5 40.70 496 41.9 1185.39 39.3 145.0100R-3, 82 19.9 4.4 1225.47 42 37.60 488 42.3 70.1 21.6100R-3, 82 17.3 31.5 4.02 1227.52 51 118.0 42.5 45.50 231 52 20.1 70.3 42.00 29.4 23.9 1227.52 98.0 11.7 52 1020 101.0 68.6 68.7 37.80 3.12 55.9 68.7 16.3 22.52H-1, 105 5.76 52 119.0 56.6 52.50 58.5 17.2 54.1 61.7 15.42H-1, 106 1.72 301.0 68.5 12.1 56.4 34.7 41.7 58.8 43.7 14.02H-1, 106 9.11 4.2 53.1 27.4 470.0 169.0 22.6 62.2 10.26 57.1 44.9 42.22H-1, 108 8.06 64.6 36.9 243.0 45.00 232 10.26 323.0 99.1 39.8 28.6 8.8 51.9 4.48 44.8 19.70 7.03 7.25 10.26 28.8 313.0 0.15 49.6 52.4 1.42 65.7 12.1 46.6 31.7 18.00 5.13H-3, 85 10.26 36.0 305.0 0.15 50.6 39.4 3.73 3.24 37.0 1.33 197 20.14H-3, 115 7.97 341.0 0.15 33.2 2.64 41.1 50.9 7.26 337.0 4.81 6.24 59.0 2657H-4, 135 7.96 0.15 20.6 41.4 77.1 0.8 272.0 78.4 176.0 23.9 16.9 1.09 263 6.67H-4, 135 5.91 21.35 2.95 45.7 0.876 8.06 404.0 5.12 8.33 31.15 80.1 34.0 44.8 5.95 216 0.87H-4, 135 2.67 30.0 249.0 9.48 61.35 83.4 37.3 9.47 8.39 0.845 5.1 5.18 4.69 0.3 1.6 1.530 8.24 61.35 90.7 0.44 5.51 0.3 174 2.43 53.0 9.27 8.32 15.3 7.49 10.30 89.94H-3, 115 61.35 0.87 224 1.0 20.8 9.74 5.84 41.0 0.912 69.4 35.0 6.25 4.3 1.45 2.010 1975H-3, 99 10.20 0.87 9.82 114 222 2.10 10.7 9.22 1.20 1.790 78.3 10.9 5.5 2.89 40.4 2.55 2606H-1, 43 0.87 103.65 35.6 175 63.9 9.95 5.43 39.9 10.80 8.57 7.58 1.15 1.650 265 10.6 2.66H-1, 43 6.28 36.3 2.01 7.43 12.9 36.0 9.69 112.99 3.24 2.94 7.73 1.69 5.93 2846H-1, 43 9.55 44.8 1.47 36.0 0.1 1.2 16.8 118.93 4.13 9.26 2.50 2.52 1.25 7.39 1.0706H-6, 87 33.3 8.84 2.65 3.37 39.3 1.340 31.5 0.441 118 118.93 1.6 4.31 1.3 172 0.1 8.89 2.44 7.55 1.6206H-6, 87 1.82 4.70 9.37 54.9 31.2 80.9 118.93 1.69 14.00 212 82.3 6.35 13.107H-1, 37 0.439 0.57 2.81 17.6 7.33 2.52 4.60 3.17 132.0 0.503 6.99 2.04 22.4 0.934 8.04 126.87 1.69 217 11.30 314 0.664 12.3 9.27 2.21 5.83 131.0 21.1 278 2.430 4.49 0.54 126.87 2.41 1.69 8.22 35.0 230 0.17 0.32 1.960 0.443 12.10 15.1 1.070 0.599 0.210 0.830 3.54 1.47 25.6 221 128.37 1.59 1.8 5.41 35.1 94.2 1.980 1.56 13.30 0.838 3.76 14.00 0.64 0.12 1.82 12.70 25.4 217 120 0.191 1.8 35.8 2.060 0.608 0.507 2.95 9.97 1.23 17.2 0.151 7.8 36.7 8.44 12.80 1.000 1.1 1.7 1.11 1.8 4.10 37.6 2.250 0.697 58.3 8.75 17.3 28.4 241 4.45 2.230 6.44 12.30 0.91 3.42 0.736 0.061 18.00 4.81 57.8 251 241 29.0 4.0 0.6 2.65 80.4 187 0.785 11.30 6.31 13.90 5.09 0.679 20.8 64.4 0.439 2.77 1.600 0.4 124.0 0.41 15.30 275 34.1 0.687 7.95 43 150 2.230 0.710 2.69 2.88 9.49 126.0 7.06 7.89 0.450 1.250 147 0.715 1.8 0.17 8.12 14.8 3.26 40.6 0.579 143.0 24.5 7.06 9.49 8.41 24.4 0.09 15.70 3.30 44.3 0.621 7.2 8.3 36.0 0.19 0.224 1.710 184 8.17 7.58 24.9 0.154 5.49 36.8 35.7 1.250 6.88 0.221 0.05 184 37.9 1.310 10.40 8.64 27.4 38.5 6.56 0.430 0.75 1.87 36.1 57.4 0.46 0.502 1.710 2.85 23.4 8.84 16.6 88.5 229 0.92 1.200 1.440 58.6 17.1 8.78 26.8 1.450 1.38 38.9 159.0 1.450 1.520 0.29 4.88 2.340 1.090 0.038 2.07 56.4 121.0 3.54 6.23 16.8 7.49 1.150 0.278 8.49 38.9 2.32 2.48 1.550 8.0 2.190 7.65 1.820 1.770 34.7 0.073 6.98 2.47 92.3 4.13 19.3 30.0 0.833 1.040 24.6 0.17 1.360 8.11 21.0 8.10 1.370 12.30 90.4 2.33 19.3 11.6 3.2 2.450 2.010 10.70 17.6 0.679 33.0 0.19 33.1 1.85 16.8 1.150 98.0 1.460 68.1 5.62 1.170 1.610 6.59 0.04 38.3 51.2 16.5 0.40 98.0 2.73 1.490 7.53 0.841 136.0 2.35 1.200 7.69 1.860 0.07 9.75 40.4 18.6 9.23 8.31 9.03 1.40 1.93 0.11 8.18 7.56 39.6 0.789 1.61 18.6 1.09 8.61 2.39 25.6 10.5 1.480 2.44 0.15 5.21 0.009 44.4 40.2 1.69 1.66 4.06 9.35 0.295 2.58 33.7 5.1 44.4 4.02 1.63 8.51 0.013 59.8 7.70 3.25 0.028 9.42 0.714 24.5 1.200 6.32 0.439 8.63 10.20 4.14 5.07 7.37 0.605 1.540 6.32 0.007 4.98 0.022 24.0 1.620 7.53 0.792 1.90 0.88 0.000 1.400 0.719 0.712 6.87 8.53 28.2 2.36 0.000 2.46 4.75 0.693 7.85 0.000 28.2 6.73 8.18 0.506 0.662 33.6 5.38 3.52 12.50 8.57 1.96 1.56 7.94 10.00 0.637 0.96 1.37 7.94 0.238 0.478 0.820 0.526 1.92 1.41 2.060 8.66 10.00 1.29 1.640 0.486 4.77 2.64 2.89 0.710 4.50 2.64 3.40 1.350 9.34 9.20 4.62 10.50 2.10 0.682 3.85 8.95 0.346 0.416 0.586 1.280 0.606 8.95 11.70 8.26 0.78 0.729 1.89 1.86 0.541 1.440 0.410 0.427 0.449 1.460 1.440 0.169 7.97 0.750 1.74 2.5 interval (cm) Core, section 152-918D- 152-919B- 152-919A- 152-919B-

78 TRACE AND RARE EARTH ELEMENT CHEMISTRY

Table 3. Representative analysis of a basaltic glass shard from Sample MORB (N-MORB), which typically show relative depletion in these 152-918A-8H-2, 34 cm, showing uncertainties for typical chemical com- elements compared to the less incompatible elements on the right of position. the diagram (Fig. 8A). A flat or incompatible enriched trace element pattern on the spider diagram is seen for Icelandic basalts and may Concentration be produced by either extracting a small degree (<1%) of partial melt Element (ppm) % Error from a normal asthenospheric source, or by large degrees of melting Rb 6.4 2.98 of a less depleted source. Figure 8A shows a basalt from the modern Sr 203 0.52 volcano at Krafla in the eastern rift zone of Iceland (Hemond et al., Y41.30.94 Zr 251 0.67 1993), whose form can be seen to be similar to that of more depleted Nb 25.2 2.00 Pleistocene ashes at Site 918. This similarity of trace element chem- Ba 96.9 1.27 La 18.6 2.33 istry supports the Icelandic origin of the ashes. Since tholeiites are Ce 45.8 1.57 generated by moderate to large degrees of partial melting, the incom- Pr 5.97 3.88 Nd 30 4.93 patible-enriched characteristics suggest that the drill core ashes and Sm 6.83 9.33 modern Icelandic lavas are derived from a plume mantle source. Eu 2.72 5.14 The middle Miocene spider diagrams in Figure 8C show greater Gd 9.95 4.23 Tb 1.69 4.77 enrichment for most elements compared to primitive mantle than the Dy 10 4.78 younger examples shown. This is especially true for the most incom- Ho 1.78 5.42 Er 5.14 6.64 patible elements. This does not reflect more incompatible-enriched Tm 0.675 8.61 volcanism at that time, but instead a concentration of incompatible el- Lu 0.556 9.49 U0.2831.60 ements by fractional crystallization during the formation of these rhy- olites. All younger glasses plotted were from basaltic shards and did not show the effects of this process. Fractional crystallization also ex- 1000 plains the large degrees of depletion seen in Sr, P, and Ti relative to the other elements. Sr, P, and Ti are removed respectively by the crystallization of plagioclase, apatite, and spinel prior to eruption. The oldest Eocene glasses from Site 918 show different trace patterns that are noticeably flatter and incompatible-element depleted than the younger grains. On the left hand side of the diagram (Fig. 8D), the stronger hydration of these grains is shown by the strong en- 100 richment of water mobile elements. Sample 152-918D-100R-3, 82 cm, in particular shows a clear depletion in the more incompatible el- Zr (ppm) ements compared to the less incompatible elements on the right of the diagram. The slope to the spider diagram is typical of lavas derived by melting from a depleted mantle source, and is similar to those plotted for the basalts on which these sediments lie. 10 1 10 100 Rare Earth Elements Nb (ppm) Rare earth elements are often considered in isolation from the oth- Figure 6. Variations in the concentrations of Zr with Nb contents from basal- er trace elements and are shown arranged in order of increasing atom- tic glass grains at Sites 918 and 919. Note the correlation and inferred high ic number (Fig. 10). In this diagram, it is conventional to normalize analytical precision at all measured concentrations. Site 918 = solid circles concentrations against a chondritic source, with the values of Sun and and Site 919 = open circles. McDonough (1989) being used in this study. It is noteworthy that in all the examples shown in Figure 10 the general slope of the curve shows an enrichment in light REE (LREE) relative to heavy REE F (HREE). The relative HREE depletion may indicate melting in the presence of garnet, which preferentially retains the HREE. Again, the Miocene rhyolitic glasses shown in Figure 10C show greater enrichment relative to the normalizing standard for all elements, due to the effects of fractional crystallization. The relative Eu depletion is char- acteristic of plagioclase fractionation. Nonetheless, the generally incompatible nature of the REE means that while total concentrations rise during crystallization, the slope of the line remains largely un- changed. In the basaltic examples, we see that while the HREE show concentrations of 20× to 30× chondrite, the degree of LREE enrich- tholeiitic ment over HREE is variable over short time spans. The general rela- calc-alkaline tively high incompatibility of the REE means that the slope on the REE diagram may be representative of the source mantle if the degree of partial melting remains constant. The REE diagrams suggest either short-term variability of the mantle source of the ashes or variation in the degree of melting.

Temporal Evolution A M The temporal variability of elemental enrichment and depletion Figure 7. Triangular alkali/iron/magnesium (AFM) diagram showing the vast can be charted by plotting key element ratios against age for both majority of glass grains from Sites 918 (solid circles) and 919 (open circles) Sites 918 and 919. Enrichment of incompatible elements in both the falling within the tholeiite field, as defined by Irvine and Baragar (1971). high field strength and REE can be modeled using Zr/Nb and Ho/La,

79 P.D. CLIFT, J.G. FITTON

A C 1000 1000 918A-2H-2, 34 cm 918D-39R-4, 14 cm 918A-4H-2, 48 cm 918D-40R-2, 36 cm 919A-8H-2, 34 cm 918D-40R-3, 114 cm Krafla N-MORB

100 100

10 10 Glass/Primitive Mantle Glass/Primitive Mantle

1 1 Ba Rb K Nb La Ce Sr Nd P Sm Zr Ti Tb Y Tm Ba Rb K Nb La Ce Sr Nd P Sm Zr Ti Tb Y Tm B D 1000 918A-24X-3, 128 cm 1000 918D-88R-1, 42 cm 918A-14H-3, 0 cm 918D-100R-3, 82 cm 918A-33X-2, 59 cm

100 100

10 1 Glass/Primitive Mantle Glass/Primitive Mantle

1 .1 Ba Rb K Nb La Ce Sr Nd P Sm Zr Ti Tb Y Tm Ba Rb K Nb La Ce Sr Nd P Sm Zr Ti Tb Y Tm

Figure 8. Spider diagrams of basaltic glass grains from (A) Pleistocene at Site 918, including N-MORB (Sun and McDonough, 1989) and a basalt from Krafla in the eastern rift zone of modern Iceland (Hemond et al., 1993); (B) Pliocene at Site 918; (C) Miocene at Site 918; and (D) Oligocene and Eocene at Site 918 (Thompson et al., 1983). respectively. These ratios reflect source characteristics and the de- insula. The Tertiary basalt pile is composed of flows large enough to gree of partial melting, but are unaffected by fractional crystallization have escaped the axial rift zone and therefore avoid burial deep in the in basalts. Figure 11 shows the spread of values measured for the post crust (Hardarson and Fitton, 1994). These large flows probably pro- 15 Ma ashes. For comparison, the values for typical N-MORB and vide a good estimate of the average composition of magma erupted ocean island basalt (OIB) are shown, as well as the range for Icelan- within the rift zone. The Snaefellsnes lavas, by contrast, were erupted dic basalts from the Tertiary lava pile and the neovolcanic zone (Har- off-axis. It is noteworthy that all the ash samples from Sites 918 and darson and Fitton, 1994). It is clear that for the most part the compo- 919 plot within the Iceland array and that most overlap the field of sition of the ashes corresponds to that of the Icelandic Tertiary lava Tertiary basalt. Three samples have higher Nb/Y and resemble basalt sequences and is broadly comparable with that of a typical OIB. from Snaefellsnes. No other neovolcanic rocks from Iceland plot in However, a considerable number of analyses show higher Zr/Nb than this part of the diagram. We conclude that the bulk of the recovered in Icelandic Tertiary basalts, although the range is not as large as in ashes were erupted from the Iceland rift zone, with two samples (152- the neovolcanic zone. 918A-2H-2, 34 cm, and 152-918A-18H-CC, 14 cm) probably erupt- Figure 12 shows the composition of Neogene ashes at Sites 918 ed from an off-axis volcano, possibly Snaefelles itself. and 919 compared with basaltic lavas from Iceland. In this diagram the value of Y used was recalculated from the Ho content because the analyzed Y/Ho values were consistently lower than the 27.7 reported DISCUSSION by McDonough and Sun (1995). Since the ratio of these elements is very stable due to their very similar compatibility, the lower than ex- The Neogene–Holocene ashes recovered from Sites 918 and 919 pected Y values suggest a consistent analytical shortfall using the ion show a predominantly basaltic character over the past 3.5 m.y. The probe for reasons that are not apparent. The parallel lines on the dia- Icelandic origin of this material is effectively demonstrated by their gram represent the limit of compositions of basalt from the Icelandic tholeiitic character, which rules out the alternative of Jan Mayen Is- neovolcanic zone (Fitton, Saunders, et al., this volume). The + sym- land. The middle Miocene ashes that have survived diagenesis are bol represents the composition of primitive mantle (McDonough and also considered to be Icelandic on the basis of their low alkali con- Sun, 1995), which plots within the Iceland array. Depleted Icelandic tent. The patchy nature of the record prior to 3.5 Ma is presumably basalt plots below and to the left of this point, but most Icelandic la- due to diagenetic alteration of volcanic glass to clay in older sedi- vas plot above and to the right. N-MORB plot below the lower bound ments, except in rare circumstances in which the grains were pre- of the array because they are more depleted in Nb with respect to oth- served by early cementation or by incorporation into impermeable er incompatible elements than are even the most depleted basalts layers. The record preserved is probably not a very complete one giv- from Iceland. en the position of Sites 918 and 919 upwind of the source volcanoes Individual data points are shown for Tertiary basalt from East Ice- in Iceland. This is thus not a good place to test models suggesting an land and for Pleistocene–Holocene basalt from the Snaefellsnes Pen- episodicity to Icelandic volcanism.

80 TRACE AND RARE EARTH ELEMENT CHEMISTRY

A 1000 919A-3H-3, 85 cm 919A-7H-4, 135 cm 919B-2H-1, 105 cm

100

10 Glass/Primitive Mantle

1 Ba Rb K Nb La Ce Sr Nd P Sm Zr Ti Tb Y Tm B 1000 919B-6H-1, 43 cm 919B-6H-1, 43 cm 919B-6H-6, 87 cm

100

10 Glass/Primitive Mantle

Figure 9. Spider diagrams of basaltic glass grains from 1 (A) Pleistocene at Site 919 and (B) Pliocene at Site Ba Rb K Nb La Ce Sr Nd P Sm Zr Ti Tb Y Tm 919.

In contrast to the East Greenland sediments, Bitschene et al. rare earth element depletion could represent melt extraction from (1989) found a dominance of rhyolitic shards in Neogene–Holocene variably depleted parts of the upwelling asthenosphere, as Hemond et sediments on the Vøring Plateau. In this area Iceland was considered al. (1993) were able to demonstrate using Sr and Nd isotopes. How- to be the principal source for the Neogene ashes. The acidic tephra ever, given that melts are being extracted from a long column of as- were sourced from both Iceland (c.f. Walker, 1966) and Jan Mayen thenosphere at any one time and mixed to a variable degree with Island, and were distinguished by their alkali contents (Sylvester, melts from other depths before eruption (McKenzie and Bickle, 1978; Maaløe et al., 1986). In the case of Sites 918 and 919, the glass- 1988), it is possible to generate the observed variability in depletion es are all tholeiitic and thus of a purely Icelandic origin. Volcanism by erupting melts extracted from different depths, while keeping the in East Greenland is believed to have continued until the early Mio- general composition of the plume mantle constant. In this case the cene (Gleadow and Brooks, 1979), but cannot have been the source least depleted rocks would be derived from melts mixed from the for the Neogene ashes analyzed here. length of the melting column. The more depleted rocks could repre- The record from Sites 918 and 919 shows that there is no long- sent melt extractions from just the higher parts of the column, from term variability in the relative depletion of Icelandic volcanism and mantle that has already undergone melt extraction at lower levels. that over short periods of time liquids of both more and less depleted Elliot et al. (1991) have demonstrated such variability from lavas character were being erupted. The variability in high field strength el- from the Reykjanes Peninsula and from Theistareykir in northern Ice- ements enrichment at approximately constant LREE enrichments in- land. However, the most depleted Icelandic lavas form small-volume dicates that the degree of partial melting is variable. The degree of flows confined to the rift axis. These will not be preserved in the ob- variability in Zr/Nb is not as high as seen in the modern neovolcanic servable part of the Tertiary lava pile, nor are they likely to contribute zones (c.f. Nicholson, 1990; Furman et al., 1991; Elliot et al., 1991), to the ashes studied here. but is more comparable to the variability found in the Tertiary lava pile (Hardarson and Fitton, 1994). This is compatible with the predic- tions of the Pálmason (1981; 1986) spreading model, which would ACKNOWLEDGMENTS predict the deep burial of small volume flows erupted close to the rift axis. Exactly what the variability in the degree of enrichment repre- PC would like to thank John Craven, Richard Hinton, and Stuart sents is uncertain without isotopic data. Such variations in trace and Kearns at the University of Edinburgh for their hospitality and pro-

81 P.D. CLIFT, J.G. FITTON

A 1000 918A-4H-2, 48 cm 918A-2H-2, 34 cm 918A-8H-2, 34 cm

100

10 Glass / Chondrite

1 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Y Er TmYb Lu B 1000 918A-14H-3, 0 cm 918A-24X-2, 128 cm 918A-33X-2, 59 cm

100

10 Glass / Chondrite

1 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Y Er TmYb Lu C 1000 918D-39R-4, 14 cm 918D-40R-2, 36 cm 918D-40R-3, 114 cm

100

Glass / Chondrite 10

Figure 10. Chondrite normalized REE diagrams for basaltic grains from Site 918: (A) Pleistocene, (B) 1 Pliocene, and (C) Miocene. La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Y Er TmYb Lu fessional advice during his visit to use the ion microprobe. The Ocean Taylor, E., et al., Proc. ODP, Sci. Results, 104: College Station, TX Drilling Program is thanked for providing both support in the sedi- (Ocean Drilling Program), 357−366. mentology laboratory and computer equipment to write up this work. Bouma, A.H., 1962. Sedimentology of Some Flysch Deposits: A Graphic JOI/USSAC are thanked for their financial support of this project. Approach to Facies Interpretation: Amsterdam (Elsevier). Laura Dancer, Mohit Jain, and Joel Prellop are thanked for their help Burnham, C.W., and Jahns, R.H., 1962. A method of determining the solubil- ity of water in silicate melts. Am. J. Sci., 260:721−745. in the sample preparation. Cande, S.C., and Kent, D.V., 1992. A new geomagnetic polarity time scale for the Late Cretaceous and Cenozoic. J. Geophys. Res., 97:13917− REFERENCES 13951. Clift, P.D., and Dixon, J.E., 1994. Variations in arc volcanism and sedimentation related to rifting of the Lau Basin (southwest Pacific). In Hawkins, Bitschene, P.R., Schmincke, H.-U., and Viereck, L., 1989. Cenozoic ash lay- J., Parson, L., Allan, J., et al., Proc. ODP, Sci. Results, 135: College Sta- ers on the Vøring Plateau (ODP Leg 104). In Eldholm, O., Thiede, J., tion, TX (Ocean Drilling Program), 23−49.

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Desprairies, A., Maury, R.C., Joron, J.-L., Bohn, M., and Tremblay, P., 1989. Nicholson, H., Condomines, M., Fitton, J.G., Fallick, A.E., Grönvold, K., Distribution, chemical characteristics, and origin of ash layers from ODP and Rogers, G., 1990. Geochemical and isotopic evidence for crustal Leg 104, Vøring Plateau, North Atlantic. In Eldholm, O., Thiede, J., Tay- assimilation beneath Krafla, Iceland. J. Petrol., 32:1005−1020. lor, E., et al., Proc. ODP, Sci. Results, 104: College Station, TX (Ocean Pálmason, G., 1981. Crustal rifting and related thermo-mechanical processes Drilling Program), 337−356. in the lithosphere beneath Iceland. Geol. Rundsch., 70:244−260. Donn, W.L., and Ninokovich, D., 1980. Rate of Cenozoic explosive volcan- ————, 1986. Model of crustal formation in Iceland, and application to ism in the North Atlantic inferred from deep sea core. J. Geophys. Res., submarine mid-ocean ridges. In Vogt, P.R., and Tucholke, B.E. (Eds.), 85:5455−5460. The Western North Atlantic Region. Geol. Soc. Am., Geol. of North Am. Elliot, T.R., Hawkesworth, C.J., and Grönvold, K., 1991. Dynamic melting Ser., M:87−98. of the Iceland plume. Nature, 351:201−206. Pearce, J.A., 1983. The role of sub-continental lithosphere in magma genesis Furman, T., Frey, F.A., and Park, K.H., 1991. Chemical constraints on the at destructive plate margin. In Hawkesworth, C.J., and Norry, M.J. petrogenesis of mildly alkaline lavas from Vestmannaeyjar, Iceland; the (Eds.), Continental Basalts and Mantle Xenoliths: Nantwich (Shiva), Eldfell (1973) and Surtsey (1963−1967) eruptions. Contrib. Mineral. 230−249. Petrol., 109:19−37. Schilling, J.-G., 1973. Iceland mantle plume: geochemical study of Reyk- Garcia, M.O., and Hull, D.M., 1994. Turbidites from giant Hawaiian land- janes Ridge. Nature, 242:565−571. slides: results from Ocean Drilling Program Site 842. Geology, 22:159− Schmincke, H.-U., Viereck, L.G., Griffin, B.J., and Pritchard, R.G., 1982. 162. Volcaniclastic rocks of the Reydarfjordur Drill Hole, eastern Iceland. 1: Gleadow, A.J.W., and Brooks, C.K., 1979. Fission track dating, thermal his- Primary features. J. Geophys. Res., 87:6437−6458. tories and tectonics of igneous intrusions in East Greenland. Contrib. Shackleton, N.J., Backman, J., Zimmerman, H., Kent, D.V., Hall, M.A., Rob- Mineral. Petrol., 71:45−60. erts, D.G., Schnitker, D., Baldauf, J.G., Desprairies, A., Homrighausen, Hardarson, B.S., and Fitton, J.G., 1994. Geochemical variation in the Tertiary R., Huddlestun, P., Keene, J.B., Kaltenback, A.J., Krumsiek, K.A.O., basalts of Iceland. Mineral. Mag., 58A:372−373. Morton, A.C., Murray, J.W., and Westberg-Smith, J., 1984. Oxygen iso- Hart, S.R., Schilling, J.-G., and Powell, J.L., 1973. Basalts from Iceland and tope calibration of the onset of ice-rafting and history of glaciation in the along the Reykjanes Ridge: Sr isotope geochemistry. Nature, 246:104− North Atlantic region. Nature, 307:620−623. 107. Shipboard Scientific Party, 1994a. Site 917. In Larsen, H.C., Saunders, A.D., Hay, R.L., and Iijima, A., 1968. Nature and origin of palagonite tuffs of the Clift, P.D., et al., Proc. ODP, Init. Repts., 152: College Station, TX Honolulu Group on Oahu, Hawaii. In Coats, R.R., Hay, R.L., and Ander- (Ocean Drilling Program), 107−158. son, C.A. (Eds.), Studies in Volcanology. Mem.—Geol. Soc. Am., ————, 1994b. Site 918. In Larsen, H.C., Saunders, A.D., Clift, P.D., et 116:331−376. al., Proc. ODP, Init. Repts., 152: College Station, TX (Ocean Drilling Hemond, C., Arndt, N.T., Lichtenstein, U., Hoffman, A.W., Oskarsson, N., Program), 177−256. and Steintthorsson, S., 1993. The heterogeneous Iceland plume: Nd-Sr-O ————, 1994c. Site 919. In Larsen, H.C., Saunders, A.D., Clift, P.D., et isotopes and trace element constraints. J. Geophys. Res., 98:15833− al., Proc. ODP, Init. Repts., 152: College Station, TX (Ocean Drilling 15850. Program), 257−277. Hinton, R.W., and Upton, B.G.J., 1991. The chemistry of Zircon: variations Sigurdsson, H., and Loebner, B., 1981. Deep-sea record of Cenozoic explo- within and between large crystals from syenite and alkali basalt xeno- sive volcanism in the North Atlantic. In Self, S., and Sparks, R.S.J. liths. Geochim. Cosmochim. Acta, 55:3287−3302. (Eds.), Tephra Studies: Dordrecht (Reidel), 289−316. Irvine, T.N., and Baragar, W.R.A., 1971. A guide to the chemical classifica- Sun, S.-S., 1980. Lead isotopic study of young volcanic rocks from mid- tion of the common volcanic rocks. Can. J. Earth Sci., 8:523−548. ocean ridges, ocean islands and island arcs. Philos. Trans. R. Soc. London Kennett, J.P., 1981. Marine tephrochronology. In Emiliani, C. (Ed.), The Sea A, 297:409−445. (Vol. 7): The Oceanic Lithosphere: New York (Wiley), 1373−1436. Sun, S.-S., and McDonough, W.F., 1989. Chemical and isotopic systematics Klein, E.M., and Langmuir, C.H., 1987. Global correlations of ocean ridge of oceanic basalts: implications for mantle composition and processes. In basalt chemistry with axial depth and crustal thickness. J. Geophys. Res., Saunders, A.D., and Norry, M.J. (Eds.), Magmatism in the Ocean Basins. 92:8089−8115. Geol. Soc. Spec. Publ. London, 42:313−345. Larsen, H.C., Saunders, A.D., Clift, P.D., et al., 1994. Proc. ODP, Init. Sylvester, A.G., 1978. Petrography of volcanic ashes in deep-sea cores near Repts., 152: College Station, TX (Ocean Drilling Program). Jan-Mayen Island: Sites 338, 345−350, DSDP Leg 38. In Talwani, M., Maaløe, S., Sorensen, I., and Hertogen, J., 1986. The trachybasaltic suite of Udintsev, G., et al., Init. Repts. DSDP, Suppl. to 38, 39, 40, 41: Washing- Jan Mayen. J. Petrol., 27:439−466. ton (U.S. Govt. Printing Office), 101−109. MacDonald, R., McGarvie, D.W., Pinkerton, H., Smith, R.L., and Palacz, Thompson, R.N., Morrison, M.A., Dickin, A.P., and Hendry, G.L., 1983. Z.A., 1990. Petrogenetic evolution of the Torfajökull volcanic complex, Continental flood basalts...arachnids rule OK? In Hawkesworth, C.J., and Iceland, I. Relationship between the magma types. J. Petrol., 31:2:429− Norry, M.J. (Eds.), Continental Basalts and Mantle Xenoliths: Nantwich, 459. UK (Shiva), 158−185. McDonough, W.F., and Sun, S.-S., 1995. The composition of the Earth. Walker, G.P.L., 1966. Acid volcanic rocks in Iceland. Bull. Volcanol., Chem. Geol., 120:223−253. 29:375−406. McKenzie, D., and Bickle, M.J., 1988. The volume and composition of melt Walker, G.P.L., and Croasdale, R., 1972. Characteristics of some basaltic generated by extension of the lithosphere. J. Petrol., 29:625−679. pyroclastics. Bull. Volcanol., 35:303−317. Mertz, D., Devey, C., Todt, W., Stoffers, P., and Hofmann, A., 1991. Sr-Nd- Zindler, A., Hart, S.R., Frey, F.A., and Jakobsson, S.P., 1979. Nd and Sr iso- Pb isotope evidence against plume-asthenosphere mixing north of Ice- tope ratios and REE abundances in Reykjanes Peninsula basalts: evi- land. Earth Planet. Sci. Lett., 107:243−255. dence for mantle heterogeneity beneath Iceland. Earth Planet. Sci. Lett., Moberly, R., and Campbell, J.F., 1984. Hawaiian hotspot volcanism mainly 45:249−262. during geomagnetic normal intervals. Geology, 12:459−63. Neilson, C.H., and Sigurdsson, H., 1981. Quantitative methods for electron microprobe analysis of sodium in natural and synthetic glasses. Am. Min- eral., 66:547−552. Date of initial receipt: 21 August 1995 Nicholson, H., 1990. The magmatic evolution of Krafla, NE Iceland [Ph.D. Date of acceptance: 3 July 1996 thesis]. Univ. of Edinburgh. Ms 152SR-203

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A OIB N-MORB B OIB N-MORB

Neovolcanic Zone 0 0

2 2

4 4

6 6

8 8 Age (Ma)

Range of Icelandic basalts Range of Icelandic basalts 10 10

12 12

14 14

0 5 10 15 20 25 30 35 40 0 0.1 0.2 0.3 0.4 0.5 Zr/Nb Ho/La Figure 11. Diagram showing the variability in (A) Zr/Nb and (B) Ho/La with time at Sites 918 and 919. Analyses with MgO >4% only. Icelandic values from Zindler et al. (1979), Nicholson (1990), MacDonald et al. (1990), Nicholson et al. (1991), and Hemond et al. (1993).

1 Icelandic compositions Nb/Y 0.1 MORB

0.01 110 Zr/Y Figure 12. Variations in Nb/Y vs. Zr/Y (Fitton, Saunders, et al., this volume) showing the range of values for Neogene ashes from Leg 152 drill sites (gray circles), the Icelandic Tertiary lavas (small black squares), and the Snaefelles volcano of western Iceland (small crosses) for comparision. The two parallel lines represent the range of values known from the Icelandic system, within which the ashes fall. The large cross represents primitive mantle (Sun and McDonough, 1989).