G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Article Volume 6, Number 9 10 September 2005 Q09G19, doi: /2004gc ISSN: Alteration of hyaloclastites in the HSDP 2 Phase 1 Drill Core: 2. Mass balance of the conversion of sideromelane to palagonite and chabazite Anthony W. Walton Department of Geology, University of Kansas, 120 Lindley Hall, 1475 Jayhawk Boulevard, Lawrence, Kansas 66045, USA (twalton@ku.edu) Peter Schiffman Department of Geology, University of California, Davis, One Shields Boulevard, Davis, California , USA G. L. Macpherson Department of Geology, University of Kansas, 120 Lindley Hall, 1475 Jayhawk Boulevard, Lawrence, Kansas 66045, USA [1] The Hawaii Scientific Drilling Project 2 Phase 1 core permits study of each stage of alteration of basalt glass during burial because stages of the process are separated vertically. Previous work has shown that alteration of hyaloclastite occurs progressively. The latest stage observed in the Phase 1 core involves marginal replacement of sideromelane in shards with palagonite while simultaneously forming chabazite in pores. The basic reaction at this stage is sideromelane + components from pore waters = palagonite + chabazite + components to pore waters. Mass balance calculations show that Fe was virtually immobile in this process, being retained in palagonite. Na, Ca, Ba, P, Al, and Si were lost during palagonitization and not fully consumed in making chabazite. Mg was lost during palagonitization but retained elsewhere in smectite. K, Rb, and Sr were extracted from pore waters and enriched in the sum of the alteration products. The amount of enrichment depended upon the amount of chabazite present, which depended upon the porosity when chabazite formed. Ti, Y, U, Zr, Nb, REE, and Th were enriched in palagonite, compared to sideromelane, but were absent in chabazite. Mass balance of all phases for the entire alteration process (including earlier stages) was not possible because poorly consolidated samples do not yield accurate modal values of phases, trace element analysis of smectite was not possible, and exchange with lavas and intrusions in the succession cannot be evaluated. Calculations indicate that too little of major oxides, except Na 2 O, was released during palagonitization to account for the amount of smectite observed in hyaloclastites. The results of this study, and several others published in the literature, indicate that under various circumstances palagonitization will consume particular elements from pore fluid or release them to it. Such mobility implies that the hydrology of the particular system and the composition of the dissolved solids in the pore water will control whether palagonitization is a source or sink of elements. The potential exists that palagonitization of basalt glass is an important source or sink of elements for seawater and fluids circulating in the ocean crust. Components: 16,596 words, 4 figures, 12 tables. Keywords: basalt glass alteration; Hawaii Scientific Drilling Program; mass balance; palagonitization. Index Terms: 1039 Geochemistry: Alteration and weathering processes (3617); 8404 Volcanology: Volcanoclastic deposits. Received 21 December 2004; Revised 8 June 2005; Accepted 21 June 2005; Published 10 September Copyright 2005 by the American Geophysical Union 1 of 27

2 Walton, A. W., P. Schiffman, and G. L. Macpherson (2005), Alteration of hyaloclastites in the HSDP 2 Phase 1 Drill Core: 2. Mass balance of the conversion of sideromelane to palagonite and chabazite, Geochem. Geophys. Geosyst., 6, Q09G19, doi: /2004gc Theme: Hawaii Scientific Drilling Project Guest Editors: Don DePaolo, Ed Stolper, and Don Thomas 1. Introduction 1.1. Hyaloclastites From the Hawaii Scientific Drilling Project 2 Phase 1 Drill Core [2] The Hawaii Scientific Drilling Project 2 collected a nearly complete core to a depth of just over 3 km during its first phase (HSDP 2 1 ). The lower two thirds of this well, below 1079 mbsl, sampled rocks that formed from magmas produced by the Mauna Kea eruptive center and accumulated below sea level. Of these submarine rocks, approximately 1/2 are hyaloclastites [HSDP, 2000], composed primarily of glass, with phenocrysts of olivine and opaque minerals of the spinel group plus microlites of olivine, pyroxene, and plagioclase. HSDP hyaloclastites are interpreted to have formed where lava flowed into the ocean, chilled in the sea to form glass, and fragmented into chunks of fine ash to block size. Most of the rest of the rocks from the submarine interval in the core are pillow lavas, but massive and intrusive basalts are other significant rock types [HSDP, 2000] Alteration of Hyaloclastites in HSDP Drill Core 2 [3] The purposes of this paper are to investigate the mass balance of the palagonitization process affecting hyaloclastites and to assess its potential in controlling mass fluxes within oceanic island volcanoes such as Mauna Kea. Glass fragments in the HSDP hyaloclastites were unstable when formed and have undergone alteration in a series of steps [Walton and Schiffman, 2003]. Glass grains were first fractured, probably from the stresses of shallow burial which may have reactivated fractures induced upon quenching, and then their surfaces were coated with a thin layer of smectite. At this point, microbes infected the glass to form tubules into it. Simultaneously, the glass in some shards was dissolved, but probably not enough dissolution occurred to provide material to form the smectite observed in the rock. In other shards, smectite replaced irregularly bounded marginal parts of the shards. This replacive smectite has three distinguishing features: (1) it is stained red or reddish, probably with iron oxide or hydroxide, (2) it is associated with titaniferous nodules, and (3) it is spatially associated with microbially generated dissolution features. Walton and Schiffman [2003] designated this unique form of replacive smectite as reddened smectitic grain replacement (RSGR). They described the sum of these processes (fracturing, grain-coating smectite, tubule formation, dissolution, and replacement of glass by RSGR) as the incipient stage of alteration of hyaloclastite in the HSDP 2 1 core. Walton and Schiffman [2003] reported that incipient alteration is characteristic of the interval from 1079 to 1335 mbsl. Subsequent examination of additional samples shows incipient alteration to a depth of 1462 mbsl. [4] Hyaloclastites from deeper in the core have followed one of two different paths of alteration. Smectitic alteration involved replacement of sideromelane and olivine by greenish smectite accompanied with filling of pores with smectite. Phillipsite filled in remaining pores. The other path of alteration, palagonitic alteration [Walton and Schiffman, 2003], includes conversion of the outer margins of sideromelane grains into palagonite, here identified in thin section as an isotropic, or very nearly so, orange or yellow-orange material [cf. Stroncik and Schmincke, 2001]. Palagonitic alteration was present in all hyaloclastite samples from 1573 mbsl to the deepest hyaloclastites in the core, including glassy fragments in pillow lavas. Apparently at the same time as palagonite formed, chabazite formed in primary pores, vesicles, and voids along fractures, leaving little open space in the rock. Walton and Schiffman [2003] suggested that the basic reaction at the time palagonite formed was Sideromelane þ components from pore waters ¼ palagonite þ chabazite þ components to pore waters: 2of27

3 Geosystems G 3 walton et al.: alteration of hyaloclastites, /2004GC [5] Petrography shows that the other alteration materials present in the rock, including several varieties of smectite, phillipsite, and Ca-silicate formed at earlier times than palagonite and chabazite. Components released to pore waters during palagonite formation may have formed other phases elsewhere in the rock, but they were not immediately involved in the palagonite reaction. The process of forming palagonite and chabazite may have fixed components from pore waters in the rock or may have released components to pore water or ultimately to seawater. Since the volume of hyaloclastites in the HSDP-2 core hole affected by palagonitization is large, encompassing a 1-km-thick section of hyaloclastite, palagonitization is potentially important in influencing pore fluid composition. The mass balance calculation reported here is limited to the three materials, sideromelane, chabazite, and palagonite, because only those materials formed in the palagonitization process and could be accounted for accurately in the samples Role of Palagonitization in the Chemical Mass Balance of Oceans [6] Hyaloclastite consists of fragments of basalt glass, or sideromelane. Basalt glass is a significant rock type in two marine settings, oceanic island hyaloclastites (as described here) and pillow lavas of oceanic islands and oceanic crust where glass forms rims on pillows. Hyaloclastites in the HSDP 2 1 core form approximately 50% of the submarine portion of the core. In comparison, Staudigel and Hart [1983] estimated a value of 20% glass in oceanic crust for use in calculating flux of elements during palagonitization. Because of the abundance and chemical reactivity of basalt glass, it and its alteration products have the potential to be significant sources or sinks of material in geochemical cycles involving seawater and the oceanic crust [Staudigel and Hart, 1983]. [7] Previous studies of the low temperature alteration of oceanic crust have not uniformly agreed on the role that palagonitization plays in global geochemical cycles. Honnorez [1978] reported that alteration of tholeiitic glass would remove K and Na from seawater and provide Mg and Ca to it, but concluded that formation of smectite and phillipsite as end products of alteration of basalt glass did not control composition of seawater. Bloch and Bischoff [1979] concluded that while palagonitization plays a minor role in oceanic mass balances for K, fracture-controlled alteration results in formation of K-rich phases (smectites, phillipsites), which play a major role. However, if formation of phillipsite and palagonite occurs as part of the same process, material released in palagonite formation would be consumed in the formation of other minerals [Ailin-Pyzik and Sommer, 1981]. In this paper, we demonstrate that the palagonitization process, with accompanying formation of chabazite, consumes some elements from pore waters or other sources and releases others. Those that are released may enter other minerals elsewhere in the system, or they may accumulate in the aqueous phase. While we cannot comment directly on the effectiveness of palagonitization in controlling the solute composition of seawater, we conclude below that nearly all elements involved in palagonitization are mobile on a scale much larger than a thin section, at least in this example of palagonitization of hyaloclastite in an ocean island volcano. [8] Many previous studies have attempted to calculate the geochemical mass balance attending palagonitization [e.g., Hay and Iijima, 1968a, 1968b; Honnorez, 1978; Staudigel and Hart, 1983; Furnes, 1984; Zhou and Fyfe, 1989; Jercinovic et al., 1990; Torseth et al., 1991; Daux et al., 1994; Stroncik and Schmincke, 2001]. Most of these studies have either tried to assess fluxes of major elements through the comparison of the bulk compositions of unaltered deposits versus palagonitized equivalents or through the comparison of fresh sideromelane versus its palagonitized equivalent. All of these studies have entailed some form of assumption regarding either constancy of mass or volume related to palagonitization. The present study differs from previous studies in three key ways: (1) Recent observations of the HSDP 2 1 core have defined the order and timing of alteration and allow the process of palagonitization to be considered independently of other reactions that have affected the rock [Walton and Schiffman, 2003]. (2) Analysis by laser ablation microprobeinductively coupled plasma mass spectrometer (LAM-ICPMS) allows determination of minor and trace element concentrations in small areas within thin sections. (3) Point counts of modal abundance combined with measured or assumed values of density and points 1 and 2 above allow calculation of actual fluxes during palagonitization without assuming that one or another element is 3of27

4 Figure 1. Summary stratigraphic column of the HSDP 2 1 core from 1000 m to total depth, including all of the submarine portion of the record of Mauna Kea. Core description modified from HSDP [2000]. Alteration zones modified from Walton and Schiffman [2003]; lower boundary of zone of incipient alteration shifted in light of further, but unpublished, observations on additional samples. immobile. A simple sensitivity analysis shows that the assumed densities are reasonable. 2. Methods 2.1. Samples [9] Samples for this study were selected from the major hyaloclastite intervals, and some thin hyaloclastite intervals, in the HSDP core (Figure 1, Table 1). While sampling did not follow a particular pattern, it was random in that core boxes from an interval were chosen without reference to any features, and samples within the boxes were chosen on the basis of the length of the segment in which they occurred and lack of other sampling. Some samples, not used in this study, were chosen because they were basalts or were in the contact metamorphic zones near intrusions. Several intervals in the lower 2 km of the hole lack any hyaloclastite, so our sample suite has an uneven distribution with depth, including some gaps of a few hundred meters. [10] Petrographic analysis of all samples taken resulted in a description of the alteration effects of each sample (Table 1) [Walton and Schiffman, 4of27

5 Table 1. Samples Analyzed Geochemically for This Study a Run Number and Footage Depth, mbsl Unit Number Lithology [HSDP, 2000] Alteration Features [after Walton and Age, c Schiffman, 2003] b Ka AWW Sample Number R0488, basaltic hyaloclastite (monolithologic with moderately olivine-phyric basalt clasts) incipient; in areas of hyaloclastite breccia, isopachous smectite coatings; many sideromelane shards partially dissolved R0549, highly olivine-phyric basalt. basalt shows alteration to smectite in olivine and along fractures R0620, basaltic hyaloclastite (monolithologic, moderately to highly olivinephyric basalt clasts) RSGR d and microtubules common R0624, pebbly conglomerate olivine more or less altered to smectite, smectite-chabazite-phillipsite vein fillings *R0675, basaltic hyaloclastite (polymict, aphyric to highly olivine-phyric basalt clasts) R0710, basaltic hyaloclastite (polymict, dominantly highly olivine-phyric basalt clasts) *R0714, basaltic hyaloclastite (polymict, aphyric to highly olivine-phyric basalt clasts) R0714, basaltic hyaloclastite (polymict, aphyric to highly olivine-phyric basalt clasts) *R0715, basaltic hyaloclastite (polymict, dominantly olivine-poor basalt clasts) *R0771, basaltic hyaloclastite with silty interbeds R0774, basaltic hyaloclastite with silty interbeds R0777, basaltic hyaloclastite with silty interbeds; this sample is a polymict conglomerate *R0844, basaltic hyaloclastite (polymict, dominantly olivine-poor basalt clasts) incipient followed by palagonitic alteration RSGR is very strikingly developed; tubules are not common and are poorly developed or poorly preserved RSGR is rare, but present; tubules commonly modified by conversion of adjacent glass to smectite contact metamorphism followed by palagonitic alteration; isopachous rims on shards may be metamorphosed smectite coatings from incipient alteration; tubules uncommon microtubules, RSGR, and grain dissolution rare lacks microtubules, RSGR and dissolved grains lacks microtubules, RSGR, and dissolved grains lacks microtubules, RSGR, and dissolved grains; little remaining sideromelane lacks microtubules, RSGR, and dissolved grains; no visible contact metamorphic effects from intrusion 2 m away of27

6 Table 1. (continued) Run Number and Footage Depth, mbsl Unit Number Lithology [HSDP, 2000] Alteration Features [after Walton and Age, c Schiffman, 2003] b Ka AWW Sample Number *R0852, basaltic hyaloclastite (polymict, dominantly olivine-poor basalt clasts) R0894, basaltic hyaloclastite (polymict, aphyric to moderately olivinephyric basalt clasts) R0911, e basaltic hyaloclastite (polymict, aphyric to highly olivine phyric basalt clasts) *R0918, basaltic hyaloclastite (polymict, aphyric to highly olivine-phyric basalt clasts) *R0930, basaltic hyaloclastite (polymict, aphyric to highly olivine-phyric basalt clasts). R0930, basaltic hyaloclastite (polymict, aphyric to highly olivine-phyric basalt clasts) lacks microtubules, RSGR, and dissolved grains lacks microtubules, RSGR, and dissolved grains lacks microtubules, RSGR, and dissolved grains lacks microtubules, RSGR, and dissolved grains acks microtubules, RSGR, and dissolved grains lacks microtubules, RSGR, and dissolved grains a Samples for which both electron microprobe and ICP-LAM results are available are marked with and asterisk (*). b Incipient alteration includes very early fracturing of grains and formation of a first isopachous pore-lining coating of brownish smectite, generally about 0.01 mm thick. In some samples either spotty to pervasive replacement of shard margins by reddened smectitic grain replacement (RSGR) or dissolution of glass occurs. Before or during formation of RSGR and dissolution of glass, microbial endolithic microborings develop inward from all kinds of glass surfaces: exteriors of shards, fracture faces, and vesicle walls. Borings range from absent to highly abundant in different samples and vary in abundance from place to place in samples. Palagonitic alteration postdates incipient alteration, without effacing its characteristic products. Palagonitic alteration also postdates near simultaneous formation of a second stage of pore-lining smectite, this one faintly greenish, and growth of phillipsite and Ca-silicate minerals. Palagonitic alteration here includes replacement of grain margins by nonbirefringent palagonite and growth of chabazite, which fills nearly all remaining pore space. No reaction relationship is implied between earlier-formed smectite and either palagonite or chabazite, although palagonite itself is a poorly organized form of smectite [Drief and Schiffman, 2004]. c Estimated age calculated from Age (Ka) = (0.116 * D) [Sharp and Renne, 2005]. d RSGR, reddened smectite grain replacement. A mixture of grain-replacing smectite; Ti-rich nodules; and reddish staining, probably ferric hydroxide, all in various quantities. 6of27

7 Table 2. Major Elements: Electron Microprobe Analyses, Averages by Material, and Standard Deviation of Materials Involved in Alteration of HSDP 2 Phase 1 Hyaloclastites a Sample Depth, mbsl n Na 2 O MgO Al 2 O 3 SiO 2 P 2 O 5 K 2 O CaO TiO 2 MnO FeO Sum Chabazite 5739 * * * * * * * * * Average Standard deviation Palagonite * * * * * * * * * * Average Standard deviation Phillipsite Average Standard deviation Sideromelane * * * * * * * * * Average of27

8 Table 2. (continued) Sample Depth, mbsl n Na 2 O MgO Al 2 O 3 SiO 2 P 2 O 5 K 2 O CaO TiO 2 MnO FeO Sum Standard deviation Smectite Average Standard deviation a Samples used in calculation of mass balance are marked with an asterisk (*). 2003]. From this petrographic analysis, we concluded that the variability of primary and alteration features is small, and the samples are generally representative of the hyaloclastites encountered in the well. From the suite of samples available from the hyaloclastite zones, 8 were chosen for geochemical analysis with microprobe and LAM-ICPMS. These 8 samples are distributed throughout the interval where palagonitic alteration prevails. Electron microprobe analyses of several phases are available on additional samples (Figure 1) Composition: Point Counts [11] Modal compositions of hyaloclastite samples were determined by point counts of petrographic thin sections with a petrographic microscope. The identity of minerals had been determined earlier by a combination of petrography, XRD, and electron microprobe analysis, and most points were unequivocally identified. Because of the coarsegrained nature of the hyaloclastite fragments in the samples, as opposed to the small size of the alteration minerals and phenocrysts and microlites in the fragments, only about 150 points were counted on each section, making the uncertainty of the results high relative to the percentages determined [Folk, 1980] Major Elements: Electron Microprobe Analysis [12] Sideromelane, palagonite, zeolite, and smectite were analyzed in polished thin sections on a Cameca SX-50 electron microprobe with an accelerating potential of 15 kev, a beam current of 5 or 10 na, and with a rastered beam having an effective diameter of 5 or 10 micrometers. Net intensities on unknowns and a variety of natural silicate calibration standards were converted to concentrations using conventional ZAF correction procedures [Schiffman and Roeske, 2002]. Calibration standards included natural jadeite (for Na), tremolite (Mg), anorthite (Al), quartz (Si), apatite (P), orthoclase (K), wollastonite (Ca), rutile (Ti), rhodonite (Mn), and hematite (Fe). A USNM basaltic glass standard, VG-2, was analyzed as a secondary standard prior to each session Minor and Trace Elements: LAM-ICPMS Analysis [13] Sideromelane, palagonite, and zeolite were analyzed using a VG PQII+XS quadrupole ICPMS with a Merchantek LUV 266 Gen 3 laser ablation microprobe (LAM). The LAM is a Q-switched 1064 nm laser quadrupled to 266 nm, with tophat profile. Geologic thick sections (approximately 100 mm thick) and NIST 612 standard glass were analyzed multiple times by single-point vertical ablation, with ablation conditions optimized for the solid material. Composition of NIST 612 is based upon Pearce et al. [1997] and Gao et al. [2002]. NIST 612 glass was ablated at energies slightly higher than 0.4 mj/pls (milli- Joules per pulse), sideromelane at slightly less than 0.4 mj/pls, palagonite at about 0.3 mj/pls, 8of27

9 and zeolite at about 0.2 mj/pls. All materials except zeolite were ablated at 10 Hz; zeolite was ablated at 2 Hz. [14] Signal background was subtracted from all elements, using the average signal detected for about one minute while the laser was running but blocked from ablating the sample. Signals without significant element fractionation lasted more than a minute for sideromelane analyses, seconds for palagonite, and about 20 seconds for zeolite. Quantification of these time-resolved acquisitions (TRAs) followed the method of Longerich et al. [1996]. In addition, for every phase, three internal standards (Ca, Ti, Si) were assessed for similarity to TRA for each element, and the internal standard having the most similar ablation behavior was selected for each element, and used to quantify elemental concentrations in each phase. Internal standard concentration values came from microprobe analyses (above). [15] Evaluating the reproducibility of the LAM- ICPMS method on geologic materials is difficult because of true heterogeneity in chemical composition. A test of eight analyses of sideromelane and eight of palagonite on a single thin section during a single analytical session resulted in percent relative standard deviation (% RSD) of 1 5% for 13 of 25 analytes in sideromelane. Only Lu had a %RSD greater than 10% in the sideromelane, but it also had a mean concentration of 0.22 ppm and %RSD of 11%. Therefore estimated analytical error for sideromelane is less than ±10% for concentrations greater than about 2 ppm. Palagonite results were more variable, with seven analytes having %RSD greater than 10% and six analytes having %RSD between 1 and 5%. Because this variability may represent true compositional variability, estimated analytical error for palagonite is assumed to be the same as for sideromelane. Chabazite abundance in the thin sections was low and the high water content made it difficult to analyze. We attempted to remove loosely bound water in chabazite by freeze-drying the thin sections before analysis. This technique improved the ablation of chabazite, but systematic testing of reproducibility of results was still not possible. However, the abundance of most trace and minor elements in chabazite, with the exception of a few discussed below, is so low that their quantification is not essential to the mass balance. Microprobe concentrations and LAM-ICPMS concentrations of major elements (Si, Ca, Ti) fell within about 10% of each other. 3. Results 3.1. Major Elements [16] Table 2 lists results of electron microprobe analyses of major elements in sideromelane, palagonite, and chabazite from HSDP 2 1 samples. Palagonite contains a lower concentration of Na 2 O, MgO, Al 2 O 3,SiO 2,P 2 O 5, and K 2 O than sideromelane. TiO 2 and FeO are enriched in palagonite, on average, while CaO, MnO are approximately equal in concentration in the two materials. Because the total concentration of the major oxides is much less in palagonite (82.8%) than in sideromelane (98.3%), we assume that palagonite is much more hydrous and that the concentration differences are not reliable guides to retention or enrichment during palagonite formation. [17] Chabazite is enriched in alkalis and Al 2 O 3 relative to both sideromelane and palagonite. SiO 2 is approximately equal in concentration in chabazite to that in sideromelane, but it is much more concentrated in chabazite than in palagonite. MgO, TiO 2, FeO, and MnO are virtually absent from chabazite. CaO is present in chabazite, but markedly depleted with respect to both sideromelane and palagonite. Again, because chabazite is hydrous, the concentrations of elements are not directly comparable between chabazite, sideromelane, and palagonite Calculation of Mass Balance for Major Elements [18] The results described above, while clear, are not corrected for the density differences among the three materials and their abundance. Hence they do not reflect the mass balance nor do they indicate which components have been gained or lost during alteration. By considering the modal abundance of the materials (Tables 3 and 4) and their known or assumed density as well as their chemical composition, it is possible to calculate actual mass balances: M ij ¼ C ij r j X j ; ð1þ M i;sideromelane M i;palagonite þ M i;chabazite ¼ DM; ð2þ where M is the mass per volume of a component i in a material j (i.e., palagonite, sideromelane, or 9of27

10 Table 3. Point Count Results on Hyaloclastites From Zone of Palagonitic Alteration, HSDP 2, Phase 1 Core AWW sample designation Subsea depth, mbsl Points counted Primary Grains Hyaloclasts, crystals within them, and alteration of them Sideromelane Olivine Pyroxene Plagioclase Opaque Dissolved Palagonite RSGR a Basalt fragments Crystal fragments Olivine Pyroxene 1 Plagioclase Unknown 1 Primary Pores Intergranular pores Empty Smectite rims and spherules Phillipsite Chabazite Analcime 3 Ca-silicate Vesicles Empty Smectite rims and spherules Phillipsite Chabazite Analcime Pyroxene 10 of 27

11 Table 3. (continued) AWW sample designation Subsea depth, mbsl Points counted Isotropic pore filling 4 Fractures Empty 1 1 Smectite lining Phillipsite 1 Chabazite a RSGR, reddened smectite grain replacement (see text). chabazite), C is the concentration of the component in a material, r is density of the material, and X is the modal fraction of that material in the sample or the average of samples. In equation (2), DM isthe change of mass. The units of the calculation are g/cm 3 of rock. Calculations from these formulae give (1) the mass change from a given volume of sideromelane to an equal volume of palagonite, (2) the change from a known volume of sideromelane to the same volume of palagonite plus the amount of chabazite in the sample, and (3) the amount of each component added to or removed from a unit volume of the rock Density of Materials [19] Two key unknowns in this analysis are the density of the phases and the volume change, if any, in converting from sideromelane to palagonite. Chabazite has a variable density (2.05 to 2.10 [Deer et al., 1966]). We have assumed a value of in our calculations. Because chabazite has a low abundance (8% by volume, on average), only a large error in its density would change the results of our calculations. Sideromelane is assumed to have a density of 2.8, which is similar to the value of 2.77 measured on one HSDP sample (A. Byrnes, personal communication to A.W.W., 2001). The density of palagonite, however, presents problems. Hay and Iijima [1968b] measured values of 1.93 to 2.03 g/cm 3 on palagonite from subaerial occurrences on Oahu. Daux et al. [1994] measured values of 1.94 to 2.17 on nine samples of palagonite from hyaloclastites that altered in subglacial and meteoric environments in Iceland. On the other hand, Furnes [1978] measured values of 2.1 to 2.4 g/cm 3, depending upon their water content, on samples from Iceland. [20] The water content of our samples (i.e., the difference between the sum of major oxides and 100%) suggests a density of 2.3 to 2.4 from a plot of water content versus palagonite from Furnes [1978]. However, the exact nature of the so-called palagonite from Hay and Iijima [1968a, 1968b], Daux et al. [1994], or Furnes [1978] is not well enough defined that it can be compared unequivocally to our material. Our initial calculations assumed a value of 2.0, in line with values from Hay and Iijima s [1968a, 1968b] and Daux et al. [1994]. Below we test the sensitivity of our results to a range of densities [cf. Stroncik and Schmincke, 2001]. Our results will demonstrate that assignment of a higher density to palagonite requires unrealistic conclusions about mobility of elements, 11 of 27

12 Table 4. Summary Point Count Results of Samples Selected for LAS-ICPMS Analysis Total Points Counted Average of Samples Standard Deviation Framework Grains Hylaoclasts Sideromelane % 9.95% Olivine Pyroxene Plagioclase Opaque Dissolved grains, identity unknown Palagonite Smectite Brown alteration Basalt fragments Crystals Olivine Plagioclase Sum of framework grains Cements (in Intergranular Spaces, Vesicles, and Fractures) Smectite Smectite Ca-silicate Analcime Phillipsite Chabazite Sum of cements Pores (intergranular, vesicular, fracture) Minus cement porosity ( = Total volume framework grain volume) without greatly affecting the direction of changes in element or oxide abundance Sensitivity Analysis and Density of Materials [21] A sensitivity analysis tests whether violations of our assumed density of palagonite and our assumption of isovolumetric conversion from sideromelane to palagonite affect the outcome of the mass balance calculation (Tables 5a and 5b). This table is constructed by averaging the mass balance for all samples for which suitable analyses exist, 10 in the case of sideromelane to palagonite (Table 5a) and 7 for the change of sideromelane to palagonite plus chabazite (Table 5b). The mass balances were calculated for each sample with each set of assumptions about palagonite density and volume change. Calculations of this kind also allow determination of the mineral densities that would imply an isochemical conversion during the palagonitization reaction. The sensitivity analysis allows us to explore the geochemical reasonableness of various scenarios. [22] With regard to the assumption of isovolumetric conversion, we see no petrographic evidence for any change in the volume during reaction from sideromelane to palagonite [Walton and Schiffman, 2003]. This is in accord with the conclusions of a number of other observers (as reviewed by Stroncik and Schmincke [2002]). For this reason, and because the changes owing to a hypothetical 10% plus or minus volume change are small (Tables 5a and 5b), we conclude the assumption is justified. [23] The density of palagonite is less certain, and so the mass balance calculations are done with a range of reasonable densities along with a small potential volumetric change, to investigate the consequences of different assumptions about density of palagonite to the mass balance. Tables 5a and 5b give the results of the mass balance calculation assuming values of 2.0, 2.2, and 2.4 gm/cm 3 for the density of palagonite. The results allow some generalizations. The amount of loss of oxides decreases, or the amount of gain increases, as the density of palagonite increases. For oxides present in low abundance and not involved in the forma- 12 of 27

13 Table 5a. Average of Amount of Mass Gain or Loss of Oxides During Conversion of Sideromelane to Palagonite Under Various Assumptions for the Density of Palagonite and Volume Shrinkage Factor a Density of Palagonite and Shrinkage Factor 2.0 and and and and and 1.1 Mass, gm/cm 3 % Mass, gm/cm 3 % Mass, gm/cm 3 % Mass, gm/cm 3 % Mass, gm/cm 3 % n Na 2 O MgO Al 2 O SiO P 2 O K 2 O CaO TiO MnO FeO a Average is of n samples. VSF, volume shrinkage factor. Where VSF = 1, palagonite occupies the same volume as the sideromelane from which it formed, and VSF < 1 means the palagonite occupies less volume. For reasons given in the text, no volume change is envisioned for conversion from sideromelane to palagonite, but this table serves as a sensitivity analysis for differing assumptions about both VSF and density of palagonite. tion of chabazite, the amount of change can be large; the enrichment of TiO 2 changes from 14% to 68%, depending upon the assumed density and volume change in Tables 5a and 5b. Even abundant elements like SiO 2 show marked change ( 26% to 1% over the range of assumed values). CaO, FeO, and Al 2 O 3 show similar results. On the other hand, MgO and MnO, which are greatly depleted in palagonite in this sample, compared to sideromelane, and Na 2 O and K 2 O, which are concentrated in chabazite, are less affected. In this example, the direction of change (i.e., whether material is added to or removed from the system during alteration) is insensitive to the values chosen for seven oxides; the direction of change for FeO, MnO, and CaO shifts from loss to gain with increasing density (Tables 5a and 5b). [24] If we assume the density of palagonite is high, and the density of chabazite is greater than the reported range, the calculated loss or gain of Al 2 O 3,SiO 2, and CaO can be made nearly zero. However, this makes the gain in TiO 2 unreasonably large (more than 50%) and requires substantial addition of FeO. SiO 2, and CaO are more mobile than FeO and TiO 2 in most aqueous media over a wide range of ph, so more mobility of the former during palagonitization is expected. In order for the Table 5b. Average of Amount of Mass Gain or Loss of Oxides During Conversion of Sideromelane to Palagonite Plus Chabazite Under Various Assumptions for the Density of Palagonite and Volume Shrinkage Factor a Density of Palagonite (g/cm3) and Volume Shrinkage Factor (Volume Palagonite/Volume Sideromelane) 2.0 and and and and and 1.1 Mass, gm/cm 3 % Mass, gm/cm 3 % Mass, gm/cm 3 % Mass, gm/cm 3 % Mass, gm/cm 3 % n Na 2 O MgO Al 2 O SiO P 2 O K2O CaO TiO MnO FeO a Average is of n samples. VSF, volume shrinkage factor. Where VSF = 1, palagonite occupies the same volume as the sideromelane from which it formed, and VSF < 1 means the palagonite occupies less volume. For reasons given in the text, no volume change is envisioned for conversion from sideromelane to palagonite, but this table serves as a sensitivity analysis for differing assumptions about both VSF and density of palagonite. 13 of 27

14 calculated change for TiO 2 to be zero, the density of palagonite would have to be less than 1.6, which is outside all reported ranges of values. [25] Hence the results presented in Tables 5a and 5b lead to the conclusion that the assumed density of palagonite should be in the low part of the range we investigated. If palagonite has a density of 2.0 gm/cm 3, the average change of iron is small ( 4%, assuming no volume change), while the average amount of change for TiO 2 is the smallest value (+27%). Consequently, we choose 2.0 gm/cm 3 as the assumed density of palagonite in further calculations Mass Balance Results [26] The average of the results of the mass balance calculation shows that TiO 2 is gained during alteration of sideromelane to palagonite, and all other oxides of major elements are lost, if we assume a palagonite density of 2.0 and no volume change on conversion (Table 5a). Amounts of other oxides lost ranges from 4% of the FeO and MnO in sideromelane in conversion to palagonite to 89% of the Na 2 O. Gain of TiO 2 amounts to 21% of the amount originally in the sideromelane, on average. The gain in TiO 2 is greater than passive accumulation, such as envisioned by Staudigel and Hart [1983], would imply. The 4% change in FeO is negligible for the purposes of this study. [27] Considering the whole reaction, including formation of both palagonite and chabazite, it appears that enough chabazite forms to reduce the overall loss of most components it contains, including Na 2 O, Al 2 O 3, SiO 2 and CaO (Table 5b). For example, enough Na 2 O enters chabazite to reduce the average loss of that oxide to 46% for the overall change from sideromelane to palagonite plus chabazite. Average net losses of Al 2 O 3 and SiO 2 are 21% and 20%, respectively, for the change from sideromelane to palagonite plus chabazite. On average they are similar to the loss of CaO (15%). Chabazite takes up enough K 2 O to make that oxide more concentrated in the altered than unaltered rock; the altered rock contains 15% more K 2 O than the glass released as it altered, on average. TiO 2 remains enriched in the overall rock, while the other components not taken up in chabazite remain depleted. The mass balance calculation was conducted on seven samples for the sideromelane to palagonite plus chabazite, but 10 samples for the sideromelane to palagonite. This disparity leads to some minor inconsistencies in amounts gained or lost in Tables 5a and 5b Minor and Trace Elements Rare Earth Elements [28] We analyzed a suite of rare earth elements (REE) to determine their mobility during palagonitization (Tables 6a, 6b, and 7). Most of the elements of the suite were below detection limits in all samples of chabazite. The few meaningful results indicate very low abundance of REE in this material. Therefore the chabazite REE content does not affect the mass balance of these elements. The results for the suite of REE are plotted in Figure 2, normalized to chondritic abundances. The relative abundance of the REE to each other in palagonite is very similar to that in sideromelane, showing only very slight relative enrichment of light REE in palagonite (1.75 to 1.78 on average on a sideromelane-normalized basis) compared to heavy REE in palagonite (1.62 to 1.68 on average). One individual sample (2173 mbsl) may show a Ce anomaly, but the other samples do not. Overall, the REE in palagonite from the HSDP samples are enriched by a factor of about 1.6 to 1.8 over sideromelane, on average. As this factor is greater than the density difference between the two materials, all REE are concentrated during palagonitization in the palagonite beyond passive accumulation, on average Other Trace Elements [29] Trace alkalis and alkaline earth elements are all less abundant in palagonite than in sideromelane, like the major oxides of the same groups (Figure 3, Tables 6a, 6b, and 7). On a ppm basis, Rb, and Sr are concentrated in chabazite, like CaO, Na 2 O, and K 2 O, but Ba is depleted in chabazite relative to sideromelane in all samples save one, and greatly decreased in one sample. The mobility of these elements in aqueous media, the likely presence in the pores of seawater-derived fluids that have experienced sulfate loss, and the structures of the materials involved might seem to make these changes understandable. The small amount of Ba released may have formed an undetected barite phase. [30] On an average mass balance basis, Rb and Sr were concentrated during alteration of sideromelane to form palagonite and chabazite, but the individual analyses show a great deal of variability and some samples display losses of all alkalis and Sr as well as Ba. As chabazite is pore-filling cement in these rocks, the amount of chabazite varies from sample to sample depending upon the 14 of 27

15 Table 6a. Minor and Trace Element Analyses for Materials in HSDP 2 Phase 1 Hyaloclastites: Summary by Sample mbsl 1882 mbsl 1889 mbsl 2173 mbsl mbsl mbsl mbsl mbsl Element Palago -nite Chaba -zite Sidero -melane Palago -nite Chaba -zite Sidero -melane Palago -nite Sidero -melane Palago -nite Sidero -melane Palago -nite Chaba -zite Sidero -melane Palago -nite Chaba -zute Sidero -melane Palago -nite Chaba -zite Sidero -melane Palago -nite Sdero -melane Rb Sr Y Zr Nb Cs Ba La Ce Nd Sm Eu Gd Dy Er Yb Lu Pb Th U of 27

16 Table 6b. Trace Element Concentrations From LAM-ICPMS Analysis a AWW Sample Depth, m Rb Sr Y Zr Nb Ba La Ce Nd Sm Eu Gd Dy Er Yb Lu Pb Th U Sideromelane nd nd 0.35 nd nd Palagonite of 27

17 Table 6b. (continued) AWW Sample Depth, m Rb Sr Y Zr Nb Ba La Ce Nd Sm Eu Gd Dy Er Yb Lu Pb Th U Chabazite nd nd nd 11.6 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 14.5 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 8.83 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 10.2 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 13.3 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 13.6 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 21.1 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 23.6 nd nd nd nd nd nd nd nd nd nd nd nd nd 17 of 27

18 Table 6b. (continued) AWW Sample Depth, m Rb Sr Y Zr Nb Ba La Ce Nd Sm Eu Gd Dy Er Yb Lu Pb Th U nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 22.3 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 27.7 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 0.27 nd nd nd 0.20 nd 0.38 nd nd nd nd nd nd nd 0.95 nd 41.3 nd 0.31 nd nd nd nd nd nd nd nd nd nd nd nd nd nd 6.42 nd 0.15 nd nd nd nd nd nd nd nd nd nd nd nd nd nd 9.18 nd Nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 2.17 nd 18.2 nd nd nd nd nd nd nd nd 0.61 nd nd a Concentrations are in ppm. All samples, all runs. available pore space when the mineral formed. Those samples with little chabazite have suffered loss of Rb, Sr, Na 2 O and K 2 O. [31] Several other minor and trace elements (Y, Zr, Nb, Th, and U) resemble REE in their behavior. They are somewhat concentrated in palagonite. One element, Pb, is highly concentrated in palagonite, on average, but is also highly variable. Abundance of these elements in chabazite is assumed to be negligible; concentrations were below the detection limits of the LAM-ICPMS technique Compatible Versus Incompatible Trace and Minor Elements [32] The spider diagram [Pearce, 1983] separates elements on the basis of their degree of compatibility with the solid phase (versus in the liquid phase) in petrogenetic processes. We plotted our results on such a diagram to explore how the aqueous alteration process of sideromelane may affect the abundances of elements and their utility in interpreting petrogenesis of palagonitized rocks (Figure 3a). We normalized our analytical results to E-MORB values [Sun and McDonough, 1989] for purposes of comparison. The LIL elements on the left of the diagram are depleted in palagonite, relative to sideromelane, increasing from 26% depleted for Sr on average to 68% depleted for Ba. HFS elements on the right are enriched in palagonite by 64 to 78% on average, except for P, which is depleted in palagonite by 43%. A few data on chabazite, on the other hand, show the opposite pattern, enriched in LIL elements and depleted in all HFS elements, though with marked P enrichment. [33] Considering that the fluid in this system is aqueous and not silicate magma, an alternate plot order for the elements shows that ionization potential (IP, ionic charge/ionic radius [e.g., Railsback, 2004]) is an important control on element mobility (Figure 3b). At low IP, Rb, K, Ba, and Sr are depleted in palagonite, relative to sideromelane. Elements from Ce to Nb show noticeable enrichment, relative to sideromelane. P, at high IP is depleted in palagonite, relative to glass. Elements that are soluble in water as ions and as complexes are lost during palagonitization. Chabazite again plots as a very different pattern, with alkalis and some alkaline earths enriched, and other elements depleted, relative to both glass and palagonite. The two anomalies in the chabazite elemental spectrum (Ba and P) persist on this plot. [34] Both plots confirm the pattern observed in the mass balance calculations. During hydrous alter- Table 7. Average Trace and Minor Element Concentrations (ppm) in HSDP 2 Phase 1 Hyaloclastite Samples a Sideromelane Palagonite Chabazite Rb Sr Y n.d. Zr n.a. Nb n.d. Ba La n.a. Ce n.a. Nd n.d. Sm n.d. Eu n.d. Gd n.d. Dy n.d. Er n.d. Yb n.d. Lu n.d. Pb n.d. Th n.d. U n.d. a Concentrations are in ppm. n.d., not determined, concentration below detection limit. n.a., <3 analyses. 18 of 27

19 Figure 2. (a) Plot of average abundance of rare earths in sideromelane and palagonite and the range of values, normalized to chondrite values [Sun and McDonough, 1989]. (b) Plot of abundances of rare earth elements in palagonite normalized to values in sideromelane. All samples show enrichment of REE in palagonite. Although the effect is small, the amounts of REE heavier than Eu are depleted on average in plagonite, relative to Eu and lighter elements. ation of sideromelane, the LIL or low IP elements are selectively removed and may be sequestered in zeolites, or even enriched on a whole rock basis by contributions from seawater. HFS elements are retained or concentrated in palagonite. Phosphorous shows anomalous behavior that may reflect its importance in the metabolism of organisms or its incorporation into a soluble complex anion (HP) 4 3+ in solution. The initial high abundance of P 2 O 5 in sideromelane, relative to E-MORB, is notable. 4. Discussion 4.1. Palagonitization Affinity Plot and Behavior of Elements [35] Spider diagrams and normalized REE plots do not reflect mass balance, but changes of concentration. Table 8 groups the elements according to their mass mobility during formation of palagonite and chabazite from sideromelane. The affinity plot in Figure 4 displays graphically the mass changes during palagonitization for most elements for which we have sufficient data. In Table 8, elements fall into three distinct groups with three outliers. Some elements (Group A: Ti, Y, U, Zr, Nb, REE, Th, Pb) are enriched in palagonite (relative to sideromelane) but are virtually absent in chabazite. Elements in the second group (Group B: Na, Ca, Ba, P, Al, Si) are depleted in palagonite, relative to sideromelane, but are taken up in chabazite. The concentration of these elements in chabazite is not enough to result in a net gain, so in fact these elements show a net loss during the palagonitic alteration process. K, Rb, and Sr (Group C) are so greatly enriched in chabazite that they more than 19 of 27

20 Figure 3. (a) E-MORB-normalized spider diagram [Sun and McDonough, 1989; Pearce, 1983] for major and trace elements in sideromelane, palagonite, and chabazite from hyaloclastites from the HSDP 2 Phase 1 core. Alkalis and alkaline earths are depleted in palagonite relative to sideromelane but HFS elements (except P) are enriched. Chabazite is enriched in alkalis and Sr, but not in Ba or HFS elements for which data exist, with the exception of a positive P anomaly. (b) E-MORB normalized diagram with elements arranged in order by increasing ionic potential (ionic charge/ionic radius [Railsback, 2004]). Soluble cations except Ba are depleted in palagonite but enriched in chabazite. Highly charged P forms a soluble anion and is depleted in palagonite. REE and other high-field-strength elements are enriched in palagonite but are not significant components in chabazite. overcome the depletion in palagonite. The outliers are Mg, Fe, and H. There is a large loss in Mg during palagonitization and Mg is not retained in chabazite. Fe is essentially retained quantitatively, but in palagonite. While we did not analyze it directly, we assume H was enriched in both alteration products. Clearly, substantial amounts of many elements move into or out of the altering rock during formation of palagonite and chabazite from basaltic glass, and the reactions took place in an open system. [36] In constructing Figure 4, we have grouped the elements and oxides according to the groups in 20 of 27

21 Table 8. Behavior of Elements During Formation of Palagonite and Chabazite From Sideromelane Elements Taken up in Chabazite Amount in Alteration Products Greater Than Amount in Sideromelane Amount in Alteration Products Less Than Amount in Sideromelane Elements Virtually Absent in Chabazite Elements enriched in palagonite H (assumed) Ti, Y, U, Zr, Nb, REE, Th, Pb Elements neither enriched nor depleted in palagonite Fe Elements depleted in palagonite K, Rb, Sr Na, Ca, Ba, P, Al, Si Mg Table 8 (Groups A, B, and C, plus MgO, FeO, and H), and arranged them in the order of decreasing mass loss and increasing mass enrichment in each group during formation of palagonite and chabazite from sideromelane. In Figure 4, the ordinate is the mass/volume of amount of an element or oxide in palagonite plus the mass/ volume in chabazite divided by the mass/volume in sideromelane that altered to form the palagonite in the same sample. Concentrations for many elements in chabazite are not available, however, these elements are in low enough concentrations that LAM-ICPMS analysis did not detect them reliably. The quantities of these elements in chabazite are not sufficient to affect the calculations that underlie Figure 4. [37] Some of the elements in Figure 4 have variable concentrations, notably alkalis and alkaline earths (except MgO). These elements are concentrated in chabazite, and their abundance in samples reflects the amount of chabazite present in the sample. As chabazite is the last phase to form in the pores, the amount of available pore space, rather than geochemical equilibrium, controlled the amount present. Several elements or oxides are consistently enriched in the alteration products: REE, Th, K 2 O, Y, Nb, Zr, TiO 2, Rb, U, Pb, and Sr. Figure 4. Affinity diagram showing relative gain and loss of elements in the process of converting sideromelane to palagonite plus chabazite. Ordinate is the ratio of the amount (i.e., mass/volume) of each component in chabazite + palagonite to the amount in the sideromelane that converted to palagonite. Elements arranged in order of increasing ratio within groups. Fe is little changed; Ti, Th, Zr, Y, Nb, U, Pb, and REE elements are concentrated in palagonite (Group A). Group B elements are lost during formation of palagonite but partially retained in chabazite, while Group C elements are concentrated, because they are abundant enough in chabazite to more than offset the loss during formation of palagonite. Mg is lost from the system. Variability in Na, K, Rb, Sr, and Ba partially reflects abundance of chabazite, which is a function of available pore space when its formation began. 21 of 27