surface chemistry associated with the cooling and subaerial weathering of recent basalt flows

Geofhimica et Cosmochimica Acla Vol. 56, pp. 37 I I-3121 Copyright @ 1992 Pergamon Press Ltd. Printed in U.S.A.

0016.7037/92/$5.00 + .OU

Surface chemistry associated with the cooling and subaerial weathering of recent basalt flows

ART F. WHITE ' and MICHAEL F. HOCHELLA JR.’

‘US Geological Survey, Menlo Park, CA 94720 USA *Department of Geology, Stanford University, Stanford, CA 94305 USA

(Received August 27, 199 1; accepted in revised form April 7, 1992)

Abstract-The surface chemistry of fresh and weathered historical basalt flows was characterized using surface-sensitive X-ray photoelectron spectroscopy ( XPS ) . Surfaces of unweathered 1987- 1990 flows from the Kilauea Volcano, Hawaii, exhibited variable enrichment in Al, Mg, Ca, and F due to the formation of refractory fluoride compounds and pronounced depletion in Si and Fe from the volatilization of SiF, and FeFs during cooling. These reactions, as predicted from shifts in thermodynamic equilibrium with temperature, are induced by diffusion of HF from the flow interiors to the cooling surface. The lack of Si loss and solid fluoride formation for recent basalts from the Krafla Volcano, Iceland, suggest HF degassing at higher temperatures.

Subsequent short-term subaerial weathering reactions are strongly influenced by the initial surface composition of the flow and therefore its cooling history. Successive samples collected from the 1987 Kilauea flow demonstrated that the fluoridated flow surfaces leached to a predominantly SiOl composition by natural weathering within one year. These chemically depleted surfaces were aIso observed on Hawaiian basalt flows dating back to 180 1 AD. Solubility and kinetic models, based on thermodynamic and kinetic data for crystalline AlF3, MgFz , and CaFz , support observed elemental depletion rates due to chemical weatherine. Additional loss of alkalis from the Hawaiian basalt occurs from incongruent dissolution of the basalt glass substrate during weathering.

INTRODUCIION

SUBAERIAL BASALT FLOWS represent an important primary rock type susceptible to chemical weathering, dissolution, and soil formation. Basalt weathering studies have compared elemental and mineralogical variations with depth in surti- tally weathered rinds of basalt flows (CRAIG and LOUGHNAN, 1964; COLEMAN, 1982; EGGLETON et al., 1987; SMITH et al., 1987). Results have generally documented that relative elemental mobilities during weathering occur in the following order: Ca > Na > Mg > Si > Al > K > Fe > Ti. These mobilities are functions of the susceptibility of various minerals to weathering, decreasing in the approximate order glass > olivine > pyroxene > amphibole > plagioclase > K-feld- spar. Initial alteration minerals, most commonly products of hydrolysis, leaching, oxidation, and destruction of primary minerals, include amorphous Fe oxyhydroxides and poorly crystalline clays. In addition, age determination of specific basalt flows permits assessing the effects of time and climate on weathering rates and the elemental and mineralogical trends mentioned above. For example, COLEMAN ( 1982) concluded that weathering rates of basalt decreased with time, reflecting loss of primary reactive phases, coating by stable secondary phases, and possible decreases in hydrologic permeability. Also, in detailed studies of 15 Ka Hawaiian basaltic tephra, HAY and JONES ( 1972) documented that the rates of Si and cation loss, as a result of chemical weathering, correlated positively with increasing annual precipitation.

More recent historical basalt flows can potentially provide additional data on the initial aspects of weathering, including synoptic characterization of time and climate on reaction rates. However, due to the limited weathering involved, minor chemical changes in such basalt flows are difficult to characterize by commonly employed techniques, such as bulk

3711

chemical analysis, electron microprobe, or energy dispersive X-ray (EDX) analysis. In contrast, X-ray photoelectron spectroscopy (XPS) possesses extreme surface sensitivity, making it an ideal tool to characterize the initial weathering of volcanic deposits; this has been demonstrated for the 1980 Mount St. Helens ash fall (WHITE et al., 1986). Although XPS has not been previously employed to analyze naturally weathered basalt surfaces, it has been used to investigate experimental basalt-water interactions ( THOMASSIN and TOURAY, 1979; WHITE et al., 1985) and to characterize surfical carbonaceous materials on Hawaiian basalts (TINGLE et al., 1990, 1991).

In the present paper, we describe the rather surprising surface chemistry of freshly chilled flow surfaces of Hawaiian and Icelandic basalts and document subsequent chemical changes due to subaerial weathering. In addition, we propose that these precursor weathering processes may impact longer- term basalt weathering.

METHODOLOGY

Sampling Location and Strategy

To determine the initial surface compositions ofchilled flows prior

to contact with atmospheric precipitation, glassy rinds of tholeiitic pahoehoe basalt were collected from active flow sheets associated with the Puu 00 vent situated on the East Rift Zone of Kilauea Volcano on the island of Hawaii (Fig. I ). These flows, which com- menced in January 1983, have continued essentially unabated to the present, becoming the most voluminous and destructive historical eruption of Kilauea (WOLF-E et al., 1987). Sample locations were specifically selected near the toe of these distal flows to avoid potential contamination by airfalls and aerosols from the active vent.

Samples were collected from cooling pahoehoe flow surfaces in January, 1987, at the Chain ofcraters Road, 3.2 km SW of Kalapana and approximately 8.2 km from the Puu 00 vent. Samples were subsequently collected in June of 1989 at a point where the active

3712 A. F. White and M. F. Hochella Jr.

A 010 20 30 40 50 KILOMETERS

FIG. 1. Map of the Island of Hawaii with insert showing details of the Kilauea Volcano and the East Rift Zone. Solid contours on the island-wide map correspond to 1000-m elevation, contours, and dashed lines approximate aerial distribution of lava flows from the volcanos indicated. Dates correspond to age of flows from which samples were collected for this study. (Base maps after PETERSON and MOORE, 1987; EASTON, 1987).

flow crossed Chain of Craters Road, 0.3 km SW of the Wahaula Visitor Center and 7.2 km from the vent. A final set of fresh basait samples was collected in May of 1990 near the northwest side of the Kalapana Gardens subdivision, 10 km from the vent. Flow depths at these locations were all -=5 m, resulting in relatively rapid cooling of the flow. These samples were collected prior to exposure to precipitation. Relatively smooth surfaces associated with ropey pillow structures on the tlows were selected for analyses. Small-scale s&ace heterogeneities were apparent based on visual variability in texture, color, and iridescence. In order to characterize short-term progressive weathering of a basalt flow, a series of samples from the 1987 Kilauea flow at the Kalapana site were collected periodically between January of 1987 and June of 1989. Basalt samples from older Hawaiian flows (Fig. I ) were also obtained to evaluate the chemical effects of longer- term subaerial weathering. Samples were obtained from tbe I92 1,

1959, 1974, and 1977 paboehoe Bows of the Kilauea volcano and from the 1801 and 1907 flows from sauna Loa (LOCKWOOD and LIPMAN 1987; HOKOMB, 1987).

The surface chemistry of the Kilauea flows was compared to samples collected in August of 1984 from the 1980, 1729, and -6000 BP flow sheets of tholeiitic basalt from the KraIla volcano situated on the NE rift zone in Iceland (~SKARSSON, 1984). These samples were from small distal flows less than a km from Kralla vents. In addition, an unweathered tephra sample, which had been collected from the July 1980 eruption, was obtained from the University of Iceland.

X-ray Photoelectron Spectroscopy

Samples of the flow surfaces were chemically analyzed by XPS, a technique based on the irradiation of the mineral or rock surface

Surface chemistry of fresh and weathered basalt 3713

~~low~ne~yx-mys~dthesub~uentm~urementofthe~netic energies of the core level phot~l~rons ejected near the surface (see HOCHELLA, 1988, for more information on XPS). X-ray irradiation also generates Auger electrons as a result of core vacancies in an atom. Because the travel distances of both types of electrons, without energy loss, are quite short in solids, the characteristic electrons must originate from the top several atomic layers of the surface. The escape depth is primarily a function of the electron energy, the overall detection signal dropping off exponentially with depth. For oxides in the electron energy range typically utilized in the anaiysis, two-thirds of the characteristic photoelectrons come from a depth of ~30 8, (HOCHELLA, 1988).

Sampies were prepared by separating approximately 1 cm2 X 3 mm thick sections of the glassy surface rinds from the bulk basalt. These pieces were then analyzed using either a PHI Model 590 XPS/ SAM or a VG ESCALAB Mk II XPS instrument. The following three modes of XPS analysis were conducted in the study: survey scans, semiquantitative chemical analysis, and depth profiling. Qual- itative survey scans between binding energies of 0 and 1000 eV, using Mg or Al nonmonochromatic radiation, 50 to 70 eV pass energies, and 1 eV step sixes, were obtained to identify specific elements present in the basalt surfaces. In addition, multiplex scans were conducted over narrow energy windows for specific elements. The differences in surface element concentrations are commonly obtained by ratioing the resulting peak intensities of an element to a second element, such as Si or Al, which is considered to be chemically conservative at the surface (BANCROFT et al., 1979). However, the spectra for the basalts contain major variations in peak intensities for all of the observed elements, and the use of such ratios is not particularly useful.

Semiquantitative analysis can be performed based on peak intensities and theoretically derived photoionization cross sections (Sco- FIELD, 1976) or atomic SWSitiVity factors employing cross sections and spectrometer transmission functions (WAGNER et al., 1979). The atomic percent, c, of element x, relative to other elements, i, can be derived from the relationship

where Z is the peak intensity, and S is the correction factor based on sensitivity factors or cross sections. Corn~~~n between the cross sections of SCOFIELD ( 1976) and the sensitivity factors of WAGNER et al. ( 1979) indicated a deviation between the two methods of generally -z 15%. Due to data reduction constraints, sensitivity factors were used for data from the PHI instrument while cross sections were employed for data derived from the VG spectrometer. As discussed by HOCHELLA ( 1988), other sources of analytical errors may be in- troduced in calculating atomic percentages for specific materials due to variations in escape depths and surface heterogeneity, aspects which are not considered in standardized atomic cross s&ion and sensitivity factors. Therefore, the XPS analyses reported in this paper should be considered ~miquanti~tive.

Table 1. Atomic percentages ia representative bulk samples from fresh and weathered basalt flows in Hawaii and Iceland.’

Kilauea

January, 1987

I I I 1

0 200 400 600 800 1000

Binding Energy, eV

RG. 2. XPS surface spectra of three unweathered basalt flows from the east rift of Kilauea Volcano, Hawaii. Elemental peaks are labeled with subscripted photoelectron orbitals and Auger electron peaks (A). The 1987 spectra was taken using Al radiation; and the 1989 and 1990 spectra was taken using Mg radiation, which results in respective shifts in the apparent binding energies of tbe F(A), Na( A), Mg(A), C(A), and O(A) peaks.

Elemental deptb profiles were produced by sequentially eroding the surface by sputtering with an Ar+ beam and reanalysis of the new surface. Sputtering rates can be empirically determined by depth profiling through thin oxide films of known thicknesses. The 3.0 keV beam voltage of the ion gun used on the PHI instrument, converging on an 8.0 X 8.0 mm* surface area, corresponded to a sputtering rate of a~roximately 50 A/min based on a calibration using a tantalum- tantalum oxide standard. This is in the general range of 10-100 A/ min suggested by HOCHELLA ( 1988) for SiO* at comparable voltages and surface amas. However, the actual rate of sputtering for a material such as basalt is mat~x~~ndent and may vary by a factor of 2 or more, relative to such calibrated standards. Based on the above constraints, the profiles in the present paper are reported as sputtering times rather than actual depths.

In addition to XPS analysis, the basalt samples were also characterized by X-my diffraction (XRD) and imaged using a Cambridge Stereoscan MK2 scanning electron microscope equipped with a Tra- car Northern energy dispersive X-my analyzer. Bulk samples of the basalt rinds were analyzed using X-my fluorescence (XRF) and wet chemical methods.

Hawaii Iceiaad 1987’ 1974’ 1921’” 1801w 1986 1984’” 1729’”

0 59.5 61.1 59.1 60.3 60.1 60.1 60.2 Si 18.1 18.8 17.7 18.6 20.9 18.4 20.3 Al 6.1 5.1 5.2 6.6 6.7 5.1 5.5 Fe 3.5 3.4 3.4 3.4 1.8 4.1 3.1 Mg 4.1 3.1 5.2 4.4 0.8 3.3 3.5 Ca 4.2 3.5 4.1 4.2 1.9 4.1 4.4 Na

::: 1.6 2.4

::: 0.7 1.4

Cl 1.1 0.8 1.3 1.2 F 0.3 0.3 0.2 0.3 0.3 0.3 0.2

’ analyses by XJW except for Cl and F which were determined by specific ion ekrodes.

ffresb basalt samples “weathered basalt samples

RESULTS

XPS Spectra of Fresh and Weathered Basalt SurFaces

Photoelectron and Auger electron energies indicative of surficial 0, Si, Al, Mg, Ca, Na, and Fe, were characteristic of the XPS spectra for the unweathered Kilauea flows. These elements are expected to be present based on the XRF analyses of the bulk phase (Table I 1. However, as evidenced from representative XPS survey spectra of unweathered samples from the 1987, 1989, and 1990 Kilauea flows (Fig. 2), additional elements, usually found in low concentrations in basalt, including F, S, Cl, and Cu, were also major surface components. The most striking example is F, as indicated


by the intense F,, peaks in the 1987 and 1989 spectra (Fig. 2). Cl, and Szs peaks are present in the spectra of all three samples. Both the 1989 and 1990 examples also exhibit Cusp peaks, The C,, peaks represent adventitious carbon deposited on the surface from atmospheric sources and possibly from handling in the vacuum system ( HOCHELLA, 1988). The XPS spectra of the fresh flow surfaces exhibit significant variations in elemental peak intensities for samples collected from different surfaces of the same flow as well as for samples from different flows. Small-scale surface heter~eneities have been averaged to a degree by collecting XPS data over relatively large sample areas (up to IO mm2).

The effects of subsequent weathering on the flow surfaces can be assessed as a function of time by comparing the XPS spectra of the fresh flow surfaces with comparable spectra for historical flows. As indicated by representative 1969 and 192 1 Kilauea and the 1801 Mauna Loa samples shown in Fig. 3, the surface chemistry of the historical flows is composi- tionally simpler than the fresh basalt surfaces described above (Fig. 2). Spectra of the weathered basalts are dominated by intense Sit,, SizJt and Or, peaks with minor Na(A), Mgf A) and Ti, peaks. Peaks for major cations found on fresh basalt surfaces, such as Na, Ca, Al, and Fe, are very small or absent, as are peaks indicative of volatile components such as F, Cl, and S.

Although based on fewer samples, comparisons of surface chemistries between fresh and weathered basalts from the KralIa volcano in Iceland are also possible. The surface spectra of the fresh 1980 basalt tephra is dominated by peaks from 0 and Si (Fig. 4)) which are significantly more intense than for the fresh Hawaiian samples (Fig. 2). Aluminum and Fe peaks are also prominent while alkaline, alkaline earth, and volatile elements, including F, are significantly lower or absent. Comparison of the 1980 spectra of the fresh KralIa tephra with the spectra collected from weathered 1984, 1729, and --6000 BP Krafla basalt flows (Fig. 4) indicate a strong

I I

Binding Energy, eV

FIG. 3. XPS surface spectra of three weathered Hawaiian basalt flows from the Kilauea and Mauna Loa Volcanos, Hawaii.

0 200 400 600 800 1000

Binding Energy, eV

FIG. 4. XPS surface spectra of unweathered tepha ( 1980) and weathered flows from the Krafla Volcano. Iceland.

similarity in surface chemistry. This similarity is in marked contrast with the significant loss of F and major cations and the corresponding increase in Si and 0 observed during the weathering of the Hawaiian basalt flows.

Compositional Variations during Weathering

The bulk chemical compositions for the fresh and weathered Hawaiian and Icelandic basalt Sows, as determined by XRF and wet chemical analyses, are very similar (Table 1) . Similarities in bulk F concentrations in the basalts, compiled in Table 1 and reported elsewhere ( AKOKI et al., 1981), indicate that the surface enrichment of the Hawaiian flows relative to the Icelandic flows cannot be attributed to significant concentration differences in the respective magmas. Also, a lack of any systematic chemical trends with time in the Ha- waiian samples indicates that short-term weathering (~250 yrs) had a negligible effect on the overall basalt composition. In contrast, major surficial differences in chemical compositions are apparent in the semiquantitative XPS analysis of representative samples of fresh and weathered Hawaiian basalts (Table 2). A sample of the fresh 1987 Kilauea Sow surface contained significant concen~ations of F (>40%). The Si and 0 concentrations in this sample (5 and 25%, respectively) were also much less than for Si and 0 in the corresponding bulk basalt based on XRF analysis ( 18 and 60%, respectively). These low 0 and Si concentrations require that the Al, Ca, and Mg concentrations be balanced by F, implying the presence of compounds with stoichiometries of the form AlF3, MgF2, and CaFz on the surfaces of the fresh 1987 basalt. In contrast, a sample from the 1990 Kilauea flow surface contained nearly an order of magnitude less F ( 5% ) and much lower concentrations of Al, Ca, and Mg. The con~ntration of Si on the 1990 flow surface ( 5%) was again much lower than expected for bulk basalt (Table 1).


Table 2. Comparison of atomic percentages present in unweathered and weathered surfaces of representative Hawaiian and lcelandic basalt flows based on XF’S analysis.’

1987’ Hawaii Krafla, Iceland

1996 1974w 1921’” 1801’ 198d 1984” 1729w 4ooOBPw

0 25.2 67.6 63.6 64.1 62.2 67.1 71.7 68.4 71.9 Si 5.1 4.8 35.2 33.1 34.8 26.3 21.3 24.7 21.4 Al 12.4 _* 0.5 0.7 0.5 5.5 3.8 5.5 Fe 0.8 2.4 - - - 4.2 1.2 2.1 1.2 Mg 8.3 4.2 0.7 2.1 2.4 Ca 2.3 1.9 1.5 0.5 - - Na 5.7 CU 1.6 - - - F 44.3 4.9 - 0.9 - - - Cl 0.5 0.5 s 1.1 6.4 - - -

’ atomc percentages are corrected by subtraction of adventitious carbon.

* fresh basalt surfaces ‘weathered basalt surfaces ‘hyphens denote elements Mow detection limits (co.5 atomic%)

In contrast to the fresh basal& weathered Hawaiian flows, as represented by atomic concentrations for the 1974, 192 1, and 1801 surfaces shown in Table 2, contain atomic percentages approaching the stoichiometry of SiOZ (i.e., 33-35% Si and 64-62% 0). Magnesium is the only alkali cation present in significant amounts on the weathered Hawaiian flow surfaces and may be related to the formation of a Mg-silicate phase similar to that observed during the initial weathering of ash deposits from Mount St. Helen (WHITE et al., 1986). The atomic percentages of Si in the weathered Icelandic flows were lower than the Hawaiian basalts (21-25%), approaching that of the fresh tephra composition (26%). Weathering appears to remove some Ca and Fe from Krafla basalt surfaces, with Al becoming slightly enriched.

Major chemical differences in the surface compositions of the fresh and historical Hawaiian basalt flows (Figs. 2 and 3) suggest that chemical changes associated with the onset of weathering are rapid. The weathering rates associated with these changes can be documented in detail based on samples collected from the same locality on the 1987 Kalapana flow at successively later dates. Representative XPS profile data, plotted as a function of atomic percentages vs. ion sputtering time, are shown in Fig. 5. Profile A corresponds to the fresh flow surface immediately after cooling and prior to weathering (Fig. 2, Tables 1 and 2). The remaining profiles (B-D) dem- onstrate the effects of progressive weathering in subsequent samples collected from the same flow 60, 180, and 270 days later. Except for the removal of most of the adventitious carbon layer, sputtering up to 7 min produced only minor elemental changes with depth in the samples (Fig. 5 ). Ion sputtering times were therefore insufficient to remove either the chemically distinct initial or weathered surface layers on the basalt. Although quantitative calculation of profile depths is not possible, comparison with sputter rates of approximately 50 A/min for SiOZ and TazOs standards under comparable analytical conditions indicate that the thickness of these surface layers must be at least several hundred Angstroms (>O.Ol Mm).

Representative EDX spectra, plotted as relative peak intensity vs. energy of the generated X-rays, are shown in Fig. 6 for the fresh 1987 Kalapana flow and the same flow after 270 days of weathering. These spectrum are similar to each

other and also correspond to the bulk basalt composition indicated in Table 1. The EDX analyses are therefore insen- sitive both to the initial fluorinated surface composition of the fresh 1987 basalt surface and to the SiOz-enriched surface of the sample weathered for 270 days (Fig. 6). The discrep- ancy in XPS and EDX analyses is related to depth sensitivity of the two techniques. The X-rays detected by EDX are pro- portional to the penetration depth of the electron beam, considered to be on the order of a few km for beam voltages used in our analyses (WELLS, 1974). Differences in resolution depths between XPS, coupled with ion sputtering, and EDX thereby permit an estimate of the thicknesses of weathered surfaces to be in the range of -0.01-l pm.

The time-dependent variations in weathering of the 1987 Kalapana flow are summarized in Fig. 7 from XPS data for samples sputtered for 2 min to remove the bulk of the adventitious C. Fluorine concentrations present on the initial flow surface decrease nearly linearly with time, reaching background levels after approximately 270 days. Corre- sponding large decreases also occur in Al, Ca, and Mg during the same time period. Conversely, Si progressively increases from near-background levels on the fresh flow surface to become a dominant component after 270 days. This Si increase is correlated to tripling of the 0 content, implying the formation of an SiO+ich surface. The surface chemistry of both the fresh and weathered samples bears little resemblance to the bulk chemistry of the basalt as reported in Table 1 and shown by the horizontal arrows in Fig. 7. The surface composition of the Kalapana flow after 270 days of weathering, however, does bear a striking resemblance to the surfaces of the older historical basalt flows, which are also essentially SiOz (Fig. 3, Table 2).

DISCUSSION

The evolution of the surface chemistry of recent basalt flows appears to be related to the following two processes: ( 1) high-temperature interactions involving volatile and refractory components which control the chemistry during cooling of the flow surface, and (2) low-temperature subaerial weathering processes that subsequently modify this composition.

A. F. White and M. F. Hochella Jr.

180 Days 1

FIG. 5. Variations in elemental percentages as functions of depth, produced by alternating ion sputtering and XPS analysis. Profiles (a)-(d) correspond to increasing weathering intervals for the 1987 Kalapana flow surface.

0 t 2345670123456 7

KeV KeV

FIG. 6. X-ray EDX spectrum of (a) a fresh surface of the Kalapana flow collected January, 1987, and (b) weathered surface of the same flow collected in June of 1989.

Processes Controlling Initial Surface Compositions

The high atomic percentages of F, and to a lesser extent Cl and S, on the surfaces of the fresh Hawaiian basalts imply enrichment by processes involving volatile release. These elements are commonly present as gas phases, p~nc~p~ly as HF, HCI, and H$ during active venting of basalt in Hawaii (Table 3). Fluorine, Cl, and S can be enriched in the gas phase relative to nonvolatile elements, such as Al and Ti, by factors of IO’-10’ (CROWE et al., 1987). As evidenced by the basalt chemical data for F and Cl in Table 1, the corresponding concentrations of these volatile elements in the residual bulk basalt phase are very low.

Volatile elements such as F, Ci, and S can concentrate out of the gas phase as surface sublimates, incrustations, and

Weathering, days

FIG. 7. Changes in atomic percentages of major elements as a function of exposure time for the 1987 Kalapana basalt flow. Arrows correspond to bulk basalt composition (Table 1).


Table 3. Gas analyses from recent Hawaiian basaltic eruptions (volume%).

Gas Matma Loa Kilaoea East Rift 1984’ 19832 19833

W 65.8 60.1 81.8

St& 23.3 17.4 12.1

co2 6.53 21.7 3.83

f&S 2.49 0.30 0.76 HF 0.06 0.25 0.20 HCi 0.14 0.32 0.17

’ Greeolaod (1987) ’ Greeolaod (1984) 3 Gedach and Graeber (1985)

flume condensates within active vents at Kilauea (NAUGH-

TON et al., 1974, 1978). Common minerals forming vent deposits include gypsum, anhydrite, halite, ralstonite ( NaMgAl( F,OH ) - HZ), thenardite ( Na,SO, ) , aphthitalite (K,Na( S04)2, Moedite (Na2Mg(S0,), * 4HrO), and aluno- genite ( Alzf SO4) * 1 8H20). The average electron binding energy of the Szs peak for the Hawaiian basalts f Fig. 2) is approximately 168 eV, indicative of sulfate mineralogy. The spectra for the 1989 and 1990 basah samples also clearly document the presence of Cu (Fig. 2 ), which possesses a significant volatility relative to other metals (CROWE et al., 1987; LOWENSTERN et al., 1991). Thermodynamic calculations predict that Cu stability is shifted toward in- corporation into sulfate minerals at lower temperatures (NAUGHTON et al., 1974). The sublimate mineral, kroehnkite ( NazCu( S04)* - ZHrO), is observed at cooler zones in the active vents of Kilauea. The correlation between Cu and S, as sulfate, is documented for the 1989 and 1990 basalt samples (Fig. 2).

The reported inco~omtion of volatile F, Cl, S, and Cu into surftcial sublimates has been generally confined to active vent systems in Hawaii. The present sampling program was specially designed to avoid direct contamination of the fresh basalt surfaces by such processes that produce sublimates by selecting sampling sites at the toe of active flows up to 10 km from the source vents. The dominance of surface volatile components at these remote sites implies that F, Cl, and S must continue to degas and mobilize from the interior of the flows during the cooling process.

In addition to variable amounts of volatile enrichment, a pervasive characteristic of the unweathered Hawaiian basalt flows, based on the XPS data, is a significant depletion of surface SiOZ. A reaction coupling volatiI~tion of both F and Si can be written as follows:

SiOz + 4HF + SiF., f 2H20. (2)

The sublimation temperature of SiF, is -86°C at 1 bar total pressure; therefore, SiF, is a gas under essentially all geologic conditions. As discussed by ROSENBERG ( 1973), the respective equilibrium concentrations of HF and SiF.,, as a function of temperature, can be calculated from Fqn. 2, based on free energy values. The increasing ratios of SiF, to HF as a function of decreasing temperature are shown by Lines 1 and 2 in Fig. 8. This and the following calculations are based on the the~~~amic tables of JANAF ( 1985) and ROWE

et al. ( I978 ). Although the sohd phase is dominantly a basaltic

glass as demonstrated from XRD data, the calculations as- sume reactions occur between model oxide components for which free energy thermodynamic data exist (i.e., quartz in Eqn. 2). Although this assumption will affect the net free energies, differences in the oxide components generally will not introduce variations in excess of several kJ to the free energy of the overall reactions, values which are generally small relative to temperature effects.

Line 1 in Fig. 8 represents the fit to Eqn. 2 assuming a constant log HF partial pressure (x fu~city) of -2.60 atm and log H20 of -0.09 atm, values equal to the average of the 1983 gas analysis for the Eastern Rift of Kilauea (Table 3). At the measured magmatic temperature of approximately 1 lOO”C, the reaction is dominated by species on the left- hand side of Eqn. 2 with the resulting low ratio of PsiF, to PHI (Fig. 8). This thermodynamic relationship explains why HF and not SiF4 is present in measurable concentrations in volcanic gases (Table 3). This conclusion also implies that volatilization of Si from the surface of basalt flows is not significant at high temperature. PsiF4 is equal PHF at 375°C assuming a constant log PHF of -2.60. Only below this temperature will SiF, become an important species and significant volatilization of silica will be expected to occur. During actual cooling of a basalt flow, volatile F continuously degasses, depressing the HF partial pressures and exponentially decreasing the PSiF,/PHF ratio. Line 2 in Fig. 8 describes the equilibrium between PSiF,/PHF based on Eqn. 2, assuming the log PHF equals -3.60 or a loss of 90% of the HF during initial degassing at 1100°C. The resulting temperature at which the Psin/ PHF ratio is unity is consequently depressed to < 15O’C. This temperature is probably unrealistically low for active outgasing of significant quantities of HF and volatilization of SiF, .

FIG. 8. Partial pressure ratios of PSiF,/PHF as a function of temperature. Lines correspond to ~u~ib~urn eon~tions between HF and indicated reaction products.


The XPS data for samples of the 1987, 1989, and 1990 flow surfaces shown in Fig. 2 and Table 2 indicate that Si volatilization is associated with the formation of various por- tions of Al, Ca, and Mg fluorides during cooling. Due to the extremely thin nature of surface layers, specific phases could not be identified by conventional analytical technique. Chemical balances, however, imply approximate stoichiometries of AlF3, CaF2, and MgF,. The presence of one of these phases, fluorite ( CaFz), has been demonstrated by X- ray analysis after experimental exposure of Islandic tephra to boiling solutions of HF-HZ0 (~SKARSSON, 1980). The nonvolatility of these refractory compounds are documented by sublimation temperatures of 12252300, and 22OO’C for respective pure crystalline AlF3, CaF,, and MgF, (JANAF, 1985 ). Reactions involving the corresponding model silicate components can be written as follows:

Al2 SiOs + 1 OHF + 2A1F3 + SiF, + 5H20, (3)

Casio3 + 6HF + CaF2 + SiF, + 3Hz0, and (4)

Mg,Si04 + 8HF + 2MgF2 + SiF4 + 4Hz0. (5)

The equilibrium PsiF4/ PHF ratios as functions of temperature for reactions 3-5 are plotted, respectively, as Lines 3- 5 in Fig. 8, assuming that the silicate components are ap- proximated by the respective thermodynamic stabilities of andalusite, wollasonite, and forsterite at a constant log PHF of -2.60 atm. The above calculations do not imply the presence of these specific minerals in the basalt but only that these compounds approximate thermodynamically solid components in the basalt glass.

Clearly, reactions involving the formation of refractory fluoride compounds containing Al, Ca, and Mg significantly increase the temperatures at which SiF, becomes the major volatile component relative to HF. Based on the distributions plotted in Fig. 8, CaF2 and MgFz would first be formed at approximately 750°C followed by AIF at approximately 550°C. Even outgassing of HF (>90%) produces SiF, volatilization at temperatures in excess of 500°C (not shown).

A comparable reaction describing fluoridation of a model Fe-silicate (fayalite) can be written in the following form:

FezSi04 + IOHF + 1 /202 + 2FeF3

+ SiF4 + 5Hz0. (6)

Although such a reaction also can be shown thermodynamically to promote the volatilization of Si, the ferric trifluoride phase itself is much more volatile than the Al, Ca, and Mg fluorides, with the solid sublimating to FeFj gas at a temperature of 800°C. Such volatilization explains the low atomic percentages of surficial Fe on the Hawaiian basalts (Fig. 2, Table 2). Gaseous F metasomatism of silicate minerals and volcanic glass, with corresponding loss of Fe in addition to Si, has also been observed in active volcanic vent systems (NABOKO, 1957).

The phase distributions mentioned in the preceding text, based on thermodynamic arguments, explain the XPS data showing depletion in Si and Fe and the association between F and Al, Mg, and Ca on the surfaces of fresh Hawaiian basalt flows. In addition, the presence of these phases is in agreement with available experimental data. ~SKARSSON

( 1980) demonstrated that CaF2 formed experimentally on the surfaces of tephra at temperatures in excess of 700°C a result compatible with the temperature range predicted for coupled SiOz volatilization and CaF2 formation (Fig. 8). At lower temperatures between 600 and 200°C the experiments of ~SKARSSON ( 1980) produced CaSiFs rather than CaFz . The lack of a correlation between Si, Ca, and F for the XPS data suggests the absence of such phases on the Hawaiian flows is caused by the formation of refractory fluoride compounds at temperatures > 600°C.

The thermodynamic data imply a strong degree of localized temperature control on the nature of the surface chemistry of fresh basalt flows (Fig. 8). Varying temperature gradients, caused by differences in thickness, viscosity, distance from the vent, and transport times, could therefore produce significant variability in surface chemistry in a single basalt flow or for different flows of comparable composition. If F is de- gassed from a flow surface close to magmatic temperatures ( lOOO”C), essentially all the F is lost as HF, no volatilization of Si and Fe will occur, and no refractory surface fluorides will form. The resulting surface chemistry would therefore approximate more closely the bulk chemistry of the basalt as observed for the Krafla tephra sample (Fig. 4) and to a lesser extent for the 1990 Kalapana flow sample (Fig. 2). In contrast, if HF diffuses from the high-temperature interior of the flow to a partially cooled surface, the dominant F species shifts from HF to SiF4 and FeFx, thus volatilizing Si and Fe from the surface. Thermodynamic considerations favor the coupling of such volatilization to concurrent formation of refractory CaF2, MgF2, and AlF3 compounds that appear to occur on the flow surfaces of the 1987 and 1989 Hawaiian samples (Fig. 2).

Weathering of the Basalt Surfaces

The initial surface compositions of basalt flows, controlled by the processes discussed above, are expected to have a pro- found effect on subsequent short-term weathering reactions. The 1987 Kalapana flow, exposed to less than one year of weathering, exhibited dramatic chemical changes starting with AIF,, CaF2, and MgFz as major surface components and ending primarily with SiOZ (Fig. 7). Surfaces of other Ha- waiian basalt flows, such as represented by the 1990 sample (Fig. 2), have initially lower concentrations of refractory fluoride compositions, and therefore would presumably exhibit less drastic chemical variations with weathering. However, as evidenced from the consistent composition of the older historical flows (Fig. 3), the final weathered surface composition of all these flows is a cation-depleted SiOz-enriched surface.

SEM and XRD studies failed to detect specific fluoride compounds, at least in part because of the extremely thin nature of the surface layer. However, composition data pre- sented in Fig. 2 and Table 2 suggest that aqueous dissolution reactions, initiated by rainfall onto the fresh basalt surfaces, can be represented by the following reactions:

CaF2 + CaZ+ + 2F-,

MgF2 + Mg2+ + 2F-, and

AlFx + 4H20 -t AI( + 4H+ + 3F-.

(7)

(8)

(9)


The chemical composition of the actual waters in contact with the basalt surface is not known. Attempts to collect runoff, even during major precipitation events, have been unsuc- cessful due to the extreme permeability ofthe fresh Hawaiian basalt flows. The fluid chemistry from the water table, approximately 500 m beneath Kilauea (8”(Z), indicated near- neutral pH waters with F concentrations ranging between 0.9-1.9 mg/L (TILLING and JONES, 1991). These authors suggested that observed chemical trends in groundwater re- flected availability of easily leachable salts which originated from sublimations and encrustations within the overlying basalt flows. Rapid losses of F from other fresh volcanic deposits have been documented during leaching by rain and snow of fresh ash falls from Mt. Hekla in Iceland, resulting in an order of magnitude increase in F in rivers, along with lethal fluorosis in grazing animals ( SIGURDSSON and PALS- SON, 1957; STEFANSSON and SIGURJ~NSSON, 1957).

The feasibility of reactions 7-9 in controlling the initial changes in surface chemistry of the flows can be evaluated based on solubility and kinetic constraints. Table 4 lists the thermodynamic solubility products and molar solubilities, assuming fluorite (CaF,), selliate (MgF,), and aluminum trifluoride saturation at 25°C. Potential aqueous complexa- tion reactions involving Ca, Mg, and F are not considered in the dissolution reactions. However, AlF3 dissolution is written in terms of the strong aqueous Al hydroxide complex, assuming near-neutral pH conditions for surface runoff. These data can be combined to calculate the maximum thickness of these phases, which can be dissolved in one year, based on their respective bulk densities (Table 4) and the annual average precipitation at Kalapana, Hawaii (250 cm/ yr), normalized to unit surface area. Results reported in Table 4 vary from ~0.3 pm for the less soluble AlF3 phase to >50 pm for the more soluble MgF, phase. These calculated thicknesses are approximately equal to or greater than the esti- mated surface layer thickness (0.01-l pm) based on XPS and EDX analysis (see preceding text), implying that the solubility of these fluoride phases is sufficient to explain their total removal from the flow surfaces by annual amounts of rainfall.

The second constraint is whether the leaching rates of the fluoride compounds are sufficient to explain their loss from

Table 4. Parameters and calculations used to estimate maximum leach depths for suficial fluoride minerals associated of Hawaiian basalts during the first year of weathering.

PWallleter CaF@uorite) MgF,(&aite) AIF3

Solubility product -10.95 -8.04 +7.86 (log rz,’

Density (gmscm-‘) 3.18 3.15 2.88

St&biometric 1.1.10-’ 2.1.1@3 3.0.106 solubility (moles.l-‘)

Layer thickness based on solubility @on)

6.1 51.6 0.3

Layer thickness based on dissolution @m)

49.2 1.1

I as defined by Fqs. 7, 8, and 9.

the surface over the time spans indicated in Fig. 7, assuming that the aqueous solution does not reach thermodynamic saturation. Although no quantitative data on AlF3 dissolution rates are available, data exist on rates of fluorite ( CaF2) and sellaite ( MgF2) dissolution (GARDNER and NANCOLLAS,

1976; HAMZA and NANCOLLAS, 1985; CHRISTOFFERSEN et al., 1988). Experiments indicate that aqueous dissolution is controlled by surface reactions associated with initial hydra- tion of surface Ca and Mg atoms, causing weakening and eventual breaking of the ionic bond with F. Results indicate that dissolution rate, R (moles - SC’ 1, is a function of the relationship between the ionic activity product of the solution (IAP) relative to the solubility product (K,) of the solid ( HAMZA and NANCOLLAS, 1985) such that

where

R = km” (10)

u = (k, - IAP)Jk,, (11)

and k is the rate constant (moles - cmm2 * SC’ ), s is the surface area ( cmm2), u is the relative undersaturation, and IAP is the ionic activity product in solution. At near-neutal pH, reported values of n vary between 0.8- 1 .O and 2.0-3.5 for CaF2 and MgFZ, respectively ( ZHANG and NANCOLLAS, 1990). The chemical composition of the water interacting with the basalt surface is not available, and therefore the extent of undersaturation cannot be accurately calculated. However, the relatively small mass of fluorite and sellaite present on the surfaces relative to the amount required to maintain stoichio- metric saturation (Table 4) implies significant aqueous undersaturation. Undersaturation is also expected based on relatively dilute F concentrations in groundwater resulting from the high permeability and rapid infiltration of precipitation in the basalt flows (TILLING and JONES, 199 1).

An example of calculated maximum leach layer thicknesses is shown in Table 4 assuming significant undersaturation (u = 1.0) and corresponding log k values for CaF2 of -7.2 (CHRISTOFFERSEN et al., 1988) and for MgF2 of -8.6 ( HAMZA and NANCOLLAS, 1985). If the refractory fluoride compounds are, in reality, either amorphous or poorly crystalline, these rate constants would be expected to be even more rapid. Such an open-system kinetic model produces leach thickness that exceeds thickness calculated for closed- system saturation of fluorite (Table 4) but produces thinner thicknesses relative to saturated conditions for sellaite. How- ever, in all cases, the calculated leach depths exceed estimates of the actual surface thicknesses based on XPS and EDX results. Therefore, the chemical changes in the fresh basalt surfaces due to leaching of fluoride phases by one year of rainfall is compatible with existing thermodynamic and kinetic data for these compounds.

The end result of weathering of the Hawaiian basalt flows are chemically depleted Si02-enriched surfaces (Fig. 7, Table 2). Part of the loss of the initial high concentrations of Al, Ca, and Mg can obviously be explained by leaching of the surface refractory fluoride compounds as discussed above. However, simple chemical removal of this fluoride-enhanced, silica-depleted layer alone does not explain the extent of chemical depletion observed on surfaces of the weathered flows because the underlying basalt substrate also contains


significant concentrations of alkali cations as well as Fe and Al (Table I). Mobilization and/or leaching of these elements within the underlying basalt phase is also required to explain the chemically depleted surfaces resulting from weathering. Leaching of alkali cations from basaltic glasses by aqueous solutions is a well-documented weathering process (THO- MASSIN and TOURAY, 1979; CROVISIER et al., 1987). How- ever, Fe and Al are generally much more immobile as documented in studies of more highly weathered, older basalt (COLEMAN, 1982, EGGLETON et al., 1987). The absence of Al and Fe in weathered surfaces of the Hawaiian flows suggests that the near-surface substrate was depleted during cooling by the surfical formation of AlF3 and volatilization of FeS. This depleted substrate becomes exposed during subsequent weathering. In contrast to the Hawaiian flows, subsequent exposure of the Krafla flows to weathering in excess of several thousand years results in a much lower loss of Al and Fe and less of an increase in Si and 0. Apparently, different cooling conditions for the Krafla flows, such as HF outgassing at high temperatures, prevented much of the extreme elemental fractionation associated with surface volatilization and/or fluoridation of the Hawaiian flows.

CONCLUSIONS

The preceding data indicate that significant chemical changes may occur at the surfaces of recent basalt flows during cooling and subsequent short-term subaerial weathering. The nature of such weathering depends in part on the extent of chemical fractionation involving volatilization and formation of refractory compounds on the flow surface during cooling. Fresh surfaces of Hawaiian basalts are deficient in Si and Fe and may contain high concentrations of F, Mg, Al, and Ca. Thermodynamic considerations indicate that F degasses from the flows as HF at temperatures in excess of 800°C. Within the temperature range of 600-8OO”C, F complexes with Si and Fe to form volatile gases coupled with the formation of refractory AlF3, CaF*, and MgFz compounds. These compounds are relatively soluble in water, based on thermodynamic and kinetic constraints, which explain their complete removal during the initial year of meteoric weathering. The stability of the residual leached SiOZ surface is evidenced by comparable surface chemistries of historical flows from Ki- lauea and Mauna Loa dating back two hundred years.

In contrast, basalt of similar bulk composition from the Krafla volcano exhibited less elemental fractionation on the fresh surfaces compared to the Hawaiian flows, suggesting high-temperature degassing of HF and minimal Si and Fe volatilization and refractory fluoride formation. Such surfaces also appear relatively stable during weathering, exhibiting only gradual cation loss over periods of up to several thousand years. The preceding data demonstrates that the initial phases of weathering of basalt flows are inexorably linked to the cooling conditions of a specific flow. Questions remain as to the extent that the cooling history dominates weathering processes in even older flows and if such histories can be separated from weathering parameters related to time and climate conditions.

Acknowledgments-The authors express their gratitude to Walter Dudley and Bill Ebersole of the University of Hawaii at Hilo; Chris

Farrar and Ken Hon of the USGS; Hal Wollenberg of Lawrence Berkeley Laboratory; and Nit% Oskarsson of the University of Iceland who collected a number of the basalt samples. The authors also express their thanks to the Physical Electronic Laboratory of Perkin Elmer Corp. and the Center for Material Research at Stanford University, which provided instrument time for the XPS analyses. Bob Oscarson and Paul Lamote of the USGS supplied the XRF and SEM/EDX data, respectively. A portion of this research was supported by NSF Grant No. 1345-13521991. The authors also thank B. Crowe, J. L. Crovisier, B. L. Jones, and W. Fyfe for constructive reviews.

Editorial handling: R. A. Schmitt

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surface chemistry associated with the cooling and subaerial weathering of recent basalt flows

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