Water-extractable and exchangeable phosphate in Martian and carbonaceous chondrite meteorites and in planetary soil analogs

Geochimica et Cosmochimica Acta, Vol. 66, No. 17, pp. 3161–3174, 2002 Copyright © 2002 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/02 $22.00 ⫹ .00

Pergamon

PII S0016-7037(02)00910-9

Water-extractable and exchangeable phosphate in Martian and carbonaceous chondrite meteorites and in planetary soil analogs MICHAEL N. MAUTNER1,2,* and SOKRAT SINAJ3 1

Soil, Plant and Ecological Sciences Division, Lincoln University, Lincoln New Zealand Department of Chemistry, University of Canterbury, Christchurch 8001, New Zealand Institute of Plant Sciences, Swiss Federal Institute of Technology Zurich (ETHZ), Postfach 185, CH-83-15 Eschikon Lindau, Switzerland 2

3

(Received April 6, 2001; accepted in revised form March 6, 2002)

Abstract—Aqueous extraction contributes to the formation and weathering of planetary materials and renders electrolytes such as phosphate available for biology. In this context, the solubility of phosphate is measured in planetary materials, represented by the Mars meteorites Nakhla, Dar al Gani 476 (DaG 476), Elephant Morraine 79001 (EETA 79001), and terrestrial analogs, and in the Murchison CM2 and Allende CV3 carbonaceous chondrites. The Mars meteorites contain high levels of phosphate that is readily extracted by water, up to 15 mg kg⫺1 in Nakhla and DaG 476 and 38 mg kg⫺1 in EETA 79001, while the terrestrial analogs and the carbonaceous chondrites contain 0.5 to 6 mg kg⫺1. Correspondingly, high phosphate concentrations of 4 to ⬎28 mg L⫺1 are obtained in extracts of the Mars meteorites at high solid/solution ratios, exceeding the concentrations of 0.4 to 2.0 mg L⫺1 in the extracts of the terrestrial analogs. A wide range of planetary conditions, including N2 and CO2 atmospheres, solid/solution ratios of 0.01 to 1.0 kg L⫺1, extraction times of 1 to 21 d, and temperatures of 20 to 121°C affect the amounts of extractable phosphate by factors of only 2 to 5 in most materials. Phosphate-fixing capacity and exchangeable phosphate are assessed by the isotopic exchange kinetics (IEK) method, which quantifies the amount of P isotopically exchangeable within 1 min (E1min) and between 1 min and 3 months (E1min-3m) and the amount of P that cannot be exchanged within 3 months (E⬎3m). The IEK results show that the DaG 476 Mars meteorite and terrestrial analogs have low P-fixing capacities, while the carbonaceous chondrites have high P-fixing capacities. Aqueous processing under early planetary CO2 atmospheres has large effects on the available phosphate. For example, the fraction of total P that is exchangeable in 3 months increases from 1.6 to 11%, 13 to 51.6%, and 43.9 to 90.4% in the DaG 476 Mars meteorite, Allende, and Murchison, respectively. The results show that solutions with high phosphate concentrations can form in the pores of planetary lava ash and basalts and in carbonaceous asteroids and meteorites. These solutions can help prebiotic synthesis and early microbial nutrition. The Martian and carbonaceous chondrite materials contain sufficient phosphate for space-based agriculture. Copyright © 2002 Elsevier Science Ltd 2000) and CM2 carbonaceous chondrites (Fuchs et al., 1973; Bunch and Chang, 1980; Barber, 1981; Brearley and Jones 1998) show evidence for aqueous processing in the parent bodies. Aqueous processes also contribute to the extraction of meteorites imported to planets by in-fall (Mautner et al., 1995). Aqueous extraction releases electrolytes that become available for geochemical processes and biology. The released phosphate participates in the basic cellular functions of reproduction through DNA and RNA, energy processing by ADP/ATP, and phospholipid membranes. As a result, phosphate is essential both in early biology and in future space-based agriculture (O’Neill, 1974; O’Leary, 1977; Ming and Henninger, 1989; Lewis, 1993). In these respects, the present paper examines the extractable phosphate in planetary materials that are represented by meteorites and some terrestrial analogs. The requirements of early life-forms suggest that phosphate, possibly at high concentrations, was present in early environments. In fact, concentrated solutions of electrolytes and organics can form in igneous rocks and lava ash, in the pores of meteorites that land in aqueous planetary environments, in their parent bodies during aqueous alteration, and in porous interstellar dust particles (IDPs), all of which are extracted at high solid/solution ratios in natural environments (Kruger and Kissel, 1989; Chyba and Sagan, 1992; Mautner et al., 1995;

1. INTRODUCTION

The prospects for past and future life in planetary environments can be assessed by applying soil nutrient studies to planetary, including asteroid, materials. Such studies are possible, on a limited scale, using meteorite materials. The initial studies on carbonaceous chondrites showed that as in many terrestrial environments, phosphate is a limiting nutrient in these materials (Mautner 1997a, Mautner et al., 1997). The present work therefore addresses extractable and exchangeable phosphate in these materials in more depth and extends the initial studies to Martian materials in meteorites and some terrestrial analogs. These studies include aqueous extractions under plausible planetary conditions and isotope exchange kinetics (IEK) measurements. The amounts of extractable and exchangeable phosphate are often used to assess the short- and longer term phosphate available for plants in agricultural and natural soils. Aqueous processes contribute to the weathering of rocks in aqueous planetary environments. Both the Mars meteorites (Gooding, 1978; Gooding et al., 1988; Gooding et al., 1991, Gooding et al., 1992; Krot et al., 1998; Bridges and Grady, * Author to whom correspondence ([email protected]).

should

be

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Meurette et al., 1995; Mautner, 1997a, 2002a,b; Jull et al., 2000). The amounts of electrolytes extracted from rocks and the concentrations of the resulting solutions depend strongly on the solid/solution ratio. A large excess of water can remove all of the extractable soluble materials, even those with limited solubilities, but lead to dilute solutions. However, when the solid is in large excess, the process may be controlled by the solubility constants, leading to more concentrated solutions but removing only a small fraction of the extractable components. In nature, rocks may be extracted at a wide range of solid/ solution ratios. For example, rocks exposed in freshwater rivers and lakes and IDPs and meteorites falling in large bodies of water are extracted at virtually infinite excess of water. On the other hand, rocks and meteorites are extracted at a large excess of solid by water trapped in the pores of these solids. For example, the porosities of ⬃20% (by volume) of carbonaceous chondrite objects (Corrigan et al., 1997) leads to an ⬃10:1 solid/solution ratio (by weight) at which the meteorite solids are extracted by water trapped in the pores. The resulting concentrated solutions of electrolytes (⬎3 mol L⫺1) and organics (10 g L⫺1) trapped for long periods in the presence of catalytic mineral surfaces can be conducive to complex prebiotic synthesis and to the nutrition of early life-forms (Mautner, 2002a,b). A further important planetary factor in phosphate extraction is the high-pressure CO2 atmosphere, up to 1 to 10 bars, that may have been present on early Earth and Mars (Kasting et al., 1986; Kasting, 1993, Kasting 2000). Evidence for more recent atmospheric CO2 on Mars, at lower pressures, is indicated by the Nakhla meteorite, which shows evidence of aqueous processing under CO2 pressures of 30 to 100 mbars (Bridges and Grady, 2000). The solutions of carbonic acid formed under these atmospheres can affect the solubilities of rock components, as will be demonstrated for phosphate below and for other ions elsewhere (Mautner, 2002a). Soil science estimates soil available phosphate using a variety of methods that include extraction with water (van der Pauw, 1971), dilute acids and bases (Kamprath and Watson, 1980), anion exchange resin (Sibbesen, 1978), and isotopic exchange (Fardeau, 1996; Frossard and Sinaj, 1997). According to White and Beckett (1964), P availability is governed by three factors: (a) the intensity factor, which is the activity of phosphate ions (H2PO4⫺; HPO2⫺ 4 ) in the soil solution, represented by the CP factor in the IEK measurements; (b) the quantity factor, which is the amount of phosphate ions that can be released into soil solution from the solid phase of the soil during the interval of time considered for plant growth, related to the E(t) factor in the IEK measurements; and (c) the buffer capacity, which describes the ability of a soil to maintain the intensity factor (i.e., activity) constant when the quantity of releasable phosphate varies, related to the R/r(1) factor in the IEK measurements. Thus, three parameters are required to describe P availability. The available soil P and isotopically exchangeable P have the same physical identity (Frossard et al., 1994; Morel and Plenchette, 1994). The three factors characterizing soil P availability can be therefore deduced from the thermodynamic parameters describing the isotopic exchange process (Fardeau, 1996). The IEK method (Fardeau, 1996) has been used exten-

sively to asses inorganic P availability and associated biologic P dynamics in a variety of soils and agricultural systems (Oberson et al., 1996; Sinaj et al., 2001; Chen et al in press). Therefore, this method can be considered as a reference method and was used in this study together with aqueous extractions and extractions in H2O/CO2, which represent planetary processes. A main objective of the present paper was to investigate the effects of varying solid/solution ratios and of extraction in neutral H2O/N2 and in acidic H2O/CO2 solutions on the shortand long-term available phosphate in planetary materials, as characterized by aqueous extractions and the IEK method. 2. MATERIALS AND METHODS 2.1. Materials The rock samples used were Martian meteorites and terrestrial analogs. The Dar al Gani 476 (DaG 476) meteorite and Elephant Morraine 79001 (EETA 79001) lithology A are both basaltic shergottites. DaG 476 contains a fined-grained pyroxene and feldspathic glass ground mass with presence of sulphides and phosphates. Both meteorites contain olivine, orthopyroxene, and chromite. Phosphate minerals in the shergottites include merrillite (Ca)19Mg2(PO4)14 and whitlockite (Ca,Mg)3(PO4)2 (International Center for Diffraction Data, 1976) or Ca9(Mg,Fe)Na(PO4)7, which are the dominant phosphate-bearing materials and constitute 0.5 to 1.5% of the meteorite, and small amounts of chlorapatite [Ca5(PO4)3Cl]. EETA 79001 contains carbonate intermixed with Mg phosphate, which suggests evaporite origins (Gooding et al., 1988). The shergottites also contain small amounts of impact glasses that contain phosphate that could be of Martian or terrestrial origin (McSween and Jarosewitz, 1983; McSween and Treiman, 1998). The comparative mineralogy of DaG 476 and EETA 79001 was discussed recently (Zipfel et al., 2000). Note that DaG 476 was subject to extensive weathering in the Sahara, and it includes carbonates and desert clays (Wadhwa et al., 2001). Some of the measured phosphate may originate from the terrestrial materials. Nevertheless, the phosphate contents display similarities to the other Martian meteorite EETA 79001 found on Antarctica, which was exposed to quite different terrestrial conditions, suggesting that the phosphate results reflect indigenous properties (see below). Although weathered, DaG 476 was used because Martian materials are limited, although more pristine samples would be of course desirable. The majority of Martian meteorites are igneous rocks, including basaltic shergottites, cumulate igneous rocks, nakhlites, and Chassigny, a dunite. For comparison, we examined several terrestrial rocks of these types. The terrestrial basalt used was from Timaru, New Zealand. An X-ray diffraction (XRD) peak area analysis (using TRACES 4 software, Diffraction Technology, Australia) showed that it contains 65 ⫾ 5% labradorite feldspar, 25 ⫾ 5% clinopyroxenite, and 10 ⫾ 5% magnetite. The Nakhla meteorite is a cumulate igneous rock. Its main component is augite, with some olivine and other minor minerals, including chlorapatite, which is the major carrier of rare earth elements (McSween and Treiman, 1998). As only small amounts of Nakhla were available, we also used a terrestrial cumulate, the Theo’s Flow lava formation in Canada, which was described as a close mineralogical analog (Friedman, 1998). As Nakhla contains mainly augite, we also used a clinopyroxenite sample from the Mandamus intrusion, Canterbury, New Zealand, as an analog. It contains 80% augite, 20% feldspar, 5% olivine, and 1% ilmenite. A further cumulate igneous rock to compare with Nakhla was a sample from Black Island, Antarctica, containing ⬃85% diopside, 10 to 15% olivine, and some magnetite. As a possible analog of Chassigny, we used a dunite sample from Mount Erebus, Antarctica. It contains ⬃90% olivine with ⬃10% serpentine rims, 10% clinopyroxenite, and ⬃1% chromite, similar to the composition of Chassigny (McSween and Treiman, 1998). Terrestrial analogs of lunar and Martian soils distributed by NASA were also examined. The Mars soil simulant JSC Mars-1 is a sample of surficially altered volcanic ash from a zone at a depth of 40 to 60 cm from the Pu’u Nene volcano in Hawaii, dried at 80°C and sieved to

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Table 1. Particle size distributions in natural and hand-ground materials. Soil

⬍1

1 to 2

2 to 5

5 to 10

10 to 20

20 to 40

⬎40

Murchison ground Theo’s Flow ground Lunar simulant lava ash Templeton soil

0.36 0.15 0.17 0.16

0.21 0.38 0.39 0.31

0.24 0.28 0.19 0.28

0.12 0.11 0.14 0.16

0.04 0.06 0.08 0.05

0.01 0.01 0.02 0.007

0.011 0.007 0.015 0.040

Population of particles in the size range indicated (␮m), observed by scanning electron microscopy, as a fraction of the total particle population. The particle sizes indicated refer to the longest axis of the generally ovoid or irregular particles observed.

separate the ⬍1-mm fraction. It is composed of finely crystallized and glassy particles with alteration rinds of various thicknesses. It contains a magnetic fraction composed of feldspar and Ti-magnetite along with minor pyroxene, olivine, and glass. A more altered nonmagnetic fraction contains the same materials and amorphous material ferric oxide similar to that inferred for Martian soil. Altogether, it contains 3000 mg kg⫺1 phosphorus. It is believed to approximate the mineralogy, chemical composition, and grain size of Martian soil. (Allen et al., 1998) However, it is a weathered and biologically processed CB soil horizon. Whether it is an appropriate model for Martian soil fertility will be discussed below. The lunar simulant JSC-1 is a glass-rich volcanic ash from the San Francisco volcanic field near Flagstaff, Arizona (McKay et al., 1993). The elemental composition is similar to Apollo 14 soil sample 14163 and contains plagioclase, clinopyroxene, orthopyroxene, olivine, magnetite, ilmenite, and apatite. A terrestrial soil used for comparison was a fine sandy loam, Udic Ustochrept (US classification) formed from weakly weathered greywacke alluvium from the Templeton area in New Zealand. Results for two carbonaceous chondrites, the Allende CV3 and the Murchison CM2 meteorites, are also presented for comparison. The mineralogy of both is well known (Jarosewich, 1971; Fuchs et al., 1973; Bunch and Chang, 1980) and was reviewed recently (Brearley and Jones, 1998). The Allende meteorite contains small amounts of merrillite and apatite and small amounts of calcium phosphates in dark inclusions. The main component of Murchison CM2 is a phyllosilicate matrix formed by aqueous alteration. Small amounts of phosphates were observed in the matrices of CM chondrites, and metal inclusions in these chondrites are enriched in phosphate (Brearley and Jones, 1998). Samples of the Allende and Murchison meteorites were obtained from the Smithsonian Institution and commercial sources. A sample of the Mars meteorite Nakhla was gifted by The Natural History Museum, London (Dr. M. M. Grady). A sample of the Mars meteorite DaG 476 was obtained from a commercial source, and sawdust residues of this meteorite were provided by the Max Planck Institute fu¨ r Chemie (Dr. Jutta Zipfel). As a terrestrial analog of the Nakhla Martian meteorite, the cumulate igneous rock Theo’s Flow from Canada was gifted by the University of Hawaii (Prof. G. Taylor). Clinopyroxenite from the Mandamus intrusion, North Canterbury, New Zealand, and dunite from Matai River, New Zealand, were gifted by the University of Canterbury (Prof. S. Weaver). A cumulate igneous rock from Black Island, Antarctica, was gifted by the Victoria University of Wellington (Prof. J. Gamble), and a sample of Hawaii lava Martian soil analog and a lunar soil analog were obtained from the NASA Johnson Space Center. 2.2. Sample Preparation, Extraction, and Analysis The efficiency of extractions depends on the exposed surface area and therefore on the particle size distributions. Some of our samples are soils and lava ash used with a natural size distribution, while some of the meteorites and analogs are solids that must be ground for extraction. We employed hand grinding with an agate mortar and pestle rather than mechanical mills, which give too fine powders. The resulting powders were analysed by scanning electron microscopy. The size distributions summarized in Table 1 confirm that grinding the Theo’s Flow rock yields a particle size distribution comparable to a natural soil and lava ash. The ground Murchison powder contains a larger fraction of particles smaller than 1 ␮m, which may be due to the small grains and

inclusions released from the matrix by grinding. In all the samples, over 50% of the particles are in the range 1 to 5 ␮m, in the same range as the 3.2-␮m mean diameter of Martian dust particles (Pollack et al., 1995). Most samples were extracted in deionized water for 1 to 21 d. Exposure times of several weeks simulate natural weathering in aqueous environments, which may lead to chemical changes on the mineral surfaces. To preclude oxidation of the mineral surfaces, most extractions were carried out in water that was deoxygenated by bubbling and stirring for 20 min with 1 atm N2 (denoted as H2O/N2) or with CO2 (denoted as H2O/CO2) to simulate early planetary conditions. All extractions were carried out at 20°C with vortex shaking of the samples two to four times in the 24-h extractions and once in 48 h in the longer extractions. Note that the method of processing should affect only the rates of extraction but not the equilibrium concentrations of interest in this study. Comparing phosphate concentrations in the extracts after 1, 4, and over 8 d suggests that constant solution concentrations are achieved in 4 d, followed in some samples by slower exposure effects. In extractions longer than 4 d, a drop of toluene was added to the solutions to inhibit microbial growth. Olsen reagent extractions were performed in 0.5 mol/L NaHCO3 at a solid/solution ratio of 0.05 kg L⫺1 for 30 min (Olsen and Sommers, 1982). Analysis of total phosphate content was performed on 10- to 100-mg samples digested with 10 mL of 70% HNO3 solution overnight, adding 4 mL of 11.6 mol/L HClO4 and heating to 210°C, cooling and diluting appropriately for analysis by the malachite green reagent (van Veldhoven and Mannerts, 1987; Subbao Rao et al., 1997). Since the solubility of phosphate is pH dependent, we measured the pH of extracts obtained after 1 d of extraction. The pH values measured for the meteorites at a solid/solution ratio of 0.01 kg L⫺1 were 7.4, 7.9, and 8.1 for Allende, Murchison, and DAG 476, respectively. For the terrestrial analogs, the pH was measured at solid/solution ratios of 0.01, 0.1, and 0.4 kg L⫺1, and the following values were obtained: JSC Mars-1 simulant: 7.6, 7.4, and 7.4; JSC-1 lunar simulant: 7.7, 9.4, and 9.2; basalt: 7.4, 7.6, and 8.0; Black Island cumulate: 7.6, 9.6, and 9.4; clinopyroxenite: 8.5, 8.9, and 8.6; Theo’s Flow cumulate Nakhla analog: 8.0, 8.7, and 8.1; and Templeton soil: 7.1, 6.4, and 5.9. A pH value of 3.9 was measured for the solutions saturated with CO2 at 1 bar. 2.3. Isotopically Exchangeable Phosphorus The experimental procedure conducted on a soil-solution system in a steady state with a soil/solution ratio of 0.1 kg L⫺1 has been recently described (Fardeau, 1996; Frossard and Sinaj, 1997). In this study, because of the small quantities of solid materials, experiments were conducted as described by Sinaj et al. (1997) with a solid/solution ratio of 0.01 kg L⫺1. After an addition of carrier-free 33PO4 ions to a solid-solution system in steady state, the solution is sampled four times from 1 to 60 min. When 33PO4 ions are added to a soil-solution system at a steady-state equilibrium, the radioactivity in solution decreases with time according to the following equation (Fardeau et al., 1985): r共t兲/R ⫽ 共r 共1兲兲/R) ⫻ 关t ⫹ 共r 共1兲/R兲 1/n兴 ⫺n ⫹ r 共 x兲/R

(1)

where R is the total introduced radioactivity (⬵0.1 MBq); r(1) and r(⬁) are the radioactivity (MBq) remaining in the solution between 1 min and infinity, respectively; and n is a parameter describing the rate of disappearance of the radioactive tracer from the solution after 1 min. The parameter n is calculated as the factor of the linear regression

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between log[r(t)/R] and log(t). The ratio r(⬁)/R, which is the maximum possible dilution of the isotope, is operationally approximated by the ratio of the water-soluble P to the total P of the soil (PT, expressed in mg P kg⫺1 soil; Fardeau, 1996). Thus, r 共⬁兲/R ⫽ 10 ⫻ C p /P T

(2)

The quantity of isotopically exchangeable P at time t, E(t) (mg P kg⫺1 material), was calculated from Eqn. 3, assuming that 31PO4 and 33 PO4 had the same fate in the system and that at a given time, the specific activity of the phosphate in the solid-solution system was identical to that of the solid isotopically exchangeable phosphate (Fardeau et al., 1985): E共t兲 ⫽ 100 ⫻ C p ⫻ R/r共t兲

(3)

E 1min ⫽ 100 ⫻ C p ⫻ R/r 1

(4)

Table 2. Comparison of Olsen phosphate (PO4-P, mg kg⫺1) measured by the International Soil Analytical Exchange Programme (ISAEP) and by the present microanalytical method (50-mg samples). ISAEP soil

This work

ISAEPa

ISAEP range

Sandy soil 955 Regolith 982 Clay 954 Clay 965 River clay 921 Sandy soil 981

3.5 5.3 10.2 73 120 197

2.0 (1.0) 4.3 (2.6) 7.2 (0.8) 51 (10) 99 (14) 145 (24)

0.6 to 3.1 1.8 to 8.0 5.0 to 9.7 21 to 79 68 to 125 81 to 188

a

Median and median of absolute deviations.

For t ⫽ 1 min,

where CP is the concentration of water-soluble phosphate (mg phosphate L⫺1), R is the total introduced radioactivity (⬵0.1 MBq), and r(t) is the radioactivity (MBq) remaining in the solid-solution system after t minutes. The factor of 100 arises from the solid/solution ratio of 1 g of solid in 100 mL of water, so that 100 ⫻ CP is equivalent to the water-soluble P content of the solid materials expressed in milligrams per kilogram. The term R/r1 expresses the fraction of labeled P that is exchanged in the first minute. A high rate of exchange between solution and the solid corresponds to a high fixing capacity. Accordingly, this ratio estimates the P ion fixing capacity of soils (Tran et al., 1988; Frossard et al., 1993). For R/r1 higher than 5, the P fixing capacity is considered to be high, between 2.5 and 5 medium, and below 2.5 low (Fardeau et al., 1991). The measured kinetics of isotope exchange during 1 h can be extrapolated to longer times, as was verified experimentally (Fardeau at al., 1985). In the present work, the kinetics are extrapolated to 3 months. On this basis, the IEK data can be interpreted in terms relevant to biology (Fardeau, 1993; Barber, 1995), as phosphate pools of varying availability: 1. The pool of phosphate exchangeable within 1 min (E1min) Ions present in this pool are composed of ions in the soil solution and those ions that are adsorbed on the solid phase of the soil but have the same kinetic properties as those in solution (Fardeau et al., 1985). Phosphate ions located in this compartment are completely and immediately bioavailable. These ions may be directly exchanged with ions located in the other pools (Fardeau, 1996; Sinaj et al., 1999). 2. The pool of phosphate exchangeable between 1 min and 3 months (E1min-3m) corresponds to the quantity of phosphate exchangeable during a period equivalent to the time of active P uptake by the entire root system of an annual crop The phosphate content of this pool was calculated as described by Fardeau (1996). 3. The pool of P that cannot be exchanged within 3 months (E⬎3m) This pool contains P that is slowly or not exchangeable, for instance, P occluded in minerals or strongly adsorbed onto solid particles. The P content of this pool was calculated as the difference between the total P and the amount of P exchangeable within 3 months (E⬍3m). By applying the IEK method and the above considerations of the various phosphate pools, we can obtain the following information: 1. The phosphate concentration in the soil solution (CP), which corresponds to the intensity factor; 2. the quantity of isotopically exchangeable phosphate, E(t), which gives information on the quantity factor; and 3. the R/r(1) ratio and n value, which correspond to the capacity factor. The relevance of these factors to phosphate availability is explained below. In relation to the present measurements that were carried out at a soil/solution ratio of 0.01 kg L⫺1, we note that this ratio affects CP but not E(t) as the R/r(1) ratio changes also, as was demonstrated for Zn (Sinaj et al., 1999).

2.3.1. Phosphate analysis Phosphate analysis was performed colorimetrically by developing the solutions with malachite green reagent at 5:1 sample/reagent solution ratios for 1 h and measuring the absorbance at 630 nm (van Veldhoven and Mannerts, 1987; Ono and Zibilske, 1991; Subba Rao et al., 1997). Samples with high concentrations were diluted to the optimal range of 0.04 to 0.8 mg P L⫺1 before measurement to avoid a green complex that precipitates at higher concentrations. A major limitation on the measurements of meteorites is the small available sample size, which had to be kept at 10 to 100 mg in most cases. In some samples, this results in concentrations near 0.02 mg P L⫺1, at the lower limit of reliable measurement. The small sample size may also result in possible inhomogeneity from various locations in the samples, which will be examined below. Most measurements were replicated two to six times. No literature data are available on the aqueous extracts of any of our materials for comparison. However, data are available on Olsen extractions (0.5 mol/L NaHCO3) from the International Soil Analytical Exchange Programme (ISAEP) of the University of Wagenningen, the Netherlands, on samples analysed usually by 10 to 40 laboratories worldwide. To check our measurements, we measured the Olsen extractable P (Olsen and Sommers, 1982) in six samples from the ISAEP with a wide range of phosphate contents. We extracted samples of 50 mg in 1 mL of 0.5 mol/L NaHCO3, similar to the amounts of the meteorite samples. Our results, compared with the reported data in Table 2, are within or near the higher end of the reported range, usually higher by 20 to 30% than the median. This may be due to the somewhat different shaking procedure in our extractions and the small sample sizes. These comparisons and the reproducibility of our measurements on replicate samples in this and preceding studies suggest an estimated uncertainty of ⫾30% in the results, which is comparable to the range of ISAEP analyses, as shown in Table 2. The uncertainty in our analyses is due to the small sample sizes and limited amounts of meteorite material for replicates. 3. RESULTS AND DISCUSSION

3.1. Phosphate Distribution in Meteorites Meteorites represent samples of asteroids or planetary materials. It is of interest whether the phosphate distribution is homogenous in these materials. Microscopic homogeneity in the Nakhla meteorite was tested by combined scanning electron microscopy– energy dispersive spectroscopy (SEM-EDS). Fig. 1 shows an SEM picture of a Nakhla fragment. Some of the spots lacked detectable P, while in most spots, the elemental abundance was 1000 to 2000 mg kg⫺1. However, some spots were richer in P, up to 6000 mg kg⫺1. These spots were mostly associated with deposits of finer sediment, possibly feldspar. Some small fragments analysed separately contained up to 9000 mg P kg⫺1.

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Fig. 1. Scanning electron micrograph of a fragment of the Nakhla Martian meteorite. The phosphate contents obtained by X-ray fluorescence of spots 1 to 8 shown are 5000, 3000, 3600, 2300, 1800, 1500, 100, and 1600 mg kg⫺1, respectively, while spots on another piece of comparable size gave 5700, 1000, 1400, 1300, 2100, 1000, 700, and 1500 mg kg⫺1, respectively.

The homogeneity of total P in the Allende meteorite was tested by acid digestion of six samples of 10 to 20 mg, two of which were fragments of samples provided by the Smithsonian Institution and four of which were fragments of various commercial samples. The measured total P ranged from 900 to 1310 mg kg⫺1, with an average of 1200 ⫾ 280 mg kg⫺1. Similarly, four samples from the Murchison meteorite were analysed, two from the Smithsonian and two from commercial sources, and gave total P in the range of 900 to 1130 mg kg⫺1 with a average of 1050 ⫾ 100 mg kg⫺1. The homogeneity of extractable PO4-P in the carbonaceous chondrites was tested by Olsen extraction of five 25-mg samples of Allende, two from the Smithsonian and three from commercial sources. The results were in the range of 6.20 to 8.55 mg kg⫺1, with an average of 7.32 ⫾ 0.88 mg kg⫺1. Four 25-mg samples of the Murchison meteorite were extracted similarly by the Olsen reagent. The results were in the range of 6.99 to 8.70 mg kg⫺1, with an average of 7.83 ⫾ 0.84 mg kg⫺1. The 10- to 25-mg fragments were obtained from several locations in 10-g piece samples from the Smithsonian Institution and from various commercial sources. These samples, especially the commercial samples, whose histories are not documented, may represent locations widely distributed in the original meteorites and their fragments. Nevertheless, all the results, both for the total and the extractable phosphate, were equal within the estimated uncertainty of ⫾30%. These results suggest that the distribution of total and soluble phosphate is homogenous on the 10-mg scale throughout the meteorites. However, fusion crusts yielded significantly higher levels of

phosphate. Olsen extraction of samples containing ⬃50% fusion crust yielded 15 mg P kg⫺1, suggesting extractable phosphate of ⬃30 mg kg⫺1 in the fusion crust itself, an increase by a factor of 4 over the interior. This is consistent with the large increase in extractable phosphate in samples subjected to pyrolysis in air at 550°C that we observed previously (Mautner, 1997a). The effect may be due to the oxidation of minerals and possibly increased porosity and accessibility of water to larger surface areas because of the pyrolytic processes. Heating and oxidation during in-fall can therefore increase the available phosphate imported by meteorites. This may be especially significant in relation to the imported phosphate by micrometeorites and Interplanetary Dust Particles (IDPs) that have large surface areas and that are impacting the Earth in large amounts (Chyba and Sagan, 1992). 3.2. Phosphate Concentrations in the Water Extracts at Varying Solid/Solution Ratios Freshly formed igneous materials such as volcanic ash and other pristine minerals such as asteroids and meteorites are extracted in nature at a wide range of solid/solution ratios. The results of the aqueous extractions may be considered in two forms: (a) the concentration of water-soluble P, called the intensity factor, denoted here as CP (mg P L⫺1), and (b) the amount of P extracted by water per unit weight of the solid, called the quantity factor, denoted as Pw (mg P kg⫺1 solid), which is an important measure of the P that is readily bioavailable. In a given extraction, both relate to the same amount of

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Fig. 2. (a) Phosphate concentrations in solution CP (mg P L⫺1) obtained by extractions in H2O/N2 at various solid/solution ratios (rsolid[kg]/H2O[L]). Extraction times as shown. (b) Phosphate concentrations in solution CP (mg P L⫺1) obtained by extractions in H2O/CO2 and by Olsen extraction at various solid/solution ratios (rsolid[kg]/H2O[L]). Extraction times as shown.

phosphate extracted from the solid into the solution, one expressed as the resulting concentration in solution, the other expressed as the fraction of total solid that was extracted into solution. The two factors are related by a simple equation, and both can vary with the solid/solution ratio (rs/w) (kg L⫺1) used in the extraction: Pw (mg kg⫺1) ⫽ Cp (mg L⫺1)/r s/w (kg L⫺1).

(5)

Given Eqn. 5, in the range of low rs/w where CP increases linearly with rs/w (Figs. 2a and b), the value of Pw, the amount of extractable phosphate per kilogram solid, remains constant. Conversely, in the range of high rs/w where CP levels off, it follows from Eqn. 5 that the amount of extractable phosphate

per kilogram solid decreases with further increasing solid/ solution ratio. Using Eqn. 5, the solution concentrations observed in the present experiments (mg L⫺1) can be obtained from Table 3 by multiplying the quoted Pw values by 0.01 for the extractions at the 0.01 kg L⫺1 solid/solution ratio and by 0.1 for the extractions at the 0.1 kg L⫺1 solid/solution ratio. Figures 2a and 2b show plots of CP vs. rs/w in the extractions of several of the present materials. It is convenient to consider the solid/solution ratios in terms of various amounts of solid added to a constant amount of water. In the initial portions of the plots with small amounts of solid, all of the extractable phosphate may pass into the solution. In this range, CP increases linearly with rs/w. On the other hand, at high solid/

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Table 3. Extractable phosphate (mg P kg⫺1) from meteorites and simulated planetary materials. Pwa Extractant

H2O/N2

Solid/solution (kg L⫺1)

0.01

0.1

3 to 5 d or ⬎6 d

Extraction time (d)

1d

Carbonaceous chondrites Allende

1.1d

Murchison

0.8d

2.8 2.8l 1.5 0.8l

Mars meteorites DaG 476

12.7d

9.3j

EETA 79001 Nakhla Terrestrial analogs Theo’s Flow (Nakhla analog)

38.6 10.0 d

1.2

1d

3 to 5 d or ⬎6 d

1.9

3.9

5.0

0.7

1.1

1.1

6.3

22.6

15.0

124.9 10.6

0.5

0.8

5.8

1.1k 2.6h

2.2l Basalt (Timaru, New Zealand) JSC-Mars1

2.2d

JSC-1 lunar simulant Black Island Clinopyroxenite

6.0

Dunite Templeton soil

0.3d 4.6

0.5d 3.3d

0.01

39.2 2.1

3.6 2.8l

1.0

2.8i 2.2m

2.7 2.2k

H2O/N2b, 121°C

POlsenc

0.01

0.05

3 to 5 d or ⬎6 d

15 min

30 min

5.1

7.5e

4.8

5.0e

7.3f (13.5)g 7.8f (15.5)g

H2O/CO2

3.0

1d

0.4

0.1

3 to 5 d or ⬎6 d

1d

3.8 0.7d,l 3.8 1.5d,l 24.2i 33.8l 82.8i

19.4 37.1

1.8

1.0i 1.3l

3.2 0.3j 4.7

0.8 1.1

0.5 1.0

0.9 5.3

46.2 5.2

k

0.3

0.2

1.1

2.6

0.4k 3.5h

13.3

1.8l 0.9

14.8

0.6

0.5i 1.0l

1.6

3.0

4.4

12.0

0.4 2.9

3.4

1.1 1.5k 0.2 0.8

3.4

2.6 1.5 0.9 5.3

a Extractions in deionized water deaerated by N2 or CO2 and extracted under 1 atm N2 or CO2, respectively, at 20 ⫾ 1°C for extraction times of 1 d or 3 to 5 d. Data for longer extractions times are indicated in notes h/m. Solid:water ratios (kg L–1) are shown in column headings. b Extractions at 121°C for 15 min at solid/solution ratio of 0.1 kg L–1. c Extracted in 0.5 mol L⫺1 NaHCO3 for 30 min at a solid/liquid ratio of 0.05 kg L–1. d From measurements in conjunction with the Isotopic Exchange Kinetics experiments, after extraction by water in H2O/N2 or in H2O/CO2 for the time period as indicated. e Average of present measurement (Allende: 5.1 mg kg–1, Murchison: 4.5 mg kg–1) and literature results (Allende: 9.8 mg kg–1, Murchison: 5.5 mg kg–1) (Mautner, 1997a). f With Allende, average of six samples and with Murchison, average of four samples from the Smithsonian Institution and commercial sources, all within ⫾0.8 mg kg–1 of the average. g Samples containing ⬃50% fusion crust. h 6 d. i 8 d. j 9 d. k 14 d. l 21 d. m 38 d.

solution ratios, the solution becomes saturated with respect to the adsorption/desorption equilibrium, no further P is released to the solution with added solid, and CP remains constant (Figs. 2a and 2b). At low solid/solution ratios, most of the extractable phosphate is dissolved, and the concentration increases approximately linearly with added solid. In the H2O/N2 extractions (Fig. 2a), this range applies up to solid/solution ratios of 0.1 to 0.4 kg L⫺1 and CP becomes constant at ratios above 0.1 kg L⫺1 for the basalt and 0.4 kg L⫺1 for the lunar simulant lava ash materials, with CP values about of 0.2 and 0.4 mg L⫺1, respectively. In comparison, in H2O/CO2 (Fig. 2b), the solution concentrations become constant already at lower solid/solution

ratios of 0.05 kg L⫺1 for the JSC Mars-1 simulant lava ash and 0.1 kg L⫺1 for the basalt sample at somewhat lower aqueous concentrations of 0.1 and 0.3 mg L⫺1, respectively. The lower equilibrium concentrations in the samples extracted by H2O/ CO2 may be due to adsorption on carbonates formed in these systems or on Fe and Al oxides at this low pH. In the Olsen extractions of the JSC Mars-1 simulant, in 0.5 mol/L NaHCO3 saturation also occurs at a low solid/solution ratio of 0.05 kg L⫺1 at a concentration of ⬃0.3 mg L⫺1 (Table 3). The upper limits of 0.1 to 0.4 mg L⫺1 phosphate concentrations obtained at high solid/solution ratios are higher than the median range of 0.005 to 0.05 mg L⫺1 in soil solutions (Bo-

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M. M. Mautner and S. Sinaj Table 4. Aqueous phosphate concentrations obtained at high solid/solution ratios. Soil

Allende Murchison DaG 476 EETA 79001 JSC-Mars l JSC-1 lunar simulant Theo’s Flow Black Island Clinopyroxenite Dunite Templeton soil

Extractions in H2O/N2

Extractions in H2O/CO2

Solid/solution ratio (kg L–1) and extraction time (d)

Cp (mg P L–1)

1.0 (4 d) 1.0 (4 d) 1.0 (4 d) 1.0 (4 d) 0.4 (8 d) 1.0 (6 d) 1.0 (4 d) 0.4 (9 d) 1.0 (4 d) 1.0 (4 d) 1.0 (4 d)

4.4 5.1 6.9 ⬎28.8 1.9 0.4 0.5 0.1 0.8 0.6 0.8

wen, 1966; Barber, 1995) but are well within the overall range of soil solutions, 0.001 to 3 mg L⫺1 (Reisenauer, 1964; Barber, 1995; Sinaj et al., 1998; Frossard et al., 2000).

Solid/solution ratio (kg L–1) time and extraction time (d)

Cp (mg P L–1)

1.0 (4 d)

10.3

0.4 (1 d)

0.1

1.0 (4 d)

0.2

1.0 (1 d) 1.0 (4 d)

1.0 0.2

and correspondingly low soluble phosphate. The Templeton Soil and Hawaii lava ash are both weathered and may contain Fe and AL oxides and organic materials that strongly bind phosphate and make it insoluble (Sinaj et al., 2002).

3.3. Extractable Phosphate The extraction results are summarized in Table 3 in terms of the extractable phosphate content in the solids. The main interest is to compare the various meteorites and analogs. The data illustrate the effects of solid/solution ratio (0.01 or 0.1 kg L⫺1), extraction and exposure time (1 to 4 to 21 d), or H2O/N2 vs. H2O/CO2 extraction and extraction at 20°C vs. mild hydrothermal conditions (121°C for 15 min). Under all conditions, most of the terrestrial samples showed water-extractable phosphate contents of 1 to 6 mg kg⫺1. The two rocks with high olivine content, the Black Island and dunite rocks, were lower, 0.2 to 0.5 mg P kg⫺1. The most remarkable result is the high extractable phosphate content in all three Mars meteorites. Under extraction by H2O/ N2, Nakhla yielded 15.0 mg P kg⫺1 in the 4-d extraction, DaG 476 yielded 12.7 mg P kg⫺1, and EETA 79001 yielded 38.6 mg P kg⫺1 in the 1-d extractions. Increasing the extraction times from 1 to 4 to 8 d resulted in increases up to a factor of 2 in most samples. Table 3 (notes h to m) shows several examples for which extraction times up to 9 to 38 d also had no significant further effects. The hydrothermal extractions were carried out in H2O/N2 at 0.1 kg L⫺1 solid/solution ratio. As may be expected, hydrothermal processing at 121°C extracts more phosphate than aqueous extraction at 20°C, with an increase up to a factor of 2 for most materials. Extraction by the Olsen reagent, 0.5 mol/L NaHCO3, also gave high phosphate yields comparable to the H2O/CO2 and hydrothermal extractions. Significant changes were also observed in the extractions by H2O/CO2 compared with H2O/N2, as discussed below. The amounts of extractable phosphate in Tables 3 and 4 seem to be related to the total phosphate content and to the amount of aqueous exposure experienced by the samples. All the meteorites have relatively high total P contents and experienced little aqueous terrestrial weathering, retaining most of their original soluble phosphate contents. The Theo’s Flow and Black Island cumulates and the dunite sample have low total

3.4. Extractions at High Solid/Solution Ratios The prebiotic synthesis of phosphate-bearing molecules should be facilitated by high concentrations of phosphate that may form at high solid/water ratios. As noted in Fig. 2, the solution concentrations, CP, level off starting at solid/solution ratios of 0.1 to 0.4 kg L⫺1. Table 4 summarizes the limiting CP values at these ratios. The limiting P concentrations in the meteorite extracts are significantly higher than in most of the terrestrial samples measured. The carbonaceous chondrites yield solutions containing over 4 mg P L⫺1, and the Mars meteorites yield solutions containing over 6 to 28 mg P L⫺1, higher than the usual range of soil solutions. Since Ca is the main cation in the extracts, we measured for comparison the CP in saturated solutions of the monocalcium phosphate (CaHPO4) and tricalcium phosphate (Ca3[PO4]2). At 20°C, we obtained concentrations of 113 and 96 mg P L⫺1, respectively, higher than in the meteorite extracts. This suggests that the lower limiting concentrations obtained in the meteorites may be due to adsorption/desorption equilibria on the mineral surfaces rather than to the solubility constants of the phosphate minerals in the meteorites. 3.5. IEK Parameters and Exchangeable Phosphate The IEK results show large differences among the materials. The factor R/r(1), which correlates with soil P-fixing capacity (Frossard et al., 1993), varies from 1.0 to 14.8 in the unweathered materials, compared with 1 to ⬎100 in agricultural soils; n varies from 0.059 to 0.362 compared with 0.05 to 0.5 in soils; and CP varies from 0.0027 to 0.13 compared to 0.001 to 3 mg L⫺1 in agricultural soils (Table 5) (Frossard et al., 1993; Sinaj et al., 1997; Oberson et al., 1999; Sinaj et al., 2001; Chen et al in press). In all of these kinetic parameters, the present planetary materials are within the range, although near the lower limits, of terrestrial soils. In our materials, the carbonaceous chondrites Murchison and Allende have the highest R/r(1) and

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Table 5. Total phosphorus, kinetic parameters, and isotopically exchangeable P in meteorites and in model terrestrial rocks and soils. Cp (mg P L–1)

R/r1

n

E1min (mg P kg–1)

Fresh extractions Allende 0.0113 4.0 0.3139 4.5 Murchison 0.0078 14.8 0.3621 11.6 Dar al Gani 476 0.127 1.2 0.0597 15.2 Hawaii lava 0.0218 1.9 0.2165 4.1 Theo’s Flow 0.0116 1.2 0.2325 1.4 Black Island 0.0051 1.0 0.1646 0.5 Clinopyroxenite 0.0332 1.1 0.0822 3.7 Dunite 0.0027 1.2 0.3061 0.3 Water-exposed materials (exposed to H2O/N2 or H2O/CO2 for 21 d) Allende/N2 0.0282 1.6 0.3384 4.6 Allende/CO2 0.0068 42.2 0.3228 28.7 Murchison/N2 0.0077 10.1 0.4271 7.7 Murchison/CO2 0.0154 24.1 0.4741 37.2 Dar al Gani 476/CO2 0.338 1.0 0.1686 33.8 Hawaii lava/N2 0.0284 1.5 0.2949 4.2 Hawaii lava/CO2 0.0131 3.3 0.4885 4.3 Theo’s Flow/N2 0.0216 1.3 0.4976 2.8 Theo’s Flow/CO2 0.0175 4.1 0.6351 7.1

E1min-3m (mg P kg–1)

E⬎3m (mg P kg–1)

Pt (mg P kg–1)

E⬍3m (% of Pt)

152.0 449.5 15.1 46.5 18.2 2.8 5.9 1.4

1043.6 588.9 1849.6 1959.4 290.4 84.6 1580.3 0.3

1200 1050 1880 2010 310 88 1590 2

13.0 43.9 1.6 2.5 6.4 3.8 0.6 85.8

199.1 591.3 548.3 911.8 183.7 121.6 808.6 233.6 295.4

996.3 580.0 493.9 101.0 1662.6 1884.2 1197.1 73.5 7.5

1200 1200 1050 1050 1880 2010 2010 310 310

17.0 51.6 53.0 90.4 11.6 6.3 40.5 76.3 97.6

Pt: Total phosphorus in the solid measured by colorimetry after an acid digestion. For the meaning of the other isotopic exchange kinetics factors, see text. N2 denotes exposure to H2O/N2, and CO2 denotes exposure to H2O/CO2; see text.

n values, having P-fixing capacities similar to terrestrial soils with high contents of Fe, Al, and Ca oxides. The 33PO4 added in these samples is rapidly exchanged with 31P in the solid phase (Figs. 3a and 3b), corresponding to the relatively high fixing capacity and low concentration of ion phosphates. The lowest n and R/r(1) and highest CP values are observed in the DaG 476 Martian basalt, exhibiting a low P-fixing capacity and correspondingly a high concentration of phosphate in the solution (Table 5). The 33PO4 added in this sample is at first instantaneously diluted in a large pool of 31PO4 in the solution and afterward exchanges slowly with the 31PO4 on the solid phase (Fig. 3b). The rest of the materials show intermediate kinetic parameters. The data in Table 5 show that the kinetic parameters of these materials are in the range of agricultural soils. Similar to the variation of the R/r1, n, and CP parameters, the P contents of different compartments (Table 5) also vary strongly among the materials. The quantity E1min represents the pool of ion phosphates that is exchanged during the first minute of the experiment. The quantities of P in this pool for both carbonaceous chondrites and in the DaG 476 samples were higher than 4 mg P kg⫺1 solid, below which agricultural soils are phosphate limited (Tran et al., 1988; Morel et al., 1992). The E1min values of Hawaii lava and clinopyroxenite were also close to this value, while the other terrestrial materials have little available phosphate. In addition to the free phosphate (E1min pool), other pools of exchangeable phosphate exist in the present materials (Table 5). The pool of phosphate exchangeable between 1 min and 3 months (E1min-3m) can contribute significantly to the long-term phosphate bioavailability. Table 5 shows that quantities of exchangeable P in this pool in the unweathered Allende and Murchison sample were very high, 152 and 445 mg P kg⫺1, respectively, indicating an important long-term reserve. In the other samples, these values were below 50 mg P kg⫺1. The highest quantities of slowly exchangeable P (E⬎3m, higher than

1.5 g P kg⫺1 solid) are observed in DaG 476, Hawaii lava ash, and clinopyroxenite. 3.6. The Effects of Exposure to Water Under N2 and CO2 Atmospheres Rocks may be exposed to water under aqueous conditions on planets, after the in-fall of meteorites to Earth, or following the terraforming of Mars and asteroids. The early exposure to water and aqueous conditions constitutes the first phases of weathering, which was simulated in this work by extraction and exposure up to 21 d and in a few cases up to 38 d (Table 3, notes h to m). With respect to early planetary conditions, exposure to water saturated with CO2 is of interest, as the atmospheres of early Earth and Mars may have contained high partial pressures of carbon dioxide (Kasting et al., 1986; Kasting, 1993, Kasting 2000). Correspondingly, the early oceans may have contained dissolved carbonic acid and may have had a low pH, for example, a pH of 3.9 at 1 bar CO2 atmosphere. Table 5 shows the IEK parameters (CP, R/r1, and n) of materials exposed for 21 d in H2O/N2 and H2O/CO2, and Table 6 summarizes these effects in terms of the ratios of these parameters to the unweathered materials. Exposure to H2O/N2 for 21 d decreases somewhat the phosphate-fixing capacities and increases the P concentration as reflected by R/r1 and CP, respectively, maybe by dissolution of phosphate-binding sites. The short-term and midterm exchangeable phosphate pools increase moderately, except for Theo’s Flow, for which a large effect is observed. Tables 3 and 6 show that phosphate concentrations in the extracts of both carbonaceous chondrites and the Martian meteorites increase under H2O/CO2 vs. H2O/N2 extractions. The effect is particularly large in Allende and in the DaG 476 and EETA 79001 Martian basalts. Conversely, in the terrestrial igneous materials, including the lava ash, clinopyroxenite, and the Theo’s Flow cumulate, the extracted phosphate decreases in

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Fig. 3. (a) Kinetics of the isotopic dilution (r[t]/R) of Murchison meteorite, i.e., the fraction of radioactivity remaining in solution as function of time after adding the P33 tracer. The meteorite was exposed to and equilibrated with the solution in the indicated solutions during the time indicated in the figure, before adding the labelled tracer. (b) Kinetics of the isotopic dilution (r[t]/R) of the Allende and Dar al Gani (DAG) 476 meteorites, i.e., the fraction of radioactivity remaining in solution as function of time after adding the P33 tracer. The meteorites were exposed to and equilibrated with the indicated solutions during the indicated time indicated in the figure, before adding the labelled tracer.

the H2O/CO2 vs. H2O/N2 extractions. Although the effects are small, they are consistent under various solid/water ratios and extraction times. The effects in the terrestrial materials may be due to the pH changes. The natural pH of the extracts of these terrestrial materials in H2O/N2 is mostly in the range of 7 to 9, at which the dissolution rate and phosphate fixation in soils is low (Barrow, 1983; White, 1983), whereas the pH of the H2O/CO2 extracts is 3.9, at which phosphate tends to be precipitated as iron and aluminum phosphates (Taylor et al 1964;

Jonasson et al., 1988; McLaren and Cameron, 1990). However, the role of the pH depends also on the forms of P in the mineral. If the P is in Ca-P form, the decrease of pH below 5 (in our case with H2O/CO2) increases the solubilization of these Ca-P forms and consequently the release of P in the solution (increase of CP and E1min) and an increase of the specific surface area, leading to an increase of P adsorption, as reflected by R/r(1). Large effects due to H2O/CO2 exposure can be observed in the isotopic exchange kinetics. Exposure to H2O/CO2 increases

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Table 6. Effects of exposure to H2O/N2 or H2O/CO2 for 21 d on the isotopic exchange kinetic parameters and isotopically exchangeable phosphorus. Ratio of the measured parameters for water-exposed to fresh materials. Cpa (1 to 8d) Exposure to H2O/N2 Allende Murchison Hawaii lava Theo’s Flow Exposure to H2O/CO2 Allende Murchison Dar al Gani 476 Hawaii lava Theo’s Flow

Cpb (21 d)

R/r1c

E1minc

E1min-3mc

2.5 1.0 1.3 1.9

0.4 0.7 0.8 1.1

1.0 0.7 1.0 2.0

1.3 1.2 2.6 12.8

0.6 2.0 2.6 0.6 1.5

10.6 1.6 0.8 1.7 3.4

6.4 3.2 2.2 1.1 5.1

3.9 2.1 12.2 17.4 16.2

2.4 2.8 2.2 0.4 0.5

a The average ratio of phosphate, Pw (PO4-P) (mg kg–1), in Table 5, extractable in 1 to 8 d in H2O/CO2 vs. those extracted in H2O/N2 under similar conditions. b The ratio of phosphate extractable in H2O/N2 or H2O/CO2, respectively, in 21 d vs. the amount extractable in H2O/N2 in 1 to 8 d. c The ratio of the phosphate exchangeable in the time periods shown in materials exposed for 21 d to H2O/N2 or H2O/CO2, respectively, vs. fresh materials, from the data in Table 5.

the pool of free phosphate ions (E1min) and the pool of phosphate exchangeable between 1 min and 3 months (E1min-3m), as shown in Tables 5 and 6, in all the materials. The effects of H2O/CO2 exposure are significantly larger than those of H2O/N2 (Tables 5 and 6, Figs. 2 and 3). Significant increases are observed in the exchangeable P pools E1min and E1min-3m in DaG 476 and the Mars simulants. While only a small fraction of the total P content is available in most of the fresh materials, the majority of the total P becomes available in the materials exposed to H2O/CO2 for 21 d. For example, E1min-3m increases by a factor of over 16 in the terrestrial lava ash samples after exposure to H2O/CO2 for 21 d. Extraction of Allende in H2O/CO2 compared with H2O/N2 increased the extracted P between 1 and 8 d, but the extracted amount decreased after an exposure for 21 d. The decrease after the longer exposure paralleled the large increase in the P-fixing capacity, as measured by the R/r(1) parameter (Tables 5 and 6). These results suggest that long exposure to H2O/CO2 forms phosphate-binding materials, probably carbonates, which readsorb much of the initially dissolved phosphate (Barrow, 1983). The P-fixing capacity increases in most of the materials because of H2O/CO2 exposure and an increase in the surface specific area (Barrow, 1983; White, 1983). The large effects of exposure to H2O/CO2 may be related to the alteration of mineralogy of basalts after exposure to H2O/ CO2 for a few days (Baker et al., 2000). To check these effects in the present materials, we carried out an XRD analysis of Murchison and the JSC Mars-1 simulant, fresh or exposed to H2O/CO2, for 21 d. In these samples, no carbonates were formed above the 1% detection level. However, some of the observed effects may still be due to small amounts of carbonates formed on the mineral surfaces that can adsorb phosphate. A broadening of the XRD lines was observed, suggesting the possible conversion of some of the crystalline materials to disordered amorphous forms. The observed increase in the long-term exchangeable phosphate pools may be due to more accessible phosphate-binding sites in these materials. It may be questioned whether the observed effect of increased phosphate extraction from the meteorites is due to increased solubility of phosphates in H2O/CO2. Since Ca2⫹ is

usually the main cation in these extracts, we examined the effects of H2O saturated with CO2 on the solubilities of CaHPO4 and Ca3(PO4)2. The solubilities increased somewhat, from 113 to 204 mg P L⫺1 and from 93 to 110 mg P L⫺1 in saturated solutions at 20°C for the two compounds, respectively. This effect is similar in direction to the concentrations (CP) in Table 6 that increase in the Murchison, Theo’s Flow, and DAG 476 extracts by exposure to H2O/CO2 for 21 d, but opposite to the decrease observed in Allende and in the Hawaii lava Mars simulant extracts. In all cases, the concentrations in the extracts are much lower than the solubilities of the measured phosphate minerals. This suggests that the observed effects are due to both the solubility of phosphate minerals (the first step) and the adsorption of phosphate on the mineral surfaces after the H2O/CO2 exposure (second step). 3.7. Comparison of Meteorite and Terrestrial Materials The Martian materials may be representative of unweathered rocks and rock fragments in the soil. The carbonaceous chondrites are undifferentiated aggregates that are also not weathered by terrestrial leaching. On the other hand, the terrestrial basalts were formed in the presence of water containing some dissolved CO2, and the JSC Mars-1 analog represents weathered materials that contain nanophase ferrous materials expected in Martian soils (Bell et al., 2000). On this basis, the meteorites may be expected to contain more soluble phosphate and larger H2O/CO2 weathering effects than the terrestrial materials that were already exposed to these environments. Correspondingly, the meteorite results show higher levels of extractable phosphate, particularly the large increase of extractable phosphate, as reflected by CP, upon extraction by H2O/ CO2 in the Martian meteorites and in the carbonaceous chondrites, compared with the terrestrial materials examined (Table 6). In fact, the meteorites behave similar to each other but different from terrestrial analogs in several respects: 1. The concentrations of phosphate in extracts obtained at high solid/solution ratios are significantly higher in the meteorites than in the terrestrial materials (Table 4).

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2. Extraction by H2O/CO2 compared with H2O/N2 increases the immediately extractable phosphate in the meteorites but decreases it in most of the terrestrial analogs (Tables 3 and 6). 3. In contrast, the effect of H2O/CO2 exposure on the longterm exchangeable phosphate pools, E1min-3m, is smaller in the meteorites than in the terrestrial analogs (Tables 5 and 6). 4. The Mars materials are particularly high in rapidly exchangeable (E1min) and water-extractable phosphate (Pw), although not in total phosphate. The similarities of the meteorites vs. terrestrial materials may correspond to the different exposure histories, as discussed above. In addition, the extractable salts in the Mars meteorites may be evaporate deposits (Gooding et al., 1988; Bridges and Grady, 2000; Sawyer et al., 2000), and Murchison also contains salts deposited after water loss due to evaporation or adsorption during aqueous alteration. The JSC-Mars 1 material was proposed as a Mars analog in terms of its spectroscopy and physical characteristics. However, the present results show that it may not be a good analog of Martian materials with regard to the biological fertility. More information on planetary materials is needed for choosing reliable analogs for modeling processes in astroecology. Such information can be obtained from planetary microcosms based on meteorite materials and from return samples in the future (Mautner, in press-a, in press-b). 3.8. Implications for Prebiotic Chemistry and Space Resources Phosphate is an essential element for forming and sustaining early life, assuming that the other conditions, especially liquid water, C, N, and other macronutrients are also available. While phosphate is needed for several essential cellular functions, early life-forms probably lacked advanced mechanisms to assimilate electrolytes. Therefore, nutrients, including phosphate, may need to have been present at relatively high concentrations. These requirements could have been met by interior solutions in meteorites which also contain other prebiotic materials (Anders, 1989). The results in Table 4 show that high aqueous phosphate concentrations can be achieved from meteorites at high solid/solution ratios, such as in water-saturated pores of carbonaceous chondrite asteroids and meteorites. These concentrated solutions can help prebiotic synthesis and early microbial nutrition. In fact, extractions of Murchison showed that internal solutions of similar carbonaceous chondrite asteroids and meteorites also contained 3.8 mol L⫺1 electrolytes and 10 g L⫺1 organics, and phosphate up to 5 mg L⫺1 (Mautner, in press-a, in press-b). Together with catalytic surfaces, these conditions would have been favorable for the origins of early life-forms. The same solutions would also support microorganisms that arise in these objects locally or are delivered externally. Frequent collisions between such objects in the early solar system could have dispersed microorganisms widely in solar nebulae and eventually delivered them to early planets, facilitating natural or directed panspermia (Crick and Orgel, 1973; Mautner, 1997b, in press-a, in press-b).

High extractable phosphate content was found also in DaG 476 and EETA 79001 Martian meteorites. Nakhla has lower concentrations, but SEM-EDS analysis showed microscopic locations containing up to 0.9% P. Such inhomogeneities may be significant in microbiology, as phosphate-rich areas can allow local microbial growth. For example, dense algal populations can be obtained on surfaces of the phosphate-rich DaG 476 Martian meteorite (Mautner, in press-a, in press-b). The increased long-term accessible phosphate noted above by exposure to H2O/CO2 can be useful on early planets. In addition, increased amounts of phosphate may be available from carbonaceous chondrites that are heated during in-fall, and their fusion crusts (Mautner et al., 1995). Significant sources may also be micrometeorites and IDPs, which have large surface areas processed by heating, as these particles are imported to Earth in large amounts (Chyba and Sagan, 1992). Considering future space resources, materials similar to the present igneous and carbonaceous chondrite rocks are present in large amounts on Mars and in asteroids. We observed relatively high amounts of immediately bioavalable P in the two carbonaceous chondrites and in DaG 476, near or above 4 mg P kg⫺1, at which soils are usually phosphate limited (Tran et al., 1988; Morel et al., 1992). These materials can be processed into phosphate-rich soils in planetary terraforming and spacebased agriculture (O’Neill, 1974; O’Leary, 1977; Ming and Henninger, 1989; Lewis, 1993). 4. CONCLUSIONS

The overall extractable and exchangeable phosphate in planetary materials was measured in this work. The results are within the range of terrestrial soils, consistent with the overall soil fertility properties of meteorites (Mautner, 1997a, 1999, in press-a, in press-b). Relatively high extractable P was found in meteorite materials that did not experience prior aqueous leaching. The available P remained relatively similar over a wide range of planetary conditions, between temperatures of 20 and 121°C, from pH 7 under N2 to pH 4 under CO2 atmospheres, solid/solution ratios of 0.01 to 0.1 kg L⫺1, and extraction times of 1 to 21 d. The extracted P varied for most materials within factors of 2 to 5 in this range of planetary conditions, with the values increasing somewhat at longer exposures and high temperatures. The IEK results showed that aqueous weathering under early planetary CO2 atmospheres can increase significantly the immediate and long-term availability of phosphate. The variation of these results among the materials must relate to adsorption/desorption processes on various mineral components. The physical basis for these complex processes constitutes a challenge for future studies. The results suggest that water in the pores of igneous rocks, meteorites, and their parent asteroids forms solutions with high phosphate and other nutrient concentrations. The phosphate resources, along with the other nutrients, could have been favorable for life in the early solar system. As well, these resources can provide useful soils for future biologic populations in the solar system (Mautner, in press-a, in press-b). Acknowledgments—We thank the individuals and institutions listed above for the gifts of meteorites and planetary soil simulants. The second author acknowledges the financing of a visit to a Lincoln University, New Zealand, financed by the Swiss Federal Institute of

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