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Journal of Petrology Volume 41 Number 11 Pages 1545-1651 2000
© Oxford University Press 2000
Flood Basalts, Basalt Floods or Topless Bushvelds? Lunar Petrogenesis Revisited
DEPARTMENT OF EARTH SCIENCES, CARDIFF UNIVERSITY, PO BOX 914, CARDIFF CF1 3YE, UK
Received September 2, 1999; Revised typescript accepted March 23, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONCONTENTS AND ROUTE MAPLUNAR PETROGENESIS 1969-1999NOTESNotes added in proofAPPENDICESREFERENCES |
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There is a conspicuous dichotomy in the conventional model of lunar petrogenesis between the total intra-crustal differentiation postulated for the products of feldspathic volcanism in the lunar highlands and the near absence of differentiation postulated for the products of mare volcanism. Both the cumulate mantle model, and the selenotherm postulated to accompany genesis of alleged ‘primary’ mare magmas by remelting of those cumulates, imply supra-adiabatic thermal gradients in near-solidus materials throughout the lunar mantle 4·3–3·2 Ga ago. This should have resulted in vigorous convective motion, which has not occurred. There is no positive europium anomaly in the average lunar highland crust. That crust cannot, therefore, have formed by plagioclase flotation from a lunar magma ocean, for which there is no other requirement. There is no negative europium anomaly in the average mantle to be inherited by later mare basalts. Other rocky bodies of lunar size in the Solar System have accreted at rates that allowed incorporation of plenty of volatiles and without forming global magma oceans. Partial melting in the presence of water, followed by near-surface fractionation and volatile losses can explain the feldspathic character, high incompatible element concentrations and lack of Eu anomaly in the lunar highlands. Volcanic eruption on the Moon must have been accompanied by selective volatilization losses of sodium, sulphur and other elements similar to the process seen on Io, which can account for the major differences between terrestrial and lunar basalts. Siderophile element depletion in lunar lavas may reflect immiscible sulphide liquid and metal separation, rather than global impoverishment in such elements, and large ore bodies may have formed close to the lunar surface. Mare basalt volcanism appears to have been a protracted, low magma productivity event with few similarities to terrestrial ocean-floor, ocean-island, continental flood basalt or komatiite volcanism. At low pressure the crystallization of plagioclase well before pyroxene typifies those terrestrial mid-ocean ridge basalt, ocean-island basalt and continental flood basalt magmas. A similar sequence is demanded of the postulated lunar primary magmas. Mare basalt hand-specimen and pyroclastic glass bead compositions do not, however, display the required crystallization sequence and cannot represent the required primary melt compositions. The true erupted lava compositions which gave rise to the regolith compositions across all the maria are much more feldspathic than the majority of large hand specimens and, in common with basalts on other planets, they are close to low-pressure plagioclase-saturated cotectic residual liquids which have evolved by removal of gabbros in crustal magma chambers, or perhaps in giant lava lakes akin to topless Bushveld complexes. Any further debate could be resolved by a 100 m drill core in a few mare locations. Field provenance of samples from Mars, a planet half covered by flood basalts and products of central volcanoes, will be little better than for those from the Moon. It will be important to encourage multiple working hypotheses, rather than to rush to a consensus.
KEY WORDS: lunar; basalt; highland; magma ocean; europium
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONCONTENTS AND ROUTE MAPLUNAR PETROGENESIS 1969-1999NOTESNotes added in proofAPPENDICESREFERENCES |
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The background to this paper has been presented by O’Hara (2000)
together with the substance of the argument in extended abstract form. The material of this paper is presented in a condensed narrative form, accompanied by detailed notes referenced throughout by numbers in parentheses, and by references. The arguments to be presented are complex, with items and groups of information utilized in, or relevant to, more than one thread. |
CONTENTS AND ROUTE MAP |
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TOPABSTRACTINTRODUCTIONCONTENTS AND ROUTE MAPLUNAR PETROGENESIS 1969-1999NOTESNotes added in proofAPPENDICESREFERENCES |
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LUNAR PETROGENESIS 1969–1999 |
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TOPABSTRACTINTRODUCTIONCONTENTS AND ROUTE MAPLUNAR PETROGENESIS 1969-1999NOTESNotes added in proofAPPENDICESREFERENCES |
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Petrogenetic background
Concepts in basalt petrogenesis
Terrestrial basalts 1950–1980. The lunar surface is composed of igneous rocks and their impact metamorphosed and impact weathered derivatives. Discussion of their origins cannot be divorced from a consideration of the evolution of ideas in igneous petrogenesis as a whole (0). The Apollo lunar samples were received into a scientific community which had been brought up in the almost unchallenged belief that the several types of abundant terrestrial basalts were primary magmas (0–2). This community was ready to accept that eruption of unmodified primary partial melts of the mantle was a commonplace event on the Moon. Contamination, assimilation and hybridization, which had been extensively explored as mechanisms in igneous evolution before 1950 (1), were becoming less fashionable (2).
Terrestrial basalts 1980–1999. Two assertions that had underpinned the view that primary magmas abounded on Earth were that the common basalts were of great uniformity and that this feature would not survive variable amounts of partial crystallization. Both assertions have been shown to be untrue (0, 5, 26–28) and the problems attending both the definition (6) and eruption (31–34) of primary magmas have been appreciated.
Lunar petrogenesis—stasis and dichotomy. In striking contrast to the situation in terrestrial basalt petrogenesis, the interpretation of lunar petrogenesis which was reached in 1969–1970 (3) has changed little in the ensuing 30 years and displays a striking dichotomy (4). Almost every hand-specimen sample from the maria is supposed to be close to a little differentiated primary magma in composition. By contrast, every igneous rock contributing to the highlands is supposed to be completely differentiated to the point that no trace of its undifferentiated parent magma remains. The only igneous compositions close to the lunar highland average composition are interpreted as clast-free impact melts which formed from rehomogenized highland target materials. A further dichotomy appears in the treatment of remote-sensing data (61), where lunar highland regoliths are assumed to be representative of the chemistry of 40–120 km of underlying crust, yet mare regoliths, which uniformly indicate feldspathic average basaltic compositions, are assumed to be unrepresentative of even the topmost flow units (107–114).
Role of trace element geochemistry
Equilibrium, not perfect fractional processes dominate. Interpretations of trace element geochemistry (7–10, 13), however, apparently provided strong support for a plethora of primary magmas. Erupted basalt sequences have distinctive geochemical features (7), which are incompatible (8) with those expected in products of perfect fractional crystallization (PFC), despite the evidence of a close approach to PFC seen in some large peridotite–gabbro layered intrusion complexes. Equilibrium between liquid and crystals at low and variable mass fractions of melt, on the other hand, produces effects which match the gross variation in basalts world-wide. All workers accepted partial melting of the upper mantle as the ultimate source of most basalts and the melting process could be modelled (9) as equilibrium partial melting (EPM). Equilibrium partial crystallization (EPC) is a process equally capable of explaining the geochemical effects (10) but could be rejected as an explanation for reasons which still appear sound. Appreciation that the actual process in the upper mantle might approach perfect fractional partial melting (PFM) did not undermine these views (11, 12), because the accumulated mixed liquid products of perfect fractional partial melting (APFM) share most geochemical characteristics with the liquids of EPM or EPC [the residues of PFM should, however, look very different from most natural rocks (13) because of the anticipated extreme depletion of highly incompatible elements]. Was it necessary to delve further into basalt petrogenesis (14)?
Possible role for eutectoid PFC. One of the distinctive general features of EPM and EPC at low mass fraction of liquid is wide variation in incompatible trace element concentration with minimal change in major element concentration in the liquid. Even perfect fractional crystallization can produce this effect in the special case of eutectoid crystallization (15), when the solid assemblage separating differs little in major and minor element composition from the liquid.
Imperfect fractionation processes. The differences between the products of PFC (8) or PFM (12, 13) and those of an equilibrium process evaporate rapidly if the process is imperfect in the sense that finite rather than infinitesimal increments of solid or liquid are removed (16).
Model dependence and apparent distribution coefficient. The conclusions from all modelling of trace element geochemistry are wholly dependent on the use of appropriate bulk distribution coefficients for each element. Crystal–liquid distribution coefficient, d, itself may vary with melt composition or element speciation. The apparent bulk distribution coefficient, dAp, i.e. that which should be utilized to obtain a satisfactory description of a process or relationship in terms of a simple equilibrium (batch) model, can be greatly modified from the anticipated values of the simple crystal–liquid distribution coefficient, d, by a number of factors. These include (a) changing mutual solubility of crystalline phases at the site and temperature of melting, and (b) zone refining, magma chromatography or polybaric crystallization effects during magma ascent (18). Apparent bulk distribution coefficient may further vary because of (c) forced precipitation of exotic phases on magma arrival and mixing, (d) magma diagenesis of the growing cumulate pile, (e) diffusive differentiation of the magma (19), and (f) trapping of melt in the cumulates (20) or residues (20, 21). Above all, the apparent distribution coefficient is heavily dependent on the choice of a relevant physical model (17, and Fig. 1).
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How much melting?
Trace element requirement. The inference of small mass fractions of melt extraction in terrestrial and lunar basalt petrogenesis is derived from one interpretation of the trace element chemistry (7–13).
Major element requirement. The major element variation in terrestrial peridotite suites (22), however, requires that large mass fractions (>0·25) of partial melt have been removed. Few natural upper-mantle peridotite samples are sufficiently low in mg-number, and rich enough in incompatible trace elements, to support a single-stage EPM origin for common terrestrial basalts (see 47) even at low mass fractions of melting.
Integrated melting regimes. This contradiction between trace and major element inferences was partly resolved by recognition that partial melts must be integrated from regimes with high mass fractions of melting in the centre, but in which small mass fraction melts contributed from the periphery carry the dominant trace element signal (23). Integrated melting regimes logically invalidate many predictions about mantle source regions based on phase equilibria of a candidate primary magma composition because there is in these cases no unique ‘primary’ liquid which was ever in equilibrium with a specific residue.
Simulating and exceeding effects of EPM–EPC in magma chamber products
Integrated crystallization simulates EPC. The process of melt integration is not restricted, however, to the partial melting process. Mixing of residual liquids from variable amounts of PFC (24) yields aggregate liquids whose geochemical properties approach and transcend those of liquids from an EPC process acting on the same parent magma.
Magma recharge and discharge. Field and petrographic evidence establish the importance of magma recharge and discharge during partial crystallization processes (25). Geochemical evolution in a periodically recharged, periodically tapped, continuously crystallized (RTXC) magma chamber (26) spans the range between PFC and EPC. The process facilitates contamination, imposes low-pressure cotectic character on a body of liquid already collected close to the site of eruption, buffers output composition against short-term fluctuations in any of the inputs or the process parameters, and combines this with a simulation of the geochemical effects of EPC or EPM at low mass fractions of liquid. The ratios of recharge to discharge and crystallization are the controlling factors in this simulation. The feedstock for the chemical variations in the pseudo-equilibrium process (i.e. the bulk composition which appears to be undergoing the EPM or EPC process) is, however, the average total input to the magma chamber including contaminants, not some embarrassingly fertile upper-mantle source (27). Ponding and partial crystallization of new input magma may occur before mixing (28).
Small packet crystallization. Recognition that large bodies of magma may solidify by repetitive partial crystallization of small magma batches (29), with mingling of the residual liquid back into the larger pool (SPC), introduced additional complexities. When combined with a variety of partial crystallization–melt aggregation models in the small packets and the whole inserted as the partial crystallization process in an RTXC magma chamber startling results can be obtained (30).
Extremes of magma chamber processes. Concentrations of highly incompatible elements may be doubled in liquid products of SPC–RTXC magma chambers whereas the concentration of a highly compatible element such as Ni (d 
10), although always decreasing in concentration relative to the parent magma, may achieve values as great as four times that expected in an EPC process; 400 times that expected in a PFC process (30)—and all this with greatly enhanced discrimination between incompatible elements in a process whose cumulates would everywhere be seen to be the product of local PFC!
Difficulties with primary magma hypotheses
Physical problems. The movement of primary magmas to the surface without modification of their compositions, posed as an industrial problem, would tax the ingenuity of large research teams of chemical and mechanical engineers (31–34). Assimilation of cooler mantle and crust and partial crystallization are likely to be pervasive (31). These effects are likely to be concentrated when magmas arrive at the density contrast at the crust–mantle boundary (32). It is doubtful whether any truly unmodified primary magma can ever be erupted (33). A window of eruptability occurs in relatively magnesian basalt compositions around the condition that plagioclase is about to start crystallizing at the liquidus, but this window could be broadened to picritic compositions by vesiculation (34).
Plagioclase-saturated low-pressure cotectic character. The vast majority of basalts erupted on the surface of the Earth have compositions and temperatures which conform to those of liquids in low-pressure, plagioclase-saturated equilibria (35), an observation which admits three explanations: coincidence of high- and low-pressure equilibria (36, 37); partial crystallization at low pressures with the formation of crustal gabbro complexes (38, 39), preferred in the terrestrial case; or partial melting of gabbro, troctolite or plagioclase–wehrlite at low pressures (40, 41), which has been advocated for asteroid-sized bodies with low central pressures.
Primary magmas should be picritic. The partial melting products of likely mantle peridotites in all the terrestrial planets should be picritic in composition (42) and they may be too dense to erupt unmodified through the crust of the Earth or Moon (43) unless assisted by reduction of bulk density by vesiculation, an effect which can become influential only at low pressures. Picritic compositions also form readily from common basalt at low pressure by the accumulation of dense, early-formed crystals in the magmas (44). Over the last 40 years the number of terrestrial magmas which have been identified as picritic liquids (45) has increased greatly but this identification depends on possession of the field relations and an undisputed knowledge of the average bulk compositions of the lava, both of which are lacking for lunar mare basalts. Even fully accredited picritic liquids have generally undergone substantial partial crystallization and some crystal accumulation subsequent to eruption.
Detection of superimposed melting and crystallization. A crucial point to keep in mind here, and at the end of several previous trains of thought, is that whereas it is easy to detect the geochemical effects of a PFC process superimposed upon primary magmas developed in an EPM process, it is extremely difficult (46) to detect the operation of a partial crystallization process superimposed on the product of a partial melting process, when the first process yields a liquid approximating to an EPM product of the true source and the second process yields liquids which approximate to EPC products of the EPM liquid.
Thermal inertia problems. A simplified interpretation of a suite of samples from such a two-stage magma output might opt for a single-stage partial melting process with very small, but varying mass fractions of melting, or for an excessively enriched source region, or for some compromise between the two. However, the partial melting regime is anticipated to be a large, thermally well-insulated volume (10–100 times that of the derived magma) of material in which short-term fluctuations of the melting parameters are unlikely. The environment of crustal magma chambers is much more susceptible to variations in thermal conditions from one location to another, and to short-term fluctuations in output chemistry, especially when rates of input and output are the critical factors in simulating the appearance of small mass fractions of equilibrium partial melting. The bulk of variation in basalt geochemistry within a single province is inherently more likely to be the result of varying partial crystallization processes than varying conditions or compositions in the source region.
Caution in the use of trace element geochemistry
Increased sophistication in modelling of igneous processes over the past 30 years has invalidated the facile use of trace element geochemistry as a definitive indicator of the presence or absence of low-pressure modification of erupted basalt compositions (47, and see Fig. 1). Some combination of the field relations, petrology, major element chemistry or phase equilibria of erupted basalts frequently suggests a possibility of modification by partial crystallization at some pressure within the crust or upper mantle. In these circumstances, truly sympathetic consideration should be given to the possible effects of those processes before projecting any geochemical features into the source region or the melting process.
Extra-terrestrial petrogenesis
Irony of the lunar mafic hand specimens
Lack of proper field relations, and lack of unambiguous knowledge of the average bulk compositions of the lunar mare lavas, underlies an important part of the debate addressed in this paper (48). It is ironic then that it has been necessary to argue over more than 40 years for largely unseen picritic parental magmas on the Earth [whose existence is still contested in mid-ocean ridge basalt (MORB), volumetrically the most important terrestrial domain of all], yet simultaneously to reject the picritic samples which dominate the lunar mare hand-specimen collections, as samples modified by crystal accumulation.
Solar System exploration post-Apollo
Accepted lunar petrogenesis. ‘Conventional’ lunar petrogenesis (3) is built around a Moon which was volatile depleted from birth. It postulates the generation of a global magma ocean during accretion with flotation of the anorthositic lunar highland crust from the consolidating magma ocean. It then requires the generation of mare basalts by remelting of the feldspar-depleted lunar mantle cumulates formed by consolidation of the global magma ocean. After the Apollo program and the formulation of the ‘conventional’ interpretation of lunar petrogenesis in 1970–1971, a wealth of information, which does not yet seem to have impinged on the interpretation of lunar rocks, has become available for more remote parts of the Solar System.
Volatiles plentiful in asteroids and other Moon-sized bodies. Many asteroids (50) are small rocky bodies which acquired plenty of volatiles during their accretion. There are now six satellites known in the Solar System that have a rock and metal content which would yield a roughly Moon-sized body if all volatiles were removed (51). Four of these bodies retain thick ice crusts, demonstrating that bodies of the same order of size and mass as the Moon could, in principle, accrete with plenty of volatiles and without generating global magma oceans (52, 53 and see 77).
Current Io volcanism a model for Precambrian lunar volcanism. Io, a body closely comparable with the Moon in size and mean density, was found 20 years ago to be the most volcanically active body in the Solar System (54), losing large amounts of sulphur and sodium to space. Surely its style of violent pyroclastic silicate volcanism needs to be considered in relation to that of the Moon, yet two recent authoritative compendiums on the Moon (Heiken et al., 1991; Papike et al., 1998) made no reference to Io and presented the ‘conventional’ view of lunar petrogenesis, which excludes a significant role for reduction and volatilization during eruption.
Ancient feldspathic crust on at least three bodies. Io, together with part of Mars, Venus and the Earth, has preserved little or no early (>3·8 Ga) crust and we can learn little about early planetary evolution from these bodies (55). The Moon, Mercury and parts of Mars all developed and preserved an ancient, heavily cratered light-coloured and probably feldspathic crust (56). The early crust on Mars developed in the presence of water and other volatiles, and appears to be of calc-alkaline and at least partly of pyroclastic volcanic character (57). Neither the presence of volatiles nor the eruptive volcanism lends any support to the concept of formation of a global magma ocean and feldspathic crust flotation during the early history of Mars.
Global melting or anorthosite flotation unnecessary elsewhere. Some small bodies (58), particularly those of the inner asteroid belt, have undergone extensive partial melting, but none of these smaller bodies have developed anorthositic crustal materials (see also 64). Neither internal heating by short-lived isotope decay nor accretional energy is appealing as the cause of this melting because similar-sized bodies (50) have manifestly undergone no melting, and Callisto (4), a much larger body, may have evaded internal differentiation entirely. A localized external heat source is required. Heating by tidal deformation (59), which plays a significant role in Io, Europa and Ganymede today, may have been much more important in the evolution of small bodies such as Vesta and even the Moon in the early years of the Solar System. If heat sources were marginally adequate for melting to occur at all it is reasonable to expect petrogenesis in those bodies which have been melted to be dominated by partial rather than total melting (60).
Lunar highland crust
Petrological composition. Remote sensing of the composition of the lunar highland crust (61) combines with petrological data from recovered samples and lunar meteorites to show that the average highland composition is anorthositic norite, an average that contains a significant ferroan anorthosite (FAN) component. There is also a substantial petrographically identified component of magnesian gabbros and norites and a minor component of petrographically identified mare basalts which are up to 4·2 Ga in age. The relationship of Fe to Th/Ti ratio, which can be obtained from remote sensing and calibrated by sample analysis, requires a much more substantial mare component to be present in the average lunar highland composition, raising a question about the possible mare parentage of some of the magnesian gabbro–norite suite cumulates.
No positive europium anomaly. It had already been decided that the Moon was volatile-poor from birth in the light of the Apollo 11 basalt samples. A global magma ocean in the Moon was proposed in the light of the Apollo 14 highland material return because plagioclase flotation from a large body of basaltic melt seemed the most plausible way to generate a thick plagioclase-rich crust in a volatile-poor body. The widely publicized large positive Eu anomaly in the lunar highlands apparently supported this interpretation, but the remote-sensing data confirm what has always been evident from the original data (Figs 2 and 3). The positive Eu anomaly does not exist (62). This fact, well appreciated at least since 1988, is not mentioned in either of the lunar compendiums mentioned above. There is probably a small negative Eu anomaly in the average highland composition. The bulk of the rare earth elements (REE) in the lunar highlands reside in the KREEPy component, which was apparently excavated by the Imbrium impact and is localized in its vicinity. This leaves open the possibility that the highlands elsewhere do indeed have the low REE contents and large positive Eu anomaly required by the conventional model. The latest survey of Th concentrations across the whole lunar highland surface, however, shows values which are predominantly in the range where small negative or only small positive anomalies would be anticipated. The possibility that a deep-seated KREEPy component is more generally distributed but rarely excavated has also to be considered.
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No plagioclase flotation. Plagioclase crystallization and accumulation takes place predominantly at the floor of terrestrial basic magma bodies (63). Plagioclase formed elsewhere in the magma cannot float or sink significantly because of the greatly increased viscosity of magmas as they approach plagioclase saturation. Upward transport of suspended plagioclase by convection in a dry magma ocean should lead to resorption of plagioclase, not formation of an anorthositic crust. Terrestrial magmas are richer in alkalis and more oxidized than lunar magmas, but cumulates from the fragmented howardite–eucrite–diogenite and mesosiderite parent bodies (64) also display accumulation of feldspar with denser ferromagnesian minerals, and provide no evidence of plagioclase flotation or accumulation to form anorthosite.
No lunar magma ocean. Combining these thoughts with those of (62), an origin for the lunar highland composition by plagioclase flotation is excluded (65) and there is, therefore, no petrological support or requirement for the former existence of a global magma ocean in the Moon (66) and grave doubt (from 53, 57, 60 also) whether any body the size of Mars or less developed a magma ocean during its accretion (67). The manifest differentiation into crust and mantle in the terrestrial planets is, in the present state of knowledge, equally well accounted for by partial melting and serial volcanism.
Wet mantle yields feldspathic melt. Leading on from the pyroclastic, possibly andesitic character of early volcanism on Mars (57), the partial melting of peridotites at moderate (>0·1 GPa) pressures in the presence of excess water yields liquids poor in potential olivine which will have plagioclase as an early liquidus phase (68) on eruption and water loss. This crystallization sequence abounds in terrestrial calc-alkaline volcanics but might be complicated in the lunar case by the simultaneous loss of alkalis on eruption. We return now to the conclusions that partial melting is more probable than total melting (60), that the Moon could have accreted plenty of volatiles (51), and that some other origin than plagioclase flotation is required for the lunar highlands (65). The high content of highly incompatible trace elements in the average lunar highland composition (69) points to the early crust being a small average mass fraction partial melt of the whole lunar interior, which would then have had to form under moderate water vapour pressures (70). The small negative Eu anomaly inferred for the average lunar highland composition then suggests the presence of either amphibole or a trace of plagioclase in the residual lunar mantle assemblage. Some might view this suggestion as preposterous in the absence of any hydrous minerals in recovered lunar samples. There can, however, be few more efficient mechanisms for thoroughly dehydrating materials than repeatedly spraying them as hot igneous or impact-generated particles into a vacuum.
Evolving lunar volcanism. Combined with (61) the above leads to two complementary propositions. The early lunar highland crust may have formed by lower-temperature serial water-rich volcanism yielding feldspathic partial melts (37), which had KREEP as one of its differentiation products. Activity may have evolved into water-poor basaltic volcanism as temperatures rose and the body degassed (71). The lunar highland crust is then the accumulated, differentiated partial melt product of that evolving serial volcanism, incorporating a contribution from mare-related plutonics (72).
Feldspathic contribution from early mare component also. The mare components might have contributed further to plagioclase enrichment of the highland crust (see 93 below) if they were derived from a mantle which was close to saturation with an alumina-rich phase. Although this effect alone could have implanted a tendency towards a positive Eu anomaly into the average lunar highlands it would be much subdued if the average lunar highlands contain the pulverized residues of substantial pre-4·1 Ga mare volcanism.
Sulphur, carbon and their gases
Io pyroclastic volcanism driven by these gases. Gases in the sulphur–carbon–oxygen system have driven the pyroclastic volcanism of Io (54), probably for the past 4·5 Ga. The contents of S and C even in consolidated lunar mare basalts are higher than in terrestrial basalts and would sustain volatile fugacities much higher than the lunar surface confining pressure at magmatic temperatures (73).
Lunar pyroclastic volcanism guaranteed. Pyroclastic volcanism similar to that on Io was guaranteed (81) on the Moon, yet this fact receives only a one-sentence comment by Papike et al. (1998) without reference to Io or to the probable consequences for alkali contents of lunar basalts. There is a fuller consideration by Heiken et al. (1991), again without reference to Io, and tending to play down the role of reduction by sulphur loss and possible losses of alkalis during pyroclastic volcanism (see also 80).
Sulphide saturation and siderophile depletion. The lunar basalts were close to saturation with an immiscible sulphide melt (74) and the widely publicized siderophile element depletion in lunar rocks is also predominantly a chalcophile element depletion (75). Any cumulate gabbros underlying the lunar maria might contain sulphide-rich horizons which concentrate the chalcophile trace elements (78) as well as possible metal-rich horizons [a few fragments of both might then be expected in lunar highland breccias if the speculation in (72) is correct]. Overall chalcophile element depletion in lunar surface rocks might owe much to this. Also relevant to the sulphur–chalcophile element story is confirmation that the Moon contains a small core which can be expected to be sulphur bearing if not sulphide rich (76). The strong influence of oxygen fugacity on wetting angles of sulphide melts suggests that segregation of a sulphide liquid to form such a core would have been difficult in a reduced lunar interior but easy in an oxidized Moon (76).
Cerium anomalies and lunar oxygen fugacities
Oxygen fugacities higher than have accompanied either terrestrial calc-alkaline volcanism or Martian volcanism are required at some stage in lunar evolution by the presence of small positive Ce anomalies in many lunar samples, another facet of lunar petrogenesis which has been largely ignored, yet never satisfactorily explained within the ‘conventional’ petrogenesis (77). Similar anomalies in a variety of Antarctic meteorites have fuelled the wild?! (chess terminology) idea that the early lunar crust might have evolved beneath an ice-layer like that on Io or Ganymede (52). If volatiles were relatively abundant in the early Moon, and assuming no major entrapment of hydrogen gas in the body, separation or losses of water from the silicate fraction would be an oxidizing process (79). Subsequent losses of carbon and sulphur as oxide gases will tend to be strongly reducing and the effects of any alkali loss are currently undetermined (80). Given the many orders of magnitude range in the intrinsic oxygen fugacities of terrestrial basalts and the changes which might accompany eruption, there is no a priori reason to think that the oxygen fugacities of lunar basalts reflect those of the lunar interior or that the latter was necessarily uniform spatially or temporally.
Eruption style on a small planet
Fire-fountaining, frothing and volatilization losses. A line of thinking led to the postulate of a volatile-bearing, progressively devolatilizing lunar mantle (71). Given that liquidus vapour pressures even of the final (already much degassed) consolidated mare basalts would have exceeded surface confining pressures (73) it is logical to expect basalts erupted on small planets and asteroids to fire-fountain, to form ash- or droplet-emulsion flows which would spread out with very low effective viscosities, and to froth at their top surfaces even once relatively condensed (81). Such eruptions would maximize the surface area of the melt, maintain the surface temperature of each droplet with minimal cooling in a black-body environment, and minimize the diffusion distances to be covered by components seeking to volatilize (82). That this scenario probably affected lunar basalts is demonstrated by their high intrinsic vapour pressures at liquidus temperatures (73) and by the observation that even after eruption, flow and consolidation, some lunar basalts are highly vesicular. Even in the waning stages of mare volcanism there were enough volatiles to power the pyroclastic eruptions which produced the dark mantle glass bead deposits (83).
Effects of sodium loss. Loss of soda by volatilization has a dramatic effect on the CIPW norm of the residual melt (84), releasing alumina and silica from original albite molecule. These recombine with lime from augite, releasing hypersthene molecule to join that being created by reaction of released silica with original olivine molecule. Terrestrial basic melts subjected to major volatilization losses of sodium would be expected to be transformed into anorthite–pigeonite basalts like those found on the Moon (90).
Asteroidal basalts. The HED (howardite–eucrite–diogenite) parent body (?Vesta) has also yielded a range of anorthite + calcium-poor-pyroxene basalts and dolerites which are low-pressure plagioclase-saturated cotectic compositions (85). The geochemical features of these can be interpreted as products of either primary partial melting (40) or partial crystallization (47) processes. The partial crystallization interpretation is favoured by the presence of cumulate-textured gabbro–norite samples and abundant orthopyroxene cumulates (87), and perhaps by the lack of identified olivine-rich types which might represent the complementary residual mantle. Like lunar basalts, HED basalts are relatively sulphide-rich (88), may have undergone considerable volatilization losses during eruption and must have undergone major losses of volatiles and sodium if the original body was related to chondrite in composition (89). The mantle residues from which the magmas evolved might not then be easily recognizable. The character of the lavas is exactly what would be expected (90) if basic melts of familiar terrestrial or even more alkaline compositions had been subjected to reduction and volatile losses on eruption at the surface of a small planet.
Plagioclase saturation in lunar basalts
No negative Eu anomaly in the lunar mantle. We return now to the complementary conclusion which arises from remote sensing of the average lunar highlands composition (61, 62). The average lunar mantle composition must reflect extraction of the highland crust (91) and must have a small complementary positive Eu anomaly if the bulk Moon has chondritic ratios of the REE. This conclusion stands independent of the debate whether the lunar highland REE signal is dominated by the KREEPy component.
Imposed, not inherited, negative Eu anomalies in mare basalts. Mare basalts cannot then inherit their variable but in many cases very marked negative Eu anomalies as a primary magmatic feature (92) during partial melting of such a mantle. Extensive plagioclase fractionation during partial crystallization at low pressures is the most probable cause of these Eu anomalies.
Low-F, moderate-P primary magmas precipitate plagioclase before pyroxene. The effect of elevated pressure on plagioclase-saturated phase equilibria in the dry basalt–peridotite system is to displace the liquid compositions rapidly towards higher normative plagioclase within the first 0·2 GPa and less rapidly towards higher normative olivine (Fig. 4). The implications are profound—plagioclase should precipitate before pyroxene from ascending primary melts—and seem to have been overlooked in discussions of mare basalt petrogenesis (93). If oxygen fugacities were low and plagioclase were a residual phase in the lunar mantle during partial melting, the Eu anomalies of mare basalts might have been explained as a primary feature, but the major element compositions of pyroclastic glass beads, hand specimens and even the feldspathic basalts are too poor in plagioclase (93) to support this mechanism.
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MORB, CFB crystallize plagioclase early. Terrestrial basaltic partial melts have formed by larger average mass fractions (0·1) of partial melting than are supposed in the lunar case. They almost certainly separated from alumina-undersaturated, feldspar-free harzburgites. Nevertheless, these melts are typically so rich in potential plagioclase that precipitation of extensive olivine and plagioclase, the common phenocryst phases in MORB and continental flood basalt (CFB), precedes the arrival of the residual melt at the low-pressure cotectic equilibria where augitic pyroxene also begins to precipitate (94, 95).
Incorrect low-P phase equilibria of alleged mare primary magmas. None of the lunar pyroclastic glass beads or alleged primary magma hand-specimen compositions displays the required crystallization sequence and most encounter pigeonite saturation before the entry of either augite or plagioclase (96). If these had been the parental magma compositions, and given (91), there would be no way of generating the observed negative Eu anomalies (97).
Alleged primary magmas do not display required moderate-P phase equilibria. A corollary of the issues discussed in (93) is that true primary magmas formed in the manner required in the ‘conventional’ lunar petrogenetic scheme should show simultaneous saturation with plagioclase, pyroxene and olivine at their pressure of formation (98), but none do.
Lunar primary magmas and absence of tectonic deformation
Global magma ocean cumulates unstable from birth. Deposition (solidus) temperatures in cumulates from a global magma ocean would decline towards the surface because of two effects—the declining liquidus temperature of any dry mafic magma with declining pressure, and the decreasing mg-number of a differentiating magma ocean. Liquidus thermal gradients in dry mafic and ultramafic materials are typically supra-adiabatic, an effect which underwrites the partial melting of mantle plumes in the terrestrial mantle. The thermal gradient at deposition of a thick global cumulate would greatly exceed the adiabatic gradient, promoting convective motion which would be enhanced by the density inversion implicit in a plagioclase-free cumulate sequence of declining mg-number, especially where later cumulates contained ilmenite. There is no tectonic evidence to suggest that the anticipated deformations took place.
Supra-adiabatic thermal gradient still required 1 Ga later. The two-phase (olivine + orthopyroxene in most cases) saturation which occurs in the putative mare basalt primary magma compositions at a variety of pressures and temperatures has no special petrogenetic significance (99). If petrogenetic significance is imputed, the array of co-saturation conditions would define a pressure–temperature gradient which is grossly supra-adiabatic (100), has to be supposed to have persisted over at least a billion years through 500 km depth of mantle, and yet has produced no evidence of tectonics caused by convective motion (101) even where the lunar crust is no thicker than the continental crust of the Earth. Consequently, the glass-bead and hand-specimen compositions cannot be primary partial melts of the alleged cumulate mantle (102). No problems with global supra-adiabatic thermal gradients need arise in the alternative petrogenesis proposed here.
Average compositions of mare basalts
Quench crystal sinking. The low viscosity of the alkali-poor mare basalt lavas, even in their condensed state, ensures significant sinking of crystals, including the large zoned metastable phenocrysts formed during quenching (103). Settling rates may have been more rapid if quench phenocrysts formed during the pyroclastic phase in the eruption. Quench texture throughout a lunar hand specimen is no criterion of the former existence of its bulk composition as a liquid. Some, possibly many, of the samples must be enriched in ferro-magnesian phases by accumulation of the quench phenocrysts (104). Petrographic variability at each site is more extensive than was then known at individual terrestrial sites (but see 133).
Flow thicknesses. The general lack of flow fronts in the maria points either to very fluid flows (103) or to flow thickness (105) comparable with or less than the regolith depth (5 m), in which case the average regolith composition should be close to the average lava composition (106). Estimates of cooling rates required to produce the observed petrographic textures in hand-specimen samples, on the other hand, suggest much thicker cooling units (107), in which case the regolith, being restricted to the top 5 m, may preferentially sample a phenocryst-depleted zone and not represent the average composition of the lava. Small impacts into the maria excavating to 100 m depth (108) should, however, sample the average lava composition accurately whichever estimate of flow thickness is correct, and the same should be true of materials exposed along the walls of the large rilles. There is no evidence to suggest that the regolith surface is not representative of materials to several tens, even a hundred metres deep in the maria (109).
Persistently feldspathic regolith compositions. Remote sensing of the mare surfaces (110) indicates average compositions much more feldspathic than all but a few of the large hand specimens from the mare landing sites (Fig. 2). These regoliths are expected to comprise about 95% locally derived material. The remote-sensing results are in good agreement with the limited ground truth established by returned regolith samples (111). They also agree with the feldspathic basaltic compositions of lithic fragments, breccia fragments, lunar mare meteorites and impact-generated glass groups in the regolith (112–116), which leads on to the conclusion, reinforced by (103–109), that the hand-specimen samples do not represent the average consolidated liquid compositions (117).
Composition bias in hand specimens. The hand-specimen compositions are inevitably biased towards materials excavated relatively recently from the top of the bedrock at 5 m depth, as well as towards more cohesive samples from shallower depths, because longer exposed and more friable materials have become preferentially comminuted (118). It is quite proper to accord all such samples equal weight when selecting them for investigation, but potentially misleading to accord each of them equal weight when arriving at an average mare basalt composition which ignores the regolith contribution. This entire line of reasoning (103–118) reinforces the phase equilibria, petrographic and geophysical arguments (93–101) pointing to the conclusion that hand-specimen compositions cannot be primary partial melts of the alleged cumulate mantle (102).
Low-P plagioclase-saturated cotectic average compositions. Information and samples made available in a random manner (110–116) indicate that the average mare basalt composition is close to that of a low-pressure cotectic, plagioclase-saturated basalt which could be the residual liquid of a low-pressure gabbro–norite crystallization process (120) during which enrichment in incompatible elements such as REE and titanium and negative Eu anomalies could be generated. These magmas might have been contaminated by highland crustal materials during that process.
Misfits in experimental data. Objections to this interpretation based on small misfits in the experimental data, specifically small mismatches in the mg-number of liquidus olivines and the failure of armalcolite crystallization to overlap with that of plagioclase in some results, can be discounted because of the problems in precise control of charge compositions and oxygen fugacities (119).
Petrogenesis of lunar high-titanium basalt. The petrogenesis of the high-titanium basalts of Mare Tranquillitatis and Taurus Littrow collected by the first and last manned missions of the Apollo program encapsulates the discussion about mare basalts and is pursued at greater length in Figs 5–9. These rocks are alkali-poor basalts very rich in TiO2, relatively rich in FeO relative to MgO, relatively rich in incompatible trace elements (REE 20–100 x chondritic and higher in Apollo 11 than Apollo 17 samples), with marked negative Eu anomalies (Eu/Eu* 0·8–0·3). These magmas have been suggested to represent either (i) very small mass fraction partial melts (1% or less by mass) of the postulated cumulate mantle, or (ii) late residual liquids (last 50–10% of the parent magma assuming a 10% initial partial melt) of differentiation involving removal of plagioclase and other minerals at low pressure.
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The first hypothesis is incompatible with the high-pressure phase equilibria of the hand-specimen compositions (93–100) and will also be incompatible with the high-pressure phase equilibria of the average erupted magma compositions advocated here (110–117), but less strikingly so. To validate the second hypothesis the phase equilibria of the average erupted magma compositions are required to demonstrate near-simultaneous entry at the liquidus of all the mineral phases required to be present in the hypothetical cumulates, i.e. plagioclase, pyroxene, ilmenite, probably olivine, possibly armalcolite and spinel as well. If the hand specimens represent samples of average liquid enriched in phenocrysts of dense olivine, ilmenite, armalcolite and spinel formed during quenching after eruption as advocated here then, subject to the points raised in (119), the hand-specimen samples should show near-simultaneous entry of pyroxene and plagioclase when the phenocryst phases are still in equilibrium with the liquid.
Data presented in (121), comprising with those published by O’Hara et al. (1970a, 1970b) by far the largest datasets for low-pressure crystallization of these rock types, are interpreted in Figs 5–8, and demonstrate that the phase equilibria are appropriate for the second hypothesis, making due allowance for the presence of a small amount of highland component in the soils and of some pyroclastic bead material in the Apollo 17 soils.
Cognate crustal cumulates. The presence of extensive bodies of cognate gabbro–norite and probably peridotite–pyroxenite is predicted somewhere within the crust beneath the mare surfaces (105, 122), some older members of which may be sampled in the highland breccias (72).
Comparative petrogenesis
Meantime, back on Earth ...
Poor terrestrial controls. Most of the rock materials now forming the lunar highland crust were in place, the period of heavy bombardment and large basin formation was over, and the filling of the western maria complete by 3·7 Ga ago. The terrestrial record from that period is sparse and fragmentary, providing little in the way of control over interpretations of lunar geology.
Magma production in large impacts. Large basin-forming impacts undoubtedly produce large volumes of impact melt (123), part at least of which may become clast free. In the small-basin-sized terrestrial impact basin at Sudbury (124) the whole body of norite and gabbro with its associated nickel sulphide deposits may be a pool of impact melt which cooled slowly enough to undergo extensive fractionation. Mafic magma may also form within the target body by partial melting triggered by a combination of pressure release accompanying the basin excavation and shock-implanted energy. This internally generated magma would have to rise through shattered upper-mantle and crustal rocks in the breccia lens, when it would have a high probability of becoming contaminated (125). If it arrived soon enough, it would be liable to hybridize with the purely impact-generated melt in the crater.
Bushveld complex a model for mare filling? The 2·2 Ga Bushveld complex of southern Africa has been put forward as a possible large terrestrial impact basin filling (126) and it has several of the features one might seek: lack of tectonic association with an internal plume-generated event, extremely high magma production rate, pervasive magma contamination, and mare-like dimensions. Extreme differentiation by partial crystallization of norites and gabbros in this body is complicated by many magma recharge events, accompanied by copious chromite and sulphide precipitation. Large outflows of magma, whose compositions were constrained to be multiply saturated with plagioclase, pyroxenes and other phases at low pressure, are deduced. These factors outline an environment similar to that required to explain the lunar mare basalts (122). The Bushveld lacks, however, any unambiguous evidence of origin as an impact basin and there may be some differences in timing which distinguish it from the lunar maria. R. G. Cawthorn (personal communication, 1999) has, moreover, drawn attention to the impoverishment of the cumulate section in sulphur.
Komatiite–greenstone analogies. Some of the terrestrial komatiite–greenstone sequences date from a period overlapping that of lunar mare filling (127) and it has been suggested that they might be analogous features. Expanding knowledge of both komatiite–greenstone and continental flood basalt provinces, however, has not strengthened the desired connection. The stratigraphic records in these provinces typically open with the eruption of more primitive magma types, which have a better claim than average to approximate to primary magma compositions. They then proceed through thick sequences of basalts whose compositions are controlled by partial crystallization somewhere within the crust and terminate with types which may be seriously contaminated or hybridized.
Realities of mare basin filling. Mare basalt filling took place over a period of about 2 Ga from >4·2 Ga ago (128) and was at least partly independent of the basin-forming process because large basins are known which are virtually devoid of basaltic fill. Magma supply was sustained over a long period of time. Even the last few surface flows at a single site may span as much as 250 Ma, but there are few constraints on the time required to emplace some 99% of the underlying mare fill, beyond noting that in the western maria the oldest surface basalts are almost as old as the basins themselves (129), consistent with very rapid initial filling of the basins. There is no association with plume-like motions in the underlying mantle. Individual flow volumes must have been very large but total magma production was low.
Dubious continental flood basalt analogies. From the outset, majority opinion among Apollo scientists favoured an analogy between lunar mare basalts and CFBs, partly under the mistaken