Lunar and Planetary Science XXIX
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MAGMA MIXING IN THE PETROGENESIS ALKALI SUITE ANORTHOSITES: REVERSE ZONING IN PLAGIOCLASE, 14305 ,303. J.W. Shervais and J.J. McGee, Department of Geological Sciences, University of South Carolina, Columbia, S.C. 29208 (
[email protected],
[email protected]). Introduction: One of the prime goals of lunar petrologic studies is to unravel the petrogenetic processes that controlled the evolution of lunar highland rocks, so that we may better understand how planetary crusts form. We have recently studied an alkali anorthosite (probe mount 14305 ,303) in which large “phenocrysts” of plagioclase are reverse zoned, while the surrounding “groundmass” feldspar shows only minor normal zoning. This relationship has not been documented in other lunar anorthosites, but we believe that this unusual sample provides us with extraordinary insights into the petrogenetic processes that shaped the evolution of the post-magma ocean highland crust.
autometasomatized by K-rich fluids along healed fractures that emmanate from the K-rich interstices between the cumulus feldspars. Discussion: We examine three possible models for the 91
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Figure 1. Photomicrograph of 14305,303 showing large zoned plagioclase grain in groundmass of smaller grains. Largest plagioclase os 1.7mm across. Data: The alkali anorthosite represented by probe mount 14305,303 was first discribed and analyzed by Warren and others [1]; the corresponding whole rock analysis is sample 14305,283. This sample consists of about 95% modal plagioclase, 2.5% modal whitlockite, and about 1% to 2% each of pigeonite and augite, with traces of K-feldspar, ilmenite, troilite, silica, and Fe-metal [1]. Most plagioclase (•An87-89) forms small (0.3-0.5 mm), subequant cumulus grains with minor normal compositional zoning, and a decussate texture. Whitlockite, pigeonite, and augite are all small, post-cumulus phases, along with the trace minerals. The sample is dominated by a few large subhedral to euhedral plagioclase “phenocrysts” with sizes ranging from about 0.8 mm up to 1.7 mm across (figure 1). These grains typically display reverse compositional zoning, with cores •An85 (up to 1 mm across) surrounded by mantles of more calcic plagioclase (•An91). These mantles are normally zoned to •An88 — similar to “groundmass” plagioclase (figure 2). The outermost rims may be as sodic as •An67. Large area X-ray maps for Na and K show the effects of this zoning and its distribution (figure 3). The core of the largest “phenocryst” is not only Na and K-rich compared to its mantle, but contains abundant tiny inclusions of ilmenite, pyroxene, whitlockite, and silica, as described by Warren et al [1]. These inclusions, which indicate an alkali suite affinity for the plagioclase core, are not found in the surrounding mantle of more calcic plagioclase. This may indicate that the calcic mantle does not have alkali suite affinities. The X-ray maps also show that the larger feldspars have been
Figure 2. Electron microprobe traverse of large reverse zoned plagioclase “phenocryst” in 14305,303. Normal zoning in outer rim is same composition as surrounding groundmass plagioclase. origin of these relations: vapor pressure variations, assimilation of plagioclase from preexisitng anorthosites, and magma mixing with more calcic primitive magmas. Vapor Pressure: Variation in the vapor pressure of a confined magma, generally in response to magma eruption and concomittant degassing of the magma chamber, is often called on to explain oscillatory zoning in plagioclase grains that are not obviously related to magma mixing [2]. In these cases, plagioclase grains may exhibit fine-scale oscillatory zoning with numerous compositional reversals that would seem to require numerous magma mixing events to form. However, similar reversals in the compositions of coexisting mafic grains are not observed, and the scale of the oscillations seem too small to have formed from significant mixing events. The small scale of the plagioclase oscillations, and the lack of similar zoning in the mafic phases, is consistent with variations in the plagioclase solidus in response to variations in vapor pressure. This explanation does not seem to hold for 14305,303 because (a) the large “phenocryst”-like crystals have only a single, thick zoning oscillation, not the fine scale repetetive oscillations associated with vapor pressure changes, and (b) lunar magmas are characterized by low volatile element contents, in keeping with the volatile-depleted bulk composition of the moon. As a result, it seems unlikely that the variation of vapor pressure within a shallow lunar magma chamber could account for the scale and style of the observed zoning. Assimilation: Assimilation of more calcic plagioclase from preexisting lunar highland cumulates will move the parent magma to more calcic compositions, but it will not cause a reversal of zoning in early-formed plagioclase primocrysts within the magma. In order to increase the anorthite content of the equilibrium plagioclase in a melt, it is necessary to raise the temperature of magma. Since the amount of heat required to
Lunar and Planetary Science XXIX
1706.pdf
MAGMA MIXING AND ALKALI ANORTHOSITES: J.W. Shervais and J.J. McGee
Figure 3. Large area X-ray maps for Na (left) and K (right) in plagioclase, 14305 ,303. Large plagioclase phenocryst 1.7mm across is reverse zoned with Na+K rich core and Ca-rich mantle. Outer rim is normally zoned to more Na+K-rich plagioclase. Core of phenocryst is inclusion rich. Note healed fractures of K-rich feldspar that emanate from interstices and cross the large plagioclase grain. melt a phase during assimilation is greater than the heat released by coeval crystallization, the net thermal effect of assimilation will be a decrease in temperature of the magma system (e.g.,[3]). Assimilation of calcic plagioclase will cause the parent magma to become more calcic, but this increase in anorthite content of the magma will simply result in more plagioclase being crystallized, with either the same composition as before (if temperature is constant) or with a more sodic composition (if there is a decrease in temperature during assimilation) [2]. In a 3-phase system (e.g., plagioclase-diopside-melt), the magma will be pulled off of the 3-phase cotectic (if it was on the cotectic), and the overall proportion of plagioclase will be increased, but the composition of the equilibrium plagioclase will never be more anorthitic than that of the equilibrium plagioclase prior to assimilation [2]. Magma Mixing: The only process which seems to be consistent with the observed reversal in plagioclase zoning in 14305 ,303 is magma mixing [4]. Mixing of an evolved alkali suite magma with a hotter, more primitive, and probably more calcic magma will result in a mixed magma that is hotter than the original alkali suite magma and has a higher Ca/Na ratio. Such a magma would crystallize a mantle of more calcic plagioclase over the more sodic core, thanks to its hotter liquidus temperature. Subsequent fractionation of the mixed magma would result in the formation of an outermost rim that is more evolved than the mantle, and in the formation of additional “groundmass” plagioclase as the magma solidified. There are two possible scenarios for the provenance of the hotter, more primitive magma postulated by this explantion. First, the primitive melts may represent the influx of new, juvenile parent magma from the mantle source region (similar to mixing events in MORB magma chambers). Alternatively, the primitive magma may result from convective overturn in a
partially crystallized, zoned, magma chamber, which mixes primitive bottom melts with the more evolved melts and crystals found near the ceiling of the magma chamber. The composition of the anorthitic mantle is about An91 — similar to the most primitive alkali anorthosites found, and to the “evolved” gabbronorites and norites of the Mg-rich suite (e.g., James, [5]). Since this reflects the composition of the mixed magma, however, the composition of the added melt must have been even more primitive than the parents of the Mgsuite gabbronorites and norites. This implies that the primitive magma which mixed with the alkali suite magma was in fact related to the Mg-rich suite. If so, this mixing could have been fortuitous and not the result of a recurring petrogenetic process. Alternativey, this may suggest a more formal relationship between the two suites, as proposed by Snyder and others [6,7]. Conclusions: The model proposed here explains the petrographic and mineral chemical characteristics of one alkali suite cumulate, but suggests a more general petrogenetic process that may have been important in the formation of many alkali suite cumulates. This model also suggests a relationship between alkali suite anorthosites and Mg-suite norites and gabbronorites. References: [1] Warren, P., et al., 1983,. J. Geophys. Res. Supl., 88, A615, [2] Morse, S.A., 1980, Basalts and Phase Diagrams, Springer, 342p, [3] Hess, P.C., 1994, J. Geophys. Res., 99, 19083 [4] Russell, 1990, Rev Min., 24, 153 [5] James, O.B., 1980, Proc. Lunar and Planetary Science Conf. 11th, 365, [6] Snyder, G., et al, 1995, J. Geophys. Res. Planets, 100, E5, 9365 [7] Snyder, G., et al, 1995, Geochim. Cosmochim. Acta, 59, 1185.
