Textural remanence: A new model of lunar rock magnetism

Earth and Planetary Science Letters, 29 (,1976) 131 - 145 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

TEXTURAL REMANENCE: A NEW MODEL OF LUNAR ROCK MAGNETISM AVIVA BRECI IER Department of l:,arth attd Planetary Sciences. Massachusetts Institute o/ Technoh;gy. Cambridge, Mass. (USA /

Received I"ebruary 25, 1975 Revised version rcccivcd October 22, 1975

hi reexamining lhe acculnulaled magnetic data on lunar rocks, several cOnllnon patterns of Inagnelic behavior are recognized. Their joint occurrence strongly suggests a new model of lunar rock magnetism, whict~ appeals only to partial preferred textural alignment of the spontaneous moments of magnetic grains, without requiring the existence of ancient lunar magnetic fields. This magnetic fabric, mimetic to locally oriented petrofabric, gives rise to an apparent "'textural remanent magnetization" ('I'XRM). In order to account for the observed intensity of "'stable remanence" in lunar rocks, only a minute fraction (10-3 to 10-s) of the single-domain iron grains present need be preferentially aligned. Several mech~,nismsoperating on the lunar surface, including shock and diurnal thermal cycling, appear adequate for producing the required type and degree of magnetic alignment in all lunar rock classes. The model is supported by a wide variely of direct and indirect evidence and its predictions {e.g. regarding anisotropic susceptibility and remanence acquisition} can be experimentally tested.

2. A model of lunar rock magnetism

I. Introduction 2. 1. The m o d e l and its observational basis

Recent reviews of the accumulated laboratory and field evidence pertaining to lunar magnetism (e.g. [1,2]) put in perspective its still controversial nature. In light of the mounting experimental evidence linking the magnetic behavior of lunar rocks with their thermomechanical history, on the one hand, and the negligible permanent dipole moment of the Moon and apparently random nature of local anomaly fields, on the other hand, it appears doubtful that ancient global lunar magnetic fields must be invoked in order to explain the body of magnetic data [2]. Although "zero-field" hypotheses were deemed "unpalatable" [ 1], the present paper proposes just such a model of lunar rock magnetism. This new model is compellingly suggested by several converging lines of evidence, drawn from magnetic data already available in the literature and is, as well, directly supported by some recent experiments. After outlining the model and detailing its observational basis, its nature and quantitative requirements are examined and finally, a number of further tests of its validity are discussed.

Right at tile outset, the following model is proposed as a framework for interpreting the bulk of magnetic data on lunar rocks: A magnetic fabric, mimetic to locally oriented petrographic textures, exists to various degrees in lunar rocks of all types. The partial preferential alignment of the spontaneous magnetic moments of metal grains, or of (elongated) magnetic grains themselves, gives rise to an apparent remanence, here called "textural remanent magnetization" (TXRM). The existence and extent of such a texturally-controlled "remanence" in lunar rocks were indicated by the pervasive occurrence of several patterns of magnetic behavior, listed in Table 1 as TXRM recognition criteria and copiously illustrated with examples from published literature in Figs. 1 and 2. In critically reexamining the magnetic data on lunar rocks [ 1], it appeared that these common magnetic behavior patterns should no hmger be regarded as "magnetic noise" masking a weak and presumably stable natural remanence (NRM) [ 1,3,4], but could 131

132 TABI.E I Textural rcmanence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7...............................

Selected examples

Recognition criteria

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. lnhomogeneity in NRM intensity, direction, or both for subsamples

100581;12063J;140531;14321; 150761;15595t:622353; 66055: 770353; 76315

2. "Undemagnctizable'" remancncc: irrcgt,lar NRM intensity chat,ges in progressive AF demagnetization

100621;120531:14066;143102;155561;603352:684152; 742751; 770173

3. Common patterns of NRM directional change in AF and/or thermal demagnetization: (a) rotation in a plane, typically along great circle (bj oscillations in a plane: frequent reversals at const. D. or in azimt, th at const. 1 (c1 convergence to a "stable" directitm of pinning 4. Magnetic anisotropy reflecting petrographic texture: (a) of low-field susceptibility (b) of high-field satt, ration and remanencc (c) of TRM or ARM acquisition 1 Mare;

2 non-lllare

igneous;

3 metac'lastic;

4 calaclastic;

10048;120381:14053~;14318:15459; 15595~;.60255; 660952 ; 65901 ( 100 kbar) 100201; 14066; 14318; 15076 I 155561; 15459; 66055 14321; 15086; 154154; 15498; 603352; 67016; 68415'; 73215; 770173 10059.24 72415.174; 659(11,10 (t00 kbar)

r e s t arc" b r e c c i a s .

more fruitfully be viewed as the "signal". The inhomogeneily of (typically both) NI¢M intensity and direction in subsamples of any lunar rock (Table 1) can be related, in this context, to tire highly localized thermal and/or mechanical effects of various lunar surface processes which may p,oduce petrographic and magnetic texture (such as defects, dislocations, deformation, recrystallization of metal grains resulting from shock fracturing, shearing and partial melting of surface rocks at impacts -- subsection 2.2). The irregular jumps in NRM intensity, accompanied by large directional changes, during progressive demagnetization [ 3 - 5 ] (Table 1)indicate that the magnetic moments are " p i n n e d " m preferred (foliation) planes or ahmg (lineation) axes and cannot be truly randomized by A F cleaning. As clusters of grain moments rotate, reverse in, or spring back to. lhe planes and axes of textural pinning, the residual intensity of NRM alternately increases and decreases (criterion 2). The corresponding directional changes conform to one of the patterns listed under criterion 3. depending on the mutual orientation of the (usually arbitrary) platte of projection for NRM and the preferred plane of magnetization. Often, when lunar sample fragments split-off from larger specimens along natural cleavage or fracture planes are examined, the NRM and cleaned direc-

tions are distributed along great circle arcs, either smoothly rotating, or oscillating in tile corresponding planes. The frequent reversals indicate that no preferred NRM direction exists ahmg the "easy" axis or in the "easy" planes, although there are some exceplions, namely the strong-pinning limit cases (Table 1 ). When the sample coordinates and magnetic projection plane are arbitrarily oriented with respect to any textural features which might control the NRM, directional changes may appear random: the existence of a preferred magnetization plane is indicated when changes in one of the angle coordinates (inclination or azimuth) is much larger than in the other (Table 1). To best illustrate each of the above characteristics, Table 1 lists samples representing all classes of lunar rocks, selected from an extensive list compiled in surveying tim inagnetic literature. Both the intra- and inter-group range of NRM behavior patterns corresponding to the TXRM recognition criteria I - 3 are illustrated in Figs. 1 and 2 [1,6-91. These magnetic behavior patterns are individuall)' consistent with, but not necessarily diagnostic of, TXRM. I lowever, their occurrence typically in conjunction strongly supports the TXRM hypothesis. A fourth characteristic logically required by the TXRM model is the existence of magnetic anisotropy,

133

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Fig. 1. "Fhis collage of published magnetic data [ 1,6 -9] illustrates the patterns of magnetic behavior listed in Table 1. (a) The NRM directions in two soil breccias (10048,55-mare and 60255-highland) and in the shock-lithified soil 65901,10 r~tate along great circle arcs with progressive demagnetization. In the latter, the " s h o c k - r e m a n e n c e " (SRM) migrates a w a y from ambient field directions (crosses) in the plane containing them.

134

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Fig. 1 tcontinued). (b) These illustrate tim irregular NRM mtensity changes with progressive AF cleaning, and tim corresponding directional behavior: rotation in planes, along a great circle, with reversal for 15086, and at constant inclination for 15459. which is usually a sensitive indicator of petrofabric in terrestrial rocks. Some direct evidence of low-field anisotropy was provided by the earliest magnetic studies of returned Apolh:) 11 lunar samples [ 101 (T. 1). Doell et al. [10] attributed to the large anisotropy of susceptibility in breccia 10059,24, its "apparent remanence" of high coercivity (i.e. stability) and also noted that the viscous remanence (VRM) and NRM vectors were coplanar, defining a preferred magnetization plane. The patterns of NRM reversal and movement ah:)ng a great circle arc are also evident m this breccia [ 10], which may thus be regarded as a fi,st paradigm of TXRM. The magnetic anisotropy can be directly linked to shock effects, via the preferential acquisition of saturation remanence in the plane of the shock wave by the (100 kbar) shock-lithified soil

65901,10 [11 ]. This high-field anisotropy was attributed to the generation in situ of elongated iron grains and/or to shock deformation of iron-rich glass spherules [1 I]. As a test of the assumption that petrographic shock and defonnational features correspond to a magnetic fabric, evidence for high-field anisotropy was sought and obtained [ 12] for the shocked dunite cube 72415,17 [ 13]. The st riking similarity in the patterns of NRM rotation along great circle segments in the mare (10048,55) and highland (60255) soil breccias, to those of shock-remanence (SRM) in the shocklithified soil 65901,10 under both AF and thermal cleaning (Fig. 1 and [111), constitutes further evidence that textural pinning of magnetization to preferred planes is related to shock. Significantly, the SRM rotates a w a y from the directions of the ambient fields at shock and post-shock cooling, but along a great circle containing them, indicating that the role played by the magnetic fields present is secondary to the texturing effects of the 100-kbar shock. The high "stability" of both NRM and IRM s in the shocked metaclastic highland rock 77017,45, which alone showed some directional coherence of NRM among a suite of rocks rep,esentative of the Apollo 17 site [14], also attests to the importance of shock textures in determining the magnetic behavior. Our experiments [14] simulating the effects of superposed brief shock-metamorphism events on lunar rocks, indicated that: (a) most lunar rock types are incapable of blocking directionally coherent thermoremanence (TRM) in laboratory fields, on experimental timescales, and (b) the AF demagnetization behavior of this "TRM" is similar It:) that of NRM, revealing strong textural control: not until the original petrofabrics were thoroughly annealed did basalts and breccias become able to acquire TRM in the conventional sense [14l. Thus, textural pinning of grain moments and the behavior patterns identified in Table 1 are evident in both "NRM" and "TRM", so that the relative stability of remancnce may simply result from competition between thermal annealing and defect hardening el: fects, cor,esponding to various shock histories. The few lunar rocks exhibiting "stable" NRM, most of which are low-grade soil breccias [ 1 ], probably ,epresent the limit of strong mechanical textural pinning without any annealing. The TXRM model provides a much more plausible

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Fig. 1 (continued). (c) The basalt 14053 .Samples exhibit the magnetic syndrome corresponding to criteria 1- 3 in Table 1 : subsamples show an inhomogeneous NRM, whose directions either rotate ahmg great circle segments, as seen for 14053,48 in thermal demagnetization, or oscillate, in planes, reversing repeatedly and returning to the original "pinning" axis. as seen for 14053,35.

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framework for explaining the NRM behavior of oriented subsamples from the Apollo 17 foliated Boulder 1, station 2 [15] and of station 6 boulder [16], than the standard TRM interpretation. In the Boulder 1, station 2 samples, the NRM directions differ significantly, and under AF cleaning reverse and migrate along great circle arcs intersecting at ~ 130". It is difficult to ac-

cept the suggestion that the lunar magnetic field rotated so substantially on the short time scale for cooling successive foliation layers in the boulder [15]. The TRM hypothesis is further weakened by the almost complete loss of NRM, in heating to only 250°C, accompanied by considerable coercivity spectrum chang. es, which - I suggest - are due to annealing of the de-

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Fig. 3. "lhe directional changes of NRM, during zero-field storage followed by At. demagnetization, in two mutually oriented 73215 breccia cubes, are clearly controlled by their main lithologic banding, shear and flow planes (see also figs. 15 and 16 in ref. [ 17b]). These are indicated here only by their trace declinations (S~, S2 , $3) projected onto the sample slab face [171. fects pinning the "remanence". Our data on a 72275 sample [ 14] conform indeed to TXRM constraints. In Boulder 6, the distribution of NRM directions in samples with well-developed foliation (e.g. 76125, 76295) indicates confinement tcr a plane of preferred magnetic orientation, as the authors are forced to conclude, although they doubt that a cause-effect relation between petrofabric and magnetic fabric exists [ 16]. However, my own examination of the consortimn petrography and sampling maps * has convinced me that Boulder 6 samples exhibit typical TXRM, controlled by shear and flow banding. Recently obtained direct evidence for the validity of TXRM (Fig. 3) has been presented in more detail in refs. [ 12] and [ 17]. The availability of two large, mutually oriented, cubic samples of consortium brec-

* Courtesy of Dr. W.C. Phinney.

cia 73215 offered the opportunity to test this model directly [12]. These samples have distinct gray and black matrix lithologies, which exhibit pronounced foliation due to flow and shear [17]. Their NRM directions are clearly different, each being unambiguously controlled by one of the main shear systems. The shear planes serve as "easy" planes of magnetization, in which the NRM freely and repeatedly reverse during zero-field storage and to which they are "pinned" up to 300 Oe cleaning fields. Above 300 Oe, the directional control passes to other main shear systems and the NRM directions end up ~110 ° apart (Fig. 3 and figs. 15, 16 in ref. [17b]). The phenomena of planar confinenlent o1" NRM directions and "zig-zag" intensity changes during AF demagnetization have been noted in 15535,28, representative of certain lunar basalts [3 -51. Ilowever, these were attributed to the presence of only "a few, large disk-shaped multidomain grains" [5]. In contrast, we propose here that this TXRM syndrome is due to partial alignment of magnetic grains over the entire size spectrum and is common in all lunar rock types with anisotropic petrofabric. Moreover, we found strikingly similar patterns of magnetic behavior in terrestrial metamorphic rocks in general, and in shocked terrestrial rocks and meteorites in particular [ 1 8 - 2 0 ] where textural control of NRM also prevails. In the context of TXRM the paleointensities estimated for a number of lunar rocks, which range broadly from 100 7 to : > 1 0 e , reflect merely tile relative contents of single-domain (SD) grains and their degree of alignment, and thus diffe,ent efficiencies of laboratory TRM acquisition as a function of ambient field. For example, a paleointensity of ~2200 "f was obtained for 15498,36 a low metamorphic grade breccia, rich in SD grams [91. Its unusually strong (~10 -4 emu/g) and "stable" NRM, though fairly coherent directionally, rotated in a constant inclination plane by :>30 ° in AF cleaning to 400 Oe (Fig. 2). This NRM may represent the strong magnetic alignment limit. In contrast, a paleointensity of 1 . 2 0 e was obtained by the same double-heating technique for 62235,53 a metaclastic highland rock, rich in coarse multidomain (MD) metal grains [7]. The direction of its similarly strong, hut considerably softer NRM, also moved roughly along a great circle, by :>>30° in I at constant azimuth. Obviously, a much stronger equivalent field is needed to magnetize and align the moments of MD grains to

140

the reqt, ired remanence level. Our data on the shocktextured 77017 metaclastic rock [ 141 shows that the apparent paleointensity changes upon repeated laboratory heating, reflecting tire degree of textural anneal and the efficiency of TRM acquisition by the carrier grains. It is also suggestive that zero-field (30 kbar) shock ot" a lunar soil rich in MD grains was associated with considerable magnetization, and that field-enhancement by factors up to X 10 was seen in konar basalts shocked by meteoritic impact in earth's field [ 11 ]. Although l loffman and Banerjee [5] recognized that the type of magnetic behavior encountered in 15535,28 "'would render any paleointensity determination of little value", Banerjee and Mellema [211 published a paleointensity of 7600 7 for this very sample, without any evidence for a '+stable" component of NRM [41. Thus, the question of whether hmar field paleomtensities can be taken literally and whether postulated ancient magnetic fields are indeed necessary to account for observational evidence is still open.

2. 2. The types and sources o f pr
Anisot topic shapes or distribution of magnetic grains have been reported for a variety of lunar rock types. For example, the soil breccia 14313 and the glassy breccia 15015 are known to contain shape-anisotropic acicular or whisker SD metal grains 124,25]. The deep-seated coarse-grained lunar norite 78235 and troctolite 76535 show oriented arrays of shock-reduced metal rods in plagioclase [26,271 and subpar~,llel veins of metal, apparently oriented with respect to fracture and cleavage crack systems, were seen in other noritic fragments [28]. Much more commonly occurring are nearly subparallel shock-melted silicate glass veins, with flow banding delineated by concentrated trams of tnetal spherules, which fill shock-induced crack systems. Examples are the Apollo 16 shocked basalts "rusty rocks" (fig. 9 in 129]) and rake samples [30], an Apollo 11 shock-fused basaltic breccia (fig. 7 m [31]) and the deep-seated crushed dunite sample 74215, in which metal beads decorate phmar strainbands [ 13]. This type of continuous Iqow structures, marked by finely disseminated, concentrated and aligned metal spherules, has been reproduced experimentally by (500-kbar) shock-melting of regolith analogs (fig. 8 in [32]). Such preferentially oriented metal veins or grain assemblages are often seen to be continuous o n a inm-cm scale, and will certainly give rise to magnetic anisotropy. Even if only a few large grains of ilmenite [311 or plagioclase [26] contain parallel rods or veins of metal, reduced in situ by shock heating, a small but sufficient degree of anisotropy may still obtain. Oriented arrays of defects or rhabdite needles, such as seen within some large, sub,-ounded metal grams (fig. 6 in [30]) might also suffice to pin the easy plane or axis of magnetization. Planar oriented fracturing and sheeting are much more obvious in boulders sampled, as are radial-linear st ructt, res of geologic units. In particular, the White Rocks near Cone Crater at the Apollo 14 Fra Mauro site and the large boulders surrounding the TaurusLittrow valley at the Apollo 17 site - are all conspicuously layered [33,34 I. Samples 14310 through 14321, whose magnetic behavior is consistent with TXRM (Table 1), show ma,ked shearing and foliation (U. Marvin and O. James, private communication), originating in the layered breccia boulders typical of the Cone (7rate, ejecta. The strongly foliated breccia boulders at station 2 are representative of the North Massif formation, whose "dominant fabric.., is apparently

141 that of roughly horizontal structural units, that ,nay be depositional or intrusive layers" [34]. The magnetic behavior of such Apollo 17 breccia and boulder samples (stations 2 and 6) was discussed above as best illustrating TXRM. The petrologic characteristics of the station 8 noritic boulder, i.e. the systems of fractures conjugate to planar shear-sheeting and veining, which are interpreted to reflect sedimentary processes [35 ], are probably the macroscopic counterpart of the crystallographically oriented arrays of metal rods [26]. Similarly, most rocks from the Van Serg (?rate, Rim (station 9) are toliated soil breccias with characteristic platy fracturing (U. Marvin, private communication). Many bulk-magnetic properties of Apollo 16 and 17 rocks bespeak the homogenizing effects of impactproduced regional metamorphism. The local magnetic anomalies typical of the lunar surface [2] may result from coherently deposited and cooled ejecta blankets, in which partial orientation of metal grains precipitated during shock-melting and/or recrystallized during shock metamorphism may obtain. Some general mechanisms which probably produce textural anisotropy in lunar rocks are as follows: (1) Low-temperature cataclasis, involving mechanical shearing, fracturing, and crushing, which will tend to introduce oriented arrays of defects, dislocations and cracks, aligning the MD grains and pinning domain walls. (2) Thermal naetamorphism, either at impact, or during post-shock cooling in ejecta deposits, which will tend to orient or deform the magnetic grains themselves. Flow and shear features may be established simultaneously with crystallization from a partial melt as in some A17 highland b.reccias [17], or may result from later reworking, as in recrystallized breccias and metaclastic rocks. (3) Diurnal thermal cycling, over a range of ~300°K, which is an important potential source of anisotropic stress and fracture systems in lunar crystalline rocks. Although some direct effects of thermal fatigue on the magnetization of lunar soils and rocks have been considered previously [ 1], the significance of indirect, mechanical-hardening effects has not been addressed. Thermally-induced stress levels in surface rocks suffice to give rise to periodic moon-quake activity [36]. Obviously, different lateral versus vertical thermal gradients can conceivably lead to anisotropy of strains and microcracks. It has been

experimentally demonstrated that low-temperature thermal fatigue, due to such cycling, introduces in chondritic meteorites deformation effects usually ascribed to shock; this is particularly effective in regions surrounding metal grains, due to differential dilation [37]. (4) An initial anisotropy in thermal gradients would similarly occur, and be frozen-in during cooling, if each lunar mare consisted of thin, tabular basalt flow units, as recently proposed by Brett [38] and supported by other data [39], and if breccia blankets forming ejecta strata were similarly thin deposits. (5) It is also of interest to note that lunar soil grains show strong preferred orientation and anisotropy in shear strength and compressibility, related to the layered fabric [40]. Breccias produced from impactlithified or compacted soils are likely to preserve this preexisting texture. Shape-anisotropic metal grains probably contorm to textural constraints of other elongated soil grains [41 ], so that the "stable remanence" shown by low-grade soil breccias (e.g. 14313) may be a carry-over "depositional remanence" DRM. Although it may be contended that such a DRM implies the presence of magnetic fields, recent experimental data on the magnetic fabric produced during laboratory deposition of silts and sands (similar in grain size to lunar soils) confirm that gravity and tlow dominate in producing the anisotropic susceptibility indicative of preferred magnetic texture [42]. In reality, the petrofabric of lunar rocks is likely to result from an often complex superposition of all these factors. Several factors may select a preferred direction from amongst the "easy" magnetic directions texturally defined by plane(s) and axes. An apparently "stable" direction to which NRM converges or is pinned during progressive (AF or the,real) cleaning then results. First, the processes discussed above, which imprint petrographic textures, may have vectorial components. Such are lot example, shock-propagation and/or postshock decomp,ession in rocks, or radial ejecta deposition, when accompanied by thermal gradients: intersecting shear planes and flow lines seen petrographically as schlieren bands, when accompanied by thermal reworking and/or partial melting, etc. Second, a magnetic direction may take precedence over other easy directions as a result of any perturbation magnetic field, be it plasma generated transients or the lunar surface,

142 or contamination fields experienced in transit to, or residence on, Earth 11,2 I. Viscous (VRM) or other (I RM, pTRM, ARM) secondary remanence are often much stronger than the intrinsic "NRM'" signal and curiously, either coplanar or coaxial with it [ 1 l, both very likely oriented ahmg the maximum susceptibility axis [ 10]. Magnetic interactions, between grain moments and/or components can also lead to a preferred direction of magnetization, llowever, as clearly seen for breccia 73215 [12,17] and for other textured Apolh) 17 rock samples [14 I, zero-field storage will lead to repeated NRM reversals along the preferred axis or in the easy plane, indicating that no stable NRM direction exist s.

Z3. Quantitatire evahtalhm of the model It is possible to estimate senti-quantitatively the fractiolaal align*nent (x) of SD grain moments required to account for the "stable" part of NRM (NRM lo0. surviving 100 Oe AF cleaning) thought to represent the indigenous lunar magnetization (Table 2). The overall iron metal (Fe °) content and its grain-size distributi(m are known for each group of lunar rocks. and. in the case of breccias, appear to correlate directly with metamorphic grade (e.g. [43 ]). Reduced saturation remanence ratios (Jrs/Js) provide an upper-limit value to the fraction of SD grains present (]Sl)),which are spontaneot, sly magnetized to saturation (.Is). [laving the highest coercivity, they are the potential carriers of stable remanence (NRM,oo). The required degree of overall preferred alignment (ix) of SD grains ranges from I0 -s in low-grade breccias to 10 -3 in igneous rocks. Numerically, this is not a stringent constraint at all, particularly since for simplicity, we overlook tire probably important contributions to NRM from the more abundant, coarse, biD metal grains, whose

domain walls may be easily pinned by oriented arrays of defects and dislocations. Using the numbers in Table 2, and aSDsize-range of Fe °grains of 1 5 0 - 3 0 0 A the model requires ~ 1 0 '~ out of both the < 10 ''~ SIt grains/g in breccias and the "-_:10 ~2 SD grains/g iI~ igneous rocks to be oriented preferentially in some direction along "easy" axes and/or planes. Based on the usually weak intensities and " r a n d o m " directions of NRM residuals after AF or thermal cleaning, it might reasonably be presumed that the magnetization of lunar rocks provides a classic example of "random moments" first noted in some unstable terrestrial basalts [44]. Although these were attributed to statistical aligmnent of noninteracting domains in MD grains, an interpretation convincingly challenged [45], we can use the statistics of Irving et al. [44] for comparison purposes. In our case, we consider n randomly oriented SD magnetic grains/g of lunar rock, each of saturation moment m s, to evaluate the minimum (random) moment: Mini n = 0.89 n l
J ABI.E 2 Degree of single-domain grains alignment (x) Rock type

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0. I 0.2 10-s

X

=

NRM l o o •Is "/'l'e0 ' tSl) .

.

.

.

.

.

143 ic but typically textured terrestrial (sedimentary and/ or metamorphic) rocks, the "random moment.., is not a remanence in the normal sense, but a state of least energy dependent on coupling between domains and on anisotropy" [47]. indeed, our parallel magnetic studies on a variety of terrestrial metamorphic and shocked rocks have revealed magnetic behavior patterns strikingly similar to those encountered in many lunar rocks [14,18,19], It is clear that if some degree of systematic magnetic alignment, related to petrofabric, exists in lunar rocks, it can be best detected and quantified by magnetic anisotropy measurements [42,48]. Derived anisotropy of susceptibility parameters can index both the type and the degree of magnetic grain alignment, particularly for cubic-isotropic minerals like (Ni)-Fe. It is anticipated that trends in the variation of magnetic fabric among lunar rock types will indicate to what extent textures resulted from primary (e.g. flow and shear at crystallization, shock lithification) and/or secondary (shock brecciation, recompaction and recrystallization in post-shock metamorphism) processes.

3. Concluding remarks Several commonly occurring patterns of lunar rock magnetic behavior were identified and shown to be consistent with strong textural control of magnetization. A new framework was proposed for integrating the bulk of lunar magnetic data, namely a model in which an apparent textural remanent magnetization (TXRM) results from the'partial preferred alignment of magnetic grain moments. This magnetic fabric has been related to the metamorphic petrology and shockthermal history of lunar rocks. The TXRM model incorporates and develops in a new coherent context some previous ideas. It is related conceptually to "piezoremanence" [49] and "shock remanence" [ 11,50], insofar as it recognizes the key role of shock and stress effects in determining the magnetic properties of lunar samples. Unlike these, however, the present model does not require the existence of hypothetical lunar magnetic fields. In emphasizing the importance of a magnetic texture mimetic to an oriented petrofabric, which gives rise to magnetic anisotropy, the model is akin to an early defect-model

developed to explain stable remanence in terrestrial rocks [ 51 ] and relies on a phenomenology remarkably similar to that of terrestrial sedimentary and metamorphic rocks [52-55] or shocked rocks [I 8,50]. Magnetic "contamination" fields [ 1] or vectorial components of lunar surface processes which produce an oriented rock fabric (subsection 2.2) sometimes select a preferred direction for TXRM, along easy axes or in the easy magnetization planes. Whereas the recent experimental evidence discussed in subsection 2.1 provides convincing support for textural control of magnetization and the associated magnetic anisotropy, the "zero-field" aspect of the TXRM model is still conjectural. Theretore, a crucial test of the model will be the outcome of a current study of zero-field vs. field dependent magnetization of textured, stressed, or sheared synthetic samples. The TXRM model of lunar rock magnetism proposed here has both explanatory and predictive power. Its advantage over global lunar magnetic field theories, the evidence for which is neither compelling, nor internally consistent, is the fact that it is experimentally testable. In order to factually consolidate and further test the validity of this model, considerable experimental work must be done, relating the directionality of NRM and other laboratory remanence (TRM, ARM, IRMs) and the anisotropy of susceptibility to the oriented petrofabric.

Acknowledgments I/bank Drs. A.J. Irving and O. James fi)r helpful discussions. This work was supported by NASA grant NGR-22-009-551, under the sponsorship of Prof. R.G. Burns. Drs. R. Brett and R. Merrill kindly previewed the manuscript, and the reviewers' comments helped improve tile original version. Ms. R. Regan and Ms. D. Chouet are thanked for preparation of the manuscript.

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