Mantle garnets: A cracking yarn

et Cosmochimica Acfa Vol.56. pp. 2633-2642 Copyright 0 1992 Pergamon PressLtd. Printed in U.S.A.

Geockrmica

0016-7037/92/$5.00

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Mantle garnets: A cracking yam M. MATTHEWS,’ B. HARTE, ’ and D. PRIOR* ‘Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, Scotland *Department of Earth Sciences, University of Liverpool, PO Box 147, Liverpool, L69 3BX, England (Received August 30, 1990; accepted in revisedform April 13, 1992)

Abstract-Garnets showing variation in chemical composition occur in the metasomatised peridotitic wallrocks to intrusive pyroxenitic “dikes” in mantle xenoliths from the Matsoku kimberlite pipe. They have been examined by scanning electron microscope (SEM) and imaged using both high contrast and electron channelling backscattered electron (BSE) methods. The images revealed intricate variation in the backscatter coefficient, q, across garnets. The pattern of variation is one of very slightly d&se. “brightwhite” (high v) lines surrounded by paler diffise auras, grading into darker (lower 7) areas away from the bright lines. The bright lines are usually irregular in detail and show branching and braiding; but in some cases they form regularly spaced parallel to subparallel sets which reflect crystallographic orientation. Electron and ion microprobe analyses, including a highly exhaustive 40,000 data point electron microprobe survey across one garnet, have correlated the change in the backscatter coefficient with compositional variation. The compositional changes largely involve enrichment in Fe and Ti, and decrease in Mg of the garnet forming the bright lines and pale auras, and are consistent with those of metasomatic garnets identified by previous work on an extensive suite of Matsoku xenoliths. The high Ti and Fe garnet is also enriched in Y, Zr, MREEs, and HREEs and is considered to have formed by direct crystallisation from the melt causing metasomatism, whilst areas away from the high r) lines show compositions similar to those ofgamets in unmetasomatised rock. The bright (high Q) lines are interpreted as delineating a pattern of fractures along which the metasomatising melt was able to penetrate the garnet, and from this melt new garnet crystallised to heal the fractures and form the high Fe-Ti garnet. Diffusion ofelements outward from these melt-filled and subsequently healed fractures into the main body of the garnet produced the more diffuse zonation pattern imaged by BSE methods on the SEM. INTRODUCTION

port of melt material from the adjacent sheets. Variation in the chemistry of the wallrock garnets provides clear evidence of this in that a single garnet in the wallrock may show a range of compositions from that typical of garnets in common coarse, depleted peridotites to that of garnets in Fe-Ti rich pyroxenite sheets ( HARTE and GURNEY, 1975; HARTE et al., 1977, 1987). Electron microprobe analyses identified the metasomatic event to be largely character&d in the garnets by Fe and Ti enrichment accompanied by Mg depletion, and commonly with some depletion in Cr and enrichment in Al (HARTE et al., 1987). Although the mineral and bulk compositions of the Matsoku metasomatised wallrock peridotites are unusual for normal coarse garnet peridotites, GIJRNEYand HARTE( 1990) pointed out that their compositions showed close similarities to those of widespread deformed high-temperature peridotites and they suggested that these widespread peridotites might also be a product of metasomatism, rather than representing relatively primitive upper mantle material, little depleted in basaltic constituents ( BOYD and NIXON, 1975). Following up the proposed metasomatic origin for the high-temperature peridotites, HARTE and GURNEY ( 198 1) and HARTE ( 1983 ) suggested that they formed in the metasomatised envelope of magmas precipitating the Cr-poor megacrysts found in kimberlites. Evidence that the high-temperature peridotites are indeed commonly metasomatised has come in recent years from several studies on detailed chemical variation within these rocks at the thin section scale (SMITH and BOYD 1989; GRIFFIN et al., 1989; HOPS et al., 1989). Further geochemical evidence supporting a genetic link between the

AN EXTENSIVESUITE OF xenoliths from the Matsoku kimberlite pipe in northern Lesotho, southern Africa, shows evidence of melt intrusion, infiltration, and metasomatism occurring in garnet peridotites in situ in the upper mantle. Some of the garnet peridotites show normal coarse textural and depleted major-minor element characteristics whilst others can be seen to have been affected either by modal metasomatism or by Fe-Ti enrichment without modal change (HARTE et al., 1975, 1987; GURNEY et al., 1975; HARTE and GURNEY, 1975). In addition, there are composite xenoliths of coarse garnet peridotite cut by sheet-like bodies of finer grained garnet pyroxenite-peridotite. These garnet peridotite facies sheets have been interpreted as being layers along which magmatic fluid has been injected ( HARTEand HUNTER, 1986; HARTE et al., 1977, 1987), and they show obvious similarities to the common Al-augite bearing (type II) “dykes” described in association with spine1 peridotite xenoliths from basalts (e.g., WILSHIREand SHERVAIS,1975; FREY and PRINZ, 1978; MENZIES, 1983; HARTE and HAWKESWORTH,1989). The garnet peridotite wallrocks to the Matsoku pyroxenite sheets are modally and texturally indistinguishable from the common coarse and depleted peridotites (the common peridotites, CP, ofCox et al., 1973; HARTE et al., 1975, 1987). However, they have unusual bulk and mineral chemistries for common coarSe peridotites. Their mineml chemistries are particularly enriched in Fe and Ti and are closely similar to those of the adjacent intrusive pyroxenite sheets, suggesting metasomatism of the peridotite wallrocks as a result of trans2633

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M. Matthews, B. Harte. and D. Prior

metasomatised Matsoku wallrock peridotites and the formation of Fe-Ti rich, high-temperature peridotites has been found in trace elements, which indicate similar signatures for the magma sources causing metasomatism. Thus, clinopyroxene and garnet from Cr-poor megacryst suites and the Matsoku intrusive pyroxenite sheets show similar trace element compositions which are compatible with equilibrium with OIB-like magmas ( HARTE,1983; HARTEet al., 1991). Some further comparisons between the Matsoku wallrocks and the high-temperature peridotites will be made at various places in the following discussion. This paper examines in detail how the metasomatic enrichment of the Matsoku wallrock garnets varies spatially within individual garnets, and thus provides basic evidence for geometrical controls on the enrichment process and thereby the mechanism of its occurrence. Previous work on the Matsoku xenoliths ( HARTEand GURNEY,1975; HARTE et al., 1987) established the occurrence of variation of garnet compositions within rocks and individual garnets. Using a limited number of electron probe point analyses, it was shown that metasomatised garnet rim compositions relatively rich in Fe, Ti, and often Al, surrounded unmetasomatised garnet core compositions, and thus indicated that the dominant garnet compositional control might be diffusion from the rims inwards. The detailed studies on spatial compositional variation reported below have utihsed both high density point analysis by electron microprobe and SEM backscatter electron imaging techniques. They reveal that Fe-Ti enrichment is controlled in the first instance by a process of garnet fracturing and healing in the presence of melt, and that volume diffusion effects are secondary to this. METHODS Electron Microprobe The data were gathered in the Department of Geology and Geophysics, Edinburgh University, using a Cameca Camebax Microbeam instrument operating four crystal spectrometers. Operating conditions were 20 KV and 20 nA. Detailed investigations of compositional variation within the garnets revealed that, although rims were generally Fe-Ti enriched relative to cores, the spatial variation was more complex in detail than a regular core-to-rim zoning. Therefore, an extremely detailed survey was carried out by point analysis on one garnet, which gave a much improved resolution of compositional variation, though this resolution was subsequently further improved by the application of SEM backscatter techniques. In this very detailed electron microprobe analytical study, a series of forty parallel line traverses were run at 100 pm intervals across the garnet. Each line was made up of 1000 data points, 4 pm apart, making a total of 40,000 data points. Under the operating conditions used (20 KV, 20 nA), the electron beam excites X-rays from a volume 4-5 pm in diameter (REED, 1975). At point spacings of less than 4-5 pm, overlapping of the excited volumes begins to occur and numerical deconvolution of the data is frequently necessary (GANGULY et al., 1988). At each data point in the traverses X-ray, counts were collected for 10 set periods, the four crystal spectrometers locked onto one element peak position each. Mg, Ca, Cr, and Ti were chosen as being the best and the four most convenient elements to represent the major chemical variation exhibited by the garnets, given the knowledge that exchange of Mg with Fe and Cr with Al am common features of variation (HARTEet al., 1987). The extremely detailed survey for four elements was backed up by less frequent analyses for Fe, Al, Mn, Na, and Si, in addition to Mg, Ca, Cr, and Ti.

SEM Backscatter Electron Imaging Recent improvements in SEM imaging capabilities led the authors to attempt to image directly the patterns of compositional variation in the wallrock garnets, using a Series 4 Camscan with electron channelling capability at the University of Leeds, England. The high contrast and electron channelling backscattered electron (BSE) techniques proved highly successful. These arc outlined briefly below, but full descriptions are given by LLOYD (1987) in his review of backscattered electron techniques. The high contrast BSE method rasters an area to produce an image of variation in 7, the backscatter coefficient, and is capable of imaging changes of less than 0. I % in values of n of the order of 0.1. The value of n is a function of ( I ) the mean atomic number, Z (and therefore composition) and (2) the beam to lattice orientation, 8. Therefore, backscatter images of single crystals are qualitative maps of compositional zonation and lattice distortion. In the electron channelling technique the beam is rocked about a small area or point on the specimen surface to produce an electron channelling pattern (ECP) for that point. This is lattice orientation dependent. Bv comparina ECPs from different points it is possible to-detect differences in ohentation of the order of a few minutes of rotation. By combining this with the high contrast images it is possible to determine qualitatively how much n-variation is compositional and how much is orientational. This can be further resolved by electron microprobe analysis, as described above, determining exact compositional differences. Ion Microprobe Analyses were conducted on the Cameca ims4f in the NERC facility at Edinburgh University using an O- primary beam and collecting high energy positive secondary ions. In making the high energy discrimination, the voltage at the peak for *‘Al was initially displaced away from the high energy ion side until a 10% drop in Al counts was achieved, and then from this position the voltage was displaced by 100 to move it well into the high energy selection sector. The energy window was + I9 eV. A duoplasmatron supplied the O- primary ions, which had a total impact energy on the target (specimen) of 14.5 KeV. The primary beam current and spot size were 8-10 x 10m9A and 20-30 pm, respectively. Isotopes measured were MSi, [email protected]. r40Ce. 14’Nd. ‘49Sm. ‘S’E~. ‘?IJ . 16’Dv1and 16’Dy. All specimens were given a d.l-rmithick ‘gold coating to reduce charging. Trace element abundances were calculated relative to Si and calibration was made against NBS610 glass and natural garnet standards from Kakanui and Dutsen Dusnowo (MASONand ALLEN, 1973; IRVINGand FREY,1978). Similarities in ion yields for the NBS glass and garnets have been established (Harte et al., unpubl. data), and in calculating oxide molecular overlaps for REE data a NdO/ Nd ratio of 0.11 was used, as determined by measurements on synthetic pyrope glasses (J. A. Craven and E. Cairns, pers. commun.) .

RESULTS Optical and BSE Characteristics

The data presented in this paper are all from a single garnet (Fig. la), which has been studied exceptionally intensely, from the metasomatised peridotite wallrock (LBM 129) to one of the intrusive pyroxenite sheets described in the introduction Similar observations to those presented have been made on the garnets in other wallrocks and in the other me&somatic zones, described by HARTE et al. ( 1987) as showing similar metasomatic effects. The garnet described here was studied in particular detail in order to establish the small scale spatial form of the variations in n (backscatter coefficient) revealed in BSE images and their relation to compositional features shown by extensive electron microprobe point analyses.

Geochemistry of cracked garnet A high contrast BSE image of the walhock garnet is shown in Fig. 1b. The image shows the subhedral garnet approximately 4000 pm across, surrounded by a fringe of kelyphite, with the kelyphite showing up as the white speckled material. Within the garnet can be seen black lines, some of which also contain kelyphite and show a small scale speckled ap pearance. These black lines are fractures and can be directly correlated with those visible in the photomicrograph of the same garnet in Fig. la. These dark fractures obviously formed at a late stage, probably during kimberlitic eruption. Crosscut by these fractures in Fig. 1b is a network of thin, very slightly diffuse, “white” or “bright” (high backscatter coefficient) branched lines, which are surrounded by more ditTuse pale auras. Although there is considerable variation in their length and distribution, all the larger bright-white lines either originate by the garnet rim or branch from a line that does, and in this respect they resemble the late-stage fractures. The bright-white lines and dark (late-stage) fracture lines run parallel or subparallel to each other in some parts of the garnet, but in detail the late-stage fractures can be seen to cut and interrupt the continuity of the bright lines in several areas. Generally, despite some similarities in form and distribution, the two sets of lines appear to display no control on each other. Also the bright lines do not represent areas of significant structural heterogeneity and show no comparable optically visible features to the late stage fractures (Fig. 1a). Examination under high-powered optical microscopes reveals only a few small isolated bubbles in positions where bright lines are known to image and the material of the bright lines appears to be homogeneous garnet. In general, no evidence of inclusions or exsolutions corresponding to the bright lines has been detected in optical or electron microscopy or by electron microprobe analysis. The overall form and distribution of the bright-white lines, as described for Fig. 1b, is suggestive of a fracture pattern; but if this is the origin of the pattern, it is evident that the fractures, after having formed, have been healed and removed by regrowth of garnet. The high backscatter coefficient (v), bright-white lines of Fig. 1b are often complimented by a zone of moderate rl with a general decrease in tl towards grey-black areas where brightwhite lines are scarce or absent. The gradual fading in the intensity of backscattering away from the bright-white lines is particularly evident where there are many such lines in close association (e.g., lower right side of garnet in Fig. 1b) and broad pale zones that surround them. Clearly in these areas the variation in backscatter coefficient suggests a chemical composition change whereby there is a gradual increase in one or several components causing high v, which culminates in the bright-white lines themselves. Again, all material involved appears to be garnet and is shown to be such by electron probe point analyses in the following section. All of the zoned wallrock garnets examined by BSE imaging display similar patterns of bright-white lines and surrounding pale auras. The pattern and density of bright lines varies considerably from sample to sample, but all exhibit a common range of features. In more detail, these features range between braided, through branching, to regularly spaced parallel sets of bright lines. The following three examples showing this detail are all taken from the garnet shown in Fig. la, but similar features can be found, to differing degrees of devel-

2635

opment, in all the wallrock garnets imaged. Figure 2a shows an example of a braided pattern. The black and white speckled irregular bands are the late-stage kelyphitised fractures and can be seen quite clearly to cross-cut the earlier bright-white line pattern. The bright lines form a broad irregular band, varying between 50 and 100 pm across, in which multiple branching and rejoining, or braiding, of lines forms a “marbled” pattern. The slightly diffise nature of the bright lines is quite clear against the kelyphitised late stage fractures. In Fig. 2b, the branching bright lines are more linear in nature, with more simple branching. This frame also exhibits a particularly high density of smaIl bubbles/inclusions, showing up as the small dark dots, which can infrequently be found along the path of bright lines. The detail recorded by the SEM picks out fine lines, only 1 to 2 pm wide, which emanate from and surround the main bright lines. In many instances the pale auras (Fig 1b) surrounding the larger bright lines can be discerned as being made up of clouds of these often short, irregular, and discontinuous fine bright lines. In appearance, these clouds of fine bright lines show a strong resemblance in pattern to “process zones” encountered in brittle fracture experiments in rocks and ceramics (ATKINSON, 1987, pg. 12). In Fig. 2b many of the bright lines (former microfractures ) are quite straight and show roughly parallel trends. This form of lineation is exceptionally well shown in Fig. 2c. Here again are seen the irregular latestage kelyphitised fractures, overlying a series of left to right downward trending subparallel bright lines. These bright lines show some branching, and possibly a little braiding, but running between them, almost like rungs on a ladder, are a multitude of parallel lines spaced roughly 5 to 10 pm apart. These parallel fine lines show varying degrees of development in different specimens, but are relatively rare. They are also unusual in indicating crystallographic control by defining one, or sometimes two, consistent orientations, and ECPs of such lineations show that they lie parallel to lattice planes. Electron and Ion Microprobe Analyses The electron microprobe point analyses show that the variation in backscatter coefficient (Q), visible as the brightwhite lines and their auras (Fig. 1b), is a result of compositional heterogeneity of the garnet. Figure lc and d show plots of the variation in Ti and Mg, respectively, detected in the 40,000 point electron microprobe survey (see methods section) across the same garnet as shown in Fig. la and b. In both plots the microprobe data have been converted into a grey scale. Note that lighter greys represent higher Ti concentrations in Fig. lc, but that the scale is inverted in Fig. Id, with lighter gmys representing lower Mg. Given that decreasing Mg is largely correlated with increasing Fe, the lighter shades in both Fig. lc and d may be correlated with increasing Ti and Fe. Comparison of these figures with the backscatter image in Fig. 1b demonstrates the direct correlation between composition and backscatter coefficient. Note that although there is some general correspondence of the shadings in Fig. Ic and d, the Ti and Mg distributions do not bring out identical features. The high Ti distribution (pale zones) in Fig. lc corresponds closely to the positions of the bright-white

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M. Matthews, B. Harte, and D. Prior

FIG. 1. (a) Photomicrograph of a garnet section (specimen LBM 129); for scale see Fig. 1b. The dark linear features are late-stage cracks which contain alteration material (kelyphite) which also fringes the outer margins of the garnet. (b) High contrast backscatter electron SEM image of the garnet section in (a). Variations from white to grey to black are a function of backscatter coefficient (7). The late stage cracks of (a) again show as dark linear features. The garnet

Geochemistry of cracked garnet lines in the backscatter image (Fig. 1b). The Mg distribution in Fig. Id does not bring out such sharp changes, the variations in concentration being more diffuse in nature and suffering from a poorer signal to noise ratio. It is evident that the Mg distribution better matches the broader diffuse variations in brightness in the backscatter image (Fig. 1b) rather than the thin bright lines. In particular, note the two dark areas at the centre and left side of the garnet in Fig. 1d matching similar features in Fig. lb. Three quantitative electron microprobe point analyses, illustrating major-minor element variation, are given in Table 1. The analyses “A, ” “F,” and “X” come from adjacent to the line marked on Fig. 1b, with “A” and “F,” respectively, from the left and right of the late-stage fractures at “A” and “F,” whilst “X” comes from the very dark area by X. Thus, the three points come from areas of decreasing backscatter coefficient in alphabetical order. The increase in Ti and Fe and decrease in Mg with increasing 4 is evident. On the brightwhite lines themselves, whose position corresponds closely with the Ti distribution (Fig. lb,c), the TiOz reaches 0.41 wt%. This is an eight-fold increase relative to the dark areas (“X”) but is a small total amount and demonstrates the effectiveness of high contrast BSE imaging. The relative increase of Fe is less than that of Ti, but the absolute change of over 1 wt% is greater than that for Ti. The increase in Fe and decrease in Mg are not so focussed on the bright lines of the BSE image as the Ti increase, and they occur significantly in the pale-grey areas of the BSE image (see above comparision of Fig. lb,c,d). Stoichiometric analysis (i.e., divalent and trivalent site occupancy) of the extensive new electron microprobe data indicates that the Fe enrichment in the garnets is composed of increases in both Fe’+ and Fe3+ (Matthews, unpubl. data), with calculated Fe3+/ ZFe values rising to 6%. However, Fe3+ contents calculated in this way are known to be prone to being inaccurate (WOOD and VIRGO, 1989; LUTH et al., 1990)) so these values should be treated with caution. Ion microprobe analyses show that trace elements follow similar zonation patterns to the major-minor elements. Zr, Y MREEs, and HREEs enrichment mimic the Ti-Fe enrichment. Figure 3 shows the relative variations revealed in an ion microprobe traverse across the garnet in Fig. 1 along the line shown in Fig. lb. Quantitative trace element point analyses are given in Table 1 and are of the same small areas of reasonably constant t (“A,” “F,” and “X”) as the electron probe analyses presented in Table 1.

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The graph (Fig. 3) shows relative variations in the trace elements, Sr, Y, Zr, Ce, and Dy, in addition to the majorminor elements Si and Ti. Along the line of the traverse, the locations of late-stage kelyphitised fractures (A-F) and high v lines (K-R) are marked on both Figs. lb and 3. Note that high 1 line locations may be very thin individual bright lines (e.g., at K and 0) as well as broader zones (e.g., at N and Q) formed by several bright-white lines. The location of the high 7 lines (locations K-R on Figs. 1b and 3) are marked not only by high Ti and Fe, as was detected from the electron microprobe analyses, but also increases in Y, Zr, and Dy. In addition, the zone of lowest Ti in the central region of the garnet (around X on the traverse line) has the lowest Y, Zr, and Dy. In the late-stage kelyphitised fractures (locations AF) the concentrations of Y, Zr, and Dy are little affected; but, in contrast, the concentrations of Sr and Ce increase sharply, in conjunction with a drop in Si. Thus, the late-stage fractures have very different geochemical signatures to the bright backscatter lines and zones. The quantitative analyses in Table 1 show an increase of Zr, Y, Sm, Eu, Tb, and Dy in the sequence “x” to “F” to “A,” correlating with increasing 7. The LREE change little, with Nd remaining essentially constant, and a possible very slight fall in Ce with increasing 7. The compositional changes (major, minor, trace elements) described in the preceding text for wallrock garnet are in close harmony with the general changes in mineral and bulk compositions associated with the metasomatism of the wallrocks (HARTE et al., 1987). DISCUSSION Evidence of a Healed Pattern of Fractures Preserved in Garnet Chemistry

The spatial distribution of compositional variation uncovered in the wallrock garnets of the Matsoku suite displays a distinct range of morphological features. The overall morphology seen in high contrast BSE images is one of a network of “bright-white” branched lines which either originate by the garnet rim or branch from a line that does. The geometrical form of the bright-white lines (Fig. 1b) resembles that of a set of fractures and microfractures. Individual lines may be both branched and braided or may be surrounded by sets of small regularly spaced parallel lines. Commonly, individual lines may show various degrees of development of some or all of these features. All the bright lines are surrounded by

matrix shows large scale variation from light to dark areas. The palest parts of the garnet are thin “bright-white” lines, which are widespread, but particular concentrations of them are seen running from upper right to lower left through Q and towards upper left from N. Pale auras surround the bright-white lines especially where there are concentrations of lines. The line and letters indicate the position and labels of the ion microprobe traverse and electron and ion microprobe point analyses referred to in the text (Table 1 and in Fig. 3). (c) Plot of Ti variation across the garnet section in (a) and (b). The plot is composed of 40 X 500 data points taken from the 40 X 1000 matrix generated by the highly detailed electron microprobe survey described in the main text. Lighter greys correspond to higher concentrations of Ti. Comparison of the distribution of Ti with the backscatter image in (b) shows that the areas of high Ti correspond directly with the bright-white, high v, lines. The pale auras surrounding the bright lines are not so evident in the Ti plot. (d ) Plot of Mg variation across the garnet section in (a) and (b) . This plot is generated from a 40 X 500 matrix from the same 40 X 1000 data set as (c). Note that the grey-scale is reversed with respect to (c), with lighter greys corresponding to lower Mg concentrations. Comparison with (b) shows a correlation between areas of lower Mg and the pale auras in the backscatter image. The bright, high 9, lines visible in (b) are not visible in the Mg variation as they are in the Ti variation (c). The dark lines that are visible in (d) match more closely the late stage fractures seen in (a) and are thought to result from the kelyphitic infilling of these fractures rather than the metasomatic zoning.

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M. Matthews, B. Harte, and D. Prior

pale auras. In some of the more detailed images the pale auras can be resolved as being made up of short, mostly irregular, discontinuous bright lines, with a pattern very similar to that of the process zones of brittle fracture mechanics (ATKINSON, 1987). In all but a few instances, these networks of bright lines show no control on or by the position and shape of the late-stage kelyphitised fractures. Thus, the large- and small-scale geometrical form of the bright-white BSE lines is that of a pattern of fractures. However, the material forming this pattern is physically continuous garnet with a widespread lack of evidence of exsolution of inclusion features. It is suggested, therefore, that subsequent to their formation, a set of fractures became healed by regrowth of garnet such that they became optically and mechanically invisible, but remain apparent in BSE images because the garnet growing within them was different in detailed chemistry to the original garnet. Chemical Features of the Healed Fracture Pattern and Diffusion Effects

FIG. 2. High contrast backscatter electron images of areas in the garnet section shown in Fig. 1. In all images the white-rimmed black and white speckled areas are kelyphitised late-stage fractures, whilst isolated white spots with black haloes are dust spots and should be ignored. (a) Multiple branching and rejoining of the fine bright-white lines running slightly upwards from left to right produces a braided type of pattern for the healed fractures. (b) Branching and subbranching of bright-white lines representing healed fractures and cracks. Just visible, along more prominent white lines in the centre, are some small trapped bubbles. (c) A series of faint parallel healed fracture sets, trending N-S, may be seen to occur between larger,

Detailed electron and ion microprobe studies prove beyond a doubt that the backscatter variation detected in wallrock garnets (Fig. 1b) is directly correlatable with chemical variation in the garnet composition. The principal changes always involve a general increase in Ti and Fe and decrease in Mg (Fig. 1, Table 1) as 4 increases, with lesser variations (differing from rock to rock) of Ca, Al, and Cr. The bright-white (high q) lines of the BSE image are associated most particularly with a large relative increase in Ti02 (ca. 0.05 to 0.4 wt%), but the total compositions are consistently those of garnets and not of any separate exsolution or inclusion phase (in agreement with optical and electron microscope observations). Furthermore, the detailed changes in composition within the garnets are the same as those seen occurring generally as a result of metasomatism of the wallrocks adjacent to intrusive pyroxene sheets in the Matsoku suite ( HARTE et al., 1987). Thus, the compositions of the BSE bright-white lines in the altered wallrock garnets are very similar to those of garnets occurring within the intrusive sheets (see compendium of data in HARTE et al., 1987) and thereby suggest crystallisation from melt which has infiltrated the wallrocks from the sheets. Therefore, we believe that the bright-white line features of the BSE images of the wallrock garnets represent the patterns of fracture arrays, which became filled with melt, which was infiltrating and metasomatising the wallrocks. The fracture arrays within the garnets were then infilled by new garnet growing on the walls of the fractures, in equilibrium with the infiltrating melt. Thus, the fracture arrays became invisible to observation other than by the compositional variation of the garnet. The high r) garnet compositions are also marked by higher trace-element compositions for Zr, Y, MREEs, and HREEs. This is believed to result from the fact that the infiltrating melt had higher equivalent Zr, Y, MREE, and HREE com-

more dominant sublinear, subparallel healed fractures (trending downwards left to right). Ignore the late-stage cracking effects given by the higher contrast, black-white speckled areas.

Geochemistry of cracked garnet Electron and Ion mlcropmba point gunet showing CNckhg and hedIng.

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42.08 0.07 21.70

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3.52 6.66 0.32 21.08 5.17 0.01 100.62

3.52 7.62 0.33 20.44 5.04 0.03 100.83

3.52 7.95 0.36 20.36 5.07 0.05 101.24

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X closest to iniUal (pre-metasomatic) composition and ‘A’most modified. Detailed locations for points given in text and Figs. 1 and 3.

positions than the phases with which the original garnet grew. Thus, the new garnet, healing the cracks and growing in contact with the infiltrating melt, attained higher trace-element compositions than the original garnet by operation of normal partition coefficients between the new garnet and the infiltrating melt. It is not suggested that the new garnet grew from a melt of its own composition, but merely in equilibrium with the infiltrating basic-ultrabasic melt. HARTE et al. ( 199 1) show that this melt, originating in the intrusive sheets, yielded clinopyroxene and garnet with trace-element signatures similar to those of Cr-poor megacryst suites, which may be in equilibrium with OIB-like magmas (HARTE, 1983). The changes in garnet trace-element composition noted here, particularly increasing Zr, Y, and HREEs with increasing Ti and Fe, were also recorded for garnet core-rim zoning in high-temperature peridotites by GRIFFIN et al. ( 1989) and HKKMOTT (1989). These authors, following SMITH and BOYD ( 1989), have explained the enhanced trace-element contents of the garnet rims as being due to melt infiltration into the high-temperature peridotites. Geochemical similarities between these metasomatic effects and those found in the Matsoku wallrocks may be expected given the wider evidence for similarities in petrogenesis (e.g., HARTE, 1983; and other references in introduction ) . Whilst the bright white-lines of the BSE image (Fig. lb) show the greatest metasomatic enrichment, the dark areas show the least. Thus, they respectively give the best preserved compositions representing the garnet crystallising direct from the infiltrating melt and the garnet present before melt-metasomatism. The pale auras of Fig. 1b represent transition zones between these two garnet compositions. These zones are partly produced by clusters of exceptionally thin bright lines (Fig. 2), but it is also within these transition zones that diffusion effects may be seen within the garnet. In the transition zones

the composition is a function of the distance from the bright white-line. This is graphically seen in the Ti and Mg concentration plots in Fig. lc and d, respectively. The Mg concentration front defines a broader zone, extending as far as the outer edge of the pale auras visible in Fig. 1b, whereas Ti is more closely restricted to the bright lines seen in Fig. 1b. Magnesium and its substitutional twin Fe are generally accepted as having a higher diffusivity than Ti (e.g., see FREER, 198 1, and the references therein), although data are very limited. On this basis Mg and Fe therefore diffuse faster and move a greater distance over a given time than Ti. The patterns of compositional variation seen are consistent with the compositional fronts originating at the bright lines and diffusing outward from them. Thus, the bright lines represent the location of the sources of the metasomatic enrichment within the garnet and would have acted as such sources both when they were filled by melt and after they had been healed and filled with new enriched garnet. Greater diffusional mobility for Fe and Mg than Ti in garnet is also consistent with the data of SMITHand EHRENBERG( 1984) on zoned garnets. The fractures would have allowed instant access into the garnets of metasomatic fluid (melt) infiltrating the wallrock. This is substantiated by the effectively constant composition along the fractures, as shown by their even illumination (e.g., Fig. 2c and d) and by probe analyses. Transport along these paths must therefore have been virtually instantaneous in relation to the transport by diffusion into the garnet body. This would have produced the extreme diffusive anisotropy recorded by the compositional zoning. The lack of significant structural heterogeneity related to the lines poses few problems since, in an environment where active diffusion was taking place, rapid and efficient annealing could be expected to occur when active fracture growth had ceased. The incursion of melt along the fractures has dominated

M. Matthews, B. Harte, and D. Prior

2640

41

K

L

M

0

N

30 Points

Q

XP

40

R

50

60

FIG. 3. Ion microprobe traverse of 60 points at 50 pm spacing along the line indicated in Fig. 1b. Relative concentrations

are shown for Si, Ti, Sr, Y, Zr, Ce, and Dy. Note the correlation of Y, Zr, and Dy with Ti, and that high concentrations (points K-R) correspond with the high 7 lines in Fig. lb, whilst the lowest Ti, Y, Zr, and Dy zone at X corresponds with the dark (low 7) area in Fig. lb. In contrast Sr and Ce are strongly changed along the late-stage kelyphitised fractures at locations A to F.

the changes in garnet composition seen within individual crystals and reduced the volume diffusion effects to a subordinate role whose spatial distribution was controlled by the fracture lines. HARTE et al. ( 1987, pp. 207-213) debated the relative roles of infiltration and diffusion metasomatism within the mantle in causing the overall changes in wallrock composition adjacent to the intrusive sheets, but they assumed that volume (or lattice) diffusion was the only process operating within individual crystals. The present results show that this assumption was wrong and that an infiltration process involving cracking and healing affected individual wallrock garnets. Rarely some optically visible evidence of this is seen in the Matsoku metasomatised peridotites by the distribution of ilmenite inclusions ( HARTE and GURNEY, 1975, Fig. 83; HARTE et al., 1987, Fig. 6). To what extent cracking and healing has been important in changing the compositions of other minerals in these rocks is very difficult to assess, because they do not show compositional heterogeneity like the garnets (HARTE et al., 1987). Considering other data on compositionally zoned garnets in the literature, it is possible that cracking and healing, rather than diffusion alone, has played a role in other cases. Thus, SMITH and EHRENBERG( 1984) report some very irregular compositional profiles in some garnets from peridotites

showing Fe-Ti metasomatism. A preliminary BSE survey by ourselves of the garnets reported by Hops et al. ( 1989) also shows some irregularities in compositional variation rather than simple core-to-rim zoning alone. GRIFFIN et al. ( 1989) provided evidence of distinct corerim zonation in high-temperature peridotites and noted that the profiles of compositional change suggested growth around the margins of the garnets rather than diffusion. In the present Matsoku case, garnet growth in the cracks has also played a significant role in developing the compositional variation seen in individual crystals. Thus, in both studies the role and extent of volume diffusion is less than previously expected. This means that relatively short times must have been available for diffusion and that lower rather than higher experimental estimates of Fe-Mg diffusion coefficients are likely to be appropriate (see discussions in SMITH and EHRENBERG, 1984; HARTE et al., 1987; SMITH and BOYD, 1989). Formation of the Healed Fractures

The formation and subsequent healing of fractures in minerals within rocks is widely documented in the literature concerned with fluid inclusions (e.g., ROEDDER, 198 1) and microcracks (SIMMONS and RICHTER, 1976; SPRUNT and

Geochemistry of cracked garnet NUR, 1979). In these cases partial trails of inclusions (fluid or solid) commonly mark the site of a former fracture, and it is evident from their distribution that considerable crystallisation of the host mineral must have occurred subsequent to fracturing in order to fill the fracture and isolate the inclusions. Where the fracturing and regrowth take place in the presence of silicate melt, it is known that large volumes of host mineral may crystallise on the walls of the original fracture ( ROEDDER,1979, 198 1) . In the present case perhaps the most unusual feature is the relative poverty in fluid inclusions in the garnet, although they are seen occasionally (Fig. 2b). This poverty in inclusions probably results from the high temperature and slow cooling conditions applying in the mantle situation. As shown by NICHOLSand MULLINS (1965a,b), the lack of uniform surface curvature along the solid interfaces of a crack will alone result in processes of diffusive material transport in order to reduce interfacial energy. Even if the total volume of crack-generated “space” within a mineral were to remain constant, a conversion from planar crack to spherical shape would produce a reduction in overall surface energy, and such healing processes are considerably aided by the presence of fluid (SMITH and EVANS,1984). In the case under consideration the presence of melt, associated with the metasomatism, would have both supplied material for additional growth of garnet on the cracked interfaces and provided a fluid medium in which diffusive transport would be rapid by comparison with solid silicate. The circumstances giving rise to the initial fracturing may be connected with the incursion of melt into the rocks. The injection of melt into intrusive sheets or “dikes” has already been documented, and the infiltration of such melt along grain edges into the wallrocks of the sheets is to be expected from interfacial energy/surface tension relationships ( HARTE and HUNTER, 1986; MCKENZIE, 1989). The fracturing of the rocks and garnets may be a result of hydraulic fracturing consequent upon the increase in fluid pressure caused by melt infiltration ( HUBBERTand RUBEY, 1959).

formed peridotites and confirm earlier predictions (GURNEY and HARTE, 1980; HARTE, 1983 ) that similar metasomatism has played an important role in generating both rock groups. The data also show the effectiveness of using BSE imaging in delineating small chemical variations in minerals and detecting an otherwise hidden history of events. Neither by optical examination nor by all but the most detailed of microprobe analyses was it possible to even hint at the form and complexity of me&somatic compositional variation recorded by the wallrock garnets of the Matsoku suite. Even the highly detailed microprobe survey, which consisted of 40,000 data points and took over 130 probe hours and a good deal of computer time to complete, could produce only a broad outline of the pattern of chemical variation. The SEM imaging techniques outlined above, however, in a matter of a few minutes are able to produce maps of the heterogeneity over whole mineral grains. Where it is required, the resolution of the SEM map can be increased more, to almost the submicron scale, by imaging smaller areas and building up a photomontage. By using these qualitative images as microprobe maps, quantitative zonation patterns can be rapidly constructed by selective electron microprobe analyses. Records of similar features to those described above involving mineral composition variation are rare in the literature, although mineral cracking and healing is well known from studies of fluid inclusion trails. The authors strongly believe that many more features similar to those recorded will come to light when more use is made of high contrast BSE imaging capabilities. Acknowledgments-We

are grateful to Dr. Condliffe for his help and guidance with the imaging techniques used on the SEM at Leeds University. We would also like to thank Dr. Elphick, Dr. Hill, and Dr. Kearns for their help with the electron microprobe techniques, and Dr. Hinton and Dr. Craven for their assistance with the ion microprobe. This research has been funded by a Natural Environmental Research Council grant. Editorial handling:

1) fracturing of the garnets; 2) melt incursion along the fractures; 3) crystallisation of new garnet (enriched in Fe, Ti, Zr, Y, MREEs, and HREEs relative to the earlier garnet) in equilibrium with the melt along the walls of the fractures, thereby healing and sealing the cracks in the original garnet; 4) diffusion of certain elements, particularly Fe and Mg, from the locations of the cracks into the original, pre-fracture garnet.

F. A. Frey REFERENCES

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