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Ion microprobe analysis with a sub-micrometer diameter spot reveals a sharp, 2 [micro]m gradient in oxygen isotope ratio proving that oxygen diffusion in zircon is slow even under prolonged high-grade metamorphism. The data are consistent with an oxygen diffusion coefficient of [10.sup.-23.5[+ or -]1] [cm.sup.2]/s. Furthermore, this gradient is found in a zircon that contains clear textural evidence of recrystallization in nearby regions. This finding shows that through careful textural and chemical analysis, primary information can be extracted from a zircon that has also undergone partial recrystallization. The oxygen isotope ratios found in zircon have been used to infer magmatic and pre-magmatic histories, including the presence of liquid water on the surface of earliest Earth. Recently, these interpretations have been questioned with the assertion that zircon may not retain its primary oxygen isotope signature through metamorphism. The slow diffusion confirmed by these results supports interpretations that assume preservation of magmatic compositions. Keywords: Ion microprobe, SIMS, zircon, diffusion, oxygen isotopes, stable isotopes, granulites facies, migmatites

Detailed electron- and ion-microprobe analysis ofdiopsides extracted from a block of marble collected in the Mt. Marcy anorthosite massif (Adirondack Highlands, New York) shows that single crystals have preserved Al-Si zoning features from early contact metamorphism later modified by regional metamorphism. Diopsides show Al and Si complementary zoning from core to rim; average A1 concentration in rims is constant (0.11 pfu), but values in the cores vary with crystals from 0.06 to 0.14 pfu. A [[delta].sup.18]O profile measured with the ion microprobe shows no zoning. Modeling of the Al-diffusion profiles affected by regional metamorphism of the Grenville orogeny shows that original Al zoning was sharp and corresponds to a rapid change of crystal-growth conditions during early contact metamorphism: (1) diopside cores crystallized with variable Al contents due to restricted fluid circulation or differing sedimentary composition, (2) then pervasive fluid infiltration crystallized the homogeneous rim of diopside, and (3) core to rim Al zoning was later smoothed by granulite-facies metamorphism. Homogeneity of [[delta].sup.18]O in core and mantle (near rim) of diopsides suggests that the crystals were isotopically homogeneous prior to and during regional metamorphism. Values of Al-Si diffusion coefficient deduced from modeling are in agreement with low-temperature extrapolation of experimental data. Keywords: Diopside, zoning, granulite, diffusion, ion probe, oxygen isotopes, Adirondack

Analysis of [[delta].sup.18]O in igneous zircons of known age traces the evolution of intracrustal recycling and crust-mantle interaction through time. This record is especially sensitive because oxygen isotope ratios of igneous rocks are strongly affected by incorporation of supracrustal materials into melts, which commonly have [[delta].sup.18]O values higher than in primitive mantle magmas. This study summarizes data for [[delta].sup.18]O in zircons that have been analyzed from 1,200 dated rocks ranging over 96% of the age of Earth. Uniformly primitive to mildly evolved magmatic [[delta].sup.18]O values are found from the first half of Earth history, but much more varied values are seen for younger magmas. The similarity of values throughout the Archean, and comparison to the composition of the 'modern' mantle indicate that [[delta].sup.18]O of primitive mantle melts have remained constant ([+ or -] 0.2[per thousand]) for the past 4.4 billion years. The range and variability of [[delta].sup.18]O in all Archean zircon samples is subdued ([[delta].sup.18]O(Zrc)=5-7.5[per thousand]) ranging from values in high temperature equilibrium with the mantle (5.3 [+ or -] 0.3[per thousand]) to slightly higher, more evolved compositions (6.5-7.5[per thousand]) including samples from: the Jack Hills (4.4-3.3 Ga), the Beartooth Mountains (4.0-2.9 Ga), Barberton (3.5-2.7 Ga), the Superior and Slave Provinces (3.0 to 2.7 Ga), and the Lewisian (2.7 Ga). No zircons from the Archean have been analyzed with magmatic [[delta].sup.18]O above 7.5[per thousand]. The mildly evolved, higher Archean values (6.5-7.5[per thousand]) are interpreted to result from exchange of protoliths with surface waters at low temperature followed by melting or contamination to create mildly elevated magmas that host the zircons. During the Proterozoic, the range of [[delta].sup.18]O(Zrc) and the highest values gradually increased in a secular change that documents maturation of the crust. After ~1.5 Ga, high [[delta].sup.18]O zircons (8 to > 10[per thousand]) became common in many Proterozoic and Phanerozoic terranes reflecting [[delta].sup.18]O(whole rock) values from 9 to over 12[per thousand]. The appearance of high [[delta].sup.18]O magmas on Earth reflects non-uniformitarian changes in the composition of sediments, and rate and style of recycling of surface-derived material into magmas within the crust.

Oxygen isotope compositions of mineral separates were determined from two metaluminous granitoids (Emas and Sao Rafael plutons) from northeastern Brazil. The I-type Emas pluton has high [[delta].sup.18]O (WR) values (11.5-11.8[per thousand]), whereas the Sao Rafael pluton has low [[delta].sup.18]O (WR) values (7.5-8.1[per thousand]), but Sr and Nd are characteristics of S-type granitoids. Measured mineral-mineral fractionations suggest continuous sub-solidus inter-mineral isotope exchange among all minerals except zircon. There is a large and consistent quartz-epidote fractionation that gives apparent temperatures that are much lower than anticipated closure temperatures for epidote. Oxygen isotope fractionation between natural zircon and magmatic epidote is opposite to that predicted from theoretical determinations, as [[delta].sup.18]O (epidote)

We present X-ray absorption near-edge structure (XANES) spectroscopy of 11 silicate and aluminosilicate minerals and two glasses at the SiK and Si[L.sub.2.3,] and OK edges. The similar nearest-neighbor environments lead to similar spectral lineshapes at each edge, but the fine-structure differences allow individual and groups of structurally similar minerals to be distinguished. By combining spectra and their first energy derivative from three absorption edges, we show that every mineral studied is distinguishable with XANES. This background work, combined with X-ray PhotoElectron Emission spectroMicroscopy (X-PEEM), allows non-destructive in situ, sub-micrometer (to 35 nm) X-ray analysis of materials, including silicate inclusions, which has not been possible previously. Images and spectra from a 7 [micro]m x 3.5 [micro]m quartz inclusion in zircon are presented as a test of this novel technique in geology.

Infiltration of a metabasite sill from Islay, Scotland by an [H.sub.2]O-C[O.sub.2] fluid caused (1) modification of [delta][sup.18]O and (2) carbonation at the sill margins. Maps of [delta][sup.18]O and reaction progress were constructed from a 20 x 47.7 metre sample grid across the sill. The grid consisted of 300 samples, spaced at m, dm and cm intervals, many of which were analysed for both [delta][sup.18]O and reaction progress. The [delta][sup.18]O was determined by laser fluorination of whole rock silicate powders and reaction progress was determined by rapid field-based measurement of % calcite ('fizz-o-meter', Skelton et al. 1995). Reaction and isotope fronts outlined tube-like features that emanate from the sill margin and discrete nodes that, although detached from the sill margin in two dimensions, are thought to represent sections through similar tubes in three dimensions. We envisage that these protrusions are the fossil record of metamorphic 'fluid pathways' whereby fluid permeated the sill. Isotope and reaction fronts are found to correlate spatially as predicted by a modified form of the chromatographic equation which describes this envisaged geometry, that is where isotopic and reactive transport in the fluid phase are facilitated by advection along specific fluid pathways and transverse diffusion in the surrounding rock. These fluid pathways consist of bundles of anastomosing grain boundary channels or micro-cracks, which are thought to propagate through transient cyclic infiltration, reaction, porosity enhancement and fracturing. This mechanism is self-perpetuating and accentuates random perturbations at the sill margin to form the observed tubes. We argue that this is the earliest stage of the infiltration process which has affected metabasites of the SW Scottish Highlands and that subsequent shear deformation of the reacted rims of these pathways, has caused their re-orientation and juxtaposition to form the reacted sill margins described by Skelton et al. (1995).

A study of high-grade metamorphic rocks by a combined ion microprobe and bulk mineral analysis with diffusion modeling revealed the significant influence exerted by interdiffusion in stabilizing the nature of arrangement of stable isotopes in gradually cooling rocks. Assessment of multiple fractionations in rocks is essential in predicting the nature of retrogression in the rocks.

The Scourian complex of northwest Scotland underwent granulite-facies metamorphism at 2.7 Ga. Granulite-facies orthogneisses have whole-rock delta-18O values similar to those of igneous rocks of comparable compositions (ultrabasic gneisses 5.5-5.9 0/00, basic gneisses 6.4-6.8 0/00, tonalitic gneisses 9.5-9.7 0/00), and steep 18O gradients of up to 3 0/00 in 10 cm are recorded at two lithological contacts. These data suggest that large volumes of pervasive metamorphic fluids have not infiltrated the rocks, but do not preclude localized fluid-rock interaction. Scourian paragneisses have lower delta-18O values (8.5-9.9 0/00) than typical metasediments, and may have undergone fluid-hosted isotopic exchange with the surrounding orthogneisses. Felsic sheets within tonalitic and basic gneisses have similar delta-18O values to their hosts: these data, combined with petrological and geochemical information, suggest that the sheets were formed by local anatexis during granulite-facies metamorphism. Fluid infiltration during the 2.5 Ga amphibolite-facies Inverian retrogression caused homogenization of delta-18O values of the Scourian gneisses. Unaltered tholeiitic Scourie dykes have low delta-18O values of 2.0 0/00. It is unlikely that heated, surface-derived fluids infiltrated the Scourian complex, and these dykes were probably emplaced as low-delta-18O magmas.