In situ, laser microprobe analyses of millimeter-wide quartz veins has revealed the largest δ18O gradients (9‰/mm) yet documented in terrestrial samples. The systematic isotopic variations indicate a crack-seal growth mechanism that variably interleaved 18O-enriched quartzite porphyroclasts with newly precipitated, 18O-depleted vein quartz. The isotopically light quartz requires open-system infiltration of meteoric fluids into the quartzite during its incorporation into the Ruby Gap duplex (central Australia). Documentation of the fluid source in these small veins would not be possible without submillimetre isotopic sampling capabilities. Application of this laser-based technology to other geologic problems should result in similar documentation of large isotopic gradients and provide new insight into fluid-rock interactions.