We introduce a novel method to measure the masses of galaxy clusters at high
redshift selected from optical and IR Spitzer data via the red-sequence
technique. Lyman-break galaxies are used as a well understood, high-redshift
background sample allowing mass measurements of lenses at unprecedented high
redshifts using weak lensing magnification. By stacking a significant number of
clusters at different redshifts with average masses of ~1-3x10^14M_sun, as
estimated from their richness, we can calibrate the normalisation of the
mass-richness relation. With the current data set (area: 6 deg^2) we detect a
magnification signal at the >3-sigma level. There is good agreement between the
masses estimated from the richness of the clusters and the average masses
estimated from magnification, albeit with large uncertainties. We perform tests
that suggest the absence of strong systematic effects and support the
robustness of the measurement. This method - when applied to larger data sets
in the future - will yield an accurate calibration of the mass-observable
relations at z>~1 which will represent an invaluable input for cosmological
studies using the galaxy cluster mass function and astrophysical studies of
cluster formation. Furthermore this method will probably be the least expensive
way to measure masses of large numbers of z>1 clusters detected in future
IR-imaging surveys.Comment: 5 pages, 1 figure, 1 table, accepted by ApJL, minor revision
