Galaxy mergers are expected to have a significant role in the mass assembly of galaxies in the early Universe, but there are very few observational constraints on the merger history of galaxies at $z>2$. We present the first study of galaxy major mergers (mass ratios $>$ 1:4) in mass-selected samples out to $zapprox6$. Using all five fields of the HST/CANDELS survey and a probabilistic pair count methodology that incorporates the full photometric redshift posteriors and corrections for stellar mass completeness, we measure galaxy pair-counts for projected separations between 5 and 30 kpc in stellar mass selected samples at $9.7 < log_{10}(rm{M}_{*}/rm{M}_{odot}) < 10.3$ and $log_{10}(rm{M}_{*}/rm{M}_{odot}) > 10.3$. We find that the major merger pair fraction rises with redshift to $zapprox6$ proportional to $(1+z)^{m}$, with $m = 0.8pm0.2$ ($m = 1.8pm0.2$) for $log_{10}(rm{M}_{*} / rm{M}_{odot}) > 10.3$ ($9.7 < log_{10}(rm{M}_{*}/rm{M}_{odot}) < 10.3$). Investigating the pair fraction as a function of mass ratio between 1:20 and 1:1, we find no evidence for a strong evolution in the relative numbers of minor to major mergers out to $z<3$. Using evolving merger timescales we find that the merger rate per galaxy ($mathcal{R}$) rises rapidly from $0.07pm 0.01$ Gyr$^{-1}$ at $z < 1$ to $7.6pm 2.7$ Gyr$^{-1}$ at $z = 6$ for galaxies at $log_{10}(rm{M}_{*}/rm{M}_{odot}) > 10.3$. The corresponding co-moving major merger rate density remains roughly constant during this time, with rates of $Gamma approx 10^{-4}$ Gyr$^{-1}$ Mpc$^{-3}$. Based on the observed merger rates per galaxy, we infer specific mass accretion rates from major mergers that are comparable to the specific star-formation rates for the same mass galaxies at $z>3$ - observational evidence that mergers are as important a mechanism for building up mass at high redshift as in-situ star-formation.
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