Arenado's solo shot came off Seattle right-hander Taijuan Walker in the fourth inning. Walker pitched a perfect third inning before giving up three runs on four hits and three walks in the fourth.
The 21-year-old Arenado went 2 for 3, lifting his spring batting average to .411 with four home runs and eight RBIs. Arenado has never appeared in the big leagues and was expected to begin the season at Triple-A, while fourth-year veteran Chris Nelson starts at third base.
Seattle leads all teams with 24 home runs.
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