The M52B Body Armor[1] is the standard issue body armor that the Marines use for protection.
Worn by UNSC Marines, the M52B Body Armor consists of armor plating over combat fatigues, and includes a number of ammunition pockets, holsters, and other features. There seem to be a number of varieties. The equipment used in the Battle of Alpha Halo was a gunmetal grey[2], while that used during the First Battle of Earth was olive green[3]. Even later, during the Second Battle of Earth, the UNSC Marine Corps uses a different type of armor altogether, though it is still labelled as M25B on containers[4]. It may be an upgrade or new variant to the prior armor, introduced due to the needs of Earth's defenders, or even a continuity retcon by Bungie. Unlike the radically more advanced MJOLNIR Armor worn by the Spartan-II soldiers, the M52B armour does not include a dynamic recharging energy shield, providing far inferior protection against directed-energy and projectile weapons.
M52B armor crates of 12 units weigh 173 lbs altogether. Assuming the weight does not include the crate, that would indicate that the M52B armor could weigh about 14.5 lbs.