Degassing of water from magmatic systems is key to transporting metals from magmas to form ore deposits, but elements like chlorine, through the formation of anion complexes, can be important in solubilizing and mobilizing these metals into water-rich fluids. Reconstructing the Cl systematics of evolving magmas is thus an important step towards understanding the origins of ore deposits, but the magmatic record is not well preserved because Cl can be lost during degassing. Here, we reconstruct the pre-degassing history of Cl in subduction zone (arc) magmas through amphiboles, which incorporate Cl directly into their crystal structures, preserving pre-eruptive magmatic signatures. Amphibole-reconstructed Cl contents indicate that magmatic differentiation can lead to a 4-fold increase in concentration due to Cl's incompatible behavior. The amphibole-reconstructed Cl contents of arc magmas are also significantly higher than values reported from melt inclusions, suggesting that many melt inclusions may have been trapped after magmas had already lost some Cl. We show that such Cl loss is likely associated with preferential partitioning of Cl into hydrous fluids degassed from the magma during crustal storage or ascent. The extent of Cl depletion can thus be used to estimate how much water was lost during early degassing. If Cl is important to certain ore deposits, magmatic water content may play an indirect role. Magmas too rich in water will lose water and hence Cl at greater depths, rendering such magmas less able to transport metals to the upper crust. By contrast, drier magmas may not produce enough Cl-rich fluids to mobilize metals. Thus, magmas with intermediate water contents may produce enough Cl-rich fluids at the right depths for certain types of ore deposits.