Magmatic volatiles (for example, water) are abundant in arc melts and exert fundamental controls on magma evolution, eruption dynamics and the formation of economic ore deposits. To constrain the H₂O content of arc magmas, most studies have relied on measuring extrusive products and mineral-hosted melt inclusions. However, these methods have inherent limitations that obfuscate the full range of H₂O in arc magmas. Here, we report secondary-ion mass spectrometry measurements of volatile (H₂O, F, P, S, Cl) abundances in lower-crustal cumulate minerals from the Kohistan palaeo-arc (northwestern Pakistan) and determine H₂O abundances of melts from which the cumulates crystallized. Pyroxenes retained magmatic H₂O abundances and record damp (less than 1 wt% H₂O) to hydrous (up to 10 wt% H₂O) primitive melts. Subsequent crystal fractionation led to formation of super-hydrous melts with approximately 12–20 wt% H₂O, predicted petrologically yet virtually absent from the melt-inclusion record. Porphyry copper deposits are probably a natural eventuality of fluid exsolution from super-hydrous melts, corroborating a growing body of evidence.

电弧熔体中丰富的岩浆挥发物（例如水）对岩浆演化、喷发动力学和经济矿床的形成具有根本性的控制作用。为了限制弧形岩浆的 H _{2 O 含量，大多数研究依赖于测量喷出产物和矿物包裹的熔体包裹体。}然而，这些方法具有固有的局限性，会混淆弧形岩浆中 H ₂ O的全部范围。_{在这里，我们报告了科希斯坦古弧（巴基斯坦西北部）下地壳堆积矿物中挥发性（H 2} O、F、P、S、Cl）丰度的二次离子质谱测量，并确定了熔体的H ₂ O 丰度堆积物从中结晶。辉石保留岩浆H ₂O 丰度和记录潮湿（小于 1 wt% H ₂ O）到含水（高达 10 wt% H ₂ O）原始熔体。_{随后的晶体分馏导致形成具有约 12-20 wt% H 2} O的超含水熔体，这在岩石学上预测，但在熔体包裹体记录中几乎不存在。斑岩铜矿床可能是流体从超含水熔体中溶出的自然结果，这证实了越来越多的证据。