H 2 O and CO 2 in magmas from the Mariana arc and back arc systems

We examined the H 2 O and CO 2 contents of glasses from lavas and xenoliths from the Mariana arc system, an intraoceanic convergent margin in the western Pacific, which contains an active volcanic arc, an actively spreading back arc basin, and active behind‐the‐arc cross‐chain volcanoes. Samples include (1) glass rims from Mariana arc, Mariana trough, and cross‐chain submarine lavas; (2) glass inclusions in arc and trough phenocrysts; and (3) glass inclusions from a gabbro + anorthosite xenolith from Agrigan (Mariana arc). Glass rims of submarine arc lavas contain 0.3–1.9 wt % H 2 O, and CO 2 is below detection limits. Where they could be compared, glass inclusions in arc phenocrysts contain more H 2 O than their host glasses; most arc glasses and phenocryst inclusions contain no detectable CO 2 , with the exception of those from a North Hiyoshi shoshonite, which contains 400–600 ppm. The glass inclusions from the Agrigan xenolith contain 4–6% H 2 O, and CO 2 is below the detection limit. Glasses from the cross‐chain lavas are similar to those from the arc: H 2 O contents are 1.4–1.7 wt %, and CO 2 is below detection limits. Volatile contents in Mariana trough lava glass rims are variable: 0.2–2.8 wt % H 2 O and 0–300 ppm CO 2 . Glass inclusions from trough phenocrysts have water contents similar to the host glass, but they can contain up to 875 ppm CO 2 . Volatile contents of melt inclusions from trough and arc lavas and from the xenolith imply minimum depths of crystallization of ∼1–8 km. H 2 O and CO 2 contents of Mariana trough glasses are negatively correlated, indicating saturation of the erupting magma with a CO 2 ‐H 2 O vapor at the pressure of eruption (∼400 bars for these samples), with the vapor ranging from nearly pure CO 2 at the CO 2 ‐rich end of the glass array to nearly pure H 2 O at the H 2 O‐rich end. Degassing of these magmas on ascent and eruption leads to significant loss of CO 2 (thereby masking preeruptive CO 2 contents) but minimal disturbance of preeruptive H 2 O contents. For submarine Mariana arc magmas, depths were low enough that degassing on ascent and eruption led to loss of both H 2 O and CO 2 ; as a result, H 2 O contents are positively correlated with water depth for these samples. The H 2 O contents of primitive Mariana trough magmas richest in the slab‐derived component (i.e., the most “arc‐like” magmas) are ∼2 wt %. Although evolved glasses with up to 4–6 wt % H 2 O are present among Mariana arc samples, we interpret the glass inclusion data as indicating that primitive Mariana arc liquids contain 1–3 wt % H 2 O. The preeruptive H 2 O contents of primitive cross‐chain seamount liquids are >1–2 wt %.