Tracing the evolution of a submarine arc-hosted hydrothermal system through phosphate mineralization; Brothers Volcano, Kermadec Arc

Tobias W Hoefig; Chao Zhang; David W Peate; Susan E Humphris; Lawrence K. S Horkley

IODP Expedition 376 drilled the submarine Brothers Volcano to gain insights into the subsurface structure of an arc-hosted hydrothermal system. Its life cycle can be tracked by accessory phosphate minerals found in cores from Hole U1530A on the NW caldera wall. At this location, subseafloor alteration mineral assemblages indicate an older, highly acidic, magmatically influenced hydrothermal system (> or =225 m below seafloor [mbsf]) that is partially overprinted by seawater-dominated, moderately acidic, higher temperature hydrothermal activity forming a stockwork zone at the present-day seafloor (de Ronde et al., 2019, Geology 47, 762-766).IODP Expedition 376 drilled the submarine Brothers volcano to gain insights into the subsurface structure of an arc-hosted hydrothermal system. Its life cycle can be tracked by accessory phosphate minerals found in cores from Hole U1530A on the NW caldera wall. At this location, subseafloor alteration mineral assemblages indicate an older, highly acidic, magmatically influenced hydrothermal system (> or =225 m below seafloor [mbsf]) that is partially overprinted by seawater-dominated, moderately acidic, higher temperature hydrothermal activity forming a stockwork zone at the present-day seafloor (de Ronde et al., 2019, Geology 47, 762-766). Primary (magmatic) phosphates from fresh caldera floor volcaniclastics (Hole U1529B) and Lower Cone lavas (Hole U1531E) are euhedral fluor-chlorapatites (average F/Cl = 0.78 + or - 0.07 [1sigma ]) up to approximately 200 mu m in length, found in the matrix or as inclusions in clinopyroxene and plagioclase phenocrysts. In contrast, secondary (metasomatic) phosphates derived from hydrothermal fluid-rock interaction are mostly represented by subhedral to anhedral fluorapatites (F/Cl = 35.8 + or - 21.5) in massive mineralization aggregates (up to 1 mm in size) interstitial to and intergrown with alteration minerals, ubiquitous quartz and varying proportions of chlorite+illite (seawater-dominated) and pyrophyllite+rutile (magmatically influenced). Rare minute (<2 mu m) inclusions of REE-phosphates monazite (LREE) and xenotime (HREE) suggest occasional slight alteration of fluorapatite patches ( approximately 335 mbsf). In contrast, the Hole U1530A stockwork zone (< or =20 mbsf) just contains fluor-chlorapatites and fluorine-rich magnesium-hydroxyphosphates. Primary (magmatic) phosphates from fresh caldera floor volcaniclastics (Hole U1529B) and Lower Cone lavas (Hole U1531E) are euhedral fluor-chlorapatites (average F/Cl = 0.78 + or - 0.07 [1sigma ]) up to approximately 200 mu m in length, found in the matrix or as inclusions in clinopyroxene and plagioclase phenocrysts. In contrast, secondary (metasomatic) phosphates derived from hydrothermal fluid-rock interaction are mostly represented by subhedral to anhedral fluorapatites (F/Cl = 35.8 + or - 21.5) in massive mineralization aggregates (up to 1 mm in size) interstitial to and intergrown with alteration minerals, ubiquitous quartz and varying proportions of chlorite+illite (seawater-dominated) and pyrophyllite+rutile (magmatically influenced). Rare minute (<2 mu m) inclusions of REE-phosphates monazite (LREE) and xenotime (HREE) suggest occasional slight alteration of fluorapatite patches ( approximately 335 mbsf). In contrast, the Hole U1530A stockwork zone (< or =20 mbsf) just contains fluor-chlorapatites and fluorine-rich magnesium-hydroxyphosphates. The hydrothermal fluorapatite, however, does not exclusively characterize the first stage of fluid-induced phosphate mineralization, as suggested by the sporadic occurrence of disseminated florencite [REEAl (sub 3) (PO (sub 4) ) (sub 2) (OH) (sub 6) ] as skeletal, relic crystals and altered aggregates. Florencite is an aluminum phosphate-sulfate (APS) mineral in the alunite supergroup, and indicates low-pH and low-temperature formation conditions of an initial magmatic fluid-dominated stage, similar to Brothers' resurgent Upper Cone. At this site, Hole U1528D has revealed APS minerals replacing plagioclase in a presently active magmatic-hydrothermal system.

Tracing the evolution of a submarine arc-hosted hydrothermal system through phosphate mineralization; Brothers Volcano, Kermadec Arc

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