• journal_title：Geology
• Contributor：F. M. Stuart ; G. Turner ; R. C. Duckworth ; A. E. Fallick
• Publisher：Geological Society of America
• Date：1994-
• Format：text/html
• Language：en
• Identifier：10.1130/0091-7613(1994)022<0823:HIATOT>2.3.CO;2
• journal_abbrev：Geology
• issn：0091-7613
• volume：22
• issue：9
• firstpage：823

Helium isotopes have been measured in fluids trapped in minerals from three mid-ocean ridge hydrothermal systems at bare-rock sites and at the sediment-covered site at Middle Valley, northern Juan de Fuca Ridge. Fluid-inclusion ³He/⁴He ratios from the bare-rock sites are indistinguishable from those of the vent fluids and are close to the local basalt values. Values for δ³⁴S of the host sulfide are consistent with variable contributions of seawater sulfate S to the basalt-derived S in the hydrothermal fluids. This does not affect inclusion-hosted ³He/⁴He because of the high He concentration of the hydrothermal fluids relative to seawater. Fluid-inclusion ³He/⁴He ratios from Middle Valley are a mixture of basalt-derived He and radiogenic He acquired from pore fluids in the overlying Pleistocene turbidite sediments after seawater-basalt interaction. The radiogenic He accompanies isotopicalty heavy S, demonstrating that He isotopes may be used to distinguish pore fluids from seawater in hydrothermal systems.