Phase separation and fluid mixing in subseafloor back arc hydrothermal systems: A microthermometric and oxygen isotope study of fluid inclusions in the barite-sulfide chimneys of the Lau Basin

<p>The Mangatolu Triple Junction (MTJ) is an intraoceanic back-arc spreading center that is host to at least 3 distinct hydrothermal systems. It is located in the NE Lau Basin, which opened due to rollback of the Pacific plate along the Tonga-Kermadec trench. At the MTJ, three spreading centers meet in a ridge-ridge-ridge (RRR)-type triple junction separating the Tonga plate in the east, the Niuafo&#8217;ou microplate in the southwest, and an unnamed microplate in the north. The MTJ is directly linked to the formation and evolution of the Northeast Lau microplate mosaic, as plate fragmentation inevitably results in the formation of triple junctions, but it remains unclear whether the spreading centers are the drivers of plate fragmentation or a consequence of stress relocation related to microplate rotation. Detailed investigation of the geology and structural setting of the MTJ therefore provides valuable insight into the development in the northeast Lau Basin. Here we present the first comprehensive 1:200,000 geological map of the MTJ, based on a compilation of marine geophysical data (hydroacoustics, magnetics, and gravity) derived from 7 research cruises that have investigated the region between 2004 and 2018. Analysis of the mapped geological formations at the MTJ shows the importance of relict arc crust originating from the Tofua Arc in the architecture of the triple junction, which includes three stages of back-arc crust development and extensive off-axis volcanism. The spreading centers along each arm of the MTJ exploit pre-existing crustal weaknesses, interpreted to have formed during initial Lau Basin opening. A reconstruction of the basin opening, based on the mapped features and published spreading rates, revealed that initiation of the MTJ commenced approximately 180,000 years ago, consistent with the very recent and ongoing dynamic evolution of the NE Lau Basin and emerging microplate mosaic. Intersecting fabrics indicate sequential evolution of the 3 arms of the triple junction, with extension along the northeast arm dominant in the early history and more recent extension along the southern and western arms. The results of this study contribute to our growing understanding of the tectonic framework of the northeast Lau Basin and the role of triple junctions in microplate formation.</p>

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Fluid inclusion evidence for subsurface phase separation and variable fluid mixing regimes beneath the deep-sea PACMANUS hydrothermal field, Manus Basin back arc rift, Papua New Guinea

Journal of Geophysical Research Atmospheres ◽

10.1029/2003jb002579 ◽

2004 ◽

Vol 109 (B3) ◽

Cited By ~ 23

Author(s):

David A. Vanko

Keyword(s):

Phase Separation ◽

Fluid Inclusion ◽

Papua New Guinea ◽

Deep Sea ◽

New Guinea ◽

Fluid Mixing ◽

Hydrothermal Field ◽

Pacmanus Hydrothermal Field ◽

Manus Basin ◽

Back Arc

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Evidence for Sources of Salinity and Dissolved Gases, Fluid Mixing and Phase Separation, from Laser Microprobe Analysis of Cl, K, Br, I, Ar, Kr, and Xe in Fluid Inclusions

Applications of Microanalytical Techniques to Understanding Mineralizing Processes ◽

10.5382/rev.07.12 ◽

1997 ◽

pp. 217-231

Keyword(s):

Phase Separation ◽

Fluid Inclusions ◽

Microprobe Analysis ◽

Fluid Mixing ◽

Dissolved Gases ◽

Laser Microprobe

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Subcritical Phase Separation and Occurrence of Deep-Seated Brines at the NW Caldera Vent Field, Brothers Volcano: Evidence from Fluid Inclusions in Hydrothermal Precipitates

Geofluids ◽

10.1155/2020/8868259 ◽

2020 ◽

Vol 2020 ◽

pp. 1-22

Author(s):

Alexander Diehl ◽

Cornel E. J. de Ronde ◽

Wolfgang Bach

Keyword(s):

Phase Separation ◽

Fluid Inclusion ◽

Fluid Inclusions ◽

Hydrothermal Vents ◽

Massive Sulfide ◽

Growth Conditions ◽

Hydrothermal Systems ◽

Small Subset ◽

Large Area ◽

Black Smoker

The northwestern caldera wall of Brothers volcano in the southern Kermadec arc features several clusters of hydrothermal venting in a large area that extends from near the caldera floor (~1700 mbsl) almost up to the crater rim (~1300 mbsl). Abundant black smoker-type hydrothermal chimneys and exposed stockwork mineralization in this area provide an excellent archive of hydrothermal processes that form seafloor massive sulfide deposits. Using sulfate precipitates from chimneys and stockwork recently recovered by remotely operated vehicles, we conducted fluid inclusion microthermometry and Sr isotope studies to determine the role of phase separation and mixing between vent fluid and seawater. The variability in the vast majority of fluid inclusion salinities (i.e., 0.1–5.25 wt.% NaCl eq.) and entrapment temperatures of up to 346°C are indicative of phase-separated hydrothermal fluids. Large salinity variations in samples with entrapment temperatures mostly below the boiling temperature for the sample’s depth show that the majority of fluids ascending below the NW Caldera are phase separating in the subsurface and cooling, prior to discharge. In several samples, entrapment temperatures of over 343°C suggest that phase-separating fluids have at least sporadically exited the seafloor at the NW Caldera site. Isobaric-isenthalpic mixing trends between coexisting phase-separated vapors and brines with seawater are consistent with phase-separated fluids at near-seafloor pressures of ~170 bar and suggest that the vast majority of the ascending fluids continue to phase separate to within tens to hundreds of meters below seafloor prior to mixing with seawater. A small subset of the most saline fluid inclusions (up to 18.6 wt.% NaCl eq.) is unlikely formed by near-seafloor phase separation and is considered to be produced either by supercritical phase separation or by the contribution of a magmatic brine from near the magmatic-hydrothermal interface. 87Sr/86Sr values of sulfate samples range from 0.7049 (i.e., near hydrothermal end-member) to 0.7090 (i.e., near seawater) and show that the crystals grew from vapor- and brine-derived fluids in a hydrothermally dominated mixing regime. Our work provides new insights into mineral growth conditions, mixing regimes, and in particular, the extent and character of subseafloor phase separation during the formation of hydrothermal vents and their underlying stockwork in seawater-dominated, arc-related hydrothermal systems.

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