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Guo, W, He H, Hilton DR, Zheng Y, Su F, Liu Y, Zhu R.  2017.  Recycled noble gases preserved in podiform chromitites from Luobusa, Tibet. Chemical Geology. 469:97-109.   10.1016/j.chemgeo.2017.03.026   Abstract

We report noble gas (He, Ne, Ar) signatures of chromite and olivine separates from the Luobusa chromitites in Tibet to better understand the volatile compositions trapped in the minerals, and further to trace the origin of melts responsible for formation of the chromite deposits. The studied samples can be divided into two groups based on petrography and distinct noble gas signatures. Group I samples are free of carbonates and have 3He/4He ratios from 0.81 to 2.36 Ra (where Ra is the 3He/4He ratio of air=1.4×10−6) and air-like Ne and Ar isotopic compositions irrespective of chromitite structure types. Most 3He/4He ratios of Group I samples are higher than air, suggesting apparent presence of mantle volatiles. Given the 4He/20Ne and 3He/36Ar several orders of magnitude higher than air, negligible contributions are from the atmospheric helium. The observed He isotope compositions thus can be regarded as a two-component mixture of mantle-derived and radiogenic He. A broadly positive correlation between 3He and 36Ar in nodular chromitite samples indicates a source mixing between mantle and recycled noble gases but not due to shallow air contamination. In addition, the wide distribution range of 20Ne/36Ar also supports a subduction-related origin of neon and argon. Combined with major element data, the most appropriate tectonic setting to generate such noble gas signatures in Group I samples is subduction zone (probably forearc) where favorable conditions are present for the formation of the chromitites. In contrast, Group II samples containing carbonates have much more radiogenic 3He/4He ratios of 0.03 to 0.3 Ra but much less radiogenic 40Ar/36Ar ratios of 344 to 420. In combination with the occurrence of carbonate veins it is suggested that Group II samples are predominated by supracrustal components that may be imparted during or after the emplacement stage. A comparison of these two group samples indicates that the primary noble gas signatures reflecting the characteristics of ore-forming melts can be preserved in chromite and olivine grains (Group I samples) and thus used to trace the origin of podiform chromitites.

Tryon, MD, Wheat CG, Hilton DR.  2010.  Fluid sources and pathways of the Costa Rica erosional convergent margin. Geochemistry Geophysics Geosystems. 11   10.1029/2009gc002818   AbstractWebsite

The margins community has only relatively recently begun to examine the tectonics and associated hydrologic systems of erosive convergent margins, which are substantially different as compared with accretionary margins. In this respect, the type example erosive margin is the Costa Rica system, which has been the subject of numerous recent large-scale investigations. Here pore fluids expelled at the wedge toe and at midslope mounds and mud volcanoes have been interpreted to have a common deep source of dehydrating clays, analogous to that at accretionary margins. However, we report unusually high B/Li molar ratios in pore fluids from a recent mudflow on Mound 11, offshore Costa Rica, which, together with unusually low B/Li ratios previously reported at the wedge toe, reveal that alternative fluid sources and/or processes must be operating at the Costa Rica margin. As serpentine formation is the only subduction zone process that significantly fractionates B and Li, we propose that the difference in fluid chemical composition is the result of erosion of upper plate serpentinites, ongoing serpentinization, and serpentine mineral phase transitions in the subduction channel. These processes provide both a source of fluids and fluid pathways that lead to the unique geochemical signature observed at this erosional margin. This conclusion is compatible with, and supported by, the current view of the tectonics, geology, and hydrogeology of the Costa Rica margin and the similarity of the pore fluid to that of two other convergent margins, both with known fluid/serpentinite interactions.

Barnes, JD, Sharp ZD, Fischer TP, Hilton DR, Carr MJ.  2009.  Chlorine isotope variations along the Central American volcanic front and back arc. Geochemistry Geophysics Geosystems. 10   10.1029/2009gc002587   AbstractWebsite

Volcanic outputs (ash, tephra, and lava samples) of 23 volcanic centers from the volcanic front, secondary front, and back arc in Central America were analyzed for their delta(37) Cl values with the goal of using chlorine isotopes as a tracer of fluids sourced from the subducting Cocos slab. delta(37) Cl values range from -2.6 to +3.0% with systematic variations along the length of the front. Values from the northernmost (Guatemala and El Salvador) and southernmost (Costa Rica) ends of the front have mantle-like signatures. In contrast, delta(37) Cl values are both positive and negative in the center of the front (Nicaragua), implying a sediment and/or serpentinite-derived component. Geophysical observations are consistent with extensive hydration of the Cocos plate offshore of Nicaragua, in support of a serpentinite-derived fluid source. Fluids from dehydrating serpentinites may also incorporate Cl from overlying sediments, resulting in a multiple-source chlorine signature.