Chemical and isotopic analyses of hydrocarbon-bearing fluid inclusions in olivine-rich rocks

Abstract

We examined the mineralogical, chemical and isotopic compositions of secondary fluid inclusions in olivine-rich rocks from two active serpentinization systems: the Von Damm hydrothermal field (Mid-Cayman Rise) and the Zambales ophiolite (Philippines). Peridotite, troctolite and gabbroic rocks in these systems contain abundant CH₄-rich secondary inclusions in olivine, with less abundant inclusions in plagioclase and clinopyroxene. Olivine-hosted secondary inclusions are chiefly composed of CH₄ and minor H₂, in addition to secondary minerals including serpentine, brucite, magnetite and carbonates. Secondary inclusions in plagioclase are dominated by CH₄ with variable amounts of H₂ and H₂O, while those in clinopyroxene contain only CH₄. We determined hydrocarbon abundances and stable carbon isotope compositions by crushing whole rocks and analysing the released volatiles using isotope ratio monitoring-gas chromatography mass spectrometry. Bulk rock gas analyses yielded appreciable quantities of CH₄ and C₂H₆ in samples from Cayman (4-313 nmol g^-1 CH₄ and 0.02-0.99 nmol g^-1 C₂H₆), with lesser amounts in samples from Zambales (2-37 nmol g^-1 CH₄ and 0.004-0.082 nmol g^-1 C₂H₆). Mafic and ultramafic rocks at Cayman exhibit δ¹³C_CH4 values of -16.7‰ to -4.4‰ and δ¹³C_C2H6 values of -20.3‰ to +0.7‰. Ultramafic rocks from Zambales exhibit δ¹³C_CH4 values of -12.4‰ to -2.8‰ and δ¹³C_C2H6 values of -1.2‰ to -0.9‰. Similarities in the carbon isotopic compositions of CH₄ and C₂H₆ in plutonic rocks, Von Damm hydrothermal fluids, and Zambales gas seeps suggest that leaching of fluid inclusions may provide a significant contribution of abiotic hydrocarbons to deep-sea vent fluids and ophiolite-hosted gas seeps. Isotopic compositions of CH₄ and C₂H₆ from a variety of hydrothermal fields hosted in olivine-rich rocks that are similar to those in Von Damm vent fluids further support the idea that a significant portion of abiotic hydrocarbons in ultramafic-influenced vent fluids is derived from fluid inclusions. This article is part of a discussion meeting issue 'Serpentinite in the Earth system'.

Keywords: abiotic organic synthesis; fluid inclusions; hydrothermal systems; methane; serpentinization.

Figure 1.

Bathymetric map of the eastern flank of the Mount Dent oceanic core complex on the Mid-Cayman Rise showing the locations of dive tracks and stations from the Cayman Trough Project. The distribution of rock types is shown for each numbered station. Samples used in this study are displayed as red circles. The approximate location of the Von Damm hydrothermal field (yellow star) is shown for comparison. Figure modified from Stroup & Fox [27]. (Online version in colour.)

Figure 2.

Map of the Zambales Ophiolite Complex in the Philippines displaying sampling locations and gas seeps. Figure modified from Dimalanta et al. [28] with gas seep GPS locations from Vacquand et al. [19]. (Online version in colour.)

Figure 3.

Schematic diagram of the stainless steel crushing device in line with an isotope ratio monitoring–gas chromatography mass spectrometry (irm-GCMS) system.

Figure 4.

Representative thin section photomicrographs showing trails of CH₄-rich secondary fluid inclusions cross cutting olivine in troctolite from the Mid-Cayman Rise (Sample ALV 623-3-2; plane-polarized light). (Online version in colour.)

Figure 5.

False-colour hyperspectral Raman maps showing the locations of mineral and gaseous species in secondary fluid inclusions hosted in olivine (a,b) and plagioclase (c) in Mid-Cayman Rise samples ALV 624-5-1 (a,c) and ALV 621-3-1 (b). Colours represent relative intensities of the characteristic Raman band for a particular phase, with warmer colours indicating higher signal intensities. Dashed lines represent the approximate outlines of the fluid inclusions. Brc, brucite; Cal, calcite; Ctl, chrysotile; Mag, magnetite. (Online version in colour.)

Figure 6.

Plot of ³He/⁴He (R/R_a) versus CH₄/³He for whole-rock samples from the Mid-Cayman Rise and Zambales ophiolite. Except for the gneissic metagabbro sample, all samples exhibit CH₄/³He ratios that are significantly higher than the average $\mathrm{\Sigma }{\text{CO}}_2{/}^3\text{He}$ ratio for mid-ocean ridge basalt (MORB). High CH₄/³He ratios may reflect inefficient helium extraction by crushing during the helium analyses or addition of He to the vent fluid from a source other than fluid inclusions. See text for discussion. (Online version in colour.)

Figure 7.

Ranges in carbon isotopic compositions of CH₄ from whole-rock samples in this study, Zambales gas seeps, mid-ocean ridge (MOR) vent fluids and Southwest Indian Ridge (SWIR) gabbros. δ¹³C_CH4 values in mafic and ultramafic rocks from Cayman encompass the δ¹³C value of dissolved CH₄ in Von Damm endmember fluids (−15.4‰), suggesting that CH₄ budgets at Von Damm may be influenced by interaction with olivine-rich rocks in the subsurface. δ¹³C values of CH₄ issuing from Zambales gas seeps at Los Fuegos Eternos (−7.0 ± 0.4‰) and Nagsasa (−5.6‰) lie in between the ranges of δ¹³C_CH4 in serpentinized dunite and peridotite from the ophiolite, suggesting that CH₄ gas in Zambales may be sourced from these two rock types. See text for discussion. Data are from [,,,,,,,–82].

Figure 8.

Ranges in carbon isotopic compositions of C₂H₆ from whole-rock samples in this study and ultramafic-influenced MOR vent fluids. Symbols with arrows denote minimum values due to C₂H₆ contamination detected from the crushing device (see text for details). The δ¹³C value of dissolved C₂H₆ in Von Damm fluids (−12.7‰) is similar to δ¹³C_C2H6 values for most Cayman samples, with the exception of ALV 620-5-1. See text for discussion. Data are from [14,16,71,72,81,82].

Figure 9.

Range in CH₄/C₂H₆ ratios from whole-rock samples in this study, Zambales gas seeps and ultramafic-influenced MOR vent fluids. CH₄/C₂H₆ ratios in Zambales gas seeps are similar to measured ratios in serpentinized dunite and peridotite from the ophiolite. By contrast, CH₄/C₂H₆ ratios in Von Damm vent fluids are more than three times higher than those in Cayman rocks. See text for discussion. Data are from [13,14,16,17,81].