H/ H measurements of amphiboles and nominally anhydrous minerals (clinopyroxene, garnet and diamond) using high-temperature continuous flow elemental analyser/pyrolysis/isotope ratio mass spectrometry.

Rationale: We have used a high-precision, high-efficiency method for the measurement of the H/ H ratios of hydrous silicates (amphiboles) and nominally anhydrous minerals (NAM) such as clinopyroxene, garnet and diamond, which are usually extremely resistant to pyrolysis. This opens up new fields of investigation to better understand the conditions of formation for deep-Earth minerals.

Methods: The technique described here involves Isotope Ratio Mass Spectrometry (IRMS) on-line in continuous flow mode with an Elemental Analyser (EA) using "purge and trap" technology rather than separation by conventional packed column gas chromatography (GC). The system is equipped with a special high-temperature furnace reaching 1500°C, with a longer hot zone and improved temperature stability. Emphasis is put on the efficiency of the system to reliably pyrolyse refractory minerals difficult to analyse with other conventional systems.

Results: While conventional systems usually fail to generate hydrogen suitable for isotopic analyses, with the technique presented here we were able to measure H/ H ratios from four diamond samples (δ H = -60, -77, -84 and -79‰ V-SMOW; average SD = 4.5‰; n = 2), three garnet samples (δ H from -70 to -63‰), and nine clinopyroxenes (δ H from -92 to -58‰) associated with seven amphiboles (δ H from -76 to -27‰) from single mantle rock.

Conclusions: The possibility of using such a system to reliably measure H/ H ratios from refractory minerals, which are usually extremely difficult to analyse, offers a new tool of investigation for providing us with unrivaled clues to study the deep interiors of Earth.