37 CRITICAL MINERAL DEPOSITS AND ORE SYSTEMS OF THE NORTHEAST

This session will showcase new findings pertaining to critical mineral resources and hydrothermal-magmatic ore systems across the northeast. We welcome submissions involving fieldwork, geochemical, petrologic, geochronologic, and other approaches from micro- to macroscopic scales.

Tuesday, 24 March 2026
8:00 AM - 12:00 PM
CCC, Room 25

Helium Isotopes in Beryl Suggest Mantle Involvement in Genesis of Evolved Granitic Pegmatites

Paper number: 37-8
Program: Technical Sessions
Time: 10:20 AM – 10:40 AM

Authors

Abstract

Although high concentrations of helium in beryl have been known since 1908, the processes behind this enrichment have not been fully explored. We report new 3He/4He isotopic results from a pilot study of representative beryls from four continents, which range from 2800 to 7 Ma. Nine beryls are from pegmatites, one is from a rhyolite, and one (an emerald) is from a hydrothermal vein. Beryl fragments were packed in 1 mm niobium foil and degassed under ultrahigh vacuum via diode laser heating at 1000°C for 3 minutes. Gas was cryogenically purified and measured on a quadrupole mass spectrometer. 

4He and 3He were observed in all samples, with replicate analyses yielding 4He concentrations ranging from 3.2x10-5 to 7.4x10-2 cm3 at STP/g, and 3He concentrations between 1.2x10-7 to 2.5x10-4 cm3 at STP/g. These data exhibit complex trends in age versus 4He and in age versus 3He; so far we have not confirmed the positive He vs age correlation reported in 1933 by Rayleigh, which used necessarily qualitative age groupings. 3He/4He values show a narrow range from 2.9–4.1x10-3 and possibly a weak negative correlation with age. We suggest that these trends together reflect degassing of primordial 3He and 4He through Earth history, with the growth of radiogenic 4He outweighing the loss of primordial 4He. Additional factors may include tectonic setting and post-emplacement He loss or gain by diffusion. Near uniformity in 3He/4He suggests that processes of helium enrichment affect both isotopes similarly across different settings and timescales.

Beryls of pegmatitic, volcanic, and hydrothermal origins all have elevated 3He/4He values, consistent with mantle involvement. Independent evidence for a mantle heat source is available for two samples: slab rollback in the case of beryl from the Elba pegmatite, delamination in the case of beryl from the Palermo pegmatite in New Hampshire. Mechanisms for conveying mantle helium to sites of crustal melting remain a mystery. Four general processes might have been involved yielding the measured abundances of 3He and 4He in a beryl crystal: initial capture during crystallization, subsequent ingrowth by radioactive decay, diffusion-in, and diffusion-out. Ongoing work is exploring stepped-heating degassing measurements to better constrain beryl helium diffusion parameters, and 3He/4He sampling profiles across the pegmatitic crystals to determine helium concentration gradients.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411438

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