Determination of the trapping temperatures and pressures of high-internal pressure CO2-bearing boiling fluid inclusion assemblages in the carbonatite-related REE deposit at Weishan, Shandong Province, China

• HDAC enables microthermometry of high pressure fluid inclusions in REE deposits. • A new P - T correction method minimizes pressure effects on temperature measurements. • Boiling FIA trapping conditions were 682–692 °C and 646–734 MPa for Weishan deposit. • CO 2 -bearing supercritical fluids were key to REE transport and mineralization. • Advance high pressure inclusion study in carbonatite-related REE deposits globally. Carbonatite-related rare earth element (REE) deposits are the world’s primary source of light REEs. The Weishan deposit in Shandong Province, China—ranked third in the country in rare earth oxide reserves—exemplifies such mineralization. Petrographic studies reveal abundant CO 2 -bearing fluid inclusions in its high-grade ore bodies. Due to their high-internal pressure, these inclusions frequently decrepitate or leak when heated on conventional heating-freezing stages (e.g., the Linkam THMSG600) at atmospheric pressure, preventing the acquisition of total homogenization temperature ( T h,tot ) data. This limitation has hindered understanding of the evolutionary process and thermodynamic properties of the ore-forming fluids. To address this, we employed a hydrothermal diamond anvil cell (HDAC) to perform high pressure microthermometry on CO 2 -bearing boiling fluid inclusion assemblages (FIAs) from the Weishan deposit. By maintaining external pressure on the high-internal pressure fluid inclusions throughout heating, we successfully obtained the T h,tot s and introduced a newly established two fitted lines intersection method to correct the T h,tot s measured with the HDAC, minimizing the influence of external pressures on the measured T h,tot s. As a result, the trapping temperatures and pressures of the CO 2 -bearing boiling FIAs in the Weishan deposit were determined to be 682–692 °C and 646–734 MPa, respectively. These results define the temperature and pressure conditions under which depressurization immiscibility and fluid exsolution occurred within the magmatic system, and confirmed the existence of CO 2 -bearing supercritical fluids in the deposit. In the future, the experimental method established herein is well-suited for thermodynamic measurements of CO 2 -bearing boiling FIAs in carbonatite-related REE deposits, which often contain high-internal pressure fluid inclusions with complex multicomponent system such as H 2 O-Na 2 SO 4 -Na 2 CO 3 –CO 2 .