Publication

How Mineral Infillings Crystallize In Multiphase Inclusions From UHP Fluid Phase: First In Situ Synchrotron X-ray Measurements

Authors: Nadia Malaspina (University of Milano Bicocca) , Matteo Alvaro (University of Padua) , Fabrizio Nestola (University of Padova) , Marcello Campione (University of Milano Bicocca)
Co-authored by industrial partner: No

Type: Conference Paper
Conference: European Current Research On Fluid Inclusions (ECROFI-XXIII)
Peer Reviewed: No

State: Published (Approved)
Published: July 2015
Diamond Proposal Number(s): 8754

Abstract: Remnants of the fluid phase at ultrahigh pressure (UHP) in subduction environments may be preserved as primary multiphase inclusions in UHP minerals. Saline aqueous inclusions with variable solute load prevail in high pressure (HP) rocks (e.g. [1,2]), whereas multiphase solid inclusions in some UHP rocks have been attributed to silicate-rich fluids or hydrous melts at supercritical conditions (e.g. [3-7]). These inclusions are frequently hosted by minerals stable at mantle depths, such as garnet, and show the same textural features as fluid inclusions (Fig. 1). The mineral infillings of the solid multiphase inclusions are generally assumed to have crystallized by precipitation from the solute load of dense supercritical fluids equilibrating with the host rock. Notwithstanding the validity of this assumption, the mode of crystallization of daughter minerals during precipitation within the inclusion and/or the mechanism of interaction between the fluid at supercritical conditions and the host mineral are still poorly understood from a crystallographic point of view. A case study is represented by garnet orthopyroxenites from the Maowu Ultramafic Complex (China) deriving from harzburgite precursors metasomatised at ∼ 4 GPa, 750 °C by a silica- and incompatible trace element- rich fluid phase. This metasomatism produced poikilitic orthopyroxene and inclusion-rich garnet porphyroblasts. Solid multiphase primary inclusions in garnet display a size within a few tens of micrometers and negative crystal shapes. Infilling minerals (spinel: 10–20 vol.%; amphibole, chlorite, talc, mica: 80– 90 vol.%) occur with constant volume ratios and derive from trapped solute-rich aqueous fluids [5]. To constrain the possible mode of precipitation of daughter minerals, we performed for the first time a single-crystal X-ray diffraction experiment by means of Synchrotron Radiation at Diamond Light Source. In combination with electron probe microanalyses, this measurement allowed the unique identification of each mineral phase and, most importantly, of their reciprocal orientation with uncertainties typically smaller than 2° (see [8] for further details). Applying this methodology for the first time to multiphase solid microinclusions, we have been able, for example, to infer possible epitaxy and to quantify preferred crystallographic orientation. We demonstrated the epitaxial relationship between spinel and garnet (Fig. 1) and between some hydrous minerals. Epitaxy drives a first-stage nucleation of spinel under near-to-equilibrium conditions, likely promoted by a dissolution and precipitation mechanism between the UHP fluid and the host garnet. A second-stage nucleation involved hydrous phases (amphiboles, chlorite and phlogopite), which nucleate in a non-registered manner and under far-from-equilibrium conditions. From the mineral chemistry of the mineral infillings and the crystallization sequence, nucleation and subsequent precipitation of the mineral phases occurred as a consequence of a fluid/garnet interaction. This conclusion is in agreement with previous studies on multiphase solid inclusions from UHP rocks which claimed that the precipitation process is due to post-entrapment modifications, such as dissolution and precipitation of the host mineral walls [9] and/or passive water diffusion from the inclusion to the host [10]. Such information is discussed in relation to physico-chemical aspects of nucleation and growth shedding light on the mode of mineral crystallization from a fluid phase at supercritical conditions.

Subject Areas: Earth Science


Instruments: I15-Extreme Conditions

Added On: 08/04/2016 13:41

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