Formation Of Melt Pockets Research Articles

Shock‐induced compaction, melting, and entrapment of atmospheric gases in Martian meteorites

The strongest evidence for a Martian origin of the SNC meteorites is the strong correlation between the rare gas abundances and isotopic compositions in shock‐induced melt pockets, and those measured for the Martian atmosphere. However, the formation of melt pockets and the entrapment of atmospheric gases remain poorly understood. Here we report the presence in the melt pockets of three Martian meteorites of the hollandite‐structured high‐pressure polymorph of feldspar. These occurrences set constraints on the continuum pressure (21–25 GPa), the local temperature increase (2000–2500 K) and the energy delivered during shock. We then test a mechanism for melt pocket formation by compaction of pre‐existing porous precursors. The model explains the local temperature increase required for melting and the presence of an atmospheric component in pores that were connected with the Martian atmosphere prior to the shock event.

Mantle Mush Compaction: a Key to Understand the Mechanisms of Concentration of Kimberlite Melts and Initiation of Swarms of Kimberlite Dykes

Kimberlite pipes or dykes tend to occur in clusters (a few kilometres in diameter) within fields 30–50 km in diameter. They are generally considered to originate from low degrees of partial melting of carbonated peridotite within zones of ascending mantle. Numerical modelling shows that at the depth of formation of kimberlite melts ( 200 km), mantle compaction processes can result in the formation of melt pockets a few tens of kilometres across, with melt concentrations up to 7%. The initiation of swarms of kimberlite dykes at the top of these melt pockets is inevitable because of the large excess pressure between the melt and the surrounding solid, which exceeds the hydraulic fracturing limit of the overlying rocks. After their initiation at mantle depth the swarm of dykes may reach the surface of the Earth when the entire cratonic lithosphere column is in extension. We propose that kimberlite fields represent the surface envelope of dyke swarms generated inside a melt pocket and that kimberlite clusters represent the discharge of melt via dykes originating from sub-regions of the pocket. This model reproduces the worldwide average diameter of kimberlite fields and is consistent with the observation that some of the main kimberlite fields display age ranges of c. 10Myr. It is deduced that, at the scale of the Kaapvaal craton, different fields such as Kimberley, N. Lesotho and Orapa, dated at 80–90Ma, probably result from synchronous melt pockets forming inside an ascending mantle flow. The same model could apply to the fields of the Rietfontein, Central Cape and Gibeon districts dated at 60–70Ma. It is suggested that the same mantle flow that produced the Kimberley, N. Lesotho and Orapa fields migrated over 20–30Myr a few hundred kilometres westward to form the Rietfontein, Central Cape and Gibeon fields.

Experimental generation of shock‐induced pseudotachylites along lithological interfaces

Abstract— To understand the mechanism of formation of shock‐induced pseudotachylites and particularly the role that rock heterogeneities and interfaces play in their formation, shock recovery experiments were carried out on samples consisting of two distinct lithologies (dunite and quartzite). It was possible to generate melt veins of 1–6 μm width along lithological interfaces at moderate shock pressures (6 to 34 GPa). The magnitudes of displacement along the interface, strain rate, and the kinetic heat production indicate that friction is an important heat source that largely contributes to the energy budget of the melt veins. The experimentally produced veins resemble natural S‐type pseudotachylites. The geometry of the veins depends on the orientation of the interface with respect to the shock front and includes strong variations in thickness, formation of melt pockets and injection veins, sudden changes in vein orientation, and sharp vein margins. Two types of melt were observed: vesicle‐free and vesicular melts. Dense vesicle‐free melt rock is likely to represent high‐pressure melts. Vesicular melts were also generated during shock compression, but they remained in a molten state during pressure release and continued shearing. Intermingling of comminuted olivine and melt suggests that ultracataclasis of olivine induced by a dynamic tensile failure is a precursor stage to frictional melting. Shock wave interferences at the lithological interface provide the necessary stress conditions to start dynamic failure of olivine. The composition of the frictional melts ranges from olivine‐normative to enstatite‐normative and is, thus, largely determined by olivine melting. The validity of the sequence of friction melting susceptibilities of rock‐forming minerals inferred from tectonically‐produced pseudotachylites is confirmed and can now be applied to ultra‐high strain rates during shock compression.