Transport and eruption of mantle xenoliths creates a lagging problem

Crustal and mantle xenoliths, incorporated into magmas, are common in volcanic deposits. Crustal xenoliths provide insights into crustal age, origin, stratigraphy, thermal history, and conditions within magmatic systems. They also inform on conduit erosion and fragmentation during eruptions. Mantle xenoliths, found in alkaline magmatic deposits (nephelinites, basanites, lamprophyres, kimberlites), are crucial for understanding the subcontinental mantle lithosphere's mineralogical, geochemical, thermal, structural, age, and origin characteristics. Direct studies of the mantle lithosphere depend on successful xenolith sampling, entrainment, and eruption by mantle-derived melts. Mantle xenoliths also constrain magma ascent rates; their thermal equilibration with host magma, but not full chemical equilibration, indicates insufficient time for chemical equilibrium during transport. Various indicators of chemical disequilibrium have been used to estimate magma ascent rates, including high-temperature experiments quantifying mineral-melt reaction rates. The authors focus on the premise that magma must rise faster than xenoliths settle, implying a substantial lag time between the eruption of the sampling magma and its xenolithic cargo. This study explores the extent of this decoupling and its implications for heterogeneous and biased mantle material distributions, and diamondiferous mantle cargo transport.

Previous research extensively utilizes crustal and mantle xenoliths to understand various aspects of the Earth's crust and mantle. Studies on crustal xenoliths have focused on determining the age and origin of the crustal lithosphere, constructing stratigraphic cross-sections of the mid-to-lower crust, constraining its thermal history and state, and documenting the physical-chemical conditions within trans-crustal magmatic systems. In physical volcanology, these xenoliths provide insights into syn-eruptive conduit erosion and fragmentation depths during explosive eruptions. Concerning mantle xenoliths, research has emphasized their petrological, geochemical, and structural properties to understand the composition, thermal state, structural properties, age, and origins of the subcontinental mantle lithosphere. However, the decoupling between magma and xenolith transport and the implications of this lag time for the interpretation of xenolith data have not been comprehensively addressed.

The study uses Stokes' Law to calculate the terminal settling velocities (V_T) of mantle xenoliths in magmas, considering magma viscosity (η), xenolith radius (r), and the density contrast (Δρ). The relationship between magma viscosity and Stokes settling velocity is analyzed for various xenolith sizes. The critical viscosity (η_c) and settling velocity (V_Tc) are determined based on particle Reynolds numbers (Re_p). The concept of yield stress and its impact on xenolith-magma coupling are evaluated using the Yield number (Y). The critical xenolith diameter required to overcome the yield stress is calculated, considering various crystal volume fractions (φ). The analysis also incorporates a relationship between yield stress and particle volume fraction to determine the extent of xenolith-magma coupling during transport. The study explores the lag time (Δt) that develops between the surface eruption of magmas and their xenolith cargo. The lag time is calculated as a function of sampling depth (X), magma ascent velocity (V_m), and xenolith rise velocity (V_x = V_m + V_T). The model incorporates calculations for various xenolith sizes and sampling depths to evaluate the impact of these parameters on lag times. The study further investigates the influence of magma ascent rate on lag times for xenoliths of varying settling velocities. The concept of minimum magma supply (Dm) required to transport lagging xenoliths to the surface is explored. Finally, the implications of the lag time concept are discussed in the context of xenolith modification during transport, sorting, mixing, and biased sampling. Calculations were performed using MATLAB, with codes accessible via Github/Zenodo.

The analysis shows that significant lag times can occur between the eruption of a magma and the xenoliths it carries, especially for larger and deeper-sourced xenoliths. For a 25-cm diameter xenolith sampled at 50 km, the lag time can be approximately 3 hours, increasing to 5 hours at 80 km depth, assuming a magma ascent velocity of 4 m/s. Larger xenoliths show significantly longer lag times, and xenoliths larger than 36.5 cm may not reach the surface with a 4 m/s ascent rate. The model demonstrates that increasing magma ascent rates can drastically reduce lag times. The study highlights that deeper-sourced xenoliths are often erupted by later, deeper-situated magmas. The lag time concept explains the heterogeneous distribution of xenoliths in volcanic deposits and the potential for biased sampling of the mantle lithosphere. Extended residence times due to lagging lead to xenolith modification, including mechanical abrasion, partial chemical re-equilibration, and assimilation. The lag time concept is particularly significant for kimberlites, as deeply sourced, potentially diamond-bearing xenoliths can erupt hours after eruption onset. The eruption duration and volume are critical; short durations or low volumes may prevent deep-seated xenoliths from reaching the surface. For a kimberlite magma ascending at 4 m/s, approximately 11 hours of transit time are required from a 160 km depth, and a 25-cm xenolith sampled at this depth has a 7-hour lag time, resulting in an 18-hour total transit time. The study suggests a minimum erupted volume of 100 × 10⁶ m³ for transporting deep-seated, diamondiferous xenoliths, contrasting with smaller, diamond-poor kimberlites.

The findings address the research question by demonstrating the significant role of lag time in the transport and eruption of mantle xenoliths. This lag time, a consequence of the density difference between xenoliths and magma and the low viscosity of alkaline magmas, leads to a temporal decoupling between the eruption of the magma and the xenoliths. The significance of the results lies in their implications for interpreting mantle xenolith data. The heterogeneous distributions of xenoliths often observed in volcanic deposits can be explained by this lag time phenomenon, highlighting the importance of considering the eruption duration and volume when interpreting the source region of the xenoliths. The findings also emphasize the need to consider the potential for biased sampling of the mantle lithosphere, where deeper-sourced xenoliths may be underrepresented if the eruption is short-lived.

This study demonstrates the significant influence of lag time on the transport and eruption of mantle xenoliths. This lag time, driven by the density contrast between xenoliths and low-viscosity magma, explains heterogeneous xenolith distributions and potential biased mantle sampling. Eruption duration and volume are crucial factors in determining the successful eruption of deep-seated xenoliths, particularly in diamondiferous kimberlites. Future research should investigate the quantitative relationship between lag time, magma properties, and xenolith characteristics across a wider range of volcanic systems. Further research could focus on developing more sophisticated models incorporating factors such as magma rheology, bubble dynamics, and xenolith fragmentation.

The study relies on simplified models of xenolith settling and magma ascent, using Stokes' Law and assuming constant magma properties. The actual magma ascent velocities and rheology could be more complex, potentially affecting the accuracy of the lag time calculations. The model does not consider the potential for interactions between xenoliths during transport and the complex dynamics of magma ascent in real volcanic systems. The availability of quantitative field data on xenolith abundance, size distribution, and stratigraphic position is limited, relying on qualitative observations in some cases.