3:30 PM - 3:45 PM
[PPS05-06] Geochemical end-members around the Caloris basin on Mercury: Insights into magmatic diversity and evolution
Keywords:Mercury, Elemental composition, Volcanic smooth plain, Crystal fractionation
Since the observation data from MESSENGER revealed that a large part of Mercury’s crust was formed by multi-stage volcanisms [e.g., 1,2], the surface heterogeneity is a key to understanding the interior evolution. Geological studies showed that the Caloris impact basin has interior and exterior volcanic smooth plains, which have distinct crater densities from surrounding impact ejecta and more densely cratered terrain [3,4]. VIS-NIR spectra showed a difference between the interior and exterior smooth plains, although the origin of this spectral difference has not yet been clarified [5,6]. Elemental composition maps have been produced from MESSENGER X-Ray Spectrometer (XRS) data and suggested a compositional difference between the interior and exterior of the Caloris basin [e.g.,7], but their lower spatial resolutions compared with spectral and geological data make a comprehensive interpretation difficult. To reveal the interior processes that produced the surface heterogeneity, this study aims to determine the end-member compositions associated with different geologic contexts and surface spectra through a detailed analysis of the footprint geometry of elemental composition observations.
We used the observation footprint data of XRS around the Caloris basin. First, the spatial extent of end-member units was defined based on spectral and geological units. A homogeneous elemental composition was assumed within each unit. Next, we modeled the observed elemental compositions as the spatial mixing of different end-member compositions within footprint areas. Mixing ratios were calculated by comparing footprint areas and the end-member unit distributions. Finally, we calculated the end-member compositions that best reproduce the observation data by least-square fitting.
We obtained 5 end-member units: Caloris Interior Plain, Northern Volcanic Plain, Caloris Exterior Plain, Intercrater Plain (IcP), and Ejecta. The derived end-member compositions constitute the observed surface compositions. The Caloris Interior Plain unit, where the smallest Mg/Si ratio was expected from previous elemental composition maps [7], represents a smaller Mg/Si ratio of 0.18±0.09 than the previous estimate of 0.28±0.03 [8]. The Caloris Exterior Plain unit shows a distinct composition from the IcP and Ejecta units, which was not resolved in previous spectral [5] and elemental composition maps [7].
The end-member units related to the young volcanism show similar compositions with low Mg/Si ratios of <0.4. These compositions agree with an evolutionary trend of magma during crystal fractionation predicted in an experimental study [9], suggesting that these separate volcanic activities shared a single common magma source. In contrast, IcP and Ejecta units associated with older geological ages show high Mg/Si ratios of >0.5 and are not consistent with the same trend, suggesting the existence of another magma source with a higher Mg content. According to our results, we propose a crustal formation scenario around the Caloris basin: The IcP unit formed from a high-Mg magma source, followed by the Caloris-forming impact along with the deposition of the Ejecta unit. Subsequently, magma eruptions from a low-Mg source formed the Caloris Exterior and Interior Plains sequentially with compositional change due to crystal fractionation. The two different magma sources could have originated from a common mantle source with different temperature-pressure conditions or from different mantle sources.
Reference [1] Byrne (2020) Nat. Astron., 4, 321-327. [2] Vander Kaaden & McCubbin (2015) JGR:Planets, 120, 195-209. [3] Denevi et al. (2013) JGR:Planets, 118, 891-907. [4] Rothery et al. (2017) JGR:Planets, 122, 560-576. [5] Caminit et al. (2023) JGR:Planets, 128, e2022JE007685. [6] Ernst et al. (2015) Icarus, 250, 413-429. [7] Nittler et al. (2020) Icarus, 345,113716. [8] Peplowski & Stockstill-Cahill (2019) JGR:Planets, 124, 2414-2429. [9] Namur et al. (2016) EPSL, 439, 117-128.
We used the observation footprint data of XRS around the Caloris basin. First, the spatial extent of end-member units was defined based on spectral and geological units. A homogeneous elemental composition was assumed within each unit. Next, we modeled the observed elemental compositions as the spatial mixing of different end-member compositions within footprint areas. Mixing ratios were calculated by comparing footprint areas and the end-member unit distributions. Finally, we calculated the end-member compositions that best reproduce the observation data by least-square fitting.
We obtained 5 end-member units: Caloris Interior Plain, Northern Volcanic Plain, Caloris Exterior Plain, Intercrater Plain (IcP), and Ejecta. The derived end-member compositions constitute the observed surface compositions. The Caloris Interior Plain unit, where the smallest Mg/Si ratio was expected from previous elemental composition maps [7], represents a smaller Mg/Si ratio of 0.18±0.09 than the previous estimate of 0.28±0.03 [8]. The Caloris Exterior Plain unit shows a distinct composition from the IcP and Ejecta units, which was not resolved in previous spectral [5] and elemental composition maps [7].
The end-member units related to the young volcanism show similar compositions with low Mg/Si ratios of <0.4. These compositions agree with an evolutionary trend of magma during crystal fractionation predicted in an experimental study [9], suggesting that these separate volcanic activities shared a single common magma source. In contrast, IcP and Ejecta units associated with older geological ages show high Mg/Si ratios of >0.5 and are not consistent with the same trend, suggesting the existence of another magma source with a higher Mg content. According to our results, we propose a crustal formation scenario around the Caloris basin: The IcP unit formed from a high-Mg magma source, followed by the Caloris-forming impact along with the deposition of the Ejecta unit. Subsequently, magma eruptions from a low-Mg source formed the Caloris Exterior and Interior Plains sequentially with compositional change due to crystal fractionation. The two different magma sources could have originated from a common mantle source with different temperature-pressure conditions or from different mantle sources.
Reference [1] Byrne (2020) Nat. Astron., 4, 321-327. [2] Vander Kaaden & McCubbin (2015) JGR:Planets, 120, 195-209. [3] Denevi et al. (2013) JGR:Planets, 118, 891-907. [4] Rothery et al. (2017) JGR:Planets, 122, 560-576. [5] Caminit et al. (2023) JGR:Planets, 128, e2022JE007685. [6] Ernst et al. (2015) Icarus, 250, 413-429. [7] Nittler et al. (2020) Icarus, 345,113716. [8] Peplowski & Stockstill-Cahill (2019) JGR:Planets, 124, 2414-2429. [9] Namur et al. (2016) EPSL, 439, 117-128.