Investigation of Magmatic Activities on Early Mars Using Igneous Mineral Chemistry in Gale Crater, Mars

One objective of rover missions is exploring the geological context of the surroundings. Over the years, igneous petrology and sedimentology have been disconnected, the first investigating magmatic processes and volcanic activities, and the second seeking environmental conditions in the past and assessing the habitability of the planet. Although different, one is related to the other: igneous rocks are altered and broken down, leading to the formation of sedimentary rocks, which can in turn be used to back out the nature of their magmatic source. The Curiosity rover that landed in the 3.7 Gyr old impact crater Gale is traveling through sedimentary rocks. About fifty float rocks have been observed, and several of them with ambiguous texture and composition have been classified as igneous or sedimentary depending on studies such as Jake_M. The composition of several unambiguous igneous rocks has been analyzed [4- 6] but their heterogeneity at a larger instrumental (measurement size < 2 cm) scale prevents the measurement of a bulk composition as performed on Earth. An original approach avoiding these two last issues is to consider igneous mineral chemistry analyzed within igneous and sedimentary rocks to assess magmatic processes that could have formed them. Most Curiosity data are used to explore ancient environmental conditions, and a significant number of compositional analyses are under-explored for constraining magmatic activities. We will present how we can make use of sedimentary data for investigating igneous processes in the vicinity of Gale crater.

Geological Context: We focus on the first 750
martian days, corresponding to measurements in a coherent
lacustrine sedimentary unit called Bradbury,
because all sedimentary rocks were sourced from the
same watershed and appear to have a consistent source
with minimal alteration [2-3]. Igneous detrital minerals
including feldspar and pyroxene, are observed in sedimentary
rocks. Monte Carlo models showed that minimal
cation loss is observed based on the composition
of all Bradbury rocks, implying negligible weathering
[3]. Although clay minerals are detected in few rocks
[7], chemical compositions of rocks can be explained
by a mixture of primary igneous minerals [3]. Variation
of composition within Bradbury rocks can be explained
by mineral sorting and one distinct source
component. While a common magmatic source is suggested,
Bradbury sediments likely come from several
volcanic eruptions from a single magmatic chamber [9-
10]. The occurrence of alkali minerals like sanidine
and K-rich rocks throughout Bradbury supports the
presence of a potassic component, likely trachytic,
while plagioclase and a mafic composition suggest a
basaltic component [8-9].
Instruments: Mineral chemistry can be estimated
by three instruments onboard Curiosity. The CheMin
instrument enables detection of mineral assemblages
using X-ray diffraction (XRD). Using Rietveld refinement,
each mineral is identified according to their 1D
XRD pattern [11]. Note that distinction between pyroxene
minerals is challenging with the CheMin instrument
due to overlapping peaks on XRD patterns
and low angular resolution of the instrument [12].
Then, using least square regression and optimization
algorithms based on unit-cell parameters, mineral
chemistry has been estimated by [11]. Plagioclase
composition has been estimated using the NaAlSi3O8-
CaAl2Si2O8 system and alkali feldspar is based on the
NaAlSi3O8-KAlSi3O8 system (stars in Fig.1). Two
mudstone samples (John Klein and Cumberland) and
one sandstone sample (Windjana) were analyzed by
CheMin at Yellowknife Bay and Kimberley, respectively.
Figure 1. Ternary diagrams of feldspars (top) and pyroxene
(bottom) quadrilateral. Stars correspond to CheMin composition
and the gray patches to ChemCam composition. The
colored dots are the composition of feldspar and pyroxene
that crystallized during fractional crystallization at FMQ+1 of a melt extracted at distinct melting degree during the adiabatic
ascent of a primitive mantle composition, without any
water (left panels) and with 0.5 wt.% of water (right panels)
at distinct pressure.
The ChemCam instrument enables the analysis of
the chemical compositions of rocks at hundreds of micrometer
scale (350-550 μm) using laser induced
breakdown spectroscopy (LIBS), which may provide
the composition of minerals when they are larger than
the beam spot (>550 μm) [13]. Within >5000 LIBS
points, we performed a typical stoichiometric filtering
allowing us to distinguish 56 feldspar and 10 pyroxene
mineral compositions (grey patches in Fig. 1).
Finally, the Alpha Particle X-ray Spectrometer
(APXS) analyzes the composition of rocks with a 1.6
cm diameter spot size. Monte Carlo mass balance
modeling allowed [3] to decipher a feldspar range varying
between An30 and An40 (Fig. 1).
Discussion: Although there could be a more complex
history and other ways to form the whole compositional
range of igneous minerals analyzed within the
Bradbury formation, we are presenting here simple
magmatic pathways commonly occurring on Earth
using the thermodynamical softwares pMELTS and
rhyoliteMELTS [14]. The objective is to find reasonable
igneous processes that produce minerals that parallel
the compositions of feldspar and pyroxene analyzed
by the Curiosity rover. As commonly observed for
mid-ocean ridge basalts, the adiabatic ascent of a primitive
mantle composition [15] partially melting at 2
GPa has been modeled, followed by the extraction of a
liquid at distinct degrees of partial melting, which undergoes
fractional crystallization at an oxygen fugacity
+1 log unit above the fayalite-magnetite-quartz (FMQ)
buffer within the crust (0.02-0.4 GPa) with H2O = 0-
0.5 wt. %. These latter conditions correspond to those
recorded within igneous clasts from the Noachian martian
breccia NWA 7034 and paired and within Gale
igneous rocks (colored dots in Fig. 1) [16-17]. To
check the reliability of these 2-step models, we also
tested fractional crystallization at similar conditions
(FMQ+1; P=0.02-0.4 GPa; H2O = 0-0.5 wt. %) of
starting compositions corresponding to that of magmas
with distinct melting degrees obtained from isobaric
experiments at 2 GPa [18]. Mineral compositions obtained
from both models are similar.
As shown on Fig. 1, the whole range of observed
feldspar compositions cannot be reproduced by fractionation
of one magma only. Indeed, while alkali feldspar
and Na-plagioclase likely crystallized from fractional
crystallization of a low-degree melt (here
<15%), plagioclase and pyroxene can only be formed
by fractional crystallization of a higher degree melt
(here >19%). The corresponding liquid descent lines
are broadly in agreement with compositions estimated
by ChemCam corresponding to float igneous rocks
(Fig. 2) [4-6].
Figure 2. Silica versus alkali content. Lines show the liquid
lines of descent from magmas with distinct degrees of melting.
Gray patches represents the composition of Gale igneous
rocks [4-6].
Trachytic to rhyolitic magmas crystallize alkali
feldspar, and andesite to dacite magmas likely form
plagioclase. Therefore, at least two starting magmas at
distinct melting degrees, which could easily come from
a single mantle source, are necessary to explain the
whole compositional range of feldspar and pyroxene
analyzed within Bradbury rocks.
Conclusion: Because rocks from the Bradbury
formation are likely originating from the same magmatic
source with minimal weathering as supported by
several studies using different approaches, igneous
mineral chemistry analyzed by CheMin and ChemCam
allows us to back out reasonable magmatic pathways
that could have crystallized them. Fractional crystallization
of at least two starting magmas originating from
distinct melting degrees of a single mantle source can
explain the whole range of feldspar and pyroxene
composition. Both alkaline and sub-alkaline liquids
can be produced, with compositions corresponding to
those of the igneous rocks analyzed by ChemCam
within the Bradbury formation, highlighting the complexity
of Mars magmatism.