Volatile abundances of coexisting merrillite and apatite in the martian meteorite Shergotty: Implications for merrillite in hydrous magmas

Abstract

Whitlockite and merrillite are two Ca-phosphate minerals found in terrestrial and planetary igneous rocks, sometimes coexisting with apatite. Whitlockite has essential structural hydrogen, and merrillite is devoid of hydrogen. Whitlockite components have yet to be discovered in samples of extraterrestrial merrillite, despite evidence for whitlockite-merrillite solid solution in terrestrial systems. The observation of merrillite in meteoritic and lunar samples has led many to conclude that the magmas from which the merrillite formed were "very dry." However, the Shergotty martian meteorite has been reported to contain both apatite and merrillite, and recently the apatite has been shown to contain substantial OH abundances, up to the equivalent of 8600 ppm H 2 O. In the present study, we determined the abundances of F, Cl, H 2 O, and S in merrillite from Shergotty using secondary ion mass spectrometry (SIMS). We determined that the merrillite in Shergotty was properly identified (i.e., no discernible whitlockite component), and it coexists with OH-rich apatite. The absence of a whitlockite component in Shergotty merrillite and other planetary merrillites may be a consequence of the limited thermal stability of H in whitlockite (stable only at T <1050 °C), which would prohibit merrillite-whitlockite solid-solution at high temperatures. Consequently, the presence of merrillite should not be used as evidence of dry magmatism without a corresponding estimate of the T of crystallization. In fact, if a whitlockite component in extraterrestrial merrillite is discovered, it may indicate formation by or equilibration with hydrothermal or aqueous fluids.

Validity of the Apatite/Merrillite Relationship in Evaluating the Water Content in the Martian Mantle: Implications from Shergottite Northwest Africa (NWA) 2975

“…McCubbin et al [12,58] and Vander Kaaden et al [59] define the order of preference as follows: F >> Cl > OH. Hence, F plays an important role in stabilizing magmatic apatite, and during degassing, apatite compositions evolve towards the F-rich member.…”

“…Some grains contain Cl amounts at the limit of detection (<50 ppm). Trace abundances of halogens in extra-terrestrial merrillites have been reported [6,12,50].…”

“…Phosphates that occur in extra-terrestrial mafic rocks may provide information regarding water (volatiles) content in the environment leading to their formation and transformation (e.g., [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]). The magma source for Martian basalts is the Martian mantle; thus, the basaltic phosphates provide information on the Martian mantle composition as well as its evolution in the course of melting progression [11,[18][19][20][21].…”

“…The magma source for Martian basalts is the Martian mantle; thus, the basaltic phosphates provide information on the Martian mantle composition as well as its evolution in the course of melting progression [11,[18][19][20][21]. Volatile-bearing apatite (Ca 10 (PO 4 ) 6 (Cl, F, OH) 2 ), volatile-free merrillite (Ca 18 Na 2 Mg 2 (PO 4 ) 14 ) and the high-pressure phase tuite (γ-Ca 3 (PO 4 ) 2 ) are frequently observed in extra-terrestrial bodies ( [1,3,5,6,12,[22][23][24][25] and references therein). These minerals are thought to be of different origins due to their joint or separate occurrence and their chemical and structural transformation processes.…”

Calcium orthophosphates (CaPO 4 ): Occurrence and properties

“…Occasionally, other types of natural CaPO 4 are found as minerals, for example clinohydroxylapatite (Chakhmouradian and Medici 2006), staffelite (synonyms: staffelit, staffelita) belonging to carbonate-rich fluorapatites (chemical formula: Ca 5 [(F,O)(PO 4 ,CO 3 ) 3 ]) (Mason et al 2009; http:// www.mindat.org/gallery.php?min=9293) and DCPD (Klein 1901;Kaflak-Hachulska et al 2000). Furthermore, CaPO 4 were found in meteoric stones (Merrill 1917;McCubbin and Nekvasil 2008;McCubbin et al 2014). The world deposits of natural CaPO 4 are estimated to exceed 150 billion tons; from which approximately 85 % belong to phosphorites and the remaining *15 % belong to apatites (Cook et al 2005).…”