Phosphate and feldspar mineralogy of equilibrated L chondrites: The record of metasomatism during metamorphism in ordinary chondrite parent bodies

Abstract

In ordinary chondrites (OCs), phosphates and feldspar are secondary minerals known to be products of parent body metamorphism. Both minerals provide evidence that metasomatic fluids played a role during metamorphism. We studied the petrology and chemistry of phosphates and feldspar in petrologic type 4-6 L chondrites, to examine the role of metasomatic fluids, and to compare metamorphic conditions across all three OC groups. Apatite in L chondrites is Cl-rich, similar to H chondrites, whereas apatite in LL chondrites has lower Cl/F ratios. Merrillite has similar compositions among the three chondrite groups. Feldspar in L chondrites shows a similar equilibration trend to LL chondrites, from a wide range of plagioclase compositions in petrologic type 4 to a homogeneous albitic composition in type 6. This contrasts with H chondrites which have homogeneous albitic plagioclase in petrologic types 4-6. Alkali-and halogen-rich and likely hydrous metasomatic fluids acted during prograde metamorphism on OC parent bodies, resulting in albitization reactions and development of phosphate minerals. Fluid compositions transitioned to a more anhydrous, Cl-rich composition after the asteroid began to cool. Differences in secondary minerals between H and L, LL chondrites can be explained by differences in fluid abundance, duration, or timing of fluid release. Phosphate minerals in the regolith breccia, Kendleton, show lithology-dependent apatite compositions. Bulk Cl/F ratios for OCs inferred from apatite compositions are higher than measured bulk chondrite values, suggesting that bulk F abundances are overestimated and that bulk Cl/F ratios in OCs are similar to CI. IntroductionOrdinary chondrites (OCs) show a sequence of equilibration of both textural and chemical characteristics. These changes are described by petrologic types 3-6 and are generally accepted to represent progressive degrees of metamorphism which occur within 65 million years after accretion (Van Schmus and Wood 1967;Huss et al. 2006). Textural changes include recrystallization of the matrix, crystallization of chondrule mesostasis, blurring of the chondrule-matrix texture, and growth of secondary phases. Olivine and pyroxene show a wide range of compositions in type 3. Due to differences in the MgFe interdiffusion rate for olivine and pyroxene, equilibration in olivine occurs by petrologic type 4 followed by pyroxene which is equilibrated by type 5.The metamorphic environment on OC parent bodies is only poorly constrained and peak temperatures are only roughly defined. Peak temperatures for petrologic type 3 are not thought to exceed 500 ºC, types 4-5 range from 500-800 ºC, and type 6 from 800 ºC to the onset of melting at ~1000 ºC (Scott and Krot 2014). Additionally, although the presence of fluids has been inferred from mineralogical observations, the importance of fluids and their role during metamorphism is still poorly understood. Low petrologic type chondrites show evidence for the action of fluids from the presence of phyllosilicates (Alexander...

Multiple impact events on the L‐chondritic parent body: Insights from SIMS U‐Pb dating of Ca‐phosphates in the NWA 7251 L‐melt breccia

“…The chemical compositions of olivine (host Fa 24.9±0.7 , IMP Fa 25.4±1.2 , CIMV Fa 24.2±0.9 , and PIMV Fa 24.9±0.6 ) and low‐Ca pyroxene (host Fs 21.1±0.3 , IMP Fs 18.4–20.5 , CIMV Fs 18.3–25.3 , and PIMV Fs 15.8–20.6 ) from all the domains confirm that NWA 7251 is a typical L chondrite. The compositions of apatites from the host are also consistent with those of L chondrites (Lewis and Jones ) (Fig. ).…”

“…Histograms of Mg# (Mg/(Mg + Fe) × 100) values of merrillites in the CIMV (a), PIMV (b), IMP (c), and host (d). The Mg# values of merrillites from the Santa Barbara L4 chondrite (e) are plotted for comparison (Lewis and Jones ). The abbreviations are as in Fig.…”

“…; Lewis and Jones ). For example, merrillites from L5–6 chondrites typically have a relatively narrow range of Mg# values (L5: Mg# = 89–95; L6: Mg# = 84–94), whereas grains from L4 chondrites exhibit a large compositional range (Mg# = 69–95) (Lewis and Jones ). In the host of NWA 7251, Ca‐phosphate grains closely coexist with metals/sulfides and have a medium grain size (10–60 μm), as expected for formation during thermal metamorphism on the parent body.…”

“…Ca‐phosphates, both apatites and merrillites, are the main U‐bearing minerals and are ubiquitous in chondrites (e.g., Göpel et al. ; Lewis and Jones ). They have relatively low closure temperatures for Pb diffusion (450–500 °C; Cherniak et al.…”

“…On the other hand, in situ secondary ionization mass spectrometry (SIMS) U-Pb dating at the micrometer scale, integrated with secondary electron microscopy (SEM) images of U-bearing accessory minerals, is a powerful tool for reconstructing the complex metamorphic histories of samples (e.g., Wu et al 2008;Li et al 2014). Ca-phosphates, both apatites and merrillites, are the main U-bearing minerals and are ubiquitous in chondrites (e.g., G€ opel et al 1994;Lewis and Jones 2016). They have relatively low closure temperatures for Pb diffusion (450-500°C; Cherniak et al 1991) and can potentially record strong shock heating events.…”