This study investigates how a professional learning approach that draws on elements from collaborative autoethnography (CAE) can support science teachers' learning about argumentation. It provides an account of how six secondary science teachers collectively explored their views and understandings of the importance of relationships for fostering argumentative sensemaking in classrooms. The educators partnered across four sessions to identify themes that emerged from their autoethnographic writings and discussions. The construct of "diffraction" later helped provide a situated, entangled analysis of how ideas traveled within the group over time. Findings highlight how teachers surfaced the importance of cultivating trusting classroom relationships (between teachers and students as well as between students with one another) to foster the social dialogic elements of argumentation and collective sensemaking. This insight is one not generally emphasized in teacher professional development related to argumentation and has only recently been examined in the research literature. Teachers also reclaimed the idea of "rigor" to encompass discourse that is connected to students' lives and engages them in knowledge-building with others. This study demonstrates how a CAE-inspired teacher professional development model that emphasizes teacher agency and professional knowledge can help educators develop nuanced understandings of argumentation. As more classrooms focus on engaging students in argumentative practices, this study suggests the need for the field of science education to shift its focus to attend more fully to the role of classroom relationships, vulnerability, and trust. This study also suggests promising strategies for helping teachers increase their commitment to enacting productive and expansive classroom argumentation practices that center students' experiences, value diverse sensemaking, and increase equitable opportunities for learning.