“All I Think About Now,” the ninth song on the new Pixies album, Head Carrier, was borne of an iPhone mishap. Earlier this year, the band’s frontman, Black Francis (aka Charles Michael Kittridge Thompson IV and Frank Black), and guitar player, Joey Santiago, recorded a demo of a different song and sent it to their bass player, Paz Lenchantin. “We didn’t mix it very well,” Francis explained by phone last week. “The lead guitar was too loud or something. So she kind of heard it differently, and then composed this more complex bass part that she thought was going to fit in with this new song.”

When it didn’t fit, Lenchantin suggested they salvage the part. “She said, ‘You know, Charles, I like it. Maybe this can be a new song,’” Francis said. “So I said, ‘Yes, so it shall be. It will be a new song. Why don’t you sing it?’ And she says, ‘Okay. I’ll sing it if you write the words.’ I said, ‘Okay, I’ll write the words if you tell me what to write about.’ She said, ‘Write about Kim.’ And I said, ‘What do you mean write about Kim?’ She said, ‘Why don’t you write her a letter? Like a thank-you letter.’ And I was like, ‘Okay.’ And I did.”

The Kim to whom the song-letter is addressed is, of course, Kim Deal, the founding bass player of The Pixies (and frontwoman of The Breeders), who left the band in 2013 after a second reunion tour, reportedly due to tensions with Francis. Deal’s shoes were filled first by Kim Shattuck, the lead singer, guitarist, and songwriter of The Muffs; then by Lenchantin, an Argentina-born bassist and violinist who had toured with Santiago’s side project, The Martinis. Head Carrier, released on Friday, is the first full-length LP The Pixies have made without Deal, and it welcomes Lenchantin as an official member of the band. All of which has made “All I Think About Now” a focus of intrigue.

“If I was going to sing anything on this record, I wanted it to be about Kim,” Lenchantin said by phone the other day. “It wasn’t something we planned.” In that Pixies way of turning contrary forces into complementary ones, the song marries an upbeat, sunny melody with melancholy lyrics tinged with regret: I try to think about tomorrow / But I always think about the past / About the things that didn’t last. A few verses later: Remember when we were happy? / If I’m late, can I thank you now? / I’m gonna try anyhow.